Chemical, biological, and trace gas detection and measurement with a newly developed integrating Cavity Enhanced Raman (iCERS) technique

Raman spectroscopy is routinely used in the laboratory for detection, chemical identification, and quantitative measurements of complex molecular compounds. One key advantage of the method is that a single laser wavelength can be used to identify and measure several different molecular compounds simultaneously. While Raman spectroscopy is a powerful technique, it is a very inefficient process where only one in 1011 scattered photons contain the desired vibrational information. Several techniques have been developed to enhance Raman scattering, which are typically applied to liquids and solids such as surface enhanced Raman spectroscopy and coherent anti-Stokes Raman spectroscopy. For gas phase measurements, photonic crystals, cavity enhanced Raman spectroscopy and functional waveguides have been developed to provide Raman enhancement. However, Raman spectroscopy has seen limited use in commercial and military applications due to instrument complexity, sample preparation, acquisition time, and spatially localized point measurements. A recently developed technique to enhance spontaneous Raman scattering utilizing a highly reflective integrating cavity is presented. Elastically scattered light circulates within the cavity volume continuously interacting with the sample, whether a bulk sample or gas, resulting in significant Raman enhancement. In addition, the Raman scattered light is collected from all directions before being coupled out of the cavity. Enhancements of 10⁷ have been realized with the use of inexpensive low power diode lasers and a modest CCD based spectrometer. Application of the iCERS technique operating near 400 nm providing near real-time detection and measurement of trace gases, chemicals, and biological compounds is discussed.