This review highlights recent and novel trends focused on metallic (plasmonic) and dielectric metasurfaces in photoconductive terahertz (THz) devices. We demonstrate the great potential of its applications in the field of THz science and technology, nevertheless indicating some limitations and technological issues. From the state-of-the-art, the metasurfaces are, by far, able to force out previous approaches like photonic crystals and are capable of significantly increasing the performance of contemporary photoconductive devices operating at THz frequencies.
We have developed a method of solid immersion THz imaging-a non-contact technique employing the THz beam focused into evanescent-field volume and allowing strong reduction in the dimensions of THz caustic. We have combined numerical simulations and experimental studies to demonstrate a sub-wavelength 0.35k 0-resolution of the solid immersion THz imaging system compared to 0.85k 0resolution of a standard imaging system, employing only an aspherical singlet. We have discussed the prospective of using the developed technique in various branches of THz science and technology, namely, for THz measurements of solid-state materials featuring sub-wavelength variations of physical properties, for highly accurate mapping of healthy and pathological tissues in THz medical diagnosis, for detection of sub-wavelength defects in THz non-destructive sensing, and for enhancement of THz nonlinear effects.
IntroductionOver the last decade, THz spectroscopy has been used with success in a variety of fundamental and practical applications, [1,2] which among others include: spectroscopy of condensed matter [3][4][5] and gases, [6] medical diagnosis [7,8] and therapy, [9] Terahertz (THz) frequency range opens significant opportunities in various fundamental and applied fields including condensed matter physics and chemistry, biology and medicine, public security and nondestructive testing. Despite significant advances in THz instrumentation, the problem of THz sensing in harsh environments, particularly at high temperatures and pressures, remains acute due to the lack of THz materials and optical components capable for operation under the extreme conditions. To address this problem, the THz hollow-core photonic crystal sapphire waveguides that are fabricated using shaped crystal growth technique are developed. Numerical analysis and experimental study show that the proposed waveguides operate in a fewmode regime and allow for the broadband transmission of THz pulses with small dispersions and low propagation losses. Thanks to the unique physical properties of sapphire, the proposed waveguides are capable of operating in a variety of aggressive environments. As an example, the developed waveguides are used to conduct the intra-waveguide interferometric sensing of phase transitions in sodium nitrite films at high temperatures. It is believed that the proposed sapphire-based material's platform has strong potential for developing THz guided optics for applications in intra-waveguide spectroscopy, interferometry, and remote sensing in aggressive environments.
Sapphire Terahertz Waveguides
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