U.S. patent application number 15/805278 was filed with the patent office on 2018-04-12 for systems, methods, and apparatus for radiation detection.
The applicant listed for this patent is Nathaniel C. Brandt, Vladimir Bulovic, Wendi Chang, Harold Young Hwang, Keith Adam Nelson, Brandt Christopher Pein. Invention is credited to Nathaniel C. Brandt, Vladimir Bulovic, Wendi Chang, Harold Young Hwang, Keith Adam Nelson, Brandt Christopher Pein.
Application Number | 20180100767 15/805278 |
Document ID | / |
Family ID | 56849678 |
Filed Date | 2018-04-12 |
United States Patent
Application |
20180100767 |
Kind Code |
A1 |
Pein; Brandt Christopher ;
et al. |
April 12, 2018 |
SYSTEMS, METHODS, AND APPARATUS FOR RADIATION DETECTION
Abstract
A radiation detection technique employs field enhancing
structures and electroluminescent materials to converts incident
Terahertz (THz) radiation into visible light and/or infrared light.
In this technique, the field-enhancing structures, such as split
ring resonators or micro-slits, enhances the electric field of
incoming THz light within a local area, where the
electroluminescent material is applied. The enhanced electric field
then induces the electroluminescent material to emit visible and/or
infrared light via electroluminescent process. A detector such as
avalanche photodiode can detect and measure the emitted light. This
technique allows cost-effective detection of THz radiation at room
temperatures.
Inventors: |
Pein; Brandt Christopher;
(Cambridge, MA) ; Hwang; Harold Young; (Cambridge,
MA) ; Chang; Wendi; (Annandale, VA) ; Nelson;
Keith Adam; (Newton, MA) ; Bulovic; Vladimir;
(Lexington, MA) ; Brandt; Nathaniel C.;
(Minneapolis, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pein; Brandt Christopher
Hwang; Harold Young
Chang; Wendi
Nelson; Keith Adam
Bulovic; Vladimir
Brandt; Nathaniel C. |
Cambridge
Cambridge
Annandale
Newton
Lexington
Minneapolis |
MA
MA
VA
MA
MA
MN |
US
US
US
US
US
US |
|
|
Family ID: |
56849678 |
Appl. No.: |
15/805278 |
Filed: |
November 7, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15061308 |
Mar 4, 2016 |
9810578 |
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15805278 |
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62216583 |
Sep 10, 2015 |
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62201274 |
Aug 5, 2015 |
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62129105 |
Mar 6, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J 5/0815 20130101;
G01J 5/0837 20130101; G01J 1/58 20130101; G01J 3/04 20130101; G01J
5/046 20130101; G01J 3/42 20130101; G01J 3/0245 20130101; G01J
3/0216 20130101 |
International
Class: |
G01J 5/04 20060101
G01J005/04; G01J 1/58 20060101 G01J001/58; G01J 3/02 20060101
G01J003/02; G01J 5/08 20060101 G01J005/08; G01J 3/04 20060101
G01J003/04; G01J 3/42 20060101 G01J003/42 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with Government support under Grant
No. N00014-13-1-0509 awarded by the Office of Naval Research. The
Government has certain rights in the invention.
Claims
1. An apparatus for detecting electromagnetic radiation including a
first spectral component having a first frequency within a range of
about 100 GHz to about 100 THz, the apparatus comprising: a
conductive structure defining a first gap to receive and trap the
electromagnetic radiation so as to generate an enhanced electric
field in response to the first spectral component; an
electroluminescent (EL) material disposed at least partially within
the first gap to emit light in response to the enhanced electric
field; and a silicon oxide layer, disposed between the conductive
structure and the EL material, to electrically insulate
the-conductive structure from the EL material.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. application Ser.
No. 15/061,308, filed Mar. 4, 2016, entitled "SYSTEMS, METHODS, AND
APPARATUS FOR RADIATION DETECTION." U.S. application Ser. No.
15/061,308 claims priority to U.S. provisional application Ser. No.
62/129,105, filed Mar. 6, 2015, entitled "SCINTILLATOR FOR IMAGING
TERAHERTZ LIGHT", U.S. provisional application Ser. No. 62/201,274,
filed Aug. 5, 2015, entitled "SCINTILLATOR FOR IMAGING TERAHERTZ
LIGHT", and U.S. provisional application Ser. No. 62/216,583, filed
Sep. 10, 2015, entitled "SCINTILLATOR FOR IMAGING TERAHERTZ LIGHT."
Each of the above mentioned applications is hereby incorporated
herein by reference in its entireties.
BACKGROUND
[0003] Electromagnetic radiation at gigahertz and terahertz
frequency ranges (e.g., about 100 GHz to about 100 THz) can
penetrate many packaging materials from a distance and identify
material contained within. For example, terahertz frequencies can
facilitate identification of possibly hazardous substances
contained within packaging materials. Examples of such packaging
materials include shipping containers, storage containers, trucking
compartments, etc., that are made of non-conductive materials or
sufficiently low conductivity materials.
[0004] There are also sizeable economic and social interests in
improved security screening methods. Government spending on
domestic security alone is estimated at around $75 billion per
year. Current screening technologies generally focus on supplying
spatial information. For example, the most frequently used security
technologies in airports, federal institutions, and other public
arenas are x-ray scanners. These technologies can show images of
concealed hazards (like knives and guns). However, they provide
little to no information about the composition of a potential
hazard. Examples of those hazards include explosives, chemical
agents, or biological agents. Given that x-rays can be ionizing
radiation, there is also the potential for harm to living
tissue.
[0005] Spectroscopic imaging in the gigahertz and terahertz
frequency ranges can be used to identify both the existence of a
concealed hazard and its chemical composition. In addition, it is
presently believed that electromagnetic radiation in the gigahertz
and terahertz frequency ranges does not cause apparent damage to
living tissue.
[0006] Current terahertz or gigahertz spectroscopic imaging
techniques can be time consuming and thus impractical for security
screening. Also, there are very few single element or array
detectors for these frequency ranges. These include Golay cells,
bolometers, and pyroelectric detectors. Each kind of detector has
limitations in its ability to be useful both in a wide range of
frequencies and as an array. In addition, these kinds of detectors
use a thermal response to measure terahertz or gigahertz power.
These detectors can be expensive (on the order of $10K to $100K)
and slow (response times on the order of millisecond). While
photocurrent methods have been employed for detection in the
infrared and visible ranges, these photocurrent methods often
depend on an above bandgap excitation to create electron-hole pairs
which then generate a measureable change in the current or voltage
in the device.
SUMMARY
[0007] Embodiments of the present invention include apparatus,
systems, and methods of detecting electromagnetic radiation. In one
example, an apparatus for detecting electromagnetic radiation is
disclosed. The electromagnetic radiation has at least one spectral
component having a frequency within a range of about 100 GHz to
about 100 THz. The apparatus includes at least one conductive
structure defining at least one gap to receive the electromagnetic
radiation and generate an enhanced electric field in response to
the at least one spectral component. The apparatus also includes an
electroluminescent (EL) material disposed at least partially within
the at least one gap to generate visible light in response to the
enhanced electric field.
[0008] In another example, a method of detecting electromagnetic
radiation is disclosed. The electromagnetic radiation has at least
one spectral component having a frequency within a range of about
100 GHz to about 100 THz. The method includes receiving the
electromagnetic radiation at a field-enhancing structure (FES) and
generating an enhanced electric field at the FES in response to the
at least one spectral component. The method also includes emitting
visible light from an electroluminescent (EL) material
electromagnetically coupled to the FES in response to the enhanced
electric field and detecting the visible light emitted by the EL
material.
[0009] In yet another example, an apparatus for detecting Terahertz
radiation includes a substrate and a field-enhancing structure
(FES), disposed on the substrate, to receive the Terahertz
radiation. The FES includes an array of interdigitated conductive
strips defining a plurality of micro-slits to generate an enhanced
electric field in response to the Terahertz radiation. The array of
interdigitated conductive strips has a pitch of about 10 .mu.m to
about 100 .mu.m and each micro-slit in the plurality of micro-slits
having a width of about 0.1 .mu.m to about 10 .mu.m. The apparatus
also includes a first electrode electrically coupled to a first
plurality of metal strips in the plurality of interdigitated metal
strips and a second electrode electrically coupled to a second
plurality of metal strips in the plurality of interdigitated metal
strips to apply a direct current (DC) electric field across the
plurality of micro-slits. An electroluminescent (EL) material is
disposed at least partially within the plurality of micro-slits to
generate light at a wavelength of about 450 nm to about 700 nm in
response to the enhanced electric field. A detector is in optical
communication with the EL material to detect the visible light.
[0010] It should be appreciated that all combinations of the
foregoing concepts and additional concepts discussed in greater
detail below (provided such concepts are not mutually inconsistent)
are contemplated as being part of the inventive subject matter
disclosed herein. In particular, all combinations of claimed
subject matter appearing at the end of this disclosure are
contemplated as being part of the inventive subject matter
disclosed herein. It should also be appreciated that terminology
explicitly employed herein that also may appear in any disclosure
incorporated by reference should be accorded a meaning most
consistent with the particular concepts disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The skilled artisan will understand that the drawings
primarily are for illustrative purposes and are not intended to
limit the scope of the inventive subject matter described herein.
The drawings are not necessarily to scale; in some instances,
various aspects of the inventive subject matter disclosed herein
may be shown exaggerated or enlarged in the drawings to facilitate
an understanding of different features. In the drawings, like
reference characters generally refer to like features (e.g.,
functionally similar and/or structurally similar elements).
[0012] FIG. 1 shows a schematic of a radiation scintillator
including a field-enhancing structure (FES) and an
electroluminescent (EL) material.
[0013] FIGS. 2A-2B show micro-slits that can be used as the FES in
the radiation scintillator shown in FIG. 1
[0014] FIGS. 2C and 2D show a perspective view and a cross
sectional view of a radiation scintillator including
micro-slits.
[0015] FIG. 3 shows a cross sectional view of a radiation
scintillator including micro-slits defined by gold and chromium
strips.
[0016] FIG. 4A shows interdigitated micro-slits that can be used in
radiation scintillators shown in FIGS. 1-3.
[0017] FIG. 4B is a photo of interdigitated micro-slits like those
shown in FIG. 4A.
[0018] FIGS. 5A-5B show split ring resonators (SRRs) that can be
used as the FES shown in FIG. 1.
[0019] FIG. 5C shows a cross sectional view of a scintillator
including a splint ring resonator disposed on a phase change
material for resonant frequency tuning.
[0020] FIG. 6A shows emission spectra of three EL materials
including Alq.sub.3, Alq.sub.3 doped with DCM, and CdSe:CdS quantum
dots.
[0021] FIG. 6B shows emission spectra of PdS quantum dots.
[0022] FIG. 7 shows a cross sectional view of a radiation
scintillator including a dielectric layer disposed between a FES
and an EL material.
[0023] FIG. 8 shows a radiation scintillator including a light beam
to enhance the sensitivity.
[0024] FIGS. 9A-9B illustrate the electro-absorption process of
Terahertz (THz) radiation and the distortion of bandgap of EL
materials due to electro-absorption.
[0025] FIGS. 9C-9D illustrate photoluminescence of EL material due
to the distortion of bandgap shown in FIGS. 9A-9B.
[0026] FIG. 10 shows a perspective view of a radiation scintillator
using an array of light emitting diodes (LED) to enhance
sensitivity.
[0027] FIG. 11 shows a radiation detection system including a
radiation scintillator.
[0028] FIG. 12 shows a radiation detection system using a
scintillator having a plurality of split ring resonators arrayed in
a Bayer filter configuration.
[0029] FIG. 13 shows a cross sectional view of a radiation
detection system including a scintillator that uses micro-slits as
an FES and is disposed on an imager.
[0030] FIG. 14A shows a perspective view of a radiation detection
system including a scintillator that uses micro-slits as an FES and
is disposed on an imager.
[0031] FIG. 14B is a photo of the radiation detection system shown
in FIG. 14A.
[0032] FIG. 14C shows the micro-slits used in the radiation
detection system shown in FIG. 14A.
[0033] FIG. 14D is a photo of electroluminescence originating from
quantum dots within the gaps of the micro-slit array.
[0034] FIG. 15A shows a radiation detection system including a
scintillator that uses split ring resonators as an FES and buses to
transmit visible light generated by the scintillator to a
detector.
[0035] FIG. 15B shows a schematic of the FES used in the radiation
detection system shown in FIG. 15A.
[0036] FIG. 15C is a photo of electroluminescence generated by the
scintillator in the radiation detection system shown in FIG.
15A.
[0037] FIGS. 16A-16B illustrate radiation detection with an FES
assisted by visible and near infrared light.
[0038] FIGS. 17A-17B are microscope images of scintillators made of
split ring resonators.
[0039] FIGS. 17C-17D are microscope images of electroluminescence
generated by the scintillators shown in FIGS. 17A-17B in response
to THz radiation.
[0040] FIGS. 18A-18B are microscope images of scintillators made of
micro-slits.
[0041] FIGS. 18C-18D are microscope images of electroluminescence
generated by the scintillators shown in FIGS. 18A-18B in response
to THz radiation.
[0042] FIGS. 19A-19B show scintillators using quantum dots as the
EL material.
[0043] FIGS. 19C-19D show scintillators using Alq.sub.3 doped with
DCM as the EL material.
[0044] FIG. 19E shows electroluminescence intensity as a function
of incident THz field strength for the four examples of
scintillators shown in FIGS. 19A-19D.
[0045] FIGS. 20A-20D illustrate radiation detection assisted by
Direct Current (DC) fields.
[0046] FIGS. 20E-20F show experimental results of radiation
detection with and without assistance of DC fields.
[0047] FIGS. 21A-21B show experimental results of radiation
detection with and without assistance of near-infrared light.
DETAILED DESCRIPTION
[0048] Overview
[0049] Light in the terahertz (THz) band has compelling uses for
high-resolution imaging, security screening, explosives detection,
industrial quality control, biomedical testing, and
high-performance wireless communications. All of these applications
can benefit from low-cost, high-sensitivity, and high-speed THz
detectors, which have remained nontrivial. In addition, current THz
detection technologies usually use deep cryogenic cooling, which
can be costly and bulky. Portable room temperature detection of THz
radiation therefore is highly desirable.
[0050] To address, at least partially, the challenges in THz
detection, systems, methods, and apparatus described herein employ
an approach that includes THz field enhancing structures (FESs) and
electroluminescent (EL) materials to form a photonic device, which
links the THz band with the visible band via THz-to-visible light
up-conversion. This approach utilizes the electric field of
incoming THz light, enhanced by the FES, to induce an EL material
to luminesce in the visible frequency band. This photonic device
can function as a THz-to-visible light scintillator, which can then
be coupled to a conventional visible light sensor. Visible light
sensing technology is a mature field where single- and
multi-element sensors, such as avalanche photodiodes (APDs) and
intensified charge-coupled devices (ICCDs), are capable of
detecting single photons with nanosecond response times. As a
result, this approach can allow cost-effective detection of THz
radiation at room temperature, potentially removing one of the
major limitations in current THz technologies.
[0051] FIG. 1 shows a schematic of an apparatus 100 for radiation
detection (including THz detection) using field enhancing
structures (FESs) and electroluminescent (EL) materials. The
apparatus 100 includes a conductive structure 110, also referred to
as a field-enhancing structure (FES) which further includes a first
portion 110a and a second portion 110b. The two portions 110a and
110b define a gap 115. An electroluminescent (EL) material 120 is
disposed in the gap 115. A substrate 130 is used to hold the
conductive structure 110 and the EL material 120. Although FIG. 1
shows two disconnected portions 110a and 110b of the conductive
structure 110, in practice, the two portions 110a and 110b can be
in a single piece. For example, the conductive structure 110 can be
a split ring (i.e., a conductive ring with a gap) and the two
portions 110a and 110b can be portions of the ring on opposing ends
of the gap.
[0052] In operation, the conductive structure 110 and the EL
material 120 are exposed to the incident radiation 101, which
includes spectral components in the frequency range of about 100
GHz to about 100 THz. Upon receiving the incident radiation 101,
the conductive structure 110 concentrates the incident radiation
101 within the gap 115, leading to a strong local enhancement of
the electric field of the incident radiation 101. The enhanced
electric field causes the EL material 120, via the
electroluminescence process, to emit visible light 102, (also
referred to as visible radiation 102) which can be easily detected
by imagers or even human eyes. Since the apparatus 100 can convert
the otherwise invisible THz radiation 101 into visible radiation
102, the apparatus 100 is also referred to as a THz
scintillator.
[0053] The width of the gap 115 (i.e., distance between portions
110a and 110b) is usually comparable to or shorter than the
wavelength of the incident radiation 101. For example, the width of
the gap 115 can be less than 30 .mu.m (e.g., less than 30 .mu.m,
less than 10 .mu.m, less than 5 .mu.m, less than 1 .mu.m, less than
200 nm, less than 100 nm, less than 50 nm, less than 10 nm, etc.).
In general, a smaller width of the gap 115 can induce stronger
enhancement of the electric field within or near the gap 115 (e.g.,
close to the edge of the first portion 110a and/or second portion
110b). Therefore, decreasing the gap width can lower the threshold
intensity of the incident radiation 101 to initiate the
electroluminescence process and accordingly increase the
sensitivity of the radiation detection.
[0054] On the other hand, it can be helpful to maintain the
capacitive nature of the gap 115 without direct current flow (or
discharge) between the first portion 110a and the second portion
110b of the conductive structure 110, because potential discharge
can dissipate the enhanced electric field. Therefore, the gap 115
can have a finite width to maintain the enhanced electric field
without discharge. In practice, the width of the gap 115 can be
greater than 1 nm.
[0055] The thickness of the gap 115 is defined by the thickness of
the conductive structure 10 and can depend on several factors. For
example, the thickness of the gap can be greater than 5 nm or so to
mitigate nonlocal and quantum effects. On the other hand, it can be
helpful for the thickness of the gap 115 to be less than the skin
depth (e.g., on the order of 300 nm) of the conductive material
constituting the conductive structure 110. Otherwise, the incident
radiation 101 may interact only with a surface portion of the
conductive structure 110, rendering the rest of the conductive
structure 110 unused.
[0056] The thickness of the gap 115 also relates to the
manufacturing methods used to make the conductive structure 110.
The material for the conductive structure 110 can be deposited
using thermal evaporation, electron-beam evaporation, physical
vapor deposition (PVD), chemical vapor deposition (CVD),
plasma-enhanced chemical vapor deposition (PECVD), or laser metal
deposition (LMD), among others. The gap 115 can be defined by
nanofabrication technologies, such as, optical lithography,
electron-beam lithography, and/or focused ion beam milling.
[0057] The Conductive Structure
[0058] The enhanced electric field, which causes the EL material
120 to emit visible light, is created in the gap 115 via the
interaction of the incident radiation 101 with the conductive
structure 110. The configuration of the conductive structure 110
(e.g., geographic shape, dimensions, material, etc.) can directly
affect the field enhancement and therefore the performance of the
apparatus 100. Examples of field-enhancing structures that can be
used in the apparatus 100 are described below with reference to
FIGS. 2A-9.
[0059] Field-Enhancing Structures Using Micro-Slits
[0060] FIGS. 2A-2B show a top view of a micro-slit array that can
be used as the field-enhancing structure for THz scintillation.
FIGS. 2C-2D show a perspective view and cross sectional view of a
THz scintillator using the micro-slit array shown in FIGS.
2A-2B.
[0061] The micro-slit array 210 includes a plurality of conductive
strips 212a, 212b, . . . , 212(n-1), and 212n, collectively
referred to as conductive strips 212. Each pair of adjacent
conductive strips 212 defines a slit 215a, 215b, . . . , 215(n-1),
collectively referred to slits 215. When incident radiation
interacts with the conductive strips 212, part of the electric
field of the incident radiation can be trapped within the slits 215
and enhanced therein. As shown in FIG. 2B, the enhanced electric
field is usually stronger close to the edge of the conductive
strips 212 that define the slit 215. The enhancement of THz field
in the slits 215 is non-resonant. Therefore, the micro-slit array
210 can be responsive to a broad range of frequencies (e.g., for
radiation having wavelengths greater than the slit width) and
accordingly can be used for broadband THz detection.
[0062] The conductive strips 212 can be made of various conductive
materials. Generally, it can be helpful to use high-conductivity
materials to mitigate potential losses of the incident radiation
when interacting with the micro-slit array 210. In one example, the
conductive strips 212 can include metal materials, such as gold,
platinum copper, tantalum, tin, tungsten, titanium, tungsten,
cobalt, chromium, silver, nickel or aluminum, or a binary or
ternary system of any of these conductive materials. In another
example, the conductive strips 212 can include a conductive metal
oxide, such as TiN, TiB.sub.2, MoSi.sub.2, n-BaTiO.sub.3, (Fe,
Ti).sub.2O.sub.3, ReO.sub.3, RuO.sub.2, and IrO.sub.2, among
others. In yet another example, the conductive strips 212 may use
carbon-based conductive materials, such as graphene.
[0063] The width of the slits 215 can range from several nanometers
(usually referred to as nano-slits) to tens of microns (usually
referred to as micro-slits), depending, for example, on the target
frequency to be detected. For example, for certain gap sizes
ranging down to nanometers, the response can be tuned from the THz
band into the far and mid infrared (IR), thereby allowing detection
of these wavelengths or very broadband operation of the resulting
detector. The length of the conductive strip 212 can range from a
millimeter to centimeters (e.g., 1-2 cm) depending on, for example,
feasibility of fabrication and form factors of the resulting
device.
[0064] FIG. 2A shows multiple slits 215 and accordingly multiple
conductive strips 212 for illustrating purposes only. In practice,
the micro-slit array 210 can include only one slit 215 defined by
two conductive strips 212a and 212b (e.g., shown in FIG. 2B). Since
the conductive strips 212 and slit 215 can be made using advanced
nano-fabrication techniques, the size of a single slit scintillator
can be on nanometer scale, even less than pixel sizes in existing
imagers. Therefore, a high-resolution THz detector can be
constructed by an array (one dimensional or two dimensional) of
single slit scintillators.
[0065] Alternatively, the micro-slit array 210 can include multiple
slits 215 and accordingly multiple conductive strips 212. In one
example, the multiple slits 215 can be arranged periodically having
a pitch of about 10 .mu.m to about 100 .mu.m. In general, given the
same slit width, increasing the pitch (i.e. increasing the width of
the conductive strips 212) can increase the enhancement of the
electric field within the slit 215. In another example, the
multiple slits 215 can be arrayed in an aperiodic manner to have,
for example, different sensitivities at different areas of the
resulting scintillator.
[0066] In one example, all the slits 215 in the micro-slit array
210 have the same width to form a grating-like structure and can
create frequency-independent enhancement of electric field. In
another example, different slits 215 can have different widths such
that some slits are more sensitive to one wavelength while other
slits are more sensitive to another wavelength. As a result, the
micro-slit array 215 can facilitate spectroscopic detection.
[0067] FIGS. 2A-2B also show that the polarization direction of the
incident radiation is perpendicular to the longitudinal direction
of the slits 215. In one example, as shown in FIGS. 2A-2B, all the
slits 215 are along the same direction and are tuned to detect
incident radiation of a specific polarization. In another example,
the micro-slit array 210 can include slits 215 along different
directions. For example, the micro-slit array 210 can have a first
layer of slits 215 along the x direction stacked on a second layer
of slits 215 along they direction (perpendicular to the x
direction). Additionally or alternatively, the two layers of slits
215 can be juxtaposed (e.g., neighboring each other in the same
plane). In this case, the micro-slit array 210 can detect incident
radiation of arbitrary polarization.
[0068] FIGS. 2C-2D illustrate a scintillator 200 that uses a
micro-slit array 210 as the field-enhancing structure for radiation
detection. The micro-slit array 210 is disposed on a substrate 230.
The micro-slit array 210 includes a plurality of conductive strips
212a, 212b, and 212c defining slits 215a and 215b. An emissive
layer 220 comprising electroluminescent materials is deposited on
the substrate 230 and fills the slits 215. In FIGS. 2C-2D, gold is
used to make the conductive strips 212 and silicon oxide is used to
make the substrate 230.
[0069] FIG. 3 shows a cross sectional view of a scintillator 300
using a micro-slit array 310 that use composite metal strips. The
micro-slit array 310 is disposed on a substrate 330. The micro-slit
array 310 includes a plurality of metal strips 312a, 312b, . . . ,
312n (collectively referred to as metal strips 312) defining a
plurality of slits 315 in between. Each metal strip 312a, 312b, . .
. , 312n includes a gold layer 313a, 313b, . . . , 313n disposed on
a chromium layer 314a, 314b, . . . 314n. The chromium layers 314a
to 314n are in contact with the substrate 330 and can function as
an adhesion layer to help the gold layer 313a to 313n to stay in
position. A luminescent coating 320 comprising electroluminescent
materials is disposed on the substrate 330 to fill the slits
defined by the metal strips 312a to 312n. In this example, the
metal strips 312 have a thickness of about 160 nm and the
luminescent coating 320 has a total thickness of about 480 nm, with
320 nm above the metal strips 312.
[0070] FIG. 4A shows a schematic of a micro-slit array 400
including interdigitated metal strips 410. A first half 412a of the
metal strips in the plurality of interdigitated metal strips 410 is
connected to a positive electrode 440a. A second half 412b of the
metal strips in the plurality of interdigitated metal strips 410 is
connected to a negative electrode 440b. The positive electrode 440a
and the negative electrode 440b are connected to a power source
450. Neighboring metal strips are usually connected to opposite
electrodes to create a direct current (DC) field across the slits
defined by the metal strips 410. Without a DC bias, the electric
field in the slits is supplied by the enhanced THz field alone, as
described above. When a DC bias is applied, however, an additional
DC component to the in-gap field is introduced, which can increase
the luminescence output of the scintillator or reduce the threshold
intensity to generate electroluminescence. In addition, the
electrodes 440 can also be employed to apply an alternating current
(AC) field to mitigate degradation of the field-enhancing
structure. FIG. 4B is a photo of an example micro-slit array
including interdigitated metal strips (using gold as the strip
material). Metal strips in the example micro-slit array have a
length of about 4 mm.
[0071] Field-Enhancing Structures Using Split Ring Resonators
[0072] FIGS. 5A-5B show schematics of a field-enhancing structure
(FES) 500 including split ring resonators (SRRs). More
specifically, the FES 500 includes an array of split ring
resonators 510a and 510b (two are labeled; collectively referred to
as split ring resonators 510). Each split ring resonator 510
includes a metal structure 512 defining four gaps 515a, 515b, 515c,
and 515d. When the incident radiation has a polarization along the
vertical direction (y direction in FIG. 5A), the two gaps 515b and
515d can trap and enhance the electric field of the incident
radiation, as shown in FIG. 5B. In contrast, when the incident
radiation has a polarization along the horizontal direction (x
direction in FIG. 5A), the two gaps 515a and 515c can trap and
enhance the electric field of the incident radiation. Diagonally or
elliptically polarized radiation produces enhanced fields in all
four gaps 515. Therefore, one split ring resonator 510 can detect
incident radiation of arbitrary polarization.
[0073] In the example shown in FIGS. 5A-5B, each split ring
resonator 510 has four gaps. In another example, the split ring
resonator 510 can have less than four gaps. For example, the split
ring resonator 510 can include a simple conductive ring having one
gap. Alternatively, the split ring resonator 510 can be
substantially similar to the one shown in FIG. 5B but has only two
or three of the four gaps. In yet another example, the split ring
resonator 510 can have more than four gaps.
[0074] The split ring resonator 510 typically enhances the electric
field of the incident radiation via resonance. In other words, the
split ring resonator 510 is tunable to respond to and enhance
electric fields oscillating at a specified THz frequency. As a
result, the field-enhancing structure 500 can include split ring
resonators 510 having different resonant frequencies so as to
perform frequency-resolved imaging and sensing. For example, a
THz-frequency Bayer filter array for a camera sensor can be
constructed with split ring resonators 510 resonant at specified
THz frequencies (e.g., see FIG. 12). In addition, each split ring
resonator 510 may resonate at multiple frequencies by, for example,
having different width for different gaps in the split ring
resonator 510.
[0075] The resonant frequency of the split ring resonator 510 can
also be dynamically tuned during operation of the scintillator,
therefore allowing flexible adjustment of the scintillator in
response to changing target frequencies.
[0076] FIG. 5C shows a scintillator 501 including a split ring
resonator 511 disposed on a phase change layer 541 supported by a
substrate 531. An emissive layer 521 covers the split ring
resonator 511. The scintillator 501 also includes an actuator 551
disposed underneath the phase change layer 541 to cause phase
transition of the phase change layer 541. The phase change layer
541 comprises a phase change material, which can undergo
significant change of conductivity (e.g., four orders of
magnitudes) when changing from one phase to another. The thickness
of the split ring resonator 511 and the underlying phase change
layer 541 can be smaller than the pitch of the array of split ring
resonators in the scintillator 501, such that they form an
effective metamaterial layer. In this manner, the underlying phase
change layer 541, as part of the metamaterial, can affect the
overall response of the field-enhancing structure (i.e. split ring
resonator 511). Therefore, by changing the phase change layer 541
from one phase to another using the actuator 551, the resonant
frequency of the split ring resonator 511 can change
accordingly.
[0077] Various phase change materials can be deposited under the
split ring resonator 511 to change the resonant frequency. In one
example, the phase change material includes
germanium-antimony-tellurium (GeSbTe, also referred to as GST),
which can have good phase-change repeatability and large optical
differences between the amorphous phase and crystalline phase. In
another example, the phase change material includes a Mott
insulator (e.g., VO.sub.2) that can be switched between a metal
phase and an insulator phase. In yet another example, the phase
change material includes one or more of AgInSbTe, InSe, SbSe, AsSe,
GeSbSe, InSbTe, AgInSbSeTe, and GeSbTeSe.
[0078] The phase transition of the phase change material can be
induced via temperature change. In one example, a micro-scale hot
plate can be disposed nearby the phase change material as the
actuator 551 to adjust the temperature. In another example, the
phase transition can be obtained by direct current injection
through the phase change material. In yet another example, a phase
transition in the phase change materials can be triggered via
mechanical actuation, optical actuation (e.g., plasmonic absorption
using meta-material or photonic crystal), electric field driven
(non-heating) transformation, resistive heating, laser
annealing/heating, or magnetic actuation. More details of frequency
tuning in split ring resonators can be found in U.S. Pat. No.
8,836,439, which is incorporated herein in its entirety.
[0079] Conductive materials (e.g., metal, conductive metal oxide,
graphene, etc.) are used to make the field-enhancing structures
described above. In practice, semiconductors can also be used to
make the field-enhance structures to detect strong THz radiations,
which can metalize the semiconductor materials. This semiconductor
field-enhancing structure can be useful for higher field strength
THz detection where a normal metal may produce too strong a field
and damage the emissive material (EL material).
[0080] Micro-slit arrays and split ring resonators are described
herein as illustrating and non-limiting examples of field-enhancing
structures that can be used in THz scintillators. More examples of
field-enhancing structures can be found in U.S. Pat. No. 9,000,376,
which is incorporated herein in its entirety.
[0081] The EL Materials
[0082] The EL material 120 shown in FIG. 1 (or the emissive layer
220 shown in FIG. 2C, or the luminescent coating 320 shown in FIG.
3) can be formed of many suitable materials. Organic or quantum dot
(also simply referred to as QD) light emitting diodes (OLEDs or
QDLEDs, respectively) can be employed to construct an emissive
layer sandwiched between two electrodes. Such a device can function
through radiative recombination of electrons and holes injected
from the biased electrodes. By placing an EL material into the
field-enhancing gaps of split ring resonators or micro-slits, a
THz-driven light emitting device can be formed, in which the
incident THz electric field transiently biases the FES gaps,
injecting into the emissive layer electrons and holes that
radiatively recombine.
[0083] In one example, the EL material can include
Tris(8-hydroxyquinolinato)aluminium (formula Al(C.sub.9H.sub.6NO),
widely abbreviated as Alq.sub.3), which is usually used as organic
light emitting materials. In another example, the EL material can
include Alq.sub.3 doped with
4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran
(DCM), also referred to as Alq.sub.3:DCM.
[0084] In yet another example, the EL material includes quantum
dots. QDs are nanometer-scale semiconductor crystals and are
defined as particles with physical dimensions smaller than the
exciton Bohr radius. Quantum dots usually include a core surrounded
by a shell. The QD core can include elements from the II-VI group
(e.g. CdSe, CdTe, CdS, ZnSe), III-V group (e.g. InP, InAs), or
IV-VI group (e.g., PbSe). The shell can include CdS, CdTe, or ZnS,
among others.
[0085] Quantum dots can be tuned to luminesce from the near UV down
to the infrared (IR) and near IR. Infrared emission of quantum dots
can be useful because IR cameras can be used to collect the emitted
light. IR quantum dots usually have a smaller band gap, and devices
with these IR quantum dots can have much higher sensitivity.
[0086] The dimensions of the quantum dot(s) can vary from single
nanometers to tens of nanometers. Quantum dots of different sizes
can generate emissions at different wavelengths. The dimensions of
quantum dots can also affect the efficiencies to luminesce. Without
being bound by any particular theory or mode of operation,
confinements affect the energy levels of particles. In this case,
quantum dots confine electrons. Therefore, changing the size of
quantum dots can change the confinement and accordingly change the
energy levels of the electrons. This in turn can change the band
gap energy, thereby making a new emission wavelength. Quantum dots
can emit throughout the visible spectrum and into the IR by tuning
the sizes.
[0087] Efficiency of quantum dots is usually determined by
nonradiative pathways that an excited electron can relax, such as
auger recombination. The dimensions such as the core and shell
thickness can affect auger recombination and thereby affect the
emission efficiency.
[0088] In one example, the quantum dots can be directly applied
over the field-enhancing structure. In another example, the quantum
dots can be encapsulated in a polymer layer to mitigate or prevent
outside damage.
[0089] In one example, the EL material 120 can include a single
species of quantum dots to emit a single wavelength. In another
example, different quantum dots can be used to emit different
wavelengths. For example, a THz scintillator can use split ring
resonators as the field-enhancing structure. One species of quantum
dots can be applied over split ring resonators having one resonant
frequency can and another species of quantum dots can be applied
over split ring resonators having another resonant frequency. In
this case, the wavelength of the incident THz radiation can be
determined by the color or wavelength of the emitted light.
[0090] FIG. 6A shows electroluminescence spectra originating from
micro-slits coated with Alq.sub.3, Alq.sub.3:DCM, and CdSe/CdS
core/shell QDs. As shown by FIG. 6A, Alq.sub.3 can emit visible
light spanning from about 450 nm to about 650 nm. Doping Alq.sub.3
with DCM can shift the emission spectra to the range between about
550 nm and about 700 nm. In contrast, quantum dots typically emit
light at wavelengths centered about 590 nm, with a bandwidth of
about 50 nm to about 60 nm. These three spectra, taken together,
span nearly the entire visible region, thereby allowing flexible
operations of THz scintillators at different output wavelengths.
The emission spectra of the EL material can also be extended to
near IR and IR region by, for example using different materials for
the quantum dots or adjusting the size of the quantum dots. FIG. 6B
shows the emission spectrum of PbS (Lead sulfide) quantum dots. The
spectrum has a center wavelength at about 1430 nm, which is in the
near IR and IR region.
[0091] Increase Sensitivity of THz Scintillators
[0092] As described above, reducing the gap size in the
field-enhancing structure can increase the enhancement of electric
field within the gap and therefore increase the sensitivity of the
scintillators. In addition, applying a DC electric field across the
gap can also help increase the output luminescence and accordingly
the sensitivity of the scintillator (see, e.g., FIGS. 4A-4B and the
associated descriptions). Other than these two approaches, there
are at least two other approaches that can be used to improve
sensitivity of THz scintillators based on field-enhancing
structures. These two approaches are described below with reference
to FIGS. 7-10.
[0093] FIG. 7 shows a scintillator 700 with a dielectric layer 740
to separate the field-enhancing structure and the EL material. The
scintillator 700 includes conductive structures 710a and 710b (can
be a single conductive piece as described above with reference to
FIG. 1) disposed on a substrate 730 to define a gap 715. A
dielectric layer 740 is conformally disposed on the conductive
structure 710a and 710b, covering both the top surfaces and the
side surfaces of the conductive structures 710a and 710b. An
emissive layer 720 comprising EL material covers the dielectric
layer 740 and fill the gap 715. The dielectric layer 740 can
electrically insulate the conductive structures 710a and 710b from
the emissive layer 720. When quantum dots are used as the EL
material for the emissive layer 720, the luminescence yield can be
greater when the dielectric layer 740 is used.
[0094] In one example, the dielectric layer 740 can include silicon
oxide (e.g., SiO.sub.2). In another example, the dielectric layer
740 can include other materials that can be conformally deposited,
such as A.sub.12O.sub.3, ZrO.sub.2, HfO.sub.2, among others known
in the art.
[0095] FIG. 8 shows a schematic of a scintillator 800 that uses a
visible or near infrared beam 845 to improve the sensitivity. The
scintillator 800 includes a field-enhancing structure 810 disposed
on a substrate 830. An emissive layer 820 comprising EL materials
is disposed on the field enhancing structure 810. A light source
840 is employed to provide an optical beam 845 to illuminate the
emissive layer 820.
[0096] The scintillator 800 can operate in at least two different
modes. In the first mode, the optical beam 845 is not used. The
scintillator 800 operates by creating visible light 802 through
electroluminescence of the emissive layer 820 in response to the
enhancement of electric field in the field-enhancing structure 810.
In other words, the incident THz radiation 801 normally provides
all the electric field that drives the electroluminescence.
[0097] In the second mode, when the optical beam 845 is applied,
the incident THz radiation 801 does not directly cause the emission
of the emissive layer 820. Instead, the enhanced electric field of
the THz radiation 810 distorts the energy bands of the emissive
layer 820 such that visible light 802 is generated by the optical
beam 845 via photoluminescence in the emissive layer 820. This
electric field to induce the energy based distortion in the second
mode is typically lower than the electric field to induce
electroluminescence in the first mode. Therefore, in the second
mode the scintillator 800 can detect THz radiation with lower
intensities. In other words, the sensitivity of the scintillator
800 in the second mode is higher.
[0098] FIG. 9A-9D illustrate the electroasorption process by an
optical beam at 800 nm in quantum dots. FIG. 9A-9B show the energy
diagrams of quantum dots when the optical beam is absent and
present, respectively. FIG. 9C-9D show observed spectra from the
quantum dots and the optical beam. In the case of the CdSe/CdS QDs
used as the emissive layer 820, the 800 nm photon energy is below
the band gap, so no luminescence is generated with this light
alone. FIG. 9A shows that, in the absence of THz light, the band
gap is greater than the photo energy so typically no transition
occurs. FIG. 9C shows that the spectra include only the quantum dot
absorption spectrum and the 800 nm incident optical beam.
[0099] When a THz pulse is coincident on the scintillator, the THz
electric field distorts the ground-state and excited-state
electronic potentials of the QDs through the electroabsorption
process, resulting in a quantum-confined Stark shift of the band
absorption edge all the way from about 550 nm to 800 nm, as shown
in FIG. 9B. The THz-induced absorption of the 800-nm pulse allows
generation of excitons that give rise to photoluminescence 930 as
shown in FIG. 9D.
[0100] FIG. 10 shows a schematic of an apparatus 1000 using optical
beams 1045 to assist the THz detection. The apparatus 1000 includes
a field-enhancing structure 1010 disposed on a substrate 1030. An
emissive layer 1020 is disposed on the field-enhancing structure
1020 and fills any gaps defined by the field-enhancing structure
1020. A light emitting diode (LED) array 1040 is disposed in
optical communication with the substrate 1030 to provide an array
of optical beams 1045. The optical beams 1045 are propagating
substantially parallel to the surface of the substrate 1030 to
reduce potential interference with visible light created by the
emissive layer 1020. The substrate 1030 can include waveguide
structures to propagate the optical beams 1045 and increase the
interaction time between the optical beams 1045 and the emissive
layer 1020 so as to increase photoluminescence yield.
[0101] Systems of Radiation Detection Including Scintillators
[0102] Based on scintillators described above, various radiation
detection systems can be constructed. FIG. 11 shows a schematic of
a radiation detection system 1100 using THz scintillators. The
system 1100 includes a scintillator 1110 to receive incident THz
light 1101 and convert the otherwise invisible THz light 1101 into
visible light 1102. A detector 1120 is disposed in optical
communication with the scintillator 1110 to receive the visible
light 1102, which represents presence (or absence) and/or
properties of the incident THz light.
[0103] The scintillator 1110 can be any of the scintillators
described above with reference to FIGS. 1-10. The detector 1120 can
include various types of sensors, such as charge-coupled devices
(CCDs), complementary metal-oxide semiconductor (CMOS detectors),
photomultiplier tubes (PMT), and avalanche photodiodes (APDs), and
Geiger-mode APDs (GM-APDs), among others. The detector 1120 can
match the emission wavelength of the scintillator 1110. For
example, when IR light is emitted by the scintillator 1110, an IR
camera can be accordingly employed to detect the IR light.
[0104] The optical communication between the scintillator 1110 and
the detector 1120 can be established by at least two approaches. In
the first approach, the scintillator 1110 can be directly deposited
on the detector 1120 (also referred to as direct integration). In
the second approach, one or more relay optics can be used to
transmit the visible light 1102 to the detector 1120 (also referred
to as relay integration). The direct integration approach can have
a higher light collection efficiency that allows incoming THz light
to be imaged at high frame rate (e.g., 25 frames per second, the
maximum frame rate of the some cameras). The relay integration
approach can have a higher spatial resolution capable of resolving
finer features such as features within individual split ring
resonators.
[0105] FIG. 12 shows a schematic of a radiation detection system
1200 including a scintillator 1210 using split ring resonators 1215
as the field-enhancing structure. The scintillator 1210 is directly
attached to a sensor 1220 (e.g., camera CCD). The scintillator 1210
includes an array of splint ring resonators (1215a, 1215b, and
1215c) arranged in a Bayer filter configuration. Split ring
resonators with three different resonant frequencies are used. Each
pixel in the camera detector 1220 has an individual SRR FES above
it with a unique resonant frequency forming a THz
frequency-resolved camera.
[0106] In this example, the split ring resonators 1215 in the
scintillator 1210 fall into three categories. The first category
1215a resonates at 4 THz, the second category 1215b resonates at 2
THz, and the third category 1215c resonates at 1 THz. In the
scintillator 1210 including nine split ring resonators, the third
category 1215c is disposed in the center of the sensor 1220. The
other eight splint ring resonators at the periphery surrounding the
center resonator include alternating resonators of 1215a (first
category) and 1215b (second category). In this configuration, the
radiation detection system 1200 can resolve different "colors" of
THz radiation, in a similar manner of color detection as in visible
range by color sub-pixels in color detectors.
[0107] FIG. 13 shows a cross sectional view of another radiation
detection system 1300 using the direct integration approach. The
system 1300 includes a scintillator 1310 directly deposited on a
glass housing 1330 of a CMOS detector 1320. The scintillator 1310
includes an array of micro-slits as the field-enhancing structure.
A fused silica layer 1340 is disposed on the scintillator 1310 to
protect the scintillator 1310 from damage. Most commercially
available CMOS cameras come with them a protection glass in front
of the sensors and this protection glass can be used as the glass
housing 1330 in the system 1300. The system 1300 allows convenient
integration of scintillators into available cameras to construct
cost-effective THz detection systems.
[0108] FIG. 14A illustrates a perspective view of a third radiation
detection system 1400 using the direct integration approach. The
system 1400 includes a scintillator 1410, deposited on a camera
detector 1420, to receive THz light 1401 and convert the THz light
1401 into visible light. The scintillator 1410 includes a
micro-slit array. FIG. 14B is a photo taken in front of the system
1400, showing the scintillator 1410 and the detector 1420 behind
the scintillator 1410. FIG. 14C shows the schematic of the
micro-slit array. FIG. 14D shows an experimental image of
electroluminescence originating from quantum dots within the gaps
of the micro-slit array.
[0109] FIGS. 15A-15C shows a schematic of a radiation detection
system 1500 using the relay integration approach. The system 1500
includes a scintillator 1510 to receive incident radiation 1501 and
convert the incident radiation 1501 into visible light 1502. A pair
of lenses 1530a and 1530b transmits the visible light 1502 from the
scintillator 1510 to a detector 1520. The pair of lenses 1530a and
1530b can form a relay imaging optic to relay the image of the
scintillator 1510 to the detector 1520. In this case, high
resolution of radiation detection can be achieved. For example,
FIG. 15B shows a schematic of the scintillator 1510 including an
array of split ring resonators. FIG. 15C shows an experimental
image, in which illustrations of four individual SRRs are overlaid
to demonstrate that the visible light originates from the
capacitive gaps. It can be seen from FIG. 15C that the experimental
image can resolve features within individual SRR.
[0110] FIGS. 16A-16B illustrate a radiation detection system 1600
including optical beams to increase the sensitivity. FIG. 16A shows
the system when THz radiation is absent and FIG. 16B shows the
system when THz is detected. The system 1600 includes a
field-enhancing structure 1610 sandwiched between a substrate 1630
and an emissive layer 1620 comprising an EL material. A light
source 1640 is employed to provide optical beams 1645 that
propagate substantially parallel to the substrate 1630 but can
interact with the emissive layer 1620. The substrate 1630 can
include a waveguide structure that guides the optical beams 1645. A
detector 1660 is disposed toward the surface of the substrate 1630,
i.e., the optical axis of the detector 1660 is substantially
perpendicular to the propagation direction of the optical beams
1645. A filter 1650 is placed between the emissive layer 1620 and
the detector 1660 to filter out the optical beams 1645.
[0111] The photon energy of the optical beams 1645 can be lower
than the bandgap of the EL material in the emissive layer 1620.
Therefore, in the absence of THz radiation 1601, the optical beams
1645 typically do not create photoluminescence in the emissive
layer 1620. In addition, any diffuse or scattered light from the
optical beams 1645 can be filtered out by the filter 1650.
Therefore, the detector 1660 detects no signal when THz radiation
1601 is absent.
[0112] When THz light 1601 illuminates the field enhancing
structure 1610, the electric field of the THz light 1601 distorts
the energy diagram of the EL material in the emissive layer 1620
via electroabsorption such that the bandgap is lower than the
photon energy of the optical beams 1645. In this case, the optical
beams 1645 creates photoluminescence in the emissive layer 1620.
The created visible light 1602 normally has a wavelength APL
different from the wavelength .lamda.LED of the optical beams 1645.
Therefore, the visible light 1602 can pass through the filter 1650,
which can be a narrowband filter transmissive at APL but absorptive
or reflective at other wavelengths including .lamda..sub.LED. The
detector 1660 then picks up the visible light 1602 to detect the
presence of the THz light 1601.
[0113] The radiation detection systems described herein can operate
in several different modes. In on example, the radiation detection
system can perform only threshold detection to detect either the
presence or the absence of radiation at a target wavelength or
within a target wavelength range. In this mode, observation of
visible light by the detector can indicate the presence of target
radiation. In another example, the radiation detection systems can
perform amplitude detection to determine the strength of target
radiation. In this mode, the intensity of the acquired visible
light can be proportional to the strength of incident target
radiation, but some calibration may be performed to determine the
absolute intensity (or power) of incident radiation. In yet another
example, the radiation detection system can perform
frequency-resolved imaging and sensing, in which case the detection
distinguishes not only different intensities of incident radiation
but also difference wavelengths. As described above, using an array
of split ring resonators tuned at different resonant frequencies
can allow this spectroscopic detection.
[0114] Characterization of Radiation Detections Systems Including
Scintillators
[0115] FIGS. 17A-17D are microscope images of scintillators made of
split ring resonators fabricated by patterned deposition of gold,
onto which an EL materials (Alq.sub.3:DCM or CdSe/CdS QD) is
deposited. FIG. 17A is a microscope image of the scintillator when
THz is absent. FIG. 17B shows an enlarged view of an individual
split ring resonator. The capacitive gaps are 3 .mu.m wide and the
SRR width is 75 .mu.m. FIG. 17C is a microscope image of the
scintillator when THz light is focused onto the SRRs.
Electroluminescence is visible within the gaps. FIG. 17D shows the
enlarged view of EL from a single gap.
[0116] FIGS. 18A-18D are microscope images of scintillators made of
micro-slits. FIG. 18A shows the image of a 7.times.7 mm array of
micro-slits. FIG. 18B shows the enlargement of some of the gold
strips (also referred to as gold lines) and the gaps between them.
The gaps are 1 .mu.m wide and the line spacing is about 100 .mu.m.
FIG. 18C shows that when THz light is focused onto the micro-slits,
electroluminescence is visible in the portion of the gaps within
the THz focus which is about 1 mm in diameter. FIG. 18D shows the
enlargement of EL from part of a single gap region. On the micron
scale, the luminescence from SRR and micro-slits appears as rows of
horizontal dots and horizontal bars respectively. In both cases the
brightest luminescence is localized at upper and lower boundaries
of each individual gap, where the THz field enhancement is the most
pronounced.
[0117] The experimental observation of electroluminescence from the
above scintillators are unexpected and surprising. For
electroluminescence to occur, electrons and holes usually meet in
the EL medium within the microns-wide capacitive gaps of the FESs.
The electron mobility and hole mobility in quantum dots and organic
EL materials are typically low. For Alq.sub.3 and QD films,
reported values can range from about 10.sup.-9 cm.sup.2 V.sup.-1
s.sup.-1 to about 1 cm.sup.2 V.sup.-1 s.sup.-1. An electric field
oscillating at 1 THz can "switch" on and off in about 0.5 ps. Even
at the high end of those values, it takes a field of about 100
MV/cm for an electron or hole to traverse 1 .mu.m in ps. This field
is far higher than the enhanced field levels (usually less than 10
MV/cm) in typical field-enhancing structures. Therefore, it is
unexpected and surprising to observe electroluminescence at these
parameters. One possible explanation can be attributed to nonlinear
THz field effects that may occur in the gaps.
[0118] FIGS. 19A-19E show experimental results of four different
micro-slit scintillators using an organic or QD emissive layers as
well as an addition of a SiO.sub.2 layer that insulates the
micro-slits. FIG. 19A shows the schematic of the first scintillator
1901 including gold strips 1911 having a thickness of about 150 nm.
The gold strips 1911 fine a slit having a width of about 1 .mu.m.
Only one gap is shown, but in practice the scintillator 1901 has
multiple gaps arrayed at a periodicity of about 100 .mu.m. An
emissive layer 1921 made of CdSe/CdS core/shell quantum dots is
disposed over the gold strips 1911 and the gap. A substrate 1931 is
used to hold the gold strips 1911 and the emissive layer 1921.
[0119] FIG. 19B shows a second scintillator 1902 similar to the
first scintillator 1901 except that a 30 nm thick layer of
SiO.sub.2 is used to suppress charge injection from the gold into
the emissive layer. More specifically, the scintillator 1902
includes a substrate 1932 holding gold strips 1912 that define
slits having a width of about 1 .mu.m and a pitch of about 100
.mu.m. A SiO.sub.2 layer 1942 is conformally deposited onto the
gold strips 1912 and sandwiched between an emissive layer 1922 and
the gold strips 1912 for insulation.
[0120] FIG. 19C shows a third scintillator 1903 similar to the
first scintillator 1901 except that the emissive layer is made of
Alq.sub.3 doped by 2% of DCM. More specifically, the scintillator
1903 includes an emissive layer 1923 disposed on gold strips 1913
that define slits having a width of about 1 .mu.m and a pitch of
about 100 .mu.m. A substrate 1933 is disposed underneath the gold
strips 1913 to provide mechanical support.
[0121] FIG. 19D shows a fourth scintillator 1904 similar to the
third scintillator 1903 except that a 30 nm thick layer of
SiO.sub.2 is sued to separate the gold strips from the emissive
layer. More specifically, the scintillator 1904 includes a
SiO.sub.2 layer 1944, which is sandwiched between gold strips 1914
disposed on a substrate 1934 and an emissive layer 1924. The
emissive layer 1922 fills gaps defined by the gold strips 1914 and
covers the top surfaces of the SiO.sub.2 layer 1944.
[0122] FIG. 19E shows the scintillator THz responses for the four
different device shown in FIGS. 19A-19D as a function of THz field
strength. The THz-driven luminescent response is highly nonlinear
with respect to the incident THz field strength and varies between
the different designs. The degree of nonlinearity can depend on the
scintillator design. One observation is that covering the
micro-slits with a layer of insulating SiO.sub.2 does not halt or
reduce emission from the QD layer in the second scintillator,
suggesting that the EL in this device is primarily field-driven and
does not require charge injection from the gold. In fact, the
luminescence yield is greater when the insulating layer is present.
On the other hand, the organic emissive layer can benefit from
physical contact with the gold in order for luminescence to occur
(e.g., in the fourth scintillator). The threshold field strength
(E.sub.th), the minimum THz field that can cause luminescence above
the noise floor, can also depend on the design.
[0123] FIGS. 20A-20F show experimental results of scintillators
with and without DC field assistance. FIG. 20A shows the schematic
of a scintillator without DC field assistance. The scintillator
2000 includes an interdigitated micro-slit array 2010, in which a
first half 2012a of the slits is connected to a first electrode
2040a and a second half 2012b of the slits is connected to a second
electrode 2040b. However, neither the electrode 2040a nor the
electrode 2040b is connected to a power source and therefore there
is no DC field across the slits. FIG. 20B shows the schematic of
the scintillator 2000 when a variable voltage source 2050 is
connected to the electrodes 2040a and 2040b, creating a DC field
across the slits.
[0124] FIG. 20C shows the luminescence intensity as a function of
in-gap field when DC field is not used as shown in FIG. 20A.
Without a DC bias, the field within the slits is supplied by the
enhanced THz field alone. FIG. 20D shows the luminescence intensity
as a function of in-gap field when the DC field is applied as shown
in FIG. 20B. In this case, an additional DC component to the in-gap
field is introduced which substantially increases the luminescence
output of the scintillator.
[0125] FIGS. 20E-20F show luminescent responses of scintillators
using a CdSe/CdS QD emissive layer without and without DC
assistance. Applying a 10, 20, and 30 V bias increases the EL
intensity and reduces the threshold incident THz field strength
required to generate luminescence. The threshold THz field can be
decreased from about 130 kV/cm to as low as about 70 kV/cm.
[0126] FIGS. 21A-21B show experimental results in which the
addition of 800-nm pulses can reduce the minimum detectable THz
field strength from about 100-150 kV/cm to about 30-40 kV/cm. The
corresponding experimental setup is similar to the one shown in
FIG. 8. Using illumination wavelengths shorter than 800 nm would
further decrease the minimum detectable field.
CONCLUSION
[0127] While various inventive embodiments have been described and
illustrated herein, those of ordinary skill in the art will readily
envision a variety of other means and/or structures for performing
the function and/or obtaining the results and/or one or more of the
advantages described herein, and each of such variations and/or
modifications is deemed to be within the scope of the inventive
embodiments described herein. More generally, those skilled in the
art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the inventive teachings is/are used. Those
skilled in the art will recognize, or be able to ascertain using no
more than routine experimentation, many equivalents to the specific
inventive embodiments described herein. It is, therefore, to be
understood that the foregoing embodiments are presented by way of
example only and that, within the scope of the appended claims and
equivalents thereto, inventive embodiments may be practiced
otherwise than as specifically described and claimed. Inventive
embodiments of the present disclosure are directed to each
individual feature, system, article, material, kit, and/or method
described herein. In addition, any combination of two or more such
features, systems, articles, materials, kits, and/or methods, if
such features, systems, articles, materials, kits, and/or methods
are not mutually inconsistent, is included within the inventive
scope of the present disclosure.
[0128] The above-described embodiments can be implemented in any of
numerous ways. For example, embodiments of designing and making the
technology disclosed herein may be implemented using hardware,
software or a combination thereof. When implemented in software,
the software code can be executed on any suitable processor or
collection of processors, whether provided in a single computer or
distributed among multiple computers.
[0129] Further, it should be appreciated that a computer may be
embodied in any of a number of forms, such as a rack-mounted
computer, a desktop computer, a laptop computer, or a tablet
computer. Additionally, a computer may be embedded in a device not
generally regarded as a computer but with suitable processing
capabilities, including a Personal Digital Assistant (PDA), a smart
phone or any other suitable portable or fixed electronic
device.
[0130] Also, a computer may have one or more input and output
devices. These devices can be used, among other things, to present
a user interface. Examples of output devices that can be used to
provide a user interface include printers or display screens for
visual presentation of output and speakers or other sound
generating devices for audible presentation of output. Examples of
input devices that can be used for a user interface include
keyboards, and pointing devices, such as mice, touch pads, and
digitizing tablets. As another example, a computer may receive
input information through speech recognition or in other audible
format.
[0131] Such computers may be interconnected by one or more networks
in any suitable form, including a local area network or a wide area
network, such as an enterprise network, and intelligent network
(IN) or the Internet. Such networks may be based on any suitable
technology and may operate according to any suitable protocol and
may include wireless networks, wired networks or fiber optic
networks.
[0132] The various methods or processes (outlined herein may be
coded as software that is executable on one or more processors that
employ any one of a variety of operating systems or platforms.
Additionally, such software may be written using any of a number of
suitable programming languages and/or programming or scripting
tools, and also may be compiled as executable machine language code
or intermediate code that is executed on a framework or virtual
machine.
[0133] In this respect, various inventive concepts may be embodied
as a computer readable storage medium (or multiple computer
readable storage media) (e.g., a computer memory, one or more
floppy discs, compact discs, optical discs, magnetic tapes, flash
memories, circuit configurations in Field Programmable Gate Arrays
or other semiconductor devices, or other non-transitory medium or
tangible computer storage medium) encoded with one or more programs
that, when executed on one or more computers or other processors,
perform methods that implement the various embodiments of the
invention discussed above. The computer readable medium or media
can be transportable, such that the program or programs stored
thereon can be loaded onto one or more different computers or other
processors to implement various aspects of the present invention as
discussed above.
[0134] The terms "program" or "software" are used herein in a
generic sense to refer to any type of computer code or set of
computer-executable instructions that can be employed to program a
computer or other processor to implement various aspects of
embodiments as discussed above. Additionally, it should be
appreciated that according to one aspect, one or more computer
programs that when executed perform methods of the present
invention need not reside on a single computer or processor, but
may be distributed in a modular fashion amongst a number of
different computers or processors to implement various aspects of
the present invention.
[0135] Computer-executable instructions may be in many forms, such
as program modules, executed by one or more computers or other
devices. Generally, program modules include routines, programs,
objects, components, data structures, etc. that perform particular
tasks or implement particular abstract data types. Typically the
functionality of the program modules may be combined or distributed
as desired in various embodiments.
[0136] Also, data structures may be stored in computer-readable
media in any suitable form. For simplicity of illustration, data
structures may be shown to have fields that are related through
location in the data structure. Such relationships may likewise be
achieved by assigning storage for the fields with locations in a
computer-readable medium that convey relationship between the
fields. However, any suitable mechanism may be used to establish a
relationship between information in fields of a data structure,
including through the use of pointers, tags or other mechanisms
that establish relationship between data elements.
[0137] Also, various inventive concepts may be embodied as one or
more methods, of which an example has been provided. The acts
performed as part of the method may be ordered in any suitable way.
Accordingly, embodiments may be constructed in which acts are
performed in an order different than illustrated, which may include
performing some acts simultaneously, even though shown as
sequential acts in illustrative embodiments.
[0138] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0139] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0140] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Multiple elements listed with "and/or" should be construed in the
same fashion, i.e., "one or more" of the elements so conjoined.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified. Thus, as a
non-limiting example, a reference to "A and/or B", when used in
conjunction with open-ended language such as "comprising" can
refer, in one embodiment, to A only (optionally including elements
other than B); in another embodiment, to B only (optionally
including elements other than A); in yet another embodiment, to
both A and B (optionally including other elements); etc.
[0141] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e., "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of" "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0142] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0143] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
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