High frequency-bandwidth optical technique to measure thermal elongation time responses of near-field scanning optical microscopy probes

A near-field scanning optical microscopy (NSOM) probe elongates when light is coupled into it. The time response of this thermal process is measured here by a new optical technique that exploits the typical flat-apex morphology of the probe as a mirror in a Fabry-Perot type cavity. Pulsed laser light is coupled into the probe to heat up the tip, while another continuous wave laser serves to monitor the elongation from the interference pattern established by the reflections from the flat-apex probe and a semitransparent metal-coated flat sample. A quarter wave plate is introduced into the interferometer optical path in order to maximize the signal to noise level, thus allowing the elongation of the tip to be monitored in real time. This optical technique, unlike other methods based on electronic feedback response, avoids limited frequency bandwidth restrictions. We have measured response time constants of 500 and 40 mus. The technique presented here will help determine the power levels, operating probe-sample distance, and pulse repetition rate requirements for safe operation of NSOM instrumentation. In addition to NSOM, the instrumentation described in this article could also impact other areas that require large working range, accuracy, and high-speed response. (C) 2002 American Institute of Physics.