Integrating Cavity Enhanced Spectroscopy for Liquid and Gas Sensing

Abstract

With continued advances in optical instrumentation coupled with new highly specific contrast methods, optical based spectroscopic techniques continue to find new applications in a wide range of fields from medical diagnostics to analytical chemistry. Fluorescence based detection has been used in a variety of medical and environmental sensing applications. Raman spectroscopy has long been used in a variety of industries and applications due to its unique ability to provide label free chemically specific information about a molecule. While both of these techniques have tremendous potential in both medical diagnostics and environmental sensing, often cost efficiency and sensitivity limit more wide spread use.

To overcome these limitations, we explore the use of integrating cavity enhanced spectroscopy as a means to greatly enhance the detection sensitivity of both Raman and fluorescence spectroscopy based detection. Using a new diffuse reflector, we have constructed novel integrating cavities that provide significant enhancement of linear optical processes. Enhancement of these processes stems from the long optical pathlength light experiences inside the cavity combined with the relatively large volume that can be probed when compared to traditional spectroscopic approaches. Integrating cavities have several advantages over other sources of signal enhancement in that there is no need for high cost laser sources, and the broad range of wavelengths over which the cavity is reflective affords the ability to use ultraviolet (UV) and blue excitation wavelengths. Raman spectroscopy greatly benefits from the use of more blue shifted sources. Additionally, this allows for one to take advantage of the endogenous fluorescence of many organic molecules that fluoresce under UV excitation.

The enhancement of both spontaneous Raman generation as a result of the integrating cavity is demonstrated by measuring the sensitivity of detection for three aromatic hydrocarbons. Fluorescence enhancement is demonstrated by exploring the detection limits of an urobilin zinc phosphor complex, a biomarker that has been shown to be a viable indicator of human and animal waste contamination in water sources. Finally, the design of an integrating cavity system for gas phase analysis will be demonstrated.