  Hetero-epitaxial crystalline films underlie many electronic and optical
technologies but are prone to forming defects at their hetero-interfaces.
Atomic-scale defects such as threading dislocations that propagate into a film
impede the flow of charge carriers and light degrading electrical-optical
performance of devices. Diagnosis of subsurface defects traditionally requires
time consuming invasive techniques such as cross sectional transmission
electron microscopy. Using III-V films grown on Si, we have demonstrated
noninvasive, bench-top diagnosis of sub-surface defects by optical
second-harmonic scanning probe microscope. We observed a high-contrast pattern
of sub-wavelength hot spots caused by scattering and localization of
fundamental light by defect scattering sites. Size of these observed hotspots
are strongly correlated to the density of dislocation defects. Our results not
only demonstrate a global and versatile method for diagnosing sub-surface
scattering sites but uniquely elucidate optical properties of disordered media.
An extension to third harmonics would enable irregularities detection in
non-X(2) materials making the technique universally applicable.
