Abstract

We present two methods for detecting subangstrom periodic vibrations of an arbitrarily rough surface. In one method, the laser speckle pattern reflected off of a rough surface is focused into a photorefractive crystal of barium titanate, and the intensity of the transmitted beam is measured. A pump laser beam, coherent with the speckle beam, is also incident on the crystal so as to deplete strongly the speckle beam by two-wave mixing. Any change in either the phase or the amplitude of the speckle pattern caused, for example, by a slight displacement of the reflecting surface will modulate the overall intensity of the transmitted speckle pattern. The fractional change in the intensity of the depleted probe beam yields the amplitude of the vibration. The second vibration-sensing method uses a phase-conjugate Michelson interferometer. Here a Gaussian beam incident on a rough surface produces a speckle pattern on reflection. This complicated pattern is focused into a photorefractive self-pumped Cat conjugator. The phase conjugator converts the incident speckle pattern into its time-reversed replica, which propagates back toward the scattering surface, where it is scattered a second time to reconstruct a nearly Gaussian beam. This double pass off the vibrating surface imparts a uniform phase shift to the reconstructed Gaussian beam, which is then detected by interferometry. We can detect periodic displacements as small as 0.04 Å of a rough aluminum surface using a lock-in detector with a 1-s integration time.

© 1988 Optical Society of America