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Using photoconductivity for coherence domain imaging

We report a new method to detect coherence waves by using photoconductivity in semi-insulating semiconductors. The method is based on a low coherence interferometry that uses a superluminescence light emission diode as the light source. In a two-wave mixing configuration, a photorefractive multiple quantum wells (PRQW) device serves as the beam combiner. When the signal beam and the reference beam interfere in the PRQW, it forms an intensity fringe pattern along the device surface. In a transverse geometry of the PRQW, photocarriers in the bright region of the intensity fringe move to the dark region. The space charge distribution causes changes of local electric field in the PRQW. It leads to the changes in absorption and index of refraction as well as photoconductivity. Conventional coherence domain imaging using PRQW is based on the diffraction of one of the beams in multi-wave mixing, such as two-wave mixing and four-wave mixing. Our innovation is to use the photoconductivity to detect coherent signals. We tested the concept by changing the optical delay and measuring the photocurrent. The change of the local electric field causes the change of the photocurrent in PRQW even when the totally incident light density stays the same. We studied the photocurrent under various externally applied electric fields and incident light densities. The relative change of photocurrent is about 10 times higher than the relative change of diffraction in two-wave mixing, which is the highest diffraction efficiency in multi-wave mixing configurations. The change of photocurrent is also proportional to the incident signal light density while the reference intensity keeps the same and the total intensity is relatively low. This method provides a potential solution of coherent signal detection using PRQW for biomedical optical imaging applications.