Microscopic mapping of subnanometric motion in semitransparent systems

Abstract

Holographic methods and apparatus for the study of object displacements that may be complex even on a microscopic scale are described. Dual reference beams are employed, and polarization-sensitive interferograms of the displacement field in a specimen are obtained by twin holographic exposures sandwiched about a shift of 1/2+δ or 1/2−δ wavelength in the reference path, with |δ|≪1. Interferograms that correspond to algebraically opposite values of δ are recorded simultaneously; during reconstruction they are projected simultaneously or alternately upon a CCD array. Changes in object wave amplitude arising from ultramicroscopic reorganization of the specimen are suppressed by electronic processing of the signals from the array. The processed signals are reconstituted into a display in which the intensity of any pixel is related linearly to the mean displacement of the specimen portion focused upon it. Extremely small displacements can be imaged in this way, and the resolution with which they can be mapped is limited only by the numerical aperture under which the object wave is gathered before being recorded on the hologram. Displacements smaller than 0.6 nm are evident in images we present, and the small objects that undergo them are readily resolved from their stationary fiducial surround.