Metasurface Terahertz Perfect Absorber with Strong Multi-Frequency Selectivity.

In this paper, we design a metasurface terahertz perfect absorber with multi-frequency selectivity and good incident angle compatibility using a double-squared open ring structure. Simulations reveal five selective absorption peaks located at 0-1.2 THz with absorption 94.50% at 0.366 THz, 99.99% at 0.507 THz, 95.65% at 0.836 THz, 98.80% at 0.996 THz, and 86.70% at 1.101 THz, caused by two resonant absorptions within the fundamental unit (fundamental mode of resonance absorption, FRA) and its adjacent unit (supermodel of resonance absorption, SRA) in the structure, respectively, when the electric field of the electromagnetic wave is incident perpendicular to the opening. The strong frequency selectivity at 0.836 THz with a Q-factor of 167.20 and 0.996 THz with a Q-factor of 166.00 is due to the common effect of the FRA and SRA. Then, the effect of polarized electromagnetic wave modes (TE and TM modes) at different angles of incidence (θ) and the size of the open rings on the device performance is analyzed. We find that for the TM mode, the absorption of the resonance peak changes only slightly at θ = 0-80°, which explains this phenomenon. The frequency shift of the absorption peaks caused by the size change of the open rings is described reasonably by an equivalent RLC resonant circuit. Next, by adjusting two-dimensional materials and photosensitive semiconductor materials embedded in the unit structure, the designed metasurface absorber has excellent tunable modulation. The absorption modulation depth (MD) reaches ≈100% using the conductivity of photosensitive semiconductor silicon (σSI-ps), indicating excellent control of the absorption spectrum. Our results can greatly promote the absorption of terahertz waves, absorption spectrum tunability, and frequency selectivity of devices, which are useful in the applications such as resonators, bio-detection, beam-controlled antennas, hyperspectral thermal imaging systems, and sensors.

Tunable and coherent terahertz source based on CdSiP2 crystal via collinear difference frequency generation.

CdSiP2 (CSP) crystals have attracted increasing attention as efficient optical conversion media. Herein, the optical properties of a CSP crystal grown with the vertical Bridgman method are measured by a terahertz time-domain spectrometer (THz-TDS) at 0.2-3 THz. For the first time, to the best of our knowledge, the broadband, tunable, coherent, monochromatic THz radiation from 0.08 to 1.68 THz (3775-178 µm) is generated experimentally via this crystal, which is pumped by a nanosecond Q-switched Nd:YAG laser and an optical parametric oscillator (OPO) and based on difference frequency generation (DFG) technology. The output power and its corresponding conversion efficiency at 0.74 THz are 26.6 mW and 1.4 × 10-7, respectively. Our work demonstrates that the CSP crystal is a potential efficient terahertz DFG candidate for out-of-door applications.

Terahertz dispersion using multi-depth phase modulation grating.

We present a multi-depth phase modulation grating (MPMG) in the terahertz range making real-time multichannel Fourier-transform spectroscopy available in a stationary manner. The calculation of the Fraunhofer diffraction field distribution and diffraction efficiency of an MPMG indicates that the zeroth-order diffraction light of an MPMG carries phase information and its diffraction intensity is modulated by the groove depth. A good agreement is found between the measurements of the 0th- and ±1st-order diffraction efficiency at 0.5 and 0.34 THz and the simulation. The frequencies of the terahertz source retrieved from the zeroth-order diffraction intensity at 0.5, 0.4, and 0.34 THz are identical to the actual frequencies.

Terahertz communication windows and their point-to-point transmission verification: publisher's note.

This publisher's note amends the author affiliations in Appl. Opt.57, 7673 (2018)APOPAI0003-693510.1364/AO.57.007673.

Terahertz communication windows and their point-to-point transmission verification.

Terahertz communication is recognized as a transformational technology that can meet the future demands of point-to-point communication. The study of terahertz atmospheric transmission characteristics is important for guiding the terahertz communication window selection process. In this report, based on the modified ITU-R P.676-10 model, we determined that the terahertz communication windows above 100 GHz were located at the bands at approximately 140, 220, 340, 410, and 460 GHz, which is verified by recent experiments. We also verified the feasibility of indoor point-to-point communication by the 110 m transmission experiment through the communication window around 460 GHz.

Directly tailoring photon-electron coupling for sensitive photoconductance.

The coupling between photons and electrons is at the heart of many fundamental phenomena in nature. Despite tremendous advances in controlling electrons by photons in engineered energy-band systems, control over their coupling is still widely lacking. Here we demonstrate an unprecedented ability to couple photon-electron interactions in real space, in which the incident electromagnetic wave directly tailors energy bands of solid to generate carriers for sensitive photoconductance. By spatially coherent manipulation of metal-wrapped material system through anti-symmetric electric field of the irradiated electromagnetic wave, electrons in the metals are injected and accumulated in the induced potential well (EIW) produced in the solid. Respective positive and negative electric conductances are easily observed in n-type and p-type semiconductors into which electrons flow down from the two metallic sides under light irradiation. The photoconductivity is further confirmed by sweeping the injected electrons out of the semiconductor before recombination applied by sufficiently strong electric fields. Our work opens up new perspectives for tailoring energy bands of solids and is especially relevant to develop high effective photon detection, spin injection, and energy harvesting in optoelectronics and electronics.

Extreme Sensitivity of Room-Temperature Photoelectric Effect for Terahertz Detection.

Extreme sensitivity of room-temperature photoelectric effect for terahertz (THz) detection is demonstrated by generating extra carriers in an electromagnetic induced well located at the semiconductor, using a wrapped metal-semiconductor-metal configuration. The excellent performance achieved with THz detectors shows great potential to open avenues for THz detection.

High performance of Mn-Co-Ni-O spinel nanofilms sputtered from acetate precursors.

Mn-Co-Ni-O (MCN) spinel oxide material, a very important transition metal oxide (TMO) with the best application prospects in information and energy fields, was discovered over five decades ago, but its applications have been impeded by the quality of its films due to the magnitude of deposition challenge. Here we report that high quality of MCN nanofilms can be achieved by sputtering deposition via acetate precursors whose decomposition temperatures are matched to the initial synthesis temperature of the MCN thin films. Excellent performance of MCN nanofilms is demonstrated, combining for the first time preferred orientation, high temperature coefficient of resistance, and moderate resistivity. The film devices show an intrinsic recombination with a much faster rate of the order of a microsecond for the laser-pumped carriers, which is ~3 orders of magnitude larger compared with that of the ceramic material. The electronic structure of the thin films confirms that it is indeed of n-type nature, exhibiting appropriate electronic states consistent with the levels of metal electrodes and semiconductors. The results offer a vital avenue for depositing high performance TMO thin films for advanced oxide devices, and will have great significance for exploiting new applications in modern oxide electronics and optoelectronics.

Room-temperature photoconductivity far below the semiconductor bandgap.

A concept to stimulate photoconductivity in a semiconductor well below its bandgap in a metal-semiconductor-metal structure with sub-wavelength spacing is proposed. A potential well is induced in the semiconductor by external electromagnetic radiation to trap carriers from the metals. This opens an avenue to generate carriers by photons without adequate excitation energy and is expected to have great significance in modern materials.