Photonic crystals slow light for better sensing

Abstract

Photonic-crystal (PC) structures offer an important platform for optofluidic devices, photonic circuits, and dispersionengineered metamaterials. These devices promise lab-on-a-chip capabilities with considerable flexibility for managing light. They are also likely to reduce the cost of integrated optical circuits and devices. PC circuits have primarily been defined by their periodicity and the architecture of the holes or columns fabricated in the transparent material. This has encouraged the development of stamping and printing technologies that can produce prototypes quickly and make large quantities of sensors inexpensively. In addition to being manufacturable, PC platforms have two attractive attributes for sensing applications. First, a gas or liquid analyte can flow directly through the nanoscopic holes within the photonic structure. Second, ’slow light’ can boost the device’s sensitivity. Optical sensors that exploit spectroscopic analysis—such as surface-enhanced Raman phenomena—are already theoretically capable of molecular sensitivity: PC sensors using slow light could increase even this sensitivity. Because controlling the speed of light has a host of important applications, engineers have learned to manage the dispersion characteristics of a material to control it or, more specifically, to control the group velocity of light. In an isotropic linear medium, the velocity of light is the same in all directions and is defined by the angular frequency (ω) divided by the wavenumber (k). The plot of ω versus k provides the phase velocity of the light. An optical pulse—which one can consider as the envelope or constructive interference arising from a number of frequencies— has a group velocity that is the derivative of ω with respect to k. If this derivative is zero, then at that frequency and for the small band of frequencies in its neighborhood, a pulse has a group velocity that is close to zero. This interests us because slow-light pulse propagation can enhance the energy density of the electromagnetic field within Figure 1. A carefully designed periodic stack with sub-wavelength features can slow certain wavelengths to a crawl and increase the resonant field intensity. The form-birefringent structure in the photo is made of Fullcure 720, a UV-cured material with a refractive index of 1.9 at 8GHz.