  For measurements designed to accurately determine layer thickness, there is a
natural trade-off between sensitivity to optical thickness and lateral
resolution due to the angular ray distribution required for a focused beam. We
demonstrate a near-field imaging approach that enables both sub-wavelength
lateral resolution and optical thickness sensitivity. We illuminate a sample in
a total internal reflection geometry, with a photo-activated spatial modulator
in the near-field, which allows optical thickness images to be computationally
reconstructed in a few seconds. We demonstrate our approach at 140 GHz
(wavelength 2.15 mm), where images are normally severely limited in spatial
resolution, and demonstrate mapping of optical thickness variation in
inhomogeneous biological tissues.
