Depth resolved nonlinear optical nanoscopy

An electromagnetic field forced to vary along a plane with a spatial scale d much smaller than its free space wavelength lambda decays exponentially along its normal with a decay length similar tod. This decay, similar to that of the wavefunction of tunneling electrons, has allowed the development of scanning near-field optical microscopes (SNOMs), reminiscent of scanning tunneling and atomic force microscopes, which have been able to resolve structures in the nanometer scale. However, existing SNOMs are unable to determine the depth below the surface from which the optical signals arise due to the monotonic decay of the optical evanescent probe fields. In this paper we study the optical second harmonic generation (SHG) produced by mixing of the evanescent fields produced by a SNOM tip. We show that employing an appropriately spatially-patterned tip, a non-monotonic non-linear probing field may be produced which has a maximum at a given distance beyond the tip, yielding a novel microscopy which may attain depth resolution with nanometric lengthscales. We estimate the size of the optical signal and we compare it with that arising in the usual SHG-based surface spectroscopy of centrosymmetric materials. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim