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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.

Terahertz waves through a split-ring resonator (SRR) can induce the resonant absorption, and this can be explained by using the theory of LC resonant circuit and the model of half wave resonance. However, in the dual-ring structure without the split gap, we still observe the resonant absorption in the THz frequency range. By analyzing the phenomenon, we think that it can be explained by using a the model of half wave resonance. Furthermore, it is found that if the structure is fabricated on quartz crystal substrate, we can obtain the angle-dependent terahertz waveforms using terahertz time-domain spectroscopy (TDS) when the sample is rotated in plane, as well as the frequency domain spectra. But this phenomenon does not exist in the silicon-based structure, which may be attributed to the birefringence effect of the quartz crystal substrate on the subwavelength metal dual-ring structure. The main purpose of this article is to explain the physical process of the effect.

In this paper, a new method using reflective pulsed thermography to measure defect depth, thermal wave reflection coefficient and thermal diffusivity is presented. First, a brief description of the pulsed thermography in terms of theoretical background and quantitative measurement is given. One stainless steel 304 structure machined several flat-bottom holes in which it is filled with different materials are used as experimental sample, the measured results of defect depth, thermal diffusivity and reflection coefficients at defect interface under different conditions are given. The agreement between the results obtained by using pulsed thermography and the value presented in the literature or measured by other techniques appears satisfactory within errors of 5%, and possible reasons for affecting the measurement precision are discussed.

This work aims at developing a thermal wave imaging method for testing the insulation layer of superconducting busbar in thermonuclear experimental reactor. In the paper, a film bag filled with hot water is used to heat the sample surface. The approximate theoretical model of transient heat conduction under an instantaneous cylindrical surface source in cylindrical structure is given. Through simulation and analysis of the theoretical model, it is found that differential processing of the thermal cooling signals can improve the signal contrast between defects and non-defects areas. Compared with the flash pulse excitation method, the contact excitation method can detect deep defects.

In this work, we present a quantitative method of predicting defect depth within carbon fiber reinforced plastics (CFRP) composite with unknown thermal property of specimen. The method discussed in this paper is derived from theoretical model of ideal one-dimensional heat conduction in a flat plate sample. The principle of depth prediction using pulsed infrared thermography is presented; the time based on the peak second-derivative of the temperature difference in the logarithmic scale is selected as the characteristic time; the calibration curve based on data of single defect depth can be easily affected by random error. The calibration curve is put forward in the paper and obtained by designing a stepped reference sample, and it can be obtained by a curve fit of different depths and corresponding characteristic time using polynomial function. The polyfitting curve is used as the calibration curve for depth prediction when the relative error square is minimum. The experimental results show that the depth measured by the calibration curve based on many reference defect depths is more accurate than by single reference defect depth. Defect depth in CFRP composite can be measured by the method without knowing thermal property of specimen.

Through the FDTD simulation of the transmittance in terahertz band of the split-ring resonator (SRR) with different opening shapes, we found a mean gap effect of SRR during the resonance and a particular conduction phenomenon in case of very sharp edge of the split.

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.