THz Oscillators Research Articles

Low phase noise THz generation from a fiber-referenced Kerr microresonator soliton comb

THz oscillators generated via frequency-multiplication of microwaves are facing difficulty in achieving low phase noise. Photonics-based techniques, in which optical two tones are translated to a THz wave through opto-electronic conversion, are promising if the relative phase noise between the two tones is well suppressed. Here, a THz (≈560 GHz) wave with a low phase noise is provided by a frequency-stabilized, dissipative Kerr microresonator soliton comb. The repetition frequency of the comb is stabilized to a long fiber in a two-wavelength delayed self-heterodyne interferometer, significantly reducing the phase noise of the THz wave. A measurement technique to characterize the phase noise of the THz wave beyond the limit of a frequency-multiplied microwave is also demonstrated, showing the superior phase noise of the THz wave to any other photonic THz oscillators (>300 GHz).

Spatially nonuniform oscillations in ferrimagnets based on an atomistic model

The ferrimagnets, such as GdxFeCo(1-x), can produce ultrafast magnetic switching and oscillation due to the strong exchange field. The two-sublattices macrospin model has been widely used to explain the experimental results. However, it fails in describing the spatial nonuniform magnetic dynamics which gives rises to many important phenomenons such as the domain walls and skyrmions. Here we develop the two-dimensional atomistic model and provide a torque analysis method to study the ferrimagnetic oscillation. Under the spin-transfer torque, the magnetization oscillates in the exchange mode or the flipped exchange mode. When the Gd composition is increased, the exchange mode firstly disappears, and then appears again as the magnetization compensation point is reached. We show that these results can only be explained by analyzing the spatial distribution of magnetization and effective fields. In particular, when the sample is small, a spatial nonuniform oscillation is also observed in the square film. Our work reveals the importance of spatial magnetic distributions in understanding the ferrimagnetic dynamics. The method developed in this paper provides an important tool to gain a deeper understanding of ferrimagnets and antiferromagnets. The observed ultrafast dynamics can also stimulate the development of THz oscillators.

Resonant Tunneling and Negative Differential Resistance in Black Phosphorus Vertical Heterostructures

AbstractResonant tunneling diodes with negative differential resistance (NDR) have attracted significant attention due to their unique quantum resonant tunneling phenomena and potential applications in terahertz emission/detection and high‐density logic/memory. In this paper, resonant tunneling devices, where the carriers tunnel through a hexagonal boron nitride (hBN) barrier sandwiched by two black phosphorus (BP) layers, are explored. The resonance occurs when the energy bands of the two black phosphorus layers are aligned. The conductive atomic force microscopy (CAFM) measurements reveal prominent NDR peaks with large peak‐to‐valley ratios at room temperature. It is found that the positions of the NDR peaks are very sensitive to the amplitude and the shape of the voltage waveform used in CAFM, which can be explained by the charge trapping effect. Furthermore, resonant tunneling transistors are demonstrated based on BP/hBN/BP stacks in which the locations of the NDR peaks are tunable by the electrostatic gating. As compared to the traditional tunneling diodes based on bulk materials, the tunneling devices based on thin boron nitride tunneling barrier and high mobility black phosphorus offer ultra‐high‐speed response. This feature, together with the NDR characteristics, provides the potential for applications in THz oscillators and multi‐value logic devices.

Frequency Limitations of Resonant-Tunnelling Diodes in Sub-THz and THz Oscillators and Detectors

The review outlines the basic principles of operation of resonant-tunnelling diodes (RTDs) and RTD oscillators followed by an overview of their development in the last decades. Further, we discuss different types of RTDs and RTD oscillators, the limitations of RTDs due to parasitics, inherent limitations of RTDs and operation of RTDs as detectors. We also give an overview of the present status of sub-THz and THz RTD oscillators and give several examples of their applications.

Micromagnetic modeling of terahertz oscillations in an antiferromagnetic material driven by the spin Hall effect

The realization of THz sources is a fundamental aspect for a wide range of applications. Over different approaches, compact THz oscillators can be realized taking advantage of dynamics in antiferromagnetic (AFMs) thin films driven by spin-Hall effect. Here we perform a systematic study of these THz oscillators within a full micromagnetic solver based on the numerical solution of two coupled Landau-Lifshitz-Gilbert-Slonczewski equations, for the case of ultra-thin films, i.e. when the N\'eel temperature of an AFM is substantially reduced. We have found two different dynamical modes depending on the strength of the Dzyaloshinskii-Moriya interaction (DMI). At low DMI, a large amplitude precession is excited where both the magnetizations of the sublattices are in a uniform state and rotate in the same direction. At large enough DMI, the ground state of the AFM becomes non-uniform and the antiferromagnetic dynamics is characterized by ultrafast domain wall motion.

Thermal imaging of Bi2212 THz oscillator

Bi2Sr2CaCu2O8+δ (Bi2212) mesas are promising candidates for THz oscillators, which can fill the frequency range around “THz gap”. However, it is known that Bi2212 mesas show self-heating effects (hot spots) when the current is passed along the c-axis due to the low thermal conductivity in this direction. Although several previous studies reported the relation of the hot spot and THz emission, consistent answer has not been obtained yet. In order to address this issue, imaging of temperature distributions on Bi2212 mesas is expected to be very effective. Here, we set up fluorescent thermal imaging (FTI) method for visualizing the surface temperature distribution on the Bi2212 mesa. We have succeeded in observing hot spots in the Bi2212 mesa with high spatial resolution.

A 0.54 THz Signal Generator in 40 nm Bulk CMOS With 22 GHz Tuning Range and Integrated Planar Antenna

This work presents the design and measurements of a 0.54 THz signal generator implemented in a 40 nm bulk CMOS technology. An LC-VCO operating at 180 GHz is connected to a nonlinear buffer, which is designed to generate the wanted third harmonic at 540 GHz. The third harmonic is coupled via a transformer to the output. The developed techniques are implemented on two different chips: one with a probe pad for on-wafer measurements and one with an on-chip planar dipole. The measured peak output power using a WR-1.5 probe is -31 dBm at 543 GHz, for 16.8 mW of dc power consumption. By changing value of the parasitic I-MOS varactors of the LC-VCO (“parasitic tuning”), the output frequency can be tuned from 561.5 to 539.6 GHz, resulting in a 21.9 GHz tuning range, which is the highest reported so far for CMOS THz oscillators. The 3-dB output bandwidth is 5.5 GHz. The 540 GHz signal generator with on-chip antenna is used for THz imaging without the use of substrate or focusing lenses, demonstrating some of the capabilities of CMOS for low-cost, mass-produced THz imagers.

Internally Integrated Active-Type Patch Antenna for Semiconductor Superlattice THz Oscillators

Semiconductor superlattices are well known to exhibit negative resistance i.e., gain medium like properties at high frequencies. In order to exploit these gain-like properties as an oscillator, one has to compensate very large inductive impedance (Im[Z] ≥ 150 |Re[Z]|). In this paper, we present a novel integrated active-type patch antenna to design the semiconductor superlattice THz oscillators. Within this integrated model, the active source is embedded within a benzocyclobutene dielectric cavity sandwiched between gold metal layers. The metal layer underneath provides THz/DC ground whereas the top metal functions as a radiating antenna simultaneously providing DC bias to the embedded superlattice active source. The design principle is based on satisfying the self-consistent oscillator impedance relationship, along with the efficient radiation of resonating cavity mode. The two oscillator-type active antenna configurations proposed are capable of matching impedance within few ohms of Im[Z] while achieving an antenna gain of >; 5 dBi for a TM10 cavity resonating mode.

Temperature dependences of geometrical and velocity-matching resonances in Bi2Sr2CaCu2O8+xintrinsic Josephson junctions

We study temperature dependence of geometrical (Fiske) and velocity-matching (Eck) resonances in the flux-flow state of small ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ mesa structures. It is shown that the quality factor of resonances is high at low $T$, but rapidly decreases with increasing temperature. We also study $T$ dependencies of resonant voltages and the speed of electromagnetic waves (the Swihart velocity). Surprisingly it is observed that the Swihart velocity exhibits a flat $T$ dependence at low $T$, following $T$ dependence of the $c$-axis critical current, rather than the expected linear $T$ dependence of the London penetration depth. Our data indicate that self-heating is detrimental for operation of mesas as coherent THz oscillators because it limits the emission power via suppression of the quality factor. On the other hand, significant temperature dependence of the Swihart velocity allows broad-range tunability of the output frequency.

Ultra-short suspended single-wall carbon nanotube transistors

We describe a method to fabricate clean suspended single-wall carbon nanotube (SWCNT) transistors hosting a single quantum dot ranging in length from a few 10 s of nm down to ≈3 nm. We first align narrow gold bow-tie junctions on top of individual SWCNTs and suspend the devices. We then use a feedback-controlled electromigration to break the gold junctions and expose nm-sized sections of SWCNTs. We measure electron transport in these devices at low temperature and show that they form clean and tunable single-electron transistors. These ultra-short suspended transistors offer the prospect of studying THz oscillators with strong electron-vibron coupling.

Breakpoint region in the IV-characteristics of intrinsic Josephson junctions

We study theoretically the IV-characteristics of intrinsic Josephson junctions in HTSC. We solve numerically a set of differential equations for N intrinsic Josephson junctions and investigate the nonlinear dynamics of the system. The charging effect is taken into account. We demonstrate that the breakpoint region in the current-voltage characteristics naturally follows from the solution of the system of the dynamical equations for the phase difference. In the breakpoint region the plasma mode is a stationary solution of the system and this fact might be used in some applications, particularly, in high frequency devices such as THz oscillators and mixers.

Microwave distribution in stacked Bi2Sr2CaCu2O8+x intrinsic Josephson junctions in a transmission-line geometry

The microwave distribution inside a rectangular stack (15 μm×0.72 μm×60 nm) of Bi2Sr2CaCu2O8+x intrinsic Josephson junctions (IJJs) was studied. The stack was microfabricated into a transmission-line geometry, with a-few-hundred-nm-thick Au layers deposited on the top and bottom of the stack. The microwave distribution was monitored by measuring the anomalous suppression of the tunneling critical current of the IJJs with varied microwave power at frequencies in the W band. This technique can provide valuable information on the microwave transmission modes inside the sandwiched stack of IJJs, which is utterly important for the high-frequency device applications using IJJs, such as fluxon-flow THz oscillators.

Operation Principle of Resonant Tunneling THz Oscillator at Fixed Bias Voltages

Based on time-dependent numerical simulation of an double barrier quantum well structure, a time-dependent energy coupling model is presented to account for the operational principle of a new kind of resonant tunneling THz oscillators.

High-frequency oscillators using phase-locked arrays of Josephson junctions

We present a basic description of Josephson junctions and discuss their use as GHz and THz oscillators. The resistively shunted junction model is used to calculate the available power, linewidth, and operating frequency of the oscillators. We discuss how phase-locked arrays of junctions are used to achieve higher power and narrower linewidth. Two experimental examples of phase-locked emission are shown: one from on-chip detection circuits at 150 GHz and one detected off-chip showing a 13-kHz linewidth at 88.8 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>