Nonoptical tip sample distance control for scanning near-field optical microscopy

We propose and demonstrate a novel nonoptical technique for regulation of tip-sample distance in a scanning near-field optical microscope (SNOM). The fiber tip for the SNOM is attached to one prong of a quartz tuning fork. The fork is dithered with a gated sinusoidal signal. The vibration of the freely oscillating fiber tip, which manifests as the induced piezoelectric voltage on the fork electrodes, is monitored during the half-period of the gated sinusoid for which the fork is not driven. The time-multiplexing scheme, thus, allows the tuning fork to serve as a dither and a sensor with high Q factor, simultaneously. The gating technique could also potentially allow the SNOM to be used for the investigation of surface relaxation dynamics with high spatial resolution and submillisecond time resolution. (C) 1996 American Institute of Physics.