Apertureless near-field optical microscope based on second-harmonic generation

The paper reports on the achievement of material optical contrast on the nanoscale by means of apertureless near-field optical microscopy ANSOM with second-harmonic generation (SH-ANSOM). A grazing incidence illumination configurations adopted, with excitation of the tip dipole along its axis, and backscattering collection arrangement along the maximum emission direction. This optimized geometry resulted in a 2/3 orders of magnitude increase of the efficiency (up to 10⁶ photons/s) with respect to previous experiments, allowing standard scan rates for NSOM imaging. An extended-cavity Ti:sapphire oscillator is developed that generates 25-fs pulses at 800 nm with 25 MHz repetition rate, obtaining a fourfold increase in pulse energy at given average power, minimizing thermal effects on the tip.