Abstract: A non-destructive, non-intrusive characterization of angstrom-level roughness characteristics of subsurface interfaces is performed by applying femtosecond light pulses from a laser onto a surface, and analyzing the contents of the reflected pulses. After impinging on the surface being analyzed, the pulses pass through optical filters, which attenuate the fundamental and third harmonic frequencies of the pulses, but keep a substantial portion of the second harmonic. Analysis of the second harmonic signals provides rapid, non-contact, interface-specific characterization of the angstrom-level interfacial microroughness of the subsurface. For semiconductor devices, the second harmonic signals can be used to detect strain, contamination, and trapped charges in the Si/Si(O.sub.2) interface.