We report measurements of third-harmonic generation from ultrathin crystall
ine silicon layers of gradually varying thickness. Both the angular and thi
ckness dependence of the third-harmonic light generated in transmission at
normal incidence are consistent with negligible surface contribution to thi
rd-harmonic generation in silicon, even under tight focusing. This work ill
ustrates a method for distinguishing surface and bulk contributions to harm
onic generation. (C) 2001 American Institute of Physics.