Sub-terahertz Frequency Range Research Articles

Terahertz detection in a slit-grating-gate field-effect-transistor structure

We have fabricated a grating-gate InGaAs/GaAs field-effect transistor structure with narrow slits between the grating gate fingers. The resonant photoconductive response of this structure has been measured in the sub-terahertz frequency range. The frequencies of the photoresponse peaks correspond to the excitation of the plasmon resonances in the structure channel. The obtained responsivity exceeds the responsivity reported previously for the grating-gate FET detector with wider slits of the grating-gate by two orders of magnitude due to enhanced coupling between incoming terahertz radiation and plasmon oscillations in the slit-grating-gate field-effect transistor structure.

Ultra-compacted sub-terahertz bandpass filter in 0.13 µm SiGe

The design of an ultra-compacted silicon-based bandpass filter in the sub-terahertz frequency range is explored. A filter with combline topology consisting of multilayered resonators and a cross-coupling capacitor has been designed in 0.13 µm SiGe. The dimensions of the implemented filter are 40×130 µm2, which are even smaller than a ground–signal–ground probing pad with 100 µm pitch. Meanwhile, the filter achieves a 3 dB bandwidth of about 32.0 GHz (22.0%) and an insertion loss of −7.7 dB at the centre frequency of 143 GHz.

Characteristics of a New Carbon Nanotube Antenna Structure With Enhanced Radiation in the Sub-Terahertz Range

In this paper, a new antenna configuration is proposed to operate efficiently in the sub-terahertz frequency range (10-1000 GHz). The proposed antenna is composed of a bundle of single-walled carbon nanotubes (SWCNTs) surrounded by two dielectric jackets; the inner of which is dielectric foam with relative permittivity close to unity. The outer one is a metamaterial layer with permittivity much smaller than the free-space permittivity. Rigorous analysis shows that the metamaterial layer helps to reduce the axial propagation constant of the surface wave mode traveling along the antenna. The radiation resistance and the radiation efficiency of a resonant dipole antenna are derived. Numerical results show that these parameters are considerably enhanced at the sub-terahertz frequencies relative to the SWCNT bundle antenna.

Quantum cascade laser: a compact, low cost, solid-state source for plasma diagnostics

Quantum cascade lasers (QCL) are unipolar injection lasers based on intersubband transitions in a modular semiconductor heterostructure. The first THz QCL, operating at 67 μm (4.3 THz), was demonstrated in 2002; the wavelength range now extends beyond 250 μm (1.2 THz) and is entering the sub-terahertz frequency range for devices operated in external magnetic field. Although a number of different quantum designs have been demonstrated, increasing the operating temperature remains a major challenge: the maximum temperature is still ∼ 195 K, and recently approached 225 K in high magnetic fields. Nevertheless, compact continuous wave systems operating within Sterling coolers already ensure ample portability and turn-key operation and QCLs represent then the THz solid-state radiation source that actually shows the best performance in terms of optical output power, which can reach more than 100 mW average, and linewidth, typically in the tens of kHz for single mode devices. THz QCLs have then a realistic chance to deeply impact technological applications such as process monitoring, security controls, and bio-medical diagnostics. They are ideally suited though for plasma polarimetry and interferometry, thanks to their high polarization selectivity, excellent stability and ruggedness, and ease of high-speed modulation. Their compact size and monolithic cavity arrangement allows placement in the very proximity of the plasma to be monitored, easing requirements of stability against vibrations etc. Furthermore, the long coherence lengths should be easily compatible with interferometric arms of even very different lengths, a geometry ideal for coupling to a plasma reactor. The possibility of direct current modulation at MHz if not GHz frequencies ensures then an excellent temporal resolution of the meaurements, and a large low-frequency noise rejection. New analysis schemes also become feasible, for instance employing two-color lasers, operating at the same time at two appropriately chosen wavelengths, or exploiting the laser emission tunability for implementing frequency modulation techniques. This paper will discuss the current state-of-the-art in THz QCL technology and applications, focusing on those aspects of greatest relevance for plasma diagnostics.

Sub-Terahertz Magnetic and Dielectric Excitations in Hexagonal Ferrites

Magnetic and dielectric resonances in the sub-terahertz (sub-THz) frequency range are observed in pure and Al-substituted hexagonal barium ferrite. A resonator based on magnetic excitations has been fabricated and its performance characteristics have been studied. The possible use of the resonator at sub-THz frequencies has been demonstrated. The resonator exhibited a loaded Q-factor of 150-330 in the frequency range 97-108 GHz. Dielectric resonances in a single-crystal barium hexaferrite are observed in the frequency range 75-110 GHz. The modes excited by circularly polarized electromagnetic waves show nonreciprocal propagation characteristics. The dielectric resonances may occur at a much higher frequency than ferromagnetic resonance. It is shown that degeneracy in the dielectric modes is lifted with an applied magnetic field H and that the modes can be tuned by 10 GHz or more with H. Data on frequencies of the modes versus H shows hysteresis. Theoretical predictions on H-tuning characteristics of the principal dielectric E11δ mode are in agreement with the data. The dielectric modes are of importance for the realization of low-loss devices, including resonators, isolators and phase shifters.

Design of a Compact Sub-Terahertz Gyrotron for Spectroscopic Applications

In this paper we present the initial design of a novel and versatile high frequency gyrotron with parameters suitable for application to various spectroscopic studies that require coherent radiation in the subterahertz frequency range (such as NMR/DNP spectroscopy, ESR spectroscopy, spectrometer based on the X-ray detected magnetic resonance etc.). The most characteristic feature of the design is that it utilises a compact, cryogen-free 8 T superconducting magnet. As a result, the overall dimensions of the entire device are considerably reduced in comparison with the previously developed tubes belonging to the Gyrotron FU and Gyrotron FU CW series. This makes the novel gyrotron highly portable to diverse laboratory environments and easily embeddable to different measuring systems. The electron-optical system (EOS) of the tube is based on a compact low-voltage magnetron injection gun (MIG), which has been specially designed and optimized together with the resonant cavity using our problem-oriented software package GYROSIM for CAD of gyrotrons. The tube operates at the second harmonic of the cyclotron frequency and generates a radiation with an output power of about 100 W and a frequency tunable up to around 424 GHz, respectively.

Competition between the barrier and injection mechanisms of nonlinearity of the current-voltage characteristic in Mott-barrier detector diodes

We obtain an analytical solution to the problem of transverse injection current in an undoped semiconductor i layer of arbitrary thickness with account of self-consistent boundary conditions. Charge transfer in the semiconductor bulk is described in the drift-diffusion approximation. Current transfer through the boundaries of an undoped layer is described in terms of the thermoelectron emission theory. Thus, the generalized thermoemission-diffusion approach applies for semiconductors with both low and high mobilities of charge carriers. On the basis of the obtained solution, we analyze the characteristics of the current nonlinearity of the Mott-barrier diodes. The generalized approach is used for describing current transfer in low-barrier diodes based on Mott contacts with near-surface δ-doping. Characteristics of detection of low-barrier diodes are analyzed. Limiting values of the volt-watt and threshold sensitivities of the detectors based on these diodes in the subterahertz frequency range (up to 1 THz) are determined.

A study of the transport of thermal acoustic phonons in CaF2 single crystals and ceramics within the subterahertz frequency range

A study of the transport of thermal acoustic phonons in CaF2 single crystals and ceramics within the subterahertz frequency range

Focal-plane micro-bolometer arrays for 0.5 THz spatial room-temperature imaging

The possibility of using commercial classical near-infrared (∼30 THz) focal-plane micro-bolometer arrays in the sub-terahertz frequency range is studied. Real-time room-temperature pixilated detector imaging of a 0.5 THz (λ=600 µm) spatial beam profile emitted from a backward-wave oscillator is presented. Images, with a signal-to-noise ratio of 5 are demonstrated, leading to an estimate of the noise equivalent power of 11 nW · Hz−1/2.

High field electron dynamics in dilute nitride Ga(AsN)

We investigate the high electric field dynamics of conduction electrons in dilute nitride GaAs1−xNx diodes. At low temperature (T<40K), we show that the trapping of hot electrons at localized states leads to low frequency oscillations (<1Hz) of the current at high bias. This slow dynamics is replaced at higher temperatures by a fast response of the current in the subterahertz frequency range, which we relate to the interaction of hot electrons with resonant nitrogen-related states in the conduction band.

Static tuning band gaps of three-dimensional photonic crystals in subterahertz frequencies

Microscale three-dimensional photonic crystals with different dielectric volume fractions (β) were fabricated to tune the band gaps in the subterahertz frequency range. When changed β from 33% to 20% in the SiO2–Al2O3 ceramic-resin photonic crystals with a lattice constant of 500μm, the measured first band gaps were increased from 380–450to450–520GHz. After dewaxing and sintering, the lattice constant of SiO2–Al2O3 ceramic photonic crystals were shrunk to 380μm, and the observed band gaps correspondingly appeared from 470–540GHzto520–600GHz. These experimental results were further confirmed by calculation of band diagrams using the plan wave expansion method.

Superconducting subterahertz fast nanoswitch

A superconducting thin-film nanoswitch for the subterahertz frequency range has been proposed, developed, fabricated, and tested. The switch makes it possible to modulate the microwave signal or switch it between two branches of a circuit with low losses and high speed. The switch can be naturally integrated with superconducting high-sensitive detectors. Its application makes it possible to avoid the use of massive slow mechanical modulators and to improve the measurement accuracy in decisive astrophysical experiments such as the investigation of the anisotropy of the cosmic microwave background.

Ultrafast optical study of amorphous Ge thin films for superconductor/semiconductor hybrid devices

We investigated the optical response of amorphous (a-) Ge thin films by a time-resolved reflectivity measurement and the observation of terahertz radiation from a-Ge photoconductive switch coupled with Au/Cr transmission lines. The results show that the lifetime of photocarrier in the a-Ge is about 0.9 ps, and the pulse width of terahertz beam radiated from the photoconductive switch is about 1 ps. These results indicate that a-Ge can be used as an optical-to-electrical converter in sub-terahertz frequency range. However, the peak width of a-Ge photoconductive switches with YBCO transmission lines is about 2 ps and broad compared with the result with Au/Cr transmission lines.

Frequency-selective Josephson detector: Power dynamic range at subterahertz frequencies

The power dynamic range of an YBa2Cu3O7−x frequency-selective Josephson detector for the Hilbert-transform spectroscopy was experimentally studied in the subterahertz frequency range. At a temperature of 80 K, the dynamic range of 2×105 was achieved at a noise equivalent power of 8×10−15 W/Hz1/2.

Cerenkov generation of high-frequency confined acoustic phonons in quantum wells

We analyze the Cerenkov emission of high-frequency confined acoustic phonons by drifting electrons in a quantum well. We find that the electron drift can cause strong phonon amplification (generation). A general formula for the gain coefficient α is obtained as a function of the phonon frequency and the structure parameters. The gain coefficient increases sharply in the short-wave region. For the example of a Si/SiGe/Si device, it is shown that the amplification coefficients of the order of hundreds of cm−1 can be achieved in the subterahertz frequency range.