Ultra-Sensitive Absorption Measurements through Cavity-Enhanced Spectroscopy

Citation

McGarvey, Raymond Timothy James (2006) Ultra-Sensitive Absorption Measurements through Cavity-Enhanced Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/CGYD-6J27. https://resolver.caltech.edu/CaltechETD:etd-09202008-110124

Abstract

The desire to increase the sensitivity of solution-based absorption spectroscopy is motivated by the need for label-free biosensing (which provides a more authentic indication of the state of a biological system) and by the usefulness of characterizing the kinetics of biologically-relevant reactions (which may not be accurately characterizable at reagent concentrations required by standard methods. There are a number of techniques by which such increasingly sensitive measurements have been made, including cavity ringdown spectroscopy, incoherent cavity-enhanced spectroscopy, microsphere-based whispering-gallery mode sensing,and our cavity-enhanced measurements, which are the most sensitive to date and which can be conducted in real time with high bandwidth. Our current device has a demonstrated detection threshold of 1.7x 10^{-7}/sqrt{Hz} (4.36x10^{-6}cm^{-1}), which could with further technical work be improved to a shot-noise limited sensitivity of 1.93x 10^{-10}/sqrt{Hz} (1.06x10^{-8}cm^{-1}). The latter would correspond to an average of 700 strong absorbers (epsilon = 10^5 M^{-1}cm^{-1}) in the optical beam volume. The shot-noise limited detection threshold of our measurement method could potentially be improved by up to two orders of magnitude by incorporating state-of-the-art optical mirrors. With such mirrors, cavity-enhanced absorption experiments performed with gas-phase samples have previously demonstrated single molecule sensitivity. We have established that solution-based cavity-enhanced absorption measurements are more sensitive than standard single-pass measurements by the predicted enhancement factor for our present device (~ 20,000). These measurements provide the proof-of-principle for solution-based, cavity-enhanced spectroscopy and serve as the intermediate step towards the attainment of the theoretical sensitivity of this technique. We believe that this device will be of broad interest to the scientific community, because it is presently the most sensitive solution-based spectroscopic device. It can make real-time absorption measurements which would allow monitoring of the kinetics of chemical reactions in which the spectral properties of reactants change by even a small amount, and, near its theoretical limit of sensitivity (given currently available mirrors), such a device could potentially resolve single-molecule absorption events on the sub-millisecond timescale and below.

Thesis Files