Reading Through Closed Books: THz Time-Gated Spectral Imaging for Content Extraction Through Layered Structures

We exploit the sub-picosecond time resolution along with spectral resolution provided by terahertz time-domain spectroscopy to extract occluding content from layers whose thicknesses are wavelength comparable. The method uses the statistics of the THz E-field at subwavelength gaps to lock into each layer position and then uses a time-gated spectral kurtosis to tune to highest spectral contrast of the content on that specific layer. To demonstrate, occluding textual content was successfully extracted from a sample similar to a closed book down to nine pages without human supervision. The method provides over an order of magnitude enhancement in the signal contrast and can impact inspection of structural defects in wooden objects, plastic components, composites, drugs, and especially cultural artifacts with subwavelength or wavelength comparable layers.