Terahertz Oscillators Research Articles

Analysis of the eight parameter variation of the resonant tunneling diode (RTD) in the rapid thermal annealing process with resistance compensation effect

The rapid thermal annealing process is a key technology to control the parameters of the resonant tunneling diode (RTD) and to achieve high performance for the device. In this paper, the rapid thermal annealing process on the planar RTD has been investigated experimentally. In the experiment, the annealing sample chips of different annealed times have been recorded from the annealing equipment and their I–V characteristics have been measured accordingly. From the I–V characteristics, the negative resistance and the series resistance of the RTD can be obtained. Thus, the relationship between these parameters and annealing time can be established. Finally, by analyzing the concept of the resistance compensation effect, this study explains fully and in detail the dependency of the RTD parameter variation on the annealing time. VP and Vi are significantly reduced, greatly lowering RS, which in return also reduces the heat loss of the circuit and the power consumption of the RTD digital circuits as well as the RTD terahertz oscillator. As VV decreases, negative resistance RN is increased, and thus, the output power of the RTD terahertz oscillator is increased. These results are very useful in the study of RTD devices and fabrication technology.

Physical-Based Simulation of the GaN-Based Grooved-Anode Planar Gunn Diode.

In this paper, a novel gallium nitride (GaN)-based heterostructure Gunn diode is proposed for the first time to enhance the output characteristics of Gunn oscillation waveforms. A well-designed grooved anode contact is adopted to separate the long-channel diode into two short-channel diodes in parallel. If the grooved anode contact is positioned in the middle of the device, the output power nearly doubles in the grooved-anode diode compared with the single-channel ones, as does the output frequency. Based on the numerical results, the best output characteristics are obtained at the 2.0-µm symmetrical grooved-anode diode, which produces nearly 5.48 mW of power at the fundamental frequency of 172.81 GHz, with 3.13% efficiency of power conversion. If the grooved anode contact is not positioned in the middle of the diode, the harmonic frequency would be enhanced. The GaN heterostructure grooved-anode Gunn diode has been demonstrated to be an excellent solid-state source of terahertz oscillator.

Hydrodynamic simulation of chaotic dynamics in InGaAs oscillator in terahertz region**Project supported by the National Natural Science Foundation of China (Grant No. 11604126) and China Scholarship Council (Grant No. 201808695016).

Hydrodynamic calculations of the chaotic behaviors in n+nn+ In0.53Ga0.47As devices biased in terahertz (THz) electric field have been carried out. Their different transport characteristics have been carefully investigated by tuning the n-region parameters and the applied ac radiation. The oscillatory mode is found to transit between synchronization and chaos, as verified by the first return map. The transitions result from the mixture of the dc induced oscillation and the one driven by the ac radiation. Our findings will give further and thorough understanding of electron transport in In0.53Ga0.47As terahertz oscillator, which is a promising solid-state THz source.

Terahertz emission spectrum of polar antiferromagnet Fe<sub>2</sub>Mo<sub>3</sub>O<sub>8</sub>

In polar materials, the transition of electrons in momentum space will change the spontaneous polarization. When excited by femtosecond pulse laser, the transient modulation of the electric polarization will radiate electromagnetic wave at terahertz frequency. In a magnetic ordered system, the coherent excited spin wave radiates electromagnetic waves of the same frequency in the process of precession and relaxation. The investigation of the terahertz emission spectra of these materials not only helps us to understand the ferroelectric and magnetic ordered dynamic processes of materials, but also provides a reference for searching for new terahertz sources. We study the terahertz emission spectrum of the polar antiferromagnet Fe<sub>2</sub>Mo<sub>3</sub>O<sub>8</sub>. Under the pumping of 800 nm laser, electrons in the material are excited across the band gap leading the electric polarization to be ultra-fast modulated. The broadband terahertz excitation spectrum from 0.1 to 3.5 THz is observed, and the direction of the terahertz electric field is along the inherent electric polarization direction of the material. After entering into the magnetic order state, two new single-frequency terahertz oscillations are observed, located at 1.25 THz and 2.7 THz respectively, which correspond to the excitation of the two antiferromagnetic spin waves of Fe<sub>2</sub>Mo<sub>3</sub>O<sub>8</sub>.

Синтезатор дискретной сетки частот на основе антиферромагнитного спинтронного осциллятора

Dynamics of an antiferromagnetic spintronic oscillator phase-locked loop-based frequency synthesizer, which generates terahertz oscillations in a wide frequency range under the influence of direct current, was investigated. By the method of slowly varying amplitudes, a dependence of the gain on the ring of the phase locked loop on the difference in frequencies of synchronized oscillations at different division coefficients in the feedback circuit is obtained. It is shown that as the division coefficient increases, the band of synchronized oscillations decreases with unchanged parameters of the antiferromagnet-based spintronic oscillator.

Theoretical analysis of subharmonic injection locking in resonant-tunneling-diode terahertz oscillators

Subharmonic injection locking of a resonant-tunneling-diode terahertz oscillator is theoretically analyzed for the purpose of output phase control which can be utilized for beam steering in a phased array. Analytical formulae for the dependences of the locking range and output phase on input and output powers, parameters of the resonator, and bias voltage are obtained. The calculated locking range for the second subharmonic injection reasonably agrees with the reported measurement.

Electromagnetic physical modeling of a gallium nitride distributed transferred electron based planar waveguide structure THz oscillator

The potential of a planar waveguide structure terahertz oscillator based on a gallium nitride distributed transferred electron device is theoretically investigated. The circuit numerical physical modeling relies on a two-dimensional time-domain electromagnetism/transport simulator. It is based on the coupled solution of the Maxwell and energy-momentum macroscopic transport equations. The study is focused on the analysis, from the space-time electromagnetic and electron transport quantities, of the complex CW operation of an oscillator, designed and DC biased, to optimally operate at one terahertz. The analysis is performed following a full electromagnetic approach in the time and frequency domain, at the local scale, for the description of the physical phenomena, as well as at the functional scale in order to obtain the quantities interesting the oscillator designer and user.

Resonant-tunneling-diode terahertz oscillator with a cylindrical cavity for high-frequency oscillation

We proposed and fabricated resonant-tunneling-diode (RTD) terahertz oscillators integrated with a cylindrical cavity. Oscillation frequency of 3 THz is expected from theoretical analysis. As a preliminary experiment, the proposed oscillator was fabricated using electron-beam lithography with a three-layer-resist process. An oscillation of up to 1.79 THz was obtained in the fabricated oscillators, which was lower than the theoretical expectation. This was because of a parasitic capacitance of the metal post connecting the cavity and the RTD. Theoretical calculations, including this parasitic capacitance, agreed well with the experiment. The parasitic capacitance can be suppressed by adding a simple process to the cavity fabrication.

Hydrodynamic simulation of InGaAs terahertz oscillations

Terahertz (THz) oscillations in n[Formula: see text]nn[Formula: see text] In[Formula: see text]Ga[Formula: see text]As diodes have been simulated with the use of a time-dependent hydrodynamic model. Under proper biased voltage and doping concentration, THz self-oscillations show up. The current self-oscillations originate from the formation and propagation of electric field domains in In[Formula: see text]Ga[Formula: see text]As diodes. The In[Formula: see text]Ga[Formula: see text]As device studied here may be presented as an excellent candidate as a solid-state THz source for monolithic integration.

About 250/285 GHz push–push oscillator using differential gate equalisation in digital 65‐nm CMOS

This study presents a push-push oscillator architecture based on differential gate equalisation to enhance the oscillation frequency while providing relatively high output power with ultra-compact layout form factor. The frequency enhancement is derived as a function of the equivalent RLC model of the oscillator's main constituents. The proposed principle is applied to a terahertz oscillator in the 200-300 GHz range to mitigate the excessive substrate and skin effect losses in standard digital 65-nm complementary metal-oxide-semiconductor technology at such high frequencies. The design concept is validated using two single-stage push-push oscillators. The first oscillator shows -8.1 dBm output power at 250 GHz oscillation frequency and -106.8 dBc/Hz phase noise at 10 MHz offset while consuming 76 mW power from 1.5 V DC supply voltage. The chip area is 200 × 250 μm 2 . The second oscillator provides -14.8 dBm output power at 285 GHz and -106 dBc/Hz phase noise at 10 MHz offset with 80 mW power consumption from 1.5 V DC supply. The chip area is 200 × 200 μm 2 .

Generation of Terahertz oscillations by thin superconducting film in fluctuation regime

Explicit analytical expressions for conductivity of a superconducting film above and below critical temperature in an arbitrary electric field are derived in the frameworks of the time dependent Ginzburg-Landau theory. It is confirmed that slightly below critical temperature the differential conductivity of superconducting film can become negative for small enough values of electric field. This fact may cause generation of electromagnetic oscillations if the superconducting film is appropriately coupled of with a resonator. Their maximal frequency is proportional to the value of critical temperature of superconducting transition. The obtained results can stimulate the development of Terahertz generators on the basis of high temperature superconducting films.

Nonlinear Dynamics of an Antiferromagnetic Spintronic Oscillator

We study nonlinear dynamics of the spintronic nanosized antiferromagnetic terahertz oscillator consisting of an antiferromagnetic layer with easy-plane anisotropy (hematite) and a normal-metal (platinum) layer. Normal oscillation frequencies, namely, ferromagnetic and antiferromagnetic (terahertz) ones, are found. Their dependence on the value of a static magnetic field parallel to the sample plane is obtained. An approximate mathematical model in the form of the equations for the Neel-vector rotation angle in the azimuthal plane is developed for describing the oscillator dynamics. The adjustment characteristic, i.e., the dependence of the antiferromagnetic-mode frequency on the value of the direct current flowing in the platinum layer is obtained.

On The Generation of Terahertz Pulses Using Thermoelastic Effect

Time-resolved terahertz spectroscopy can be proposed as a method of non-destructive testing of various parts, for example, aircraft composite materials. The theoretical background and the possibility of exciting pulses of electromagnetic waves in the range of terahertz oscillations using picosecond acoustic pulses excited by laser pulses based on the thermoelastic effect are studied.

A 269‐278 GHz voltage‐controlled oscillator with uniform output power of 5 dBm

AbstractA terahertz oscillator design with an emphasis on the output power performance is presented. The 270‐GHz oscillator fabricated using InP 250‐nm DHBT technology adopts a common‐base cross‐coupled configuration. The design focus is on the input varactor termination by utilizing simulated power and frequency load‐pull contours. The measurement results show an improved peak output power of 5.8 dBm with an outstanding power variation of only 0.8 dB while maintaining moderate frequency‐tuning range of 269‐278 GHz.

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.

Off-axis terahertz parametric oscillator

We study an off-axis terahertz (THz) parametric oscillator and show its superior performance over a conventional THz parametric oscillator. Furthermore, by pumping an off-axis lithium-niobate oscillator with a pulsed Nd:YAG laser having an axial-mode spacing matched to the THz oscillator’s mode spacing, we generated an ultra-broad red-shifted frequency comb between 1065 and 1085 nm at the output, which corresponds to a four-octave-spanning THz-frequency comb in the idler spectrum. We also demonstrated a signal-seeded off-axis THz parametric oscillator, which generates 8 nJ narrow-line radiation at 2.1 THz and achieved 46% pump depletion in a 6.5 cm long crystal with only 4 mJ pump energy in a 460 ps pump pulse width. Furthermore, tuning between 3.6 and 4.9 THz for the generated radiation is also demonstrated in this work.

Hydrodynamic simulations of terahertz oscillation in double-layer graphene * * Project supported by the National Natural Science Foundation of China (No. 11604126).

We have theoretically studied current self-oscillations in double-layer graphene n+nn+ diodes driven by dc bias with the help of a time-dependent hydrodynamic model. The current self-oscillation results from resonant tunneling in the double-layer graphene structure. A detailed investigation of the dependence of the current self-oscillations on the applied bias has been carried out. The frequencies of current self-oscillations are in the terahertz (THz) region. The double-layer graphene n+nn+ device studied here may be presented as a THz source at room temperature.

Repeatable Room Temperature Negative Differential Resistance in AlN/GaN Resonant Tunneling Diodes Grown on Sapphire

AbstractResonant tunneling diodes (RTDs) are candidates for high power terahertz oscillators, and form the basis for understanding the quantum confinement and vertical transport in quantum structures such as quantum cascade lasers and quantum cascade detectors. In this work, repeatable negative differential resistance (NDR) is achieved in AlN/GaN RTDs grown on sapphire substrate by plasma‐assisted molecular‐beam epitaxy. Two reproducible NDR regions sequentially following two preresonance replicas are demonstrated at room temperature. A current region exhibiting negative correlation with temperature and oscillation‐like features is first identified under reverse bias, which is interpreted as a combined contribution of weak resonant tunneling channels through different bound states in the well. The revealed peak‐to‐valley current ratio ranges from 1.1 to 1.8, and peak current density ranges from 5 to 164 kA cm−2. Using an analytic model, resonant tunneling transports in both bias directions are quantitatively characterized and show good agreements with experiment results, demonstrating the capability of accurate quantum transport control using III‐nitride grown on sapphire substrate. The findings will promote the implementation of low cost III‐nitride monolithic microwave circuits and resonant tunneling structures based on sapphire, SiC, and even silicon substrates.

Slot Lens Antenna Based on Thin Nb Films for the Wideband Josephson Terahertz Oscillator

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.

Resonant-tunneling-diode terahertz oscillator integrated with a radial line slot antenna for circularly polarized wave radiation

We proposed a resonant-tunneling-diode (RTD) terahertz (THz) oscillator for circularly polarized radiation, which is convenient for THz wireless communication. The oscillator is composed of an RTD, a cross-slot resonator, and a radial line slot antenna (RLSA). The output of the RTD in the cross-slot resonator laterally propagates in the substrate, and is radiated from the RLSA in the upward direction of the substrate as a circular wave. In theoretical calculations, the axial ratio of the circular polarization was 1.6 dB and the directivity of the radiation pattern was 23 dBi at the oscillation frequency of 500 GHz. In a preliminary experiment, we fabricated this oscillator, and obtained circularly polarized THz radiation with the axial ratio of 2.2 dB and the directivity of 15 dBi for the oscillation frequency of 507 GHz. The measured radiation pattern was wider than the theoretical calculation, probably because the radiation from the substrate edge is mixed into the output from the RLSA. Using the fabricated oscillator, we demonstrated 1 Gbps wireless data transmission. The observed bit error rate was almost independent of the rotation angle of the oscillator around the transmission direction, indicating an alignment-free transmission for the polarization axis.