Marcus Cicerone

Exploring Biology and Materials with Coherent Raman Methods

Over the past ten years, we have developed a spectroscopic coherent Raman imaging (CRI) method that allows us to rapidly obtain broadband Raman spectra from specimens such as biological systems or engineering materials. Raman spectra can provide rich functional information pertaining to complex biological systems, but is rarely used due to inefficiency of spontaneous Raman scattering. Most vibrational spectroscopic information is found in the fingerprint region where spontaneous Raman methods typically require acquisition times on the order of seconds; too slow for imaging. Coherent Raman methods have previously been unable to acquire high quality fingerprint spectra. We have overcome this limitation by developing a highly efficient signal excitation paradigm and appropriately harnessing the nonresonant background (NRB) signal that accompanies the resonant signal of interest. With these and other innovations, we have developed a CRI approach based on broadband coherent anti-Stokes Raman scattering (BCARS) that provides an unprecedented combination of speed, sensitivity, and chemical selectivity [1]. Using this system we are able to obtain high quality Raman spectra in the fingerprint and CH stretch regions from biological specimens at 3.5 ms, enabling rapid, label-free chemical imaging of even delicate samples. I will describe key technical features of the imaging system we have developed, and provide application examples in materials and biology. I will also put our approach in context with the broader CRI field and will briefly speculate on ultimate performance limits for coherent Raman imaging. Time permitting; I will also discuss recent work using coherent Raman and other nonlinear optical methods to investigate transport and relaxation processes, revealing a fundamental origin of dynamic heterogeneity commonly observed in dynamics of supercooled liquids, glasses and proteins.