Broadband coherent anti-strokes Raman spectroscopy characterization of polymer thin films
Lee J. Richter, M C. Gurau, Zachary Schultz
Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of the polymer thin film materials. A simple modification to a commercially available 1 kHz SFG spectrometer permits acquisition of CARS spectra for polymer thin films with an aspect ratio relevant to device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained consistently demonstrate a 1-2 orders of magnitude signal enhancement compared to spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The Fresnel coefficients associated semiconducting (Si), conducting (Au), and insulating (Quartz) substrates has a dramatic effect on the detected signal intensity. The strongest signals are obtained from Quartz surfaces. Similar to surface enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS no surface roughening is necessary. The CARS measurements present here demonstrate methodology for the rapid acquisition of high sensitivity spectra for polymer thin film samples.