This PDF file contains the front matter associated with SPIE Proceedings Volume 8195, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We propose two methods for avoiding misplacement phase error in terahertz reflection timedomain spectroscopy (THz-RTDS). The absorption signatures of the materials are extracted directly from the second-order derivative of the phase of the sample beam or the first derivative of the relative reflectance with respect to frequency. The techniques provide straightforward and fast solutions to solve the phase-retrieval problem in RTDS.

We numerically studied the terahertz wave generation in quasi phase matched Gallium Arsenide (QPM-GaAs) based on difference frequency generation (DFG) process between resonating signal and idler waves of the type-IIphase matched periodically poled LiNbO3 (PPLN) optical parametric oscillator (OPO). The optimal GaAs length for maximum conversion efficiency was obtained considering the wave propagation loss in the GaAs crystal by adding a loss term in the coupled wave equations.

The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using 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 planes perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on axis but also inhibit focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to be a terahertz filter. Moreover, the terahertz pulses travelling as basic mode of Gaussian beam is discussed in detail.

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption and dispersion of polycrystalline α - and γ -glycine in the spectral region 0.5-3.0 THz. The spectra exhibit distinct features in these two crystalline phases. The observed far-infrared responses are attributed to intermolecular vibrational modes mediated by hydrogen bonds. We also measure the Raman spectra of the polycrystalline and dissolved glycine in the frequency range 28-3900 cm^-1. The results show that all the vibrational modes below 200 cm^-1 are nonlocalized but are of a collective (phonon-like) nature. Furthermore, the temperature dependence of the Raman spectra of α -glycine agrees with the anharmonicity mechanism of the vibrational potentials.

In this paper, we present measured results and theoretical modeling of a 1.5 THz waveguide NbTiN HEB mixer, which is fabricated by in situ process. The NbTiN HEB mixer with dimensions of 1 μm in width and 0.15 μm in length is measured to be as low as 570 K in noise temperature (7hν/k) even with a relatively thick NbTiN film (10.8 nm). The intrinsic mixer noise temperature is 300 K after the correction of the losses of quasi-optical and IF amplifier chain. At the same time, we adopt an optimized hot spot model to simulate the DC and RF behaviors of the NbTiN HEB mixer by combining phonon-cooling and diffusion-cooling mechanisms together. The measured and calculated current-voltage curves are in good agreement. The calculated intrinsic mixer noise temperature is 100 K and IF gain bandwidth is 2.7 GHz at the optimum bias point. The difference between the measured and simulated mixer noise temperature is factor of 3 times, which is probably due to misalignment of quasi-optical RF coupling system and air loss. The calculated and measured results show the two cooling mechanism work together to lower the noise temperature of the waveguide NbTiN HEB mixers. The details will be presented in this paper.

The detector array of THz synthetic aperture imaging system is consists of many individual detectors. At a specific distance, arranging the curvature of the detector array to match with wavefront can reduce the phase shift. There is not exist perfect detected object in reality. Given the specification resolution of a system, cut-off frequency is determined to design the longest baseline and to ensure an efficient coverage of the u-v plane. The shorter of the baseline, the more influence on u-v spectrum is. To be pointed out, throwing away the high frequency component can improve the resolution of the object. In this paper, by arranging the detector array to make the curvature of imaging array match with the wavefront and to throw away the high frequency component when the baselines are lost, the result of research proves the above way is right.

Acquiring messages from the mail but not destroying the envelope is a big challenge in the war of intelligence. If one can read the message of the mail when the envelope is closed, he will benefit from the message asymmetry and be on a good wicket in the competition. In this paper, we presented a transmitted imaging system using THz time-domain spectroscopy technology. We applied the system to image the mail inside an envelope by step-scanning imaging technology. The experimental results show that the THz spectroscopy can image the mail in an envelope. The words in the paper can be identified easily from the background. We also present the THz image of a metal blade in the envelope, in which we can see the metal blade clearly. The results show that it is feasible of THz Time-Domain Spectroscopy Imaging for mail inspection applications.

Frequency selective surface is a monolayer or multilayer 2D periodic structure which is composed of multiple resonance units scattering by a two-dimensional periodic array on dielectric layer. FSS can't absorb radio frequency energy, but can filter the frequency which is therefore applied in microwave technique or stealth technology. The relative research on curved FSS is relatively scarce since the curved FSS structure can be obtained only when FSS is attached on the materials surfaces of curved structures in engineering application. However, curved FSS is widely applied in practical engineering; therefore, the research on curved FSS structure has important significance. In this paper, a curved FSS structure model of Y-pore unit is established and numerical simulated by means of FDTD. The influence of curvature on FSS transmission characteristics is studied according to the analysis on the changing of radar cross section (RCS). The results show: the center frequency point of the plane band pass FSS structure drifts after the curve surface deformation of the structure; the center frequency point of the curved band pass FSS structure drifts with the changing of the curvature radius, i. e. with the decreasing of curvature radius, the frequency point drifts towards high points and the transmittance decreases. The design of FSS radome demands of accurate and stable center resonance frequency; therefore, the actual situation of curved surface should be considered in practical engineering application when band pass FSS is made into frequency selection filtering radome. The curvature radius should be long enough to avoid center frequency drifting and transmittance deceasing.

The submillimeter (submm) regime, ranging from 100 to 1000um, is an important frequency band for radio astronomy. A large number of astronomical spectral lines are located in this frequency region. Compared with ground-based observation, which is limited by the atmospheric absorption of signal, space borne platform provides perfect condition for submm observation. Here we introduce some preliminary results for a compact 500GHz SIS (Superconductor- Insulator-Superconductor) heterodyne receiver system developed for future space borne observation. Considering low power consumption requirement for space applications, we adopt a high critical temperature (Tc) NbN/AlN/NbN SIS tunnel junction for the mixer, a key component of the receiver system, which may work at relatively high temperature around 10 K. All the components, including the SIS mixer, HEMT low noise amplifier and optical lens, are assembled into a compact system. The whole system is cooled by a close-cycled 4K cryo-cooler in laboratory and test result shows a good noise performance, less than 250K at the 500GHz band. Detailed simulation and experimental results will be presented in this paper.

4nm single-walled carbon nanotubes ( SWNTs ) of three possible structures were fabricated. The frequency-domain transmission spectrum of SWNTs was obtained by Fourier transform in terahertz time-domain spectroscopy (THz-TDS) system. By analyzing the spectrum, we can see that the absorption coefficient of SWNTs augments with an increase in the frequency. It increases slowly at low frequency, and almost be linear. But after 1.1THz, the absorption coefficient rises rapidly with the frequency. The result is generally corresponding with the theory Drude-Lorentz model combined with the Maxwell-Garnett model. But there is some difference at low frequency, further discuss is needed.

Interaction between the high temperature (high-T_c) Josephson junctions and 0.14 THz nanosecond pulse has been numerically investigated in this paper. A general equivalent circuit based on resistively-shunted junction (RSJ) model was applied to simulate a typical high-T_c Josephson junction under the radiation of 0.14 THz narrow-band pulse with pulse duration of 2 ns. The varying ratio of phase difference of electron wave functions between the two sides of the junction, the current-voltage characteristics, and the voltage responses were determined at several specific times during the interaction. The Shapiro steps were clearly observed but distorted, and then the irradiation frequency was derived, coinciding with the simulated frequency. Also discussed were the effects of some parameters, including the pulse power, the normal resistance and the critical current of junction, on the current-voltage characteristics and the voltage responses. All the results showed that the high-T_c Josephson junctions probably could be used for the direct frequency measurements of narrow-band terahertz pulses under some specific conditions.

In this work, vibrational properties of a series of parent polycyclic aromatic hydrocarbons (PAHs) and nitropolycyclic aromatic hydrocarbons (NPAHs) compounds with the substituted nitro group at different positions have been investigated using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The experimental results show large difference among absorption spectra of several NPAHs and their corresponding parent PAHs compounds in 0.1~2.2 THz region, which probably originated from the difference of NPAHs' molecular structure and lattice vibrational modes. All the experimental NPAHs vibrational modes showed distinct fingerprint absorption in THz region. The study indicates that THz-TDS technology can not only give a new experimental method to identify and analyse such different kinds of NPAHs environmental pollutants, but also provide a useful suggestion for further assessing the possible relationships between the NPAHs optical properties and the effects of the substituted nitro group position to better know their photoreactivity and biochemical activities.

A biological sensor based on the resonant transmission characteristics of metal meshes has been proposed in the terahertz (THz) region. We simulated THz transmission characteristics of two kinds of structure by FDTD method: arrays of circular hole and rectangular hole. Their resonant frequencies at the interface of metal and air are 1.42THz and 1.225THz respectively. We find that the refractive index and thickness of the sample commonly affect the surface plasma's resonant frequency. When coupled with a layer of sample of 50 micron on the metallic meshes, the resonance frequency would shift to a lower frequency with an increasing refractive index. Sensitivity of two kinds of structure is 0.64 THz/RIU and 0.507 THz/RIU respectively. The sensitivity differences of two kinds of structure lie mainly in the different dominant resonant transmission mechanism. The thickness also affected detection sensitivity. When thickness of sample is less than the corresponding resonance wavelength, we need to consider the influence of thickness. This theoretical simulation can be good reference for the following experiment.

Terahertz radiation has been studied extensively both experimentally and theoretically within the past decades. A number of methods have been developed to generate terahertz radiation using ultra-short laser pulses, including photoconductive effect, optical rectification, and so on. The photoconductive effect has been utilized to generate THz radiation from a piece of semiconductor with antenna structure. The bias electric field between the electrodes of photoconductor is limited by the breakdown field of the semiconductor. In order to enhance the THz radiation power, the distribution of electric field on the surface of the substrate is simulated and the relationship between the geometrical structure of antenna and bias electric field is analyzed. The results show that the bow-tie antenna works more safely than other antennas under the same condition, and the breakdown risk of the PC antenna is reduced and the capability of resistance to voltage is improved.

We report the design and fabrication of terahertz quantum cascade lasers at about 3 THz that use semi-insulating surface-plasmon waveguide for optical mode confinement The single-mode operations are realized by employing one-dimensional first-order distributed-feedback (DFB) and photonic crystal distributed-feedback (PCDFB) resonators, respectively. The single-mode operation of up to 88 K and peak power of 9.5 mW at 10 K for the DFB devices is obtained. The PCDFB devices also emit a single frequency at above 50 K.

Light scattering effect can be commonly occurred in the terahertz transmitting spectra of various powders (particle sizes are average less than 50μm), include pure materials and mixtures. Light scattering of various materials distort absorption spectra. We demonstrate the effect using mixture of two kinds of powders with different refractive index. According to the Mie theory, we explain the reason of light scattering effect in subwavelength domain, relying on refractive index between the powders and their surrounding medium. Furthermore, we show a wavelet method which can extracts terahertz spectral information without considering scattering effect.

In this paper, for explosives classification problem in safety inspection field, feature extraction and recognition from the radiation data as the core, combined with dual-energy X-ray transmission technology, low-energy forward scatter and low-energy back scatter technology to get eigenvalue R associated with effective atomic number and eigenvalue L associated with density. Synthesized R and L, get the discriminate, decision-making plane and distinguish rule based on the least mistake probability. The distribution of materials with regularity in this plane, there is a curve, most illicit materials drop into the area under the curve. Then get the criterion to classify the explosives. A given object may be randomly placed anywhere on the conveyor belt, resulting in a variation in the detected signals. Both an adaptive modeling technique and least squares method are used to decrease this distance effect. This is the magnitude contribute for recognition of solid explosive.

In material classification, distilling eigenvalue will use the object's true gray levels. The problem is objects in a bag almost always overlap with others. Being able to identify the object of interest and remove the overlap effects becomes the key issue that needs to be solved. First, the paper took an n-object-overlapping problem simplified to a two-object-overlapping problem. So the research focus turned to computing true gray levels for two-object-overlapping problem. It was necessary to develop models that can be used to remove the background object overlapping effects. The author took back scatter images for example, discussed the development of the mathematical model for removing the overlapping effects, solved the model parameter by experiment and analyzed model error. This method has been used in x-ray security inspection equipment of DT Inspection equipment limited company. The results of application show that the algorithm is feasible. This is a unique contribution to the explosive detection community.

The numerical simulation of terahertz generation and detection in the interaction between femtosecond laser pulse and photoconductive material has been reported in this paper. The simulation model based on the Drude-Lorentz theory is used, and takes into account the phenomena that photo-generated electrons and holes are separated by the external bias field, which is screened by the space-charge field simultaneously. According to the numerical calculation, the terahertz time-domain waveforms and their Fourier-transformed spectra are presented under different conditions. The simulation results indicate that terahertz generation and detection properties of photoconductive antennas are largely influenced by three major factors, including photo-carriers' lifetime, laser pulse width and pump laser power. Finally, a simple model has been applied to simulate the detected terahertz pulses by photoconductive antennas with various photo-carriers' lifetimes, and the results show that the detected terahertz spectra are very different from the spectra radiated from the emitter.

In this paper, the low-frequency vibrational modes of crystalline benzoic acid (BA) have been investigated by terahertz time-domain spectroscopy (THz-TDS) and theoretical simulations based on the linearity combination of atomic orbital within the Density Functional Theory (DFT) as well as ab initio molecular orbital method at second-order Moller-Plesset Perturbation Theory (MP2) level for single molecule and dimer. Experimentally, a series of prominent absorption features of pure benzoic acid relevant to intra- and inter-molecular vibrational modes have been obtained below 4 THz at room temperature. For the theoretical simulations, geometry-optimization results of bond lengths and dihedral angles in both BA monomer and dimer are very close to experimental neutron diffraction measurements. Furthermore, the simulation results demonstrate absorption profile centered at 1.89 THz contains low-frequency modes of Ph-COOH twisting due to intramolecular motion and cogwheel owing to intermolecular motion. All the intra- and inter-molecular vibrational modes measured have also been assigned.

We have investigated the effect of a moderately strong magnetic field (generated by superconducting coil, up to 380Gauss, 6 quantum fluxes within a single junction; generated by one piece of permanent magnet; generated by two pieces of permanent magnets respectively) on Nb superconducting tunnel junctions at different temperatures. Both experimental and numerical results show that the superconducting tunnel junction DC Josephson current can be significantly reduced by a magnetic field. A relatively strong magnet field may suppress the gap voltage as well as the sharp nonlinearity at the gap. We also investigated the leakage current and DC Josephson current of the Nb SIS junction at different temperatures.

We present a technique for calibrating spectrum measurement of terahertz time domain spectrometers (THz-TDS) using absorption lines of carbon monoxide (CO). The mean error between the peak positions measured by our THz-TDS and JPL database is 3.1 GHz. The results show that CO is suitable for frequency calibration of THz-TDS.

Promising Crystals of Generating THz have been discussed according as the theory of solid state physics. We found that PbB₄O₇ (PTB) crystal has probability of generating about 3.12THz, and Bi₄Ge₃O₁₂ (BGO) crystal has probability of generating and 2.7, 3.7, 6.0 and 10.8THz radiation.

Terahertz time-domain spectroscopy was used to measure the optical properties of ZnO nanoparticles (NPs) in the composite samples. The complex conductivity of pure ZnO NPs was extracted by applying Bruggeman effective medium theory. We find that the real part of the complex conductivities increase with the increasing diameter of the ZnO NPs and are governed by a restricted mean free path for NPs. The results demonstrate that carriers become localized with a backscattering behavior in nano-structured ZnO.

With the development of terahertz related technologies, the terahertz imaging technology will show its greater practical value in more areas. In this paper, we describe a terahertz imaging system that uses a single pixel detector in combination with a series of random masks to enable high-speed image acquisition. The image formation is based on the theory of compressed sensing (CS). When the scene under view is compressible by an algorithm like JPEG or JPEG2000, the CS theory enables us to stably reconstruct an image of the scene from fewer measurements than the number of reconstructed pixels. In this manner, we achieve sub-Nyquist image acquisition. CS theory mainly includes signal sparse representation, encoding measurement and reconstruction algorithm. CS combines sampling and compression into a single non-adaptive linear measurement process. Rather than measuring pixel samples of the scene under view, we measure inner products between the scene and a set of test functions. CS permits the reconstruction of a N-by-N pixel image using much fewer than N² measurements. This approach eliminates the need for raster scanning of the object or the terahertz beam, while maintaining the high sensitivity of a single-element detector. We demonstrate the concept using a backward wave oscillator (BWO) which is a continuous-wave terahertz source and get a preliminary test result.

The transmission spectra of sub-wavelength cross metallic fractal slits in terahertz (THz) frequency region are presented by means of finite-difference-time-domain (FDTD) simulation. The transmission spectra with multiple pass bands and stop bands are observed. To understand the physical mechanism of the enhanced transmissions, we simulated the electric field distribution of THz radiation within the metallic slits at the resonance frequencies by the electromagnetic design software named CONCERTO. Further analysis reveals that the two transmission peaks in the low frequency is the local resonance of electric field of the two cross slit, respectively. The third transmission peak is the co-effect of the two level cross slits. Our simulation is helpful for the understanding of THz wave propagation and THz transmission through the cross metallic fractal structures. It is also useful for the development of THz photonic devices.

Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail under various pump powers. The pump-induced attenuation of terahertz transmission and the carrier relaxation time are found to be correlated with photo-injected carrier density. These effects can be fully explained in terms of the enhanced photoconductivity and surface states filling in GaAs. Moreover, abnormal reversal and increase of terahertz reflected waves transmitted from GaAs are observed, and these phenomena arise primarily from the pump injected carriers. One-dimensional pump scans of terahertz transmission of both the main peaks and the reflection peaks are discussed to reveal the more complete studies of the dynamics of carrier transfer in GaAs.

The transient absorption bleaching and velocity overshoot of photoexcited carriers in GaAs and Si have been observed by the intense few-circle terahertz (THz) probe pulse in the optical pump-terahertz probe (OPTP) configuration. The free-carrier THz nonlinearity is attributed to the transient electron redistribution in conduction band induced by the strong THz electric field component.

Resonance and absorption peak of transmission spectrum can be achieved when terahertz pulses transmit through the split-ring resonators (SRRs). Using the theory of LC oscillating circuit and the model of half wave resonance, we can account for the phenomenon. We once fabricate metal subwavelength dual-ring structure which is different from the SRRs and there aren't splits in. The resonance and absorption peak, however, still appear in the THz frequency range. In this time, we respectively fabricate the dual-ring structure and reverse dual-ring structure on quartz crystal substrate and silicon. When the metal structure is fabricated on quartz crystal, Using terahertz time-domain spectroscopy (TDS), we find the characteristic spectra exist as well as time domain waveform and frequency domain spectra of transmitted terahertz pulses present obviously periodic variation with rotating the sample round the normal of its surface. However, if silicon is used as substrate, the phenomena will not appear. In this article, we will analyze the phenomena and obtain the reason resulting in it. We hold that the metal dual-ring structure in itself can't lead to the variation of the terahertz pulses by changing the sample's angle due to isotropy of the dual-ring structure in surface of the sample. We think the phenomenon may originate from quartz crystal substrate.

We report measurements for hollow core kagome microstructured Terahertz (THz) fiber characterized with the THz-time domain spectroscopy (THz-TDS). To achieve good mode overlap between the input beam and the fiber mode distribution, we incorporate specially designed THz lenses in our experimental setup. The experimental observations show that only the fundamental mode is guided in the fiber core. The time domain scans and their spectral information show air-guidance over a large frequency range from 0.6 to at least 1.1 THz. Within this frequency bandwidth, low transmission amplitudes at certain frequencies are identified as the frequency cut-offs for the kagome structure is observed. The measured transmission of the kagome fibers shows the characteristics of the inhibited coupling mechanism. We estimate high coupling efficiencies, as high as 60%, have been achieved in our experiments.

In this paper, ZnGeP2 (ZGP) crystal was pumped by a dual-band tunable CO2 laser. Based on the theory of nonlinear optical (NLO) frequency effect, a new THz light was generated. Under the conditions of three-frequency collinear nonlinear interaction and type-II phase matching (o+e→o), formulas of phase-matching angle, efficient nonlinear coefficient, the wave vector direction (θ, Φ), walk-off angle and acceptance angle were derived and relevant curves were analyzed with software. In this paper, the schematic diagram of THz generation based on ring cavity optical parametric oscillator (OPO) was designed. Theory analysis showed that the new ring cavity OPO was reasonable and efficient.

The precision of the conventional terahertz transmission measurement system with a parallel-plate sample will be reduced due to the interference effect, when the frequency of the THz source drifts. In order to solve such problem, a frequency-drift insensitive transmission measurement system composed of double-path is proposed in this paper, and an F-P interference model is built. The parameters are optimized, and the effects of the terahertz source's frequency drift on the precision of both the single- and double- path transmission measurement are analyzed and compared. The results demonstrated that such kind of measurement system could avoid the influence of the frequency-drift of the THz source, thus enhance the measurement accuracy.

Terahertz (THz) has received significant attentions in recent years, as it promises tempting perspectives in multiple applications, such as sensing, imaging and spectroscopy, etc. However, most THz systems rely on free space propagation, where THz wave would experience significant loss. Moreover, most materials have high absorption loss in this spectral region. A new type of low loss THz waveguides is needed in order to promote the application of THz technologies. In this paper, we exploit a Teflon solid-core photonic crystal fiber (PCF) and its transmission characteristics have been numerically investigated in the THz band by using finite element method. Results show that in a large pitch length (E=2.0mm) dispersion could be as low as 0.01 ps/ (THz·cm) in the range of 1.0 to 2.5 THz. When d/E is larger than 0.45 and E larger than 1.0mm, the modal loss is as low as 0.25 cm-1. It can also be seen that confinement loss can be eliminated by proper design. The results can provide theoretical references for further studies in this area.

In this paper, we have determined water-ethanol mixtures and Chinese spirit with terahertz time-domain transmission spectroscopy (THz-TDS). Eleven samples of water-ethanol mixtures with varying alcohol degree from 0% to 100% (v/v), increased in 10%-steps, were used as the calibration set. Several Chinese spirit samples were used as the prediction set. We measured alcohol degree by alcohol meter, with measurement accuracy of 0.2% (v/v). Partial least square (PLS) model was employed for regression. The refractive index, absorption coefficient and mixed models by PLS regression together with alcohol degree were established, respectively. The predictive values of alcohol degree were very close to the reference values determined by alcohol meter. To assess the quality of the model, we calculated the correlation coefficient R-square, the root mean square errors of the calibration set (RMSEC) and the prediction set (RMSEP) of the three models. The results show that the absorption coefficient PLS regression model is the best method for determining alcohol degree of Chinese spirit with THz-TDS transmission measurement.

Soluble solids content is an important index for fruit quality. One of the traditional methods in determining soluble solids content of fruits is refractometry which measures the refractive index in visible or near infrared. Here we reported the use of terahertz time-domain transmission spectroscopy (THz-TDTS) technique for the determination of soluble solids content in apple products. Not only the refractive index, but also the absorption coefficient is used in regression model. In method one, sucrose solutions were for the calibration set , root mean square of validation set (RMSEP) was 0.168% for absorption coefficient model, 0.741% for refractive index model. In method two, apple products were for the calibration set, RMSEP was 0.143% for absorption coefficient model, 0.648% for refractive index model. Less absolute error of 0.2% between predicted and refractometer value has been both obtained from two methods with absorption coefficient model. This result proved the THz-TDS technique is quite potential for nondestructive detection on food quality.

Terahertz (THz) technology has huge potential in detection of concealed objects as well as in medical imaging for non-invasive diagnostics. Recently thanks to improvement of THz sources and detectors, the THz imaging technology has become the hotspot. This paper presents a real-time imaging system for THz radiation from BWO source. The key component of the system is the IR-FPA which is a bi-material MEMS detector array optimized for infrared wave lengths. In order to get a better THz image, the paper presents a modified FPA for THz region. Two main methods were explored to improve the detector performance in the THz region. The first way identified was the use of novel optical readout system based on SPR. The second detector improvement approach involved the change of the micro-cantilever structure that the cantilever of each pixel can shift in vertical direction. A model of one pixel was set up to optimize the structure. Computer simulations results show that both approaches are effective. The manufacturing of the proposed detector are also discussed. The technological investigation shows that it is feasible to employ sacrificial layer technique related to MEMS (Micro-Electro-Mechanical Systems) technology.l.+

Terahertz radiation, with wavelengths from 30 to 3000 microns, has big potential for applications such as biomedical sensing, noninvasive imaging and spectroscopy. Current researches on photonic crystal fibers as THz waveguide has been widely presented, but the sorts of waveguide usually need special fabrication technique and always be difficult to get perfect structure. We report on a new type of polymer photonic crystal fibers for low-loss guidance of THz radiation with general fabrication technique. The use of the cyclic olefin copolymer (COC) Topas, results in THz fibers with relatively low loss and low material dispersion in the THz range. We manufactured a class of THz waveguides with different structures. Then we use a THz time-domain spectroscopy system (THz- TDS) to characterize the Material loss and index of refractive of the waveguides.

We developed a narrow line width terahertz and mid-infrared source based on difference frequency generation(DFG) in GaSe crystal. The source could be easily continuous-tuned in a terahertz region of 68.1-1386 μm (0.22-4.4 THz) and a mid-infrared region of 16.1-26.5 m without altering any component. The maximal peak power of terahertz waves is 9.1 W at 143 μm (2.1 THz), which corresponds to a power conversion efficiency of 2.8~10^-5. Using a FTIR spectrometer, we verified the wavelength and the line width of the source.

We present an experimental terahertz (THz) spectroscopic investigation of amino acid using an air-breakdown-coherent detection (ABCD) system. The strong and ultra-broadband (0.1 to 10THz) terahertz radiations generated by two-color laser induced air plasma and measured by coherent heterodyne detection. The broadband THz reflection spectra of L-Lysine (C₆H₁₄N₂O₂) and L-Arginine (C₆H₁₄N₂O₂) are obtained. To solve the phase-retrieval problem in RTDS, the absorption signatures of the materials are extracted directly from the first derivative of the relative reflectance with respect to frequency. The absorption features of the two amino acids are characterized in the 0.5~6 THz region. It is found that both the two amino acids have an absorption peak at 1.10 THz.

Terahertz (THz) radiation has been widely applied in the areas of spectroscopy, sensing and imaging. With the advancement of THz technologies, THz imaging has been progressed significantly and has huge potential applications in medical diagnosis, environmental control, chemical and biological identification. In the past, people mainly focused on overcoming the obstacles in the hardware, such as the limitation of resolution, accuracy and speed in THz imaging system. In this paper, image enhancement techniques are employed to enhance the contrast and get the better profile of THz image. Our results clearly demonstrate that digital image processing techniques can suppress noise effectively, extract target edges robustly, and therefore improve the quality of THz image.181

The challenges of terahertz (THz) technology exist in the development of new or improved sources of THz radiation and detection technologies. In general, inexpensive, fast, and room temperature operation THz detectors are not readily available. A glow discharge neon lamp can serve as a THz detector, and holds great potential for various THz sensing and imaging applications. Neon lamps THz detector has many advantages such as low cost, fast response time, large dynamic range, broad spectral range, and room temperature operation. In this paper, the performance of the THz detector based on glow discharge neon lamps was evaluated. The detector was successfully used in a THz imaging system, the THz images obtained by the detector was demonstrated and compared with that obtained by a Schottky detector.at

Terahertz time domain spectroscopy (THz-TDS) is used to the pure active ingredient of three benzodiazepine sedative hypnotics with similar molecular structure. The absorption spectra of them are studied in the range of 0.2~2.6THz. Based on the experiment, the theoretical simulation results of diazepam, nitrazepam and clonazepam are got by the Gaussian03 package of DFT/B3LYP/6-31G* method in single-molecule models. The experimental results show that even if the molecular structure and medicine property of them are similar, the accurate identification of them can still be done with their characteristic absorption spectra. Theoretical simulation results are well consistent with the experimental results. It demonstrates that absorption peaks of them in THz range mainly come from intra-molecular forces and are less affected by the intermolecular interaction and crystal effects.ô

This paper describes a new concept related to the MEMS(Micro Electro Mechanical system) imaging-array sensors with the structure of micro-cantilever-arrays for detecting far IR and THz radiation. The measure principle is based on an improved optical lever and the core component is a set of micro-displacement measuring device with nano-degree displacement measurement. The amplification coefficient of this improved optical cantilever can reach 10²~10³ times, combined with a high resolving power to 10^-10m. Compared with focal plane arrays sensors, these tape sensors have the ability to measure deformations of micro-cantilever-arrays caused by far IR or THz radiation directly, which can increase the radiation detector sensitivity. The validity of this method is proved by practical experiments. Imaging-array sensors, based on this measure principle, can be made into a new-type MEMS Far IR or THz sensors.

In this paper, ZnGeP2 (ZGP) crystal was pumped by a dual-band tunable CO2 laser. Based on the theory of nonlinear optical (NLO) frequency effect, a new THz light was generated. Under the conditions of three-frequency collinear nonlinear interaction and type-II phase matching (o+e→o), formulas of phase-matching angle, efficient nonlinear coefficient, the wave vector direction (θ, φ), walk-off angle and acceptance angle were derived and relevant curves were analyzed with software. In this paper, the schematic diagram of THz generation based on ring cavity optical parametric oscillator (OPO) was designed. Theory analysis showed that the new ring cavity OPO was reasonable and efficient.

We present a method to extract the polarization of terahertz wave by placing a 4-mm-thick quartz crystal on the terahertz beam. The polarization and electric field amplitude are varied by a terahertz polarizer. The quartz crystal is fixed at 45 degree with respect to the horizontal direction and the terahertz polarization angle can be extracted by the ratio of the two electric field amplitude corresponding to the ordinary and extraordinary waves that transmitted through the quartz crystal. The result shows this method is valid to measure polarization of terahertz wave, and it is more convenient and saving time for only once measure.A

We present a new design of an antenna-coupled superconducting transition-edge sensor (TES) bolometer. The incident radiation is coupled to a membrane-based twin-slot antenna and then transmitted through lossless microstrip lines to lossy meander-like microstrip lines in which a NbSi film is located between strip and dielectric layer. The NbSi film serves as both power absorber and temperature sensor (thermometer). With the help of the electromagnetic simulation software High Frequency Structure Simulator (HFSS), we have studied the performance of the twin-slot antenna. The simulation results for radiation pattern and return loss show promising prospects for this TES bolometer design. The power absorption in NbSi film is another important aspect which will be modeled in the future.

In this paper, the terahertz time domain spectroscopy (THz-TDS) technology is used to study the terahertz (THz) spectral characteristics of the Yungang Grottoes weathered material samples winch collected from No. 4, 9 and 39 these three different caves dug in different periods, A.D.520, A.D.494 and A.D. 484 years. The available effective wave band is between 0.2-2.6 THz. It is found that the transmission coefficient is high in low frequency region but low in high frequency region for all of the three kinds of samples. Experimental results also indicate that all of the samples have weak absorption in low frequency region, but the absorption coefficient increase quickly by exponential function as the frequency increasing. This character is more obvious especially for the absorption spectrum of the samples from the cave of late period. The refraction coefficient for the three kinds of samples has obvious differences in the whole THz wave band. The mean refraction coefficient of them is 1.53, 1.57 and 1.51 separately. According the results above, different periods of caves weathered samples can be identified. In addition, we can also offer the various optic constant of Yungang Grottoes samples in THz wave band for other studies work.

The amplitude modulation/direct detection (IM/DD) method is adopted to achieve communication in the practical optical fiber communication system at present. By comparison with the traditional direct detection system, coherent optical transmission system has a lot of advantages, such as high sensitivity, long relay distance, good selectivity, large communication capacity and a variety of modulations. Based on coherent detection principle, ASK heterodyne asynchronous demodulation system is researched and designed. The signal light and the local oscillator light are mixed through the mixer. Then through the photoelectric conversion the intermediate frequency (IF) current signal is obtained when the difference frequency signal is input to detector. And then the signal in form of baseband signal is output after photocurrent signals passing through the electrical signal processing system. In paper, the circuit of receiver is implemented. And the correctness of circuit design is demonstrated by experiment. It lays a solid groundwork for further study of Heterodyne Asynchronous Demodulation System of coherent optical communications.

There is a close correlation between dielectric loss and complex dielectric constant of non-magnetism medium, and the former is the macroscopical representation for the later. Relaxation is one of important cause of the dielectric loss. This paper educes the complex dielectric constant and complex refractive index of non-magnetism medium in Terahertz spectrum on the base of initial Drude Model, analyzing their characteristics; it also calculates the Argand Picture of Drude Model, comparing with the counterpoint of Debye Model.

Leaf water content is one of the most important indicators of the plant growth status. Thus, for a long-term research of the plants, the nondestructive and contactless measurement methods are preferable. Recently, the terahertz (THz) frequency region from 0.1THz to 10THz (or 3-333cm^-1) has seen a flurry of research activity in the fields of physics, chemistry and biology. Terahertz wave is very sensitive to water. This characteristic suggests the capability of THz spectroscopy for monitoring the water content of the plants. In this paper, the influence of varying water content in different zones of the same plant on the absorption and refraction coefficients was given. The influence of varying water content in the different locations of the mesophyll and mid-vein on the absorption and refraction coefficient was described. And the variations mainly result from the different thicknesses of leaf. The absorption and refraction coefficients with different relative water content in the green and yellow leaves were given. These comparisons between green leaf and yellow leaf reveal that the complex permittivity of leaf is as a complex function of the water content of the leaf. So our measurements tend to demonstrate that the dielectric material parameters will be employed to determine the leaf water status in plant leaves. All measurements are in the range of 0.2 to 2.6 THz. These researches will contribute to the future work of a compact and contact-free THz spectrometer to avoid any damage of the leaf surface.

We report on analysis of epoxy composite reinforced with a glass fiber mat by means of Time Domain Spectroscopy (TDS) technique in reflection way. The samples consisted of 2 or 3 layers with the beveling angle of the layers equal to 90° and thickness up to 2 mm. Two samples with artificially induced air-bubbles (~1-2 mm in diameter) and a sample which consisted of the 2 layers of epoxy and a layer of an air-bubble foil between them were also investigated. The measurements shown that all layers as well as defects can be identified.

We study a coaxial terahertz waveguide with low transmission loss and broad bandwidth. The coaxial transmission line is composed of an aurum line in the center, which ento-ectad covered with Teflon layer, wire-mesh, and the dielectric layer for protection. In this paper, we utilize the finite element method (FEM) to numerically analyze the transmission mode, the attenuation constant, and the bandwidth of the waveguide. The major transmission properties of the coaxial terahertz waveguide are calculated for different diameters of aurum axes and different thicknesses of dielectric layers. We also compare two waveguides with different Teflon thicknesses for getting broader transmission bandwidth. The simulation results indicate that the optimum dimension of coaxial terahertz waveguide is the aurum axes of 700μm diameter, the Teflon of 900μm thickness. With these parameters, the average attenuation constant of the waveguide can be as low as 2.44 x 10^-4 dB/m in the frequency range 1.0^-2.2THz. As these excellent transmission properties and simple structure of the coaxial waveguide, it can be expected that it will be widely used in terahertz wave communications and the fabrication of terahertz system integration.

In this paper, complex Morlet wavelet transform is introduced to analyze the mode character and the group velocity dispersion of the metallic cylindrical waveguide in the THz time-domain spectroscopy system. The result shows that the dominant mode TE₁₁ is separated from the higher mode, and its intension is above 90% of the total. This analysis method is universal for any THz time-domain spectroscopy system.

In this paper, the optical characteristics of crude oil samples from selected territories in Tuha oil field have been investigated with THz-TDS at room temperature in nitrogen environment. The absorption coefficient and refraction index of crude oil are calculated in the frequency range of 0.20~2.5 THz. No distinct absorption peak has been found in the spectra, which may result from the complexity of the constitution of the samples and the water content. The absorption coefficients increase with the content of wax in the samples with different slopes, which means the intensity of absorption in THz wave band increases with enhanced density of the sample and unaltered thickness. The average refractive indices were 0.62070, 0.59767 and 0.59290 corresponding to TuLuFan, ShanShan and WenMi territories, respectively. The refractive values show the phenomenon of abnormal dispersion, which meant the samples had the characters of strong selective absorption in this wave band. In contrast, we have also utilized Fourier transform infrared spectroscopy (FTIR) in the 400-4000cm^-1 to measure the mid-IR absorption spectra, and the results revealed that the crude oil is more active in the range of Terahertz than in the mid-IR band. The experiments demonstrated that THz-TDS is a powerful tool to complement the conventional analytical approaches and can be applied to detect and analyze the features of the constituent in crude oil.

We have developed a novel THz detector which uses the cantilever technology and surface plasmon resonance (SPR) technology to achieve a high sensitivity. The Micro Electro Mechanical System (MEMS) technology is adopted to fabricate the detector, which comprise thermo-sensitive bi-material micro-cantilever, prism and optical readout system. The bi-material of Si3N4 and Al is used to fabricate the micro-cantilever because of the good absorption characteristic for THz of Si3N4 and the great difference in thermal expansion coefficient of the bi-material for the deformation of the micro-cantilever. In order to increase the deformation of micro-cantilever, the method of computer simulation is used to obtain the optimal structure of micro-cantilever and the thickness of Si3N4 and Al. The function of the glass prism is to make the incident light generate total reflection under certain conditions. The gold film is sputtering on the top of glass slide using the method of magnetron sputtering and it is necessary for the generation of SPR performance. The optical readout system can make the change of cantilever bending convert to the change of reflection luminous intensity proportionally. The heat on the micro-cantilever coming from the THz radiation can lose easily in the air, so the detector is placed vertically in a cylindrical vacuum chamber which is sealed with quartz glasses and polyethylene lamina at the two end surfaces respectively. The quartz glass is used for the incidence of visible polarized light and the polyethylene lamina for the THz radiation. In order to maintain the vacuum performance of the chamber, the mechanical pump and molecular pump are adopted. In static mode, THz radiation absorption raises the temperature of micro-cantilever, so it bends proportionally. The micro-cantilever bending changes the thicknesses of the gap between the micro-cantilever and the metallic thin film on the micro-prism. It will result in a shift of the SPR angle. Therefore, the surface plasmon excitation efficiency and therewith the measured at a fixed incident angle reflectance of a metallic film will be changed almost proportionally to the cantilever bending. Consequently, the radiation energy of THz can be determined via the metallic film reflectivity change. Finally, the technology of image processing is introduced in detail.

An enhanced measurement system for terahertz pulsed imaging is presented that is capable of effectively performing en-face and three-dimensional scans of planar and curved surfaces. The bending of the scanning surface in one direction is compensated by synchronously advancing the mechanical delay time with the deflection of a scanning mirror. The second scanning mirror for the fast scanning direction forms a telecentric system and does not exhibit phase distortion.

THz spectroscopy has recently been used to examine human disease because of its a higher sensitivity and signal-to-noise ratio. In this work, the interaction mode between THZ radiation and biological tissue in the THz frequency range is given. Clinical Intralipid-10% is diluted into different concentrations to use as tissue phantoms. And they are firstly tested based on the terahertz time domain spectroscopy in the condition of room temperature. Optical parameters of phantom solution including absorption coefficient and refractive index are gotten, and the frequency properties of these optical parameters are analyzed. The function relations can be firstly found. The conclusions is meaningful to make the further research of interaction between THZ radiation and human tissue, related cancers.

This paper illustrates the fundamental transmission characteristics of periodic air hole metal array utilizing FDTD simulations. A metal-dielectric structure with periodic hole arrays perforated in the metal layer is presented. By changing the values of parameters, such as dielectric constant, thickness of the metal film and hole periodicity, the transmission responses are explored at terahertz regime. The surface plasmon polaritons play a significant role in the transmission. Based on the analysis, a wide band-pass filter containing metal-dielectric-metal layers is designed with square lattices of air holes in metal layers. The parameters of the structures are carefully selected to give a filtering characteristic with relatively wide and flat pass-band exceeding 200 GHz, over 95% THz transmittance and easy manufacture process. This kind of sample structure can be the potential candidate of filters in terahertz systems.

This paper reports a hot colloidal synthesis method toward the synthesis of CuInS₂/ZnS hybrid nanoparticles. First, CuInS2 quantum dots (QDs) were synthesized in the noncoordinating solvent octadecene using copper nitrate, indium acetate, myristic acid, oleylamine and dodecanethiol. CuInS₂ QDs with diameter of 2~10 nm were obtained by tuning the reaction temperature and time. Secondly, CuInS₂/ZnS core/shell QDs with different thickness of ZnS shell were synthesized by seed-mediated growth method, which can prevent the CuInS₂ QDs oxidized and improve their photo luminance. Low-toxic cadmium-free CuInS₂/ZnS QDs with high PL efficiency are expected to be used for bio-label.

Terahertz spectroscopy contains rich physical, chemical, and structural information of the materials, and one unique attribute of terahertz is their ability to sensitively measure the induced molecular dipole moments of many liquid crystals. In this paper, the transmission spectra of several liquid crystals 5CB, TEB300, LC203000 LCs were measured by THz time-domain spectroscopy technique and free-space electro-optic sampling method. The refractive indexes of the mentioned liquid crystals in the THz range are calculated. Furthermore, the optical constants are compared and analyzed, expecting to fill the spectrum gap of liquid crystals in the THz range and provide the experimental and theoretical foundation for the application of liquid crystals.

In this paper, a compact terahertz stop band filter is presented, which consists of two parallel metallic surfaces corrugated with rectangular groove arrays of period a. When a metallic surface is periodically corrugated, surface electromagnetic modes can be excited by incident terahertz waves. These modes are surface plasmon polaritons with an effective plasma frequency controlled entirely by the surface geometry. Because of the corrugated grooves and geometry induced highly confined surface plasmon polaritons, the fundamental mode of the parallel metallic plates splits into two modes and there is a band gap between the two modes. The band stop filtering functionality is realized by rejecting all frequencies in the gap. Simulation results calculated by finite element method show that the proposed structure has a band gap ranging from 0.315c/a to 0.350c/a for groove depth d = 0.5a, groove width l = 0.5a and gap width of the two parallel metallic surfaces w = 2a. Transmission spectra also show zero transmission within the band gap region where no guided modes are supported. By varying the gap width w and groove depth d, different filtering bandwidths with different center frequencies can be achieved.

Half-Wave Loss (HWL) plays an important role in the optical effects and devices based on the thin film interference coatings, such as in fiber optical telecommunication, electron displays, optical storage as well as polarizing beam splitter with very high extinction ratios in both reflection and transmission. Using the reflective terahertz time-domain spectroscopy (THz-TDS), we present a direct and unambiguous observation of the polarity reversal due to the half-wave loss by comparing the single-cycle THz pulse normally reflected from the quartz-air interface with that from the quartzwater interface. In addition, a quantitative measurement of the phase shift is provided and the impact of the imaginary part of the complex refraction index on the phase shift is discussed. The measured results are in excellent agreement with the theoretical predictions.

A terahertz (THz) imaging system based on air ionizing in two color laser fields is demonstrated. A 1mm thick (110)-oriented ZnTe is used for electro-optic sampling crystal. The depth of modulation of THz is 0.8%, which is much lager than that of optical rectification in ZnTe. Minus the background, the transmission images of the THz focus and a moving metal rod are demonstrated. The imaging speed is 34 fps. It is very useful to observe the movement of objects with terahertz in real time.

The terahertz radiation from the crystal of Cadmium Telluride (CdTe) can be achieved in difference frequency generation (DFG) experiment of 1064 nm nanosecond laser for collinear configuration. For the isotropic crystal CdTe, the exact phase matching could not be fulfilled by the two NIR lasers. However, if the interaction length is smaller than the coherent length, quasi-phase matching could be achieved. In order to understand the property of the coherent length in CdTe, the property of the refractive index and the absorption coefficient at the THz region is analyzed by two kinds of transmission spectra: one from Terahertz Time-Domain Spectroscopy (TDS) and the other from Fourier Transform Infrared Spectrometer (FTIRS). From the transmission spectra of FTIRS, four absorption lines are detected: 2.1 Thz, 4.2 Thz, 7.4 Thz, and 8.6 Thz. Also additional two little absorption peaks occurred at 1.6 Thz and 1.8 Thz. This maybe explained by the phone mode LO-LA and LA-TA, respectively. Below 1.0 Thz, the absorption coefficient is small and constant (about 5cm^-1). Based on the refractive index spectra of THz-TDS, the coherent length calculated increases linearly with the THz wavelength in the region of 200μm to 900μm, with its' value 3.6 mm at 300μm. Then interaction length can be long enough to satisfy the quasi-phase matching condition. A high power and compactable terahertz source can be obtained from CdTe under 1 Thz, which can be tuned continually and operated under room temperature.

Nowadays, heavy metals pollution is becoming a serious problem in agriculture. This paper reports a preliminary work on a feasibility study of applying terahertz (THz) technology for heavy metal ions detection in soil. This study was first conducted at Oklahoma State University, and then carried out at China Agricultural University and Capital Normal University. Pure soil was collected in an experimental field, which contains nearly no heavy metal ions from standard detection; in the experiment, heavy metal ions were mainly Pb²⁺, Cr³⁺, Zn²⁺ and Cu²⁺ from chemical compounds. Based on the National Standard for Heavy Metals Pollution, a set of soil samples with different polluted levels were prepared in the lab. The metal ions concentration levels were selected as 50ppm, 300ppm and 700ppm. Each soil sample was pre-processed by collecting, weighing, mixing, drying, grinding and labeling before measurements. The thickness of soil samples was selected as 1.5mm and 3mm. The absorption spectra for the soil samples with different heavy metal ions were collected using THz time-domain spectroscopy (THz-TDS) equipments separately at Oklahoma State University and Capital Normal University. The test results showed that soil samples with Pb²⁺, Cr³⁺, Zn²⁺ and Cu²⁺ had different absorption characteristics within the bandwidth of 0.1-1.1THz. A narrow bandwidth only up to 1.1THz was got because the soil particles showed much absorption and scattering properties to the THz spectroscopy. Different soil samples with different concentration of heavy metal ions also showed much difference and it could be used to predict the heavy metal concentration in the future. The results from the preliminary study show a potential of THz technology applied for heavy metal ions detection in agricultural fields environment. However, since the high scattering features of samples and high cost of equipments, the measurement methods and practical issues needs to be further investigated and improved to make the THz technology a feasible tool for soil heavy metal ions detection.

The influence of an absolute phase of the THz pulse with a few cycles on the spectrum and spectral dynamics of signal reflected from or transmitted through the explosive or paper sheet is analyzed for problems of the detection and identification of explosive. The signal was measured on the long-term interval with duration about 180 ps and contains both the main reflected THz pulse and the subpulse with less intensity. Two cases of the reflection (normal and specular reflection) of the THz pulse from the RDX are considered. For the THz pulse interaction with the paper sheet the transmission mode is analyzed. Dependence of the nonlinear response of substance from the absolute phase of pulse is illustrated by examples of SHG or self-action of laser pulse with a few cycles. We showed that the absolute phase of pulse plays one of the key roles for the detection and identification of the substances at the reflection mode. It is necessary to investigate this problem with big attention during next few years.