Photonic crystal slabs for optical micro electro mechanical systems (OMEMS)

Photonic crystals are materials with a periodic variation of composition in one, two or three directions. For some optical frequency intervals, they behave as homogeneous transparent materials where optical waves are allowed to travel without scattering, but these frequency intervals are intervened by photonic band gaps in which the propagation of light is forbidden. Photonic crystals are the photonic analogue to traditional crystals, where the periodic arrangement of atoms allows electrons and light to travel without scattering. We present some fundamental properties of two-dimensional photonic crystals in form of photonic crystal slabs, with emphasis on their design, fabrication and incorporation into micro-electro-mechanical systems (MEMS). Photonic crystal slabs have become a very popular research topic because of their compactness, which makes them easily integrated with MEMS, and also because their fabrication is relatively easy compared to that of their three-dimensional relatives. The photonic crystal slabs support oscillating resonator modes [Fan and Joannopoulos, 2002], and the modes of the photonic crystal slabs can be designed to couple weakly or strongly to the outside, and give rise to interferences in the reflection or transmission spectra of the photonic crystal slabs. These interferences manifest themselves as areas of high reflectivity or transmissivity that, depending on the design, can be both very sharp ( 150 nm). In other words, photonic crystal slabs can be used to make both broad-band mirrors and narrow optical filters [Lousse et al., 2004, Kim et al., 2007]. We have taken the fabrication technology for photonic crystal slabs one step further by developing the GOPHER-generation of photonic elements by reactive ion etching (RIE) process [Hadzialic et al., 2007b]. This is a simple method for making photonic crystals in monolithic materials. The process consists of only one lithographic step in which the photonic crystal pattern is defined. The pattern is then transferred to the underlying material by RIE. In order to achieve guided resonance modes in the photonic crystal slabs we need to have a refractive index contrast to the substrate. This is achieved by undercutting the structure in an isotropic RIE. The advantages are low internal stress, no temperature expansion coefficient mismatch, and compatibility with MEMS and CMOS processing. By integrating photonic crystal slabs with MEMS, we can make the photonic crystal slab move with respect to its mechanical support, and make devices such as tunable optical filters, scanners, displacement sensors, and more. We demonstrate one such device based on a mechanically tunable photonic crystal designed for highly position-sensitive reflection [Hadzialic et al., 2007a]