MEMS Tunable Resonant Leaky-Mode Filters for Multispectral Imaging Applications

Abstract

Multispectral imaging refers to a combination of spectroscopy and photography. By using rapidly tunable filters and two-dimensional (2D) image planes such as those provided by charge-coupled device (CCD) detectors, data sets containing spatial (x, y) and spectral information are acquired. The resulting spectral image cubes contain intensity and wavelength (┣) data at each pixel in the 2D image (Gat, 2000). Under time-varying conditions, the data cube would be multidimensional in (x, y, ┣, t) space. Hyperspectral imaging is a similar concept principally differentiated from multispectral imaging in that many more wavelengths and narrower spectral passbands are employed. Thus, in multispectral imaging, relatively few wavelengths are used to carry the spatial information, whereas in hyperspectral imaging, the number of wavelength channels may reach ~100 (VoDinh et al., 2004). Each of these methods is connected with a plethora of useful applications. Examples include spatio-spectral diagnostics in agricultural crop management, true-color night vision, forensics, and archaeology and art (Gat, 2000). In medicine, hyperspectral invivo diagnostics of tissue may avoid excision and permit in situ analysis (Vo-Dinh et al., 2004). Its application to real-time guidance in surgery is promising (Vo-Dinh et al., 2004). The capability of the tunable filters central to these spectral imaging methods defines the quality of the data sets collected. Gat lists principal attributes of ideal tunable filters and describes examples of filters employed to date (Gat, 2000). Among these, tunable liquidcrystal and acousto-optical filters represent two prominent device classes (Gat, 2000; VoDinh et al., 2004). The former is based on stacks of birefringent liquid-crystal plates integrated with polarizers, whereas the latter is diffractive in nature. In the present contribution, we introduce a new tunable filter concept for potential application in multispectral and hyperspectral imaging systems. In short, we employ a resonant waveguide grating supporting leaky modes that is tuned by micro-electromechanical (MEMS) means. We begin this chapter by summarizing the physical basis for this class of tunable filters. Then, we provide numerical spectral characteristics of resonance elements based on exact electromagnetic models of the devices with representative materials. We investigate theoretically the operation of MEMS-tunable resonant elements.