Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/iedm50854.2024.10873497

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz60956.2024.10697695

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz60956.2024.10697537

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz60956.2024.10697566

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz60956.2024.10697846

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz60956.2024.10697719

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Oscillators based on resonant tunneling diodes (RTDs) are able to reach the highest oscillation frequency among all electronic THz emitters. However, the emitted power from RTDs remains limited. Here, we propose linear RTD oscillator arrays capable of supporting coherent emission from both in-phase and anti-phase coupled modes. The oscillation modes can be selected by adjusting the mesa areas of the RTDs. Both the modes exhibit constructive interference at different angles in the far field, enabling high-power emission. Experimental demonstrations of coherent emission from linear arrays containing 11 RTDs are presented. The anti-phase mode oscillates at ∼450 GHz, emitting about 0.7 mW, while the in-phase mode oscillates at around 750 GHz, emitting about 1 mW. Moreover, certain RTD oscillator arrays exhibit dual-band operation: changing the bias voltage allows for controllable switching between the anti-phase and in-phase modes. Upon bias sweeping in both directions, a notable hysteresis feature is observed. Our linear RTD oscillator array represents a significant step forward in the realization of large arrays for applications requiring continuous-wave THz radiation with substantial power.

DOI： 10.1063/5.0213695

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

During the last two decades, rapid advancements in RT oscillators that use resonant tunneling diodes (RTDs) have been reported, with operations approaching the limits of electronic device oscillators. Although RTD devices are known for HF operation, milliwatt-level high-output powers have been recently obtained using a single device. Moreover, interesting operations using feedback and injection locking phenomena are also emerging. This paper outlines the basic oscillation principles, oscillation characteristics, and applications of RTD devices. Unlike previous reviews, the basic parts include harmonic signal generation, the construction of resonators and antennas, and bias circuits, which have been newly summarized. A graphical method for determining oscillation is introduced, and the oscillator characteristics are summarized in terms of new indicators, such as power density. This paper also includes the modulation characteristics of the intrinsic part of the device, spectral changes owing to feedback, and the characteristics of the RTD device as a receiver.

DOI： 10.35848/1882-0786/ad5c27

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ad5c27/pdf

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We proposed and fabricated a terahertz resonant-tunneling-diode oscillator integrated with two offset slot-ring antennas for high-output power and high-directivity radiation. In this device, the length of the antenna, approximately half the wavelength of the oscillation frequency, enables efficient terahertz radiation. The increased radiation conductance, resulting from the offset and the two slot-ring antennas, enables higher output power. Additionally, radiation directivity can be improved by using two slot-ring antennas. The fabricated device generated high-power oscillation of 1.29 mW at 412 GHz. This is the highest output power of a single electronic device oscillator in the 400 GHz range.

DOI： 10.35848/1882-0786/ad308b

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ad308b/pdf

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/gemic59120.2024.10485344

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

This study investigates the heat dissipation structures for resonant tunneling diodes (RTDs). The n⁺-InGaAs conductive layer beneath the RTD double-barrier layer, which possesses low thermal conductivity and disrupting heat dissipation, has been replaced with n⁺-InP, which has high thermal conductivity. We manufactured simple RTD mesa structures with varying areas to analyze the impact of heat dissipation. Additionally, we conducted a study to explore the relationship between mesa area and power consumption at the RTD mesa under current–voltage measurements. The results clearly indicate that the proposed structure, incorporating an n⁺-InP layer, can function over an area twice as large without experiencing heat-induced destruction. By integrating this proposed structure, we successfully fabricated terahertz oscillators equipped with rectangular-cavity resonators. These oscillators achieved relatively high output power, approximately 0.2 mW was achieved at a frequency of 0.53 THz, all without any heat-induced damage, even within a large-area RTD device.

DOI： 10.35848/1347-4065/ad169a

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ad169a/pdf

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

The use of terahertz (THz) electromagnetic waves is anticipated for high-capacity communications, sensing, imaging, and spectroscopic analysis. However, the output power of semiconductor THz signal sources remains low, limiting THz applications. Resonant tunneling diodes (RTDs) are promising candidates for THz sources due to their compactness and high-frequency operation [1]. For high-power operation, RTD oscillator arrays have been studied for power combination. Recently, a very high output power of 11.8 mW at 450 GHz was achieved by a 36-element coherent array using mutual injection locking [2]. In addition to power combination using arrays, attempts to increase the output power of a single oscillator have also been conducted. In this presentation, recent studies on high-power RTD THz oscillators will be introduced.

DOI： 10.1364/jsapo.2024.18p_b2_1

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/icicdt59917.2023.10332334

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz57677.2023.10299317

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz57677.2023.10299163

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz57677.2023.10298970

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz57677.2023.10299125

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz57677.2023.10299277

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tthz.2023.3270672

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We developed a two-coupled resonant-tunneling-diode (RTD) terahertz (THz) oscillator with a high output power to fill the THz gap. We arranged two RTD mesas in a low-loss air-bridged transmission line for strong mutual coupling and generation of a unique operation mode and integrated a planar ring-slot antenna for efficient THz radiation while satisfying the impedance matching condition. The device structure was fabricated using a multilayer resist process and a carefully controlled wet etching process. The fabricated device exhibited coherent operation and a high output power of approximately 0.24 mW at a high frequency of 925 GHz.

DOI： 10.35848/1882-0786/acdb2d

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/acdb2d/pdf

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tthz.2023.3239809

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/iedm45625.2022.10019547

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Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.2632368

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Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We investigate the coherent coupling of metamaterial resonators with hydrogen-like boron acceptors in Si at cryogenic temperatures. When the resonance frequency of the metamaterial, chosen to be in the range 7–9 THz, superimposes the transition frequency from the ground state of the acceptor to an excited state, Rabi splitting as large as 0.4 THz is observed. The coherent coupling shows a feature of cooperative interaction, where the Rabi splitting is proportional to the square root of the density of the acceptors. Our experiments may help to open a possible route for the investigation of quantum information processes employing strong coupling of dopants in cavities.

DOI： 10.1364/ol.466392

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tthz.2022.3170516

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50927.2022.9895697

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50927.2022.9896012

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50927.2022.9895508

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50927.2022.9895762

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s10762-022-00863-5

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Other Link： https://link.springer.com/article/10.1007/s10762-022-00863-5/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

A structure-simplified resonant tunneling diode (RTD) oscillator eliminates metal–insulator–metal capacitors, resulting in a simple and brief fabrication process. However, the structure dependence of oscillation characteristics has not been identified. We revealed the structure dependence and obtained an oscillation frequency of up to ∼1 THz using a short antenna. We found that an increase in radiation conductance using offset-fed structure combined with coplanar stripline antennas is effective for high output power, and achieved up to ∼220 μW of output power at 500 GHz. We also clarified the dependence of the oscillation frequency on the stabilization resistor.

DOI： 10.35848/1882-0786/ac5b32

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac5b32/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Vacuum Society  

More and more novel applications are appearing in the almost-unexplored-up-to-recent-times frequency range of around 0.3–3 terahertz (THz), where sub-millimeter radio waves meet far-infrared optical waves. Resonant tunneling diodes (RTDs) are considered one of the promising compact and coherent room temperature signal sources for terahertz applications. In this work, the fabrication process and fabrication challenges for an RTD THz oscillator with a cylindrical resonant cavity are discussed. Successful fabrication of 3D metallic structures with a height of 2–5 μm and a feature size down to 0.5 μm was achieved by combining the traditional trilayer resist process with the dose-modulated (gray-tone) electron-beam (EB) lithography process. It was shown that two-step EB exposure could be used in thick (>2.4 μm) PMMA resist to achieve predictable and controllable fabrication of V-shaped metallic structures with lateral sizes down to 0.5 μm. Applicability of the described fabrication approach was proven by the measurement of oscillation characteristics for the fabricated RTD THz oscillators. Successful operation of the RTD oscillator devices confirms good electrical contact between the top contact of the RTD mesa structure and the RTD pillar structure as well as between the resonant cavity and antenna parts. The fabrication approach described in this work allowed us to eliminate parasitic capacitance formed around RTD mesa in the first fabrication trial and achieve a frequency increase of up to 200 GHz for RTD THz oscillators operating at frequencies 1.5–1.7 THz. The described fabrication approach may also be applicable for the fabrication of 3D metallic structures with a feature size less than 0.5 μm and a height more than 2 μm with EB energies above 50 keV.

DOI： 10.1116/6.0001647

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Publishing type：Part of collection (book)   Publisher：Springer International Publishing  

DOI： 10.1007/978-3-030-73738-2_24

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

An increasing number of novel applications has appeared in the previously unexplored frequency range of 0.3–3.0 THz, where sub-mm radio waves meet far-infrared optical waves. Resonant-tunneling diodes (RTDs) are considered as one of the promising compact and coherent room-temperature signal sources for terahertz (THz) applications. In this work, dependencies of output power on the resonator dimensions and output power limitation factors are analyzed for an RTD THz oscillator with a cylindrical cavity resonator, which can oscillate above 2 THz. Analysis of the output power dependencies on radius and height of the cylindrical resonant cavity shows that a decrease in the resonant cavity size could lead to an increase in the output power at a fixed frequency for this type of RTD oscillator. Moreover, in addition to the high-frequency oscillation limit, a rapid decrease in the output power in the lower frequency region was found for oscillator devices with larger RTD mesa areas. Rapid decrease in output power may occur even at frequencies around 1 THz, which could considerably limit the operational range for RTD oscillators with cavity-type resonators. To determine an approach for output power optimization and understand the nature of output power drop at lower frequencies, the output power behavior and connection with resonant cavity parameters were explained in detail. Results of the output power analysis and numerical calculation indicate that for the RTD structure and circular-resonator geometry considered in the present study, output powers up to 45 μW at 1.5 THz and up to 0.25 μW at 2.5 THz could be expected for single oscillator design.

DOI： 10.35848/1347-4065/ac3721

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac3721/pdf

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567337

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567471

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567234

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567210

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567317

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567063

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9566843

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/irmmw-thz50926.2021.9567622

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electronics, Information and Communications Engineers (IEICE)  

DOI： 10.1587/transele.2020ecs6020

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Publisher：Wiley  

DOI： 10.1002/9781119460749.ch11

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/led.2021.3082577

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Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

We used a resonant-tunneling-diode (RTD) oscillator as the source of a terahertz-wave radar based on the principle of the swept-source optical coherence tomography (SS-OCT). Unlike similar reports in the terahertz range, we apply the stepwise frequency modulation to a subcarrier obtained by amplitude modulation instead of tuning the terahertz carrier frequency. Additionally, we replace the usual optical interference with electrical mixing and, by using a quadrature mixer, we can discriminate between negative and positive optical path differences, which doubles the measurement range without increasing the measurement time. To measure the distance to multiple targets simultaneously, the terahertz wave is modulated in amplitude at a series of frequencies; the signal returning from the target is detected and homodyne mixed with the original modulation signal. A series of voltages is obtained; by Fourier transformation the distance to each target is retrieved. Experimental results on one and two targets are shown.

DOI： 10.3390/s21134367

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Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Optical imaging is a powerful tool for nondestructive inspection, with high spatial resolution and low invasiveness. As light–material interactions vary a great deal depending on the wavelength, it is difficult to select the best imaging wavelength without prior knowledge of the optical properties of the material. To overcome this difficulty, we constructed a hybrid optical imaging system using three different wavelengths: near-infrared (NIR), mid-infrared (MIR), and terahertz (THz) regions. The same imaging optics were integrated with different light sources and detectors. Depending on the light–material interaction and detection sensitivity, NIR and THz imaging indicated some potential for nondestructive inspection, but MIR imaging showed difficulty. A combination of NIR and THz imaging will be a powerful tool for optical nondestructive inspection.

DOI： 10.1364/ao.415131

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Other Link： https://opg.optica.org/viewmedia.cfm?URI=ao-60-10-B100&seq=0

Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

A compact source is important for various applications utilizing terahertz (THz) waves. In this paper, the recent progress in resonant-tunneling diode (RTD) THz oscillators, which are compact semiconductor THz sources, is reviewed, including principles and characteristics of oscillation, studies addressing high-frequency and high output power, a structure which can easily be fabricated, frequency tuning, spectral narrowing, different polarizations, and select applications. At present, fundamental oscillation up to 1.98 THz and output power of 0.7 mW at 1 THz by a large-scale array have been reported. For high-frequency and high output power, structures integrated with cylindrical and rectangular cavities have been proposed. Using oscillators integrated with varactor diodes and their arrays, wide electrical tuning of 400–900 GHz has been demonstrated. For spectral narrowing, a line width as narrow as 1 Hz has been obtained, through use of a phase-locked loop system with a frequency-tunable oscillator. Basic research for various applications—including imaging, spectroscopy, high-capacity wireless communication, and radar systems—of RTD oscillators has been carried out. Some recent results relating to these applications are discussed.

DOI： 10.3390/s21041384

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