The advent of robust, ultrafast lasers has enabled the application of coherent nonlinear spectroscopies to routine sample characterization. Coherent anti-Stokes Raman scattering, CARS, is the nonlinear equivalent of conventional spontaneous Raman scattering. Conventional Raman is a scattering process, and the signal is highly dependent upon collection efficiency. The coherent nature of the CARS process gives rise to a directional signal simplifying collection and improving the sensitivity. Surprisingly, there has been little work exploring the utility of CARS as a thin film diagnostic. Surface enhanced Raman scattering, SERS, has been used to probe the interfacial region of thin polymer films on metal surfaces. However, limitations inherent in SERS leave much unknown. For example, SERS enhancement often requires roughened coinage metal (Au, Ag, Cu) substrates. It is difficult to characterize film surface interactions when enhancement only comes from particular surface sites. Additionally, SERS enhancement only extends a few nm into the film, making it sensitive to the interface, but blind to contributions from further into the bulk. CARS alleviates both of these restrictive conditions as nonlinear mixing occurs throughout the focal overlap region of the incident beams and is not dependent upon substrate enhancement. We have demonstrated a simple modification to an existing nonlinear spectrometer that enables CARS characterization of thin polymeric films. We find that this methodology permits rapid acquisition of high signal to noise (S/N) spectra.
Coherent anti-Stokes Raman Scattering (CARS) is a high sensitivity alternative to conventional Raman spectroscopy. Ultrafast lasers are used to enable multiplex CARS, with demonstrated advantages for the study of ultra-thin films.
Additional Technical Details
Raman Spectroscopy Spectrum Illustration
Additional experiments for films of various polymers have been performed on diverse substrates (Si, Au, glass). The film thickness was varied from ~ 20 nm to ~300 nm. In all cases, high quality vibrational spectra were acquired, with demonstrated improved SNR compared to conventional Raman spectra. A mathematical framework for the quantitation of the CARS spectra, accounting for the influence of the substrate dielectric response, was developed and shown to be consistent with the measurements.