Terahertz heterodyne imaging is an established technique that offers the potential for extremely large dynamic range and high signal-to-noise ratio while maintaining fast data acquisition, stable magnitude and phase measurements, reasonable frequency flexibility and mm level penetration in tissue and other materials. The authors have set up an imaging system based around a custom fabricated 2.5 THz planar Schottky diode mixer pair and two optically pumped far IR lasers. One laser is used for the signal beam and supplies as much as 70mW at 2.5 THz. The other laser acts as a local oscillator (LO) source for the two mixers. Line pairs very close to each other (CH3OH and CH2F2) are chosen to provide a workable intermediate frequency output (IF=24 GHz). Broader RF bandwidth is possible with tunable signal sources and wider IF band amplifiers. A novel frequency stabilization scheme has been implemented to track and calibrate the laser power (magnitude and phase) over a sample run. The system uses the second THz mixer, a low frequency (GHz) reference oscillator and a lock-in amplifier to monitor and normalize the two lasers (LO and Signal). Stability of ~0.1 dB and <5 degrees have been achieved with a dynamic range of more than 90dB. The present system scans the sample through the focused beam and measures transmission or reflection at a fixed RF frequency. Applications to date include establishing contrast mechanisms in a range of test and biological materials. The measurement system is described and the merits and demerits discussed. Early results on a variety of samples are presented as well as plans to enhance the performance in the near future.

One dimensional photonic bandgap structures can be used to enhance the efficiency of nonlinear optical parametric processes. Structural dispersion can be used to achieve phase matching, and resonant effects can be used to increase the intensity of the pump beams in the nonlinear optical material. In this paper these ideas are used to derive for the a continuous wave or a quasicontinuous wave THz signal and show that an improvement of three orders of magnitude in the output intensity can be achieved for a structure involving as few as four layers of ZnTe.

We describe a tunable, narrow-band, coherent THz radiation source based on parametric down-conversion in a photonic crystal. Our design is based on down-conversion mixing and a local-field enhancement mechanism that is available by tuning each of the two driving laser fields either to band-edge or to a defect mode in the band gap. The frequency of the down-converted signal can be tuned by intersecting two non co-linear laser sources. The polarizations are degenerate at normal incidence and have sub-THz down-conversion maximum. The peak conversion efficiency for both polarizations is enhanced by over two orders of magnitude.

Quantum theory shows that tunneling electrons have a resonant interaction with a radiation field, and because of this resonance a highly-focused amplitude-modulated laser diode (670 nm, 30 mW) changes field emission current enough to be seen with an oscilloscope. The emitting tip is much smaller than the optical wavelength, so the potential of the tip follows each cycle of the radiation field. Electron emission responds to the total electric field (DC + radiation) with a delay τ < 2 fs, and the current-voltage characteristics of field emission are highly nonlinear. Thus, quantum simulations show that two lasers can cause current oscillations by photomixing, which can be tuned from DC to 500 THz (1/τ) by shifting the frequency offset of the lasers. Microwave prototypes for 1-10 GHz are being tested. The output power is proportional to the resistive part of the impedance that is seen by the current oscillations. However, the electric field at the mixer frequency, which is proportional to this impedance, causes negative feedback to reduce the current oscillations, so there is an optimum impedance for maximum output power. Analyses with equivalent circuits are used to optimize the design. Simulations suggest that 1 μW may be obtained in CW operation, or 10 mW in pulsed operation, using 10 mW laser diodes as the pump sources.

Pulsed THz (100 GHz - 30 THz) Imaging Spectroscopy combines three ways of mine detection in one system, high resolution radar, depth ranging, and infrared spectroscopy. It allows minefield detection, single mine imaging, and near-zero false alarm due to the capabilities of explosives / plastic identification using spectroscopy with working distances to 1000 feet. We have previously demonstrated imaging capabilities with 1 mm spatial resolution on a rubber O-ring embedded in sand. The estimated transmission depth in moist sand is 1 to 3 cm, which should be sufficient for imaging anti-personnel mines. In this work, we present initial results investigating the feasibility of THz spectroscopy in the frequency range from 1 to 10 THz to detect and identify explosives and related compounds (ERCs). A major component of this effort is chemical modeling to obtain spectroscopic information on ERCs and environmental background. A time-domain THz system using femtosecond laser pulses is also being developed.

The THz spectra of the high explosives, HMX, RDX, PETN, and TNT were measured using the technique of Time Domain THz (TD-THz) spectroscopy, and resonances attributed to phonon bands were observed. The TD-THz methods used to obtain these spectra are described and strategies for improved data collection methods are outlined. Concepts for through container DIfferential Absorption Lidar (DIAL) are outlined and the suitability of TD-THz methods for DIAL sensing is discussed.

We have developed a novel basic technology for terahertz (THz) imaging, which allows detection and identification of chemicals by introducing the component spatial pattern analysis. The spatial distributions of the chemicals were obtained from terahertz multispectral transillumination images, using absorption spectra previously measured with a widely tunable THz-wave parametric oscillator. Further we have applied this technique to the detection and identification of illicit drugs concealed in envelopes. The samples we used were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.

Pulsed THz (T-ray) spectroscopy is sensitive, non-invasive tool for studying materials from physics to biology, but transmission measurements of liquid samples, especially water, have been limited by noise. This paper shows that the accuracy of T-ray material parameter measurements of liquid samples can be greatly increased, especially for highly-absorbing liquids, by using a rapid modulation of the liquid in the T-ray beam path, coupled with a novel implementation of mean and amplitude detection to T-ray spectroscopy. The experiments are supported by calculations quantifying the sources of uncertainty. Liquid transmission T-ray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension.

High-sensitivity terahertz direct detectors using superconducting tunnel junctions were fabricated. They were designed for detecting terahertz radiation in the frequency range of 0.4 and 0.65 THz with the fractional bandwidth of above 10 percent. The results of their performance evaluation of five detector elements are presented. We show the results of the frequency response as well as that the absolute efficiency ranged from 10 to 30 percent and that the the sensitivity was 1.9 x 10^-16 W Hz^-0.5 in noise equivalent power.

A new CW photoconductive integrated photomixer/antenna THz source is presented. A THz signal is generated in the DC-biased photoconductive strip by employing optical heterodyne photomixing, and at the same time the size of the photoconductive strip on the grounded dielectric substrate is designed to have an efficient broadside radiation. Analytical expressions for the photo-induced current as well as the radiation power are calculated in detail, which make it possible to evaluate the performance of the structure made by different photoconductive materials. The typical μW output power can be obtained by mW laser pump power for frequencies up to 1 THz.

We have proposed and demonstrated a nondestructive and non-contact inspection method for electrical faults using laser-Terahertz (THz) emission microscopy (LTEM). By measuring the position dependence of the amplitude of the THz emission from integrated circuits (IC) excited with femtosecond (fs) laser pulses, it is possible to investigate the electrical faults in IC. By improving the spatial resolution of the system, we successfully observed the THz emission image of a microprocessor on standby mode. The LTEM system has a spatial resolution about 3µm and it can localize electrically defective sites in the chip to within a ten square microns.

Magnetic flux letters are visualized by a laser terahertz emission microscope (LTEM). LTEM excites terahertz (THz) radiation from electronics materials with a femtosecond laser and maps the amplitude of the THz waves while scanning laser beam on the sample. Magnetic flux letters written in high-Tc superconductors are successfully imaged without destroying them.

A new type of optical-microwave conversion system has been proposed as high-stabilized oscillators and optical interfaces for superconducing circuits. The photomixer with heterodyne mixing of two lasers was applied to generate the electromagnetic waves and high-T_C grain boundary Josephson junction detector was used to monitor the frequency of electromagnetic waves in real time. The current-voltage properties of Josephson junction detector were examined and the system operation up to 50 GHz was demonstrated. We also described in detail the two types of optical-microwave conversion system.

We demonstrate the operation of a superlattice GaAs/AlGaAs quantum cascade laser emitting at λ = 103 μm. The maximum operating temperature is 95K in pulsed mode and 70K in continuous wave. At 4K, we measured a peak output power in the tens of mW range and a threshold current density of 110 A/cm² (300 A/cm² at 90K). We attribute this excellent performance to a low ratio between the lower and upper state lifetimes, as well as to a low leakage current. These characteristics are highlighted by a pronounced decrease of the differential resistance at threshold and by the fact that the slope efficiency remains constant up to approximately 70K. At any temperature, we observe an abrupt decrease of the output power at the breaking of miniband alignment, corresponding to a strong negative differential resistance feature in the current/voltage characteristics. Ultimately, this effect limits the operation of the device at high temperatures. By comparing this laser with a previous design, we will outline the importance of (i) having a diagonal rather than vertical laser transition in real space, and (ii) avoiding possible intersubband re-absorption of the emitted radiation.

We demonstrate an all-optoelectronic continuous-wave terahertz (cw-THz) imaging system using technology based on low-cost and compact diode lasers. THz radiation is generated by photomixing two near-infrared lasers (830 nm) in a photoconductive emitter and is tunable from 0.1-2 THz. Images are captured in reflection geometry, and a phase-sensitive photoconductive detection scheme is used, which operates at room temperature. We have optimised the growth and annealing of low-temperature gallium arsenide (LT-GaAs), and achieved state-of-the-art material with 100 fs carrier trapping lifetimes. The photomixers load resonant antennas, which efficiently couple out monochromatic THz radiation. Images are captured in the time-domain using a real-time rapid scan delay line capable of data acquisition at 15 Hz, with both amplitude and phase information available. There are a number of advantages in using continuous-wave imaging systems, compared to the more established pulsed technologies. In particular, the combination of diode lasers and photoconductive detection demonstrates, for the first time, an imaging system that is compact, robust, genuinely turn-key and of low cost. Such a system would be well suited for routine THz imaging in both medical and non-medical applications.

We observe ultrafast polarization dynamics in strongly internally biased InGaN/GaN multiple quantum wells during intense femtosecond optical excitation by means of time-resolved detection of THz emission, correlated with time-integrated photoluminescence measurements. We demonstrate that in the case of strong enough excitation the built-in bias field (on the order of MV/cm) can be completely screened by the carriers excited into spatially separated states. This ultrafast screening of the initial bias field across the quantum well leads to dynamical modification of the band structure of the sample, and consequently to dynamical modification of the optical absorption coefficient within the duration of the excitation pulse. We show that such an optically induced dynamical screening of the biased quantum well can be described in terms of discharging of a nano-scale capacitor with a femtosecond laser pulse. The electrostatic energy stored in the capacitor is released via THz emission. A realistic quantum-mechanical model of the temporal evolution of the polarization inside the quantum wells shows that due to its nonlinearity such a process may lead to emission of a THz pulse with bandwidth significantly exceeding that of the excitation pulse.

A new calorimetric absolute power meter has been developed for THz radiation. This broad band THz power meter measures average power at ambient temperature and pressure, does not use a window, and is insensitive to polarization and time structure of THz radiation. The operation of the power meter is based on the calorimetric method: in order to determine the power of a beam of THz radiation, the beam is used to illuminate a highly absorbing surface with known BRDF characteristics until a stable temperature is reached. The power in the incident beam can then be determined by measuring the electric power needed to cause the sample temperature rise. The new power meter was used with laser calorimetry to measure the absorptivity, and thus the emissivity, of aluminum-coated silicon carbide mirror samples produced during the coating qualification run of the Herschel Space Observatory telescope to be launched by the European Space Agency in 2007. The samples were measured at 77 Kelvin to simulate the operating temperature of the telescope in its planned orbit around the second Lagrangian point, L₂, of the Earth-Sun system. The absorptivity of both clean and dust-contaminated samples was measured at 70, 118, 184 and 496 mm and found to be in the range 0.2 - 0.8%.

Recent events have led to dramatic changes to the methods employed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new Recent events have led to dramatic changes to the methods employed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new technologies that can be applied to security screening, either to simplify or speed up the checking process, or to provide additional functionality. Terahertz (THz) technology is a promising, emerging candidate. In previous publications we have shown how our THz pulsed imaging systems can be used to image threat items, and have demonstrated that explosive materials have characteristic THz spectra. We have also demonstrated that nonmetallic weaponry can be imaged when concealed beneath clothing. In this work we examine more closely the properties of barrier and potential confusion materials. We demonstrate that barrier materials have smooth spectra with relatively low attenuation. We further demonstrate that the terahertz spectra of several common chemicals and medicines are distinct from those of threat materials.