In the last ten years terahertz techniques have become increasingly common laboratory and industrial tools. This progress has been made possible by over thirty years of concentrated effort. In this talk we discuss our recent work combining time-domain terahertz imaging with advanced signal-processing to obtain unprecedented depth information in a nondestructive fashion about subsurface damage in both glass and carbon fiber composites and in coatings on metals. In addition, we present an example characterizing the stratigraphy of an art painting to illustrate the technique to measure thicknesses in a multilayer coating. Other optically opaque materials, including polymers, glass, textiles, paper, and ceramics, are transparent to terahertz radiation, and thus terahertz imaging may access information in these materials below the surface.
Signal processing techniques are needed to unleash the power of terahertz imaging to measure thin layers of thickness on the order of 10 microns. These approaches permit us to gain information about thin layers that are obscured in the raw signals. That is, when the time duration of the terahertz pulses is longer than the optical delay to traverse a given layer, the terahertz echoes associated with reflections off the various interfaces may temporally overlap. Specifically, we have successfully employed frequency-wavelet domain deconvolution, sparse deconvolution, and autoregressive deconvolution for a range of problems.