Design and analysis of reconfigurable photonic crystals and whispering gallery mode resonators based on optical force

Abstract

Photonic crystals (PCs) and whispering gallery mode (WGM) resonators are two of the most versatile photonic structures in the field of photonics and cavity quantum electrodynamics. A long standing challenge in the design of these structures has been their dynamic tunability, which is the capability of varying their resonant characteristics without going through the fabrication process. In this work the prospect of dynamic tunability of polystyrene (PS) microparticle based PCs and WGM resonators is studied by spatially reconfiguring these structures employing optical force. The behavior of optical force exerted by monochromatic illumination via dual counter-propagating Gaussian beams is analyzed using Maxwell Stress Tensor and Finite Difference Time Domain (FDTD) analysis techniques. Both one dimensional (1D) and two dimensional (2D) water submerged periodic PS microsphere arrays are found to exhibit spatial re-orientation in the presence of optical force. Detailed analysis shows that such optical force induced reconfiguration can be controlled by varying the illumination intensity, incident wavelength, and also the surrounding medium of the PS microparticles. For illumination with 980 nm wavelength, 1D array results in an inhomogeneous stable spacing of microspheres whereas periodically reconfigured arrays can be attained by illuminating in direct correlation with the resonant wavelength of the structure. Changing illumination intensity enahnces the magnitude of optical force acting on the particles, though it does not alter their stable locations. On the other hand, variation of refractive index of the background medium alters the stable interparticle separation of particles. Band structures and transmission characteristics of the PS array are observed to evolve in accordance with the spatial reconfiguration. A complete bandgap with nearly 100% reflection is attained for the 1D array whereas nearly 94% reflectivity is obtained for the 2D array. Besides photonic crystals, WGM resonators comprising of polystyrene (PS) microspheres are also analyzed and their Q factors are compared with those of single solid monospherical WGM resonators. The highest Q-factor of 91224 is obtained for a water submerged 20μm monosphere WGM micro resonator whereas for multiple microsphere based circularly symmetric structures no WGM is observed. By changing the surrounding medium, an exponential rise in Q factor is achieved which peaks for air with a WGM mode having Q factor of 408. For water suspended hexagonal configurations of the microspheres, the WGM mode disappears in the presence of optical force, thereby resulting transition from a highly reflective to a nearly transparent medium. Based on such optical force mediated reconfiguration, conveniently tunable photonic crystals and whispering gallery mode resonators can be designed and implemented for novel sensing, detection and lasing applications.