Transmission spectra were measured over the range 90-140 GHz and 300-4200 GHz for 20 soil samples that span a number of soil orders that have extensive worldwide distribution. A vector network analyzer equipped with 16 degree horn antennas covered the spectral range 90-140 GHz. Transmission measurements were also taken for some organic materials in the 90-140 GHz range. A Fourier spectrometer equipped with Hg arc lamp, pellicle beamsplitter, and Si bolometer collected transmission spectra over the range 300 to 4200 GHz. Transmittance ranged from 10^-7 to almost 1. In all cases, transmission drops to zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. In samples of mixed particle size, low transmittance in the 90-140 GHz range was found to be caused by the coarse component. This work is relevant to mine detection using THz and millimeter wave (mmW) radiation.

We investigate small peptides using standard terahertz (THz) time-domain spectroscopy. As a test set we examine the tripeptides glutathione, gly-gly-gly and enalapril maleate at room temperature. While earlier investigations of short-chain polypeptides with a conventional FTIR spectrometer were performed at higher THz frequencies, we present first measurements between 150 GHz and 2 THz and compare our measurements to density functional theory (DFT) calculations in order to assign the measured resonances to distinct molecular motions. DFT calculations obtained for a single molecule (glutathione), dimers (gly-gly-gly) and ion pairs (enalapril maleate) coupled via H-bonds, reproduce correctly the number of resonances observed in the experiment.

Terahertz time-domain spectroscopy (THz-TDS) was used to measure the far-infrared absorption spectra of some important biological materials such as Bacillus subtilis spores and dipicolinic acid in the frequency region of 0.2 to 2.0 THz (6.7-67 cm-¹). A distinct absorption peak at 1.538 THz was observed in the B. subtilis spores. A dominate absorption mode at 1.54 THz was also observed in DPA powder.

We report on experimental and numerical studies of free space terahertz (THz) propagation in strongly scattering random dielectric media. The on-axis ballistic and small angle scattered transmission is measured through media of varying thickness. The experimental variations of the terahertz pulse group delay and scattering-induced effects such as temporal pulse distortion, spectral decay, and power attenuation as a function of sample thickness are well described by a Monte Carlo photon migration model. The transmitted pulses are analysed using the classical Bruggemann effective medium approximation (EMA). It is found that the effective medium approximation underestimates the accumulated pulse phase acquired by the high frequencies during pulse propagation. An empirically modified EMA provides accurate description of the random dielectric medium.

We report the use of a terahertz pulsed imaging technique for three-dimensional chemical mapping. Terahertz radiation reflected from a sample was measured pixel-by-pixel in time domain using a terahertz pulsed imaging system developed at TeraView Ltd, UK. The recorded terahertz waveforms were then transformed into frequency domain using time-partitioned Fourier transform. Structural maps of samples were obtained by analyzing the terahertz time-domain data whilst chemical maps were obtained from terahertz spectral data sets. For a sample comprising chemical A at the surface of a polyethylene pellet and chemical B buried inside the pellet, we have separated the component spatial patterns of the two chemicals using their spectral fingerprints. The reconstructed three-dimensional chemical maps not only locate the chemicals in the object, but also identify each chemical. We also demonstrate the capabilities of terahertz pulsed imaging for non-destructive analysis of coating thickness and quality, and for detecting and identifying explosive materials such as RDX.

We have reviewed our most recent results. Let us consider a second-order nonlinear medium in which perfect phase-matching can be achieved at one particular output wavelength for the process of the THz generation from a train of ultrafast laser pulses based on difference-frequency generation. We assume that the coherence lengths for the THz generation are sufficiently long within a wide bandwidth around this perfect phase-matching wavelength due to a slight dispersion in the THz region (i.e. broadband phase-matching). In this theoretical work, we show that quasi-single-cycle THz pulses can be efficiently generated. An efficient conversion for the THz generation is made possible not only by utilizing the broadband phase-matching but also by optimizing the pulse width for each peak THz frequency. We have investigated the regime of the strong pump depletion and found the limits to the conversion efficiencies. We have ruled out the significant contributions due to the effects of multiphoton absorption, free-carrier absorption, and nonlinear refractive indices.

A new geometry for the intersubband THz laser on delta-doped multi-layer Ge thin films with in-plane transport of carriers in crossed electric and magnetic fields is proposed. A remarkable increase of the gain compared to existing bulk p-Ge lasers is based on spatial separation of light and heavy hole streams, which helps to eliminate scattering of light holes on ionized impurities and the majority of heavy holes. Inversion population and the gain have been studied using Monte-Carlo simulation. The terahertz transparency of a CVD-grown delta-doped Ge test structure has been experimentally studied by intracavity laser absorption spectroscopy using a bulk p-Ge laser. A practical goal of this study is development of a widely tunable (2-4 THz) laser based on intersubband hole transitions in thin germanium films with the gain sufficient to operate at liquid nitrogen temperatures.

We suggest a new structure for THz generation based on coupled quantum wells of InAs-GaSb. This structure uniquely combines the advantages of both the p-n junction laser and the cascade laser. Actual generation results from optical transitions between the e3 and e2 levels in InAlAs quantum well, and resonant with them effective band gap between the conduction band of InAs and the valence band of GaSb quantum well, e1-hh1. The separation between e1 and e2 equals to LO phonon energy that provides population inversion between e3 and e2. We consider two ways of structure design that differ by the carrier dispersion: W-shaped dispersion of the carriers in ground states and regular V-shaped dispersion. All these structures bring in the advantages of the system with equidistant levels, i.e., good temperature characteristics and high probability of radiative transition leading to low threshold current compared to alternative designs. We present a comparative analysis of various mechanisms of carrier relaxation (LO phonons and electron-electron scattering) and point out an optimal band structure favoring high efficiency of THz emitter. Corresponding band structure calculations supply one with the range of quantum well parameters providing all the features presented above.

In this work we show that improved performances of small-aperture terahertz antennas can be obtained using an ion implantation process. Our photoconductive materials consist of high resistivity GaAs substrates. Terahertz pulses are generated by exciting our devices with ultrashort laser pulses. Ion implantation introduces nonradiative centers which reduce the carrier lifetime in GaAs and modify the shape of our terahertz pulses. The introduction of charge defects also induces a redistribution of the electric field between the antenna electrodes. The overall process is optimized to better control the dynamical field screening effect which has a huge influence on the amplitude of the radiated terahertz field. Results obtained as a function of the laser excitation power is discussed and comparison of the performance of these devices with conventional small-aperture antennas is given.

In this work we have investigated the operation of a recently demonstrated bound-to-continuum quantum cascade laser emitting at 2.9 THz under different active region doping densities. In addition, we have studied the injection efficiency as a function of the thickness of the Al0.15Ga0.85As barrier controlling the tunnel coupling between the superlattice miniband and the upper state of the laser transition. By tuning these two parameters threshold current densities as low as 52 A/cm2 and 83 A/cm2 were obtained at 5 K, corresponding to a reduction over the base design of 55% and 25% respectively. In both cases we attribute the improved threshold performance to a reduction of the parasitic leakage current. The decrease in threshold was always at the expense of a smaller laser dynamic operating range, which ultimately limited maximum operating temperatures in pulsed and continuous wave mode to 95 K and 80 K respectively.

Reflecting the flourishing THz science and technology in the past decade, a number of groups in Taiwan have initiated research activities in this area. All major research universities as well as the semi-public Industrial Technology Research Institute are represented. Current research topics include THz devices (emitters, detectors, phase shifters) and the applications of THz time-domain spectroscopy to the study the far-infrared optical properties of various materials. Highlights of recent research activities are outlined.

External cavity lasers (ECL) based on semiconductor diode gain elements and Fiber Bragg Gratings (FBG) have been developed for Telecom (OC-48) nd Analog (CATV, QAM) applications. They possess very narrow linewidth (100 kHz) and exceptional wavelength stability. These qualities makes them attractive platform for implementation of heterodyne sources and Optical Phase Locked Loops (OPLL) for Microwave Photonics applications. We discuss two types of such heterodyne sources: heterodyne oscillator based on heterodyning of two ECL, and fixed frequency heterodyne oscillators based on ECL with FBG written in the polarization maintaining fiber. All two types of heterodyne sources were built based on industry standard 14-pin butterfly package. All of them exhibited excellent wavelength stability (less than 1 pm/mA and 1-2 pm/°C). Fixed frequency sources provided beat oscillation around 40 GHz. We present performance characteristics and measurement data on (linewidth, phase noise, heterodyne mixing, etc.) and discuss the merits of ECL use as heterodyne sources for Microwave Photonics applications.

Superconducting Hot Electron Bolometer (HEB) mixers are a competitive alternative to Schottky diode mixers or other conventional superconducting receiver technologies in the terahertz frequency range because of their ultrawide bandwidth (from millimeter waves to the visible), high conversion gain, and low intrinsic noise level, even at 77 K. A new technological process has been developed to realize HEB mixers based on high temperature superconducting materials, using 15 to 40 nm thick layers of YBa₂Cu₃O_7-δ (YBCO), sputtered on MgO (100) substrates by hollow cathode magnetron sputtering. Critical temperature values of YBCO films were found in the 85 to 91 K range. Sub-micron HEB bridges (0.8 μm x 0.8 μm) were obtained by combining electronic and UV lithography followed by selective etching techniques. Realization of YBCO HEB coupling to planar integrated gold antennas was also considered.

We present the results of our studies of NbN phonon-cooled HEB mixers at terahertz frequencies. The mixers were fabricated from NbN film deposited on a high-resistivity Si substrate with an MgO buffer layer. The mixer element was integrated with a log-periodic spiral antenna. The noise temperature measurements were performed at 2.5 THz and at 3.8 THz local oscillator frequencies for the 3 x 0.2 μm² active area devices. The best uncorrected receiver noise temperatures found for these frequencies are 1300 K and 3100 K, respectively. A water vapour discharge laser was used as the LO source. The largest gain bandwidth of 5.2 GHz was achieved for a mixer based on 2 nm thick NbN film deposited on MgO layer over Si substrate. The gain bandwidth of the mixer based on 3.5 nm NbN film deposited on Si with MgO is 4.2 GHz and the noise bandwidth for the same device amounts to 5 GHz. We also present the results of our research into decrease of the direct detection contribution to the measured Y-factor and a possible error of noise temperature calculation. The use of a square nickel cell mesh as an IR-filter enabled us to avoid the effect of direct detection and measure apparent value of the noise temperature which was 16% less than that obtained using conventional black polyethylene IR-filter.

We have assembled an imaging system using quantum cascade lasers at frequencies of both 3.4 and 2.3 THz. Images at the two frequencies and the resulting absorption coefficients are compared. We demonstrate imaging in both reflection and transmission. The lasers are operated in a closed-cycle refrigerator and we use a peak output power of >2.5 mW at 7-10% duty cycle and 22-40 K operating temperature. The focal spot size is approximately 300 microns for both lasers and is not diffraction-limited. The signal is detected with a single-element DTGS detector, and images are captured by scanning the sample. Applications enabled by longer wavelengths are demonstrated, as well as the determination of chemical information through imaging at two wavelengths.

Polymers are often mixed with other additives or fillers to yield compounds with modified physical properties. In most cases a homogeneous mixture is desired. Yet, it often remains difficult to verify the degree of homogeneity of the resulting compound especially for nano-scaled fillers. We present initial experiments to evaluate the potential of terahertz (THz) spectroscopy for the quality control of polymeric compounds. We study low-density polyethylene (LDPE) samples which contain titanium dioxide nanospheres with a typical diameter of 270 nm which themselves are coated with even smaller silver nanoparticles with a typical diameter of 20 nm. Images obtained with a standard terahertz time-domain spectrometer show significant inhomogeneities in the compound on a millimeter scale. The imaging results indicate imperfectly mixed material regions. On the other hand, we show that also fluctuations in the sample thickness can lead to inhomogeneous terahertz images. A final conclusion, if the inhomogeneities observed in our LDPE/Ag-TiO2 samples result from variations in the compound composition or from thickness fluctuations, cannot be drawn at this point.

The application of near-field interferometric imaging to the Terahertz frequency range for detection of concealed objects is discussed. A circular array architecture can be employed to compensate for near-field distortions and increase the field of view and depth of focus. The lateral and focusing errors of this imaging method are discussed as well as the trade-offs of interferometric imaging compared to a focal plane array architecture.

A simple, compact CW sub-THz imaging system, utilizing a 0.2 and 0.6 THz Gunn diode source is presented. A silicon beam lead diode detector and a Golay cell are used for the detection. Various results are presented, which show that the CW THz imaging modality is suitable for diverse applications, such as non-destructive testing and security. The key components of the system include the Gunn diode assembly, an optical chopper, a polyethylene lens, a detector, a lock-in amplifier, and two translation stages. The beam from the Gunn diode is focused on the sample being imaged by the polyethylene lens, the transmitted or reflected beam is measured by the detector. The energy transmitted through the sample at each point in the plane of the sample is detected. Since the system has relatively few components compared to pulsed THz imaging systems, it is less expensive and easier to design and operate, although it does not provide depth or spectral information about the sample. Since no time-delay scans take place, scanning can be done quickly compared to a time-domain system, limited by the maximum velocity of the translation stages and response of the detectors. It provides information about the macroscopic features of hidden structures within materials that are transparent to sub THz radiation, such as space shuttle insulating foam, articles of clothing, and luggage.

Terahertz (THz) radiation has important applications in spectroscopy, imaging, and space science. For some of these applications, in particular spectroscopy, a flexible fiber optic could potentially simplify the THz system and enable the user to transmit radiation to remote locations without excessive absorption by atmospheric moisture. To date THz fiber optics and waveguides have been limited to rigid hollow metallic waveguides, solid wires, or short lengths of solid-core transparent dielectrics such as sapphire and plastic. In this paper we report on flexible, hollow polycarbonate waveguides with interior Cu coatings for broadband THz transmission fabricated using simple liquid-phase chemistry techniques. The losses for these hollow-core guides were measured using a tunable, cw single-mode far IR laser. The losses for the best guides were found to be less than four dB/m and the single mode of the laser was preserved for the smaller bore waveguides. Loss calculations of the loss for the HE11 mode reveal that the metal-coated hollow waveguides have much higher loss than for waveguides coated with both metallic and dielectric thin films.

This paper describes a new method for carrying increased information in an optical network via imparting that information onto a THz frequency carrier and then "up-shifting" the resulting THz signal into a frequency band where: long-range transmission is facilitated, and there are a wide array of existing components which can be applied. The architecture described here takes the approach of carrier suppression as a method of improving extinction ratio. These authors believe that this general architecture has not been applied in the optical communications field in the past. The focus of this paper will be to describe the proposed approach and to provide estimates of the required performance of each major element of the system. For each element the required performance will be compared with published results to show that the majority of the basic technology to prove-out this approach exists today. While the description which follows is focused on a long-range fiber transmission system, the same approach could also be applied to ultra-high-data-rate local networking as well.

Short range wireless communication systems are expanding at rapid rate, finding application in offices, congested urban areas and homes. Development of wireless local area networks is accompanied by a steady increase in the demand for ever higher data rates. This in turn entails the necessity to develop communication systems which operate at higher frequencies. Currently WLAN works at a few GHz, while systems operating at several ten GHz appear already feasible. It can be expected that wireless short-range communication networks will soon push towards the THz frequency range and that systems which handle high-density information and support wider bandwidth communications will be developed in a few years time. Since THz radiation is strongly absorbed by the atmosphere, working distances may be short and individual THz pico-cells may cover only single rooms or at most one building. For an indoor system of practical importance it must be robust against shadowing. Recently, flexible all-plastic mirrors, supporting specular reflections in the THz range have been demonstrated. They are cheap and easy to produce and can be used as frequency selective wall-paper to enhance the reflectivity of walls and hence facilitate non-line-of-sight communication in a THz cell. For this case the spatial and temporal characteristics of the indoor THz propagation channel in a room with randomly placed objects and moving people are derived with ray-tracing methodology and Monte Carlo simulations. Our simulations show that high-gain antennas will be needed for the realization of THz communication in indoor environments. Furthermore, indirect transmission paths between transmitter and receiver, supported by dielectric mirrors make the communication channel much more robust against shadowing.

We have observed for the first time the dynamics of a speckle field generated in bulk scattering partly ordered media or low-mode optical fibers probed by a laser diode ( λ = 654 nm) and a compact diode-pumped YAG:Nd laser ( λ = 532 nm) in the regime of frequency tuning. The dynamic coherent speckle-photo-chromic effect can be observed provided that the deviation of the probing radiation frequency is comparable with or greater than the effective phase delay difference between modes in the optical fiber of between waves in the random scattering medium. Using the proposed photochromic speckle technique, it is possible to determine the intermode dispersion of fibers with a length on the order of one meter and the dispersion of phase delay in test fluoroplastic structures in the regimes of several fold scattering (for a sample thickness of 10-20 μm) or multiple scattering (for a thickness of up to ~ 2 cm).