Real-time photodisplacement imaging using parallel excitation and parallel heterodyne interferometry

Abstract

A parallel photodisplacement technique that achieves real-time imaging of subsurface structures is presented. In this technique, a linear region of photothermal displacement is excited by a line-focused intensity-modulated laser beam and detected with a parallel heterodyne interferometer using a charge-coupled device linear image sensor as a detector. Because of integration and sampling effects of the sensor, the interference light is spatiotemporally multiplexed. To extract the spatially resolved photodisplacement component from the sensor signal, a scheme of phase-shifting light integration combined with a Fourier analysis technique is developed for parallel interferometry. The frequencies of several control signals, including the heterodyne beat signal, modulation signal, and sensor gate signal, are optimized so as to eliminate undesirable components, allowing only the displacement component to be extracted. Two-dimensional subsurface lattice defects in silicon are clearly imaged at a remarkable speed of only 0.26s for an area of 256×256pixels. Thus, the proposed technique allows for real-time imaging more than 10 000 times faster than conventional photoacoustic microscopy.