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In this paper, a multifunctional device and a design method are proposed based on the vanadium dioxide (VO2)-assisted metamaterial structure. The structure comprises several layers arranged from top to bottom, including a VO2 patch layer, a polyimide (PI) dielectric layer, an elliptical metal layer, a VO2 thin film layer, another PI dielectric layer, and a bottom metal layer. The research results show that the metamaterial structure enables linear-to-linear (LTL) polarization conversion and linear-to-circular (LTC) polarization conversion across multiple frequency bands when the VO2 is in the insulating state. Moreover, as the VO2 material undergoes a transition from the insulating state to the metallic state, the multifunctional structure can function as a broadband absorber, exhibiting an absorption rate of over 90% within the frequency range of 1.751–3.853 THz, with a relative bandwidth of 75%. This versatile conversion device holds great potential for applications in terahertz system and smart system fields. [ABSTRACT FROM AUTHOR]

We investigate the time domain characterization of a coplanar waveguide (CPW) based on an on-chip electro-optic sampling (EOS) system for millimeter waveform metrology. The CPW is fabricated on a thin layer of low-temperature gallium arsenide (LT-GaAs), and the substrate material is GaAs. A femtosecond laser generates and detects ultrashort pulses on the CPW. The forward propagating pulses are simulated using a simplified current source for the femtosecond laser at different positions on the CPW for the first time. Then, the influences of the CPW geometry parameters on the measured pulses are discussed. The varying slot width has larger influences on the amplitude of millimeter wave pulses than the center conductor width and the pumping gap. Finally, in the frequency range of 10 GHz to 500 GHz, the transfer functions calculated by the time domain pulses are in good agreement with the transfer functions calculated by the frequency domain ports. The above results are important for improving the measurement precision of the millimeter waveform on the CPW for millimeter waveform metrology. [ABSTRACT FROM AUTHOR]

Research on the interaction between nanoscale materials and light holds significant scientific significance for the development of fields such as optoelectronic conversion and biosensing. The study of micro- and nano-optics has produced numerous outstanding research achievements by utilizing the dielectric optical coupling mechanism and plasmon effects to enhance the interaction between light and matter. These findings have demonstrated tremendous potential for applications in the field of molecular fingerprint sensing. This review focuses on a retrospective analysis of recent research studies in the enhancement of wide-band trace terahertz absorption spectroscopy. The physical mechanisms of using waveguide structures, dielectric metasurfaces/meta-gratings, and spoof surface plasmon polaritons (SSPs) to improve the interaction between light and trace-amount matters are introduced. The new approaches and methods for enhancing broad-band terahertz absorption spectroscopy of trace samples using microstructure designs are discussed. Additionally, we elucidate the scientific ideas and exploratory achievements in enhancing terahertz fingerprint spectroscopy detection. Finally, we provide an outlook on the research and development direction and potential practical applications of absorption spectroscopy enhancement detection. [ABSTRACT FROM AUTHOR]

Metasurface optical devices have shown a tendency to gradually supersede the conventional bulk devices in applications requiring compactness and light weight. Typical metasurface-based flat lenses that manipulate the wavefront rely on phase mapping using a limited number of subwavelength structures. However, this approach greatly reduces the efficiency when limited structures are mapped for a high numerical aperture (NA). Here, we theoretically and experimentally propose low refractive index meta-grating-based multifunctional devices fabricated by three-dimensional printing technology. The meta-grating arrays effectively bend the incident polarized terahertz wave to the designed diffraction angle of 67°, with transmission efficiencies of 74.1% (S-dimer) and 54.2% (P-dimer) at the frequency of 0.1 THz. Then, the meta-lens based on the proposed meta-grating arrays achieves the measured focus efficiency of 32.1% with NA = 0.902, corresponding to a maximum collection angle of 64.5°. The designed flat devices with excellent characteristics in terms of efficiency and NA represent a new paradigm in high efficiency, low-cost and multifunctional terahertz devices. [ABSTRACT FROM AUTHOR]

Terahertz (THz) absorption spectroscopy is widely used for molecular label-free fingerprinting detection, but it is not capable of efficiently analyzing trace-amount sample materials. For improving the efficiency of terahertz absorptance spectroscopy detection, we propose a sensing strategy by treating the geometry sweeping spoof surface plasmon polariton (SSP) of the stretchable metasurface. For the first time, the geometry sweeping can be realized by dynamically stretching the polydimethylsiloxane (PDMS) flexible substrate, leading to the resonant frequency variation of the unit cell. This design provides a significant absorption enhancement factor about 270 times for a 0.1 μm lactose film in a broad terahertz band, enabling the unambiguous identification of different trace-amount samples. The designed method exhibits a novel solution for the enhancement of broad-band terahertz absorption spectroscopy and great application potential in the field of detecting trace-amount samples. [ABSTRACT FROM AUTHOR]

We used modulating retro-reflector (MRR), hybrid frequency-phase-keying (FPK) modulation, and Reed-Solomon (RS) code technology to improve the performance of the terrestrial-satellite laser communication link. A system using FPK modulation and RS codes based on the Fisher-Snedecor (F) distribution atmospheric turbulence model is proposed for the MRR-assisted terrestrial-high-altitude-platform (HAP)-satellite laser communication, in which MRR is installed on the HAP. The laser beam without signal is transmitted to HAP through the forward path. The MRR loads the optical signal from the terrestrial-HAP link onto the optical carrier, which is then reflected back to the satellite terminal along the backward path. The combined effects of light intensity scintillation of atmospheric turbulence, beam wander of uplink, atmospheric attenuation, pointing error, and HAP random jitter on the terrestrial-HAP (equipped with MRR)-satellite (T-HMRR-S) laser communication link are considered. The expression of the BER of RS codes and FPK modulation (RS-FPK) system under weak atmospheric turbulence is derived and compared with terrestrial-HAP-satellite (T-H-S) system with different modulation and without MRR. At the same time, the effects of modulation coding parameters, zenith angle, the field-of-view angle, and the coverage area of the MRR on the performance of the T-HMRR-S system are simulated and analyzed. The simulation results show that the T-HMRR-S system using RS-FPK technology has better performance. These works provide a theoretical basis for the research of modulation coding technology of T-HMRR-S link systems. [ABSTRACT FROM AUTHOR]