U.S. patent application number 10/877220 was filed with the patent office on 2005-02-10 for near field optical apparatus.
This patent application is currently assigned to Research Investment Network, Inc.. Invention is credited to Hesselink, Lambertus, Shi, Xiaolei, Thornton, Robert L..
Application Number | 20050030993 10/877220 |
Document ID | / |
Family ID | 34118182 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050030993 |
Kind Code |
A1 |
Thornton, Robert L. ; et
al. |
February 10, 2005 |
Near field optical apparatus
Abstract
A near field optical apparatus comprising a conductive sheet or
plane having an aperture therein with the conductive plane
including at least one protrusion which extends into the aperture.
The location, structure and configuration of the protrusion or
protrusions can be controlled to provide desired near field
localization of optical power output associated with the aperture.
Preferably, the location, structure and configuration of the
protrusion are tailored to maximize near field localization at
generally the center of the aperture. The aperture preferably has a
perimeter dimension which is substantially resonant with the output
wavelength of the light source, or is otherwise able to support a
standing wave of significant amplitude. The apparatus may be
embodied in a vertical cavity surface emitting layer or VCSEL
having enhanced nearfield brightness by providing a conductive
layer on the laser emission facet, with, a protrusion of the
conductive layer extending into an aperture in the emission facet.
The aperture in the emission facet preferably has dimensions
smaller than the guide mode of the laser, and the aperture
preferably defines different regions of reflectivity under the
emission facet. The depth of the aperture can be etched to provide
a particular target loss, and results in higher optical power
extraction from the emission facet.
Inventors: |
Thornton, Robert L.; (Los
Altos, CA) ; Shi, Xiaolei; (Niskayuna, NY) ;
Hesselink, Lambertus; (Atherton, CA) |
Correspondence
Address: |
DISCOVISION ASSOCIATES
INTELLECTUAL PROPERTY DEVELOPMENT
2355 MAIN STREET, SUITE 200
IRVINE
CA
92614
US
|
Assignee: |
Research Investment Network,
Inc.
Irvine
CA
92614
|
Family ID: |
34118182 |
Appl. No.: |
10/877220 |
Filed: |
June 25, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10877220 |
Jun 25, 2004 |
|
|
|
09650969 |
Aug 29, 2000 |
|
|
|
60151492 |
Aug 30, 1999 |
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Current U.S.
Class: |
372/43.01 |
Current CPC
Class: |
H01S 5/18375 20130101;
H01S 5/18327 20130101; H01S 5/18377 20130101; H01S 5/18338
20130101; H01S 5/18355 20130101; H01S 2301/18 20130101; H01S
5/18391 20130101 |
Class at
Publication: |
372/043 |
International
Class: |
H01S 005/00 |
Claims
What is claimed is:
1. A near field optical apparatus comprising: a light source; and a
conductive layer defining a subwavelength aperture, the aperture
having a perimeter, the conductive layer having at least one
protrusion extending into the aperture at the perimeter, wherein
the protrusion into the aperture is of sufficient size to produce a
transmission mode with high throughout.
2. The near field optical apparatus of claim 1, wherein the
aperture has a perimeter dimension which is substantially resonant
with an output wavelength of light passing through the
aperture.
3. A near field optical apparatus comprising: a light source; and a
conductive plane proximate to the light source, the conductive
plane having a subwavelength aperture positioned such that light
from the light source passes through the aperture, the conductive
plane including at least one protrusion which extends into the
aperture, wherein the protrusion into the aperture is of sufficient
size to produce a transmission mode with high throughput.
4. The near field optical apparatus of claim 3, wherein the
aperture has a perimeter dimension which is substantially resonant
with an output wavelength of the light source.
5. A semiconductor laser apparatus comprising a light source and an
emission facet having a conductive surface, the conductive surface
having a subwavelength aperture therein, the conductive surface
including at least one protrusion extending into the aperture,
wherein the protrusion into the aperture is of sufficient size to
produce a transmission mode with high throughput.
6. The near field optical apparatus of claim 5, wherein the
aperture has a perimeter dimension which is substantially resonant
with an output wavelength of the light source.
7. A near field optical apparatus comprising a light source and a
conductive plane having a subwavelength aperture therein, the
aperture including a plurality of spaced apart slots, and at least
one connector region joined to each adjacent the spaced apart slot,
wherein the slots of the aperture are of sufficient size to produce
a transmission mode with high throughput.
8. The near field optical apparatus of claim 7, wherein the
aperture has a perimeter dimension which is substantially resonant
with an output wavelength of the light source.
9. A semiconductor laser apparatus, comprising: a laser active
region; a first reflective region adjacent a first side of the
active region; a second reflective region adjacent a second side of
the active region; an emission face proximate to the first
reflective region, the emission face including a reflective,
conductive layer thereon; and the emission face including a
subwavelength aperture extending through the reflective conductive
layer and into at least a portion of the first reflective region,
the reflective conductive layer including at least one protrusion
which extends into the aperture, wherein the protrusion into the
aperture is of sufficient size to produce a transmission mode with
high throughput.
10. The near field optical apparatus of claim 9, wherein the first
slot includes a first end, the second slot includes a first end,
and the connector region is positioned adjacent the first ends of
the first and second slots.
11. A semiconductor laser comprising: a laser active region; an
first conductivity type upper reflective region adjacent an upper
side of the active region; a second conductivity type lower
reflective region adjacent a lower side of side active region; and
an emission facet adjacent the upper reflective region, and
emission facet having a subwavelength aperture therein, the
aperture smaller than a guide mode of the semiconductor laser, the
upper reflective region having at least one protrusion extending
into the aperture, wherein the protrusion is of sufficient size to
produce a transmission mode with high throughput.
12. The near field optical apparatus of claim 11, wherein the first
slot includes a first end, the second slot includes a first end,
and the connector region is positioned adjacent the first ends of
the first and second slots.
13. An apparatus comprising: a conductive layer positioning in
association with a light source wherein radiation of the light
source passes through an aperture having a perimeter; the
conductive layer having at least one protrusion extending into the
aperture to produce a transmission mode with high throughout.
14. The apparatus of claim 13, wherein the aperture has a perimeter
dimension which is substantially resonant with an output wavelength
of light passing through the aperture.
15. An apparatus, comprising: a conductive plane proximate to a
light source, the conductive plane having a subwavelength aperture
positioned such that light from the light source passes through the
aperture, the conductive plane including at least one protrusion
which extends into the aperture, the protrusion being of sufficient
size to produce a transmission mode with high throughput.
16. The apparatus of claim 15, wherein the aperture has a perimeter
dimension which is substantially resonant with an output wavelength
of light passing through the aperture.
17. An apparatus comprising a light source and an emission facet
having a conductive surface, the conductive surface having a
subwavelength aperture, the conductive surface including at least
one protrusion extending into the aperture, the protrusion being of
sufficient size to produce a transmission mode with high
throughput.
18. The apparatus of claim 17, wherein the aperture has a perimeter
dimension which is substantially resonant with an output wavelength
of the light source.
19. An apparatus comprising a light source and a conductive plane
having a subwavelength aperture, the aperture including a plurality
of spaced apart slots, and at least one connector region joined to
each adjacent the spaced apart slot, wherein the slots of the
aperture are of sufficient size to produce a transmission mode with
high throughput.
20. The apparatus of claim 19, wherein the aperture has a perimeter
dimension which is substantially resonant with an output wavelength
of the light source.
21. An apparatus, comprising: a first reflective region adjacent a
first side of an active region; a second reflective region adjacent
a second side of the active region; an emission face proximate to
the first reflective region, the emission face including a
reflective, conductive layer, the emission face including a
subwavelength aperture extending through the reflective conductive
layer and into at least a portion of the first reflective region,
the reflective conductive layer including at least one protrusion
which extends into the aperture, wherein the protrusion is of
sufficient size to produce a transmission mode with high
throughput.
22. The apparatus of claim 21, wherein the aperture has a perimeter
dimension which is substantially resonant with an output wavelength
of the light source.
23. A semiconductor laser comprising: a laser active region; a
first conductivity type upper reflective region adjacent an upper
side of the active region; a second conductivity type lower
reflective region adjacent a lower side of active region; and an
emission facet adjacent the upper reflective region, the emission
facet having a subwavelength aperture smaller than a guide mode,
the upper reflective region having at least one protrusion
extending into the aperture, wherein the protrusion is of
sufficient size to produce a transmission mode with high
throughput.
24. The apparatus of claim 23 wherein the aperture has a perimeter
dimension which is substantially resonant with an output wavelength
of the light source.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation of U.S. Non-provisional
patent application Ser. No. 09/650,969, filed on Aug. 29, 2000,
which claims benefit to U.S. Provisional Patent Application Ser.
No. 60/151,492, filed on Aug. 30,1999, and all of which are
incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention pertains generally to near-field optical
devices and methods, and more particularly to an optical emitter
apparatus and method utilizing a conductive plane with a resonant
aperture configured for near field localization. The invention may
be used with edge emitting, corner emitting and surface emitting
semiconductor lasers as well as other near field radiation
sources.
[0004] 2. Description of the Background Art
[0005] Near-field optical techniques require the use of radiation
source apertures and distances on the order of generally less than
the wavelength .lambda. of the radiation source. Near field optical
technologies offer high optical data storage densities, increased
microscopic resolution, and other advantages. Near field
technologies have utilized surface and edge emitting lasers,
tapered optical fibers with metallized surfaces, and solid
immersion lens techniques.
[0006] One class of semiconductor lasers which has appeared
promising for near-field applications are vertical cavity, surface
emitting lasers or VCSELs. These surface or top-emitting lasers
generally have laser cavity or "post" with an active region
surrounded by stacks of interleaved quarter wave semiconductor
layers which define mirrors about the active region laser cavity.
The active region can be in bulk form or have single or multiple
quantum well, quantum wire and/or quantum dot structures therein.
There are also p- and n-type conductive regions included on
opposite sides of the active region, and the VCSEL can be turned on
and off by varying the current through the p-n junction diode.
[0007] Top surface emitting GaAs, AlGaAs, AlGaInP, InGaAs, InGaAsP
and InP VCSEL devices are relatively easy and inexpensive to
manufacture, and generally can be produced via low-cost, high
volume semiconductor IC fabrication methods using metal organic
vapor phase epitaxy (MOVPE), liquid phase epitaxy (LPE) or
molecular beam epitaxy (MBE) techniques. The laser cavity
structures are typically deposited or grown vertically on a
substrate and have an emission face defined by ion implantation,
lateral oxidation, by polyimide or other dielectric, or
free-standing post. The lateral oxidation and dielectric
encapsulant techniques will generally provide for a lower effective
refractive index in the region bounding the core of the laser
structure, resulting in improved optical confinement relative to
the ion implanted emission face. However, VCSEL devices of both
types generally have been demonstrated with good reproducibility,
uniformity and reliability.
[0008] Currently available VCSEL devices can be designed to
effectively provide transverse and longitudinal mode laser light
with a relatively high degree of intrinsic polarization. VCSELs
also provide a radially symmetric Gaussian near-field with low
divergence angle, which simplifies coupling to optics or fibers. An
important drawback of VCSEL devices, however, is that they provide
relatively low single mode optical power output compared to edge
emitting lasers. The limitation on power output has limited the use
of VCSEL devices in near-field technologies. Edge-emitting diode
laser devices are known which provide higher optical power, but
such devices are much more difficult and expensive to manufacture,
and further require relatively large drive currents for
operation.
[0009] For the purposes of near field optical recording, the total
power requirement is modest and within the capability of the VCSEL
device. However, the power density at the emission facet is greatly
reduced relative to the edge emitter. Since in near field
applications it is the power density which is operative, the
conventional edge emitter is advantaged over the conventional
VCSEL.
[0010] There is accordingly a need for a near field optical device
which provides high optical power density, which provides good near
field localization, and which can be embodied in vertical cavity
surface emitting lasers, edge emitting lasers and other radiation
sources. The present invention satisfies these needs, as well as
others, and generally overcomes the deficiencies found in the
background art.
SUMMARY OF THE INVENTION
[0011] The present invention pertains to a near field optical
apparatus which provides high output power with effective near
field localization. In its most general terms, the invention is a
near field optical apparatus comprising a conductive sheet or plane
having an aperture therein, with the conductive plane including at
least one protrusion which extends into the aperture. The location,
structure and configuration of the protrusion or protrusions can be
controlled to provide desired near field localization of optical
power output associated with the aperture. Preferably, the
location, structure and configuration of the protrusion are
tailored to maximize near field localization at generally the
center of the aperture. The aperture preferably has a perimeter
dimension which is substantially resonant with the output
wavelength of the light source, or is otherwise able to support a
standing wave of significant amplitude.
[0012] By way of example, and not of limitation, the near field
apparatus will also preferably include a light source. The
conductive layer or plane may comprise a layer or sheet of gold,
silver, platinum or other highly conductive metal or metal alloy
associated with the light source. The protrusion of the conductive
plane generally defines first and second regions in the aperture
which are separated, or at least partially separated, by the
protrusion. The protrusion also defines generally a waist or
connecting section which joins or connects the first and second
regions.
[0013] The first and second regions may be elongated in the
direction of polarization of the light source. The protrusion may
be in the form of a stub or tab which is rectangular, rounded, or
pointed in shape or formed as a truncated point. The first and
second regions may comprise first and second slots or slits which
are substantially parallel to each other, with the slots separated
by the protrusion of the conductive plane, and joined together by
the connector region. The protrusion may be insular in nature, such
that it is electrically isolated from the surrounding conductive
plane.
[0014] In some presently preferred embodiments, the light source
comprises a semiconductor laser, with the conductive plane
comprising a metal layer associated with the emission facet of the
laser, such that the invention provides a semiconductor laser
apparatus with enhanced near field brightness. The invention may be
embodied in a vertical cavity surface emitting laser comprising a
laser apparatus having a laser active region, a first or upper
reflective region adjacent one side of the active region, a second
or lower reflective region adjacent the opposite side of the active
region, a conductive and reflective metal layer adjacent the outer
surface of the first reflective region, and an aperture in the
metal layer which extends inward or downward through a portion of
the reflective region, and which has dimensions which are generally
smaller than the guide mode of the laser apparatus. The aperture is
configured such that at least one protrusion in the metal layer
extends into the aperture.
[0015] By way of example, and not of limitation, the active region
of the laser apparatus preferably comprises a plurality of quantum
well and quantum barrier structures. In one preferred embodiment of
the invention, the upper reflective region preferably comprises a
first or upper set of distributed Bragg reflector or DBR mirrors,
and the lower reflective region preferably comprises a second or
lower set of DBR mirrors. The upper DBR mirror set preferably
comprises a plurality of p-doped, quarter wave dielectric layer
pairs, and the lower DBR mirror set preferably comprises a
plurality of n-doped quarter wave layer pairs. A p-doped
semiconductor layer is preferably included between the quantum well
active region and the upper, p-doped DBR mirror set, and an n-doped
semiconductor layer is preferably included between the quantum well
active region and the n-doped DBR mirror set. The conductive layer
is preferred highly reflective, and may act as a mirror together
with the upper DBR mirror set.
[0016] Preferably, a semiconductor contact layer is positioned
between the reflective conducting layer and the upper DBR mirror
set. Means are provided for optimizing adhesion of the reflective
metal layer to the semiconductor contact layer, and means are
provided for reducing reactivity between the reflective conducting
layer and semiconductor contact layer. The adhesion optimization
means and reactivity reducing means preferably comprise an oxide
layer, preferably TiO.sub.2, and an AlGaAs layer, positioned
between the reflective conducting layer and the semiconductor
contact layer.
[0017] The aperture in the emission face preferably extends inward
from the emission face into the upper reflective layer such that a
region of lower reflectivity is defined beneath the aperture. In
one embodiment, a smaller number of dielectric layer pairs are
present between the bottom of the aperture and the active region
than are present between the surrounding emission face and the
active region. The aperture in the emission face may extend inward,
for example, to a depth such that there are between about one half
and one quarter fewer quarter wave dielectric layer pairs between
the bottom of the aperture and the active region than are between
the emission face and the active region. In the embodiment with a
metal reflective layer, the aperture preferably extends inward
through reflective metal layer, such that the emission face
surrounding the aperture has reflectivity from the reflective metal
layer as well as a plurality of dielectric layer pairs, while the
area beneath the aperture has reflectivity only from the plurality
of dielectric layer pairs.
[0018] The aperture is dimensioned smaller than the guided mode of
the laser apparatus, such that the aperture defines a region with a
different reflectivity than the surrounding portions of the
emission face, and so that the emission face overall presents two
regions with different reflectivities. The region surrounding the
aperture provides a higher reflectivity, due to greater thickness
of the upper DBR mirror set and/or the presence of reflective metal
layer, and presents a region having generally reduced laser loss
and reduced threshold current, but with relatively reduced
efficiency. The region of the upper DBR mirror set under the
aperture has relatively greater mirror losses and higher laser
threshold current due to the smaller number of dielectric layer
pairs and/or the absence of a metallic reflective layer.
[0019] The inclusion of an aperture in the emission facet allows
access to the high E.sup.2-field region within the upper DBR mirror
set, and increases the power density of the laser apparatus of the
invention. The size and depth of the aperture can be varied to
provide a selected or target loss. By reducing the number of
dielectric layer pairs in the upper DBR mirror, then depositing a
highly reflective metal layer on top of the DBR mirror to make up
the reflectivity difference, and then etching an aperture through
the reflective metal layer to access the high E.sup.2-field region
immediately inside the reflective metal layer, power densities
similar to edge-emitting laser devices can be achieved over the
dimension of the emitting aperture.
[0020] The aperture structure of the invention is based on several
considerations. Polarization considerations are critical to
optimizing the E.sup.2-field strength at the center of the
aperture, and the aperture is preferably configured to optimize or
take advantage of polarization effects. In the case of an elongated
slot, for example, as slot width decreases, the electric field
components which are perpendicular to the edges of the slot can be
supported more readily than electric field components which are
parallel to the edges. Output power confinement and, to a lesser
extent, geometry confinement of the aperture, are dependent on
polarization.
[0021] Resonance effects associated with the perimeter of the
aperture are also important considerations in the aperture
configuration. When the physical size or dimensions of the aperture
decrease to substantially less than the output wavelength, the
aperture structure is decreasingly able to support a standing wave
of significant amplitude around its perimeter.
[0022] The aperture configuration also must take areal effects into
consideration. Generally, as the total aperture area decreases, the
total emission throughput will increase. This consideration is most
important for apertures of dimensions which are larger than the
output wavelength.
[0023] The aperture can also be associated with an impedance value
which should be considered. The aperture may define a radiative
element in association with the laser as a transmission line which
has an effective impedance, which should ideally be matched through
the impedance of the aperture structure, to the impedance of the
region into which the radiator is being coupled.
[0024] Yet another consideration in aperture design in accordance
with the invention are local field effects. The geometry or
structure of the aperture and emission facet should be adjusted to
localize and maximize the total field intensity of
E.sub.x.sup.2+E.sub.y.sup.2+E.sub.z.sup.2.
[0025] With the above considerations in mind, the use of one or
more conductive protrusions which extend into the aperture, as
noted above, has been found to allow optimization of near field
localization and take advantage of polarization effects when used
with a semiconductor laser or other light sources. A variety of
such aperture configurations in accordance with the invention can
be easily etched into a conductive sheet or layer using focussed
ion beam (FIB) or other anisotropic etching techniques.
[0026] The conductive protrusion or protrusions define generally a
plurality of regions in the aperture, which are separated by the
protrusion or protrusions of the surrounding conductive sheet or
plane which extend into the aperture. In the preferred embodiments
wherein a single protrusion is used, the protrusion will define
generally first and second regions which are separated by the
protrusion. The protrusion also defines generally a waist or
connecting section which joins the two regions separated by the
protrusion. The regions thus defined are preferably configured such
that they are elongated in the direction of the polarization of
output light through the aperture. The elongated regions are
preferably separated by the protrusion by a distance W wherein W
<.lambda., such that the waists or connecting section and
protrusion have generally a width W. The perimeter length or
dimension of the aperture is preferably resonant with the
wavelength .lambda. of the output light.
[0027] The aperture of the invention may alternatively be
considered as comprising first and second elongated regions
connected by a waist or connecting section. Assuming that
polarization is in a transverse direction, with respect to the
aperture, the aperture may comprise a first transverse slot of
length L.sub.1, a second transverse slot of length L.sub.2, and a
connecting region or waist of width W which communicates with the
first and second slots. Preferably, the first and second slots are
of equal length, such that L.sub.1=L.sub.2, with each slot having a
width equal to the width W of the connecting region.
[0028] In some preferred embodiments of the invention, the
connecting section is centrally positioned with respect to the
transverse slots, with two conductive protrusions defining the
central connecting section. In other preferred embodiments, the
connecting region is off center with respect to the slots, as is
provided by a single conductive protrusion. In some preferred
embodiments the lengths of the slots are greater or substantially
greater than the width W of the slots and connecting region
(L.sub.1=L.sub.2>W). In other preferred embodiments, the length
of the slots is generally equal to the width of the slots and
connecting region (L.sub.1=L.sub.2=W). The slots may,
alternatively, be of different lengths and widths.
[0029] In embodiments of the invention which utilize relatively
long transverse slots (L.sub.1 and L.sub.2>W), the connector
region between the slots will define a gap between conductive
protrusions of surrounding conductive plate which are located
between the transverse slots. The gap between the conductive
protrusions will operate as a dipole radiator. In such embodiments,
the antenna or transmission line characteristics of the laser
emission facet and aperture are important, as are the dimensions of
the transverse slots, connecting region and conductive protrusions.
The aperture can define a short circuited transmission line
configuration when a single connector region is located generally
proximate the center of the transverse slots. Alternatively,
additional connecting regions may be associated with the ends of
the transverse slots such that a non-short circuited transmission
line configuration is achieved.
[0030] In embodiments wherein the conductive protrusion is insular
or electrically isolated from the surrounding conductive plane, the
aperture in effect partitions the conductive plane into two regions
which are electrically isolated from each other. This configuration
provides means for creating an electric dipole in the emission
plane of the aperture, and creates enhanced localized emission
efficiency through the small separating region. The shape of the
electrically isolated regions and separating region may be tailored
to provide for enhancement of the emission as a result of
electromagnetic resonance effects within the electrically isolated
regions.
[0031] In still another preferred embodiment of the invention, an
insular conducting protrusion will result in the aperture
comprising two transverse slots each of length L and two connecting
sections associated with the ends of the slots which define an
annular-shaped aperture is formed. In the emission face of a laser,
thus configuration provides a central core or "post" which is
electrically isolated from the surrounding region of the emission
face to create an electric dipole in the emission plane of the
aperture. The created electric dipole provides an efficient
radiator of optical energy at a given wavelength such that emission
efficiency is enhanced. The annular aperture is analogous in shape
to a coaxial waveguide structure. In yet another preferred
embodiment, the aperture comprises two transverse slots of equal
length L connected by three connector regions to define three
electrically isolated regions on the emission face of the
laser.
[0032] A goal of the invention is to provide a near field optical
apparatus having an aperture structure which provides for efficient
localization of electric field strength at optical frequencies to a
region or regions which are smaller in dimension than the output
wavelength associated with the aperture.
[0033] A goal of the invention is to provide a near field optical
apparatus having an aperture with perimeter dimensions which are
resonant with output wavelength.
[0034] A goal of the invention is to provide a near field optical
apparatus having an aperture wherein a majority of aperture edges
are substantially aligned with or parallel to the direction of
polarization of the output light.
[0035] Another goal of the invention is to provide a near field
optical apparatus which can be utilized with surface and edge
emitting semiconductor lasers, tapered metallized fibers, SIL
optics, and other near field optical systems.
[0036] Another goal of the invention is to provide a VCSEL
apparatus which is easy and inexpensive to manufacture.
[0037] Further advantages of the invention will be brought out in
the following portions of the specification, wherein the detailed
description is for the purpose of fully disclosing the preferred
embodiment of the invention without placing limitations
thereon.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The present invention will be more fully understood by
reference to the following drawings, which are for illustrative
purposes only.
[0039] FIG. 1 is a schematic illustration of a square aperture.
[0040] FIG. 2 is a schematic illustration of an aperture in
accordance with the invention wherein a protrusion in the
surrounding conductive plane extends into the aperture.
[0041] FIG. 3 is a graphical representation of relative far field
power versus aperture size for the apertures of FIG. 1 and FIG.
2.
[0042] FIG. 4 is a graphical representation of relative near field
normalized E.sup.2 versus aperture size for the apertures of FIG. 1
and FIG. 2.
[0043] FIG. 5 is a field plot of normalized E.sup.2 field for the
aperture of FIG. 1.
[0044] FIG. 6 is a graphical representation of the normalized
E.sup.2 field of FIG. 5 along the x and y axes.
[0045] FIG. 7 is a field plot of normalized E.sup.2 field for the
aperture of FIG. 2.
[0046] FIG. 8 is a graphical representation of the normalized
E.sup.2 field of FIG. 7 along the x and y axes.
[0047] FIG. 9A through FIG. 9C are field plots of the x, y and z
components of the E.sup.2 field for the aperture of FIG. 1.
[0048] FIG. 10A through FIG. 10C are field plots of the x, y and z
components of the E.sup.2 field for the aperture of FIG. 2.
[0049] FIG. 11 is schematic illustration of an alternative
embodiment aperture in accordance with the present invention.
[0050] FIG. 12 is a field plot of normalized E.sup.2 field for the
aperture of FIG. 11.
[0051] FIG. 13 is a graphical representation of the normalized
E.sup.2 field of FIG. 12 along the x and y axes.
[0052] FIG. 14A is a schematic illustration of an elongated one
dimensional slit of width W.
[0053] FIG. 14B is a graphical representation of normalized power
flux versus slit width W for the slit of FIG. 14A, for light
polarized in x and y directions.
[0054] FIG. 14C is a table illustrating normalized power flux and
beam width through the slit of FIG. 14A, for x and y polarization
at varying slit widths.
[0055] FIG. 14D is a graphical representation of Full Width Half
Maximum dimension (nm) versus slit width for x and y polarization
at varying slit widths.
[0056] FIG. 15 is a graphical representation of the S.sub.11
scattering parameter versus frequency for the aperture of FIG.
2.
[0057] FIG. 16 is a graphical representation of relative field
strength versus time for a pulsed plane wave incident on, and
output from, the aperture of FIG. 2.
[0058] FIG. 17 is a graphical representation of the Fourier
transforms of the incident and output fields of FIG. 16, shown as
relative field strength versus frequency.
[0059] FIG. 18 is a graphical representation of the normalized
spectra of FIG. 17, shown as relative intensity versus
frequency.
[0060] FIG. 19 is a graphical representation of normalized power
output versus overall aperture width for the aperture of FIG.
2.
[0061] FIG. 20 is a graphical representation of normalized E.sup.2
field versus overall aperture width for the aperture of FIG. 2.
[0062] FIG. 21 is a graphical representation of full width half
maximum dimensions in x and y directions versus overall aperture
width for the aperture of FIG. 2.
[0063] FIG. 22A through FIG. 22C are schematic illustrations of
alternative embodiment apertures in accordance with the invention
wherein a single protrusion of the surrounding conductive plane
extends into the aperture.
[0064] FIG. 23A through FIG. 23C are schematic illustrations of
alternative embodiment apertures in accordance with the invention
wherein two conductive protrusions extend into the aperture.
[0065] FIG. 24A through FIG. 24C are field plots of normalized
E.sup.2 field for the apertures of FIG. 23A through FIG. 23C.
[0066] FIG. 25A is schematic illustration of an elongated slot
aperture in accordance with the invention, and FIG. 25B is a field
plot of normalized E.sup.2 field for the aperture of FIG. 25A.
[0067] FIG. 26A is schematic illustration of an aperture in
accordance with the present invention having a conductive
protrusion which is electrically isolated from the surrounding
conductive plane, and FIG. 26B is a field plot of normalized
E.sup.2 field for the aperture of FIG. 26A.
[0068] FIG. 27 is a schematic side elevation view in cross-section
of a prior art vertical cavity surface emitting laser.
[0069] FIG. 28 is a graphic representation of the optical field
profile, shown as normalized E.sup.2 versus distance from the edge
of the laser cavity, and refractive index profile, shown as
refractive index versus distance from the edge of the laser cavity,
for the prior art vertical cavity surface emitting laser of FIG.
1.
[0070] FIG. 29 is a schematic side elevation view in cross-section
of a vertical cavity surface emitting laser in accordance with the
present invention.
[0071] FIG. 30 is a graphic representation of the optical field
profile, shown as normalized E.sup.2 versus distance from the edge
of the laser cavity, and refractive index profile, shown as
refractive index versus distance from the edge of the laser cavity,
for the area surrounding the aperture of the vertical cavity
surface emitting lasers of FIG. 3.
[0072] FIG. 31 is a graphic representation of the optical field
profile, shown as normalized E.sup.2 versus distance from the edge
of the laser cavity, and refractive index profile, shown as
refractive index versus distance from the edge of the laser cavity,
for the area under the aperture of the vertical cavity surface
emitting laser of FIG. 3 when fifteen dielectric layer pairs are
positioned between the bottom of the aperture and the top of the
optical cavity.
[0073] FIG. 32 is a graphic representation of the optical field
profile, shown as normalized E.sup.2 versus distance from the edge
of the laser cavity, and refractive index profile, shown as
refractive index versus distance from the edge of the laser cavity,
for the area under the aperture of the vertical cavity surface
emitting laser of FIG. 3 when eleven dielectric layer pairs are
positioned between the bottom of the aperture and the top of the
optical cavity.
[0074] FIG. 33 is a schematic side elevation view in cross-section
of an alternative embodiment vertical cavity surface emitting laser
in accordance with the present invention.
[0075] FIG. 34 is a graphic representation of the optical field
profile, shown as normalized E.sup.2 versus distance from the edge
of the laser cavity, and refractive index profile, shown as
refractive index versus distance from the edge of the laser cavity,
for the area under the aperture of the vertical cavity surface
emitting laser of FIG. 7 when fifteen dielectric layer pairs are
positioned between the bottom of the aperture and the top of the
optical cavity.
[0076] FIG. 35 is a graphic representation of the optical field
profile, shown as normalized E.sup.2 versus distance from the edge
of the laser cavity, and refractive index profile, shown as
refractive index versus distance from the edge of the laser cavity,
for the area under the aperture of the vertical cavity surface
emitting laser of FIG. 7 when eleven dielectric layer pairs are
positioned between the bottom of the aperture and the top of the
optical cavity.
[0077] FIG. 36 is a schematic side elevation view in cross-section
of an another preferred embodiment vertical cavity surface emitting
laser in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0078] Referring more specifically to the drawings, for
illustrative purposes the present invention is embodied in the
apparatus and graphical representations shown generally in FIG. 2
through FIG. 26 and FIG. 28 through FIG. 36. It will be appreciated
that the apparatus may vary as to configuration and as to details
of the parts without departing from the basic concepts as disclosed
herein. The invention is disclosed primarily in terms of use with a
vertical cavity surface emitting laser (VCSEL) made of AlGaAs
materials. However, it will be readily apparent to those skilled in
the art that the invention may be embodied in edge emitting or
other types of semiconductor lasers, as well as with various other
light sources for near field applications. Various other uses for
the invention will suggest themselves to those skilled in the art
upon review of this disclosure.
[0079] The invention will be more fully understood by referring
first to FIG. 1, where there is shown an infinite conductive plane
or sheet 10 having a simple square aperture 12 therein. Conductive
sheet 10 is positioned in association with a light source (not
shown) such that radiation from the light source passes through the
aperture. The scaling laws for a small square aperture 12 are well
known and are described in "Theory of Diffraction by Small Holes"
by H. H. Bethe, Physical Review, Vol. 66, pp. 163-182 (1944). For
an aperture 12 which is much smaller in dimension than the
wavelength .lambda. of incident radiation, the radiation coupled
through the aperture is proportional to the sixth power (r.sup.6)
of the radius r of aperture 12. For apertures which are
substantially larger than the wavelength .lambda. of incident
radiation, the radiation coupled through aperture 12 has only a
second power dependence (r.sup.2) on the relative area of aperture
12. The sixth power dependence has a component of scaling which is
due to the overall length or dimensions of the perimeter of
aperture 12.
[0080] Using conventional Finite Difference Time Domain (FDTD)
calculation techniques, and assuming that conductive sheet 10 is an
infinite plane of perfect electrical conductor, the relative output
power and E.sup.2 field strength can be accurately determined for
the aperture 12, as well as numerous other aperture configurations
which are described further below.
[0081] FIG. 3 graphically illustrates normalized output power
associated with aperture 12 versus size of aperture 12 (shown as
circles) as aperture width W over incident wavelength .lambda.
(W/.lambda.), for light of wavelength .lambda.=1000 nanometers
(nm), and width W=100 nm, as determined by FDTD. For clarity,
aperture size in FIG. 3 is shown as three separate regions. In
region A, the aperture size is large relative to the wavelength (W
>>.lambda.). Region B is a transition region wherein aperture
width is approximately the same as the wavelength
(W.apprxeq..lambda.). In region C, the aperture size is small
relative to the wavelength (W <<.lambda.). While the aperture
12 is relatively large with respect to the incident wavelength as
in region A, normalized output power through the aperture 12
remains relatively constant. As the width or diameter W of aperture
12 decreases to W .apprxeq..lambda. in the transition region B, the
normalized output power increases to a maximum M.sub.1 in output
power, after which the output power begins to decrease. In the
small aperture region C, normalized output power drops off sharply
and rapidly becomes vanishingly small. The maximum M.sub.1 in
output power is largely due to a resonance effect wherein the
perimeter of the aperture 12 has a dimension which is substantially
resonant with the wavelength .lambda. and can support a standing
wave at .lambda. of significant or substantial magnitude.
[0082] In FIG. 4, normalized near field E.sup.2 is shown as circles
for aperture 12, as a function of aperture size (W/.lambda.) at a
near field distance of W/2 away from aperture 12, for W=100 nm and
.lambda.=1000 nm, as determined via FDTD calculations. FIG. 4 again
shows aperture size in terms of a large aperture region A, a
transition region B, and a small aperture region C. In the large
aperture region (W>>.lambda.), field intensity remains
relatively constant. As the width W of aperture 12 decreases to
W.apprxeq..lambda. in the transition region B, the normalized field
strength increases to a maximum M.sub.2 associated with resonance
before starting to diminish, and in the small aperture region
(W<<.lambda.) the normalized field strength drops off
sharply.
[0083] The presence of the maxima M.sub.1, M.sub.2 as noted above
indicate that, in the transition region B, the decrease in output
power and near field E.sup.2 with decreasing aperture size are
significantly affected by the aperture perimeter or periphery
dimensions, rather than merely the area of the aperture. Thus, if
the overall effective size or area of the aperture could be
decreased while maintaining perimeter at a length or dimension
which is substantially resonant with the output wavelength, the
drop off in output power and near field E.sup.2 as shown in regions
C of FIG. 3 and FIG. 4 can be avoided. The present invention
advantageously provides a near field apparatus having an aperture
which can be reduced or scaled downward in size while maintaining
high near field E.sup.2 strength and output power.
[0084] With the above in mind, reference is now made to FIG. 2,
wherein a near field optical apparatus 14 in accordance with the
invention is shown. The apparatus 14 includes a conductive sheet,
layer or plane 16 having a hole or aperture 18 therein, with at
least one protrusion or tab 20 of conductive sheet which extends
into aperture 18. The overall size or area of aperture 18 can be
decreased while maintaining the perimeter length or dimensions of
aperture 18 at resonance, by varying the location, size, and
configuration of protrusion 20. The conductive plane 16 may be made
of metal, metal alloy, semiconductor or other conductor or
semiconductor material.
[0085] Protrusion 20 defines generally a first region 22 and a
second region 24 in aperture 18 which are separated, or at least
partially separated, from each other by protrusion 20. Protrusion
20 also defines a waist or connecting section 26 which joins or
connects first and second regions 22, 24. Connecting section 26
constitutes an effective localization aperture which is
substantially centrally located with respect to aperture 18
overall. In the aperture 18 of FIG. 2, the width W of the
localization aperture 26 can be decreased while maintaining the
overall perimeter length of aperture 18 at the resonance point, by
increasing the length and/or width of protrusion 20 as the aperture
width is decreased. The presence of protrusion 20 in effect creates
a folded slot configuration for aperture 18, so that the overall
size of the localization aperture 26 is scalable while allowing the
perimeter dimension to be maintained or held at a resonance
dimension with respect to output wavelength, by varying the shape
of the folded slot.
[0086] Referring again to FIG. 3, the relative output power
associated with aperture 18 versus size (width) of the connector
region or localization aperture 18 is shown as squares, taking into
account variation of the dimensions of protrusion 20 as the width
of localization aperture 26 is decreased. As can be seen, the
relative output power can be maintained at, and even increased
from, the maximum level, by keeping the perimeter dimensions of
aperture 18 resonant with the output wavelength. Referring again to
FIG. 4 as well, the relative near field E.sup.2 intensity with
respect to aperture size for localization aperture 26 (shown as
squares) can be maintained at, and even increased from, the maximum
level provided by a simple square aperture.
[0087] FIG. 5 is a field plot of normalized E.sup.2 for the square
aperture 12 of FIG. 1 at a distance of approximately W/2 from
aperture 12, as determined by FDTD calculation assuming a width
W=100 nm for aperture 12 and .lambda.=1000 nm. FIG. 6 a graphical
illustration of E.sup.2 along the x and y central axis for the
aperture 12 of FIG. 1 as determined by FDTD calculation. From FIG.
6, it can be seen that the Full Width at Half Maximum (FWHM) in the
x direction is approximately 140 nm, and is approximately 80 nm in
the y direction. The normalized near field E.sup.2 is centrally and
symmetrically localized for square aperture 12. From FIG. 5,
however, it can be seen that the localized E.sup.2 field strength
provided by aperture 12 is quite small. For a relative incident
intensity=1, the peak E.sup.2 field from aperture 12 is
2.times.10.sup.-2.
[0088] Thus, a simple square aperture is, in general, a very
inefficient way to confine radiation to a small sub-wavelength
region for near field applications. This is largely due to the
aperture boundary approaching an equipotential of the electric
field as the aperture size approaches zero. Despite this
limitation, the simple square aperture 12, or the circular
equivalent, provides a conceptually simple means for field
localization, and heretofore, such simple apertures have been
widely used in near field techniques even though the localized
field provided by the aperture rapidly becomes vanishingly
small.
[0089] Referring now to FIG. 7, a field plot of normalized E.sup.2
for the aperture 18 of near field apparatus 14 of the invention is
shown, as determined by FDTD calculation for a distance of
approximately W/2 from aperture 12, with an incident wavelength
.lambda.=1000 nm, and with light polarized in the transverse or x
direction. In the specific example of FIG. 7, the connector region
or localization aperture 26 (FIG. 2) has a width and height of
W=100 nm, while first region 22 has a length L.sub.1=220 nm and a
height h.sub.1=100 nm, and second region 24 22 has a length
L.sub.2=220 nm and a height h.sub.2=100 nm. From these dimensions,
the aperture 18 has an overall width of L.sub.1=L.sub.2=220 nm, and
an overall height of W+h.sub.1+h.sub.2=300 nm. The localized
E.sup.2 field strength provided by aperture 18, as shown in FIG. 7,
is approximately three orders of magnitude, or 1000.times., greater
than the corresponding localized E.sup.2 field resulting from
square aperture 12. For an incident intensity=1, the peak localized
E.sup.2 field strength for aperture 18 is approximately 37. Thus,
the aperture 18 provides a tremendous improvement in near field
output power over a simple square aperture 12.
[0090] The above specific dimensions for localization aperture 26
and first and second regions 22, 24 merely provide one specific
example of the possible dimensions for aperture 18, and should not
be considered limiting. Additionally, the localization region need
not necessarily be square in shape as shown in FIG. 2, and may have
a width different from its height. The particular values for
L.sub.1, L.sub.2, W, h.sub.i and h.sub.2 will necessarily vary to
optimize aperture power output and near field localization under
different wavelength conditions.
[0091] FIG. 8 is a corresponding graphical illustration of E.sup.2
along the x and y central axis for the aperture 18 of FIG. 2 as
determined by FDTD calculation. From FIG. 7 and FIG. 8, it can be
seen the that the normalized E.sup.2 is substantially centrally and
symmetrically localized despite the somewhat asymmetric shape
imparted to aperture 18 by the presence of protrusion 20. FIG. 8
shows a FWHM of approximately 128 nm in the x direction and
approximately 136 in the y direction. Thus, the aperture 18 of the
invention provides near field localization comparable to the
symmetrical square aperture 12 even though the aperture 18 of the
invention is larger overall (220 nm wide by 300 nm high) than
square aperture 12 (100 nm width), and even though the aperture 18
is not symmetric about the center of aperture 18.
[0092] The effective near field localization provided by aperture
18 will be more fully understood by considering the x, y and z
components of the normalized E.sup.2 field. FIG. 9A through FIG. 9C
are field plots of the x, y and z components of the normalized
E.sup.2 field of FIG. 5 for square aperture 12 as determined via
FDTD calculation. FIG. 10A through FIG. 10C show the corresponding
x, y and z components of the normalized E.sup.2 field of FIG. 7 for
the aperture 18 of the invention as determined by FDTD calculation.
For optimum field localization, it is necessary to maintain the
maxima of E.sub.x.sup.2, E.sub.y.sup.2 and E.sub.z.sup.2 components
as close together as possible. The shape of aperture 18 tends to
suppress one lobe of the relatively large z component, as shown in
FIG. 10C, which partially accounts for the good field localization
provided by aperture 18. The y component, while dual lobed, is
relatively weak, and is also relatively central in location, and
therefore does not substantially detract from the field
localization provided by aperture 18.
[0093] The aperture of the invention advantageously is scalable to
smaller sizes without resulting in decrease of near field power
output, as occurs in the conventional square aperture 12. Referring
to FIG. 11, an alternative embodiment near field optical apparatus
28 is shown. The apparatus 28 includes a sheet, plane or layer 30
of conductive metal or like conductive material, with an opening or
aperture 32 therein. A stub or protrusion 33 of the conductive
plane 30 extends into aperture 30, such that first and second
regions 34, 36 are defined. Protrusion 32 also defines a waist or
connecting section 38 which joins first and second regions 34, 36,
and which serves effectively as a localization aperture.
[0094] FIG. 12 shows the normalized E.sup.2 field strength for the
near field apparatus 28 at a distance of approximately W/2 from
aperture 32, as determined by FDTD calculation where w=50 nm (the
width of localization aperture 38) and .lambda.=1000 nm. The
overall width d of aperture 32 is 200 nm for the example of FIG.
12, with the length of tab or protrusion 32 being 150 nm. FIG. 13
is graphical illustration of E.sup.2 along the x and y central axis
for the aperture 32 of FIG. 11 according to FDTD calculation, and
shows the Full Width at Half Maximum (FWHM) in the x direction to
be approximately 80 nm, and approximately 70 nm in the y
direction.
[0095] From FIG. 12 and FIG. 13 it can be seen that the normalized
E.sup.2 field strength and degree of localization provided by the
aperture 32, having an overall width of 200 nm, is even greater
than that provided by the larger aperture 18 of 300 nm width. The
localization of normalized E.sup.2 field, however, is not as
centrally located with respect to the center of aperture 32, as was
the case for aperture 18 described above. The off center
localization is due at least in part to the length of tongue or
protrusion 32, which extends into and substantially across aperture
32, and which results in the individual x, y and zE.sup.2 field
components (not shown) being slightly off-center. Despite the
slightly off-center localization, the overall field strength
offered by aperture 32 and near field device 28 is highly useful in
most near field applications.
[0096] In optimizing the structure and configuration of a near
field aperture in accordance with the invention, there are several
important considerations to be kept in mind. Polarization
considerations are essential to optimizing the E.sup.2-field
strength at the center of the aperture, and the aperture of the
invention is preferably configured to optimize or take advantage of
polarization effects. In considering polarization effects,
reference is made to FIG. 14A through 14D. FIG. 14A schematically
illustrates an elongated one dimensional slit of width W. FIG. 14B
graphically illustrates normalized power density, as Poynting
vector Sz, derived from the slit of FIG. 14A, for light polarized
in the x and y directions and varying width W of the slit of FIG.
14A. FIG. 14C is a table which provides normalized power flux and
beam width for x and y polarized light through varying slit widths,
and FIG. 14D illustrates full width half maximum dimension versus
slit width for x and y polarized light.
[0097] For decreasing slit width, the electric field components
which are perpendicular to the edges of the slit can be supported
more readily than electric field components are parallel to the
edges of the slit. This can be seen from FIG. 14B and FIG. 14C,
which show that the normalized power flux Sz for x-polarized light
(polarization perpendicular or transverse to the slit) remains
relatively constant as slit width decreases, while power flux for
the y-polarized light (polarization parallel to slit) falls off or
decreases with decreasing slit width. Thus, the preferred aperture
will be structured and configured to take advantage of polarization
by having relatively large number of edges which are substantially
perpendicular to the electric field direction. The near field
apparatus 14 of FIG. 2 provides this feature. From FIG. 14D, it can
be seen that the full width half maximum dimension for both x- and
y-polarized light decreases with decreasing slit width, but with
the x-polarized light (perpendicular to slit) being slightly
greater than for y-polarized (parallel to slit) light. Thus, once
again, configuration of the aperture to keep edges perpendicular to
the direction of polarization is preferable. This is provided by
regions 22, 24 of aperture 18 being at least partially elongated in
the transverse or x direction, with polarization being generally
parallel to the transverse or x direction.
[0098] Resonance effects associated with the perimeter length or
dimension of the aperture are also important in providing good near
field brightness from a small aperture. When the physical size or
dimensions of the aperture decrease to substantially less than the
output wavelength, the aperture structure is decreasingly able to
support a standing wave of significant amplitude around its
perimeter, as noted above with regard to FIG. 3 and FIG. 4. As also
noted above, the present invention provides an aperture which is
scalable to smaller size while maintaining perimeter length at
resonance with output wavelength by providing a protrusion of
conducting surface into the aperture, which can be varied in size
and shape as the overall size of the aperture itself is
decreased.
[0099] The resonance effect associated with the aperture 18 of FIG.
2 can be observed in several aspects. FIG. 15 graphically shows the
scattering parameter S.sub.11 (decibels) versus frequency (Hertz)
for the aperture 18 of FIG. 2, for the aperture dimensions shown in
FIG. 2 and .lambda.=1000 nanometers when excited with an electrical
pulse, according to FDTD calculation. As can be seen, the
scattering parameter S.sub.11 increases substantially near
resonance at approximately 4.times.10.sup.14 Hz.
[0100] FIG. 16 graphically illustrates relative field strength
versus time for a pulsed plane wave incident on, and the resulting
output from, the aperture of FIG. 2. FIG. 17 shows graphically the
Fourier transforms of the incident and output fields of FIG. 16, as
relative field strength versus frequency. FIG. 18 graphically shows
the normalized spectra of FIG. 17, shown as relative intensity
versus frequency. From FIG. 16 through FIG. 18, the presence of a
resonance effect in the aperture 18 can be clearly seen.
[0101] In FIG. 19, normalized power output (Sz) versus aperture
width is shown graphically for the aperture 18 of FIG. 2, for
.lambda.=1000 nm. The output power density peaks generally for an
aperture width of about 220 nm for the aperture shape of FIG. 2 at
.lambda.=1000 nm. This aperture width corresponds to a perimeter
length of approximately 1280 nm for aperture 18, which, accounting
for fringing field effects, corresponds to a perimeter length of
approximately one wavelength .lambda.. FIG. 20 graphically provides
the normalized E.sup.2 field versus aperture width for the aperture
of FIG. 2 at .lambda.=1000 nm. As with power density, the
normalized E.sup.2 field peaks generally at the resonance dimension
for aperture 18, where the aperture width is approximately 220
nm.
[0102] From the above, the effectiveness of having an aperture
perimeter dimension at resonance with the output wavelength can be
seen. The particular aperture dimensions noted above are only
exemplary, and the dimensions will vary according to the desired
output wavelength and details of the aperture shape, as will be
readily understood by those skilled in the art.
[0103] While resonance is an important consideration, the aperture
configuration also must take areal effects into consideration as
well. Generally, as the total aperture area decreases, the total
emission throughput will increase, and thus overall aperture area
should remain as large as possible. Aperture areal effects are most
important for apertures of dimensions which are larger than the
output wavelength.
[0104] The localization and maximizing of near field brightness or
intensity is another consideration in aperture design which can be
controlled by aperture shape in accordance with the invention. The
size and geometry or structure of the aperture and emission facet
ideally should be configured to localize and maximize the total
field intensity of E.sub.x.sup.2+E.sub.y.sup.2+E.sub.z.sup.2 at
near field distances from the aperture. To this effect, the
inclusion of one or more protrusions into the aperture, as provided
by the invention, has been found to provide control of field
localization. The configuration of aperture 18, with a stub or
tongue 20 which extends into the aperture to define regions 22, 24,
is a particularly effective arrangement for providing near field
localization. The optimum dimensions for field localization for the
aperture configuration of FIG. 2 can be seen from FIG. 21, which
graphically illustrates the full width half maximum dimensions in
x- and y-directions versus aperture width. For the aperture 18,
optimum localization occurs at an aperture width of approximately
220 nm. This corresponds generally to the aperture width for
perimeter resonance at .lambda.=1000 nm, as noted above. The
optimum width for aperture 18 will of course vary with
wavelength.
[0105] Numerous variations on the aperture configuration of FIG. 2
will also provide good near field localization in accordance with
the invention. In FIG. 22A there is shown an alternative embodiment
near field optical apparatus 40 having a conductive plane 42 with
an aperture 44 therein, and with a tongue or protrusion 46 of
conductive plane 42 extending into aperture 44. In the apparatus
40, the protrusion 46 is trapezoidal in shape such protrusion 46 is
configured as a truncated point.
[0106] Referring to FIG. 22B, another alternative embodiment near
field optical apparatus 48 is shown. In the apparatus 48, a
conductive sheet 50 includes an aperture 52 extending therethrough,
with a protrusion 54 extending into aperture 52. In the apparatus
48, the aperture 52 is rounded in shape, and protrusion 54 is also
rounded in shape.
[0107] Referring also to FIG. 22C, another preferred near field
optical apparatus 56 in accordance with the invention is shown. The
apparatus 56 includes a conductive layer 58 with an aperture 60
therein, and a protrusion 62 of conductive plane 58 extending into
aperture 60. In the apparatus 56, the aperture 60 is rounded in
shape, and protrusion 62 is pointed or generally triangular in
shape. Various other aperture configurations based on the above
considerations will suggest themselves to those skilled in the art,
and are considered to be within the scope of this disclosure.
[0108] The effect of aperture shape on near field localization can
also be seen from the aperture structures and corresponding field
plots shown in FIG. 23 through FIG. 27. In FIG. 23A, a near field
optical apparatus 64 in accordance with the invention. The
apparatus 64 includes a conductive sheet 65 with an aperture 66
therein, with first and second protrusions or stubs 67a, 67b
extending into aperture 66 from generally opposite sides thereof.
Protrusions 67a, 67b define and separate first and second regions
or slots 68a, 68b which are joined by a centrally located waist or
connector region 69. Slot 68a has a length L.sub.1, and slot 68b
has a length L.sub.2. In the embodiment of FIG. 23A, L.sub.1 is
generally equal to L.sub.2, with L.sub.1 and L.sub.2 being equal to
the overall aperture width, and with L.sub.1 and L.sub.2 each being
greater than the width W of waist 69. In the particular embodiment
shown in FIG. 23a, L.sub.2, and L.sub.1 are equal to approximately
2W. In other embodiments, L.sub.2, and L.sub.1 need not be
equal.
[0109] FIG. 24A is a field plot of normalized E.sup.2 field for the
aperture 66 of FIG. 23A for .lambda. =1000 nm, W=100 nm, and
L.sub.2,=L.sub.1=200 nm, at a distance of approximately W/2 (50 nm)
from aperture 66. The direction of light polarization is in the
transverse or x direction. As can be seen in FIG. 24, the use of
dual protrusions 67a, 67b results in near field localization which
is generally symmetrical about the center of aperture 66, but which
defines two distinct lobes which are separated in the x direction.
The aperture 18 of FIG. 2 provides better E.sup.2 field
localization, but with localization being slightly less symmetric
about the aperture center. The aperture of FIG. 2 also provides
substantially better overall E.sup.2 field strength. It should be
noted, however, that aperture 66 provides an E.sup.2 field strength
on the order of 100.times. greater than that of the simple aperture
of FIG. 1.
[0110] FIG. 23B, a near field optical apparatus 70 is shown with a
conductive sheet 71 having an aperture 72 therein, with first and
second protrusions or stubs 73a, 73b extending into aperture 72
from generally opposite sides thereof. Protrusions 73a, 73b define
first and second transverse slots 74a, 74b which are joined by a
centrally located waist or connector region 75. Slots 74a, 74b are
respectively of lengths L.sub.1 and L.sub.2, with
L.sub.1=L.sub.2>W. As noted above, these particular dimensions
may be varied.
[0111] FIG. 24B is a field plot of normalized E.sup.2 field for the
aperture 72 of FIG. 23B, for .lambda.=1000 nm, W=100 nm, and
L.sub.2,=L.sub.1=300 nm, at a distance of approximately W/2 (50 nm)
from aperture 72, with direction of light polarization being in the
transverse or x direction. The dual protrusions 73a, 73b, together
with the longer length of transverse slots 74a, 74b provides
symmetrical near field localization but with two distinct lobes in
the E.sup.2 field which are separated in the x direction to a
greater extent than provided by the aperture 66 of FIG. 23A.
[0112] FIG. 23C shows another near field optical apparatus 76
wherein a conductive sheet 77 includes an aperture 78, with first
and second protrusions 79a, 79b extending into aperture 78 from
generally opposite sides. Protrusions 79a, 79b define first and
second transverse slots 80a, 80b which are joined by a centrally
located waist or connector region 81, with slots 80a, 80b
respectively of lengths L.sub.1 and L.sub.2, and waist of width W,
with L.sub.1=L.sub.2>>W (L.sub.1=L.sub.2=6W). The
corresponding field plot of normalized E.sup.2 field for FIG. 23C
is shown in FIG. 24C, for .lambda.=1000 nm, W=100 nm, and
L.sub.2,=L.sub.1=600 nm, at a distance of approximately W/2 (50 nm)
from aperture 78, with light polarized in the transverse or x
direction. In the case of aperture 78, the E.sup.2 field lobes are
even further separated in the x direction than occurs for apertures
66, 72. The relatively long transverse slots 74a, 74b are able to
support a comparatively large z component of the E.sup.2 field
which is somewhat poorly localized, and results in relatively
large, undesirable full width half maximum dimensions.
[0113] In the case of aperture 78, the near field apparatus 76 may
be associated with an impedance value, which should be considered
when shaping the aperture. The aperture may define a radiative
element, in association with a light source as a transmission line,
which has an effective impedance, which should ideally be matched
through the impedance of the aperture structure, to the impedance
of the region into which the radiator is being coupled. In the near
field optical apparatus 76, transverse slots 82a, 82b extend to and
communicate with the edges of the conductive plane 77, such that
protrusions 79a, 79b are electrically isolated from the remainder
of conductive plane 77. In this regard, the near field apparatus 76
has some of the features of a short-circuited transmission line.
The structure of aperture 78 could be modified so that the
apparatus 76 provides non-short-circuited transmission line
features. This would be provided by providing an additional
connector region (not shown) which joins first ends 82a, 83b of
first and second transverse slots 80a, 80b, and an additional
connector region (also not shown) which joins the second ends 82b,
83b of slots. The additional connector regions would have generally
the same dimensions of connector region 81.
[0114] Referring to FIG. 25A, there is shown a near field optical
apparatus 84 having a longitudinally elongated aperture 86 in a
conducting plane 85. In the case of apparatus 84, the aperture 86
may be considered as having first and second regions 87a, 87b
joined by a waist 88 of width W, wherein the lengths L.sub.1 and
L.sub.2 of regions 87a, 87b are such that L.sub.1=L.sub.2=W, with
no protrusions extending into the aperture 86. The corresponding
field plot of normalized E.sup.2 field for the apparatus of FIG.
25A is shown in FIG. 25B, for .lambda.=1000 nm at a distance of
approximately W/2 (50 nm) from the aperture 86, with
L.sub.1=L.sub.2=W=100 nm, with light polarized in the transverse or
x direction. In the case of aperture 86, the E.sup.2 field is
substantially localized at the aperture center, with a slight
definition of lobes in the x direction. The elongation of aperture
86 in the direction perpendicular to the polarization direction
takes maximum advantage of polarization effects on E.sup.2 field
strength. This structure provides an E.sup.2 field strength on the
order of 100.times. greater than that of the simple aperture of
FIG. 1.
[0115] Referring next to FIG. 26A, there is shown yet another
preferred embodiment near field optical apparatus 89 in accordance
with the present invention. The apparatus 29 includes a conductive
sheet 90 having an aperture 91 therein. Aperture 91 is generally
annular in configuration, with first and second transverse slots
92a, 92b joined together by first and second connector regions 93a,
93b. An insular or isolated portion 94 of conductive plane 90 is
surrounded by slots 92a, 92b and connector regions 93a, 93b, such
that conductive portion 94 is electrically isolated from the
surrounding conductive plane 90. The island 94 may be considered as
a protrusion of conductive plane 90 which has been electrically
isolated therefrom by connector regions 93b.
[0116] FIG. 26B shows the corresponding field plot of normalized
E.sup.2 field for the apparatus 89, for .lambda.=1000 nm at a
distance of approximately W/2 (50 nm) from the aperture 86, with
transverse slots 92a, 92b being 300 nm in length, with the width of
connector regions being 100 nm, and with light polarized in the
transverse or x direction. The apparatus 89 provides a relatively
large aperture area and good polarization transmission through
slots 92a, 92b which are not shorted at the ends (as in the
apparatus 76 of FIG. 23C). The E.sup.2 field is localized to a pair
of regions or lobes located to the left and right of central
conductor island 94. The apparatus 89, it should be noted, provides
an E.sup.2 field strength on the order of 100.times. that of the
simple square aperture of FIG. 1.
[0117] The near field optical apparatus of the invention may be
used with a variety of light sources, and is particularly well
suited to use in semiconductor laser devices where the conducting
plane surrounding the aperture can comprise a metal layer on the
laser emission facet. The invention, as embodied in a vertical
cavity surface emitting laser (VCSEL), is described in detail
below. It should be kept in mind, however, that a VCSEL device is
merely one preferred embodiment of the invention, and that edge
emitting and other solid state lasers, as well as other light
sources generally, may be used with the invention.
[0118] The invention as embodied in a VCSEL will be more fully
understood by first reviewing the structure and properties of
conventional vertical cavity surface emitting lasers or VCSEL
devices. Referring to FIG. 27, there is shown a prior art AlGaAs
VCSEL device 100 which is structured and configured for operation
at approximately 821.8 -821.9 nm. The VCSEL device 100 includes an
active region 112 within an optical cavity 114, an upper, first
conductivity type distributed Bragg reflector or DBR mirror 116,
and a lower, second conductivity type DBR mirror 118. Upper and
lower DBR mirrors 116, 118 each include a plurality of dielectric
layer pairs 120. The dielectric layer pairs 120 are generally
quarter wave or .lambda./4
Al.sub.0.16Ga.sub.0.84As/Al.sub.0.96Ga.sub.0.04As pairs. The
thickness of various layer components of VCSEL 10 are exaggerated
for clarity, and thus it should be understood that the particular
layer thicknesses shown are merely illustrative and are not
necessarily to scale.
[0119] The laser or optical cavity 114 is defined generally by the
lower edge 122 of upper DBR mirror 116 and the upper edge 124 of
lower DBR mirror. A p-doped Al.sub.0.6Ga.sub.0.4As layer 126 within
optical cavity 114 is positioned between upper DBR mirror 116 and
active region 112, and an n-doped Al.sub.0.6Ga.sub.0.4As layer 128
is positioned within optical cavity 114 between lower DBR mirror
118 and active region. Active region 112 is shown with a plurality
of Al.sub.0.05Ga.sub.0.95As quantum wells 130 and
Al.sub.0.4Ga.sub.0.6As quantum barriers 132, which may range in
thickness generally within the range of 2-20 .ANG.. The n-doped and
p-doped Al.sub.0.6Ga.sub.0.4As regions 124, 126 define a diode
structure so that lasing within optical cavity 14 can be turned on
and off by varying current through the active region with respect
to a threshold current in a conventional manner.
[0120] An emission facet or face 134 is defined at the upper
surface 136 of upper DBR mirror 116. A GaAs cap or coating 138 is
typically included on top of emission facet 134. The VCSEL 100 also
generally includes a bottom GaAs substrate 140 upon which the other
layers structures of VCSEL 100 are deposited or "grown". An
encapsulant material (not shown) is generally included adjacent the
sides of VCSEL 100.
[0121] The upper DBR mirror 116 generally includes fewer dielectric
layer pairs 120 than lower DBR mirror 118, so that optical power
can be extracted from emission facet 134. For example, upper DBR
mirror 116 may comprise twenty-five dielectric layer pairs 120,
while lower DBR mirror 116 comprises thirty-five dielectric layer
pairs 120 in a typical laser structure. Stacked dielectric DBR
mirrors 116, 118 have very high reflectivity, typically in excess
of 99.5%. As a result, the optical power recirculating within laser
cavity 114 is generally a factor of 100 to 10,000 greater than the
optical power extracted through emission facet 34. Upper DBR mirror
presents the dominant loss for VCSEL 110 and represents the path
through which output power is extracted.
[0122] The distribution of the optical field in VCSEL 100 as a
function of depth within the laser structure is illustrated
graphically in FIG. 28 as normalized E.sup.2-field versus distance
from the center of optical cavity 114. FIG. 28 shows an optical
field profile 42 of the longitudinal standing wave within optical
cavity 114 superimposed with a refractive index profile 144 for
upper DBR mirror 16 with twenty five .lambda./4 layer pairs.
[0123] The right hand y-axis of FIG. 2 represent relative
refractive index. The standing wave pattern of optical field
profile 42 is characterized by a series of peaks and nulls, with
the amplitude of the peaks increasing monotonically as the center
of resonance cavity 14 is approached. Optical field profile 42
exhibits a peak E.sup.2-field of about 9598 and a bottom
E.sup.2-field of about 12 normalized units with respect to the
bottom DBR mirror.
[0124] Referring also to Table 1 below, Column 1A, there are shown
several standard performance parameters calculated for VCSEL 100,
having twenty-five .lambda./4 layer pairs in upper DBR mirror 116,
thirty-five .lambda./4 layer pairs in lower DBR mirror 118, and
emitting at 821.868 nm, as would be used, for example, for data
communication applications. The E.sup.2 field parameters provided
in Table 1 are normalized relative to the bottom DBR mirror 118.
Note again that upper DBR mirror 116 presents the dominant loss for
VCSEL 100. In a well designed VCSEL device, this loss will
represent the predominant loss mechanism in the laser cavity.
[0125] If the VCSEL 100 is structured and configured such that
upper DBR mirror 116 had greater reflectivity than in the above
example, the total losses in the VCSEL, and the threshold current
for the VCSEL, would be reduced. Such a VCSEL, however, would have
reduced efficiency at moderate output levels, as the upper mirror
loss will be less predominant with respect to other cavity losses.
Referring to Column 1B of Table 1, there are shown calculated laser
performance parameters for VCSEL 10 wherein upper DBR mirror 116
comprises thirty-five .lambda./4 pairs to provide increased upper
mirror reflectivity.
1 TABLE 1 Conventional VCSEL With VCSEL (25 Increased Top
.lambda./4 pair upper DBR) Mirror Reflectivity Column 1A Column 1B
Upper .lambda./4 Pairs 25 35 Lower .lambda./4 Pairs 35 35 Threshold
Gain (cm.sup.-1) 312.586 227.824 Wavelength (nm) 821.868 821.868
External Efficiency 0.3965115 0.171773 (Total) External Efficiency
0.0337275 0.0938378 (Bottom) External Efficiency 0.362788 0.0779357
(Top) Round Trip Net Gain 0.00529864 0.00380674 Round Trip Net Loss
0.00319765 0.00315284 Round Trip Transmission 0.00210099
0.000653897 Transmission Down 0.00017871 0.000357216 Transmission
Up 0.00192228 0.000296681 Active Region Thickness 16.0488 16.0488
(nm) Gain Enhancement Factor 5.2814 5.20573 Normalized Peak E.sup.2
Field 9598.54 9590.45 Normalized E.sup.2 Field at. 9050.11 9042.51
Active Region Normalized Top E.sup.2 Field 12.0265 0.927765
Normalized Bottom E.sup.2 1.11807 1.11707 Field Surface-Peak Field
Ratio 0.001328879 0.0001026 Intensity Enhancement 1 0.077208229 Top
Surface Reflectivity 0.99807772 0.999703319
[0126]
2 TABLE 2 VCSEL With Reduced Top Mirror Reflectivity VCSEL With
Reduced (15 .lambda./4 pair Top Mirror Reflectivity upper DBR) (15
.lambda./4 pair upper DBR) Column 2A Column 2B Upper .lambda./4
Pairs 15 11 Lower .lambda./4 Pairs 35 35 Threshold Gain (cm.sup.-1)
1405.31 3522.52 Wavelength (nm) 821.875 821.889 External Efficiency
0.867193 0.947815 (Total) External Efficiency 0.00442919
0.000932462 (Bottom) External Efficiency (Top) 0.862764 0.946882
Round Trip Net Gain 0.023863 0.0599235 Round Trip Net Loss
0.00316917 0.00312712 Round Trip Transmission 0.0206938 0.0567964
Transmission Down 0.000105694 0.000055876 Transmission Up 0.0205881
0.0567405 Active Region Thickness 16.0488 16.0488 (nm) Gain
Enhancement Factor 5.29029 5.29995 Normalized Peak E.sup.2 Field
12986.3 21878.5 Normalized E.sup.2 Field at 12243.8 20627 Active
Region Normalized Top E.sup.2 Field 210.758 989.224 Normalized
Bottom E.sup.2 1.08197 0.974159 Field Surface-Peak Field Ratio
0.017213447 0.047957725 Intensity Enhancement 12.95336025
36.08886122 Top Surface Reflectivity 0.9794119 0.9432595
[0127] If VCSEL100 is structured and configured such that upper DBR
mirror 116 had reduced reflectivity, by having fewer .lambda./4
pairs, the upper mirror losses would be increased, and the laser
threshold current would be increased. Referring to Table 2 above,
Column 2A, there are shown laser performance parameters calculated
for VCSEL 10 wherein upper DBR mirror 116 comprises fifteen
.lambda./4 pairs. The E.sup.2 Field values in Table 2 are
normalized with respect to the bottom DBR mirror 118. Column 2B of
Table 2 further shows calculated performance parameters for VCSEL
100 wherein upper DBR mirror 116 includes only eleven .lambda./4
pairs. In these cases, the laser threshold current is increased to
the extent that laser operation may not occur before the advent of
excess heating and quantum well gain saturation occur which will
prevent lasing.
[0128] With the above properties for high and low reflectivity
upper DBR mirrors 116, 118 in mind, it can be seen that a VCSEL
having an upper mirror region of lower reflectivity dimensioned
smaller than the guide mode of the laser structure, surrounded by
an upper mirror region of higher reflectivity, to provide
performance parameters corresponding to both high and low upper
mirror reflectivity models. This can be achieved by etching or
otherwise creating an aperture in the emission face of the VCSEL
which extends into the upper DBR mirror and having dimensions
smaller than the dimension of the laser guide mode of the VCSEL
structure. The emission facet would then present two regions of
differing reflectivities. The region surrounding the aperture would
have a high reflectivity and properties similar to those
illustrated in Column 1B in Table 1. The region under the aperture,
having a smaller number of .lambda./4 pairs, would have a lower
reflectivity and properties like those illustrated in Columns 2A
and 2B of Table 2.
[0129] A reasonable approximation of the net upper mirror
reflectivity can be provided by a weighted average integral of
reflectivity as a function of position on the emission facet,
according to the area of the aperture relative to the size of the
optical mode. For an aperture of a particular or given area, the
depth of the aperture could be selected and adjusted such that a
particular target loss can be achieved. This can be seen by
comparing the parameters of Columns 2A and 2B of Table 2 for the
VCSEL structures having upper mirror thicknesses of fifteen and
eleven .lambda./4 pairs respectively. In particular, if the desired
or target value of loss is the same value as the mirror loss for
the conventional VCSEL structure 100 with twenty-five upper mirror
.lambda./4 pairs as shown in Column 1A of Table 1, the same power
would be extracted from an apertured upper mirror as would be from
the conventional non-apertured upper mirror. In other words, the
output power extracted through a VCSEL having an apertured emission
facet can be achieved which is equal to the output power extracted
from the conventional VCSEL 10 having a flat or planar emission
facet.
[0130] Referring now to FIG. 29, a vertical cavity surface emitting
laser or VCSEL 146 in accordance with the present invention is
shown. VCSEL 146 is shown as a GaAlAs device structured and
configured for output at approximately 821.9 nm, and it should be
readily understood that the layer thicknesses and semiconductor
materials used for VCSEL 146 may vary as required for different
applications. Thus, VCSEL 146 may be fabricated from various
semiconductor materials, including, for example, GaAs, InGaAs,
InGaAsP and InP materials, and can be structured and configured to
provide various output wavelengths. The thicknesses of various
layer components of VCSEL 146 as shown in FIG. 29 are exaggerated
for clarity, and the particular layer thicknesses shown are merely
illustrative and are not necessarily to scale.
[0131] VCSEL 146 comprises an active region 148 centered within a
laser cavity 150. An upper, p-doped distributed Bragg reflector or
DBR mirror 152 is positioned adjacent the top of cavity 150, and a
lower n-doped DBR mirror 154 is positioned adjacent the bottom of
cavity 150. Laser cavity 150 is thus defined generally by a lower
edge 156 of upper DBR mirror 152 and an upper edge 158 of lower DBR
mirror 154, and is shown as a one-wavelength .lambda. cavity. Upper
and lower DBR mirrors 152,154 each comprise a plurality of
dielectric layer pairs 160 which, in this example, are preferably
quarter wave or a .lambda./4
Al.sub.0.16Ga.sub.0.84As/Al.sub.0.96Ga.sub.0.04As layer pairs.
[0132] VCSEL 146 preferably comprises a p-doped
Al.sub.0.6Ga.sub.0.4As layer 162, which is positioned within
optical cavity 150 between upper DBR mirror 152 and active region
48, and an n-doped Al.sub.0.6Ga.sub.0.4As layer 164 which is
positioned within optical cavity 150 between lower DBR mirror 154
and active region 148. Active region 148 preferably comprises a
single or plurality of quantum well, quantum wire, quantum do,
and/or other quantum nanostructures. Active region 148 is shown
with a plurality of Al.sub.0.05Ga.sub.0.95. As quantum wells 166
and Al.sub.0.4Ga.sub.0.6As quantum barriers 168, which may range in
thickness generally within the range of 2-20 .ANG..
[0133] VCSEL 146 also comprises an emission facet or face 170,
which is defined at the upper surface 172 of upper DBR mirror 116.
A GaAs cap 174 is included on top of emission facet 170. A bottom
GaAs substrate 176 is present adjacent lower DBR mirror 154. An
encapsulant material may be included adjacent the sides of VCSEL
146.
[0134] An opening or aperture 178 is included in emission facet 170
of VCSEL 146, with aperture 178 extending through GaAs cap 174 and
into upper DBR mirror 152. As a result, there are a smaller number
of .lambda./4 layer pairs 160 positioned between the bottom 180 of
aperture 178 and lower edge 156 of upper DBR mirror 152, than are
positioned between emission facet 170 and lower edge 156 of upper
DBR mirror 152. The dimensions of aperture 178 are generally
smaller or less than that of the guide mode 182 of VCSEL 146.
[0135] The area of emission facet 170 associated with aperture 178
thus provides a region of lower reflectivity, while the region
surrounding portions of emission facet 170 present a region of
higher reflectivity, as discussed above. When upper DBR mirror
comprises thirty-five .lambda./4 layer pairs 160, the region
surrounding aperture 178 will exhibit generally the parameters of
Column 1 of Table 1. In FIG. 29, the distribution of the optical
field 184 for the thirty-five .lambda./4 layer pair thick structure
surrounding aperture 178, as a function of depth within the laser
structure, is illustrated graphically as normalized E.sup.2-field
versus distance from the edge of optical cavity 150. The refractive
index profile 186 for the thirty-five .lambda./4 layer pair, shown
as relative refractive index versus distance from edge of optical
cavity 150, is shown superimposed with the optical field profile
184. The right hand y-axis shows relative refractive index.
[0136] When aperture 178 is etched into upper DBR mirror 152 to a
depth such that fifteen .lambda./4 layer pairs 160 are present
between the bottom 180 of aperture and lower edge 156 of mirror
152, the region defined by aperture 178 will exhibit generally the
properties shown in Column 2A of Table 2. When aperture 178 is
etched into upper DBR mirror 152 to a depth such that eleven
.lambda./4 layer pairs 160 are present between the bottom 180 of
aperture and lower edge 156 of mirror 152, the region defined by
aperture 178 will exhibit generally the properties shown in Column
2B of Table 2. As noted above, the depth of aperture 178 can be
selected and adjusted such that a particular target loss is
achieved for VCSEL 146. By selecting or targeting the loss to equal
that of the mirror loss of conventional VCSEL 110, a power output
can be extracted from facet 170 of VCSEL 146 which is similar to
the power output extracted from the facet 134 of VCSEL 100. FIG. 31
illustrates graphically the optical field profile 188 and
refractive index profile 190 for the fifteen .lambda./4 layer pair
structure corresponding to Column 2A, and FIG. 32 illustrates
graphically the optical field profile 192 and refractive index
profile 194 for the eleven .lambda./4 layer pair structure of
Column 2B.
[0137] Aperture 178 will generally extend one to two microns in
depth into upper DBR mirror 152, and thus introduces some
undesirable scattering losses. One way to reduce such scattering
losses is to replace a portion of the .lambda./4 layer pairs 160 in
the region surrounding aperture 178 with a highly reflective metal
layer. The metal layer makes up for or replaces the reflectivity of
the .lambda./4 layers which have been replaced. In this case, the
aperture extends through the homogeneous reflective metal layer and
down to the DBR mirror 152. Since the reflective metal layer is
also conductive, it provides a conductive plane such that the
aperture may be structured and configured with one or more
protrusions of the conductive plane extending into the aperture, as
described above.
[0138] Referring now to FIG. 33, there is shown an alternative
embodiment VCSEL apparatus 196 in accordance with the present
invention, with like reference numbers used to designate like
parts, which utilizes a highly reflective layer as part of an upper
mirror. As in VCSEL 146 described above, the VCSEL 196 in FIG. 33
includes a laser cavity 150 comprising an active region 148
interposed between p- and n-doped Al.sub.0.6Ga.sub.0.4As layers
162, 164, which in turn are interposed between upper and lower DBR
mirrors 152, 154. In VCSEL 196, upper DBR mirror 152 has a smaller
number of .lambda./4 layer pairs 160 than described for VCSEL 146
above. In this example, upper DBR mirror 152 includes eleven or
fifteen .lambda./4 layer pairs 160. VCSEL 196 includes a highly
reflective, conductive metal layer 198, preferably of silver (Ag)
or other highly reflective metal, to make for the lessened
reflectivity of DBR mirror 152 due to the smaller number of
.lambda./4 layer pairs 160.
[0139] The reflective metal layer 198 is preferably about 40 nm in
thickness, and is positioned above GaAs cap or layer 174. GaAs
layer 174 has a thickness of .lambda./2. Preferably, a
semiconductor layer 200 of Al.sub.0.16GaAs of about 16 nm or 94 nm
thickness is deposited on top of GaAs layer 174, and insulator
spacer layer 202 of TiO.sub.2 of about 181 nm thickness is
deposited on Al.sub.0.16GaAs layer 200, such that Al.sub.0.16GaAs
layer 200 and TiO.sub.2 layer 202 are positioned between GaAs layer
174 and reflective silver Ag layer 198. A layer or coating 204 of
SiO.sub.2 of about 17 nm thickness is preferably deposited on top
of silver layer 198 to act as a passivation layer and prevent
oxidation or other reaction with silver layer 198. The particular
thicknesses noted above for reflective Ag layer 198,
Al.sub.0.16GaAs layer 200, TiO.sub.2 layer 202, and SiO.sub.2 layer
204, are the preferred thicknesses for operation of the invention
at an output wavelength .lambda.=approximately 821.8 nm. A
thickness of 94 nm is preferably used for Al.sub.0.16GaAs layer 200
when fifteen .lambda./4 pairs are used with upper DBR mirror 152,
and a thickness of 16 nm is preferably used for Al.sub.0.16GaAs
layer 200 when eleven .lambda./4 pairs are used with upper DBR
mirror 152.
[0140] The TiO.sub.2 layer 202 and Al.sub.0.16GaAs layer 200
provide means for optimizing adhesion of the reflective and
conductive Ag layer 198 to GaAs contact layer 174, and means for
reducing reactivity between the reflective and conductive Ag layer
and contact layer 174. The use of Ag for reflective layer 198 is
presently preferred due to the high reflectivity of Ag. The
TiO.sub.2 layer 202 and Al.sub.0.16GaAs layer 200 employed with the
invention aid in the adhesion of the Ag layer 198 in the layered
structure of VCSEL 196. The TiO.sub.2 layer 202 and SiO.sub.2 layer
204 isolate and passivate the Ag metal layer and avoid oxidation,
corrosion or other reaction which would otherwise reduce the
reflectivity of Ag layer 198.
[0141] An aperture 206 is etched through SiO.sub.2 layer 204 and Ag
layer 198 in order to provide for extraction of optical power.
Aperture 206 may comprise a simple square or round aperture, but
more preferably comprises an aperture wherein a portion of the
reflective, conductive metal layer 198 extends or protrudes into
the aperture in the manner described above. For reason of clarity,
only a simple aperture is shown in FIG. 33. The depth or bottom 208
of aperture 206 will nominally coincide with the center of the
TiO.sub.2 layer 202, which is subject to the highest magnitude of
electric field. The relative planarity of this structure provides
for a reduction of scattering loss when compared to the structure
of FIG. 29.
[0142] In the region surrounding aperture 206, VCSEL 196 has an
upper mirror reflectivity equal to the combined reflectivity of
metal layer 202 and the .lambda./4 layer pairs 160 of upper DBR
mirror 152, while the area under aperture 206 has a reflectivity
provided only by the .lambda./4 layer pairs 160 of upper DBR mirror
152. Thus, emission facet 206 present a region of reduced
reflectivity which is surrounded by a region of higher
reflectivity. As described above, the depth of aperture 206 can be
tailored to provide a selected or target loss.
[0143] In Table 3 below, Column 3A and Column 3B there are
respectively shown parameters for the structure surrounding
aperture 206, for an upper DBR mirror 152 having fifteen .lambda./4
layer pairs 160 and eleven .lambda./4 layer pairs 160.
[0144] FIG. 34 illustrates graphically the optical field profile
210 and refractive index profile 202 for the fifteen .lambda./4
layer pair and metal layer structure corresponding to Column 3A.
The 40 nm thick reflective silver layer 98 has a refractive index
profile 214. The 17 nm thick SiO.sub.2 layer 209 has a refractive
index profile 216. The 181 nm thick TiO.sub.2 layer 202 has a
refractive index profile 118, and the 94 nm thick
Al.sub.0..sub.16GaAs layer 200 has a refractive index profile
220.
[0145] FIG. 35 illustrates graphically the optical field profile
222 and refractive index profile 224 for the eleven .lambda./4
layer pair structure of Column 3B. The 40 nm thick reflective,
conductive silver layer 198 has a refractive index profile 226. The
17 nm thick SiO.sub.2 layer 209 has a refractive index profile 228.
The 181 nm thick TiO.sub.2 layer 202 has a refractive index profile
230, and the 16 nm thick Al.sub.0.16GaAs layer 200 has a refractive
index profile 220.
3 TABLE 3 VCSEL With Metal Enhanced Upper Mirror VCSEL With Metal
(15 .lambda./4 pair Enhanced Upper Mirror upper DBR) (11 .lambda./4
pair upper DBR) Column 3A Column 3B Upper .lambda./4 Pairs 15 11
Lower .lambda./4 Pairs 35 35 Threshold Gain (cm.sup.-1) 276.265
374.256 Wavelength (nm) 821.851 821.82 External Efficiency 0.319013
0.499923 (Total) External Efficiency 0.0942835 0.0654105 (Bottom)
External Efficiency (Top) 0.22473 0.434512 Round Trip Net Gain
0.00280533 0.00304929 Round Trip Net Loss 0.00191039 0.00152488
Round Trip Transmission 0.000894938 0.00152411 Transmission Down
0.000264497 0.000199456 Transmission Up 0.000630442 0.00132495
Active Region Thickness 16.0844 16.0844 (nm) Gain Enhancement
Factor 3.16363 2.53839 Normalized Peak E.sup.2 Field 9470.77
9482.14 Normalized E.sup.2 Field at 8930.6 8942.97 Active region
Normalized Top E.sup.2 Field 2.6734 7.45238 Normalized Bottom
E.sup.2 1.1216 1.12186 Field Nearest Subsurface Metal 300 300 Peak
Surface-Peak Field Ratio 0.031676411 0.105461425 Intensity
Enhancement 23.83694351 79.36120238 Top Surface Reflectivity
0.999369558 0.99867505
[0146] Table 2, Columns 2A and 2B respectively provide the
parameters for the structure under aperture 206 for eleven and
fifteen .lambda./4 layer pairs 160, and FIG. 31 and FIG. 32 provide
graphic representations of the optical field profiles and
refractive index profiles for these structures, as described
above.
[0147] In order to further enhance the efficiency of extraction of
optical power from the small aperture VCSEL apparatus provided by
the invention, the configuration of the aperture can be modified
such that emission efficiency is enhanced. As described above, one
presently preferred aperture configuration in accordance with the
invention includes one or more protrusions in the surrounding
conductive layer which extend into the aperture. A variety of
aperture configurations in accordance with the present invention
may be etched into the emission facet of a VCSEL device using
conventional FIB or e-beam lithographic techniques.
[0148] Referring now to FIG. 36, there is shown another embodiment
VCSEL 234 is shown, wherein like reference numbers denote like
parts. The VCSEL 234 comprises an aperture 236 which is structured
and configured in the same manner of the aperture 18 described
above and shown in FIG. 2. The aperture 236 includes a protrusion
or tongue 238 which extends into the aperture 236 from the
surrounding conductive layer 198, such that regions 239a, 239b are
defined in the aperture. Aperture 236 is etched into emission facet
204 using a focused ion beam or other anisotropic etching
technique, with the bottom 240 of aperture 236 nominally coinciding
with the center of the TiO.sub.2 layer 202, which is subject to the
highest magnitude of electric field during laser operation, as
noted above. The tongue or protrusion 238 includes the conductive
metal layer 198 as well as the layers 202, 200 and 174 positioned
beneath metal layer 198. In respects other than the configuration
of aperture 236, the structure of VCSEL 234 is the same as
described above for VCSEL 196.
[0149] As described above, the use of an aperture 236 surrounded by
a conductive metal plane 198, with a protrusion 238 of the
conductive metal plane into the aperture, provides good near field
localization, together with a higher level of near field brightness
than has previously been available in VCSEL devices. Preferably,
the perimeter length of aperture 236 is tailored to be
substantially at resonance with the output wavelength of laser 234
as noted above. Resonance effects here should take into
consideration the particular semiconductor, oxide and metal
materials of the VCSEL, which will generally modify the effective
wavelength to satisfy the resonance condition. VCSEL devices offer
a relatively high degree of intrinsic polarization, and aperture
236 is positioned to maximize the number of aperture edges which
are substantially perpendicular to the polarization direction of
the output of laser 236.
[0150] The aperture 236 may be structured and configured in the
manner of the apertures shown in FIG. 11, FIG. 22, FIG. 23, FIG. 25
and FIG. 26, as described above. If the annular aperture of FIG.
26A is used in VCSEL 234, for example, the aperture 236 in emission
face 209, would be annular in shape, with a central post or core
238 defined by the annular aperture 236. The structure of an
annular aperture 236 and post 238 is conceptually analogous to the
difference between a hollow coaxial metal waveguide, which has a
well defined frequency or wavelength above (or below) which
electromagnetic radiation evanesces rather than propagates, and a
coaxial waveguide with two electrically isolated conductors,
wherein electromagnetic radiation may propagate at arbitrarily low
frequencies (long wavelengths). The annular aperture configuration
provide two regions in the conductive layer 198 and emission face
204 that are electrically isolated from each other, and provides
for creating an electrical dipole in the emission plane of the
laser. Such an electric dipole is a much more efficient radiator of
optical energy at a given wavelength than a simple aperture
configuration, since the simple aperture provides for only one
electrical potential in the emission plane of the aperture. In
other words, the simple conducting aperture is an equipotential of
the electric field. This effect can be important for apertures of
subwavelength dimension, where, for simple apertures such as that
shown in FIG. 1, evanescent fields which do not propagate beyond
the aperture represent a substantial fraction of the total optical
power, and the evanescent power fraction is strongly wavelength
dependent.
[0151] The use of an annular aperture as described above is only
exemplary. More generally, the conductive plane of the emission
face may be partitioned into at least two regions which are
electrically isolated from each other, and with at least one region
in which the separation between the electrically isolated regions
is much smaller than the emission wavelength, and which thereby
provides for enhanced localized emission efficiency through the
region of small separation. The shape of such isolated regions may
be tailored to provide for further enhancement of the emission as a
result of electromagnetic resonance effects within the isolated
regions.
[0152] Accordingly, it will be seen that this invention provides a
near field optical apparatus which provides high output power with
effective near field localization. Although the description above
contains many specificities, these should not be construed as
limiting the scope of the invention but as merely providing an
illustration of the presently preferred embodiment of the
invention. Thus the scope of this invention should be determined by
the appended claims and their legal equivalents.
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