Abstract-Boosting Terahertz Photoconductive Antenna Performance with Optimised Plasmonic Nanostructures

Sergey Lepeshov, Andrei Gorodetsky, Alexander Krasnok, Nikita Toropov, Tigran A. Vartanyan, Pavel Belov, Andrea Alú, Edik U. Rafailov,

https://www.nature.com/articles/s41598-018-25013-7


Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today’s terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.

Abstract- Photoconductivity of an InAs/GaAs self-assembled quantum dot photoconductive THz antenna

Amit Yadav,  Andrei Gorodetsky,  Eugene Avrutin, Ksenia A. Fedorova,  Edik U. Rafailov

http://ieeexplore.ieee.org/document/8086621/


A broadband terahertz (THz) source is desirable for applications such as imaging, spectroscopy and security. Towards this, an InAs/GaAs quantum dot (QD) based photoconductive antenna (PCA) is a promising and compact solution for THz generation. Coherent THz radiation in the pulsed and the CW regime has been generated with a QD PCA under a resonant and off-resonant pumps [1, 2]. While photoconductivity of QD materials in mid- and far-IR at lower temperatures has been studied for cryogenic sensors and attributed to interlevel transitions, near-IE interband photoconductivity needs further investigation [3, 4]. In this work, we report on the photoconductive properties of an InAs/GaAs QD PCA pumped by a broadly-tunable InAs/GaAs QD external-cavity diode laser.

Abstract-Compact All-Quantum-Dot-Based Tunable THz Laser Source

 Ksenia A. Fedorova, Andrei Gorodetsky, Edik U. Rafailov

http://ieeexplore.ieee.org/document/7762827/

We demonstrate an ultracompact, room temperature, tunable terahertz (THz) generating laser source based on difference-frequency-driven photomixing in a coplanar stripline InAs/GaAs quantum-dot (QD) antenna pumped by a broadly tunable, high power, continuous wave InAs/GaAs QD laser diode in the double-grating quasi-Littrow configuration. The dual-wavelength QD laser operating in the 1150- 1301 nm wavelength region with a maximum output power of 280 mW and with tunable difference-frequency (277 GHz to 30 THz) was used to achieve tunable THz generation in the QD antenna with a photoconductive gap of 50 μm. The best THz output performance was observed at pump wavelengths around the first excited state of the InAs/GaAs QDs (~1160 nm), where a maximum output power of 0.6 nW at 0.83 THz was demonstrated.

Abstract-Boosting the Terahertz Photoconductive Antenna Performance with Optimized Plasmonic Nanostructures

Sergey Lepeshov, Andrei Gorodetsky, Alexander Krasnok, Nikita Toropov, Tigran A. Vartanyan, Pavel Belov, Andrea Alu, Edik U. Rafailov

(Submitted on 14 Jun 2017)

https://arxiv.org/abs/1706.04607

Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology and resulting in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today's terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and the distance between them, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. Moreover, we propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.

Generating tunable terahertz radiation with a novel quantum dot photoconductive antenna

Andrei Gorodetsky, Ksenia A. Fedorova, Natalia Bazieva and Edik U. Rafailov

Indium arsenide quantum dots in gallium arsenide wafers offer wider pump-wavelength range, significantly higher thermal tolerance, and higher conversion efficiency than typical terahertz radiation sources.

28 December 2016, SPIE Newsroom. DOI: 10.1117/2.1201611.006751
http://spie.org/newsroom/6751-generating-tunable-terahertz-radiation-with-a-novel-quantum-dot-photoconductive-antenna?highlight=x2404&ArticleID=x124361

The terahertz (THz) range of electromagnetic waves (0.1–10THz)—which lies between the microwave and optical regions—is of great interest. This is mainly because this band of the electromagnetic spectrum includes the frequencies of rotational and vibrational spectra of complex (e.g., biological) molecules. Most dielectric materials are transparent in the THz region, and THz waves are already used in many biomedical applications (e.g., for the detection of dangerous and illicit substances, as well as for the diagnosis and treatment of diseases). Photoconductive antennas are the most-developed room-temperature sources of THz radiation. However, ultrafast low-temperature-grown gallium arsenide (GaAs)—which is typically used as a substrate for such antennas—suffers (because of its large band gap) from low thermal efficiency, low carrier mobility, and a pump limit at a wavelength of about 850nm. An alternative substrate material is thus required so that efficient and tunable THz radiation can be generated.

Quantum dots (QDs) were first discovered in the 1980s,1 i.e., around the same time as photoconductive antennas.2 The properties of these particles are controlled by both their size and the material from which they are made. QDs exhibit discrete energy levels and are highly configurable, and QD materials are successfully used in multiple applications, e.g., for lasers,3 saturable absorbers,4 photovoltaic devices,5and biosensors.6 To generate pulsed and continuous wave (CW) THz radiation, photoeffective semiconductors that also possess a high carrier mobility and ultrashort carrier lifetime are necessary (i.e., QD wafers perfectly meet all the requirements).

In our work,7 we have shown that QD materials can be used to effectively generate THz radiation.8, 9 We used molecular beam epitaxy (MBE) in the Stranski-Krastanov regime to grow indium arsenide (InAs)/GaAs QD structures over a suitable distributed Bragg reflector (DBR). The need for this DBR is twofold, i.e., to reflect the pump beam (and thus reduce the IR power at the antenna output) and to enable full-optical cavity-type optimization of the structure. Our constructed antenna—see Figure 1—contains InAs QD layers within a GaAs lattice and has a 30nm-thick top layer of low-temperature GaAs (LT-GaAs). The top layer (i.e., of LT-GaAs) is suitable for titanium/gold metal contact deposition and significantly reduces the dark current of the antenna. In addition, each InAs QD layer (of 2.3 monolayers) is capped with a 4–5nm-thick In0.15Ga0.85As layer and is separated by a 35–36nm GaAs spacer layer (giving a total active region of 1–1.7μm).

Figure 1. Left: Schematic diagram of a quantum-dot-based photoconductive antenna. The structure—grown over a distributed Bragg reflector (DBR)—contains indium arsenide (InAs) quantum dots (QDs) within a gallium arsenide (GaAs) lattice. Right: Transmission electron microscope image of one of the InAs QDs.

We find that the pulsed THz radiation emitted/detected from our QD antenna (when it is used as a source/detector) displays a wavelength dependence, i.e., there are peaks in the vicinity of the QDs' first and second excited states (see Figure 2). Our QD-based antenna may thus also be used in conjunction with semiconductor laser-pump sources, in the 1100–1300nm wavelength range. This is possible because similar InAs:GaAs QD structures are used in both the laser-pump source and the photoconductive THz emitter, and because of the complementary emission and absorption energies exhibited by the InAs:GaAs laser and antenna.

Figure 2. The performance (at different wavelengths) of the QD antenna as both a terahertz (THz) source and detector.

In addition to pulsed THz signals, we can use our QD antenna to generate CW THz radiation (through heterodyne conversion). We have previously demonstrated CW THz emission by photomixing of 850nm laser diode pumps in QD structures.10 Building on that work, here we show an approach for the generation of tunable CW narrowband THz radiation. In this method we use our QD photoconductive antenna pumped by a tunable QD semiconductor laser in a double-Littrow configuration: see Figure 3(a). By rotating the diffraction gratings in this setup, we can achieve simultaneous and arbitrary tuning of both output wavelengths, and thus the resultant THz difference frequency. An example THz output power trend, from our QD-based antenna pumped by this laser (tuned to a difference frequency of 1THz), is shown in Figure 3(b).

Figure 3. A tunable QD semiconductor laser in a double-Littrow configuration (a) is used to pump the QD photoconductive antenna and produce a continuous wave THz signal (b). The inset to (b) shows the pump spectrum. E: Energy. λ: Wavelength. a.u.: Arbitrary units.

The thermal tolerance of our QD materials significantly surpasses that of bulk semiconductors. This means that there is the potential to enable the use of the full laser intracavity power, e.g., of an optically pumped disk laser.11 Such a layout, i.e., the intracavity placement of an antenna in a vertical-external-cavity surface-emitting-laser (see Figure 4), allows an all-semiconductor-element-based, efficient, room-temperature THz source to be contained in an ultracompact housing.

Figure 4. Schematic diagram for the intracavity antenna placement within a vertical-external-cavity surface-emitting laser.

In summary, we have proposed nanoengineered GaAs wafers (with self-assembled InAs QDs) as novel materials for photoconductive THz generation. In our antennas, we keep the carrier mobility as high as in bulk semi-insulating GaAs wafers. In addition, the InAs QDs (typically several nanometers in size) allow pumping with significantly longer wavelengths than with typical antennas. The QDs also serve as photocarrier capture sites and thus shorten the carrier lifetimes of the devices. Furthermore, our MBE-grown wafers have exceptional uniformity and quality, and thus possess nearly perfect thermal efficiency. Our materials can therefore be engineered to allow longer-wavelength and higher-power pumping, as well as ultrafast and tunable room-temperature operation in a compact layout. In our future research we will aim to produce a highly efficient and ultrafast device—without introducing compromises between factors such as carrier mobility, photoconductive gain, resistivity, and carrier lifetime (as would normally be the case with bulk-type photoconductive THz materials)—and ultracompact, electrically pumped, on-chip THz sources.

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Andrei Gorodetsky, Ksenia A. Fedorova, Natalia Bazieva, Edik U. Rafailov

Aston University

Birmingham, United Kingdom

Edik Rafailov is head of the Optoelectronics and Biomedical Photonics Group. He is the author of more than 400 articles, two books, as well as numerous invited chapters and talks. His current research includes high-power CW and ultrashort-pulse lasers, generation of UV/visible/IR/mid-IR and THz radiation, nanostructures, nonlinear and integrated optics, and biophotonics.

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References:

1. A. I. Ekimov, Al. L. Efros, A. A. Onushchenko, Quantum size effect in semiconductor microcrystals, Solid State Commun. 56, p. 921-924, 1985.

2. D. H. Auston, K. P. Cheung, P. R. Smith, Picosecond photoconducting Hertzian dipoles, Appl. Phys. Lett. 45, p. 284-286, 1984.

3. E. U. Rafailov, M. A. Cataluna, W. Sibbett, Mode-locked quantum-dot lasers, Nat. Photon. 1, p. 395-401, 2007.

4. E. U. Rafailov, S. J. White, A. A. Lagatsky, A. Miller, W. Sibbett, D. A. Livshits, A. E. Zhukov, V. M. Ustinov, Fast quantum-dot saturable absorber for passive mode-locking of solid-state lasers, IEEE Photon. Technol. Lett. 16, p. 2439-2441, 2004.

5. R. D. Schaller, V. I. Klimov, High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion, Phys Rev. Lett. 92, p. 186601, 2004.

6. F. Tokumasu, R. M. Fairhurst, G. R. Ostera, N. J. Brittain, J. Hwang, T. E. Wellems, J. A. Dvorak, Band 3 modifications in Plasmodium falciparum-infected AA and CC erythrocytes assayed by autocorrelation analysis using quantum dots, J. Cell Sci. 118, p. 1091-1098, 2005.

7. A. Gorodetsky, K. A. Fedorova, N. Bazieva, E. U. Rafailov, Towards efficient and tunable generation of THz radiation from quantum dot based ultrafast photoconductive antennae, Proc. SPIE 9934, p.993402, 2016. doi:10.1117/12.2240121

8. A. Gorodetsky, N. Bazieva, E. U. Rafailov, Quantum-dot based ultrafast photoconductive antennae for efficient THz radiation, Proc. SPIE 9737, p. 97370C, 2016. doi:10.1117/12.2222815

9. R. R. Leyman, A. Gorodetsky, N. Bazieva, G. Molis, A. Krotkus, E. Clarke, E. U. Rafailov, Quantum dot materials for terahertz generation applications, Laser Photon. Rev.10, p. 772-779, 2016.

10. T. Kruczek, R. Leyman, D. Carnegie, N. Bazieva, G. Erbert, S. Schulz, C. Reardon, S. Reynolds, E. U. Rafailov, Continuous wave terahertz radiation from an InAs/GaAs quantum-dot photomixer device, Appl. Phys. Lett. 101, p. 081114, 2012.

11. M. A. Gaafar, A. Rahimi-Iman, K. A. Fedorova, W. Stolz, E. U. Rafailov, M. Koch, Mode-locked semiconductor disk lasers, Adv. Opt. Photon. 8, p. 370-400, 2016.

Quantum dots offer new platform for fingertip terahertz devices

Credit: ITMO University
http://phys.org/news/2016-12-quantum-dots-platform-fingertip-terahertz.html
Scientists from Russia and the U.K. have developed an antenna that can aid in reducing sources of terahertz radiation down to the size of a fingertip. The antenna is a "sandwich" of semiconductor layers combined with quantum dots. The scientists demonstrated that such antennas provide a foundation for a new universal system capable of both transmitting and receiving terahertz radiation. Compact devices, operating at terahertz range, have applications in medicine and biology for tumor visualization and in the aerospace industry for high-speed communication systems. The study was published in Laser & Photonics Reviews.

The terahertz range lies between infrared and microwave spectra. Terahertz radiation can penetrate living tissues, but unlike X-rays, is not ionizing and poses no health hazard. Therefore, medical practitioners could benefit immensely from compact terahertz scanners that can obtain pictures of tissues in living organisms.

Researchers from Aston University and ITMO University used quantum dots to develop an antenna that can significantly reduce the size of terahertz sources. The work was supported by scientists from the University of Strathclyde and University of Sheffield, as well as TeraVil Ltd company and Center for Physical Sciences and Technology in Vilnius.

"It was a technological challenge," says the study's academic supervisor Edik Rafailov, professor at Aston Institute of Photonic Technologies and leading research associate at ITMO University. "We demonstrated that quantum dots are a good alternative for conventional semiconductors. This new technology gives us an opportunity to generate terahertz at room temperature. And potentially make terahertz devices compact and cheap."

Credit: ITMO University

Today, terahertz generation relies on sources that involve conversion of infrared laser beam into terahertz. The transformation is carried out with intricate systems of waveguides, semiconductor crystals or diodes. The search for alternative ways of generating and detecting terahertz waves is still underway, but such devices remain bulky, expensive and operate only at low temperatures.

The new antennas make it possible not only to use terahertz sources at room temperature, but also to miniaturize them. "We are able to create very compact sources of terahertz radiation the size of a fingertip," comments leading author of the paper Andrei Gorodetsky, researcher at the Department of Photonics and Optical Information Technology of ITMO University and research associate at Aston Institute of Photonic Technologies. "With the new antennas, we managed to remove the limitation associated with the narrow light spectrum that is used by current converts. This gives us an opportunity to combine the antennas with compact infrared lasers. Additionally, the antennas are 20 times more resistant to damage than typical semiconductor devices. Both factors allow us to incorporate the antenna into the laser instead of setting it apart."

Credit: ITMO University

The researchers suggest that their findings can be used in high-speed communication systems and also in compact terahertz scanners, which would give dynamic imaging of deep skin layers, embryo development, brain processes, and scanning of internal organs or tumors. Terahertz radiation is not harmful, as it does not scatter too much in biological tissues. As a result, terahertz systems are more informative, sensitive and fast compared to their substitutes from other parts of electromagnetic spectrum.

Credit: ITMO University

Credit: ITMO University

Abstract-Quantum dot materials for terahertz generation applications

http://onlinelibrary.wiley.com/doi/10.1002/lpor.201500176/abstract;jsessionid=1553E8F460D87428EFD923630A734846.f01t04

Compact and tunable semiconductor terahertz sources providing direct electrical control, efficient operation at room temperatures and device integration opportunities are of great interest at the present time. One of the most well-established techniques for terahertz generation utilises photoconductive antennas driven by ultrafast pulsed or dual-wavelength continuous wave laser systems, though some limitations, such as confined optical wavelength pumping range and thermal breakdown, still exist. The use of quantum dot-based semiconductor materials, having unique carrier dynamics and material properties, can help to overcome limitations and enable efficient optical-to-terahertz signal conversion at room temperatures. Here we discuss the construction of novel and versatile terahertz transceiver systems based on quantum dot semiconductor devices. Configurable, energy-dependent optical and electronic characteristics of quantum-dot-based semiconductors are described, and the resonant response to optical pump wavelength is revealed. Terahertz signal generation and detection at energies that resonantly excite only the implanted quantum dots opens the potential for using compact quantum dot-based semiconductor lasers as pump sources. Proof-of-concept experiments are demonstrated here that show quantum dot-based samples to have higher optical pump damage thresholds and reduced carrier lifetime with increasing pump power.