Direct scanning tunneling microscope detection of laser induced ultrasonic pulses with nanometer resolution

We show the use of scanning tunneling microscope (STM) with a self-designed wide-bandwidth current preamplifier (50 MHz) for direct registration of ultrasonic pulses induced by nanosecond laser pulses in substrate. The changes in tip-sample separation caused by ultrasonic vibrations are seen as perturbations in tunneling current. At moderate incident laser pulse energy of 1-2 mJ ultrasonic waves in glass rod and silica wafer were observed. This wide-band signal, low noise registration in addition with nanometer spatial resolution of a STM opens new possibilities in investigations of ultrasonic near-field distribution, elastic material properties on a nanometer scale, and measurements of the ultrasound velocity in laser ultrasonics. (C) 1999 American Institute of Physics. [S0034-6748(99)00212-9].