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

在这一年，我们演示了使用扫描力显微镜进行时间分辨作用力检测。在动态模式操作中，针尖仅在针尖接近样品表面时才与表面相互作用。交互持续时间可以估计或更短。利用这种短持续时间，通过光照射和悬臂运动的重合实现了亚微秒级的时间分辨作用力检测，由于悬臂振动的频率受到扫描过程中针尖-样品力变化的调制，因此每个激光脉冲的定时必须与悬臂运动的每一次行程同步。考察了不同偏置电压下振荡幅度与激光脉冲延迟的关系。样品为超高压条件下的Si(111)晶面。通常情况下，非接触区的振荡幅度不受偏置电压的影响。实际上，在没有激光照射的情况下，没有偏置效应。然而，在脉冲激光照射下，振荡幅度随延迟时间和偏置电压的变化而显著变化。此外，我们还发现了延迟时间内的死区。这些结果表明，脉冲激光辐照产生的瞬变电荷能够以μS级的分辨率被探测到。

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.