Optical-feedback scanning ion conductance microscopy with high-speed and low-overshoot probe control

At present, the hopping mode of scanning ion conductance microscopy (SICM) is the mainstream method for probing complex sample topography. However, conventional SICM typically relies on built-in electrical sensors for Z-axis feedback control, and the dynamic performance of this feedback path can be limited by sensor bandwidth, readout delay, and system-level disturbances during high-speed scanning. As a result, the probe is more prone to overshoot during rapid approach, which increases the risk of probe-sample collision and restricts imaging speed and stability. To address this problem, an optical feedback system based on a long-range optical lever was proposed. This system utilizes a mechanical-optical synergistic amplification mechanism to convert the tiny linear Z-axis displacement of the probe into measurable spot displacement on a high-speed image sensor, providing a displacement measurement more directly related to probe-side motion. In terms of image processing and displacement calculation, a step-down sampling and constant-parameter Gaussian analytical algorithm was adopted to achieve the low-latency and rapid extraction of sub-pixel spot displacement on FPGA hardware. Experimental results show an overall displacement-tracking linearity of up to R² = 0.9995 and a static measurement resolution better than 2.06 nm. PDMS imaging experiments further demonstrate that, compared with conventional closed-loop control, the proposed optical feedback system achieves lower ion-current overshoot, reduced repeated-imaging error, and higher allowable approach speed under comparable image quality. These results indicate that the proposed method provides an effective feedback route for improving the dynamic performance and imaging stability of SICM.