Grating light reflection spectroscopy

Abstract

A new optical technique for analytical sensing, termed Grating Light Reflection Spectroscopy (GLRS), is described and evaluated in this dissertation. GLRS relies on the existence of thresholds in diffracted transmitted light for the interrogation of the dielectric properties of the sample in contact with a transmission diffraction grating. These thresholds are created when a particular diffracted transmitted order is transformed from a traveling wave into an evanescent one. This occurs at a specific wavelength that is dependent upon the angle of incidence, grating period, and complex dielectric function of the sample. The existence of transmission thresholds produces singularities in reflected light that occur at the threshold wavelength due to the redistribution of optical energy from the threshold order to all diffracted orders. GLRS theory, developed by Dr. Anatol Brodsky, predicts that application of GLRS will yield measurements that are sensitive to optical properties such as refractive index and absorbance and in addition will allow for the deconvolution of refractive index and absorbance effects as well as surface and bulk effects.A binary metal/dielectric diffraction grating having submicron periodicity was fabricated and used in the characterization of GLRS. The GLRS singularity in reflected light is visible as a sharp change in reflection coefficient at the threshold wavelength. An evaluation scheme was devised to test the sensor response to model sample matrices and evaluate the correlation of that response with each major theoretically predicted response. Experimental evidence directly supports predictions regarding the response of GLRS to changes in the complex dielectric function of the sample. Calibration of GLRS response yielded sensitivity to refractive index changes on the order of $2\times10\sp{-6}$ and an absorbance dynamic range to 250 A.U. In addition, the sensor was exposed to mixtures, simulated fouling layers and a sample set of model scattering systems. The experimental results prove theoretical claims that the sensor is relatively insensitive to surface fouling and it is shown that GLRS may provide substantial information regarding the characteristics of colloids and suspensions including the particle shape and density distribution.