WO2016042340A1

WO2016042340A1 - Terahertz radiation detector

Info

Publication number: WO2016042340A1
Application number: PCT/GB2015/052704
Authority: WO
Inventors: John Cunningham; Alexander Giles Davies; Edmund Harold Linfield; Alexander Mayorov; Nicholas Callum HUNTER; Divyang Kishore MISTRY
Original assignee: University Of Leeds
Priority date: 2014-09-19
Filing date: 2015-09-18
Publication date: 2016-03-24
Also published as: GB201416558D0

Abstract

A photoconductive switch which can be applied as a detector (304) for terahertz radiation (316) is disclosed. The detector (304) uses a graphene sheet or flake as a body (318) for the detection of an incident terahertz radiation (316). The incident THz radiation (316) provides a biasing voltage that serves to separate carriers generated within the graphene body (318) in response to illumination with a detection laser beam (324). A current or voltage indicative of the incident terahertz radiation (316) can be detected by a first electrode (320) and a second electrode (322).

Description

TERAHERTZ RADIATION DETECTOR

Technical Field

Embodiments relate to detectors such as, for example, terahertz detectors.

Background

Advances in terahertz (THz) technology are leading, for example, to improved imaging systems and improved spectroscopy systems, as well as improvements in security. Terahertz radiation readily penetrates commonly used materials that are otherwise opaque to visible and mid- infrared light, which, in turn, allows substance-specific spectroscopy to be realised.

The terahertz region, that is, 0.3 to 3 THz, has traditionally used low-temperature gallium arsenide (LT-GaAs) detectors, in the form of photoconductive (PC) switches. LT-GaAs PC switches are also used in generating pulsed THz radiation in terahertz time-domain spectroscopy. LT-GaAs is a suitable material for such applications since it has a high mobility, high intrinsic resistivity and the photo-generated carriers have a short life-time, which can be O(10^"15s). Low temperature GaAs PC switches are, however, difficult and expensive to manufacture because careful temperature control is needed during both molecular beam epitaxial growth and subsequent annealing of the material.

Brief Description of the Drawings

Embodiments described herein are, without limitation and by way of example, described with reference to and illustrated in the accompanying drawings:

Figure 1 shows a conventional terahertz system comprising LT-GaAs PC switches;

Figure 2 depicts an LT-GaAs PC switch in greater detail;

Figure 3 illustrates a graphene terahertz system according to an embodiment;

Figures 4, 4A and 4B show graphene terahertz PC detectors according to embodiments;

Figures 5A and 5B depict graphene terahertz PC detectors according to further embodiments;

Figure 6 shows a graphene terahertz system according to an embodiment;

Figure 7 illustrates a time domain view of a detected THz pulse; and

Figure 8 depicts a frequency domain view of the detected THz pulse. l Description of Embodiments

Figure 1 shows a view of a known terahertz system 100. The system 100 comprises a terahertz source 102 for generating terahertz radiation and a detector 104 for detecting terahertz radiation. The source 102 comprises a low-temperature gallium arsenide photoconductive switch (LT-GaAs PC switch). Such an LT-GaAs PC switch is shown in greater detail in, and described below with reference to, figure 2. The system 100 comprises a waveguide 106 that couples the source 102 and the detector 104.

The source 102 comprises a low-temperature GaAs planar body 108. The body 108 is coupled to the waveguide 106. The body 108 bears first 1 10 and second 1 12 electrodes. The first 1 10 and second 112 electrodes provide a biasing voltage across the body 108. The biasing voltage serves to separate carriers generated within the body in response to being illuminated with a generation laser beam 114. The charge carriers comprise electron-hole pairs that are separated via the biasing voltage. Generating and separating such electron-hole pairs generates terahertz radiation 116. The terahertz radiation takes the form of a pulse of terahertz radiation.

The terahertz radiation 1 16 propagates via the waveguide 106 to the detector 104. The detector comprises a low-temperature GaAs planar body 1 18. The body 118 is coupled to the waveguide 106. The body 1 18 bears first 120 and second 122 electrodes. The first 120 and second 122 electrodes provide a means for detecting a current within and/or a voltage across the body 1 18. Incident terahertz radiation provides a biasing voltage that serves to separate carriers generated within the body 1 18 in response to illumination with a detection laser beam 124. The detection laser beam 124 is synchronized with the generation laser beam 114; typically the former is derived from the latter as indicated below. The charge carriers comprise electron-hole pairs that are separated via the biasing voltage. Generating and separating such electron-hole pairs produces a detectable current and voltage associated with or indicative of the incident terahertz radiation 116. The detectable current and voltage allow a system, such as that shown in figure 6, to process that current and voltage to allow, for example, an image to be formed or to allow spectroscopy to be performed.

Figure 2 shows in greater detail the structure of an LT-GaAs PC switch 200 used to form the above source 102 and detector 104. It can be appreciated that the switch 200 comprises an LT-GaAs body 202 that is overlaid with a waveguide 204 and coupled to biasing electrodes 206 and 208. The body 202 is illuminated with a generation or detection laser beam 210.

Figure 3 shows a view a terahertz system 300 according to an embodiment. The system 300 comprises a terahertz source 302 for generating terahertz radiation and a detector 304 for detecting terahertz radiation. The source 302 comprises a LT-GaAs photoconductive switch. Such a photoconductive switch has been described above with reference to figure 2. The system 300 comprises a waveguide 306 that couples the source 302 and the detector 304.

The source 302 comprises a low-temperature GaAs planar body 308. The body 308 is coupled to the waveguide 306. The body 308 bears first 310 and second 312 electrodes. The first 310 and second 312 electrodes provide a biasing voltage across the body 308. The biasing voltage serves to separate carriers generated within the body in response to illumination with a generation laser beam 314. The charge carriers comprise electron-hole pairs that are separated via the biasing voltage. Generating and separating such electron-hole pairs generates terahertz radiation 316. The terahertz radiation takes the form of a pulse of terahertz radiation.

The terahertz radiation 316 propagates via the waveguide 306 to the detector 304. The detector comprises a graphene planar body 318. The body 318 is coupled to the waveguide 306. The body 318 bears first 320 and second 322 electrodes. The first 320 and second 322 electrodes provide a means for detecting a current within and/or voltage across the body 318. Incident terahertz radiation provides a biasing voltage that serves to separate carriers generated within the body in response to illuminate with a detection laser beam 324. The detection laser beam 324 is synchronized with the generation laser beam 314; typically the former is derived from the latter as indicated below. The charge carriers comprise electron-hole pairs that are separated via the biasing voltage. Generating and separating such electron-hole pairs produces a detectable current and voltage associated with or indicative of the incident terahertz radiation 316. The detectable current and voltage allow a system, not shown, to process that current and voltage to allow, for example, an image to be formed or to allow spectroscopy to be formed.

Figure 4 shows in greater detail the structure of a graphene PC switch 400 used to form the above detector 304. It can be appreciated that the switch 400 comprises a graphene body 402 that is coupled to a waveguide 404 and coupled to detection electrodes 406 and 408. The body 402 is illuminated with a generation or detection laser beam 410. The terahertz radiation 316 is received from the waveguide 404 where it generates a time varying field within the body 402 that serves to accelerate the charge carriers generated by the detection laser beam 410. The electrodes 406 and 408 provide a means of detecting the generated charge carriers. The detection can take the form of measuring at least one of a current and voltage associated with the charge carriers. Although the illustrated embodiment shows the waveguide terminating normally to the graphene, embodiments are not limited thereto. Embodiments can be realised in which the electrodes either overlay or underlay the graphene body 402 as shown in figures 4A and 4B. Hence, referring to figure 4A, there is shown a view 400A of the structure of a graphene PC switch 400A that can be used to form the above detector 304. It can be appreciated that the switch 400A comprises a graphene body 402A coupled to a waveguide 404A and also detection electrodes 406A and 408A. It can be appreciated that the waveguide 404A has an upper portion 409A. In the embodiment illustrated, the upper portion 409A overlays the graphene body 402A. The body 402A is illuminated with a generation or detection laser beam 41 OA to generate charge carriers within the body 402A. The terahertz radiation 316 is received from the waveguide 404A where it generates a time varying field within the body 402A that serves to accelerate the charge carriers generated by the detection laser beam 41 OA. The electrodes 406A and 408A allow the charge carriers to be detected. The detection can take the form of measuring at least one of a current and voltage associated with the charge carriers. Embodiments can have the graphene 402A disposed beneath at least one of the electrodes and waveguide.

There is shown a view 400B, in figure 4B, of the structure of a graphene PC switch 400B that can be used to form the above detector 304. It can be appreciated that the switch 400B comprises a graphene body 402B coupled to a waveguide 404B and also coupled to detection electrodes 406B and 408B. It can be appreciated that the waveguide 404B has a lower portion 409B. In the embodiment illustrated, the lower portion 409B underlays the graphene body 402B. The body 402B is illuminated with a generation or detection laser beam 410B to generate charge carriers within the body 402B. The terahertz radiation 316 is received from the waveguide 404B where it generates a time varying field within the body 402B that serves to accelerate the charge carriers generated by the detection laser beam 410B. The electrodes 406B and 408B allow the charge carriers to be detected. The detection can take the form of measuring at least one of a current and voltage associated with the charge carrier movement that is responsive to the THz radiation.

Although the embodiments described herein disclose the waveguides terminating at the interface with the graphene via an underlay portion or an overlay portion, embodiments are not limited thereto. Embodiments can be realised in which the waveguide terminates normally to the graphene. For example, the waveguide could be arranged to terminate normally to a planar graphene portion. Such a waveguide could be realised in the form of, for example, a hollow- metal waveguide. Figure 5A illustrates an embodiment of a graphene detector 500A. The graphene detector comprises a graphene body 502A. The graphene body 502A bears an antenna 504A. The antenna 504A is arranged to couple free space terahertz radiation 506A to the body 502A. The antenna 504A can be, for example, a bow-tie antenna comprising a pair of inwardly directed 508A and 51 OA pointed limbs. The antenna 504A generates an electric field within the body 502A for accelerating any charge carriers generated by an incident detection laser (not shown) such as the detection lasers described above. The antenna 504A also serves as a pair of electrodes for detecting at least one of current and voltage associated with THz wave stimulated movement of charge carriers generated by the incident detection laser.

Embodiments can be realised in which the free space terahertz radiation 506A is incident upon the graphene body 502A at a predetermined angle of incidence. In preferred embodiments, normal incidence is preferred.

Figure 5B illustrates an embodiment of a graphene detector 500B. The graphene detector comprises a graphene body 502B. The graphene body 502B bears an antenna 504B. The antenna 504B is arranged to couple free space terahertz radiation 506B to the body 502B. The antenna 504B can be, for example, a bow-tie antenna comprising a pair of inwardly directed 508B and 51 OB limbs having trapeze shaped ends. The antenna 504B generates an electric field within the body 502B for accelerating any charge carriers generated by an incident detection laser (not shown) such as the detection lasers described above. The antenna 504B also serves as a pair of electrodes for detecting at least one of current and voltage associated with THz wave stimulated movement of charge carriers generated by the incident detection laser.

Embodiments can be realised in which the free space terahertz radiation 506B is incident upon the graphene body 502B at a predetermined angle of incidence. In preferred embodiments, normal incidence is preferred.

The above waveguides are realised in the form of Goubau lines, microstrip, or a coplanar waveguide.

In any of the embodiments described in this specification, the detectors can comprise a substrate on which the graphene is deposited. Preferably, the substrate is a quartz substrate. Although the embodiments described herein use a quartz substrate, embodiments can be realised that use some dielectric material. Therefore, embodiments can be realised that use materials such as, for example, benzocyclobutene, high resistivity silicone, polyimide and polyamide. Methods of fabricating embodiments of the present invention will now be described.

Referring to the embodiment shown in, and described with reference to, figures 3 and 4, the low temperature grown GaAs (LT-GaAs) body 308 was grown using molecular beam epitaxy (MBE). The body 308 comprises a predetermined thickness such as, for example, 500 μηι, of bulk semi-insulating GaAs (SI-GaAs) with a sacrificial layer having a predetermined thickness such as, for example, 100 nm, of AIGaAs that, in turn, is topped by a layer of LT-GaAs having a predetermined thickness such as, for example, 350 nm. The resulting structure was cleaved into chips having predetermined dimensions, such as, for example, 3 x 4 mm chip sizes. The chips were cleaned for a predetermined time such as, for example, 5 minutes, in acetone and then in isopropanol (I PA) in an ultrasonic bath before being plasma ashed for a predetermined period of time such as, for example, 5 min, at a respective power, such as, for example, 50 W.

The chips were then annealed. Preferably, the annealing was at a temperature of 575°C for 15 min in a rapid thermal annealer. To epitaxially lift-off the LT-GaAs from the SI-GaAs, black wax was used as a temporary host superstrate whilst the AIGaAs layer was etched away. To achieve this, the MBE structure, that is, a chip, was transferred onto a hot plate at 130°C and black wax was placed onto the surface until covering the top of the chips, and then the sample was left to cool. Tnchloroethylene soaked swabs were used to rub away the outer edge of the wax, exposing about -0.5 mm of the LT-GaAs surface around the perimeter. When submerged in a solution of

(1 :40:80) for 1 min, the unprotected LT-GaAs was removed, exposing the AIGaAs. The chip was then rinsed in deionised (Dl) water and placed in a 10% solution of HF acid (at -4°C to ensure a low etch rate). A slow etch rate is required to limit the size of hydrogen bubbles in the interface between the two layers of GaAs, which was found to cause cracks in the transferred layer.

The chip was then separated into two; the thin layer of LT-GaAs attached to the wax, and the SI-GaAs host substrate, which could then be discarded. The LT-GaAs was rinsed several times in water and placed onto the cleaned quartz host substrate, ensuring that a small film of water supported the LT-GaAs and the wax. The sample was then left to dry for 4-5 days, allowing Van der Waals bonding to take place.

After bonding, the stack was placed in tnchloroethylene until all wax was removed, leaving only ~2 by 3 mm of photoconductive material on the much larger quartz substrate. The remaining wax film was removed by a further bath in tnchloroethylene, and the sample then gently rinsed in Dl water. The sample was then dried and placed into the vacuum oven for 15 hours at 250°C to desorb any particles trapped at the quartz/GaAs interface. To etch the GaAs into rails, standard photolithography was used to protect two parallel rails 20 μηι thick and 2 mm apart. The sample was then placed into a solution of H₂S0₄:H₂0₂:H₂0 (1 :8:950) for -10 min, until the majority of the GaAs had been removed, leaving just the areas under the S1813 rails. After cleaning in acetone, I PA and water for 5 min each the device was plasma ashed for 10 min at 50 W, and placed hot plate for 10 min at 200°C to desorb any residual liquid. Note that the devices could not be placed into an ultrasonic bath due to the fragility of the LT-GaAs after Van der Waals bonding. A primer (HMDS) was used to enhance adhesion, and bi-layer photolithography was then used to define the Goubau line and probe arms. The device was plasma ashed for 30 s at 50 W after photolithography to create a clean surface for metal deposition. 5 nm titanium and 80 nm gold was deposited using e-beam evaporation and subsequently lifted-off using acetone and MF319.

Graphene, deposited by chemical vapour deposition on to a copper film, was then prepared by spinning a 600 nm layer onto a supporting layer of PMMA and then baked at 200°C for 2 min. The graphene on the backside was then removed using a reactive ion etch with 15 seem flow of 0₂ for 30 s and 80 W power. The sample was then placed copper side down on top of a solution of FeN0₃and Dl water (0.01 g/ml), floating on the surface. Once all copper has been etched away, leaving a suspended stack of graphene and PMMA, the stack was lifted from the surface of the solution using quartz and transferred to the surface of Dl water. This step was repeated several times into fresh water until any residual FeN0₃ had been sufficiently diluted. The previously prepared quartz substrate with GaAs was then cleaned in acetone and I PA for 5 minutes each and ashed for 5 minutes at 50 W and used to lift out the stack from the water. The graphene/PMMA, floating on a thin film of water on top of the device, was then gently manipulated into the desired position between the two GaAs rails and left to dry overnight in ambient conditions, bonding the graphene to the quartz/gold. The sample was then placed on a hot plate at 200°C for 5 min to soften the PMMA and reduce any wrinkles that had formed during the drying process. Afterwards the PMMA was removed using an acetone bath for at least 1 hour and then rinsed in I PA. A further heating step at 200°C for 10 min was then used. A final step of standard optical lithography was then used to mask the desired graphene regions during a plasma ash. The S1813 was left on to structurally support the graphene at the edges of the metal.

Referring to figure 6, there is shown a view 600 of a system according to an embodiment. The system comprises a laser 602 for producing a correspond beam 604. The laser 602 can be, for example, a Ti:Sapphire laser. The beam 604 is divided by a beam splitter 606. A first divided portion 608 of the beam is directed, via a respective reflector or beam splitter 610, to terahertz radiation source 612. The terahertz radiation source 612 can be realised as a conventional LT- GaAs source as described with reference to figures 1 and 2, or as a graphene switch described with reference to figures 3 to 5, suitably biased via a biasing voltage, Vbias.

The source 612 produces terahertz radiation 614. The radiation 614 is focused, via a lens 616, onto a terahertz detector 618 such as, for example, a detector as described and claimed herein with reference to any of figures 3 to 5.

A second portion 620 of the beam 604 is arranged, via a number of reflectors or beam splitters 622 to 628, to be focused on the graphene body of the terahertz detector 618. The reflectors or beam splitters 622 and 628 are arranged to introduce a sufficient delay synchronise the arrival of the terahertz radiation 614 and the second portion 620 of the beam, that is, the detection laser, at the graphene body of the detector 618. The incident radiation 614 and the incident laser generate charge carriers that are detected via at least one of a current and voltage measurement apparatus. In the present example, the apparatus is realised using an A/D converter 630 that is configured to sample the current generated by the detector 618.

The A/D converter 630 produces output data (not shown). The output data is associated with the measured terahertz radiation 614 incident on the graphene body of the detector 618. The A/D converter 630 can form part of a computer 632. The computer 632 has a processor 634 that is arranged to execute software for processing the output data.

In use, the terahertz radiation 614 received by the detector 618 will have interacted with a body (not shown) and the terahertz radiation 614 detected will be characteristic of that interaction or body. The body can be, for example, a human body and the output data can be processed to produce an associated image. Alternatively, the body can be a sample or substance and the output data can be processed to realise a form of terahertz time-domain spectroscopy.

Embodiments are provided in which the results of the processing undertaken by the processor 634 are output for further processing. The further processing can comprise, for example, display on a display 636 or rendering via some other output device.

Figure 7 shows a time domain representation 700 of a terahertz pulse 702 detected according to an embodiment. It can be appreciated that the pulse has a respective peak-to-peak amplitude 704. Embodiments can be realised in which that respective peak-to-peak amplitude is 0.3 mV. The pulse was detected by a graphene body such as any of the above described graphene bodies. The system shown in figure 6 was configured with a respective bias voltage. Embodiments can be realised in which the respective bias is 100V. The system shown in figure 6 was configured to use a pump beam 608 of 500 mW and a probe beam 620 of 500 mW. The pulse has a duration of approximately 9-10 ps. Figure 8 depicts a frequency domain representation 800 of the detected terahertz pulse 702.

Advantageously, embodiments can provide detectors and systems for detecting THz em wave using photon stimulated charge carrier movement that is responsive to a THz em wave.

It will be appreciated that the embodiments use the electric field component of the incident THz waves, or photons, to accelerate the laser generated charge carriers within the graphene bodies 402A and 402B as well as graphene bodies 502A and 502B.

One or more of the above embodiments provides a single graphene detector body 402 and 502 that can be used to detect THz waves of all THz frequencies or a predetermined range of THz frequencies.

Claims

1. A detector comprising a graphene photo-excitable charge carrier body and means to couple an electromagnetic (em) wave to the graphene photo-excitable charge carrier body.

2. The detector of claim 1 , comprising a substrate bearing at least the graphene photo- excitable carrier body.

3. The detector of claim 2, wherein the substrate is a quartz or other dielectric substrate.

4. The detector of any preceding claim, wherein the means to couple the em wave to the graphene photo-excitable charge carrier body comprises an antenna.

5. The detector of claim 4, in which the antenna is a bow-tie antenna.

6. The detector of any preceding claim, comprising an optical source to illuminate the graphene photo-excitable charge carrier body with light to generate charge carriers within the graphene photo-excitable charge carrier body in response to the em wave.

7. The detector of any of claims 1 to 3, wherein the means to couple the em wave to the graphene photo-excitable charge carrier body comprises an em waveguide.

8. The detector of claim 7, in which the em waveguide comprises a Goubau waveguide.

9. A system for terahertz imaging or sampling, the system comprising a detector as

claimed in any of claims 1 to 8, coupled to at least one of a current and voltage measurement apparatus for producing a signal associated with at least one of a current and voltage measured by the measurement apparatus and a processor, the processor being configured to process the signal.

10. A system as claimed in claim 9, in which the processor is configured to process the signal to determine data associated with a frequency spectrum of the signal.

1 1. A method, system, and detector substantially as described herein with reference to and/or as illustrated in any of figures 3 to 8.