A rapid nanometre-precision autocorrelator

Abstract

The precise measurement of a target depth has applications in biophysics and nanophysics, and non-linear optical methods are sensitive to intensity changes on very small length scales. By exploiting the high sensitivity of an autocorrelator's dependency on path length, we propose a technique that achieves $\approx$30 nm depth resolution for each pixel in 30 seconds. Our method images up-converted pulses from a non-linear crystal using a sCMOS camera and converts the intensity recorded by each pixel to a delay. By utilising statistical estimation theory and using the data from a set of 32$\times$32 pixels, the standard error of the detected delay falls below 1 nm after 30 seconds of measurement. Numerical simulations show that this result is extremely close to what can be achieved with a shot-noise-limited source and is consistent with the precision that can be achieved with a sCMOS camera.