A method for reducing the reflections from silicon optics at terahertz frequencies has been investigated. In this study, we used thin films of parylene as an anti-reflection (AR) layer for silicon optics and show low-loss behavior well above 1 THz. Transmittance spectra are acquired on double-sided-parylene-coated, high-resistivity, single-crystal silicon etalons between 0.45 THz and 2.8 THz. Modeling the optical behavior of the three-layer system allowed for the determination of the refractive index and absorption coefficient of parylene at these frequencies. Our data indicate a refractive index, , of 1.62 for parylene C and parylene D, and a reasonably modest absorption coefficient make these materials a suitable AR coating for silicon at terahertz frequencies. Coatings sufficiently thick for AR performance reduced the average transmittance of the three-layer system by 10% compared to a lossless AR coating with an ideal refractive index.

A coherent transceiver using a THz quantum cascade (TQCL) laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented.

D ~ centers have been unambiguously identified in GaAs/(Ga,Al)As quantum wells by analysis of the dependence of the observed photoconductivity spectrum on the applied magnetic field and sample orientation. Theoretical investigations show that in magnetic fields of interest the D ~ transitions do not involve photoionization of the centers as has been previously supposed but proceed from the ground state to discrete p-\-or p + i-like D~ levels, which lie above the N-0 and 7V = 1 Landau-level energies, respectively. Tilting the sample leads to a predicted anticrossing of discrete po~ and p-H-like D~ levels. Excellent agreement with experiment is obtained without any adjustable parameters.

A high precision reflectometer has been designed and implemented to measure directly the specular reflectance (R) of materials in the submillimeter (SM) region of the spectrum (300 GHz < ν < 3000 GHz). Previous laser-based measurement systems were limited to an uncertainty in R of approximately ± 1.0% due to a number of issues such as: lack of an absolute reflection standard, difficulties in the interchange of sample and standard in the laser beam, and instabilities in the laser system. A SM reflection standard was realized by ellipsometrically characterizing the complex index of refraction of high purity, single-crystal silicon to a precision such that its SM reflectivity could be calculated to better than ± 0.03%. To deal with alignment issues, a precision sample holder was designed and built to accommodate both sample and silicon reflection standard on an air-bearing rotary stage. The entire measurement system operated under computer control and included ratioing of the reflected signal to a reference laser signal, measured simultaneously, to help eliminate short-term laser instabilities. Many such measurements taken rapidly in succession helped eliminate the effects of both source and detector drift. A liquid helium-cooled bolometer was modified with a large area detecting element to help compensate for the slight residual misalignment between sample and reflection standard as they were positioned into and out of the laser beam. These modifications enabled the final measurement precision for R to be reduced to less than 0.1%. The major contribution to this uncertainty was the difficulty in precisely exchanging the positions of sample and standard into and out of the laser beam and not due to laser or detector noise or instabilities. In other words, further averaging would not help to reduce this uncertainty. This order of magnitude improvement makes possible, for the first time, high precision reflectance measurements of common metals such as copper, gold, aluminum and chromium whose predicted reflectivities exceed 99% in the SM. Furthermore, precise measurement of the high frequency losses in high temperature superconducting materials is now also possible. Measurements reported here of metals at a laser wavelength of λ = 513.01 µm (ν ≈ 584 GHz) indicate a slight discrepancy between experimental and theoretically predicted values, with measured results falling between 0.1%-0.3% below predicted values.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH(2)DOH THz CO(2) optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t approximately sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers.