This PDF file contains the front matter associated with SPIE Proceedings Volume 6840, including the Title Page, Copyright information, Table of Contents, and the Conference and Symposium Committee listings.

Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (Δν/ν<10^-6). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.

Heavy water vapor (D₂O gas) which owns special structure properties, can generate terahertz (THz) radiation by optically pumped technology, and its 385 μm wavelength radiation can be widely used. In this research, on the base of semi-classical density matrix theory, we set up a three-level energy system as its theoretical model, a TEA-CO₂ laser 9R (22) output line (λ=9.26 μm) acted as pumping source, D₂O gas molecules were operating medium, the expressions of pumping absorption coefficient G_p and THz signal gain coefficient G_s were deduced , It was shown that the gain of THz signal was related with the energy-level parameters of operating molecules and some operating parameters of the THz laser cavity, mainly including gas pressure, temperature etc.; By means of iteration method, the output power density of THz pulse signal was calculated numerically as its initial power density was known; Changing the parameter of gas pressure and keeping others steady, the relationship curve between the output power intensity (Is) of Tera-Hz pulse laser and the operating D₂O gas pressure (P) was obtained. The curve showed that the power intensity (Is) increased with gas pressure (P) in a certain range, but decreased when the pressure (P) exceeded some value because of the bottleneck effect, and there was an optimal gas pressure for the highest output power. We used a grating tuned TEA-CO₂ laser as pumping power and a sample tube of 97cm length as THz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other.

The THz spectral region, lying between the microwave and the far infrared, is currently attracting widespread interest in relation to potential applications that span many areas of pure and applied science and technology such as in security, human health and communications. The development of practical source of THz radiation is crucial to the realization of these applications. Using optical parametric oscillator with intersecting pump results in compact, low threshold THz radiation source. Lithium niobate is one of the most suitable materials for generating THz waves efficiently because of its large nonlinear coefficient and its transparency over a wide wavelength range. In this communication we theoretically analyze generation of tunable terahertz (THz) radiation which is based on optical parametric process, in particular using a non-collinear quasi-phase matched geometry in the nonlinear crystal periodically poled lithium niobate (PPLN), and where wide and continuous tuning is obtained by changing the poled period and the angle between the resonated idler wave and the pump wave. Also the THz wave should fulfill the energy and momentum conservation laws. Furthermore, the bandwidth of the wavelength and the frequency of THz wave are analyzed. The optimal THz wave parametric oscillator for efficient coupling output power can be designed according to our theoretical results

A novel nano-antenna for the generation and applications of THz wave is proposed based on the carbon nanotube. The geometric structure and radiation properties of carbon nanotube (CNT) antenna at THz span have been investigated by the finite integral numerical methods. The designed typical CNT antenna arrays (λ=3.0×10⁴nm, L=0.485λ, R=2.712nm, d=0.1λ) operate from 9.3 THz to 10.2 THz, and main resonant frequency is 9.7 THz. The simulated -10dB return loss (S₁₁) bandwidths is about 10% and the standing wave ratio is less than 1.5 at the center frequency of 9.7THz with the total radiation efficiency in excess of 85%. The maximum gains for 5×5, 15×15, 25×25 CNT antenna array are 5.92dB, 8.19dB and 8.50dB in the center frequency, respectively. The details of THz antenna characteristics such as field distribution, surface current profile, standing wave ratio, scattering coefficient and gain are presented and discussed. The overall results could be suitable for the design of CNT antenna array generating THz wave.

It is known that the increase of bath temperature results in the decrease of critical current of superconducting hot-electron bolometer (HEB) mixers owing to the depression of superconductivity, thus leading to the degradation of the mixer's sensitivity. Here we report our study on the effect of bath temperature on the heterodyne mixing performance of quasi-optical superconducting NbN HEB mixers incorporated with a two-arm log-spiral antenna. The correlation between the bath temperature, critical current, LO power requirement and noise temperature is investigated at 0.5 THz. Furthermore, the heterodyne mixing performance of quasi-optical superconducting NbN HEB mixers is examined while there is an optical-axis displacement between the center of the extended hemispherical silicon lens and the superconducting NbN HEB device, which is placed on the back of the lens. Detailed experimental results and analysis are presented.

Detection of signals in the THz frequency region is important for applications of THz waves in many areas, such as in medical imaging, forbidden-combined sensing, weapon monitoring, and wireless communications. Cooling system operating under very low temperature, for eliminating the unwanted background THz radiation that exists everywhere in room temperature, sets an obstacle for applications of conventional THz signal detecting systems. We present a combined cavity that can pick out the useful signal in high sensitivity, while the influence of the background THz radiation can be neglected in the detection. The combined cavity consists of a point-defect photonic-crystal resonator and a photonic-crystal WGR. The two resonators are coupled together through optical tunneling to form the combined cavity. Under proper operating parameters, the two resonators are in simultaneous resonance, and the field intensity in the point-defect resonator can be thousands of times of that of an incoming THz signal for a given frequency, so that the sensitivity of detection can be very high. Since the background THz radiation is not in resonance with the cavity, the influence of it to the detection of THz signals wanted can be neglected, and thus cooling systems can be omitted. Plane wave expansion method is used to determine the resonance wavelengths and mode patterns of the cavity. Finite-difference- time-domain method is used to find the quality factors and to simulate the resonance process. Parameter optimization and the conditions for simultaneous resonance of the two cavities are studied.

Na₅ [B₂P₃O₁₃] (NBP) is a promising nonlinear optical crystal. It exhibits a wider transparency range from UV to far-IR. And its absorption edge lies at 186nm. The forbidden band gap is about 6.67eV. The time domain spectra in the range of 0.25-2.5 THz were measured. The power absorption coefficient and refractive index were studied. The refractive index is linear with frequency from 0.4THz to 1.8THz. And the value increases slowly from 1.8 to 2.1THz. The properties of the sample show that it is possible to apply it to optical device.

Absorption spectra and index of refraction of aminophenol compounds are obtained using Terahertz time-domain spectroscopy (THz-TDS) in the frequency range between 0.3 and 2.0 THz at room temperature. Quantum chemical calculations are performed to better understand the origin of the absorption features observed in experiments. Different weight ratios of isomeric mixtures are analyzed by THZ-TDS with the help of linear regression technology. It is concluded that THZ-TDS is able to identify the components and their relative percentage in isomeric mixtures.

In this presentation, a system for THz en-face tomography imaging is proposed. The THz radiation reflected by the object is imaged on the ZnTe crystal, and then a CCD is used to detect the two dimensional modulated probe light. By adjusting the delay line, the information about the object can be achieved layer by layer. Expanded terahertz pulses are reflected from the different medium interfaces inside a sample, the amplitudes and time decays of these pulses can be used to determine the thickness of each medium inside the sample. In this way, the structure image of the sample can be reconstructed. The longitudinal resolution of the system is discussed by using the timing information of the reflection terahertz pulses.

A novel transmitted terahertz-emission microscopy (TTEM) is proposed and developed for improving the spatial resolution of THz imaging. An epitaxial GaAs film grown on the GaP substrate was used as THz emitter; the transmitted THz signal was collected and detected by a ZnTe electric-optical crystal. Because the thick of GaAs layer is merely 1 μm, the THz wave source has the same size of the excited point. While attaching a sample directly onto the emitter, the spatial resolution is decided by the diameter of focused pump beam, which can be achieved a few micrometers and tunable. In addition, it can avoid the loss of the spectral components. By means of the near-field detection, the intensity, spatial resolution and bandwidth of THz signal in this system can be enhanced further. The configuration and characteristics of this microscopy are described in detail.

As a new technology, the terahertz technology had made a great progress in security inspection and medical field. This paper shows the application of the terahertz time-domain spectroscopy (THz-TDS) technology on cosmetic testing. We obtain the THz spectra of three kinds of usual cosmetics powders. Two kind of powder have an obvious absorption peak at 1.14 THz, but the third one has no absorption peak. The positions of absorption peaks in the infrared spectra of three kinds of powders are approximately identical. These results show that THz-TDS technology has the advantage and potential application on the cosmetic testing. In addition, we also measure some solid and liquid cosmetic components, such as Titanium-dioxide, Magnesium Stearate, Kaolin, Glycerol, etc. THz spectra of their refractive index and absorption coefficient are obtained experimentally. We are trying to establish the fingerprint spectra database of cosmetic components for further research and application.

Although the experimental solid-state terahertz (THz) spectrum (0.2-2.5THz) of the high explosive petaerythritol tetranitrate (PETN, C₅H₈N₄O₁₂) has been presented previously, till now, the theoretical analysis of its THz spectrum is few reported before. In this paper, we used solid-state density functional theory (DFT) calculation to simulate the vibrational spectrum of PETN in THz range. The DFT calculations were performed using DMol³ (version 4.0). Solid-state DFT, employing the BP density functional, can be able to reproduce the experimentally observed solid-state structure and low-frequency vibrational motions, and final simulated spectrum has a good-to-excellent agreement with experimental THz spectrum. In order to observe more THz spectrum details of PETN, we increased the frequency-domain resolution. Furthermore, we also have obtained the refractive index, absorption coefficient of this sample by THz time-domain spectroscopy.

The spin-related phenomena in semiconductor nanostructures are currently attracting considerable interest due to the novel physics and the potential application in electronics and optoelectronics. The Rashba and Dresselhaus spin-orbit interactions play an important role in controlling and manipulating the spin and charge degrees of freedom in low-dimensional and nanoscale semiconductor systems. We present a study of quantum transport and optical properties in spintronic materials in the terahertz frequency regime and discuss the potential application of spintronic materials in terahertz optoelectronic devices.

In this paper, we present experimentally the transmission properties of THz pulse through subwavelength planar fractal slits on the copper foil. The transmission measurement is carried out using THz time-domain spectroscopy. THz spectroscopy of these fractal slits shows the featured frequency selectivity. Multiple band gaps and pass bands for THz wave are observed. THz transmission under different polarization for a 7-level fractal pattern is analyzed. The transmission spectra with the different levels of fractal slits are compared and discussed. Polarization dependent THz transmission is attributed to the localized resonance induced by the fractal pattern.

We study plasma effects in a micromachined high-electron mobility transistor (HEMT) with the microcantilever (MC) serving as the gate using the developed model. The model accounts for mechanical motion of the MC and spatio-temporal variations (plasma effects) of the two-dimensional electron gas (2DEG) system in the transistor channel. The MC mechanical motion is described in the point-mass approximation. The hydrodynamic electron transport model is used to describe distributed electron plasma phenomena in the 2DEG system. Using the developed model, we calculated the response function characterizing the amplitude microcantilever oscillations and the output electric signal as functions of the signal frequency and the bias voltage for the devices with different parameters. We find the voltage dependences of the frequency of the mechanical resonance and its damping. In particular, it is demonstrated that the amplitudes of the mechanical oscillations and output electric signal exhibit pronounced maxima at the bias voltages close to the voltage of the 2DEG channel depletion followed by a steep drop with further increase in the bias voltage. We also consider a concept of a resonant detector of modulated terahertz radiation based on a micromachined HEMT. This device can exhibit both the plasma (in terahertz range) and mechanical (in megahertz or gigahertz range) resonances.

In the terahertz (THz) spectroscopic system, the research of propagation characteristics of terahertz electromagnetic field is very important. Using a single slit which width is close to the central wavelength of terahertz pulse, we investigate the spectral distribution of terahertz beam at the location of samples. We obtain the following results by analyzing the experimental data. Firstly, the size of THz beam is different for the THz wave of the different frequency. The width of THz beam is small for the high frequency of THz radiation. At the same frequency, THz beam becomes wider for the narrower slit, that is, THz wave show the obvious diffraction phenomenon. Secondly, the frequency spectra distribution is different at the different position of THz beam. The peak position of frequency spectra appears red shift with the increasing of the distance away from the center of terahertz beam. These results provide an important reference for the further terahertz experiments of spectroscopy and imaging.

Recently some naphthoquinone derivatives have been found with anticancer or other therapeutic properties, but also have some negative side effects. Numerous research projects have been conducted to investigate their properties and therapeutic mechanisms. With Terahertz Time-Domain Spectroscopy (THz-TDS), we have successfully obtained THz spectra of 1,4-naphthoquinone and its four derivatives in a series of naphthazarin - juglone - 1,4-naphthoquinone - menadione - plumbagin, in the range between 0.2 and 2.4~2.8 THz. Although these molecules are almost identical to each other, they have very distinctive THz spectra so that they can be identified much more easily than using conventional spectroscopy. We have comparatively analyzed their THz spectra, and found some possible correlations between THz spectra and molecular structures. These THz spectra cannot only be used as spectral fingerprint, but also provide us their conformational properties that can be used in study of their interaction with biomolecules to reveal their pharmaceutical mechanisms.

Terahertz spectroscopy has tremendous potential for applications to evaluate the quality of the drugs including the TCM. In this paper, the Terahertz Time-Domain Spectroscopy investigated two active ingredients: Andrographolide and Dehydroandrographoline, isolated from Andrographis paniculata (Burm. f.) Nees. We also measured the mixtures of two active ingredients at the different ratio and the quantitative analysis is also applied to determine the contents of compound. The Terahertz spectroscopy is a potential and promising technique in identifying the components, evaluating the drugs sanitation and inspecting the quality of medicine including TCM.

We experimentally investigate the resonantly enhanced transmission of the terahertz radiation through a periodic array of subwavelength apertures on metal films. We fabricated the periodic arrays of subwavelength apertures with different periods and different diameters on silicon substrate. THz transmission spectra of these samples are measured by means of terahertz time-domain spectroscopic system. The resonantly enhanced transmissions with the frequency selectivity are observed. Our experimental results show that the resonantly transmission peak appears red shift and increases with increasing of hole diameter when keeping the period of array as a constant. On the other hand, the resonantly transmission peak with the same hole diameter appears red shift with increasing of the array period.

We report the sensing of explosive materials and illicit drugs by using terahertz time-domain spectroscopy (THz-TDS) and imaging. Several explosive materials, such as γ-HNIW, RDX, 2,4-DNT, TNT, Nitro-aniline, and illicit drugs, such as methamphetamine (MA) etc were researched here. Non-destructive testing, as one of the major applications of THz imaging, can be applied to an area of critical need: the testing of aerospace materials. Composite materials such as carbon fiber are widely used in this industry. The nature of their use requires technologies that are able to differentiate between safe and unsafe materials, due to either manufacturing tolerance or damage acquired while in use. In this paper, we discuss the applicability of terahertz (THz) imaging systems to this purpose, focusing on graphite fiber composite materials, carbon silicon composite materials and so on. We applied THz imaging technology to evaluate the fire damage to a variety of carbon fiber composite samples. Major carbon fiber materials have polarization-dependent reflectivity in THz frequency range, and we show how the polarization dependence changes versus the burned damage level. Additionally, time domain information acquired through a THz time-domain spectroscopy (TDS) system provides further information with which to characterize the damage. We also detect fuel tank insulation foam panel defects with pulse and continuous-wave (CW) terahertz system.

Many materials of interest to the forensic and security services, such as explosives, drugs and biological agents, exhibit characteristic spectral features in the terahertz (THz) frequency range. These spectral features originate from inter-molecular interactions, involving collective motions of molecules. Broadband THz time-domain spectroscopy (THz-TDS) system have been used to analyze a number of drugs-of-abuse and explosives that are of interest to the forensic and security services. These samples ranged from crystalline powders, pressed into pellets, to thin sheets of plastic explosives, and all being measured in transmission geometry in the frequency range 0.1 - 8 THz. To well understand the nature of the observed spectral features and the effects of thermal broadening on these far-infrared signatures, temperature-dependent THz-TDS measurements have also been performed at temperatures as low as 4 K, especially for two types of cocaine. Well-resolved low-frequency absorption peaks were observed in the frequency range 0.1 - 3 THz with high resolution. Some of absorption peaks were found clearly to become more intense and shift to higher frequencies as the temperature was reduced. The results confirm that the low-frequency collective modes are highly sensitive to the structural and spatial arrangement of molecules. Furthermore, a number of common postal packaging materials made from paper, cardboard, even several types of plastic, have been tested with drug sample to assess the ability of THz-TDS in a hostile detection environment.

A method was proposed to quantitatively inspect the mixtures of illicit drugs with terahertz time-domain spectroscopy technique. The mass percentages of all components in a mixture can be obtained by linear regression analysis, on the assumption that all components in the mixture and their absorption features be known. For illicit drugs were scarce and expensive, firstly we used common chemicals, Benzophenone, Anthraquinone, Pyridoxine hydrochloride and L-Ascorbic acid in the experiment. Then illicit drugs and a common adulterant, methamphetamine and flour, were selected for our experiment. Experimental results were in significant agreement with actual content, which suggested that it could be an effective method for quantitative identification of illicit drugs.

The worldwide production volume of polymers is still rising exponentially and the number of applications for plastic components steadily increases. Yet, many branches within the polymer industry are hardly supported by non-destructive testing techniques. We demonstrate that terahertz (THz) spectroscopy could be the method of choice to ensure high-quality polymer products. Applications range from the in-line monitoring of extrusion processes and the quality control of commodities in a mass production up to a total inspection of high-tech safety relevant products. Furthermore, we present an extension to THz time-domain spectroscopy in the form of a new data extraction algorithm, which derives the absorption coefficient, the refractive index and the thickness of a sample with very high precision in a single pass. Apart from that, we discuss the ability of THz systems for quality control of polymeric compounds. Here, it is essential to monitor the additive content as well as additive inhomogeneities within the mixture. Recently, we built a fiber-coupled THz spectrometer for in-line monitoring of compounding processes. Additionally, we demonstrate the potential of THz systems for the non-destructive and contactless testing of structural components. THz imaging is capable of analyzing material thicknesses, superstructures, the quality of plastic weld joints, and of detecting flaws in components. Plastics and THz form a very fruitful symbiosis. In return, plastics industry can provide THz systems with custom-tailored components, which have very attractive properties and extremely low costs. Examples of this development are photonic crystals or polymeric Bragg filters, which have recently been demonstrated.

Experimental measurement and theoretical analysis of THz spectrum for methylenedioxy amphetamine are introduced. The refractive index and absorption coefficient of the sample were observed by terahertz time-domain spectroscopy (THz-TDS) technique in the range of 0.2~2.6 THz. It exhibits obvious absorption feature at 1.40 THz and weak THz absorption at around 1.68 and 2.21 THz. The spectral absorption characteristic in THz band was useful for the inspection and identification of drugs using THz-TDS. The theoretical calculation was performed using Density functional theory (DFT) with the GAUSSIAN 03 software package. Fully geometry optimization and frequency analysis of the optimized structure were performed at the B3LYP/6-21G levels. The simulated absorption spectrum was in agreement with the experimental data, and can hence be used for the assignment of observed THz spectrum. The theoretical simulation result showed that absorption peaks mainly result from intra-molecule and inter-molecule vibrations, different absorption peaks are corresponding to different vibrational modes and intensity. So the combination of the THz-TDS and DFT is an effective way to investigate characteristic spectra of illicit drugs.

The ability to detect and measure a variety of gases under certain environmental conditions has significant potential impact in many areas; from hazardous gas detection in the industrial domain to physical sciences in academia. Gas sensing has long received attention with microwave and infrared spectroscopy. With many molecular resonances occurring in the THz (T-ray) range leading to simple, unique spectral features, THz time-domain spectroscopy (TDS) promises to be a potential tool for gas detection. This paper presents a preliminary study on real-time gas recognition with THz-TDS. In particular, a simple method is proposed that involves extracting line positions from gas species without a reference pulse and classifying them by means of the minimum Euclidean distance using the Submillimeter, Millimeter, and Microwave Spectral Line Catalog.

We present a real time imaging measurement in the terahertz (THz) frequency region. The dynamic subtraction technique is used to reduce long-term optical background drift. The reflective images of two targets, a Nikon camera's lens cap and a plastic toy gun, are obtained. For the lens cap, the image data were processed to be false color images. For the toy gun, we show that even under an optically opaque canvas bag, a clear terahertz image is obtained. It is shown that terahertz real time imaging can be used to nondestructively detect concealed objects.

The sensing of the explosives and the related compounds is very important for homeland security and defense. Based on the non-invasive terahertz (THz) technology, we have studied some pure and mixed explosives by using the THz time-domain spectroscopy and have obtained the absorption spectra of those samples. The obtained results show that those explosives can be identified due to their different characterized finger-prints in the terahertz frequency region of 0.2-2.5 THz. Furthermore, the spectra analyses indicate that the shape and peak positions of the spectra for these mixed explosive are mainly determined by their explosive components. In order to identify those different kinds of explosives, we have applied the artificial neural network, which is a mathematical device for modeling complex and non-linear functionalities, to our present work. After the repetitive modeling and adequate training with the known input-output data, the identification of the explosive is realized roughly on a multi-hidden-layers model. It is shown that the neural network analyses of the THz spectra would positively identify the explosives and reduce false alarm rates.

A terahertz continuous-wave transmission imaging system and its applications in security inspections are reported. A Gunn oscillator is utilized as emitter and an unbiased Schottky diode is employed as detector in this system. The sample is placed on an X-Y two-dimensional stage which is controlled by a computer. The intensity information of the terahertz wave after passing though the sample is collected by the Schottky diode and fed into the computer. Two-dimensional image is obtained by scanning the sample point by point. Compared with the terahertz pulse imaging system, this system is compact, simple, and portable. Tennis bat with sheath and knife in the box are imaged by using this system. The results obtained here show that this new technology can be widely used in security inspections.

Semiconductor nanostructures, such as quantum wells and quantum dots (QD), are well known, and some have been incorporated in applications. Here will focus on novel structures made of QDs and related devices for terahertz (THz) generation. Their potential advantages, such as low threshold current density, high characteristic temperature, increased differential gain, etc., make QDs promising candidates for light emitting applications in the THz region. Our idea of using resonant tunneling through QDs is presented, and initial results on devices consisting of self assembled InAs QDs in an undoped GaAs matrix, with a design incorporating GaInNAs/GaAs short period superlattice, are discussed. Moreover, shallow impurities are also being explored for possible THz emission: the idea is based on the tunneling through bound states of individual donor or acceptor impurities in the quantum well. Initial results on devices having an AlGaAs/GaAs double barrier resonant tunneling structure are discussed.

Terahertz (THz) waves have attracted intensive researches in recent years due to their potential applications. However, because of the long wavelength of this range, it is quite difficult to integrate the THz system in a small size. Surface plasmons subwavelength optics opens a new way for fabrication of THz devices within wavelength size. In this presentation, we use a simple subwavelength slit to generate the band gap in the THz range. The transmission spectrum of this slit is measured by using the THz time domain spectroscopy. It was found that when the slit width is narrow enough, the Fabry-Pèrot-like band gaps in the transmission spectrum will be generated. The passband will be moved to the high frequency direction with the slit width narrowing. The numerical simulations are also carried out based on the finite difference time domain method. It was found that the simulation results correspond to the experiment results well. It is expected that this research can provide a new way to fabricate THz device.

Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlO_x/Nb junction and Nb/SiO₂/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (J_c) of 10kA/cm². In this paper, we describe their design, fabrication and preliminary experimental results.

The experiments of ultra-wide-band electromagnetic radiation from Photoconductive Semiconductor Switches (PCSS's) triggered by high repeating frequency YAG ps laser pulse are reported. Based on the experimental results of GaAs PCSS's and InP PCSS's in the linear mode, it is indicated that the rise time of ultra-fast electric pulse generated by PCSS's depends on the rise time and the width of the triggering laser pulse. The fall time of the electric pulse is mainly effected by the lifetime of the carriers. By analyzing the relationship between the time-domain characteristic and the frequency-domain characteristic of the ultra-fast electric pulse generated by PCSS's, it is found that the frequency band of electromagnetic radiation is mainly determined by the rise time and the fall time of the ultra-fast electric pulse. The high-frequency component of frequency spectrum of electromagnetic radiation is determined by the rise time of electric pulse, and the low-frequency component is determined by the fall time. The ultra-fast electric pulse whose rise time is less than 100ps is obtained from GaAs PCSS's, and the ultra-fast electric pulse whose rise time is about 200ps is obtained from InP PCSS's.

Terahertz (THz) emission from ZnSe and ZnTe nano dots and nano gratings is experimentally studied compared with that from <111> orientation bulk ZnSe and <110> orientation bulk ZnTe. These nano dots and nano gratings are fabricated by the femtosecond laser ablation technique. Three main mechanisms coexist in the THz radiation from ZnSe and ZnTe surface nano structures as the same as from bulk sample: current surge effect (drift current), Photo Dember effect (diffusion current), and optical rectification. Moreover, it is found that nano structures not only contribute to an enhancement of THz emission, but also increase the threshold of polarity reversal of THz wave, and widen the spectrum of THz radiation. We also observe a two-fold symmetry in rotating pump polarization angle from ZnTe nano gratings, which is completely different from the three-fold symmetry observed from bulk ZnTe.

In this letter, we have proposed a novel compact terahertz wave switch based on photonic crystals. The OFF-ON mechanism of the novel terahertz wave switch is based on a dynamic shift of the photonic band gap. The two-dimensional finite-difference time-domain (FDTD) method is used to investigate switch properties of the terahertz wave switch. The simulation results show that the terahertz wave switch size is about 6.8mm. The novel terahertz wave switch can be used in future terahertz-wave communication and detection systems.

As a new attempt to expand wireless transmission channel bandwidth, a submillimeter-wave communication experiment has been carried out by using techniques of frequency multiplication of millimeter-wave signal source and high-sensitivity superconducting SIS (Superconductor-Insulator-Superconductor) receiver. The transmitter consists of a 500-GHz frequency quintupler and a 93-GHz varactor-tunable Gunn oscillator with its output frequency modulated via a 10-14 MHz signal source, while the 500-GHz SIS receiver has an IF (Intermediate Frequency) band of 1.1-1.7 GHz and an overall system noise temperature below 400 K. A theoretical analysis is given to prove the principle and engineering feasibility of the experimental communication system. The measured spectra of the received FM (Frequency Modulation) signals with modulation frequencies of 10-14 MHz on a downconverted IF carrier of 1.6 GHz show that effective direct frequency modulation on a transmitting submillimeter-wave carrier of 466 GHz and the reception of the submillimeter-wave FM signals by the 500-GHz SIS receiver have been successfully realized.

High energy terahertz pulses are produced by illuminating a biased GaAs wafer using a short pulse from a Ti:sapphire laser with a central wavelength of 800 nm, a pulse width of 50fs (FWHM) and a repetition rate of 10 Hz. We show that the peak THz amplitude scales with the bias voltage and thus the THz energy and intensity scales quadratically with bias voltage for bias fields up to 3 kV/cm. For laser pulses with an energy density of 1 mJ/cm² we observe a multiple pulse structure. We show that the polarity of the terahertz pulses is consistent with multiple reflections from the exit face of the GaAs slab and the boundary of a plasma slab inside the wafer produced by the laser. We use the standard Drude model for terahertz production from the GaAs wafer to describe multiple pulse structure due to reflections from the plasma boundary layer in the slab. The time delays between multiple pulses are consistent with a 120 μm thick slab produced by photo-produced carriers.

In current particle-in-cell (PIC) simulation software, the most important parts are several key technologies. In this paper, we discuss some technologies based on Yee-grid model and time-leapfrog model. Firstly, we induce 3-D electromagnetic discrete algorithm formulae, which include the centered-difference algorithm formulae, time-biased algorithm formulae and high-Q algorithm formulae. The application range of these electromagnetic discrete algorithms is also analyzed. Secondly, we discuss the phenomenological description technology used in emission simulations and induce the formulae which represent how to set the charge of macro particles in explosive emission. The charge-conserving algorithm used to calculate current which is produced by the particle moving and the weight algorithm used to calculate Lorentz force are also discussed. Thirdly, the ways to realize the common boundary conditions including the ideal conductor, port boundary, absorption boundaries and metal strut are discussed. The way to calculate the absorption coefficient of the absorption boundary is described in detail. The formulae of the metal strut's current and inductance under two-dimensional and three-dimensional coordinates in different orientation in space are induced and realized using finite-difference time-domain (FDTD) method. Then, the iterative process under main FDTD iteration of metal strut is given. At last, the correctness of these technologies is proved by computer simulation on the Vavilov-Cherenkov radiation (VCR) in 2-D photonic crystal. The results show that the THz radiation is excited by electron bunches in photonic crystal.

In this letter, a new type of optically controllable, terahertz wave switch using high resistivity silicon wafer is developed. A high resistivity silicon is a lossless dielectric material at terahertz wave without optical excitation. When a silicon wafer is optically excited, free carriers are generated, and the silicon wafer becomes a lossy dielectric. We study theoretically and demonstrate experimentally light controllable terahertz wave of the high resistivity silicon wafers. The results show that a more than 15dB attenuation of the novel device is obtained at frequency of 0.3THz. The proposed structure is useful for developing low cost switch in the terahertz wave region.

The terahertz waves are powerful tools for the astronomical research as for other applications. The emission from the cosmic dust particles, for example, enables us to investigate the formation of stars and planetary systems. However, detectors with high enough sensitivities to detect faint emission from the celestial bodies are not available yet. We have developed extrinsic photoconductors utilizing shallow donor levels in the GaAs for the astronomical applications. The high sensitivity detectors require very low impurity concentration in the GaAs crystal. We adopted the liquid-phase epitaxy to obtain the GaAs crystals which have high purity and enough thickness. The purest sample we have obtained to date has the carrier concentration 4x10¹³cm^-3, and high electron mobility 140,000cm²/Vs. The photoluminescence measurements showed the residual impurity elements are silicon and carbon. We have tried to fabricate terahertz detectors using three types of epitaxial layers, selenium- doped, tellurium-doped and un-doped (unintentional-silicon doped) layers. These photoconductors all have sensitivities in 1-2 THz at the operation temperature of 1.6 K. The photoconductors with the un-doped layers showed the highest responsivity 30 A/W and reached a good NEP as low as 3x10^-16 W/Hz^1/2. We have also fabricated an eight-element linear array with feed horns to serve for the actual astronomical observations. This detector array is now under performance evaluation. The performance of the photoconductors and the linear array is described in this paper.

In this report, a required aperiodically poled structure is designed, where two quasi-phase-matched conditions of optical parametric oscillator and frequency difference are required to be satisfied simultaneously. By numerical simulation, we analyze the effects of crystal structure, the cavity parameter of optical parametric oscillator, and terahertz absorption coefficient on terahertz generation. And, the terahertz wave is generated via cascaded processes, which is of great interest in fundamental and applied sciences, and is a great demand for many applications in spectroscopy, sensing, communication, medial diagnoses and biomedical imaging. In addition, the design method for poled ferroelectric crystal is universal and applicable to many other fields, particularly in those processes where multiple quasi-phase-matched conditions are required to be satisfied simultaneously.

The optical properties of Cesium Iodide (CsI) in the frequency range from 0.1 to 2.0 THz were studied by employing THz time-domain spectroscopy. The CsI samples were synthesized using CsI powder that was pressed into 2 mm thick pellets. The obtained time-domain data were transformed into the frequency domain using Fourier Transforms, and then analyzing the respective phase and amplitude spectra, the refractive index (RI) and the absorption constant of CsI were deduced. The RI of CsI is seen to increase from 2.55 to 3.15 as the frequency increases from 0.15 to 1.4 THz. For frequencies above 1.4 THz, the data was unusable due to the increased absorption. The empirical Sellmeier equation was used to fit the frequency-dependent RI and to determine the absorption wavelength λ₁.

The electrical and magnetic properties of perovskite structures have great scientific interest. In this report we present a spectroscopy study of LaCa_0.7Mn_0.3O₃/MgO thin film to determine its optical properties. Terahertz time-domain measurements were performed to obtain THz transmission and emission spectra. The LaCa_0.7Mn_0.3O₃/MgO thin film used in this work was grow on single crystal substrate MgO (100) by pulsed laser deposition (PLD) technique and the transport measurement has shown that the thin is a colossal magnetoresistance (CMR) material. The insulator to metal transition temperature, T_IM is about 150K according to the temperature dependence of the resistivity measurement. On the THz transmission measurement, the spectra of the MgO and the thin film were taken in both air and N₂ to determine the effect of water absorption. In the emission measurement, the femto-second laser is incident on either the LaCa_0.7Mn_0.3O₃/MgO thin film or on the InAs wafer. The emitted THz signals in both cases were obtained and compared.

Recently, more and more groups are concentrating on learning surface plasma on metal split with sub-wavelength of terahertz. There is a new material which is made of two materials named metamaterial. It is combined with semiconductor and metal materials. Metal parts are made with subwavelength microstructure. Abnormal phenomenon occurred when it is excited by an intense light, showing an enhancement transmission of terahertz wave. In this paper, we design a planar structure composed of gallium arsenide (substrate) and copper (metal). Copper array is plating on a GaAs wafer on order of micron with photoetching technology. The thickness of GaAs and copper layer are 650 and 0.5 micros individually. THz incidence has an enhanced transmission like surface plasmonic resonance with terahertz time-domain system when another exciting lighted on, which made current carriers in photoconductor. This resonance arises from an inductor-capacitor circuit resonance. Comparison is performed between results with and without excitation. The angle of sample plane with terahertz polarization and incidence can affect this electromagnetic response in some degree. Its transmittance with back-incidence is much stronger than that with front-incidence at certain frequencies. This kind of artificial structure has potential abilities in terahertz devices in the future. It is helpful to develop terahertz filter, polarizing film, beam reflector, phase retarder and so on.