Opt. Lett. 37, 1070–1072 (2012)

Speckle-based metrology can be used to provide versatile measurements of strain, displacement and rotation in a sample. However, the speed of such techniques is limited by the required image sensor, whose response time is often orders of magnitude slower than detectors such as fast photodiodes. E. G. van Putten and co-workers have now experimentally demonstrated an alternative fast technique for measuring displacements in complex scattering materials. By spatially controlling the wavefront of the incident light, the researchers were able to concentrate the scattered light to a focus at a designated position. The scattered wavefront acts as a unique optical fingerprint that enables precise position detection of the illuminated material simply by measuring the intensity at the focus. The researchers combined two such fingerprints to provide position detection along one in-plane dimension with a displacement resolution of 2.1 nm. Because this approach does not require an image of the scattered field, it is possible to employ fast detectors to enable the high-speed position detection of scattering materials. The researchers say that employing more than two detectors would provide multiple optical fingerprints, and that the set-up can also be configured to detect other sample movements, such as rotations.