Stroboscopic Scanning Probe Microscopy and Time-Resolved Molecular-Scale Imaging of Photoexcited Carriers

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In this year, we demonstrated time-resolved force detection by using scanning force microscopy. In the dynamic-mode operation, the tip interacts with the surface only at the moment when the tip closes to the sample surface. The interactive duration can be estimated or less. Taking advantage of this short duration, submicrosecond-class time-resolved force detection has been achieved by the coincidence between photo-irradiation and cantilever motion.Since the frequency of cantilever oscillation is modulated by the tip-sample force change during the scan, it is necessary that the timing of each laser pulse should be synchronized by each stroke of cantilever motion. We examined that the relationship between oscillation amplitude and the laser-pulse delay with various sample bias voltage. The sample was a Si(111) flushed in UHV condition. In usual, oscillation amplitude does not changed by bias voltage in non-contact region. Actually, in case of no laser irradiation, there is no bias effect. However, under the pulsed laser irradiation, the oscillation amplitude is dramatically changed as a function of both delay time and bias voltage. Furthermore, we found dead zone in delay time. These results indicate that the transient charge generated by pulsed laser irradiation can be detected with μs-class resolution.