The high-speed atomic force microscopy (AFM) have realized dynamic visualization of moving proteins in action at high spatial resolution. The highest imaging rate we have reached so far is 60 ms/frame. In order to achieve the video-rate imaging beyond the present imaging rate, the feedback control keeping the tip-surface distance constant should be improved to work much more quickly. However, it seems difficult to improve the performance of the piezoactuator-based scanner, which is used at the present system, because the scanner has macroscopic dimensions and available piezoactuators are limited.
Here, we have introduced a novel method to increase the feedback bandwidth in atomic force microscopy. An intensity-modulated infrared laser beam is used to photothermally deflect small cantilevers. The slow response of the photothermal expansion effect is eliminated by inverse transfer function compensation. By regulating the laser power and regulating the cantilever deflection, the tip–sample distance is controlled; this enables much faster imaging than that in the conventional piezoactuator-based z-scanners because of the considerably higher resonant frequency of small cantilevers. Using this control together with other devices optimized for high-speed scanning, video-rate imaging of protein molecules in liquids is achieved.
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