Terahertz Spectral Regime Research Articles

Discrimination of hydration states of hydrated salts using terahertz time-domain spectroscopy

Water-sensitive nature of terahertz radiation has been exploited here to discriminate the hydration states of different hydrated salts, such as Cobalt Chloride Hexahydrate (CoCl2.6H2O), Copper Sulfate Pentahydrate (CuSO4.5H2O), Magnesium Chloride Hexahydrate (MgCl2.6H2O), and Calcium Chloride Dihydrate (CaCl2.2H2O). Hydration states of these salts have been prepared by heating them in an oven at different temperature for specific time on the basis of thermo-gravimetric analysis. The prepared samples were analyzed using terahertz time-domain spectroscopy (THz-TDS). The variation in absorption coefficient and refractive indices of these hydrated salts in terahertz spectral regime (0.2–1.6 THz) are linked with the change of hydration states, which has been used for their discrimination. The results have been validated with X-ray diffraction analysis. The present work indicates that THz-TDs can be considered a promising technique to fulfill the ever increasing demand of the analysis of these hydrated salts.

Magnetothermally-controllable coating for scattering and absorption in the terahertz spectral regime

Plane-wave scattering and absorption characteristics of a spherical core composed of an isotropic material, and covered by InSb spherical coating are investigated in the terahertz spectral regime. The InSb coating is subjected to a magnetostatic field; hence it is a gyrotropic, uniaxial dielectric material comprised of two parameters: temperature and magnetostatic field. It is seen that the interplay of these two parameters can modify the various scattering efficiencies, depending on (i) the frequency of the incident plane wave, (ii) the incident plane-wave orientation with respect to the magnetostatic field's direction, and (iii) the identity of the core material. In particular, for a fixed orientation of the incident plane wave, the impact of the temperature and the magnetostatic field on the total scattering and the backscattering efficiencies becomes more pronounced at higher frequencies, compared to lower frequencies. The absorption efficiency, on the other hand, is strongly dependent upon these two parameters, regardless of the frequency. On fixing the two parameters of InSb, the role of the orientation of the incident plane wave with respect of the magnetostatic field' direction emerges in some spectral regimes for the total scattering and absorption efficiencies, and in high-frequency spectral regimes for the backscattering efficiency. These hold true for dielectric core, as well as for conductive core.

Non-metallic magnetostatically tunable metasurface absorber for terahertz waves

A non-metallic metasurface absorber containing VO2, InSb, and Teflon has been designed, simulated, and investigated for normally incident plane waves in the terahertz spectral regime. When the temperature exceeds 72°C, a backing layer of VO2 in its rutile phase instead of a conventional metal virtually eliminates transmission. Peak absorptance as high as 0.998, quality factor on the order of 200, and magnetostatic tunability of about 16GHz\, T−1 are achievable. Such metasurfaces can be made using standard microfabrication techniques.

2D Materials for Photothermoelectric Detectors: Mechanisms, Materials, and Devices

Abstract2D materials, with outstanding optical, thermal, and electric properties, are emerging as promising candidates for fabricating high‐performance photodetectors. Recently, impressive progresses have been made in this area and some challenges are remaining to improve the properties of photodetectors. As one important part in the mainstream photodetection mechanisms, photothermoelectric (PTE) effect is showing unique priorities in fabricating advanced photodetectors, especially broadband detection operating in the mid‐infrared and terahertz spectral regime. Here, recent progress on PTE photodetectors based on layered 2D materials is reviewed. The physical mechanism of PTE effect is first discussed and then the optical and thermoelectric properties of various 2D materials are analyzed. Furthermore, strategies to improve the photodetection performance of PTE detectors are summarized in two major categories including enhanced photothermal conversion and thermoelectric conversion processes. Finally, the challenges and prospects for future research in 2D thermoelectric materials and PTE detectors are also provided.

Multifunctional metasurface based bandstop and bandpass filters for terahertz radiation

A multifunctional metasurface using all dielectric structure comprising an array of InAs square pillar over the InSb substrate has been realized. The considered metasurface shows zero reflection and close to unity transmission at single as well as dual frequency in the terahertz spectral regime with the application of magnetic field. This metasurface also exhibits switching action from bandpass to bandstop over the designed frequency regime with the exposure of varying magnetic field from 0 to 0.2 T. The tunability in the maximum transmittance frequency is achieved by the variation in temperature of Insb substrate which acts as another control modality showing a blue shift from 2.37 THz to 2.65 THz at an average tuning rate of 0.028 THz K−1. The proposed all dielectric metasurface is designed to function as a switch, bandstop and bandpass filter for the terahertz spectral regime thus acting as multifunctional metasurface, thereby reduces inventory cost as well as additional peripheral utility devices.

Graphene-sandwich metasurface as a frequency shifter, switch, and isolator at terahertz frequencies

A graphene-sandwich metasurface for electrically controlled operation as a frequency shifter, a reciprocal switch, and a bidirectional isolator in the terahertz spectral regime was simulated using CST Microwave Studio™ 2019 software. The metasurface is a biperiodic array of identical meta-atoms, each of which comprises a silicon layer with a graphene patch on each of its two faces. The polarization-insensitive metasurface has ∼15 dB isolation loss as well as ∼15 dB extinction ratio and can be controlled to function over a wide frequency range.

Magnetically tunable metasurface comprising InAs and InSb pixels for absorbing terahertz radiation.

A magnetically tunable metasurface comprising meta-atoms with InSb-patched, InAs-patched, and unpatched pixels was simulated using commercial software to maximize the absorption of normally incident radiation in the terahertz spectral regime, with the patches decorating the illuminated face of a gold-backed polyimide substrate. Maximum absorptance of 0.99 and minimum absorptance of 0.95 can be obtained in 0.14-0.23-THz-wide bands in the 2-4-THz spectral regime, with an average tuning rate of 0.3THzT-1 and 0.24-THz dynamic range when the controlling magnetostatic field is aligned parallel to the incident electric field. The use of both InSb and InAs patches is much superior to the use of patches of only one of those materials. The design can be adapted for neighboring spectral regimes by exploiting the scale invariance of the Maxwell equations.

Dependence on excitation polarization and crystal orientation for terahertz radiation generation in a BaGa4Se7 crystal.

Optical rectification is experimentally investigated in a biaxial BaGa4Se7 crystal by considering various combinations of near-infrared excitation polarizations and crystal orientations. The highest terahertz radiation is produced along the Z crystallo-physical direction of the BaGa4Se7 crystal. Despite the optical complexity of the BaGa4Se7 crystal in the terahertz spectral regime, this systematic experimental investigation determines the optimal excitation polarization and crystal orientation for the optical rectification process.

Active phase control of terahertz pulses using a dynamic waveguide.

Control over the spectral phase of a light pulse is a fundamental step toward arbitrary signal generation in a spectral band. For the terahertz spectral regime, pulse shaping holds the key for applications ranging from ultra-high speed wireless data transmission to quantum control with shaped fields. In this work, we demonstrate a technique for all-optical and reconfigurable control of the spectral phase of a light pulse in the important terahertz (THz) band. The technique is based on interaction of a guided THz pulse with patterned photoexcited regions within a uniform silicon-filled parallel-plate waveguide. We use this platform to demonstrate broadband and tunable positive and negative chirp of a THz pulse, as well as control of the pulse carrier envelope phase.

Bicontrollable terahertz metasurface with subwavelength scattering elements of two different materials.

Transmission of a normally incident plane wave through a metasurface with bicontrollable subwavelength scattering elements was simulated using a commercial software. Some pixels composing the H-shaped scattering elements were made of a magnetostatically controllable material whereas the remaining pixels were made of a thermally controllable material, the metasurface designed to operate in the terahertz spectral regime. The copolarized transmission coefficients were found to exhibit stopbands that shift when either a magnetostatic field is applied or the temperature is increased or both. Depending on spectral location of the stopband, either the magnetostatic field gives coarse control and temperature gives fine control or vice versa. The level of magnetostatic control depends on the magnetostatic-field configuration.

Broadband terahertz generation from metamaterials

The terahertz spectral regime, ranging from about 0.1-15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1-4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 μm (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility ~10(-16) m(2) V(-1) that far exceeds thin films and bulk non-centrosymmetric materials.

Terahertz transmission through p+porous silicon membranes

We report an experimental measurement of far infrared dielectric properties of p+-doped free-standing porous silicon membranes, characterized using terahertz time-domain spectroscopy. Freshly fabricated hydrogen-terminated porous silicon samples exhibited significant absorption in the terahertz regime, and the absorption was observed to slowly drift in response to the surrounding environment and the duration of illumination. Samples that are partially oxidized, either by age or through thermal oxidation in an oven, become almost transparent in the terahertz spectral regime, and the dielectric properties stabilize to a constant value. The partially-oxidized porous silicon samples are observed to have an absorption coefficient of approximately 2 cm–1 over the range of 0–2 THz, which corresponds to an effective resistivity of 35 ± 9 Ω cm, approximately four orders of magnitude higher than that of the crystalline silicon from which they were formed. Spectral transmission measurements further confirm that p+ porous silicon has no significant absorption peaks or other spectral variation in the 0–2 THz regime. These findings suggest that p+ porous silicon could be a useful material for fabricating future THz devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)