A bi-tunable triple-band metamaterial absorber based on Dirac semimetal films (DSFs) and vanadium dioxide (VO₂) is presented. When VO₂ is in the fully metallic state, the proposed absorber presents three distinctive absorption peaks in the terahertz range with absorptance 97%. Because the conductivity of VO₂ changes from 100000 to 10 S/m, the reflectance and absorptance intensities achieve dynamic tunability at the three absorption peaks, and the proposed triple-band absorber exhibits a switchable function by the insulation-to-metal transition of VO₂. Moreover, the frequencies of the three absorption peaks can also be tuned by varying the Fermi energies of the DSFs.

We demonstrated a dynamically controlled broadband terahertz (THz) metamaterials absorber, which composed of continuous vanadium dioxide (VO₂) film, a silicon dioxide (SiO₂) layer, and a structured borophene layer. When VO₂ is in its metallic state and the armchair direction of borophene along x axis, the proposed absorber realizes an absorptivity peak value of 100% at 7.2 THz for y polarized normal incidence, and an absorptivity peak value of 79% at 8.9 THz for x polarized normal incidence. It is the anisotropic property of borophene that results in the absorptivity difference for x and y polarization in the whole frequency range. Simulated electric field distribution and surface current oscillation has been extracted to explain the physical mechanism of THz wave absorption. Through modifying the geometric parameters of metamaterials microstructure, the broadband absorption performance can be tailored passively. Additionally, the proposed metamaterials absorber has been actively controlled by manipulating the carrier density of borophene and the conductivity of VO₂, respectively. The absorptivity can be switched from 45% to 100% at 7.2 THz by changing the carrier density of borophene, and from 22% to 100% at 7.2 THz by changing the conductivity of VO₂. Moreover, the proposed absorber exhibits an excellent operation tolerance for oblique TE and TE polarized incidence from 0° to 60°. This work provides a novel approach to design dynamically controlled broadband THz absorbers, which reveals promising applications in the devices of optoelectronic switches, cloakings, filters, and sensors, etc.

The high-resolution terahertz spectra of the two structural isomers, mannose and galactose, have been measured by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.54.0THz at room temperature. Significant differences between these similar molecules have been found in their THz characteristic spectra, implying that THz-TDS is a powerful tool for identifying isomers. Structural analyses and normal mode calculations of the two systems were performed using solid-state density functional theory (DFT) with the PBE and PW91 density functionals as well as using gas-state DFT with B3LYP hybrid functional. Among these calculations, the solid-state simulated results obtained from the PBE method exhibit a good agreement with the experimentally measured spectra. According to the calculated results of PBE, the observed spectral features were assigned as primarily external lattice translations, deformations, and rotations with lesser contributions due to intramolecular motion of pyranose ring, CH₂OH group, and hydroxyl groups.

Yb,Nd:Sc2SiO5 crystal was investigated by co-doping Nd3+ ions into Yb:Sc2SiO5 crystal. The emission cross section was systematically enquired under different excitation wavelength at 808 nm, 915 nm and 980 nm. The emission cross section was enhanced by 18.9 % at excitation wavelength of 980 nm due to energy transfer between Nd3+ ions and Yb3+ ions when comparing with Yb:Sc2SiO5 crystal. The continuous-wave (CW) tunable Yb,Nd:Sc2SiO5 laser ranging over 58 nm was achieved from 1030.4 nm to 1088.4 nm. Mode-locking Yb,Nd:Sc2SiO5 femtosecond laser was obtained with output power of 240 mW, pulse duration of 367 fs and peak power of 11.5 kW.