Terahertz time-domain spectroscopy (THz-TDS) is a newly developed technique in the last decade. A new method to classify tea is proposed based on THz-TDS and pattern recognition method. Four kinds of tea are investigated to study its feasibility. Their absorption spectra are measured using the THz-TDS system. Then a pattern recognition method, support vector machine (SVM), is employed to differentiate the investigated tea based on their terahertz data. The absorption spectra between 0.2 and 2.0 THz of the investigated tea samples are selected as the feature to classify them. The correct rate of recognition for the SVM classifier is 96.88%. The results give evidence that different tea varieties can be distinguished based on their terahertz spectra. In this respect, the spectral method provides enough information to differentiate different kinds of tea.

We used terahertz time-domain spectroscopy (THz-TDS) to probe the enhancement transmission of Ag₂O-Ag compound. Using a 500μm- GaAs substrate attaching to the sample, the evanescent wave is coupled to the far field. And the transmitted amplitude is enhanced, corresponding to the frequent shift and spectra broadening.

In this work a powerful terahertz antenna array with interdigital electrodes is fabricated, and the performance of one antenna unit is compared with a conventional resonant dipole antenna. The antenna unit has a better capacity of generating THz wave compared with a conventional resonant dipole antenna at the same bias electrical field and the same laser energy. However only 23 % of THz wave transmitted through the ceramic substrate of antenna array, if there is a hole drilled through ceramic substrate to release the THz wave, the THz amplitude of entire interdigital antenna array with 8 antenna units can be more than 10 times larger than that of resonant dipole antenna. To get this result, the pump beam is focused into a linear beam by a cylindrical lens to trigger the antenna array, and the linear THz wave is focused by a polyethylene lens before it reaches ZnTe crystal.

Over the past ten years, electromagnetic terahertz (THz) frequencies region from 100 GHz to 10 THz (or wavelengths of 30 μm ~3 mm) have received extensive attention and investigation. Terahertz wave detection enables direct calculations of both the imaginary and the real parts of the refractive index without using the Kramers-Kronig relations. There are many potential applications such as radio astronomy, atmospheric studies, remote sensing, and plasma diagnostics. Shoes, neckties and sofa, etc are mainly made of skin of animal, imitated skin and artificial leather. It has important practical value to component analysis and quality assessment by measuring absorption, refractive index, and other optical parameters. In this paper, the spectral characteristics of sheepskin, imitated sheepskin and artificial leather have been measured with terahertz time-domain spectroscopy (THz-TDS) in the range of 0.1~2.0THz. The results show that there have not absorption peak in the absorption spectrum of the sheepskin. However, it is found that there are three absorption peaks in the absorption spectrum of the artificial leather at the frequency of 1.13THz, 1.21THz, and 1.36THz, respectively. The potential application of the leather in THz frequency region is also discussed.

Many materials were previously studied using far-infrared Fourier transform spectroscopy (FTS) in transmission and reflection modes. Recently, there has been a remarkable effort in employing terahertz time-domain spectroscopy (THz-TDS) for investigating material properties, including environment pollutants, semiconductors, polymers, explosive materials, and gases, etc. Since the absorption coefficient and the refractive index of the material studied are directly related to the amplitude and phase respectively of the transmitted field, both parts of the complex permittivity can be obtained by THz-TDS. In this letter, the optical properties of peanut oils in the frequency range from 0.2 to 2.5 THz were studied by employing terahertz time-domain spectroscopy. Several peanut oils, such as clean unused peanut oil, peanut oil after five minutes of boiling, and peanut oil after ten minutes of boiling were tested. The time delays of clean unused peanut oil, peanut oil after five minutes of boiling, and peanut oil after ten minutes of boiling are 8.33ps, 8.46ps and 8.46ps, respectively. The refractive indices of the three oil samples show slow a decrease as the terahertz wave frequency increases. The power absorption coefficients increase as the frequency increases within the investigated terahertz wave frequency range.

Recently, there has been a remarkable effort in employing terahertz (THz) spectroscopy for investigating material properties. THz-TDS has been employed to investigate a wide variety of materials, including environment pollutants, semiconductors, polymers, explosive materials, and gases, etc. In this letter, the spectral characteristics of cotton in the range of 0.2~2.5THz have been measured with THz time-domain spectroscopy. Its absorption and refraction spectra are obtained at room temperature in nitrogen atmosphere. It is found that cotton has the spectral response to THz waves in this frequency region. The results provided in this paper will help us to study the THz application to cotton commercial transaction inspection further.

Recently, there has been a remarkable effort in employing terahertz (THz) spectroscopy for investigating material properties. Pulsed THz time-domain spectroscopy is a coherent technique, in which both the amplitude and the phase of a THz pulse are measured. Coherent detection enables direct calculations of both the imaginary and the real parts of the refractive index without using the Kramers-Kronig relations. In this letter, the terahertz absorption spectra and the refractive indices of olive oil were measured by using terahertz time-domain spectroscopy (THz-TDS) in the frequency range extending from 0.2 to 2.5 THz. The terahertz dielectric properties of olive oil were characterized by THz-TDS, and the consistency with the known parameters was identified. A novel iterative algorithm improves the existing data extraction algorithms and further enhances the accuracy of the parameter extraction for terahertz time-domain spectroscopy. The results obtained in this study suggest that the THz-TDS method is a useful tool for vegetable oils characterization in the far infrared region. This method can be applied not only to terahertz time-domain spectroscopy but also to any kind of optical constant measurement in the time domain.

A THz time-domain spectroscopy (THz-TDS) pulse signal is a temporal response of THz reference pulse. Although the field of THz-TDS signal processing and analysis techniques is relatively unexplored, work has been reported in this field. One of those is wavelet analysis approach of terahertz signals. It has been shown that the wavelet transform is an efficient representation of THz pulses due to their pulse-like nature. Unlike Fourier analysis, which only uses infinite sinusoids as the basis functions, in wavelet analysis, there are a large number of wavelet bases for different applications, and each of these wavelet bases exhibits different properties. In this paper, the problem that how to select an appropriate wavelet basis for representation and analysis of THz-TDS signals is discussed by lots of comparing experiments. Three criterions, which are wavelet basis efficiency index (WBEI), pulse spectral relative entropy (PSRE) and pulse spectral cumulative error (PSCE), are presented to determine a preferable mother wavelet for a given THz-TDS reference pulse.

We diagnosed the polarization characteristics of Terahertz (THz) emission from a femtosecond laser filament in air in the forward direction by using a wire grid polarizer and E-O sampling technique. The experimental results have demonstrated that the investigated THz pulse is elliptical polarized. It indicates that there is a new generation mechanism for THz emission from a laser filament other than the transition-Cherenkov radiation which is radially polarized. The generation mechanism could be through four-wave optical rectification or second-order optical rectification inside the filament due to the spatial asymmetry of the generated plasma column in the experiment.

In this letter, a bi-layer metamaterial composed of SRR layer and split wires layer is proposed, and the anisotropy of THz propagation through this MTM was firstly addressed and the intrinsic physic foundation was preliminarily discussed. The metamaterial consists of SRR layer, polyimide slab, and metal wires layer on GaAs substrate. A strong absorbance of 99.93% is obtained at 0.836 THz when the THz wave propagates in the "positive" directions of the absorber while in the "negative" case this value is only 2.16%. Our results indicate a strong interaction coupling between the SRR layer and the wires layer, which significantly enhanced the value of the power loss in the space between the two metal layers, leading to the perfect absorbance. The wires layer is a wave reflector which reflects most of the THz wave thus induced the absorbance anisotropy of this metamaterial.

A complete analysis of terahertz(THZ) wave generation and detection of silicon nanowire gated field effect transistor (SNFET) is presented in this paper. Based on the developed SNFET THZ device theory, the dependence of THZ detection of SNFET on bias and structure parameters are obtained and illustrated. The THZ generation condition and various unique characteristics are also demonstrated and analyzed in details. The numerical skills utilized in this paper are also described. Based on the developed numerical tool, the evolution processes of plasma wave in generation and detection mode are also presented.

Ferromagnetic high-k dielectrics are very promising candidates to replace SiO₂ in silicon based microelectronics industry, and also could simultaneously enable semiconductor spintronic devices that harness polarized electrons. In present work, Fe doped CeO₂ was synthesized by ceramic method and the effects of Fe doping on the structure and properties were characterized by ordinary methods and terahertz-time domain spectrometer (THz-TDS) technique. Our results show that the pure CeO₂ only has a small dielectric constant (ε) of 4, while a small amount of Fe (0.9%) doping into CeO₂ promotes the densification and induces a large ε of 23. From the THz spectroscopy, it is found that for undoped CeO₂ both the power absorption and the index of refraction increase with frequency, while for Fe doped CeO₂ we measure a remarkable transparency together with a flat index curve. The absorption coefficient of Fe doped CeO₂ was less than 0.35 cm^-1 ranging from 0.2 to 1.8 THz, rendering it a potential THz optical material. Our results also illustrate that THz technique is a powerful tool to differentiate the dielectric related electronic characteristics of high k materials.

With the deepening of research on the terahertz imaging, real-time imaging is becoming more and more important. Right after Massachusetts Institute of Technology demonstrated 2.52-THz CW imaging by use of a 160×120 element microbolometer camera with spectral coverage from 7.5-14 μm, the single frequency CW THz imaging using the focal-plane array has become a new research direction. In this paper, 124×124 array transmission imaging is demonstrated. The 1.63-THz CW CO₂-pumped laser with the output power of 45mW is used as the illumination source, and a pyroelectric camera is adopted as the array detector with spectral response range of 1.06μm - >1mm. In the imaging experiments, RMB watermark, gasket and key are chosen as objects and several pieces of paper are placed before the objects to test the transmission imaging effect. The experimental results show that utilizing the experimental devices designed in this paper the 1.63-THz images generated are basically clear and objects' contours are clear. The images of gasket and key can be generated penetrating at most 4 pieces of B5 paper in the transmission distance of 0.5m when the power is 45mW and the focal length of polyethylene lens is 120mm.

THz-TDS signals can be represented as vectors in a high dimensional vector space, which are hyper-complex numbers in geometric algebra (GA). Using the language of GA, the properties of these vectors are theoretically analyzed and demonstrate the projective character of THz-TDS signals. The tangential distance of vectors is used to measure the difference of the corresponding THz-TDS signals. A novel imensionality reduction method via the projective split is presented, by which vectors of THz-TDS signals can be linear mapped from a high dimensional space into a lower dimensional space. The projective split is recursively employed and linear maps the vector space of high dimension into a sequence of sub-spaces step by step. Experiments demonstrate the feasibility and accuracy of our method.

Terahertz time-domain spectroscopy (THz-TDS) was utilized to study solid state reaction progress between α-naphthol(NP) and p-benzoquinone(BQ). Density functional theory (DFT) was employed to optimize the structure of the complex. The result shows that the reaction product is associated by hydrogen bonding between one of the oxygen atoms of BQ and one hydrogen atom of the hydroxyl group of NP; and FTIR spectra is presented as supporting evidence. Our work demonstrated that THz-TDS was a promising and complementary method in analyzing solid-state reaction between NP and BQ.

A theory for treating terahertz (THz) surface wave propagation on conical metal wire waveguides is presented. The expression for describing surface wave propagation on the conical wire waveguide is obtained based on the Sommerfeld surface wave model. According to the theory, the surface wave propagation on the conical copper wire waveguide is investigated by the numerical calculation, and the results obtained agree well with that of experimental measurement by Y. B. Ji et al. Furthermore, both the surface wave propagation properties of the conical copper wire waveguides versus the wire diameter and versus the THz wave frequency are studied by the numerical calculation. The work presented provides a theoretical basis for describing surface wave propagation on the conical THz metal wire waveguide and is quite useful for designing this kind of waveguides.

By using the free-space terahertz (THz) electro-optic (EO) sampling technique, the THz waveforms emitted from intrinsic bulk GaAs photoexcited by femtosecond laser pulses under strong bias electric fields up to 300 kV/cm were recorded. The initial acceleration signal in THz emission waveforms, which are corresponding to the acceleration of electrons in the bottom of Γ-valley, has been found to start decreasing with applied electric field above 50 kV/cm, in contrast to the simple picture of electron acceleration in the Γ-valley. This result suggests that the effective acceleration mass of electrons in the Γ-valley significantly increases with increasing F, most likely due to strong band mixing under very high fields.

The calculation of the electron-electron (e-e) scattering rate is the key point in the ensemble Monte Carlo simulation of quantum well THz lasers. Its exact calculation would take most of the whole CPU times. In this paper, we develop a parallel algorithm to speed up the simulation process: the execution of ensemble Monte Carlo algorithm is in the root node, and the calculation of e-e scattering rate is distributed into other 25 nodes. The corresponding parallel algorithm is realized in our computer cluster system using message passing interface (MPI) library. Our practice shows that this parallelization of the serial EMC is simple and easy to realize the implementation.

Terahertz radiation has high penetration capability and will not cause harmful ionization to human beings; therefore THz reflection imaging has wide application prospect in security inspection and counter-terrorism. However the reflection signal in THz active imaging is weak and the sensitivity of existing detectors in this wave band is low. In this paper the 2.52-THz laser reflection imaging system based on scanning imaging is constructed. The FIR laser of SIFIR-50FPL made by Coherent Inc in America is chosen as the THz radiation source and its power is about 30mW. The detector of P4-42 made by the Molectron Corporation is adopted, which works at the room temperature with the spectral range from 1nm to over 1mm. The imaging experiments on knives and some other objects are made utilizing this system. The images of concealed objects are obtained and the images are clearer after image preprocessing. The experimental results show that this imaging system has wide application potential in security inspection.

A design process for a second harmonic operation of a low ohm lossy TE03 mode 0.25 THz gyrotron has been presented. Mode competition and mode selection are carefully studied through the linear theory of CRM. The cavity are designed and optimized by using a time domain open cavity calculation code, and validated by using the famous FEM code HFSS. Interaction numerical investigations are carried out by using a self-consistent nonlinear theory cod. The influences of the magnetic field, current, voltage and the velocity ratio of the electron beam under the interaction between the electron beam and RF field are analyzed. The 14-kW 0.25 THz gyrotron with a predicted device efficiency of 39% is driven by a 25-kV 1.5-A (v⊥/v_// = 1.5, ▵v_z / v_z = 6%) electron beam from a magnetron injection gun. A tapered magnetic field is adopted in the large signal simulation to prompt the electron efficiency.

We construct and characterize a room-temperature tunable Terahertz(THz) liquid crystal(LC) filter. Liquid crystal is a substance which is intervenient between inerratic crystal and isotropy liquid, because their molecular long-range are not likeness crystal, so they are easily affected by exterior conditions, such as electric field, magnetic field, temperature and stress etc., accordingly, their optical characters are change. According to the electrooptic effect of liquid crystal and the theory of Lyot filter, tunable Lyot liquid crystal filter is designed and tunable F-P liquid crystal filter is designed too. It is proved that the tunable liquid crystal filter has great tuning range and narrow band.

The paper presents an electrically controlled terahertz (THz) band pass filter with liquid crystal (LC). Considering dichroic filters theory, the filter of two-dimensional metallic photonic crystals is designed as a band pass filter in THz wave band. Electric field is used to change the direction of LC molecules, so that refraction index of LC is changed and correspondingly the peak transmissivity frequency of the THz band pass filter is changed finally. By this way mentioned above electrically controlled THz band-pass filter with LC can be got. The peak transmissivity frequency of the electrically controlled THz band-pass filter is tunable from 0.18THz to 0.19THz by changing electric field.

A detector for terahertz(THz) operating at room temperature for detecting the THz signals is fabricated by using radio frequency (RF) magnetron sputtering and electronic beam lithography. This detector is composed of a planar logarithm periodic antenna of Al film and a microbolometer of Nb₅N₆ thin film, acting as both a radiation absorber and a temperature sensor, respectively which is fabricated on a high resistivity Si substrate with 100nm thick SiO₂ layer. The best attainable responsivity of this device is over 100 volts per watt at 300K at a bias current of 2 mA,and the electrical noise equivalent powers (NEP) is as low as 3.98×10^-10W•Hz^1/2.These are good enough for many detecting and imaging arrays in THz frequencies.

Based on the analytical solutions of the wave equations in the elliptical-cylindrical coordinate system, the propagation characteristics of hollow elliptical waveguide for THz Radiation are investigated, and then the mode characteristic equations are also given. Our results show that the mode characteristic equations of a circular waveguide can be treated as a special case of an elliptic waveguide. The propagation characteristics are numerically simulated depending on the refractive index of dielectrics films and the eccentricity of the elliptic waveguide. The cutoff wavelengths of guided modes in the elliptic waveguide are also presented.

The matching is not only an important part in microwave technique, but also a significant aspect in the microwave system. In this paper, it is researched and compared for the matching of different types of rectangular waveguides on the basis of the three dowels matching unit. The purpose is to establish a coincidence relation by systemizing these data such as different depth of dowels, the reflection coefficient and the equivalent impedance at the port and the type of rectangular waveguide. It is convenient to achieve the matching through adjusting the dowels to the fixed depth.

In the present study, the propagation characteristics of terahertz radiation in both metallic film-coated and dielectric film-coated hollow waveguides have been theoretically analyzed. The dominant modes in metallic film-coated and metal/dielectric film-coated hollow waveguides are respectively TE₁₁ and HE₁₁ modes. Theoretical attenuation coefficients of terahertz radiations in hollow waveguides with a bore diameter of 1mm film-coated with Au, Pb and Ni at different incident frequencies are compared. The dominant mode, i.e. TE₁₁ mode, as a function of bore diameter in hollow waveguides film-coated with different metals are calculated at a given incident frequency. An additional dielectric film with appropriate thickness on the metal films can effectively enhance the wave reflection, resulting in decreased attenuation of the terahertz radiation propagating in hollow waveguides. Calculated results indicate that the attenuation of the terahertz radiation in metal/dielectric film-coated hollow waveguide with a bore diameter of 1mm for a given incident wavelength of 200μm is about 4 times lower than that in metal hollow waveguide with the same bore diameter.

The wavelength of terahertz radiation is typically on the order of millimeters and submillimeters, and not negligible compared to the sizes of the optical components and the sample structure. Diffraction and scattering effects will be prominent in this situation, yielding spatiotemporal reshaping of terahertz pulse during its propagation. One representative example involves the propagation of a focused terahertz beam through a dielectric edge, which is frequently encountered in terahertz pulsed imaging. In this paper, we present a theoretical and experimental investigation on this edge diffraction problem. Benefiting from the presence of edge diffraction, high contrast image is likely to be obtained, which enables a weak-absorption (or transparent) object to be clearly resolved and reveals potential applications of terahertz imaging in nondestructive detection and biomedical diagnosis.

Terahertz (THz) time-domain spectroscopy (TDS) is attracting more and more attention recently, and has been growing up as a powerful technique for measuring the material parameters. Indium phosphide (InP), which with short carrier average collision time, high band-gap energy and low effective mass, is growing up as one of the best photoconductive materials for emitting and detecting THz waves. An n-type InP of 0.35 Ω•cm was measured with normal transmitted TDS system for the temperature and frequency ranges of 4.2 - 300 K and 0.2 - 4 THz, respectively. THz beam was placed in a closed box purged with dry nitrogen gas, and the sample was mounted in a MicrostatHe cryostat made by Oxford Instruments. Temperature dependence of THz properties for InP as a function of frequency was characterized.

In this article we report on studies aimed at a theoretical description and an experiment design of generation of intense THz pulsed laser. Through solving the time-independent rate equations, the absorption coefficient and the gain coefficient of intense THz pulsed laser are obtained with the assumption of the pump rate and the induced transition rate much larger than collision rate. The analysis further shows that at low pressure the absorption coefficient is proportional to the second power of pressure and is in inverse proportion to pump intensity, and that the gain coefficient has the same feature expect for being in inverse proportion to THz radiation intensity. Meanwhile the expressions of the saturation pumping intensity and THz radiation intensity are given under the same assumption. On the other hand the expression of energy absorbed from pump beam is derived from solving the time-dependent rate equations, and we demonstrate that path length, gas pressure, and pulse width are the important factors impacting on the energy absorbed from pump beam. When transmission of pump laser is determined, it is found that gain molecule mass, gas pressure and pulse width exist an inner confine relation. Finally according to the theoretical analysis and some parameters assumed, we present an experiment design of the intense THz pulsed laser.

The characteristics of spatiotemporal shaping and filtering of terahertz (THz) pulses passing through metal slits with finite thickness have been investigated by employing the finite-difference time-domain (FDTD) method. It is shown that this diffraction-based quasi-optical technique has a good influence on shaping and filtering of TEM-polarized THz pulses for thick-slits, while no effect for thin-slits. Furthermore, the numerical simulation results can be employed in the relevant experiments.

Pine wood nematode disease, namely pine wilt disease, is caused by the invasion of Bursaphelenchus xylophilus (Bx) into pines. Once susceptible pines are infected by the nematode, the disease develops rapidly, the infected pines cease to exude oleoresin and die quickly. Hence it is called pine cancer. Given the fact that there are still no good methods in diagnosing the disease, here we propose to study the spectroscopic characteristics of pine wood nematode and diseased pine wood in the THz regime in order to look for a rapid spectroscopic discrimination method for the disease. Firstly, we measure the transmittances of a Bx sample, a B. mucronatus (Bm) sample, a healthy Pinus massoniana wood sample and a diseased P. massoniana wood sample by a superconducting heterodyne mixer at 500 GHz. And their characteristics are compared. Secondly, we measure the transmission characteristics of a Bx sample and a Bm sample by terahertz time domain spectroscopy (THz-TDS). The measured time domain spectrums and corresponding frequency domain spectrums are compared to distinguish them from their absorption characteristics. Thirdly, we measure the transmission characteristics of a healthy P. massoniana wood sample and a diseased P. massoniana wood sample by THz TDS and compare their difference in THz absorption spectrum and diffraction dispersive spectrum to confirm the effect of Bx on P. massoniana by the absorption coefficient and refractive index. Some discussions are given for future development of the diagnostic technique of pine wood nematode disease based on THz spectrum.

Considering the Coulomb many-body interactions, we theoretically investigate the terahertz optical gain spectrum in an asymmetric step quantum well by employing the intersubband semiconductor-Bloch equations (SBEs). We show the effects of various many-body interactions on the THz gain spectrum. Because of the pump-induced population redistribution, optical gain saturation at larger pump intensities is predicted. The dependence of optical gain on electron density, temperature is also explored in detail.

In recent years, the advance of the elliptical resonant cavity and focus cavity is known by many people. There are homogeneous and multipatternal virtues in the focus dimensional microwave field of the elliptical resonant cavity. It is very suitable for applying the low power microwave biological effect equipment. However, when designing the elliptical resonant cavity may meet the problems of complex and huge computation need to be solved. This paper proposed the simple way of approximate processing the Mathieu function. It can greatly simplify the difficulty and decrease the scale of computation. This method can satisfy the requirements of research and development within project permitted precision.

To provide high efficient pumping source for THz laser, we designed a new active frequency stabilized system for gain switched RF CO₂ laser. According to our theoretical analysis and experimental study, the pulse setup time is varied for different output power. The pulse setup time is shortest when the output power is highest for a pulse laser. It is equivalent to stabilize the laser frequency and power if the pulse setup time is locked in the center of laser gain curve. By the method of adjusting the pulse setup time, we have stabilized a pulse rf excited CO₂ laser at the center of the laser gain curve. The frequency stability is estimated to be less than 5MHz.

We present a theoretical analysis of transmission characteristic of THz waves in liquid-crystal-filled Bragg fiber controlled by an external magnetic field. Detailed calculations on the band of the cladding and the filed distribution in Bragg fiber demonstrate the on-off switching property. Finite Element Method (FEM) is used to calculate the extinction ratio of the switch which reaches to 39.93dB.

We have generated a narrow line-width T-ray continuously tunable in the range of 0.5~4 THz by the different frequency generation in a GaSe crystal with a collinear phase-matched configuration. The peak power at 3.07 THz reaches 17.3 W, and the measured line width is less than 0.3cm^-1.

In this paper, THz-TDS has been used to measure the spectral properties of two active ingredients of Herba Ephedrae: ephedrine and pseudoephedrine, which exist in hydrochloride salts. The THz spectra of the sole-ingredient, twoingredient and three-ingredient compounds are studied. We obtained the finger-print spectra of the net active ingredients of the medicine, and also measured the mixtures of by two or three active ingredients at the different ratios. At the same time, theoretical analysis and quantitative analysis is applied to foretell the different THz spectra, identify the ingredients and infer the contents of principal components in samples. The THz spectroscopy is a potential and promising technique in evaluating and inspecting the quality of the drugs in the TCM field.

Scene simulation of radiation characteristics of targets and backgrounds is an important research topic for its benefits in the adaption and optimization of a sensor and its observation conditions. In this paper, imaging of the armored vehicle, an important and complicated military target, formed by passive terahertz sensors was studied, including calculation of the temperature field, analysis of atmospheric effects and the sensor models.

The physical mechanism of terahertz-wave generated from nonlinear crystal GaP was discussed based on polariton. The method which predicted to generate terahertz-wave in nonlinear crystal were reported. The polariton dispersion curve was physical basis of which terahertz-wave was generated from nonlinear crystal. The polariton dispersion curve of GaP crystal was depicted by using density functional theory and the polariton dispersion relation. The corresponding frequency of part polariton dispersion curve for GaP were between 2.5 THz-9.1 THz when small scattering angle about 2-11 degree were required. Namely, it showed theoretically that terahertz-wave with a range of 2.5-9.1 THz were generated from GaP crystal.

This PDF file contains the front matter associated with SPIE Proceedings volume 7277, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

A theoretical analysis of optical-to-THz conversion efficiency via optical rectification in nonlinear crystal and the conversion efficiency comparison of ZnTe, DAST, LiNbO₃ is presented; The optical-to-THz conversion efficiencies are mainly affected by following several factors; Pulse durations mostly affect the conversion efficiency: Optical-to-THz conversion efficiency approaches to a maximum when crystal length is equal to the optimal length Lc, for the periodically-inverted crystals, conversion efficiency is almost proportional to the crystal length when absorption of medium is negligible. Taking account of the absorption of crystals, effective length of crystal is L_eff=0.63/α, the conversion efficiency approaches to a constant eventually when the crystal length is increased for crystal DAST and LiNbO₃. But the optical-to-THz wave conversion efficiency of ZnTe will increase rapidly with increase of crystal length.