Sensitive refractive index detection using a broad-band optical ring resonator

Broad-band operation is investigated as an alternative means of refractive index detection in an optical ring resonator. In this work, a liquid-core (LCORR, capillary-based) design is used. Optical ring resonators have been recently demonstrated for the detection of a wide variety of analytes, including DNA, viruses, proteins, chemical vapors, and pesticides. In the field of analytical chemistry, much of their value is in the ability to provide enhanced detection of surface analytes in small volumes. Conventional analysis methods that employ a single resonant wavelength and propagation mode are highly dependent on resonator quality. By utilizing the complex and variable response of a multiple-mode resonator and the simultaneous data of more than 40 resonance peaks, impressive results exceeding the single-mode prediction are produced from a very modest quality device. The presented methods also become attractive through the use of inexpensive LED light sources and common UV-vis spectrometers, as well as the ability to also take absorbance measurements without any physical configuration changes. Complex interference spectra are produced from the convolution of multiple wavelengths and modes. Two possible methods for analyzing this type of data are presented-one using Fourier transform deconvolution to extract resonance components from interference spectra, and another using chemometrics by constructing a partial least-squares model. Using isopropyl alcohol and water mixtures, detection limits are the order of 10(-6) RIU (refractive index units), comparable to existing ring resonators devices. To study surface detectability and biomolecule detection feasibility, surface adsorption of bovine serum albumin (BSA) is also analyzed.