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Title:
PHOTODETECTOR, METHOD FOR FORMING THE SAME, METHOD FOR CONTROLLING THE SAME AND PHOTODETECTOR ARRANGEMENT
Document Type and Number:
WIPO Patent Application WO/2019/074441
Kind Code:
A1
Abstract:
According to embodiments of the present invention, a photodetector is provided. The photodetector includes a semiconductor portion, two electrical contacts arranged spaced apart from each other and electrically coupled to the semiconductor portion, and an antenna electrically coupled to the two electrical contacts, wherein, in response to an electromagnetic wave incident on the antenna, the antenna is configured to couple photons corresponding to the electromagnetic wave to the semiconductor portion to excite surface plasmon polaritons in the semiconductor portion to generate free electrical carriers, and wherein the photodetector is configured, in response to an electrical bias applied to the two electrical contacts, to generate an electrical current defined by the free electrical carriers. According to further embodiments of the present invention, a photodetector arrangement, a method for forming a photodetector and a method for controlling a photodetector are also provided.

Inventors:
ZHANG, Daohua (50 Nanyang Avenue, Singapore 8, 639798, SG)
TONG, Jinchao (50 Nanyang Avenue, Singapore 8, 639798, SG)
HUANG, Zhiming (500 Yutian Road, Shanghai 3, 200083, CN)
ZHOU, Wei (500 Yutian Road, Shanghai 3, 200083, CN)
Application Number:
SG2018/050480
Publication Date:
April 18, 2019
Filing Date:
September 20, 2018
Export Citation:
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Assignee:
NANYANG TECHNOLOGICAL UNIVERSITY (50 Nanyang Avenue, Singapore 8, 639798, SG)
SHANGHAI INSTITUTE OF TECHNICAL PHYSICS (500 Yutian Road, Shanghai 3, 200083, CN)
International Classes:
H01L31/0232; H01Q23/00
Domestic Patent References:
WO2012044250A12012-04-05
Foreign References:
US20120205767A12012-08-16
CN104157741A2014-11-19
Attorney, Agent or Firm:
MCLAUGHLIN, Michael Gerard et al. (McLaughlin IP Pte. Ltd, 112 Robinson Road #14-01, Singapore 2, 059504, SG)
Download PDF:
Claims:
CLAIMS

1. A photodetector comprising:

a semiconductor portion;

two electrical contacts arranged spaced apart from each other and electrically coupled to the semiconductor portion; and

an antenna electrically coupled to the two electrical contacts,

wherein, in response to an electromagnetic wave incident on the antenna, the antenna is configured to couple photons corresponding to the electromagnetic wave to the semiconductor portion to excite surface plasmon polaritons in the semiconductor portion to generate free electrical carriers, and

wherein the photodetector is configured, in response to an electrical bias applied to the two electrical contacts, to generate an electrical current defined by the free electrical carriers. 2. The photodetector as claimed in claim 1, wherein the two electrical contacts are arranged on the semiconductor portion at opposite ends of the semiconductor portion.

3. The photodetector as claimed in claim 1 or 2, wherein each electrical contact of the two electrical contacts is arranged on a plurality of surfaces of the semiconductor portion.

4. The photodetector as claimed in any one of claims 1 to 3, wherein a spacing between the two electrical contacts is less than a wavelength of the electromagnetic wave.

5. The photodetector as claimed in any one of claims 1 to 4, wherein the two electrical contacts are arranged spaced apart from each other by between about 20 nm and about 300 μπι.

6. The photodetector as claimed in any one of claims 1 to 5, wherein the antenna comprises a planar antenna. 7. The photodetector as claimed in any one of claims 1 to 6, wherein the antenna comprises a dipole antenna.

8. The photodetector as claimed in claim 7, wherein the dipole antenna has a half-wave dipole configuration.

9. The photodetector as claimed in claim 7 or 8, wherein the dipole antenna has a bowtie shape.

10. The photodetector as claimed in any one of claims 1 to 6, wherein the antenna comprises a log periodic antenna. 11. The photodetector as claimed in claim 10, wherein the log periodic antenna comprises a plurality of tooth elements arranged spaced apart from each other, wherein each tooth element of the plurality of tooth elements is curved.

12. The photodetector as claimed in any one of claims 1 to 11, wherein the semiconductor portion comprises a material with a negative permittivity.

13. The photodetector as claimed in any one of claims 1 to 12, wherein a length of the semiconductor portion is less than a wavelength of the electromagnetic wave. 14. A photodetector arrangement comprising a plurality of photodetectors, wherein each photodetector of the plurality of photodetectors is as claimed in any one of claims 1 to 13.

15. The photodetector arrangement as claimed in claim 14, wherein at least two photodetectors of the plurality of photodetectors are electrically coupled to each other.

16. The photodetector arrangement as claimed in claim 14 or 15, wherein respective antennas of at least two photodetectors of the plurality of photodetectors are different from each other in at least one of shape or dimension.

17. The photodetector arrangement as claimed in any one of claims 14 to 16, wherein respective semiconductor portions of at least two photodetectors of the plurality of photodetectors are different from each other in at least one of material or dimension. 18. A method for forming a photodetector comprising:

arranging two electrical contacts spaced apart from each other and electrically coupled to a semiconductor portion; and

electrically coupling an antenna to the two electrical contacts,

wherein, in response to an electromagnetic wave incident on the antenna, the antenna is configured to couple photons corresponding to the electromagnetic wave to the semiconductor portion to excite surface plasmon polaritons in the semiconductor portion to generate free electrical carriers, and

wherein the photodetector is configured, in response to an electrical bias applied to the two electrical contacts, to generate an electrical current defined by the free electrical carriers.

19. A method for controlling a photodetector as claimed in any one of claims 1 to 13, the method comprising applying an electrical bias to the two electrical contacts of the photodetector for detecting the electromagnetic wave incident on the antenna of the photodetector.

Description:
PHOTODETECTOR, METHOD FOR FORMING THE SAME, METHOD FOR

CONTROLLING THE SAME AND PHOTODETECTOR ARRANGEMENT

Cross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore patent application No. 10201708404Y, filed 12 October 2017, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] Various embodiments relate to a photodetector, a method for forming a photodetector, a method for controlling a photodetector, and a photodetector arrangement.

Background

[0003] It is well known that long wavelength photodetectors, especially for millimeter and terahertz wave ranges, have wide applications in many areas, including meteorology, astronomy, medicine, communication and biology. However, known photodetection based on photogenerated electron-hole pairs in a semiconductor is not applicable for such long wavelength photons (LWPs) due to the relatively small photon energy and strong thermal noise disturbance. Current techniques for millimeter and terahertz wave detection mainly include thermal sensing elements (Golay, pyroelectric element, bolometer), nonlinear electronic Schottky diodes, two dimensional electron gas field-effect transistors (2DEG FETs), photoconductive antenna (PCA), photothermoelectric effects in two dimensional (2D) Graphene, and tunable hot-carrier photodetection. In three terminal FET terahertz detectors which usually have a sub-micrometer scale channel length and a gate to control the carrier density in the channel, the detection is based on nonlinear property of plasma wave excited in the transistor channel. In known terahertz photoconductive antennas, the semiconductor in the gap usually has a high resistivity and an ultrashort carrier lifetime. The detection requires a local pulse laser to excite free charge carriers. The graphene based terahertz detectors are based on photogenerated electron-hole pairs or photothermoelectric effect with sensitivity exceeding 10 WW. The tunable hot-carrier photodetectors are based on hot-cold carrier energy transfer mechanism which enable a very long-wavelength infrared response up to 55 μπι.

[0004] Recently, surface plasmon polaritons (SPPs) in subwavelength structures have been attracting tremendous efforts as they can be used for many applications, including extraordinary optical transmission, manipulation of cold atoms, wavelength filtering, plasmonic devices, solar cell energy harvesting, metamaterials, modern molecular sensing and spectroscopy. One of the key properties of SPPs is the capability to induce non-equilibrium electrons. Since the plasma frequencies of metals are generally located in the visible or ultraviolet range of the electromagnetic spectrum, it is not possible to acquire intense SPPs from metals in millimeter and terahertz wave ranges. To overcome this problem, spoof SPPs have been proposed and realized from periodical holes in a metal surface. In addition, SPPs can also be generated in some semiconductors. Highly doped silicon and some narrow band gap semiconductors, such as indium antimonide (InSb), are excellent plasmonic materials for LWPs, owing to their high electron mobility, low electron density and small effective mass.

[0005] The state of the art detection techniques for LWPs lag far behind the urgent demand due to structure and performance limitation. On one hand, their architectures are complicated for large planar arrays; on the other hand, their sensitivity is not good enough to meet the application requirements.

Summary

[0006] The invention is defined in the independent claims. Further embodiments of the invention are defined in the dependent claims.

[0007] According to an embodiment, a photodetector is provided. The photodetector may include a semiconductor portion, two electrical contacts arranged spaced apart from each other and electrically coupled to the semiconductor portion, and an antenna electrically coupled to the two electrical contacts, wherein, in response to an electromagnetic wave incident on the antenna, the antenna is configured to couple photons corresponding to the electromagnetic wave to the semiconductor portion to excite surface plasmon polaritons in the semiconductor portion to generate free electrical carriers, and wherein the photodetector is configured, in response to an electrical bias applied to the two electrical contacts, to generate an electrical current defined by the free electrical carriers.

[0008] According to an embodiment, a photodetector arrangement is provided. The photodetector arrangement may include a plurality of photodetectors, wherein each photodetector of the plurality of photodetectors is as described herein.

[0009] According to an embodiment, a method for forming a photodetector is provided. The method may include arranging two electrical contacts spaced apart from each other and electrically coupled to a semiconductor portion, and electrically coupling an antenna to the two electrical contacts, wherein, in response to an electromagnetic wave incident on the antenna, the antenna is configured to couple photons corresponding to the electromagnetic wave to the semiconductor portion to excite surface plasmon polaritons in the semiconductor portion to generate free electrical carriers, and wherein the photodetector is configured, in response to an electrical bias applied to the two electrical contacts, to generate an electrical current defined by the free electrical carriers.

[0010] According to an embodiment, a method for controlling a photodetector as described herein is provided. The method may include applying an electrical bias to the two electrical contacts of the photodetector for detecting the electromagnetic wave incident on the antenna of the photodetector. Brief Description of the Drawings

[0011] In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0012] FIG. 1A shows a schematic top view of a photodetector, according to various embodiments.

[0013] FIG. IB shows a schematic view of a photodetector arrangement, according to various embodiments.

[0014] FIG. 1C shows a flow chart illustrating a method for forming a photodetector, according to various embodiments. [0015] FIG. ID shows a method for controlling a photodetector, according to various embodiments.

[0016] FIG. 2A shows a schematic perspective view of a photodetector, according to various embodiments, while FIGS. 2B and 2C show schematic views representative of the SPP-induced electrons of the photodetector at different biasing conditions.

[0017] FIGS. 3 A to 3C show results of numerical simulations for various structures including for an antenna-assisted subwavelength ohmic metal-semiconductor-metal (OMSM) structure made of gold (Au) and indium antimonide (InSb).

[0018] FIGS. 4A to 4E show results of numerical simulations for an antenna-assisted subwavelength Au-InSb-Au (gold-indium antimonide-gold) structure.

[0019] FIGS. 5A to 5G show results of characterization of antenna-assisted subwavelength Au- InSb-Au (gold-indium antimonide-gold) devices.

[0020] FIGS. 6A to 6D show two-dimensional performance maps of a device having a spacing, s, of 90 μηι at temperatures ranging from 77 to 293 K.

[0021] FIGS. 7 A to 7D show results for the temperature effects on the performance of a device having a spacing, s, of 90 μπι.

[0022] FIG. 8 shows a schematic view of a photodetector having a log periodic antenna, according to various embodiments.

[0023] FIGS. 9 A and 9B show examples of photodetector arrangements having a linear array design and a two-dimensional (2D) array design respectively.

Detailed Description

[0024] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. [0025] Embodiments described in the context of one of the methods or devices are analogously valid for the other methods or devices. Similarly, embodiments described in the context of a method are analogously valid for a device, and vice versa.

[0026] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0027] In the context of various embodiments, the term "about" as applied to a numeric value encompasses the exact value and a reasonable variance.

[0028] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0029] As used herein, the phrase of the form of "at least one of A or B" may include A or B or both A and B. Correspondingly, the phrase of the form of "at least one of A or B or C", or including further listed items, may include any and all combinations of one or more of the associated listed items.

[0030] Various embodiments may provide long wavelength photodetectors and methods for forming them.

[0031] Various embodiments may enable surface plasmon induced direct detection of long wavelength photons.

[0032] Various embodiments may provide millimeter and terahertz wave photodetectors designed based on localized surface plasmon polaritons induced non-equilibrium electrons for direct detection of such waves.

[0033] Various embodiments may provide a method and mechanism for design of millimeter and terahertz wave photodetectors based on localized surface plasmon polariton (SPP) induced non-equilibrium electrons in antenna-assisted subwavelength ohmic metal-semiconductor-metal (OMSM) structures. Such photodetectors may directly detect millimeter and terahertz wave radiation. The methods and devices of various embodiments disclosed herein may provide one or more of the following: (1) The disclosed millimeter and terahertz wave photodetectors are based on non- equilibrium electrons induced by localized surface plasmon polaritons (SPPs);

(2) The disclosed detectors may contain a narrowband semiconductor which may have a high electron mobility and a low plasma frequency;

(3) The disclosed detectors may have two ohmic contacts between the specified semiconductor and metal (i.e., antenna) to better collect photocurrent;

(4) The spacing length between two metal contacts is preferably much smaller than the wavelength to be detected, e.g., to meet subwavelength requirement;

(5) The disclosed detectors preferably have a specially designed metal planar antenna which may couple most of the detected photons into the subwavelength OMSM structure.

[0034] As compared to existing methods, devices or materials, various embodiments may provide one or more of the following:

(1) Direct detection mode

Compared to known terahertz photoconductive antennas, the devices disclosed herein are based on direct detection, in which no external femtosecond laser is required to excite the photocarriers.

(2) Can be easily designed for detecting any particular wavelength photons

By combining different antenna designs and/or different semiconductors, tunable detected peak response from millimeter to terahertz waves may be obtained.

(3) Easy to fabricate

The detectors disclosed herein may have a very simple device structure which may (only) include a thin semiconductor film, metal contacts and an antenna.

(4) Beyond the bandgap limitation

The energy of detected photons by the detectors disclosed herein may be beyond the bandgap limitation, where the relevant parameters of the semiconductor involved may include mobility and/or plasmon frequency.

(5) High performance

The detectors disclosed herein have very good detecting performance which may be monitored by controlling the structure parameters, such as the spacing between two ohmic contacts.

(6) Wide operating temperature range The detectors disclosed herein for millimeter and terahertz detection may operate from room temperature to very low temperatures.

(7) Low cost

Owing to the simple structure, simple operating mode, low bias current, capability of room temperature operation, the cost of production of the detectors disclosed herein may be very low.

(8) Fast response

As the method disclosed herein is based on photoconductive carriers, the response speed (e.g., < level) may be much faster than those (e.g., ms level) of the state of art thermal detectors.

(9) Easy to extend to large linear or 2D arrays

Linear or 2D arrays may be easily realized for future millimeter and terahertz camera applications, depending on the required applications.

[0035] Various embodiments may provide a strategy or mechanism for direct detection of long wavelength photons (LWPs) from millimeter to terahertz wave range, based on localized surface plasmon polariton (SPP) induced non-equilibrium electrons in an antenna-assisted subwavelength ohmic metal-semiconductor-metal (OMSM) structure. The subwavelength OMSM structure may be used to convert the absorbed photons into localized SPPs which may then induce non-equilibrium electrons in the structure, while the antenna is utilized to couple the photons to be detected into the subwavelength OMSM structure to improve detecting performance. In other words, the subwavelength OMSM structure is for excitation of localized SPPs which induce non-equilibrium electrons, while the antenna is for coupling more photons into the OMSM structure. The semiconductor in the structure is the platform for generation or excitation of SPPs and the induced non-equilibrium electrons. In addition to preferably low plasma frequency, the semiconductor in the structure also preferably has high electron mobility so that the SPP-induced electrons can move fast in the material, i.e., high electron mobility capability for fast transit of conduction electrons. When the device is under illumination and biased, a unidirectional flow of the SPP-induced non-equilibrium electrons forms a photocurrent. The simulation and experimental results from the antenna-assisted subwavelength OMSM structures made of gold (Au) and indium antimonide (InSb), which will be described further below, demonstrate that the structures disclosed herein may detect photons in the millimeter and terahertz wave ranges.

[0036] The energy of the detected photons is determined by the structure of the photodetector rather than the bandgap of the semiconductor, which differs from known photodetector s. The energy may be determined by one or more of the following factors:

(i) The antenna used in the photodetector. Generally, each designed antenna may (only) efficiently couple photons of one specific wavelength range;

(ii) The permittivity of the semiconductor material. The detection is based on localized surface plasmon polaritons generated in the semiconductor, and to allow this, the semiconductor itself has a negative permittivity in the detected photons range to excite localized SPPs;

(iii) The detected photon energy is smaller than the bandgap energy of the semiconductor, which is the reason the detected photon energy is not determined by the bandgap of the semiconductor.

[0037] As a non-limiting example, a photodetector may be provided, having a planar antenna for specific wavelength, a semiconductor portion, and two ohmic contacts disposed adjacent to and at opposing ends of the semiconductor portion, wherein the semiconductor portion and the ohmic contacts are arranged in the center portion of the antenna. The semiconductor portion may include indium antimonide (InSb). Alternatively, the semiconductor portion may include any suitable high mobility semiconductor materials which have surface plasmon polariton (SPP) at millimeter wave or THz (terahertz wave). The antenna and the ohmic contacts may include metallic materials, preferably gold (Au). The specific shape (for example, bowtie shape) of the antenna may be designed for coupling the most detected photons into the ohmic metal-semiconductor-metal (OMSM) structure formed by the semiconductor portion and the ohmic contacts.

[0038] FIG. 1A shows a schematic top view of a photodetector 100, according to various embodiments. The photodetector 100 includes a semiconductor portion 104, two electrical contacts 106, 108 arranged spaced apart from each other and electrically coupled to the semiconductor portion 104, and an antenna 110 electrically coupled to the two electrical contacts 106, 108, wherein, in response to an electromagnetic wave (represented by solid arrows 112) incident on the antenna 1 10, the antenna 110 is configured to couple photons corresponding to the electromagnetic wave 112 to the semiconductor portion 104 to excite surface plasmon polaritons in the semiconductor portion 104 to generate free electrical carriers, and, wherein the photodetector 100 is configured, in response to an electrical bias applied to the two electrical contacts 106, 108, to generate an electrical current defined by the free electrical carriers.

[0039] In other words, a photodetector 100 may be provided, having a semiconductor portion 104, two electrical contacts (or terminals) 106, 108, each of which may be electrically coupled to the semiconductor portion 104, and an antenna 110 electrically coupled to each of the two electrical contacts 106, 108. The two electrical contacts 106, 108 may be arranged spaced apart from each other by a spacing. The semiconductor portion 104 and the two electrical contacts 106, 108 may define an (subwavelength) ohmic metal-semiconductor-metal (OMSM) structure.

[0040] The two electrical contacts 106, 108 may be arranged on the semiconductor portion 104. The two electrical contacts 106, 108 may be provided on respective portions of a surface of the semiconductor portion 104.

[0041] Each of the two electrical contacts 106, 108 may be an ohmic contact.

[0042] The photodetector 110 may be used for detection of an electromagnetic wave. During operation, when an electromagnetic wave 112 is incident on or impinges on the antenna 110, the antenna 110 may couple incident or received or detected photons corresponding to the electromagnetic wave 112 to the semiconductor portion 104 to excite surface plasmon polaritons (SPPs) in the semiconductor portion 104 to generate free electrical carriers in the semiconductor portion 104. In this way, the semiconductor portion 104 may convert the photons into the surface plasmon polaritons (SPPs), where, in turn, energy of the SPPs may be transferred to electrical carriers in the semiconductor portion 104 to generate free or unbound electrical carriers in the semiconductor portion 104. The free electrical carriers may include or may be electrons, for example, non-equilibrium electrons.

[0043] When an electrical bias is applied to the two electrical contacts 106, 108, an electrical current (e.g., a photocurrent) defined by the free electrical carriers may be generated by the photodetector 100. The electrical current may be indicative of the electromagnetic wave 112. The electrical bias may include or may be a DC bias. It should be appreciated that the electrical bias may be applied directly to the two electrical contacts 106, 108, or indirectly via the antenna 110 which is electrically coupled to the two electrical contacts 106, 108. The two electrical contacts 106, 108, and the antenna 110 may be equipotential, for example, in embodiments where the two electrical contacts 106, 108, and the antenna 110 may be metallic. Nevertheless, it should be appreciated that the two electrical contacts 106, 108 may have or may be at different potentials.

[0044] As should be appreciated, the semiconductor portion 104 includes a semiconductor material, e.g., indium antimonide (InSb). Each of the electrical contacts 106, 108, and/or the antenna 110 may include a metal, for example, gold (Au), or chromium (Cr)/gold (Au), or titanium (Ti)/gold (Au). The choice of Cr/Au may be preferable as it may form good ohmic contact with InSb.

[0045] In the context of various embodiments, "non-equilibrium electrons" may refer to electrons excited by localized surface plasmon polaritons (SPPs) resonant with the incident photons (corresponding to the electromagnetic wave 112). They are the carriers that form a photocurrent when an electrical bias is applied to the two electrical contacts 106, 108.

[0046] In the context of various embodiments, the electromagnetic wave 112 may be in a range from millimeter wave to terahertz wave. The corresponding photons may include or may be long wavelength photons (LWPs).

[0047] In the context of various embodiments, the antenna 110 is configured to couple incident radiations. As a non-limiting example, the antenna 110 may be a planar metallic antenna configured to couple incident radiations.

[0048] In the context of various embodiments, the antenna 110 may be designed with different dimensions, depending on applications, for coupling incident photons of different quantum energies (or wavelengths or wavelength ranges).

[0049] In various embodiments, the two electrical contacts 106, 108 may be arranged on the semiconductor portion 104 at opposite ends of the semiconductor portion 104.

[0050] Each electrical contact of the two electrical contacts 106, 108 may be arranged on a plurality of surfaces of the semiconductor portion 104, including, for example, the top surface and side surfaces of the semiconductor portion 104.

[0051] In various embodiments, a spacing between the two electrical contacts 106, 108 may be less than a wavelength of the electromagnetic wave 112. This may be for the purpose of meeting subwavelength requirement. [0052] The two electrical contacts 106, 108 may be arranged spaced apart from each other by between about 20 nm and about 300 μπι, for example, between about 20 nm and about 100 μπι, between about 20 nm and about 10 μηι, between about 20 nm and about 1 μπι (1000 nm), between about 20 nm and about 500 nm, between about 20 nm and about 300 nm, between about 20 nm and about 100 nm, between about 20 nm and about 50 nm, between about 200 nm and about 500 nm, between about 200 nm and about 1 μπι, between about 1 μηι and about 300 μπι, between about 1 μπι and about 100 μπι, between about 1 μπι and about 50 μπι, and between about 100 μπι and about 300 μπι. A smaller spacing may result in a higher intensity of the SPPs.

[0053] In various embodiments, the antenna 1 10 may include or may be a planar antenna.

[0054] In various embodiments, the antenna 1 10 may include or may be a dipole antenna. The dipole antenna may be defined by two antenna sections or elements, each of which may be electrically coupled to a respective electrical contact of the two electrical contacts 106, 108. The dipole antenna may have a half-wave dipole configuration. The dipole antenna may have a bowtie shape.

[0055] In various embodiments, the antenna 110 may include or may be a log periodic antenna. The log periodic antenna may include a plurality of tooth elements arranged spaced apart from each other, wherein each tooth element of the plurality of tooth elements may be curved.

[0056] By changing the design of the antenna 110, e.g., in terms of the shape and/or one or more dimension parameters, the antenna 110 may be optimized to couple incident radiations or photons in different frequency ranges. In other words, the detected peak response for different frequencies may be achieved by providing antennas of different designs optimised for respective frequency ranges. Further, different semiconductors having a high electron mobility may be used for the semiconductor portion 104.

[0057] In various embodiments, the semiconductor portion 104 may include a material with a negative permittivity.

[0058] In various embodiments, the semiconductor portion 104 may include an indium-based material, for example, indium antimonide (InSb), indium gallium arsenide (InGaAs), indium arsenide (InAs), or indium arsenide antimonide (InAsSb). [0059] In various embodiments, a length of the semiconductor portion 104 may be less than a wavelength of the electromagnetic wave 112. This may be for the purpose of meeting subwavelength requirement.

[0060] FIG. IB shows a schematic view of a photodetector arrangement 116, according to various embodiments. The photodetector arrangement 116 may include a plurality of photodetectors 100a, 100b, 100c, for example, two, three, four or any higher number of photodetectors. Each photodetector 100a, 100b, 100c may be a photodetector as described in the context of the photodetector 100 of FIG. 1 A.

[0061] In various embodiments, respective photodetectors of the plurality of photodetectors 100a, 100b, 100c may be employed for detecting different electromagnetic waves (of different wavelengths).

[0062] The plurality of photodetectors 100a, 100b, 100c may be arranged in row(s) and/or column(s). For example, the plurality of photodetectors 100a, 100b, 100c may be arranged as a linear array or a two-dimensional (2D) array.

[0063] The plurality of photodetectors 100a, 100b, 100c may be provided or formed on a (same) substrate.

[0064] At least two photodetectors of the plurality of photodetectors 100a, 100b, 100c may be electrically coupled to each other.

[0065] Respective antennas of at least two photodetectors of the plurality of photodetectors 100a, 100b, 100c may be different from each other in at least one of shape or dimension (e.g., length and/or width). In this way, the at least two photodetectors may be employed for detecting different electromagnetic waves (of different wavelengths).

[0066] Respective semiconductor portions of at least two photodetectors of the plurality of photodetectors 100a, 100b, 100c may be different from each other in at least one of material or dimension (e.g., at least one of length, width, or thickness). In this way, the at least two photodetectors may be employed for detecting different electromagnetic waves (of different wavelengths).

[0067] FIG. 1C shows a flow chart 120 illustrating a method for forming a photodetector, according to various embodiments.

[0068] At 122, two electrical contacts are arranged spaced apart from each other and electrically coupled to a semiconductor portion. [0069] At 124, an antenna is electrically coupled to the two electrical contacts.

[0070] It should be appreciated that, for the photodetector that is formed, in response to an electromagnetic wave incident on the antenna, the antenna is configured to couple photons corresponding to the electromagnetic wave to the semiconductor portion to excite surface plasmon polaritons in the semiconductor portion to generate free electrical carriers, and, the photodetector is configured, in response to an electrical bias applied to the two electrical contacts, to generate an electrical current defined by the free electrical carriers.

[0071] FIG. ID shows a method 128 for controlling a photodetector as described herein. An electrical bias is applied to the two electrical contacts of the photodetector for detecting the electromagnetic wave incident on the antenna of the photodetector. The electrical bias may be a DC bias.

[0072] It should be appreciated that descriptions in the context of the photodetector 100 may correspondingly be applicable in relation to the method for forming a photodetector described in the context of the flow chart 120 and the method 128 for controlling a photodetector.

[0073] Design of subwavelength OMSM structure

[0074] FIG. 2A shows a schematic perspective view of a photodetector 200, according to various embodiments. The photodetector 200 may include an antenna-assisted subwavelength ohmic metal-semiconductor-metal (OMSM) structure, for example, for millimeter and terahertz waves detection. The photodetector 200 may include a semiconductor portion (or layer) 204, and two electrical (ohmic) contacts (or metallic contacts) 206, 208 that collectively define an OMSM structure 202. The OMSM structure 202 may be arranged at a central portion of the photodetector 200 (or antenna-assisted OMSM structure). The semiconductor portion 204 may include indium antimonide (InSb). Each electrical contact 206, 208 may include gold (Au) with a very thin adhesion layer of chromium (Cr). The photodetector 200 may further include an antenna 210 electrically coupled to the electrical contacts 206, 208 and which may receive incident (electromagnetic) waves that are to be detected. The photodetector 200 may further include a substrate 230, e.g., a sapphire substrate.

[0075] The antenna 210 may be metallic (e.g., including gold (Au)). As a non-limiting example, the antenna 210 may be a planar antenna and/or a dipole antenna. The antenna 210 illustrated in FIG. 2A is a planar dipole antenna, with two antenna elements arranged on opposite sides of the semiconductor portion 204. The antenna 210 may have a total length, m and width, n, and may have a bowtie shape, as illustrated in the false-coloured scanning electron microscopy (SEM) image of the photodetector or device 200 provided as an inset in FIG. 2A. The scale bar is 1 mm.

[0076] The electrical contacts 206, 208 may be electrically coupled to the semiconductor portion 204. The two electrical contacts 206, 208 may be arranged spaced apart from each other by a spacing, s. In other words, there may be a spacing, s, between the edges of the two contacts 206, 208. As shown in FIG. 2A, the electrical contacts 206, 208 may be provided or formed on the semiconductor portion 204, and may cover a number of surfaces of the semiconductor portion 204. The semiconductor portion 204 may have a length, L, a width, W, and a thickness, t. The length, L, of the semiconductor portion 204, and/or the spacing, s, between the two electrical contacts 206, 208 may be much less than the wavelength, λ, of incident waves (or incident photons) to be detected, for subwavelength requirement.

[0077] Respective electrical conductors or conductive paths 232, 234 may be provided electrically coupled to the antenna 210, and, consequently also the two electrical contacts 206, 208, for application of an electrical bias (e.g., a DC bias).

[0078] Under transverse magnetic (TM) illumination, the planar dipole antenna 210 may efficiently couple the corresponding photons into the structure 202, and localized surface plasmon polaritons (SPPs) may be excited by the coupled photons within the semiconductor portion 204, for example, near the interfaces of metal/air and semiconductor. The SPPs may then induce non-equilibrium electrons by passing the energy to some of the electrons in the semiconductor portion 204.

[0079] FIGS. 2B and 2C show schematic views representative of the SPP-induced electrons of the photodetector 200 at different biasing conditions. In FIGS. 2B and 2C, metal (M) 206b, represents the electrical contact 206 of FIG. 2A, metal (M) 208b represents the electrical contact 208 of FIG. 2A, and semiconductor (S) 204b represents the semiconductor portion 204 of FIG. 2A. Referring to FIG. 2B, without bias (or at zero bias condition), the SPP-induced electrons 240b may have a symmetric distribution. Referring to FIG. 2C, after a bias is applied (or in a biased condition) to the OMSM structure 202, the SPP-induced electrons 240b may flow through the semiconductor 204b, leading to a photocurrent. [0080] To allow excitation of localized SPPs in millimeter and terahertz wave ranges, the semiconductor portion 204, 204b may preferably have low plasma frequency (generally with a negative permittivity) close to the frequency of the incident waves, and/or may preferably possess high electron mobility to allow fast transit of the SPP-induced non-equilibrium electrons 240b.

[0081] Au antenna-assisted Au-InSb-Au device

[0082] As a non-limiting example, indium antimonide (InSb) may be employed as the semiconductor portion 204 in the OMSM structure 202 as it meets the desired requirements. InSb is a III-V semiconductor which has been used as a photodetecting material in infrared range based on interband transition, and as thermal hot-electron bolometers based on intraband free electron absorption at low temperature. The InSb layer 204 may have a bandgap of - 180 meV and an electron mobility of ~ 5.6xl0 4 cmVs "1 at room temperature. The plasma frequency of InSb is ~ 4 THz, corresponding to ~ 16.5 meV which is in the terahertz wave range.

[0083] The contacts 206, 208 may be made of gold (Au) with a very thin adhesion layer of chromium (Cr). The planar antenna 210 may be made of gold (Au) with a half-wave dipole configuration. The device 200 having the OMSM structure 202 and the planar antenna 210 may be fabricated on a sapphire substrate 230.

[0084] For fabricating the photodetector of various embodiments, a single crystal undoped InSb (1 11) material may be employed for the subwavelength OMSM devices or structures. First, an InSb wafer may be transferred and stuck onto a sapphire substrate by epoxy glue. It may then be polished into a 10 μπι thick film. By using photolithography and chemical solution etching (HF:HAC:H 2 02), a series of mesas with a width of about 50 μπι and a thickness of 10 μπι but different lengths may be formed. After that, the metallic contacts and dipole antennas may be defined by photolithography, E-beam evaporation and standard lift-off process to form antenna-assisted subwavelength OMSM structures with spacing, s, of 10 - 130 μπι. 15 nm chromium (Cr) / 400 nm gold (Au) may be deposited to form the ohmic contact and the coupled antenna. As non-limiting examples, planar Au dipole antennas may be employed (see FIG. 2A) to introduce more photons which strengthen localized SPPs in millimeter and terahertz ranges. [0085] Simulation for surface plasmon

[0086] To gain insights on the excitation of localized SPPs in the structure of various embodiments, numerical simulations were carried out using a Finite Elements Method (e.g., Comsol, RF module). In the millimeter and terahertz wave ranges, without wishing to be bound by any theory, the free electrons in the conduction band of InSb may behave as classic solid-state plasmas and its complex dielectric constant may be derived by Drude model. Gold may be regarded as a perfect conductor in the considered frequency range. The complex dielectric constant of sapphire substrate may be about 11.602 + 0.073L

[0087] In the simulations, the permittivity of InSb at millimeter and terahertz range may be described by Drude model ε((θ)=ε∞εο[1- (Ορ 2 Ι{ (0 +ί(0(Οτ)], where, f∞ is the high-frequency permittivity, 8o is the permittivity in vaccum, (Op is the plasma frequency, (O is the angular frequency, and (θτ is the average collision rate of the charge carriers. The plasma frequency (Op is defined as (Op 1 = β 2 η/(ηι * ε∞εο), where e is the electric charge unit, n is the electron concentration, and m * is the effective mass of electrons. HFSS (high frequency structural simulator, which is a finite element method solver for electromagnetic structures) may be employed to calculate the Gain of the antenna in the calibration.

[0088] Simulation was first performed using a single bare indium antimonide (InSb) slice (air- InSb-air) 304a with a length, L, of 150 μπι, a width, W, of 50 μπι, and a thickness, t, of 10 μπι as shown in FIG. 3A. TM (transverse magnetic) polarized (with electric field in the x-direction) plane wave with energy of about 0.151 meV (~8 mm) was used in the simulation, which may be experimentally verified. Then, a subwavelength Au-InSb-Au structure 301b as shown in FIG. 3B was simulated by adding an Au layer 306b, 308b on the sides and parts of the top surface of a InSb slice 304b (for the purpose of the simulation, the InSb slice 304b is the same as the InSb slice 304a with the corresponding dimensions), separated by a spacing, s, of about 90 μπι. To enhance the intensity of localized SPPs, a planar Au antenna with a half-wave dipole configuration designed for photons of 0.151 meV may be added to the structure 301b. As shown in FIG. 3C, a device 300c having a InSb slice 304c having gold contacts 306c, 308c (same as the structure 301b for the purpose of the simulation, with the corresponding dimensions), with an antenna 310c of a half-wave dipole configuration coupled thereto. [0089] FIGS. 3 A to 3C also show results of the excitation of localized SPPs, with two dimensional colour patterns for E 2 /Eo 2 in the x-y (z = 0) and x-z (y = 0) planes (where Eo is the electrical field of the incident light in air, and E is the electrical field in the structure), and the distribution of E 2 /Eo 2 along the dashed (half width) line for incident photons of 0.151 meV in the bare InSb structure 304a (L = 150 μπι, W = 50 μπι, and t = 10 μπι), the subwavelength Au- InSb-Au structure 301b (s = 90 μπι), and the antenna-assisted subwavelength Au-InSb-Au structure 300c respectively.

[0090] As shown in FIG. 3A, very strong localized SPPs at the two edge interfaces of InSb 304a and air may be clearly observed for the TM polarized incidence. However, the SPP intensity E 2 /Eo 2 is found to be very weak inside the InSb 304a, as shown in the plot 360a for E 2 /Eo 2 along the dashed line I defined in FIG. 3A. This is because most SPPs exist in the dielectric (air in this case).

[0091] Referring to FIG. 3B, it may be observed that by covering with the perfect conductor (Au) 306b, 308b, the SPP intensity E 2 /E 0 2 in the InSb 304b becomes stronger than that in the bare InSb 304a, although it is still the strongest at the two edges (traced by the two dotted vertical lines) of the spacing between the gold contacts 306b, 308b. While these simulation results for the subwavelength ohmic Au-InSb-Au structure 301b show excitation of SPPs in the subwavelength Au-InSb-Au structure 301b, the value of E 2 /Eo 2 , however, is much smaller than one (« 1), as may be observed in the plot 360b.

[0092] Referring to FIG. 3C, the results for the antenna-assisted structure 300c with the same Au-InSb-Au dimensions show about 310 times enhancement at the edges (traced by the two dotted vertical lines) and about 200 times in the center of InSb 304c in the intensity of the localized SPPs (see plot 360c), as compared for that of the structure 301b. This may be due to the antenna 310c coupling in most such photons which excite more SPPs in the structure 300c.

[0093] Simulations may be carried out for the antenna-assisted subwavelength Au-InSb-Au structure (or photodetector) 300c (which may be referred to simply as "device" below), and the results of the numerical simulations are shown in FIGS. 4 A to 4E.

[0094] Simulation was carried out for the SPP intensities for different powers of the same source. The value of E 2 /Eo 2 at the position (s/2, 0, 0) is taken as representation of the SPP intensity. As may be observed in FIG. 4A illustrating SPP intensity of the structure 300c as a function of power, a clear linear relationship may be observed, as more SPPs may be generated when the number of incident photons increases.

[0095] FIG. 4B shows the E 2 /E 0 2 values of the device 300c (FIG. 3C) at different polarization angles for incident photons of 0.151 meV, while FIG. 4C shows the E 2 /Eo 2 values of the air- InSb-air structure 304a (FIG. 3A) at different polarization angles for incident photons of 0.151 meV for comparison of the effects of polarization angle of incident photons on SPP intensity in both the antenna-assisted Au-InSb-Au structure 300c and the air-InSb-air structure 304a. As may be observed in FIG. 4C, the SPP intensity in the air-InSb-air structure 304a is sensitive to the polarization angle of the incident light with the maximum and minimum values occurring at x (TM configuration) and y (TE configuration) axis, respectively. This is because the TM polarized light has the electric field perpendicular to the air-InSb interface, which is required for excitation of SPPs. For the antenna-assisted subwavelength Au- InSb-Au structure 300c, the same polarization dependence should be observed as the antenna 310c is designed for efficient coupling of TM polarized photons of 0.151 meV, as illustrated in FIG. 4B.

[0096] The SPP intensity as a function of the energy of the incident photons was then simulated. As shown in FIG. 4D illustrating results for normalized E 2 /E max 2 (where E max refers to the maximum value of E in the simulation at the specific photon energy) as a function of incident photon energy, a sharp peak intensity may be observed at ~ 0.151 meV, indicating that the antenna designed is able to couple in most of the photons of this energy.

[0097] For devices with fixed W and t, SPP distribution of the structures was then simulated with different values of the spacing, s. FIG. 4E shows the distribution of E 2 /Eo 2 along the half width line for devices with spacings of 10, 30, 50, 70, 90, 110, and 130 μπι, respectively. Plot 460e, as an inset in FIG. 4E, shows the results for E 2 /Eo 2 as a function of the spacing, s, at the point (s/2, 0, 0). As may be observed in FIG. 4E, the intensity of SPPs increases as the spacing, s, decreases.

[0098] Characterisation of devices

[0099] To verify the technique disclosed herein and the simulation results, a set of antenna- assisted subwavelength ohmic Au-InSb-Au devices was fabricated. The first structure measured corresponds to the device 300c (FIG. 3C) with a spacing, s, of 90 μηι, a width, W, of 50 μπι, a length, L, of 150 μηι, and a thickness, t, of 10 μηι, and the Au antenna (e.g., 310c) having a length of 4 mm, a width of 0.5 mm and a thickness of 400 nm.

[0100] For the purpose of performance characterization, for responsivity measurement, the incident radiation was mechanically or electrically modulated. The detector under test was mounted in a low temperature dewar and biased by a direct current (DC). The photovoltage data were collected from a Lock-in amplifier or an Oscilloscope after a preamplifier. An Agilent E8257D microwave source combined with a horn was used as the irradiation source from photons of 0.130 meV to 0.165 meV, and a VDI WR2.2SGX source was used for photons from 1.36 meV to 1.43 meV. A Golay cell was used to calibrate the responsivity, R = V/(pA) = VAGRG/VGA, where p is the power density, V and VG are the output voltage of the OMSM device and Golay cell, respectively, A is the effective absorption area of the antenna of the device, described as A = GA 2 /(4rt) (where G is the gain of the antenna), AG is the absorption area of the Golay cell (~ 50 mm 2 ), and RG is the responsivity of the Golay cell (~10 5 V/W@ 15 Hz). For the s = 90 μηι device, at 0.151 meV incident photons, the output voltage signal of Golay is 34.5 mV, and then the calibrated p is ~ 0.69 μψ/οπι 2 . For the s = 90 μπι device at 3.5 mA, the output photovoltage is 0.0032 mV, and the calculated Gain by HFSS is ~ 1.73, and the responsivity can be determined as ~ 50 V/W.

[0101] The Current- Voltage (I-V) characteristics of the device in the range of -0.1 to 0.1 V were measured and the results are shown in FIG. 5A for room temperature I-V characteristics of the device with s = 90 μπι. The white line shown is the linear fit to the experimental data. As shown in FIG. 5A, an excellent resistance behavior may be observed, indicating excellent ohmic contacts.

[0102] The photoresponse of the same device was characterized using a 0.151 meV photon source (e.g., Agilent E8257D microwave source combined with a horn with output powers up to 50 mW). FIG. 5B shows the results of the photovoltage of the device, as a function of biased DC current, under illumination of a 0.151 meV source output power of 25 mW at a modulation frequency of 300 Hz. The white line shown acts as a guide for the eyes. The inset shows a photoresponse waveform recorded by an oscilloscope. Significant photovoltage may be observed and it increases linearly with the biased current. The photovoltage is about 0.0032 mV at a bias of about 3.5 mA and increases to about 0.01 mV at a bias of about 15 mA. These observations are the evidence for direct detection of millimeter wave. The increase in photovoltage with biased current may be due to an increased photoconductive gain g which may be expressed as g = τ/τ¾, where τ is the carrier lifetime and Tt is the transit time of the non- equilibrium electrons. For the SPP-induced electrons near the interface at the low potential side, the transit time may be be expressed as n = s I (μζ), where s is the spacing between the ohmic contacts, μ is the electron mobility and ξ is the biased electric field. For the SPP-induced electrons in the InSb, however, the transit time may be much shorter as the distance for the electrons to travel is much shorter. The photovoltage also shows a linear dependence on the source output power in the measured range up to 50 mW, as shown in FIG. 5C illustrating the results for the photovoltage of the same device as a function of source output power at 300 Hz under a DC bias of 3.5 mA. The white line shown is the linear fit. The results obtained is in excellent agreement with the simulation results shown in FIG. 4A. The linear increase in the photovoltage is because more incident photons excite more SPPs which then generate more conduction carriers for photocurrent. By using a calibrated Golay cell, the responsivity of the device obtained at a modulation frequency of 300 Hz under a DC bias current of 3.5 mA is about 50 V/W (as described above), corresponding to a thermal noise limited NEP (Noise Equivalent Power) of ~ 10 " 11 WHz "1/2 . The performance is superior to commercially available products.

[0103] The result for the polarization dependence of the photovoltage of the device for the 0.151 meV source of 25 mW measured at 300 Hz under a DC bias current of 3.5 mA is shown in FIG. 5D, which is also similar to the simulated results (see FIG. 4B). The vertical and horizontal axes are assigned as x and y, respectively. As may be observed, the photovoltage is largest when the polarization is along the x axis (TM). The photovoltage becomes smaller and smaller when the polarization deviates from the x axis, and finally disappears when the polarization is along the y axis (TE). As an additional confirmation, a linearly polarized laser source of 1064 nm (- 1.165 eV which is much greater than the band gap of the semiconductor) was used to illuminate the device and the photoresponse did not show polarization dependence. This is because for this incident source, the subwavelength features do not exist as the source wavelength of 1064 nm is much less than the spacing, s, of 90 μπι and the interband transition dominates the photocurrent in this case.

[0104] To study the effect of the spacing, s, between the two ohmic contacts on photoresponse, six more ohmic Au-InSb-Au structures (devices) with spacing, s, of 10 μπι, 30 μπι, 50 μπι, 70 μηι, 1 10 μηι and 130 μηι but with the same other parameters were fabricated and measured under the same conditions as for the device with the 90 μπι spacing. As shown in FIG. 5E illustrating the photovoltage of the devices with different values of the spacing, s, the photovoltage increases when the spacing decreases. For the device with the spacing, s, reduced to 10 μπι, the photovoltage is increased to about 0.081 mV, about 25 times of that of the device with a 90 μπι spacing. This observation is different from that observed in known photodetectors where the photovoltage increases with an increase of the semiconductor area. This is because the semiconductor in known photodetectors is the active medium where photogenerated carriers are generated from interband or intraband transitions, whereas the semiconductor (e.g., InSb) in the devices disclosed herein and measured is for excitation of localized SPPs and transit of the SPP-induced electrons under a bias. For the device with a smaller spacing, s, the intensity of the SPPs is stronger, and the average transit time is shorter, which, therefore, lead to a higher photovoltage.

[0105] To characterize the spectral response of the device having the spacing, s, of 90 μπι, the photovoltage was measured using the same source with varied photon energies from ~ 0.130 meV to ~ 0.165 meV. As expected, as shown in FIG. 5F illustrating the photovoltages of the device for incident waves with energies from 0.130 meV to 0.165 meV under a DC bias of 3.5 mA (the smoothed white line shown acts as a guide for the eyes), the result for the photovoltage shows a sharp peak close to about 0.151 meV as the antenna is designed (as a half-wave dipole with ~ 4 mm length) for this wavelength at which most photons may be coupled into the device for the excitation of SPPs.

[0106] To extend the detection to terahertz wave range, a device which has the same spacing of 90 μηι but with different antenna type and dimension (e.g., see FIG. 8 to be described further below) was designed and fabricated for resonance peak at the incident photons of 1.371 meV (0.332 THz). As shown in FIG. 5G illustrating the photovoltages of the device designed for resonance peaking at 1.37 meV (0.332 THz) photons for incident photons from 1.36 meV to 1.43 meV under a DC bias of 3.5 mA (the smoothed white line shown acts as a guide for the eyes), the photovoltage peaks at the expected wavelength with a value of about 0.0148 mV for an output power of 15 mW (VDI source as described above), as most or maximum photons at this wavelength may be coupled into the device. [0107] The devices may also be characterized at temperatures from 297 K to 77 K under the same conditions. FIGS. 6A to 6D show two-dimensional performance maps of a device having a spacing, s, of 90 μπι at temperatures ranging from 77 to 293 K. FIG. 6A shows the I-V data, FIG. 6B shows the photovoltage -bias current data, FIG. 6C shows the photovoltage-output power data, and FIG. 6D shows the photovoltage-modulation frequency data. The dashed arrows are shown as guides to represent the direction of the increase in the current (FIG. 6A) and the photovoltage (FIGS. 6B to 6D).

[0108] FIGS. 7 A to 7D show the results for the temperature effects on the performance of a device having a spacing, s, of 90 μπι. The I-V curve at 77 K of the device is shown in FIG. 7A (the white line shown is the linear fit), where an excellent ohmic contact may be observed. FIG. 7B shows the photovoltage as a function of temperature for an output power of 25 mW of 0.151 meV photons. As may be observed, the photovoltage increases when the temperarure decreases, and the value at 77 K under a bias of 3 mA is about 0.35 mV which is about 136 times of the 0.00257 mV at room temperature. FIG. 7C shows the photovoltage as a function of biased DC current at 77 K. It may be observed that the photovoltage tends to saturate from ~ 3 mA due to the saturation of electron velocity under large electric field. At this temperature, the resistance of the device is ~ 250 Ω as may be derived from FIG. 7A, and the voltage applied to the 90 μπι InSb spacing at saturation is ~ IV, corresponding to an electric field of - 1 10 V/cm. With increasing temperature, the required electrical field for saturation may be higher.

[0109] The effect of modulation frequency on the photoresponse of the device at temperatures from 77 K to 297 K may also be determined and the results obtained at three different temperatures are shown in FIG. 7D illustrating the photovoltage-frequency relationships at three temperatures of 77K, 237K and 297K. It may be observed that the photovoltage values are nearly unchanged in the measured frequency range of ~10 5 Hz at lower temperatures, demonsting a fast response character. The photoresponse speed may be comparable to known photodetectors and much faster than the thermal Golay cell detectors at millimeter wave range. At room temperature, however, the photovoltage is not only smaller but also decreases when the modulation frequency becomes high. At room temperature, the electron mobility becomes smaller and the lifetime of the SPP induced electrons becomes shorter due to enhanced lattice scattering, leading to a smaller photovoltage. For the decrease of photovoltage with the increase of modulation frequency, the lowered mobility is a factor as it results in a lower response. With an increase in modulation frequency, it may become harder and harder for the SPP-induced electrons to follow, leading to a decrease in photovoltage.

[0110] While a single photodetector with a planar dipole antenna and the corresponding results have been described, it should be appreciated that there may be various modifications that may be made.

[0111] FIG. 8 shows a schematic view of a photodetector 800 having a log periodic antenna 810, according to various embodiments. The photodetector 800 may include an OMSM structure 802 having a semiconductor portion 804 and two electrical contacts (or ohmic contacts) 806, 808 electrically coupled to the semiconductor portion 804 and arranged spaced apart from each other. A log periodic antenna 810 may be electrically coupled to the electrical contacts 806, 808. The log periodic antenna 810 and the electrical contacts 806, 808 may be metallic. The photodetector 800 may include a substrate 830. It should be appreciated that one or more materials described in the context of the photodetectors 100, 200 may be applicable to the photodetector 800.

[0112] The log periodic antenna 810 may be planar (e.g., a planar metallic antenna). The log periodic antenna 810 may have a diameter defined by R. The log periodic antenna 810 may include two central portions 850a, 850b and a plurality of tooth elements 852a, 854a, 856a, 852b, 854b connected to the central portions 850a, 850b, on both sides of the respective central portions 850a, 850b. Each of the tooth element 852a, 854a, 856a, 852b, 854b may be curved. The plurality of tooth elements 852a, 854a, 856a may be spaced apart from each other, and the plurality of tooth elements 852b, 854b may be spaced apart from each other.

[0113] Electrical conductors or conductive paths 832, 834 may be provided electrically coupled to the log periodic antenna 810, and, consequently also the two electrical contacts 806, 808, for application of an electrical bias (e.g., a DC bias).

[0114] The log periodic antenna 810 is a non-limiting example of a log periodic antenna that may be employed. The corresponding shape and/or the specific dimension parameter(s) may be optimized by simulations, for example, using software such as HFSS (high frequency structural simulator) or CST (Computer Simulation Technology), to optimize the antenna for different wavelength photons to be detected. [0115] FIGS. 9A and 9B show examples of photodetector arrangements 970a, 970b having a linear array design and a two-dimensional (2D) array design respectively. The photodetector arrangement 970a may have a linear ( 1x8) array of photodetectors (one example photodetector 900a is shown inside of the illustrated dashed line box), with 8 photodetectors arranged in one row. The photodetector arrangement 970b may have a 2D (2x8) array of photodetectors (one example photodetector 900b is shown inside of the illustrated dashed line box), with 16 photodetectors arranged in two rows. The photodetectors 900a, 900b in the respective photodetector arrangements 970a, 970b may not necessarily be connected to each other (although it may be possible) and each of the photodetectors 900a, 900b may be employed as one pixel in the respective arrays. Arrays may be used for imaging. For example, a linear array such as the photodetector arrangement 970a may form an image by scanning in one directions. A 2D array such as the photodetector arrangement 970b may form an image without scanning.

[0116] As described, various embodiments may provide antenna-assisted subwavelength OMSM structures for direct detection of LWPs in millimeter and terahertz wave ranges by making use of a low plasma frequency and high electron mobility semiconductor with subwavelength size, combined with an antenna. The performance has been verified by devices made of gold and InSb. The subwavelength Au-InSb-Au structure is used to absorb photons and excite SPPs which then generate non-equilibrium electrons, while the antenna is employed to couple most of the photons with the desired energy into the subwavelength structure. For a device (with a spacing, s, of 90 μπι) designed for ~ 8 mm wave, the responsivity is about 50 V/W at room temperature and may be increased to about 6800 V/W at 77 K. When the spacing of the 8 mm wave device is reduced to 10 μπι, the photovoltage is increased by ~ 25 times. For the device with a 90 μηι spacing designed for 0.332 THz, a photovoltage signal of 0.0148 mV is recorded. By selecting the dimension of the antenna and/or the subwavelength semiconductor size, devices may be designed for the particular wavelength in the millimeter and terahertz wave ranges, and the device performance may be optimized. In addition, such devices are easy to fabricate and operate. The technique disclosed herein may open an avenue for LWP detection and may be extended to other device applications. As non-limiting examples, for detection of 8 mm wave with a dipole-like antenna, the whole length of the antenna maybe about 4 mm, the width may be about 0.5 mm and the thickness of gold may be 300 nm, and, for detection of 0.9 mm wave with a log-period antenna (see FIG. 8), R may be about 0.95 mm, and the thickness of gold may be 300 nm.

[0117] As non-limiting examples, the detectors for millimeter and terahertz waves of various embodiments may be used in various applications including but not limited to:

[0118] Security: Linear or 2D cameras for millimeter and terahertz waves based on the disclosed method and devices may be used in security screening in public places like airport. They may uncover concealed weapons, explosives, and some other dangerous items.

[0119] Scientific research: Currently, commercial terahertz TDS systems (Terahertz time domain systems) usually use known terahertz photoconductive antennas which require expensive local femtosecond laser for normal operation. The disclosed device and/or cameras based on the disclosed method may have a very low cost of product and simple configuration, which may stimulate or lead to cheap terahertz spectroscopy systems.

[0120] Communication: Devices based on the disclosed technique may be used in communication for wireless data transmission with a large bandwidth, which may have a huge potential commercial value.

[0121] Remote sensing: Imaging elements based on the disclosed technique may be used in remote sensing for meteorology, space exploration, and national defense.

[0122] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.