Light beams with helical phase-fronts are attractive for many optical applications, such as optical tweezers, particle manipulation, and other optical applications. A superoscillatory metalens has been designed for an azimuthally polarized wave at a wavelength λ = 632.8 nm. Numerical simulation demonstrates that a superoscillation hollow dark spot is generated. The transverse inner FWHM is 0.358λ overcoming the diffraction limit, while the sidelobe ratio can be compressed to 18.3%. More importantly, for such incidence with different orbital angular momentum (OAM) ℓ = 1, 2, and 3, the proposed metalens also presents elegant subwavelength tight focusing performances (the FWHMs are 0.469λ, 0.352λ, and 0.737λ, respectively). Extensive investigations have been made to present the unique characteristics of the generated foci, the phase distribution patterns are given for different OAM values. Small size and ultrathin thickness make such lenses suitable for various optical applications.
The determination of residual water content is vital for the quality control of high explosives, such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). However, conventional analytical methods are time-consuming and inaccurate. We have proposed a simple, sensitive, and accurate method for moisture determination based on terahertz (THz) time-domain spectroscopy. A model based on effective medium theory was implemented by a third-order extension of the Landau, Lifshitz, and Looyenga model to predict the moisture content of HMX based on THz spectroscopy. The predicted values of moisture content based on the model were found to be in excellent agreement with the results obtained using gravimetric measurements. Overall, the results proved the effectiveness of the proposed method for rapid and accurate moisture determination of HMX.
We have utilized optical-pump terahertz-probe spectroscopy to analyze the relaxation process of photoinduced nonequilibrium carriers in bulk ZnTe with a crystal orientation of ⟨ 110 ⟩ . The experimental results showed that the concentration of nonequilibrium carriers increases with the increase in the energy of pump laser. When the pump energy calculated by a single exponential fitting function reached 63 μJ, the concentration of nonequilibrium carriers attained saturation. In addition, for low pump energy, the relaxation time did not change with increasing pump energy. On the contrary, when the pump energy was high, the relaxation time increased in proportion to the pump laser energy.