Terahertz Oscillators Research Articles

Efficient Fabrication of Intrinsic-Josephson-Junction Terahertz Oscillators with Greatly Reduced Self-Heating Effects

The hunt is on for sources of photons to fill the so-called ``terahertz gap'' in the electromagnetic spectrum, for applications spanning medical diagnosis, imaging, security, ultrahigh-speed communication, and biotechnology. The authors suppress the inevitable self-heating of intrinsic Josephson junctions in Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$, realizing an emitter with strongly expanded tunability and a range from 0.3 to 1.63 THz---and potentially extending to 15 THz, which would cover almost the entire gap with one device.

Enhancement of the output power of terahertz folded waveguide oscillator by two parallel electron beams

A novel two-beam folded waveguide (FW) oscillator is presented for the purpose of gaining higher power with a small-size circuit compared with the normal FW oscillator. The high-frequency characteristics of the two-beam FW, including dispersion and interaction impedance, were investigated by the numerical simulation and compared with the one-beam FW. The radio-frequency loss of the two-beam FW was also analyzed. A 3-D particle-in-cell code CHIPIC was applied to analyze and optimize the performance of a G-band two-beam FW oscillator. The influences of the distance between the two beam tunnels, beam voltage, the number of periods, magnetic field, radius of beam tunnel, and the packing ratio on the circuit performance are investigated in detail. Compared with a one-beam circuit, a larger output power of the two-beam circuit with the same beam power was observed by the simulation. Moreover, the start-oscillation current of two-beam circuit is much lower than the one-beam circuit with better performance. It will favor the miniaturized design of the high-power terahertz oscillator.

Ultrafast dynamics and THz oscillation in [Co/Pd]8 multilayers

Abstract We report the experimental observation of ultrafast demagnetization associated with a significant terahertz (THz) emission in [Co/Pd] 8 multilayers (MLs) with strong perpendicular magnetic anisotropy (PMA) by using a time-resolved magneto-optical Kerr effect (TR-MOKE) magnetometer. The THz emission is associated with subsequent remagnetization which is coupled with a collective picosecond magnetization dynamics and generation of acoustic waves in these multilayers. The demagnetization time increases slightly with decreasing Co layer thickness, while no such clear trend is observed in remagnetization timescales. The anisotropy field and corresponding precession frequency show systematic increase with the decrease of Co layer thickness. However, the acoustic wave frequencies remain unchanged with Co layer thickness. The inverse of the full width at half maximum of the THz radiation varies systematically with the anisotropy field, whereas the THz peak frequency remains almost constant at around 2 THz. The origin of THz radiation is explained in terms of excitation of THz transient phonons.

Structure dependence of oscillation characteristics of resonant-tunneling-diode terahertz oscillators associated with intrinsic and extrinsic delay times

We investigate the effect of intrinsic and extrinsic delay times on the oscillation characteristics of resonant-tunneling-diode (RTD) terahertz oscillators. The intrinsic delay time is composed of the electron dwell time in the resonant tunneling region and the electron transit time in the collector depletion region. We obtain and discuss the structure dependence of these factors in terms of the oscillation frequency and output power measured for RTD oscillators with different quantum-well and collector-spacer thicknesses and different air-bridge widths between the RTD and a slot antenna. The highest oscillation frequency achieved in this experiment is 1.86 THz for the well and spacer thicknesses of 2.5 and 12 nm, respectively, with a 1-µm-wide air bridge. In this structure, the extrinsic delay time (80 fs) estimated from the parasitic elements is more than double the intrinsic delay time (35 fs). It is shown theoretically that an oscillation frequency of over 2 THz is possible upon the reduction in the extrinsic delay time caused by the bulk and spread resistances in RTDs.

Resonant-Tunneling-Diode Terahertz Oscillator Using Patch Antenna Integrated on Slot Resonator for Power Radiation

In this paper, we proposed and fabricated resonant-tunneling-diode (RTD) terahertz oscillators integrated with slot-fed patch antennas to extract output power without using typical Si lenses. A square patch antenna with a ${\hbox{170}} \mu{\hbox{m}}$ -long side was fabricated on a ${\hbox{7}} \mu{\hbox{m}}$ -thick benzocyclobutene (BCB) layer stacked on an RTD with a slot resonator. The RTD oscillated at ${\sim}{\hbox{510}}~{\hbox{GHz}}$ , and an output power of ${\sim}{\hbox{40}}~\mu{\hbox{W}}$ radiated in the upward direction without the Si lens. An equivalent circuit model was proposed to estimate the output power, and the circuit parameters were derived from the calculated admittance by electromagnetic simulation. The dependence of the output power on the side length of the patch antenna agreed with the theoretical calculation. Higher output power was expected with the optimization of BCB thickness and RTD with a high current density. The radiation pattern was measured, and a directivity of 7 dBi was obtained.

Theoretical analysis of external feedback effect on oscillation characteristics of resonant-tunneling-diode terahertz oscillators

The resonant tunneling diode (RTD) is a candidate for a compact and coherent source in the terahertz frequency range. We show theoretically that the oscillation characteristics of RTDs are strongly affected by the external feedback of partially reflected output power. The oscillation frequency and output power largely change with a small amount of the reflected output power as a periodic function of the position of the reflection object. A change in the current–voltage curve, which can be used for the detection of the external feedback, also occurs.

Transmission and Reflection of Terahertz Plasmons in Two-Dimensional Plasmonic Devices

Plasmons in two-dimensional semiconductor devices will be reflected by discontinuities, notably, junctions between gated and non-gated electron channels. The transmitted and reflected plasmons can form spatially- and frequency-varying signals, and their understanding is important for the design of terahertz detectors, oscillators, and plasmonic crystals. Using mode decomposition, we studied terahertz plasmons incident on a junction between a gated and a nongated channel. The plasmon reflection and transmission coefficients were found numerically and analytically and studied between 0.3 and 1 THz for a range of electron densities. At higher frequencies, we could describe the plasmons by a simplified model of channels in homogeneous dielectrics, for which the analytical approximations were accurate. At low frequencies, however, the full geometry and mode spectrum had to be taken into account. The results agreed with simulations by the finite-element method. Mode decomposition thus proved to be a powerful method for plasmonic devices, combining the rigor of complete solutions of Maxwell's equations with the convenience of analytical expressions.

Direct intensity modulation of resonant-tunneling-diode terahertz oscillator up to ∼30 GHz

We proposed and fabricated resonant-tunneling-diode (RTD) terahertz (THz) oscillators with a structure for high-frequency direct modulation, which is useful for high-capacity THz wireless communications. The oscillator is composed of RTD and slot antenna. To obtain high cut-off frequency of direct modulation, the capacitance of the metal-insulator-metal (MIM) layer forming the slot antenna was reduced without decrease in THz output power which was a problem in the previous structure. A cut-off frequency of 30 GHz was obtained in direct intensity modulation of the device oscillating at 350 GHz with the reduced MIM capacitance of 0.7 pF.

Power Combination in 1 THz Resonant-Tunneling-Diode Oscillators Integrated with Patch Antennas

In this study, we propose and fabricate an oscillator array composed of three resonant-tunneling-diode terahertz oscillators integrated with slot-coupled patch antennas, and which does not require a Si lens. We measure the radiation pattern for single and arrayed oscillator, and calculate the output power using the integration of the pattern. The output power of a single oscillator was found to be ～15 µW. However, using an array configuration, almost combined output power of ～55 µW was obtained.

650-GHz Resonant-Tunneling-Diode VCO With Wide Tuning Range Using Varactor Diode

We fabricated a wide-range varactor-tuned terahertz oscillator using a resonant tunneling diode (RTD). An AlAs/InGaAs double-barrier RTD and a varactor-diode mesa were integrated into a 20-μm-long slot antenna. A wide tuning range of ~11% (70 GHz) of the center frequency of 655 GHz was achieved by changing the depletion-layer capacitance of the varactor diode with a dc sweep from -4 to 0.5 V. The dependence of the output power on the varactor-diode bias was also measured. These experimental results agreed well with theory.

Spin-dependent terahertz oscillator based on hybrid graphene superlattices

We theoretically study the occurrence of Bloch oscillations in biased hybrid graphene systems with spin-dependent superlattices. The spin-dependent potential is realized by a set of ferromagnetic insulator strips deposited on top of a gapped graphene nanoribbon, which induce a proximity exchange splitting of the electronic states in the graphene monolayer. We numerically solve the Dirac equation and study Bloch oscillations in the lowest conduction band of the spin-dependent superlattice. While the Bloch frequency is the same for both spins, we find the Bloch amplitude to be spin dependent. This difference results in a spin-polarized ac electric current in the THz range.

Frequency increase in terahertz oscillation of resonant tunnelling diode up to 1.55 THz by reduced slot‐antenna length

A report is presented on the dependence of oscillation frequency on antenna length in resonant-tunnelling-diode terahertz (THz) oscillators integrated with slot antennas. The upper limit of the oscillation frequency was maximised at an optimum antenna length. The highest oscillation frequency obtained in the experiment was 1.55 THz for a 16 μm-long antenna at room temperature.

Self-excited oscillation of rotating double-walled carbon nanotubes.

The oscillatory behavior of a double-walled carbon nanotubes with a rotating inner tube is investigated using molecular dynamics simulation. In the simulation, one end of the outer tube is assumed to be fixed and the other is free. Without any prepullout of the rotating inner tube, it is interesting to observe that self-excited oscillation can be triggered by nonequilibrium attraction of the ends of two tubes. The oscillation amplitude increases until it reaches its maximum with decrease of the rotating speed of the inner tube. The oscillation of a bitube is sensitive to the gap between two walls. Numerical results also indicate that the zigzag/zigzag commensurate model with a larger gap of >0.335 nm can act as a terahertz oscillator, and the armchair/zigzag incommensurate model plays the role of a high amplitude oscillator with the frequency of 1 GHz. An oblique chiral model with a smaller gap of <0.335 nm is unsuitable for the oscillator because of the steep damping of oscillation.

Wide-Range Varactor-Tuned Terahertz Oscillator Using Resonant Tunneling Diode

We propose a wide-range varactor-tuned terahertz oscillator using a resonant tunneling diode (RTD) and estimate the tuning range. In a slot antenna, a varactor diode is placed in parallel with an RTD and can be operated with different bias voltages. Frequency tuning is possible by changing the varactor-diode capacitance with the bias voltage. A wide frequency tuning range >200 GHz (500 to 740 GHz) is obtained with an oscillator with a 20-μm-long antenna, 1.3-μm2 RTD mesa, and 16-μm2 varactor-diode mesa by electromagnetic-field analysis including a varactor-diode model.

Fundamental Oscillation up to 1.42 THz in Resonant Tunneling Diodes by Optimized Collector Spacer Thickness

We report an increase in the oscillation frequency of terahertz oscillators using AlAs/InGaAs double-barrier resonant tunneling diodes (RTDs) by optimizing the collector spacer thickness. For high-frequency oscillation of RTDs, the electron delay time, which is composed of the dwell time in the resonance region and the transit time in the collector depletion region, must be reduced. Although the transit time is reduced by a thin collector spacer, the capacitance increases. Thus, an optimum thickness of collector spacer layer exists. In this report, we investigate the dependence of oscillation frequency on the collector spacer thickness. The RTDs were integrated with 20-μm-long slot antennas, and oscillations up to 1.1, 1.42, and 1.29 THz were obtained for spacer thicknesses of 25, 12, and 6 nm, respectively. The optimum spacer thickness for high-frequency oscillation was around 12 nm. The highest frequency in this experiment was 1.42 THz oscillation, with an output power of ~1 μW. We also extracted the electron velocity in the collector depletion region and the dwell time from the dependence of the delay time on the collector spacer thickness.

Synthesis and Characterization of Mass Produced High Quality Few Layered Graphene Sheets via a Chemical Method

Graphene is a two-dimensional crystal of carbon atoms arranged in a honeycomb lattice. It is a zero band gap semimetal with very unique physical and chemical properties which make it useful for many applications such as ultra-high-speed field-effect transistors, p-n junction diodes, terahertz oscillators, and low-noise electronic, NEMS and sensors. When the high quality mass production of this nanomaterial is still a big challenge, we developed a process which will be an important step to achieve this goal. Atomic Force Microscopy, Scanning Electron Microscopy, Scanning tunneling microscopy, High Resolution Transmission Electron Microscopy, X-Ray Diffraction, Raman spectroscopy, Energy Dispersive X-ray system were investigated to characterize and examine the quality of this product.

Terahertz Oscillating Devices Based Upon the Intrinsic Josephson Junctions in a High Temperature Superconductor

Recent developments of coherent terahertz (THz) oscillators based on the intrinsic Josephson junctions (IJJs) in mesas of the high temperature superconductor Bi2Sr2CaCu2O8+δ are reviewed. Experimental and theoretical studies of the emission from equilateral, right-angled isosceles, and acute isosceles triangular mesas are compared with those obtained from rectangular, square, and disk mesas, in order to determine the role of the mesa geometry. The superconducting properties and emission frequency f spectra are presented for a variety of triangular mesa geometries. Analytic and finite difference time domain numerical calculations of the emissions from the internal electromagnetic (EM) cavity modes of triangular mesas are compared with experiment. The experimental f always satisfies the ac Josephson relation, and its narrow linewidth arises from the synchronized emissions from many IJJs. For some mesa geometries, f also strongly locks onto an EM cavity mode frequency, enhancing the emission’s stability and output power. For other geometries, such cavity mode locking is weak, and f is highly tunable.

Harmonic phase detector for phase locking of cryogenic terahertz oscillators

We present a simple and effective way to phase lock terahertz cryogenic oscillators. Extreme nonlinearity of a superconductor-insulator-superconductor tunnel junction allows its implementation as a cryogenic high-harmonic phase detector (HPD), which is used both for mixing a terahertz oscillator signal with a microwave reference and for generating a phase error feedback signal that is directly applied to the oscillator for its phase locking. An integration of the HPD with a cryogenic flux-flow oscillator results in synchronization bandwidth as wide as 70 MHz (significantly exceeding conventional room-temperature system bandwidth), providing phase locking of 84% emitted power for 15 MHz oscillator linewidth.

Amplification of drifting semiconductor plasmons and effects of carrier collisions and diffusion

Carrier collisions and diffusion are the major mechanisms influencing loss in solid-state plasmas, and their understanding is crucial for the design of terahertz plasmonic oscillators and amplifiers. This paper presents an analysis of the role collisions and diffusion play in the amplification of plasmons reflecting from conducting boundaries in two-dimensional electron channels. The theoretical model relies on the hydrodynamic description of the electron dynamics, complete mode spectra and electrodynamic boundary conditions. Diffusion has been found to reduce the amplification coefficient due to a reduced asymmetry of the counter-propagating plasmons, and both collisions and diffusion have been found to influence the plasmon propagation loss. The distances of sustained amplification have been calculated and the implications for terahertz oscillators have been analysed.

Plasma wave instability in a quantum field effect transistor with magnetic field effect

The current-carrying state of a nanometer Field Effect Transistor (FET) may become unstable against the generation of high-frequency plasma waves and lead to generation of terahertz radiation. In this paper, the influences of magnetic field, quantum effects, electron exchange-correlation, and thermal motion of electrons on the instability of the plasma waves in a nanometer FET are reported. We find that, while the electron exchange-correlation suppresses the radiation power, the magnetic field, the quantum effects, and the thermal motion of electrons can enhance the radiation power. The radiation frequency increases with quantum effects and thermal motion of electrons, but decreases with electron exchange-correlation effect. Interestingly, we find that magnetic field can suppress the quantum effects and the thermal motion of electrons and the radiation frequency changes non-monotonely with the magnetic field. These properties could make the nanometer FET advantageous for realization of practical terahertz oscillations.