Design and fabrication of mechanochromic photonic crystals as strain sensor

A photonic crystal (PhC) is a periodic structure with periodicity comparable with the wavelength of light, having a photonic band gap in the visible range. In this contribution we discuss the possible use of PhCs as strain sensors, based on the observation that a distortion in the crystal structure produces a change in the reflected bandwidth. First, we demonstrate the feasible fabrication of a PhC having sub-micrometric polystyrene colloidal spheres in a PDMS matrix on a rubber substrate, and we demonstrate that the photonic properties change with substrate elongation according to theoretical prediction. The crystal sensitivity to strain depends directly on interplanar spacing and on Poisson's ratio. To enhance the crystal strain resolution, we propose to fabricate inverse photonic crystals with FCC structure, which are known from the literature to exhibit a high negative Poisson's ratio. We carried out a theoretical investigation to predict the opto-mechanical response of inverse PhCs, and carried out preliminary tests to demonstrate their fabrication feasibility.