Thermo-Optical Modulation of PPLN Crystal for Tunable Poisson Spot Array

Lithium Niobate is a ferroelectric material with interesting physical properties. In particular, Periodically Poled Lithium Niobate (PPLN) crystals have been used in diverse applications, such as non-linear optics or microlens array fabrication. In this work, we used a PPLN crystal having hexagonal reversed polarization domains, disposed on a square array of 200 µm period. We applied a temperature gradient to the PPLN and simultaneously observed it with a lensless incoherent holographic microscope. We observed that the phase of the inverse polarization domains varied depending on the temperature applied. Therefore, we induced a thermo-optical modulation of the PPLN crystal. We further analysed the behaviour of the PPLN, propagating the complex field beyond the crystal and plotting its intensity. We found that an elongated bright spot was formed at the centre of each hexagonal reversed polarization domain, due to diffraction. Given their shape and the nature of the phenomenon, these intensity spots are similar to Poisson spots. The intensity of the spots depended on the phase of the PPLN (hence, on the temperature applied). Therefore, we were able to generate a tunable Poisson spot array by controlling the temperature of the PPLN.