Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals

Abstract

The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip-sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 mum as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ~70 nm full width at half-maximum.

Weak signal detection for scanning near-field optical microscopy

“…The near-field near-field mode. This is usually known as the illuminationcollection mode [40]. One can also classify the collection of the optical signal of electroluminescence or photoluminescence feature under this mode, as the illumination source can be considered as a near-field source.…”