U.S. patent application number 14/705022 was filed with the patent office on 2015-11-12 for high power, broadband terahertz, photoconductive antennas with chaotic shape electrodes.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is Benjamin Graber, Christopher Kim, Dong Ho Wu. Invention is credited to Benjamin Graber, Christopher Kim, Dong Ho Wu.
Application Number | 20150325324 14/705022 |
Document ID | / |
Family ID | 54352794 |
Filed Date | 2015-11-12 |
United States Patent
Application |
20150325324 |
Kind Code |
A1 |
Wu; Dong Ho ; et
al. |
November 12, 2015 |
HIGH POWER, BROADBAND TERAHERTZ, PHOTOCONDUCTIVE ANTENNAS WITH
CHAOTIC SHAPE ELECTRODES
Abstract
A photoconductive antenna is described that includes a substrate
that includes a pair of trenches. Furthermore, a pair of
non-parallel electrodes, which can be designed with a chaotic
electrode geometry, can each be deposited in one of the trenches,
and can be configured to produce chaotic trajectories of incoherent
electric currents. Finally, an insulation layer, which can be
either a physical electrical insulation layer or an air gap, can be
included between each of the pair of non-parallel electrodes and
the trench walls. Overall, the thickness of the substrate, the
thickness of the trenches, and the thickness of the non-parallel
electrodes can each be optimized to produce a coherent terahertz
beam.
Inventors: |
Wu; Dong Ho; (Olney, MD)
; Graber; Benjamin; (Washington, DC) ; Kim;
Christopher; (Springfield, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wu; Dong Ho
Graber; Benjamin
Kim; Christopher |
Olney
Washington
Springfield |
MD
DC
VA |
US
US
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Washington
DC
|
Family ID: |
54352794 |
Appl. No.: |
14/705022 |
Filed: |
May 6, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61988968 |
May 6, 2014 |
|
|
|
Current U.S.
Class: |
250/504R ;
438/666 |
Current CPC
Class: |
H01L 31/09 20130101;
G21K 5/02 20130101; H01S 1/02 20130101 |
International
Class: |
G21K 5/02 20060101
G21K005/02; H01S 1/02 20060101 H01S001/02 |
Claims
1. A photoconductive antenna; comprising: a substrate comprising a
pair of trenches; a pair of non-parallel electrodes each deposited
in one of the trenches, and configured to produce chaotic
trajectories of incoherent electric currents; and an insulation
layer between each of the pair of non-parallel electrodes and the
trench walls.
2. The photoconductive antenna of claim 1, wherein a thickness of
the substrate, a thickness of the trenches, and a thickness of the
non-parallel electrodes are each optimized to produce a coherent
terahertz beam.
3. The photoconductive antenna of claim 1, wherein the pair of
non-parallel electrodes each comprise a chaotic electrode
geometry.
4. The photoconductive antenna of claim 3, wherein the chaotic
electrode geometries comprise one of a circle plus arc electrode
geometry; a ripple electrode geometry; a stadium concave geometry;
or a hourglass geometry.
5. The photoconductive antenna of claim 1, wherein the insulation
layer comprises at least one of a physical electrical insulation
layer or an air gap.
6. A method, comprising the steps of: etching a pair of trenches
into a substrate; depositing each one of a pair of non-parallel
electrodes into each of the pair of trenches in the substrate;
wherein the pair of non-parallel electrodes are configured to
produce chaotic trajectories of incoherent electric currents; and
configuring the pair of non-parallel electrodes to maintain an
insulation layer between each of the pair of non-parallel
electrodes and the trench walls.
7. The method of claim 6, further comprising optimizing a thickness
of the substrate, a thickness of the trenches, and a thickness of
the non-parallel electrodes to produce a coherent terahertz
beam.
8. The method of claim 6, wherein configuring the pair of
non-parallel electrodes to maintain an insulation layer, comprises
depositing a physical electrical insulation layer between each of
the pair of non-parallel electrodes and the trench walls.
9. The method of claim 6, wherein configuring the pair of
non-parallel electrodes to maintain an insulation layer, comprises
maintaining an air gap between each of the pair of non-parallel
electrodes and the trench walls.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to provisional patent
application entitled, "High Power, Broadband Terahertz,
Photoconductive Antennas with Chaotic Shape Electrodes," filed on
May 6, 2014, and assigned U.S. Application No. 61/988,968; the
entire contents of which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present disclosure relates generally to terahertz
photoconductive antennas, and more specifically relates to
optimizing a terahertz photoconductive antenna design to produce a
strongly coherent terahertz beam.
BACKGROUND
[0003] Terahertz photoconductive antennas have been used for more
than two decades. Since its invention in 1984, minor modifications
have been made to the antenna structures; however, details of the
antenna design and the parallel micro-strip-line electrodes, which
form the basic electrode structure of the conventional
photoconductive antenna, have not been modified and are still being
used.
[0004] FIG. 1 is a prior art terahertz photoconductive antenna
structure 100. The parallel electrodes 105 are typically fabricated
by depositing gold layers into two parallel trenches. Although the
trench depth in FIG. 1 and the gold electrode thickness are labeled
as 650 nm and 520 nm, the depth and thickness of the commercial
photoconductive antennas have not been optimized, and there is no
standard for these values. Therefore, it is not uncommon to see a
large variation in these parameters from photoconductive-antenna
manufacturers, and there are few guidelines for the fabrication of
electrodes. The gold electrodes can often be excessively deposited,
so that their thickness exceeds 1 .mu.m. In commercial
photoconductive antennas, the gold layers (electrodes) directly
contact the trench walls, so that electric currents can flow from
the electrode through the sidewall of the trench.
[0005] FIG. 2A is a prior art diagram representing the generation
of a terahertz pulse using a femto-second laser and a
photoconductive antenna. FIG. 2B is a prior art diagram
representing the positive and negative charges between the
electrodes during the generation of a terahertz pulse. FIG. 3 is a
prior art diagram of a cross-sectional view of photoconductive
antenna showing the photocurrents, bias currents and thermal
currents during the generation of a terahertz pulse. FIGS. 2A and 3
illustrate that the terahertz pulse can be produced by illuminating
a semiconductor slab (e.g., a GaAs substrate) with a femto-second
laser beam. The laser pulse can generate a surface plasma,
consisting of positive charges and negative charges. This
oscillating surface plasma is known as a surface plasmon. The
oscillating positive and negative charges can generate the
terahertz pulse.
[0006] If the positive and negative charges recombine immediately
after they are produced, the intensity of the terahertz pulse
becomes very weak. Therefore, in order to minimize the charge
recombination, a bias voltage can be applied to the electrodes,
which can create an electric field that separates the positive
charges from the negative charges (see FIG. 2(B1)). The positive
charges will be attracted to the negative electrode, and the
negative charges will be attracted to the positive electrode.
However, when these charges arrive at the electrodes, they will
ordinarily be discharged. To prevent such a discharge, the polarity
of the electrodes can be switched right before the charges touch
the electrodes, or just before they collide and recombine (see FIG.
2(B2) and FIG. 2(B3)). In other words, an AC bias voltage with an
optimum frequency can substantially enhance the oscillation
amplitude of the plasmon (the photocurrent) so that it increases
the terahertz pulse strength.
[0007] The ac bias voltage, however, can result in substantial bias
current flowing between the electrodes. This bias current, along
with the photocurrent, can generate considerable Joule heating. The
Joule heating, together with the thermal energy provided by the
femto-second laser beam, can create thermal electric currents,
which are incoherent in nature. The photocurrent, bias currents and
thermal electric currents can all produce Joule heat, and the Joule
heat can create more thermal currents, which can produce blackbody
radiation, such as incoherent terahertz beams and infrared
beams.
[0008] The thermal electric currents can also disrupt the coherency
of the photocurrent and the bias currents, so that it reduces the
strength of the coherent terahertz beam, and enhances the
incoherent terahertz beam. This further increases the heating and
the thermal electric currents. FIG. 4 is a prior art schematic
diagram the explains how the photocurrent, bias-current, and
thermal currents affect the production of a coherent terahertz beam
and an incoherent terahertz beam. Specifically, FIG. 4 depicts the
complex recursive process and the interactions among the three
different currents. If the heat produced through this recursive
process is excessive, it will eventually destroy the
photoconductive antenna.
[0009] When a thermal electron (or electron wave function) travels
perpendicular to the electrodes, in between a pair of parallel
electrodes, the electron (or electron wave function) is likely to
follow a bouncing ball trajectory or a standing wave pattern. FIG.
5A represents a bouncing ball trajectory for an electron wave
function. FIG. 5B represents a standing wave pattern for an
electron wave function. Therefore, the particle (the electron) or
wave (the electron wave) is likely to be trapped in between the
electrodes, unless the particle or wave travels at an oblique
angle, such as in FIG. 6A and FIG. 7A. FIG. 6A represents a
traveling ball mode trajectory for an electron wave function, and,
similarly, FIG. 7A represents a non-chaotic trajectory, such as the
traveling ball mode, for an electron wave function. Therefore, with
the parallel electrode geometry, a large number of thermal
electrons that flow incoherently can be trapped in between the
electrodes and disrupt the photocurrent. FIG. 8A represents a
slowly traveling, or virtually trapped, wave pattern for a
traveling ball mode trajectory for an electron wave function.
[0010] Additionally, FIG. 6B represents a trapped ball mode
trajectory for an electron wave function, and the associated FIG.
8B represents a standing wave mode for a trapped ball mode
trajectory for an electron wave function. Similarly to FIG. 6A and
associated FIG. 8A, FIG. 6B and FIG. 8B show how a large number of
thermal electrons that flow incoherently can be trapped in between
the electrodes and disrupt the photocurrent, and may not allow
traveling wave pattern.
[0011] Consequently, a conventional photoconductive antenna with a
pair of parallel electrodes is highly inefficient in converting the
femto-second laser pulse into a terahertz beam; and, therefore, is
not efficient in producing a strong, coherent terahertz beam.
Instead, because of the above-mentioned problems, the antenna
structure with the conventional electrodes predominantly produces
incoherent terahertz beams, and the efficiency of the conventional
photoconductive antenna is therefore very poor.
[0012] In summary, the conventional terahertz photoconductive
antennas have the following limitations and disadvantages: (1) the
design parameters, such as the trench depth, the thickness of the
electrode, and the thickness of the substrate, are not optimized;
(2) the conventional photoconductive antenna with a pair of
parallel electrodes produces a very weak, coherent terahertz beam
(<<1 mW); (3) with a strong pump-laser beam and a large bias
voltage applied to the electrodes, they produce excessively
incoherent terahertz beams, which lead to the destruction of the
photoconductive antenna; and (4) as a result, the lifetime of the
conventional photoconductive antenna is short.
[0013] Accordingly, there remains a need in the art for an improved
design of a photoconductive antenna that can dramatically improve
its conversion efficiency, and produce a strong, coherent terahertz
beam.
SUMMARY OF THE INVENTION
[0014] According to one aspect of the invention, a photoconductive
antenna is described that includes a substrate that includes a pair
of trenches. Furthermore, a pair of non-parallel electrodes, which
can be designed with a chaotic electrode geometry, can each be
deposited in one of the trenches, and can be configured to produce
chaotic trajectories of incoherent electric currents. Finally, an
insulation layer, which can be either a physical electrical
insulation layer or an air gap, can be included between each of the
pair of non-parallel electrodes and the trench walls. Overall, the
thickness of the substrate, the thickness of the trenches, and the
thickness of the non-parallel electrodes can each be optimized to
produce a coherent terahertz beam.
[0015] According to another aspect of the invention, a method is
described for etching a pair of trenches into a substrate. Next,
each one of a pair of non-parallel electrodes can be deposited into
each of the pair of trenches in the substrate; wherein the pair of
non-parallel electrodes can be configured to produce chaotic
trajectories of incoherent electric currents. Finally, the pair of
non-parallel electrodes can be configured to maintain an insulation
layer between each of the pair of non-parallel electrodes and the
trench walls. Furthermore, the thickness of the substrate, the
thickness of the trenches, and the thickness of the non-parallel
electrodes can be optimized to produce a coherent terahertz
beam.
[0016] These and other aspects, objects, and features of the
present invention will become apparent from the following detailed
description of the exemplary embodiments, read in conjunction with,
and reference to, the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The following description and drawings set forth certain
illustrative implementations of the disclosure in detail, which are
indicative of several exemplary ways in which the various
principles of the disclosure may be carried out. The illustrated
examples, however, are not exhaustive of the many possible
embodiments of the disclosure. Other objects, advantages and novel
features of the disclosure will be set forth in the following
detailed description of the disclosure when considered in
conjunction with the drawings, in which:
[0018] FIG. 1 is a prior art terahertz photoconductive antenna
structure.
[0019] FIG. 2A is a prior art diagram representing the generation
of a terahertz pulse using a femto-second laser and a
photoconductive antenna.
[0020] FIG. 2B is a prior art diagram representing the positive and
negative charges between the electrodes during the generation of a
terahertz pulse.
[0021] FIG. 3 is a prior art diagram of a cross-sectional view of
photoconductive antenna showing the photocurrents, bias currents
and thermal currents during the generation of a terahertz
pulse.
[0022] FIG. 4 is a prior art schematic diagram that explains how
the photocurrent, bias-current, and thermal currents affect the
production of a coherent terahertz beam and an incoherent terahertz
beam.
[0023] FIG. 5A represents a bouncing ball trajectory for an
electron wave function.
[0024] FIG. 5B represents a standing wave pattern for an electron
wave function.
[0025] FIG. 6A represents a traveling ball mode trajectory for an
electron wave function.
[0026] FIG. 6B represents a trapped ball mode trajectory for an
electron wave function.
[0027] FIG. 6C represents a rapidly diverging ball mode trajectory
for an electron wave function.
[0028] FIG. 7A represents a non-chaotic trajectory for an electron
wave function.
[0029] FIG. 7B represents a chaotic trajectory for an electron wave
function, in accordance with an exemplary embodiment of the
invention.
[0030] FIG. 8A represents a slowly traveling, or virtually trapped,
wave pattern for a traveling ball mode trajectory for an electron
wave function.
[0031] FIG. 8B represents a standing wave mode for a trapped ball
mode trajectory for an electron wave function.
[0032] FIG. 8C represents a rapidly diverging wave pattern for a
rapidly diverging ball mode trajectory for an electron wave
function.
[0033] FIG. 9 is a photoconductive antenna with a pair of chaotic
electrodes, in accordance with an exemplary embodiment of the
invention.
[0034] FIG. 10 is a graph comparing time domain signals from a
conventional THz emitter and THz emitter in accordance with an
exemplary embodiment of the invention.
[0035] FIG. 11 is a chart of sample results obtained from
photoconductive antennas with various geometries in accordance with
an exemplary embodiment of the invention.
[0036] FIG. 12A is an example view of thermal electron behavior in
a stadium concave geometry, in accordance with an exemplary
embodiment of the invention.
[0037] FIG. 12B is an example view of thermal electron behavior in
a stadium convex geometry, in accordance with an exemplary
embodiment of the invention.
[0038] FIG. 13 is a graphic of how a ripple electrode geometry
produces a more coherent terahertz beam, in accordance with an
exemplary embodiment of the invention.
[0039] FIG. 14 is an example of an optimally configured
photoconductive antenna, in accordance with an exemplary embodiment
of the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0040] One or more embodiments or implementations are hereinafter
described in conjunction with the drawings, where like reference
numerals refer to like elements throughout, and where the various
features are not necessarily drawn to scale.
[0041] As mentioned in the background section, the inefficiency of
the conventional photoconductive antenna stems from the thermal
electric currents, which are produced by both the thermal load of
the femto-second laser beam and the electric currents, including
the photocurrent, the bias current and the thermal electric current
itself. It is also understood that the behavior of the thermal
electric currents (and the heat production associated with these
currents) is closely related with the electrodes. For example, by
altering the design of the electrodes, one can suppress the
production of thermal electrons, and minimize the disruption of the
photocurrents by the thermal currents.
[0042] Accordingly, to improve the terahertz photoconductive
antenna, the electrode design can be changed. First, a pair of
trenches can be etched into a substrate. Then, in an exemplary
embodiment of the invention, the electrode design of the terahertz
photoconductive antenna can include two electrodes that are not
parallel, instead of two parallel micro-strip-line electrodes. Each
one of the pair of non-parallel electrodes can be separately
deposited into each of the pair of trenches in the substrate. That
is, one electrode can go in one trench, and the other electrode can
go in the other trench. Finally, the pair of non-parallel
electrodes can be configured in the trenches to maintain an
insulation layer, which can include either depositing a physical
electrical insulation layer between each of the pair of
non-parallel electrodes and the trench walls, or maintaining an air
gap between each of the pair of non-parallel electrodes and the
trench walls.
[0043] Several different, non-parallel shapes can be used in the
design for the electrodes, and these shapes can be called "chaotic
geometries" since, in contrast to a pair of parallel electrodes,
these electrodes are configured to produce chaotic trajectories
when a particle or wave bounces between the electrodes. The chaotic
trajectory means that with a minute variation of the initial
condition, the trajectory deviates, or varies, exponentially with
time. FIG. 7B represents a chaotic trajectory for an electron wave
function. In particular, FIG. 6C represents a rapidly diverging
ball mode trajectory for an electron wave function, which
represents an example of a chaotic trajectory. In association, FIG.
8C represents a rapidly diverging wave pattern for a rapidly
diverging ball mode trajectory for an electron wave function.
[0044] As explained in the background section with respect to the
prior art schematic diagram in FIG. 4, there are three electric
currents in a photoconductive antenna: the photocurrent, the bias
currents and the thermal electric currents. These three currents
interact with each other, either constructively or destructively.
For example, the bias currents (or the electric field produced by
the bias voltage) force the thermal electric currents to be in
phase with the photocurrent, so that they can produce more of a
coherent terahertz beam. However, the thermal electric currents
tend to disrupt the coherency of the photocurrent and the bias
currents, so that the strength of coherent terahertz beam is
reduced and the strength of incoherent terahertz beam is
increased.
[0045] In an exemplary embodiment of the invention, in a
photoconductive antenna with a pair of chaotic electrodes,
incoherent electric currents (e.g., thermal electric currents)
normally follow chaotic trajectories, and their interference with
the coherent electric currents become minimized. Therefore, it will
promote some bias currents and some thermal electric currents to
flow in phase with the coherent photocurrent, which can allow the
photoconductive antenna to produce a stronger coherent terahertz
beam.
[0046] In an exemplary embodiment of the invention, several
different chaotic electrode geometries can be utilized. FIG. 9 is a
photoconductive antenna 900 with a pair of chaotic electrodes 905,
in accordance with an exemplary embodiment of the invention.
Specifically, the chaotic electrodes in FIG. 9 represent an
hourglass shape geometry that leads to chaotic trajectories (as
also seen in FIG. 7B). FIG. 10 is a graph comparing time domain
signals from a conventional THz emitter and THz emitter in
accordance with an exemplary embodiment of the invention.
Specifically, FIG. 10 is a graph of time domain signals from a
conventional THz emitter with a conventional photoconductive
antenna with a pair of parallel electrode (solid line) and an
exemplary THz emitter with an exemplary photoconductive antenna
with a pair of chaotic electrode (dotted line). Note that the
exemplary THz emitter can produce a much greater coherent THz
output (i.e., at least 3 mW THz output). Furthermore, the terahertz
spectrum of the exemplary photoconductive antenna is nearly
identical to that of the conventional photoconductive antenna, even
while producing the much greater and more coherent THz output.
[0047] FIG. 11 is a chart of sample results obtained from
photoconductive antennas with various geometries in accordance with
an exemplary embodiment of the invention. Four antennas containing
chaotic electrodes (i.e., circle plus arc electrode geometry 1110;
a ripple electrode geometry 1115; a stadium concave geometry 1120;
or a hourglass geometry 1130) produce much stronger terahertz beams
when compared with three antennas with non-chaotic electrodes
(i.e., a conventional parallel geometry 1105, a stadium convex
geometry 1125, and a gap concave geometry 1135). Interestingly,
drastically different results can be obtained from two antennas,
stadium concave 1120 and stadium convex 1125, that look very
similar to each other, except one, stadium concave 1120, has sharp
edges and the other, stadium convex 1125, has round and smooth
edges. The antenna with the stadium concave 1120 chaotic geometry
leads to chaotic electron trajectories because its sharp edges
reflect the electrons and do not allow them to enter into the
slits.
[0048] FIG. 12A is an example view of thermal electron behavior in
a stadium concave geometry, in accordance with an exemplary
embodiment of the invention. The sharp edges of the stadium concave
1120 geometry force the electrons to follow spiral trajectories, as
represented in FIG. 12A. Most thermal electrons reflected by the
concave geometry follow chaotic trajectories, and diverge away from
the emitter so that they minimally interfere with the photocurrent.
Therefore, this emitter produces a relatively stronger terahertz
beam.
[0049] By contrast, the smooth edges of the antenna with the
stadium convex 1125 geometry make the electrons enter the slits, so
that the electrons can eventually be trapped in the slits. FIG. 12B
is an example view of thermal electron behavior in a stadium convex
geometry, in accordance with an exemplary embodiment of the
invention. These trapped electrons release their energy as heat,
which disrupts the coherent electron currents and hence reduces the
coherent terahertz beam output. Although the shape of the
electrodes in FIG. 12B look similar to that of FIG. 12A, because of
the convex tip of the electrodes, the thermal electrons tend to be
trapped in the gaps between the electrodes. Consequently, they
create more heat and disturb the photocurrent. Therefore, this
emitter is inefficient and produces a weak terahertz beam. FIGS.
12A and 12B are included here to illustrate how the thermal
electron behavior affects the performance of a terahertz
emitter.
[0050] In an exemplary embodiment of the invention, a
photoconductive antenna with a ripple chaotic electrode geometry
can be the most efficient. The ripple chaotic electrode geometry
includes a pair of wavy electrodes at a variable distance apart.
The ratio of the coherent terahertz power to the total (coherent
and incoherent) terahertz power for the ripple geometry is about
73% for the antenna. This is because a pair of ripple electrodes
leads to chaotic electron trajectories. Furthermore, ripple
electrodes not only minimize the interference between the
incoherent electron currents and coherent currents, but also
convert the incoherent electron currents into coherent currents,
further amplifying the coherent electron currents, which produce a
more coherent terahertz beam. FIG. 13 is a graphic of how a ripple
electrode geometry produces a more coherent terahertz beam, in
accordance with an exemplary embodiment of the invention.
[0051] In addition to altering the shapes of the electrodes as
described above, the electrodes of the photoconductive antennas can
be electrically insulated from the trench walls by air-gaps or by
an electrical insulation layer, in accordance with an exemplary
embodiment of the invention. FIG. 9 shows electrodes of the
photoconductive antennas can be electrically insulated from the
trench walls by air-gaps 905 or by an electrical insulation layer
910. With this electrical insulation, the electrodes can generate
an electric field with a minimal bias current flowing between the
electrodes. This exemplary photoconductive antenna can
significantly reduce Joule heating; and, therefore, the thermal
electrical currents, which can disrupt the coherent photocurrent.
As a result, the exemplary photoconductive antenna can generate
more of a coherent terahertz beam, and the incoherent
terahertz-beam generation is considerably suppressed. In an
exemplary embodiment of the invention, silicon-nitride (SiN) has
been used as the material for an electrical insulation layer;
however, one of ordinary skill in the art recognizes that other
materials can be used as well.
[0052] For high-efficiency terahertz emission, the surface plasmon
(the photocurrent) that produces the terahertz pulse should be
generated at an optimum depth from the surface of the
photoconductive antenna. Therefore, the depth of the electrodes
that control the surface plasmon should be accordingly configured
and optimized. In other words, the trench depth and the electrode
thickness should be configured and optimized so that the surface
plasmon is confined at the optimum depth from the surface. The
depth of confinement is dependent on the electron mobility and the
surface energy. If the trench depth is too deep or too shallow, and
the thickness of the electrode is too thick or too shallow, they
cannot control the surface plasmon effectively.
[0053] In addition, in an exemplary embodiment of the invention, a
photoconductive antenna can use an optimum thickness of the GaAs
substrate, so that the terahertz pulse can transmit through the
GaAs substrate without suffering much transmission loss. If the
GaAs substrate thickness is too thin, the femto-second laser beam
will be able to penetrate through the substrate and deposit too
much thermal energy onto the substrate, which eventually will lead
to thermal damage to the substrate and destroy the photoconductive
antenna. If the GaAs substrate is too thick, the terahertz beam
will experience significant dissipation while passing through the
substrate. To meet these design considerations the thickness of
GaAs substrate, the trench depth of the electrodes, and the
thickness of the gold electrodes for the exemplary photoconductive
antenna is optimized.
[0054] As noted, configuring an exemplary photoconductive antenna
for an optimum depth of trench, optimum thickness of electrode,
optimum thickness of the substrate is an important feature of the
invention. FIG. 14 is an example of an optimally configured
photoconductive antenna, in accordance with an exemplary embodiment
of the invention. Specifically, the exemplary photoconductive
antenna can be designed with a ripple electrode geometry with a
substrate thickness of about 350 .mu.m; a trench depth of about 160
nm; and an electrode thickness of about 50 nm. One of ordinary
skill in the art will understand that these different parameters
are just examples of an optimized exemplary photoconductive
antenna, and values greater than or less than the values disclosed
above can be utilized. Furthermore, the exemplary photoconductive
antenna can include an air-gap between the electrode and the trench
walls. The exemplary combination of features and parameters can
produce a terahertz beam of at least 3 mW (average power), which is
about 20-30 times stronger than the maximum average power a
conventional terahertz emitter can currently produce.
[0055] In an exemplary embodiment of the invention, the exemplary
terahertz photoconductive antennas described herein can be used for
terahertz spectroscopy and imaging, which can enable new
applications that were nearly impossible previously. Examples of
these applications include detection and identification of
biological and chemical agents, detection of hidden explosives, and
detection and identification of environmental contaminants at a
standoff distance. Another application currently being researched
is to use the terahertz spectrometer to detect ionized air produced
by a hidden nuclear material.
[0056] In addition to the applications discussed above, prototype
devices for pharmaceutical applications in a real environment have
been developed to see if terahertz spectrometers and terahertz
imaging devices are able to screen for counterfeit drugs. The
prototype device that consists of a conveyer belt, robotic arms,
and a high-speed terahertz spectrometer can measure the terahertz
spectrum of an unknown drug in order to determine whether its
spectrum matches that of a legitimate drug. If the spectrum does
not match, the robotic arm can reject the drug.
[0057] As noted herein, the maximum terahertz beam power produced
by prior art photoconductive antennas is limited. A weak terahertz
beam affects the spectroscopic resolution, the detection range, and
the detection speed. Accordingly, it is imperative to increase the
coherent terahertz-beam output for the above-mentioned
applications.
[0058] One of ordinary skill in the art will understand that
certain changes may be made to embodiments of the invention without
departing from the scope and spirit of the invention. For example,
while a GaAs substrate for the demonstration of the exemplary
photoconductive antenna is described herein, other substrate
materials for the photoconductive antenna can also be used. The
electrode gap size can be in the range from a few tens of
micrometers to a few hundred micrometers. The electrodes can be
made of other metals than gold. The photoconductive antenna is
described as a transmission mode terahertz emitter herein; however,
one can slightly alter the design to demonstrate a reflection mode
terahertz emitter.
[0059] In summary, the exemplary photoconductive antenna described
herein does not produce much bias current or Joule heat; therefore,
it can produce a strongly coherent terahertz beam. The exemplary
photoconductive antenna can produce at least 3 mW of coherent
terahertz radiation, whereas prior art photoconductive antennas
could generate at best only 0.16 mW of coherent terahertz
radiation. Furthermore, the exemplary photoconductive antenna can
produce a wider bandwidth (i.e., 100 GHz to 3 THz) and a
predominantly coherent terahertz beam. In addition, the lifetime of
the exemplary photoconductive antenna is much longer than that of
prior art versions. Overall, the features of the exemplary
photoconductive antenna include (1) a pair of electrodes that lead
to chaotic trajectories of incoherent electric currents, (2) an
insulating layer, or air-gap, between the electrode and the trench
walls, (3) optimum depth of trench, (4) optimum thickness of
electrode, (5) optimum thickness of the substrate.
[0060] It should be understood that the foregoing relates only to
illustrative embodiments of the present invention, and that
numerous changes may be made therein without departing from the
scope and spirit of the invention as defined by the following
claims.
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