Focusing in the second band of a flexible membrane photonic crystal

Abstract

We show experimental results of anomalous refraction through a photonic crystal membrane. The membrane layer consists of a thin polymer film suspending a triangular array of silicon pillars. Light is coupled into the photonic crystal (PC) through ridge waveguides etched onto a silicon substrate. By altering the shape of the tip of the input waveguides, we can shape the light that is incident into the PC. In this paper we show that when we shape the field to be quasi point source like, the PC focuses the incident light onto a deflection block placed behind the membrane structure. We experimentally observe focusing of both TE and TM light inside the PC. In the same structure we have previously shown that when we illuminate the PC with a much broader beam incident at an angle, the light negatively refracts through the crystal. We designed the device so that it is capable of being stretched by mechanical actuators, which will stretch the polymer film and silicon lattice and distort the photonic band structure. Mechanical stretching of the dimensions of the flexible PC makes possible a device that can dynamically change its beam steering and focusing properties.