The detection of anti-personnel mines depends on the soil transmission and scattering at a given wavelength. Transmission spectra were measured over the range 90-4200 GHz for 19 soil samples that span a number of soil orders that have extensive worldwide distribution using a vector network analyzer (90-140 GHz) and a Fourier spectrometer (120-4200 GHz). Transmission drops to nearly zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. This interpretation is supported by a fit to a standard scattering model with physically reasonable fitting parameters. Transmission spectra were also measured for various liquids (90-600 GHz) for possible index matching. These liquids were mixed with the soil sample and were found to reduce scattering and increase transmission through the soil at higher frequencies. This work is relevant to mine detection using terahertz and millimeter wave radiation for high resolution images through the soil.

Applications using millimeter wave (mmW) and THz radiation have increased during the past few years. One of the principal applications of these technologies is the detection and identification of objects buried beneath the soil, in particular land mines and unexploded ordnances. A novel active mmW scanning imaging system was developed for this purpose. It is a hyperspectral system that collects images at different mmW frequencies from 90 to 140 GHz using a vector network analyzer collecting backscattering mmW radiation from the buried sample. A multivariate statistical method, principal components analysis, is applied to extract useful information from these images. This method is applied to images of different objects and experimental conditions.

A robust metal-free intracavity fixed-wavelength selector for the cryogenically cooled far-infrared p-Ge laser is demonstrated. The device is a back mirror consisting of a thin silicon etalon and dielectric SrTiO 3 flat. A laser line width of 0.2 cm Ϫ1 is achieved, which corresponds to an active cavity finesse of ϳ0.15. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. Use of SrTiO 3 lowers the laser resonance line frequencies by ϳ1 cm Ϫ1 compared with expectations for metal mirrors. The effect is due to phase shift, which is determined from far-infrared reflectivity measurements of SrTiO 3. A p-Ge laser with such selector is free from danger of electrical breakdown and mirror oxidation during repeatable thermal cycling, which makes it more reliable than previous selection schemes for practical applications.