Terahertz Oscillators Research Articles

Generation of Terahertz Radiation Spectra by Radiation Sources Based on Solid-State Micro- and Nanostructures and Detection of Terahertz Spectra

Principles and mechanisms of generation of terahertz electromagnetic oscillations by generators based on solid-state micro- and nanostructures are discussed. Parameters of experimental models of such devices are given. A theoretical model of terahertz radiation in magnetic metal junctions is presented, and the possibility of varying the frequency of a spin-injection terahertz oscillator is demonstrated. The results of development of methods for examining the spectral characteristics of radiation in the 0.15–80 THz range with a spectral resolution of up to 1.8 GHz are detailed.

Theoretical study of terahertz active transmission line oscillator based on RTD-gated HEMT

In this paper, a new kind of terahertz oscillator is presented using plasma wave excitation in a resonant tunnel diode (RTD) gated high electron mobility transistor (HEMT). The plasma wave arising from the RTD-gated HEMT is equivalent to active transmission lines and induces negative differential conductance (NDC) of the oscillator. The proposed RTD-gated HEMT oscillator is more compact and has higher oscillation frequency than the transmission line loaded traditional RTD oscillator duo to plasma wave effect. This paper analyses and calculates the oscillation conditions, the relationships between device structures, oscillation frequency and the output power of the oscillator. The presented work may provide a new concept for fabricating terahertz oscillator.

Modulation of terahertz emission in time-domain waveform via a photoinduced phase transition in a charge ordered organic ferroelectric

The time-domain waveform of the terahertz (THz) electric field emitted from the charge-ordered organic ferroelectrics α − (ET)2I3 [ET:bis(ethylenedithio)-tetrathiafulvalene)] can be significantly modulated depending on incident fluence of femtosecond pulses. Weak irradiation triggers the emission, showing ∼1 THz oscillation in the time-domain waveform with more than 5 oscillation cycles and a corresponding narrow spectral shape. In contrast, strong irradiation almost completely suppresses the trailing portion of the waveform after the first cycle, resulting in over 3 times broader bandwidth. As revealed by time-resolved experiments, this modulation arises from the cooperative nature of photoinduced melting of the charge order accompanying quenching of infrared-activity of the intermolecular vibrations which strongly couple with the emission.

Линзовая щелевая антенна на основе тонких пленок Nb для джозефсоновского широкополосного генератора ТГц-диапазона

Abstract —An oscillator based on the distributed tunnel superconductor-insulator-superconductor junction with an ultrawide operating bandwidth of up to 100% of the central frequency seems to be a promising type of directional source of continuous electromagnetic radiation in the terahertz frequency range. In this paper, we propose a scheme of a terahertz oscillator integrated on a single microchip with a transmitting lens antenna with the slot structure in a 200-nm Nb film to radiate the signal into the open space. We also proposed and numerically simulated several designs of a planar slot antenna matched (in the input) with a Josephson oscillator and (in the output) with a silicon elliptical lens. The obtained results of the matching of the oscillator output power with the antenna of various designs operating in four frequency ranges: 250–410, 330–570, 380–520, and 420–700 GHz are presented. The antenna beam patterns and impedances are calculated as well.

Active Josephson traveling wave antennae as prospective terahertz oscillators

Active Josephson traveling wave antennae as prospective terahertz oscillators

Generation of high-power terahertz radiation by nonlinear photon-assisted tunneling transport in plasmonic metamaterials

We propose an optoelectronic terahertz oscillator based on the quantum tunneling effect in a plasmonic metamaterial, utilizing a nanostructured metal-insulator-metal (MIM) tunneling junction. The collective resonant response of meta-atoms can achieve >90% optical absorption and strongly localized optical fields within the MIM plasmonic nanojunction. By properly tailoring the radiation aperture, the nonlinear quantum conductance induced by the metamaterial-enhanced, photon-assisted tunneling may produce miliwatt-level terahertz radiation through the optical beating (or heterodyne down conversion) of two lasers with a slight frequency offset. We envisage that the interplay between photon-assisted tunneling and plasmon coupling within the MIM metamaterial/diode may substantially enhance the modulated terahertz photocurrent, and may therefore realize a practical high-power, room-temperature source in applications of terahertz electronics.

An Efficient High-Power Fundamental Oscillator Above $f_{\max }/2$ : A Systematic Design

A novel approach to design efficient high-output-power fundamental oscillators beyond $f_{\max }/2$ of the employed process is presented. The idea is to shape and maximize the unilateral power gain ( $U$ ) of the network at the desired frequency using optimum passive internal and external feedback networks. The proposed technique significantly improves the output power and dc-to-RF efficiency of the oscillator. To show the feasibility of this novel approach, a 175 GHz fundamental oscillator is designed in a 130 nm SiGe BiCMOS process ( $f_{\max }\simeq ~280$ GHz), which achieves a measured dc-to-RF efficiency of 11.7% that is markedly higher than all reported oscillators above $f_{\max }/3$ of their active device. Measurements show that the designed oscillator generates a peak power of 3 mW (4.8 dBm) with a phase noise figure of merit (FoM) of −195.4 dBc/Hz at 1 MHz offset frequency, which is the highest phase noise FoM among all reported CMOS/BiCMOS millimeter-wave and terahertz oscillators. The proposed method considers the possible process, voltage, temperature variations as well as modeling errors of the passive components in the design stage.

Measurements of temperature characteristics and estimation of terahertz negative differential conductance in resonant-tunneling-diode oscillators

The temperature dependences of output power, oscillation frequency, and current-voltage curve are measured for resonant-tunneling-diode terahertz (THz) oscillators. The output power largely changes with temperature owing to the change in Ohmic loss. In contrast to the output power, the oscillation frequency and current-voltage curve are almost insensitive to temperature. The measured temperature dependence of output power is compared with the theoretical calculation including the negative differential conductance (NDC) as a fitting parameter assumed to be independent of temperature. Very good agreement was obtained between the measurement and calculation, and the NDC in the THz frequency region is estimated. The results show that the absolute values of NDC in the THz region significantly decrease relative to that at DC, and increases with increasing frequency in the measured frequency range.

Spectral Narrowing of a Varactor-Integrated Resonant-Tunneling-Diode Terahertz Oscillator by Phase-Locked Loop

Spectral narrowing of a resonant-tunneling-diode (RTD) terahertz oscillator, which is useful for various applications of terahertz frequency range, such as an accurate gas spectroscopy, a frequency reference in various communication systems, etc., was achieved with a phase-locked loop system. The oscillator is composed of an RTD, a slot antenna, and a varactor diode for electrical frequency tuning. The output of the RTD oscillating at 610 GHz was down-converted to 400 MHz by a heterodyne detection. The phase noise was transformed to amplitude noise by a balanced mixer and fed back into the varactor diode. The loop filter for a stable operation is discussed. The spectral linewidth of 18.6 MHz in free-running operation was reduced to less than 1 Hz by the feedback.

Anesthetic Alterations of Collective Terahertz Oscillations in Tubulin Correlate with Clinical Potency: Implications for Anesthetic Action and Post-Operative Cognitive Dysfunction

Anesthesia blocks consciousness and memory while sparing non-conscious brain activities. While the exact mechanisms of anesthetic action are unknown, the Meyer-Overton correlation provides a link between anesthetic potency and solubility in a lipid-like, non-polar medium. Anesthetic action is also related to an anesthetic’s hydrophobicity, permanent dipole, and polarizability, and is accepted to occur in lipid-like, non-polar regions within brain proteins. Generally the protein target for anesthetics is assumed to be neuronal membrane receptors and ion channels, however new evidence points to critical effects on intra-neuronal microtubules, a target of interest due to their potential role in post-operative cognitive dysfunction (POCD). Here we use binding site predictions on tubulin, the protein subunit of microtubules, with molecular docking simulations, quantum chemistry calculations, and theoretical modeling of collective dipole interactions in tubulin to investigate the effect of a group of gases including anesthetics, non-anesthetics, and anesthetic/convulsants on tubulin dynamics. We found that these gases alter collective terahertz dipole oscillations in a manner that is correlated with their anesthetic potency. Understanding anesthetic action may help reveal brain mechanisms underlying consciousness, and minimize POCD in the choice and development of anesthetics used during surgeries for patients suffering from neurodegenerative conditions with compromised cytoskeletal microtubules.

Frequency-tunable resonant-tunneling-diode terahertz oscillators applied to absorbance measurement

We show that frequency-tunable terahertz oscillators with resonant-tunneling diodes (RTDs) can be applied to absorbance measurement, using allopurinol as a specimen, which has a characteristic absorption spectra at around 700 GHz. The RTD oscillators used in this experiment are seven elements covering from 420 to 970 GHz. The result was in good agreement with that of time-domain spectroscopy usually used in the terahertz frequency region.

Wide frequency tuning in resonant-tunneling-diode terahertz oscillator using forward-biased varactor diode

We report wide frequency tuning in a resonant-tunneling-diode (RTD) terahertz oscillator with a small change in the forward bias of a varactor diode integrated with the RTD. The obtained frequency change is 600–730 GHz with the voltage change of 0.4–0.8 V. The rate of this frequency change with bias voltage is much higher than that in the previously reported reverse-bias operation (680–600 GHz with −4 to +0.4 V in the present device). The result is well explained by theoretical analysis using equivalent circuits of the varactor diode and air bridge. The possibility of wider frequency tuning is also discussed.

Structures and Physics of Resonant Tunneling Diodes for Terahertz Oscillators

Resonant tunneling diode (RTD) is one candidate for compact and coherent room-temperature terahertz sources. In this paper, we introduce our recent results on RTD terahertz oscillators, including RTD materials and structures for high frequency oscillation. Up to now, oscillation at 1.98 THz has been achieved at room temperature. We also described structures for high output power and some applications of RTD oscillators.

Diced and grounded broadband bow‐tie antenna with tuning stub for resonant tunnelling diode terahertz oscillators

Radiation from antennas integrated with indium phosphide (InP)-based resonant tunnelling diode (RTD) oscillators is mainly through the substrate because of the effects of the large dielectric constant. Therefore, hemispherical lenses are used to extract the signal from the backside of the substrate. In this study the authors present a broadband bow-tie slot antenna with a tuning stub which is diced and mounted on a ground plane to alleviate the substrate effects. Here, the large dielectric constant substrate around the antenna conductor is removed. In addition, the ground plane underneath the diced substrate acts as a reflector and, ultimately, the antenna radiates to the air-side direction. Antenna integration with RTD oscillators is described in this study as well. Two-port bow-tie slot antennas were designed and characterised and showed the suitability of integration with power combining RTD oscillator circuits which are based on mutual coupling. The antennas were fabricated using electron beam lithography on a semi-insulating InP substrate. Simulated and measured bandwidth almost extends the entire frequency range 230–325 GHz. Simulations shows air-side radiation pattern and antenna gain of around 11 dB at 280 GHz. Simulations also show that the antenna may be fed with a 50-Ω or 30-Ω feed line, i.e. suitable feed lines, without compromising its performance which may prove beneficial for optimum loading of RTD oscillators.

Room-Temperature Oscillation of Resonant Tunneling Diodes close to 2 THz and Their Functions for Various Applications

Compact and coherent source is a key component for various applications of the terahertz (THz) wave. We report on our recent results of THz oscillators using resonant tunneling diodes (RTDs). To achieve high-frequency oscillation, the electron delay time of RTD was reduced with a narrow quantum well and an optimized collector spacer thickness. Conduction loss at the air bridge connecting RTD and slot antenna, which works as a resonator and a radiator, was also reduced. By these structures, a fundamental oscillation up to 1.92 THz was obtained at room temperature. Theoretical calculation shows that an oscillation over 2 THz is further expected by improved structures of RTD and antenna. Using the offset slot antenna and two-element array configuration, high output power of 0.61 mW was obtained at 620 GHz. A direct intensity modulation of RTD oscillators up to 30 GHz, which is useful for high-speed wireless data transmission, was demonstrated. By the integration of a varactor diode, wide frequency sweep of 580–700 GHz in a single device and 580–900 GHz in a four-element array were also demonstrated. This result expands possible applications of RTD oscillators.

Wireless data transmission of 34 Gbit/s at a 500‐GHz range using resonant‐tunnelling‐diode terahertz oscillator

A resonant-tunnelling-diode terahertz oscillator capable of wide-band direct modulation was fabricated, and wireless data transmission at a 500-GHz range was demonstrated. An error-free transmission up to the data rate of 22 Gbit/s and a transmission with a bit error rate less than the forward error correction limit up to 34 Gbit/s were achieved.

Plasmonic noise of field-effect transistors operating at terahertz frequencies

We analyze electronic noise in field-effect transistors associated with plasma waves in the terahertz frequency domain by using a numerical approach based on the coupled hydrodynamic and Poisson equations. The current and voltage noise spectra, calculated by means of the transfer impedance method, exhibit a series of peaks associated with two- and three-dimensional plasma resonances. The two-dimensional plasma peaks have been found to depend on the channel length including gated and ungated regions. Under asymmetric boundary conditions, electrical instabilities characterized by terahertz oscillations of the drain voltage and gate current can be reached at sufficiently high values of the total drain current. A resonant enhancement of the plasma peaks in the noise spectra has been found to be a precursor of the instability onset. Through a spatial analysis of the local contributions we have shown that the main contribution to the total noise at frequencies near the plasma resonance comes from the ungated n-region placed between the source and the gate contacts.

Oscillation up to 1.92 THz in resonant tunneling diode by reduced conduction loss

A large increase in oscillation frequency was achieved in resonant-tunneling-diode (RTD) terahertz oscillators by reducing the conduction loss. An n+-InGaAs layer under the air-bridge electrode connected to the RTD was observed to cause a large conduction loss for high-frequency current due to the skin effect. By introducing a new fabrication process removing the InGaAs layer, we obtained 1.92-THz oscillation, which extended the highest frequency of room-temperature electronic single oscillators. Theoretical calculations reasonably agreed with the experiment, and an oscillation above 2 THz is further expected with an improved structure of the slot antenna used as a resonator and radiator.

基于史密斯-帕赛尔效应的太赫兹振荡器的原理与特性分析

基于史密斯-帕赛尔效应的太赫兹振荡器的原理与特性分析

Numerical Simulation of Plasma Oscillation in 2-D Electron Gas Using a Periodic Steady-State Solver

The terahertz oscillation due to the plasma instability in the 2-D electron gas is numerically investigated using an in-house developed device simulator, G-Device. In order to overcome practical difficulties in the conventional transient simulation, a periodic steady-state (PSS) solver is implemented. The full Newton–Raphson scheme is applied to a set of discretized equations sampled at various time points during an oscillation period. Numerical results show that there is a threshold value of the injection velocity that allows the PSS oscillation. Moreover, the impact of the drain load resistance on the oscillation amplitude is estimated. It is found that considerable amplitude of the voltage oscillation can be achieved even with finite drain resistances. Although the growth rate of the voltage oscillation exhibits its maximum value with the infinite drain resistance, the maximum output power is obtained at a finite drain resistance.