Optical gain by a simple photoisomerization process

Abstract

Organic holographic materials are pursued as versatile and cheap data-storage materials. It is generally assumed that under steady-state conditions, only photorefractive holographic media exhibit a non-local response to a light-intensity pattern, which results in an asymmetric two-beam coupling or 'gain', where intensity is transferred from one beam to the other as a measure of writing efficiency. Here, we demonstrate non-local holographic recording in a non-photorefractive material. We demonstrate that reversible photoisomerization gratings recorded in a non-photorefractive azo-based material exhibit large optical gain coefficients beyond 1,000 cm-1, even for polarization gratings. The grating characteristics differ markedly from classical photorefractive features, but can be modelled by considering the influence of the Poynting vector on the photoisomerization. The external control of the Poynting vector enables manipulation of the gain coefficient, including its sign (the direction of energy exchange), a novel phenomenon we refer to as 'gain steering'. A very high sensitivity of about 100 cm2 J-1 was achieved. This high sensitivity, combined with a high spatial resolution, suggests a great technical advantage for applications in image processing and phase conjugation.