THz Frequency Regime Research Articles

III–V super-lattice SPST/SPMT pin switches for THz communication: theoretical reliability and experimental feasibility studies

A physics-based Quantum-Modified CLassical Drift–Diffusion (QMCLDD) non-linear mathematical model has been developed for design and characterisation of GaN/AlGaN asymmetrical superlattice pin based single-pole single-throw (SPST) and single-pole multi-throw (SPMT) switches for sub-MM wave communication systems. The simulator has incorporated several important physical phenomena that arise in superlattice structure, including quantum confinement effects, generation-recombination and tunnelling generation of carriers as well as scattering limited carrier mobility and velocity in GaN/AlGaN structure. In order to reduce the dislocation density and in turn the series resistance in GaN/AlGaN heterostructure, thin AlN nucleation layer and a buffer layer are considered in the simulation. It is observed that the RF series resistance of the pin device, operating around 0.2 THz frequency regime, reduces in case of proposed superlattice structure. The advantages of super-lattice pin diode over the conventional Si devices are the faster reverse recovery time (~ 9 vs 35 ns) and lowering of forward RF series resistances (0.39 vs 1.20 Ω). This study also reveals that SPST and SPMT switches made up with super-lattice devices are characterized by low insertion loss (~ 0.13 and 0.15 dB, respectively) and high isolation (~ 65.4 and 82.5 dB, respectively). A good agreement between theory and experiment establishes the superiority of the present model over the others. A comparative analysis of Si and GaN/AlGaN super-lattice pin SPST and SPMT switches establishes the potential of the later for its application in high-frequency THz-communication. In addition to this, a detailed thermal modelling of the device has also been done to make the analysis more realistic. The junction temperature of the designed GaN/AlGaN superlattice pin switch will be as high as 377 K, which is quite moderate compared to its flat profile counterpart. To the best of authors’ knowledge, this is the first ever report on QMCLDD non-linear modelling of pin SPST and SPMT switches (series-shunt combination) at THz-arena.

Towards gas sensing with vertically aligned carbon nanotubes interrogated by THz radiation pulses

When vertically aligned carbon nanotubes (VACNT) are exposed to gases, their dielectric properties may change by mechanisms such as doping effects or a modification of the scattering channels of the mobile charge carriers of the VACNT. This report provides first data of a study which aims at an exploration of such conductivity changes in the THz frequency regime. The test gases are NH3 and SO2 which are known to act as donors, respectively as acceptors when interacting with the carbon nanotubes. Our measurements confirm the doping effects. This may open the way towards gas sensors based on the VACNT interrogated by THz radiation, where the VACNTs act as an accumulation volume for certain species contained in a gas stream.

Modeling the semiconductor devices with negative differential resistance based on nitride nanowires

We proposed a numerical model of the Gunn diode and resonant diode based on a single nitride nanowire. Important model parameters needed for development of the considered devices, namely layers thickness, composition and doping level were evaluated. It has been shown theoretically that the single GaN nanowire based Gunn diode is capable to operate in over 1 THz frequency regime.

Plasmonic properties of graphene-based nanostructures in terahertz waves

Abstract We theoretically study the plasmonic properties of graphene on bulk substrates and graphene-coated nanoparticles. The surface plasmons of such systems are strongly dependent on bandgap and Fermi level of graphene that can be tunable by applying external fields or doping. An increase of bandgap prohibits the surface plasmon resonance for GHz and THz frequency regime. While increasing the Fermi level enhances the absorption of the graphene-based nanostructures in these regions of wifi-waves. Some mechanisms for electric-wifi-signal energy conversion devices are proposed. Our results have a good agreement with experimental studies and can pave the way for designing state-of-the-art electric graphene-integrated nanodevices that operate in the GHz–THz radiation.

Bulk refractive-index sensitivities of the THz-range plasmon resonances on a micro-size graphene strip

We studied numerically the potential use of a micro-size graphene strip as a surface plasmon (SP) resonance-based bulk refractive-index sensor in the THz frequency regime. Our accurate computational instrument was an in-house algorithm based on integral equations (IEs) and Nystrom discretization. The refractive-index sensitivities and figure-of-merit (FOM) values of the associated plasmon resonances were calculated. It was found that the primary plasmon mode P1 is more sensitive to the refractive-index changes than plasmons of higher orders, although the latter demonstrated much larger FOM values explained by the higher Q-factors. The FOM values of the higher-order resonances on a graphene strip in the THz range are at a level similar to the FOM values of the localized SP resonances on a noble-metal strip in the optical range.

Active terahertz metamaterials based on the phase transition of VO2 thin films

Vanadium dioxide (VO2) thin films were prepared on single crystal sapphire substrates by pulsed laser deposition. VO2 films exhibited a significant resistivity drop (>104Ω-cm) and large optical transmittance change (>60%) in the near-infrared region across their semiconductor-to-metal transition. Hybrid metamaterial devices designed for the THz frequency regime were fabricated by combining double split-ring resonators (SRRs) with phase changing VO2 films. By changing the conductivity of VO2 via temperature, the behavior of the SRR gap was adjusted from capacitive to resistive in order to modulate the THz beam transmission at their resonance frequencies. A modulation efficiency greater than 50% was achieved at the magnetic resonance frequencies (0.3THz and 0.7THz) in these hybrid SRR–VO2 metamaterial devices.

Design and analysis of perfect metamaterial absorber in GHz and THz frequencies

The new perfect metamaterial absorber (MA) based on square resonator with gap configuration is numerically (for GHz and THz frequency regime) and experimentally (for GHz frequency regime) presented and investigated. The proposed MA has a simple configuration which introduces flexibility to adjust its metamaterial properties and easily rescale the structure for other frequency regimes. In addition, polarization and incident angle independencies of the proposed model are analyzed for the THz frequency regime. The suggested structure could achieve very high absorption at wide angles of polarization and incident angles for both transverse electric and transverse magnetic waves. At resonance frequencies, absorption results are realized around 99.99% at 5.48 GHz and 99.92% at 0.865 THz for their respective frequency regimes. Besides numerical results in GHz and THz frequencies, experimental results are also obtained for GHz frequency regime. The proposed MA and its variations enable myriad potential applications in areas such as defense systems, communication, stealth technologies, and so on.

Equilateral Triangular Dielectric Resonator Nantenna at Optical Frequencies for Energy Harvesting

The last decade has witnessed a remarkable growth in the telecommunication industry. With the introduction of smart gadgets, the demand for high data rate and bandwidth for wireless applications have increased exponentially at the cost of exponential consumption of energy. The latter is pushing the research and industry communities to devise green communication solutions that require the design of energy saving devices and techniques in one part and ambient energy harvesting techniques in the other part. With the advent of nanocomponents fabrication technology, researchers are now able to tap into the THz frequency regime and fabricate optical low profile antennas at a nanoscale. Optical antennas have proved their potential and are revolutionizing a class of novel optical detectors, interconnectors, sensors, and energy harvesting related fields. Authors in this paper propose an equilateral triangular dielectric resonator nantenna (ETDRNA) working at 193.5 THz standard optical frequency. The simulated antenna achieves an impedance bandwidth from 192.3 THz to 197.3 THz with an end-fire directivity of 8.6 dBi, covering the entire standard optical window of C-band. Numerical demonstrations prove the efficiency of the nantenna at the frequencies of interest, making it a viable candidate for future green energy harvesting and high speed optical applications.

Graphene as a tunable THz reservoir for shaping the Mollow triplet of an artificial atom via plasmonic effects

Using a realistic quantum master equation, we show that the resonance fluorescence spectra of a two-level artificial atom (quantum dot) can be tuned by adjusting its photonic local density of states via biasing one or more graphene monolayers. The structured photon reservoir is included using a photon Green function theory which fully accounts for the loss and dispersion. The field-driven Mollow triplet spectrum can be actively controlled by the graphene bias in the THz frequency regime. We also consider the effect of a dielectric support environment and multiple graphene layers on the emitted fluorescence. Finally, thermal bath effects are considered and are shown to be important for low THz frequencies.

Optical properties of LiInSe_2 in the THz frequency regime

We investigate the optical properties of LiInSe2 (LISe) by THz time-domain spectroscopy (THz-TDS). The experimental results indicate that LISe has excellent transparency in the THz frequency region. We determine quantitatively the birefringence of LISe. The refractive index and absorption coefficient along the three dielectric axes of LISe are reported in the THz frequency region. The lattice vibrations of LISe are analyzed on the basis of the THz-TDS measurement in combination with infrared and Raman spectra. The origin of the low-frequency vibrational modes below 3 THz as interatomic bond-bending modes is revealed.

Detection of Harmful Residues in Honey Using Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has been applied for the detection and discrimination of harmful chemical residues in honey. Three antibiotics (sulfapyridine, sulfathiazole, and tetracycline) and two acaricides (coumaphos and amitraz) were characterized in the THz frequency regime between 0.5 THz and 6.0 THz. All chemical substances present distinct absorption peaks. THz transmission measurements of honey mixtures with antibiotics have been performed, revealing that antibiotic residues are traceable in highly absorptive food products, such as honey, at concentrations down to 1% weight percentage, thanks to their THz fingerprints. Moreover, multiple antibiotics were identified in their mixture with honey, pointing out the potential of the technique to be used in the near future as a fast, real-time technique for detecting and discriminating multi-residues strictly related to food safety issues.

Vibrational Collective Dynamics of Dry Proteins in the Terahertz Region

The coherent density fluctuations of a perdeuterated dry protein have been studied by Brillouin neutron spectroscopy. Besides a nearly wavevector-independent branch located around 5 meV, a propagating mode with a linear trend at low wavevector Q is revealed. The corresponding speed of 3780 ± 130 m/s is definitely higher than that of hydrated proteins. Above Q = 0.8 Å(-1), this mode becomes overdamped, with lifetimes shorter than 0.1 ps, in fashion similar to glassy materials. The present results indicate that dry proteins sustain coherent density fluctuations in the THz frequency regime. The trend of the longitudinal modulus indicates that in this frequency range dry biomolecules are more rigid than hydrated proteins.

Channel-switchable single-/dual-wavelength single-longitudinal-mode laser and THz beat frequency generation up to 3.6 THz

We propose and demonstrate a simple approach to realize channel-switchable single-/dual-wavelength single-longitudinal-mode (SLM) lasers and switchable beat frequency generation in the THz frequency regime. It is a compact fiber-ring laser incorporating a Fabry–Perot etalon, tunable filters, and a piece of unpumped polarization maintaining erbium-doped fiber (PM-EDF) as the saturable absorber. Two parallel-arranged tunable filters are configured inside the ring cavity to support switchable dual-wavelength SLM operation and consequent THz beat frequency generation. Switchable 19-channel 200-GHz ITU-grid-compatible single-wavelength SLM operation is implemented. Moreover, the channel-switchable dual-wavelength SLM laser and switchable THz beat frequency generation ranging from 0.2 to 3.6 THz with a tuning step of 0.2 THz are demonstrated in the experiment.

A broadband and tunable MMW–THz quasi-optical cavity

Based on the Fabry–Perot cavity, a broadband and tunable cavity is presented and investigated theoretically and experimentally. The results show that the cavity can work in the millimetre wave (MMW)–THz frequency regime. The experimental Q value is up to about 6000 at 37.5 GHz and higher than 800 at 2.54 THz. The resonant frequency of the cavity can be tuned by adjusting the distance between two mirrors, and it is capable of working at a high power. So this quasi-optical cavity may have very good potential applications in MMW–THz science and technology.

Planar THz quasioptics

Two-dimensional (2D) quasioptical elements are demonstrated as guiding structures in the THz frequency regime. A planar polyethylene lens is incorporated in a parallel metal plate structure and is shown to focus guided THz radiation to a spot comparable to that expected for a three-dimensional (3D) optical element in free space. A 2D double-slit aperture is also demonstrated to diffract the incoming THz beam as expected for 3D. The spatial resolution of the measurement techniques employed is illustrated by the sharpness of the measured diffraction pattern.

Superfluid inhomogeneity and microwave absorption in a model for thin high- Tc superconducting films

We investigate the microwave absorption arising from inhomogeneity in the superfluid density of thin high- T c superconducting films. Such inhomogeneities may arise from a wide variety of sources, including quenched random disorder and static charge density waves such as stripes. We show that both mechanisms will inevitably produce additional absorption at finite frequencies. We present simple model calculations for this extra absorption, and discuss applications to other transport properties in high- T c materials. Finally, we discuss the connection of these predictions to recent measurements by Corson et al. (Nature (London) 398 (1999) 221) of absorption by the high-temperature superconductor Bi 2Sr 2CaCu 2O 8+δ in the THz frequency regime.

Optical pulse shaping using optical coherent transients

Using multiple temporally-overlapped, frequency offset and phase-tuned, linear frequency chirps, a new method of multi-GHz optical coherent transient optical pulse shaping and processing in inhomogeneously broadened rare-earth doped crystals is proposed. Using this technique with properly chirped laser sources, multi-GHz processing can be controlled with conventional low-bandwidth electronics and optical modulators. Specifically, this technique enables pulse shaping in the MHz to THz frequency regime with time-bandwidth-products exceeding 100,000, filling the gap between the operating regimes of femtosecond pulse shaping and analog electronics. The low bandwidth (~20 MHz) proof-of-concept demonstrations presented in this paper include pulse train creation, selfconvolution, auto-correlation, and chirped pulse compression.

Synthesis and properties of dielectric Bi2(Zn1/3Nb2/3)2O7 thin films

Abstract Crystalline, Bi 2 (Zn 1/3 Nb 2/3 ) 2 O 7 , BiZN thin films can be easily obtained when the films were in-situ deposited at high enough substrate temperature 450–600 °C (30 min). The optical parameters ( N = n + ik ) measured and analyzed by optical transmission spectroscopy are insensitive to the deposition parameters, provided that the films are crystalline. The dielectric properties converted from optical parameters are e ′=4.75 and Q =325. The dielectric constant of BiZN thin films in THz frequency regime, ( e ′) f·THz =32, is markedly smaller than the e ′ value of BiZN bulk materials in microwave regime, and the quality factor of the thin films is less than 20% of the bulk materials. However, the dielectric constant of the thin films in THz region is still markedly larger than that derived from optical transmission spectroscopy in optical region.

Electrodynamic c-Axis Properties of YBa2Cu3O7?? Thin Films in the THz Frequency Regime

Electrodynamic c-Axis Properties of YBa2Cu3O7?? Thin Films in the THz Frequency Regime

Complex dielectric function of NdGaO 3 in the THz frequency regime

The dielectric properties of NdGaO 3 crystals in the temperature range between 10 K and 300 K are measured by coherent THz pulse spectroscopy. The complex dielectric function at frequencies between 0.2 and 1.2 THz is obtained directly from phase sensitive transmission measurements.