Design of high-Q Cavities in Photosensitive Material-based Photonic Crystal Slab Heterostructures

Abstract

We propose a novel concept for creating high-Q cavities in photonic crystal slabs (PCS). We show that photonic crystal slab-based double heterostructure cavities, formed by variations in the refractive index, can have large a Q-factor (up to Q = 1 £ 10 6 ), and that such cavities can be implemented in chalcogenide glasses using their photosensitive properties. In the last few years the study of optical microcavities based on photonic crystal slabs has attracted much attention (1{10). Almost all of these studies consider a PCS composed of a hexagonal array of cylindrical air holes in a high-index semiconductor slab. There are many possible device applications of compact and e-cient PCS nanocavities, such as channel drop fllters (1), low-treshold laser (5), and cavity QED experiments (6,7). The principal design aim for all these applications is to obtain a high quality factor within a small modal volume. A cavity is usually formed in either of two ways: forming a point cavity or forming a \hetero- Microcavities with the highest Q values achieved to date, have been realised through the use of photonic crystal double-heterostructures (9,10), where regions of slightly difierent lattice constant are combined in a single slab to create a cavity. Song etal., constructed double hetero- structure PCS, in which a short length of crystal (PC2) with a lattice constant stretched in one direction, interrupts the main crystal (PC1) (9,10) (see Fig. 1 ). (a) (b) Figure 1: (a) Schematic of PCS with a W1 waveguide in the i-K direction and (b) refractive index distri- bution in the plane of the structure considered. The central darker region indicates the increased index. It is possible to form heterostructures exploiting material properties, rather than the geometry of the structure. In this paper we consider a chalcogenide glass-based PCS. It has been already shown that high quality PCS can be fabricated in this material (11). The key property here is that chalcogenide glasses are photosensitive. This means that the refractive index of the material can change by 1 to 8%, depending on the type of glass, when it is illuminated by light, typically in the visible part of the spectrum (12). The concept of the cavity design in hetero-structures relies on the mode-gap efiect, a narrow frequency range for which PC2 supports a mode, but not PC1. Therefore, flrst we determine if there is a su-cient mode-gap to support a localized state between the structures having difierent refractive indices. We introduce a W1 waveguide in these structures: W11 for PC1 with n = 2:7, and W12 for PC2 with n = 2:75. Using the Plane Wave Expansion method, we obtain the dispersion curves. The results are shown in Fig. 2(a). The size of the mode-gap is comparable to those of the hetero-structures formed by geometric variation (10). This suggests that hetero-structures formed by photosensitive index enhancement should also be capable of supporting localized states.