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Field-Resolved Raman Micro-Spectroscopy

Dr. H. Fattahi, MPQ

Real-time, in vivo imaging of biological samples enhances our understanding of the molecular machinery in living systems, such as cells, organs and even whole organisms. Heretofore, imaging methods have been predominantly limited to approaches that require labeling of the system, which often distorts their normal mechanism of action. Moreover, resolving the temporal dynamics of the quantized vibrations of chemical bonds within a specific molecular species, i.e., acquiring so-called »molecular fingerprints«, has so far been hampered by the limited dynamic range of the current methodology, which is incapable of detecting small amplitude changes near the noise floor. To tackle these limitations, we are pursuing two routes:

  1. Development of a novel, broadband, high-repetition rate, laser source for label-free imaging of biological samples and temporal detection of their molecular fingerprints by means of stimulated Raman spectroscopy.

  2. Development of a new field-resolved detection scheme based on nonlinear photoconductive sampling to enhance the dynamic range of detection beyond the current state of the art. To study the spatio-temporal dynamics of biological systems, we employ the above-mentioned developments, together with "pattern recognition methodology", for label-free detection and analysis of molecular fingerprints. Here, the aim is to accurately examine the patterns of energy exchange between the Stokes laser field and the sample, due to stimulated Raman scattering.

By resolving the exact temporal evolution of the Stokes field on the shortest observable timescales - within a single light cycle - by means of electro-optic sampling or nonlinear photoconductive sampling, one obtains accurate and detailed information about the evolution of the spectral phase and amplitude of the Stokes field, which yields the required molecular fingerprint.