U.S. patent application number 14/052504 was filed with the patent office on 2017-08-10 for waveguide embedded plasmon laser with multiplexing and electrical modulation.
This patent application is currently assigned to The Regents of the University of California. The applicant listed for this patent is Ren-min Ma, Xiang Zhang. Invention is credited to Ren-min Ma, Xiang Zhang.
Application Number | 20170229843 14/052504 |
Document ID | / |
Family ID | 59496543 |
Filed Date | 2017-08-10 |
United States Patent
Application |
20170229843 |
Kind Code |
A1 |
Ma; Ren-min ; et
al. |
August 10, 2017 |
WAVEGUIDE EMBEDDED PLASMON LASER WITH MULTIPLEXING AND ELECTRICAL
MODULATION
Abstract
This disclosure provides systems, methods, and apparatus related
to nanometer scale lasers. In one aspect, a device includes a
substrate, a line of metal disposed on the substrate, an insulating
material disposed on the line of metal, and a line of semiconductor
material disposed on the substrate and the insulating material. The
line of semiconductor material overlaying the line of metal,
disposed on the insulating material, forms a plasmonic cavity.
Inventors: |
Ma; Ren-min; (Albany,
CA) ; Zhang; Xiang; (Alamo, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ma; Ren-min
Zhang; Xiang |
Albany
Alamo |
CA
CA |
US
US |
|
|
Assignee: |
The Regents of the University of
California
Oakland
CA
|
Family ID: |
59496543 |
Appl. No.: |
14/052504 |
Filed: |
October 11, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61714553 |
Oct 16, 2012 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/1042 20130101;
H01S 5/026 20130101; H01S 5/3059 20130101; H01S 5/4031 20130101;
H01S 5/1046 20130101; H01S 3/0632 20130101 |
International
Class: |
H01S 5/34 20060101
H01S005/34; H01S 5/042 20060101 H01S005/042; H01S 5/10 20060101
H01S005/10 |
Goverment Interests
STATEMENT OF GOVERNMENT SUPPORT
[0002] This invention was made with government support under
Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of
Energy and under Contract No. FA9550-12-1-0197 awarded by the U.S.
Air Force Office of Scientific Research. The government has certain
rights in this invention.
Claims
1. A device comprising: a substrate; a line of metal disposed on
the substrate; an insulating material disposed on the line of
metal; and a line of semiconductor material disposed on the
substrate and the insulating material, wherein the line of
semiconductor material overlaying the line of metal, disposed on
the insulating material, forms a plasmonic cavity.
2. The device of claim 1, wherein a width of the line of
semiconductor material disposed on the insulating material
determines a resonant condition of the plasmonic cavity and an
emission wavelength of the device.
3. The device of claim 1, wherein a metal of the line of metal
includes silver, wherein the insulating material includes magnesium
fluoride, and wherein a semiconductor material of the line of
semiconductor material includes cadmium sulfide.
4. The device of claim 1, wherein the line of metal is thicker than
about 10 nanometers, and wherein the line of metal is about 10
nanometers to 10 micrometers wide.
5. The device of claim 1, wherein the insulating material is about
0.1 nanometers to 50 nanometers thick.
6. The device of claim 1, wherein the line of semiconductor
material is about 10 nanometers to 10 micrometers thick and about
10 nanometers to 10 micrometers wide.
7. The device of claim 1, wherein the line of semiconductor
material is substantially perpendicular to the line of metal.
8. The device of claim 1, wherein the line of semiconductor
material includes a first end and a second end, the device further
comprising: a first electrode associated with the first end; and a
second electrode associated with the second end.
9. The device of claim 1, wherein the line of semiconductor
material disposed on the insulating material is configured to be
optically pumped, and wherein an end of the line of semiconductor
material emits electromagnetic radiation.
10. A device comprising: a substrate; a first and a second line of
metal disposed on the substrate, the first and the second line of
metal being substantially parallel; an insulating material disposed
each of the first and the second line of metal; and a first, a
second, and a third electrode disposed on the substrate, the lines
of metal and the electrodes arranged such that the first line of
metal is between the first and the second electrodes and the second
line of metal is between the second and the third electrodes; and a
line of semiconductor material overlaying the first line of metal,
disposed on the insulating material, forming a first plasmonic
cavity, and overlaying the second line of metal, disposed on the
insulating material, forming a second plasmonic cavity.
11. The device of claim 10, wherein a first width of the line of
semiconductor material overlaying the first line of metal
determines a resonant condition of the first plasmonic cavity and
an emission wavelength of the first plasmonic cavity, and wherein a
second width of the line of semiconductor material overlaying the
second line of metal determines a resonant condition of the second
plasmonic cavity and an emission wavelength of the second plasmonic
cavity.
12. The device of claim 11, wherein the first width is different
than the second width.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/714,553, filed Oct. 16, 2012, which is herein
incorporated by reference. This application is related to U.S. Pat.
No. 8,509,276, which is herein incorporated by reference.
FIELD
[0003] Embodiments described herein relate to the field of lasers,
and particularly relate to nanometer scale lasers.
BACKGROUND
[0004] Regarded as the key driver of ultra-dense optoelectronic
circuitry, single-molecule sensing, and ultrahigh-density data
storage, nanoscale lasers have attracted much attention. The
development of nanoscale lasers is rapidly advancing and a variety
of approaches have been explored, including Fabry-Perot lasers,
whispering gallery lasers, photonic crystal lasers, and metallic
lasers. Recently, plasmon lasers with both physical size and
optical mode confinement below the diffraction limit of light in a
different number of dimensions have been demonstrated using
localized surface plasmons bound to metal surfaces. With the
ability to generate intense electromagnetic radiation at the
nanoscale in femtosecond timescales, plasmon lasers now stimulate
the exploration of broad scientific and technological innovation at
the nanometer-scale.
SUMMARY
[0005] Embodiments of a directionally emitting waveguide embedded
(WEB) plasmon laser that efficiently convert coherent surface
plasmons from a small laser cavity into an embedded photonic
semiconductor waveguide are disclosed herein. In some embodiments,
a WEB plasmon laser has an enhanced radiation efficiency of about
35%. Effective electrical modulation and wavelength multiplexing of
WEB plasmon lasers at room temperature have been demonstrated
experimentally. The hybrid photonic and plasmonic circuit may
integrate four functions, including: multi-colored plasmon light
sources, direct electrical modulation, efficient waveguide
collection and out-coupling, and wavelength multiplexing in a
compact configuration, paving the way towards large scale on-chip
integrated hybrid optoelectronic circuitry.
[0006] Details of one or more embodiments of the subject matter
described in this specification are set forth in the accompanying
drawings and the description below. Other features, aspects, and
advantages will become apparent from the description, the drawings,
and the claims. Note that the relative dimensions of the following
figures may not be drawn to scale.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 shows an example of a top-down schematic illustration
of a waveguide embedded (WEB) plasmon laser.
[0008] FIG. 2 shows an example of a top-down schematic illustration
of an array of waveguide embedded (WEB) plasmon lasers.
[0009] FIGS. 3a and 3b show examples of a waveguide embedded (WEB)
plasmon laser with directional emission.
[0010] FIG. 4 shows an example of a SEM micrograph of a multiplexed
array of WEB plasmon lasers.
[0011] FIGS. 5a and 5b show an example of the direct electrical
modulation of a WEB plasmon laser.
DETAILED DESCRIPTION
[0012] Reference will now be made in detail to some specific
examples of the invention including the best modes contemplated by
the inventors for carrying out the invention. Examples of these
specific embodiments are illustrated in the accompanying drawings.
While the invention is described in conjunction with these specific
embodiments, it will be understood that it is not intended to limit
the invention to the described embodiments. On the contrary, it is
intended to cover alternatives, modifications, and equivalents as
may be included within the spirit and scope of the invention as
defined by the appended claims.
[0013] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
present invention. Particular example embodiments of the present
invention may be implemented without some or all of these specific
details. In other instances, well known process operations have not
been described in detail in order not to unnecessarily obscure the
present invention.
[0014] Various techniques and mechanisms of the present invention
will sometimes be described in singular form for clarity. However,
it should be noted that some embodiments include multiple
iterations of a technique or multiple instantiations of a mechanism
unless noted otherwise.
INTRODUCTION
[0015] Critical challenges remain that need to be addressed before
plasmon lasers can be utilized as integrated light sources.
Firstly, the large momentum mismatch of light inside and outside of
a deep sub-wavelength plasmon cavity results in diffraction into
all directions, inhibiting directional emission and efficient
collection of optical power from a plasmon laser for practical
applications. Furthermore, due to the intrinsic metal Ohmic loss
limited quality factor, the radiation efficiency of plasmon lasers
may be very low. The devices deliver energy to the nanoscale
plasmonic mode but release a small part of their optical energy to
the far field before it is dissipated in the metal. Lastly, scaling
down integrated photonics requires multiplexed nanolasers with
direct on-chip electrical modulation, which places constraints on
the integration of driving electronics without disturbing the
cavity mode or increasing the device footprint.
[0016] An integrated waveguide embedded (WEB) plasmon laser that
can efficiently convert surface plasmons into directional laser
emission by launching more than 70% of its radiation into a
semiconductor nanobelt waveguide is disclosed herein. Due to this
efficient conversion, the radiation efficiency of the plasmon laser
has been enhanced about 20 times to 35%. Further disclosed is an
array of five WEB lasers with different colors multiplexed onto a
single semiconductor waveguide. Each of these plasmon lasers has a
sub-micron footprint and can operate at room temperature. Moreover,
this unique design integrates electronic and photonic elements,
enabling independent direct electrical modulation of each plasmon
laser. A maximum modulation depth of 11 dB for 1 volt (V) of bias
sweep may be obtained. These unique properties demonstrate an
intriguing hybrid photonic and plasmonic circuit that integrates
multi-color nanoscopic plasmon lasers with direct electrical
modulation and wavelength multiplexing onto a single photonic
semiconductor waveguide.
APPARATUS/METHODS
[0017] FIG. 1 shows an example of a top-down schematic illustration
of a waveguide embedded (WEB) plasmon laser. As shown in FIG. 1,
the WEB plasmon laser 100 includes a substrate 105, a line of metal
110 disposed on the substrate, an insulating material (not shown)
disposed on the line of metal 110, and a line of semiconductor
material 115 disposed on the substrate 105 and the insulating
material. The line of semiconductor material 115 overlaying the
line of metal 105, disposed on the insulating material, forms a
plasmonic cavity. In some embodiments, a width of the line of
semiconductor material disposed on the insulating material
determines a resonant condition of the plasmonic cavity. This in
turn determines an emission wavelength of the device.
[0018] In some embodiments, the line of semiconductor material
includes a first end and a second end. A first electrode (not
shown) may be associated with the first end, and a second electrode
(not shown) may be associated with the second end. In some
embodiments, the first and the second electrodes may include an
indium-gold alloy or a layer of gold combined with a layer of
indium.
[0019] In some embodiments, instead of the electrodes being
associated with a first end and a second end of the line of
semiconductor material, the line of semiconductor material may
overlay each of the first and the second electrodes. The line of
semiconductor material may overlay one of the electrodes on one
side of where the line of semiconductor material overlays the
insulating material and overlay the other of the electrodes on the
other side of where the line of semiconductor material overlays the
insulating material.
[0020] In some embodiments, instead of the electrodes being
associated with a first end and a second end of the line of
semiconductor material, the first and the second electrodes overlay
the line of semiconductor material. The first electrode may overlay
the line of semiconductor material on one side of where the line of
semiconductor material overlays the insulating material, and the
second electrode may overlay the line of semiconductor material on
the other side of where the line of semiconductor material overlays
the insulating material.
[0021] In some embodiments, the line of semiconductor material
disposed on the insulating material is configured to be optically
pumped. This may cause an end of the line of semiconductor material
to emit electromagnetic radiation.
[0022] In some embodiments, the substrate may include any of a
number of different materials. In some embodiments, the substrate
may be a silicon wafer. In some embodiments, a silicon wafer may be
oxidized to produce a silicon dioxide layer, with the WEB plasmon
laser being disposed on the silicon dioxide layer.
[0023] In some embodiments, a metal of the line of metal may
include silver, gold, or aluminum. In some embodiments, the line of
metal may be about 10 nanometers thick or thicker than about 10
nanometers. In some embodiments, the line of metal may be about 10
nanometers to 10 micrometers wide.
[0024] In some embodiments, the insulating material may include
magnesium fluoride or another insulating material. In some
embodiments, the insulating material may be about 0.1 nanometers to
50 nanometers thick, about 2.5 nanometers to 7.5 nanometers thick,
or about 5 nanometers thick.
[0025] In some embodiments, the semiconductor material of the line
of semiconductor material may include cadmium sulfide. Dye
molecules that can provide an optical gain and other semiconductor
materials also may be included in the line of semiconductor
material. In some embodiments, the line of semiconductor material
may be about 10 nanometers to 10 micrometers thick. In some
embodiments, the line of semiconductor material may be about 10
nanometers to 10 micrometers wide.
[0026] In some embodiments, the line of semiconductor material may
be substantially perpendicular to the line of metal.
[0027] In some embodiments, the line of metal may be replaced with
a line of heavily doped semiconductor. In some other embodiments,
the insulating material may not be included in the WEB plasmon
laser.
[0028] FIG. 2 shows an example of a top-down schematic illustration
of an array of waveguide embedded (WEB) plasmon lasers. As shown in
FIG. 2, the array of WEB plasmon lasers 150 includes a substrate
155. A first line of metal 160 and a second line of metal 165 are
disposed on the substrate. An insulating material (not shown) is
disposed on the first line of metal 160 and the second line of
metal 165. A first electrode 170, a second electrode 175, and a
third electrode 180 are also disposed on the substrate 155. The
lines of metal and the electrodes arranged such that the first line
of metal 160 is between the first electrode 170 and the second
electrode 175, and the second line of metal 165 is between the
second electrode 175 and the third electrode 180.
[0029] A line of semiconductor material 185 is disposed on the
substrate 155 and the insulating material. The line of
semiconductor material 175 overlaying the first line of metal 160,
disposed on the insulating material, forms a first plasmonic
cavity. The line of semiconductor material 175 overlaying the
second line of metal 165, disposed on the insulating material,
forms a second plasmonic cavity.
[0030] In some embodimets, the first electrode 170, the second
electrode 175, and the third electrode 180 overlay the line of
semiconductor material 185.
[0031] In some embodiments, a first width of the line of
semiconductor material overlaying the first line of metal
determines a resonant condition of the first plasmonic cavity and
an emission wavelength of the first plasmonic cavity. A second
width of the line of semiconductor material overlaying the second
line of metal determines a resonant condition of the second
plasmonic cavity and an emission wavelength of the second plasmonic
cavity. Thus, each of the plasmonic cavities may emit a different
wavelength of electromagnetic radiation.
[0032] The array of WEB plasmon lasers 150 includes two plasmonic
cavities. More plasmonic cavities can be created by including
further lines of metal, and further electrodes may also be
included.
[0033] Embodiments of the devices disclosed herein may be used in,
for example, ultra-dense optoelectronic circuitry, on-chip photonic
interconnectors, ultrahigh-density data storage applications, new
type of sensors, and new type of displays.
[0034] Chemical vapor deposition (CVD), atomic layer deposition
(ALD), and physical vapor deposition (PVD; e.g., electron beam
evaporation) processes may be used to deposit the line of metal,
the insulating material, and the line of semiconductor material.
Electron beam lithography techniques may be used to define the
regions onto which different materials are deposited. One of
ordinary skill in the art could fabricate the devices disclosed
herein using these techniques, as well as other microfabrication
techniques.
EXAMPLE
[0035] Below is a description of the development of, experiments
performed with, and simulations of WEB plasmon lasers. The below
description is intended cover examples of the embodiments disclosed
herein, and is not intended to be limiting.
[0036] FIGS. 3a and 3b show examples of a waveguide embedded (WEB)
plasmon laser with directional emission. In some embodiments, a WEB
plasmon laser may be fabricated by crossing a semiconductor cadmium
sulfide (CdS) nanobelt waveguide over a silver strip with an about
5 nm thick magnesium fluoride (MgF.sub.2) gap layer. At the
semiconductor-metal intersection, the surface plasmon effect
induces a high effective refractive index, forming a square shaped
plasmon laser cavity. The dominant radiative loss of the cavity is
scattering into the semiconductor waveguide that guides the
majority of the laser radiation into desired directions (FIG. 3a).
A CdS nanobelt with high luminescence quantum efficiency serves as
both a gain medium in the laser cavity and a semiconductor
waveguide outside for emitted laser light. The 5 nm MgF.sub.2 gap
layer is used to pull the electric field into the gap region thus
confining the plasmonic mode significantly below the diffraction
limit of light with relatively low metal Ohmic loss.
[0037] FIG. 3b shows a SEM micrograph of a WEB plasmon laser. The
WEB plasmon laser comprises a 620 nm wide, 100 nm thick CdS
nanobelt crossing a 250 nm thick, 790 nm wide silver strip
separated by a 5 nm MgF.sub.2 gap. The footprint of the plasmon
laser was about 0.48 .mu.m.sup.2. The directional emission of the
laser was observed by optically pumping the cavity region and
imaging the scattered light. At pump intensities above the laser
threshold, the brightest light spot appeared at the end facet of
the semiconductor waveguide instead of at the excited plasmon laser
cavity region, indicating that laser emission is efficiently
coupled into the waveguide. The transition from spontaneous
emission to full laser oscillation was clearly visible by both the
rapid increase in spectral purity of the plasmon cavity modes (line
width narrowing effect) and the clear threshold behavior in
integrated light output versus pump response. Note that the current
threshold can be reduced dramatically by using a laser with longer
pulse, since the pump laser pulse width (.about.100 fs) used was
about 1/1000th of the spontaneous emission life time. The observed
lasing signal at the end of the waveguide originates from the
plasmonic cavity mode at the intersection, but is guided to the
waveguide facet by the photonic waveguide mode supported in the
semiconductor strip waveguide. The efficient conversion of
amplified surface plasmons to the directional waveguide emission
was evident from an optical image of a lasing device. Integrating
the intensity of all emitted light, it was estimated that 80% of
the light emission was coupled to the waveguide propagating in both
directions away from the laser due to the symmetry of the
structure.
[0038] It is important to emphasize that the observed plasmon
lasing behavior solely originates from the WEB plasmon cavity mode
in the intersection region. In a control experiment, the CdS
nanobelt was locally excited away from the crossed region at a pump
power of 7.4 GW cm.sup.-2. The obtained spectrum indicated that it
was a broad band-edge spontaneous emission of CdS with full width
at half maximum of about 18 nm, which is in contrast with the high
purity and intense plasmon lasing emission from the WEB plasmon
cavity region with full width at half maximum under 2 nm at the
same pump power. This is unambiguous evidence that the crossed
metal strip and semiconductor waveguide have formed a high quality
WEB plasmon laser cavity. The Fabry-Perot mode across the width of
CdS nanobelt has radiation loss (estimated to be about
4.times.10.sup.4 cm.sup.-1) much higher than the metal Ohmic loss
due to the small dimensions of width (.about.620 nm) and thickness
(.about.100 nm) which prevents the lasing from the photonic CdS
nanobelt.
[0039] In the development of WEB plasmon lasers, cadmium sulfide
(CdS) nanobelts were fabricated via a chemical vapor deposition
(CVD) process. CdS (99.995%) powders were used as the source with
pieces of Si wafers covered with 10 nm of thermally evaporated Au
catalysts used as the substrates. The laser devices were
constructed from oxidized silicon substrates (100 nm SiO.sub.2)
with 250 nm thick silver strip arrays with a 5 nm MgF.sub.2 layer
on top defined by electron beam lithography followed by electron
beam evaporation and lift-off processes. For electrical modulation
of the waveguide embedded plasmon lasers, In/Au (10/120 nm) ohmic
contact electrodes were constructed with electron beam lithography
followed by thermal evaporation and lift-off processes. The WEB
lasers were optically pumped by a frequency-doubled, mode-locked
Ti-sapphire laser with a .lamda..sub.pump=405 nm, a 10 KHz
repetition rate, and an approximately 100 fs pulse length. A
20.times. objective lens (NA=0.4) was used to focus the pump beam
to a .about.2 .mu.m diameter spot onto the sample and collect the
luminescence. All experiments were conducted at room
temperature.
[0040] The experimental observation of efficient directional
waveguide coupling from a WEB plasmon laser into a semiconductor
waveguide was supported by full wave electromagnetic simulations.
The relative momentum and spatial intensity profiles determine the
coupling strength between the plasmon cavity mode and the external
modes. The effective refractive index of the plasmonic TM mode with
a dominant electric field perpendicular to the substrate surface at
the intersection region was much higher than that of the modes of
pure semiconductor nanobelt and metal strip alone, which is
important in forming a high quality plasmon cavity in the crossed
region. Simulations were performed of the mode profiles along the
direction perpendicular to the metal surface of the
semiconductor-insulator-metal gap surface plasmon mode,
semiconductor nanobelt waveguide mode, and surface plasmon mode at
the Ag-air interface. The thickness of the CdS nanobelt was 100 nm
in the simulations. It was seen that the TE mode of the
semiconductor waveguide was the most confined mode available with
both the best momentum and spatial mode matching with the weaker
in-plane electric fields (E.sub.x and E.sub.y) of the WEB plasmon
cavity mode. Although TM modes of the semiconductor waveguide and
silver strip shared the same dominant E.sub.z electric field
component in the cavity, they were delocalized and low momentum,
leading to both poor momentum and spatial mode matching to the
cavity mode. While the weak coupling to all available modes ensured
relatively large cavity quality factors, the best momentum and
spatial mode matching was achieved for the TE semiconductor
waveguide mode leading to the observed preferential coupling to the
semiconductor waveguide.
[0041] The coupling between the WEB plasmon cavity and various
radiation channels was further studied by three dimensional
electromagnetic simulations. It was seen that there is a
square-shaped plasmon cavity formed in the intersection region due
to the high effective index contrast between the surface plasmon
mode and the surroundings. The dominant electric field, E.sub.z,
was confined well in the intersection region. The in plane fields,
E.sub.x and E.sub.y, can be efficiently coupled to the TE mode of
the semiconductor waveguide which is the most confined mode
available with both the best momentum and spatial mode matching
them. The observed significantly preferential light capture and
subsequent guiding by the semiconductor waveguide suggests that
mode coupling was stronger between this photonic waveguide and
plasmon cavity modes. Significantly, the calculations showed that
more than 70% of all the radiated energy from the laser cavity is
efficiently coupled to the waveguide with thickness above 60 nm,
and about 5% of the energy is coupled to plasmonic modes of the
silver strip. A higher efficiency was measured from the scattered
light in the aforementioned WEB laser (FIG. 3b) where the energy
coupled to the plasmonic modes was not taken into account. When the
CdS strip was thinner than 60 nanometers, the cavity still
maintains a similar quality factor due to the plasmonic confinement
effect, however, the coupling efficiency to the semiconductor
waveguide is reduced due to the decreased effective index and the
cut-off of photonic waveguide modes. For such thin waveguides, the
light scattered to free space increases and the dominant waveguide
coupling channel switches to the metal strip, whose surface plasmon
mode has the better momentum and spatial mode matching to the WEB
plasmon cavity. In this way, WEB plasmon lasers may serve as
coherent surface plasmon sources for constructing nanophotonic
circuits based entirely on surface plasmons.
[0042] FIG. 4 shows an example of a SEM micrograph of a multiplexed
array of WEB plasmon lasers. The unique architecture of the WEB
plasmon laser allows for implementing multi-color laser arrays and
multiplexing them into the same waveguide. With each laser
occupying a footprint less than a square micrometer, a five-channel
single-mode WEB plasmon laser array multiplexed onto a single
semiconductor waveguide was demonstrated by integrating a
semiconductor strip onto multiple silver strips fabricated by
E-beam lithography. The multiplexed array of WEB plasmon lasers
shown in FIG. 4 was assembled from the same CdS strip crossing five
silver strips with widths of 1 .mu.m. For electrical interface,
In/Au (10/120 nm) ohmic contact electrodes are defined through
lithography and lift-off processes.
[0043] Each laser can emit a different color because the varying
width of the waveguide tunes the resonant condition of plasmon
cavity and therefore the emission wavelength. The full width at
half maximum of a single mode plasmon laser emission can be
narrower than 1 nm. The propagating mode in the semiconductor
waveguide is the TE mode with electric field parallel to the
substrate surface, which interacts very weakly with both the
material discontinuities and the modes of neighboring WEB plasmon
cavities. As a result, the emission from each laser device can be
effectively transmitted across neighboring cavities, without
significant scattering or interference, which allows the embedded
waveguide to effectively multiplex the emission of all WEB plasmon
lasers. Note that the number of lasing modes can be tuned by the
thickness of CdS strip. The single mode operation occurs here
because that a thicker CdS strip intersecting a metal strip induced
plasmon cavity has weak effective index contrast with CdS nanobelt
waveguiding modes and thus supports less square cavity modes.
[0044] All the lasing behaviors were verified by two measures: (1)
there were clear linear-superlinear-linear transitions in the pump
intensity dependence of the total output power curves of all
measured lasing devices; and (2) the obtained intensity of lasing
cavity mode peaks exceeded the spontaneous emission background by
at least one order of magnitude. The spectra evolution, threshold
and line width narrowing behaviors of a WEB plasmon laser was
recorded. A line width narrowing from about 20 nm to lower than 1
nm around the threshold indicating the onset of lasing was
observed. With increasing pump power well above threshold, the
spectrum became asymmetrical and the line width slightly broadened.
For the large conventional cavity lasers under continuous wave
excitation, the line width will decrease inversely with the pump
power above threshold due to an increasing degree of population
inversion, i.e., the usual Schawlow-Townes behavior. However, for
microscale and nanoscale lasers under fast pulsed excitation, the
stimulated emission rate can become comparable to phonon relaxation
rate that induces electronic nonequilibrium of the gain in the
lasing regime. The resulting nonequilibrium energy distribution of
carriers gives the broadening and asymmetry of the lasing peak.
[0045] The metal and semiconductor strips forming the plasmon laser
cavity not only can serve as out-coupling waveguides, but also can
be used as electrical contacts simultaneously, allowing carriers to
be transported into and out-of the cavity free from jeopardizing
the well confined plasmon modes at all. This unique property was
employed to enable direct laser amplitude modulation here, while
the opposite operation, injecting electrons and holes into the
active cavity region can lead to an electrically pumped
semiconductor plasmon laser. FIG. 5a shows an example of the laser
spectra of a device under a peak pump intensity of 3.8 GW cm.sup.-2
under various applied biases for a 1.15 um width, 140 nm thick CdS
strip crossing 250 nm thick, 1 .mu.m width silver strip separated
by a 5 nm MgF.sub.2 gap. In/Au (10/120 nm) ohmic contact electrodes
were defined through lithography and lift-off processes. FIG. 5b
shows a schematic illustration of the direct electrical modulated
WEB plasmon laser.
[0046] As shown in FIG. 5b, two In/Au electrodes are integrated at
both ends of the waveguide to demonstrate modulation of the laser
intensity by extracting electron-hole pairs from the cavity region
by biasing the two electrodes. Remarkably, the laser peak intensity
can be modulated by 16 dB for a peak bias of 4 V by tuning the
electron-hole pair concentration in CdS, and thus the total gain of
the laser. A maximum modulation strength of 11 dBV.sup.-1 was
experimentally demonstrated. Since the applied bias changes the
density of excited carriers in the cavity, the real part of the
refractive index of CdS is also changed due to the plasma
dispersion effect. As a result, a linear shift of peak emission
wavelength was expected. It is estimated that the carrier density
in a lasing WEB plasmon cavity changes by about
4.4.times.10.sup.18cm.sup.-3 for a voltage sweep of 4 V. It is
remarkable that this value is of the same order as the inversion
density needed in bulk CdS laser despite the much higher loss of a
plasmon cavity. This is mainly due to the much higher spontaneous
emission .beta. factor, spatial gain overlap factor, and Purcell
factor of deeply confined plasmon cavity modes compared to that of
diffraction limited cavity modes.
[0047] The WEB plasmon laser can also achieve unidirectional out
coupling by cutting off one out coupling waveguide. Providing the
cut is made at the silver nanowire edge, the corresponding cavity
boundary would become total internal reflective. Simulations
indicated the excellent unidirectional coupling possible. Note that
further scaling down the waveguide size is also possible, where the
fundamental dipole resonant mode will preferentially couple to
metal strip surface plasmon waveguide. A CdS waveguide with lateral
dimensions of 60 nm.times.60 nm and a Ag waveguide with lateral
dimensions of 60 nm.times.60 nm were examined with a three
dimensional electromagnetic simulation. It was found that a
fundamental dipole mode cavity formed in the cross region and the
major emission from the cavity was captured and guided by the
sub-diffraction Ag surface plasmon waveguide.
[0048] In a plasmon cavity, the total quality factor is usually
limited by the metal Ohmic loss. Thus, the devices deliver energy
to the nanoscale plasmonic mode but release only a small part of
their optical energy to the far field before it is dissipated in
the metal. Since the fraction of energy radiating out of the cavity
depends on the radiative quality factor relative to the quality
factor of the cavity, reducing the radiation quality factor to a
certain level can increase the radiation (energy) efficiency of a
plasmon laser, while still maintain mediate total quality factor.
In the WEB plasmon laser, due to the efficient conversion of
surface plasmons in the WEB plasmon cavity to photons propagating
in photonic waveguide, the radiation quality factor can be reduced
by more than one order of magnitude compared to the plasmon square
cavity. The radiative efficiency of the cross cavity is about 35%
estimated by the radiative quality factor relative to the quality
factor of the cavity, while the estimated radiative efficiency is
just about 2% for the square cavity due to a high radiative quality
factor. The crossing waveguide configuration has significantly
enhanced the percentage of energy coupled out of the plasmon
laser.
CONCLUSION
[0049] Further details regarding the development and
implementations of WEB lasers can be found in the following
publications, all of which are herein incorporated by reference:
[0050] 1. Ren-Min Ma, Xiaobo Yin, Rupert F. Oulton, Volker J.
Sorger, & Xiang Zhang, "Directionally emitting plasmon lasers
with multiplexing and electrical modulation," FIO Postdeadline
Papers, PDPC7 (published Oct. 16, 2011); [0051] 2. Ren-Min Ma,
Xiaobo Yin, Rupert F. Oulton, Volker J. Sorger, & Xiang Zhang,
"Directionality and Integration of Nanoscale Plasmon Lasers,"
CLEO/QELS Postdeadline Papers, QTh5B.8 (published May 2012); and
[0052] 3. Ren-Min Ma, Xiaobo Yin, Rupert F. Oulton, Volker J.
Sorger, & Xiang Zhang, "Multiplexed and electrically modulated
plasmon laser circuit," Nano Letters, 2012, 12 (10), pp. 5396-5402
(published Sep. 18, 2012).
[0053] In the foregoing specification, the invention has been
described with reference to specific embodiments. However, one of
ordinary skill in the art appreciates that various modifications
and changes can be made without departing from the scope of the
invention as set forth in the claims below. Accordingly, the
specification and figures are to be regarded in an illustrative
rather than a restrictive sense, and all such modifications are
intended to be included within the scope of invention.
* * * * *