  We utilize the recently demonstrated orders of magnitude enhancement of
extremely nondegenerate two-photon absorption in direct-gap semiconductor
photodiodes to perform scanned imaging of 3D structures using IR femtosecond
illumination pulses (1.6 um and 4.93 um) gated on the GaN detector by sub-gap,
femtosecond pulses. While transverse resolution is limited by the usual imaging
criteria, the longitudinal or depth resolution can be less than a wavelength,
dependent on the pulsewidths in this nonlinear interaction within the detector
element. The imaging system can accommodate a wide range of wavelengths in the
mid-IR and near-IR without the need to modify the detection and imaging
systems.
