Shi Yu-Lei

Consistency-dependent optical properties of lubricating grease studied by terahertz spectroscopy

The optical properties of four kinds of lubricating greases (urea, lithium, extreme pressure lithium, molybdenum disulflde lithium greases) with difierent NLGL (National Lubricant Grease Institute of America) numbers were investigated using terahertz time-domain spectroscopy. Greases with difierent NLGL grades have unique spectral features in the terahertz range. Comparison of the experimental data with predictions based on Lorentz{Lorenz theory exhibited that the refractive indices of each kind of lubricating grease were dependent on the their consistency. In addition, molybdenum disulflde (MoS2) as a libricant additive shows strong absorption from 0.2 to 1.4 THz, leading to higher absorption of MoS2-lithium grease than that of lithium grease.

Transient optical modulation properties in the terahertz metamaterial of split ring resonators

The ultrafast optical modulation properties of split ring resonators are characterized by utilizing optical pump-terahertz probe spectroscopy. The experimental results show that when the terahertz electric vector is perpendicular to the gap of the split ring resonator, resonant absorption can be quenched significantly under high pump excitation. However, when the terahertz electric vector is parallel to the gap, the resonant absorption is less sensitive to pump excitation due to the structural properties of the metamaterial. Our numerical simulations also demonstrate that the pump pulse significantly influences the split ring resonator current by generating carriers in the substrate.

Far-infrared conductivity of CuS nanoparticles measured by terahertz time-domain spectroscopy

This paper reports that terahertz time-domain spectroscopy is used to measure the optical properties of CuS nanoparticles in composite samples. The complex conductivity of pure CuS nanoparticles is extracted by applying the Bruggeman effective medium theory. The experimental data are consistent with the Drude–Smith model of conductivity in the range of 0.2–1.5 THz. The results demonstrate that carriers become localized with a backscattering behaviour in small-size nanostructures. In addition, the time constant for the carrier scattering is obtained and is only 64.3 fs due to increased electron interaction with interfaces and grain boundaries.

Diffraction of terahertz waves after passing through a Fresnel lens

The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using the scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is observed. Additionally, the distribution of terahertz fields in a plane perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on-axis but also inhibits focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to being a terahertz filter. Moreover, the terahertz pulses travelling as a basic mode of a Gaussian beam are discussed in detail.

Transient surface photoconductivity of GaAs emitter studied by terahertz pump-emission spectroscopy

This paper investigates the ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs in detail by using a terahertz pump-emission technique. Based on theoretical modelling, it finds that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse. Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed after the excitation of pump pulse and we attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperature-dependent measurements are also performed to verify this trapping model.