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In this paper, we study terahertz generation through optical rectification in reflection at normal incidence in a dielectric nonlinear crystal. We first analyze, with a nonlinear optical model, the sample parameters (thickness, absorption at both laser and terahertz wavelengths, etc.) for which a terahertz optical rectification reflection scheme is preferable to the common transmission scheme. Then, we report our experimental observations of a reflected terahertz signal generated at the surface of a ZnTe crystal. The reflected terahertz signal shares all the characteristics of a signal generated in transmission but is not limited by absorption losses in the crystal, thereby providing a broader bandwidth. At high pump laser power, the signal exhibits saturation, which is caused by the decrease of the nonlinear susceptibility due to photocarriers generated by two-photon absorption. This reflection scheme could be of great importance for terahertz microscopy of opaque materials like, e.g., humid samples or samples exhibiting strong absorption bands or to study samples for which the transmitted signal cannot be recorded.

We investigate terahertz time-domain spectroscopy (THz-TDS) as a non-destructive and non-contact technique for depth profiling of dopants in semiconductors. THz temporal waveforms transmitted through silicon-ion-implanted semi-insulating gallium arsenide substrates, as-implanted or post-annealed by rapid thermal annealing, were analyzed by assuming a multi-layered Gaussian refractive index profile in the ∼sub-micrometer-thick implantation region. The implantation energy and dosages in this work were 200 KeV, 1014, 5 × 1014, and 1015 ions/cm2, respectively. The average values of real ( n) and imaginary ( κ) parts of refractive indices of an as-implanted sample in the depth range of 0–800 nm are 5.8 and 0.7, respectively, at 0.5 THz and are 6.2 and 0.2, respectively, at 1 THz. On the other hand, the refractive index profile of the post-annealed samples displays a prominent Gaussian-like form, and peak refractive indices ( n ∼ 25 and κ ∼ 32.7 at 0.5 THz and n, κ ∼17 at 1 THz) were found to be at the depth of 210 nm. Reconstructed dopant profiles in as-implanted, implanted, and post-annealed substrates were found to be in good agreement with measurements by secondary ion mass spectroscopy as well as simulation by the Monte Carlo method. We were also able to determine accurately the projected range ( Rp), straggle ( Rs), and concentration of dopants by the analysis of THz-TDS data. The spatial resolution, along the depth direction, of the THz-TDS technique for depth profiling of dopants was estimated to be as small as 8-nm. This work suggests the feasibility of using THz-TDS for nondestructive and non-contact diagnostics for profiling dopants in semiconductors.

We study optical rectification in GaSe by performing THz generation with femtosecond laser pulses whose wavelength is tuned from below to above the GaSe bandgap. As expected from a theory, we observed a first THz emission peak at 1.77 eV, where phase matching is realized. A second THz emission peak was recorded, when the pump photon energy reaches the crystal bandgap (2.205 eV). This can be attributed to a resonance of the GaSe nonlinearity. In crystals thinner than the coherence length, the bandgap peak is stronger than the phase-matched one.

By generating terahertz pulses in KTP crystals through optical rectification with a pump photon energy varying from below to above the bandgap, we observe a peak of the THz signal at the bandgap energy but also a second one around half the bandgap. This later one is attributed to a two-photon absorption enhanced nonlinearity, which is validated by the similarity of the two-photon absorption coefficient and THz peak amplitude data versus the pump photon energy. A careful analysis of the KTP sample absorption spectral dependence nearby the bandgap demonstrates that KTP is an indirect bandgap crystal, whose absorption below the bandgap involves emission of a phonon related to the symmetric Ti-O stretching vibration, i.e. the ν

We present a UV-visible pump/THz-probe study to determine the photo-carrier lifetime in GaAs. Three samples ( , and oriented) were pumped from 1.56 eV to 4.15 eV. For the three orientations we observed a strong unexpected peak of the carrier lifetime around 2.6 eV.

We perform THz generation by optical rectification in ZnTe, KTP and RKTP with femtosecond laser pulses whose wavelength ranges from UV to IR. We observe a strong THz emission when the pump photon energy reaches the crystal bandgap. This phenomenon is explained by the resonance of material nonlinear susceptibility around its bandgap. Additionally, for the first time, we report of a two-photon enhanced nonlinearity in KTP/RKTP, which leads to an enhanced THz emission near half the bandgap.

We report on the generation of THz waves through optical rectification in ZnTe of femtosecond laser pulses whose photon energy is tuned from below to above the ZnTe bandgap energy. The THz signal exhibits a pronounced peak at the bandgap energy, at THz frequencies for which losses in ZnTe remain small. This peak is likely due to the resonance of the ZnTe nonlinear susceptibility in the vicinity of the bandgap.

ZnTe is a crystal widely used to generate THz waves through optical rectification (OR) of femtosecond 800 nm laser pulses [1] . It indeed presents interesting characteristics since phase-matching is almost realized, losses are rather weak, and the birefringence issue is ignored. Its nonlinear coefficients are however quite small compared to other used crystals like LiNbO 3 for example. It should be possible to increase those nonlinear coefficients by pumping the crystal closer to its bandgap, since this enhancement has been modelled and measured for second harmonic generation [2] . Unfortunately, when dealing with THz generation, pumping at or above the bandgap excites free carriers that absorb the THz wave, leading to a decrease of THz generation efficiency. The goal of our work is to study the balance between enhanced nonlinearity and free carrier absorption by recording the THz signal versus the pump photon energy.

A frequency-selective, tunable terahertz (THz) filter based on a flexible tape is designed and characterized for a low-cost and easy-access THz filter with a wide range of center wavelengths (CWL) from 150 to 600 μm (2.0 to 0.5 THz). With more than 85% bandpass transmission and 10 dB bandstop reflection at the CWLs, the proposed filter based on metallic crossed-slot hole arrays are fabricated on a 50-μm polyimide film with low absorption and good mechanical properties, resulting in a large-area, free-standing, and flexible membrane with a low intrinsic loss. The frequency dependent characteristics of the filter are investigated by THz time-domain spectroscopy and excellent agreements between the measured and simulated results are demonstrated. The designed THz filter possesses not only a simple structure and polarization insensitivity, but also a light weight and high integration.

Nondestructive analysis of historical objects is of significance for cultural heritage conservation. In this paper, terahertz time-domain spectroscopy (THz-TDS) was used to distinguish seven red mineral pigments used in ancient Chinese artworks. Two absorption features of natural minerals HgS and four highly resolved spectral features of mineral pigment Pb3O4 were observed and identified as their fingerprints in the range 0.2 to 3.0 THz, based on which the spatial distribution of individual chemical substances including cinnabar, vermilion, and red lead were clearly revealed at various frequencies using terahertz spectroscopy imaging. Moreover, a noncontact evaluation of thickness changing and dehydration of a wet painting was monitored by inferring time delay as well as signal amplitude of THz pulses transmitted through the painting. In order to demonstrate the feasibility of THz-TDS and THz imaging for authentic artworks detection, a complete set of THz analysis of two nineteenth century wall paintings discovered in the Fuchen Temple of the Forbidden City, Beijing, was performed and the results indicate that THz measurement techniques provide a noninvasive and nondestructive solution for the care, preservation, and restoration of cultural relics.

In this work, we demonstrate a high-selectivity terahertz (THz) band-stop filter with a wide range of center wavelengths (CWLs) from 150 to 600 μm (2.0 THz to 0.5 THz). The dip transmission is lower than 10 % at the center CWLs, even to 5 % at 1.9 THz. The band-stop terahertz filter is based on periodic metallic Cross Cell (CC) structures deposited on the top of a 50 μm thick polyimide film (Kapton) with low absorption and good mechanical properties, resulting in a large-area, freestanding and flexible membrane with a low intrinsic loss. The excellent tunable terahertz band-stop filter properties are investigated using terahertz time-domain spectroscopy. The measured and simulated results are coming to an excellent agreement. The THz band-stop filter possesses not only a light weight and polarization insensitivity but also a simple structure and high integration.

A band-pass filter working in terahertz region has been experimentally proposed. The proposed filter has a 3dB bandwidth of 0.14THz and a Q factor of 3.1. The fabrication method is also proposed in this paper and this process is fit to the existing microelectronic processing technology. A comparative experiment was also carried out to further discuss the characteristic of the proposed filter.

The flow rate is calculated via ultrasonic flow meter (UFM), which is through measuring the difference of time transmitting flow between flow direction and reverse direction. This paper describes the uncertainty analysis for the method of time difference which is commonly used in ultrasonic flow measurement, and the analysis of error source of uncertainty components as well as the general method of elimination. Based on the technique of pseudo random sequence, this paper presents a precise time difference method based on digital correlation technology, and its principle, realization way and uncertainty evaluation are introduced. On the DSP and FPGA system platform, an ultrasonic flow meter scheme based on the digital correlation technology is suggested.

TDCNTP is a time series forecasting model, which is an improved network traffic prediction model using two-dimensional correlation and the Single exponential smoothing, the TDCNTP model proposed in this article is demonstrated and deduced in detail. Combined with character of the network and the TDCNTP model, the experimental result shows that, compare to the traditional network traffic prediction model, the prediction efficiency and accuracy has increased significantly.