Validating 3D photonic crystals for structural health monitoring

A photonic crystal (PhC) is a periodic structure with nanometric periodicity comparable with the wavelength of light, having a photonic band gap in the visible range: in practice, it reflects selectively only a band of the incident light, thus appearing to the observer of a determinate color. In this contribution, we propose to use photonic crystals as a colorimetric sensitive material for Structural Heath Monitoring. The idea is based on the observation that any distortion in the crystal structure produces a change in the reflected light bandwidth, resulting in turn in a change in its apparent color, visible to naked eyes. In a near future, we will be able to speed a photonic sensitive material on the surface of a structure in the form of a thin paint layer, and directly measure any variation in the strain field by simply observing change in color. To demonstrate this concept, we first we introduce the basic formulation that controls the photo-mechanical behavior of a 3D photonic structures. Next, we demonstrate the feasibility of the fabrication of a PhC made of sub-micrometric polystyrene colloidal spheres in a PDMS matrix on a rubber substrate. Through laboratory experiments, we show that the photonic properties of the crystal change with substrate elongation according to theoretical prediction. Lastly, we introduce a Finite Difference Time Domain (FDTD) analysis method to simulate the opto-mechanical response of a generic photonic crystal design, through direct integration Maxwell's equations, and validated the method compering the numerical results to the experimental data.