Surface-enhanced femtosecond stimulated Raman spectroscopy for detection of trace chemicals in mixtures

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Timely diagnosis is one of the most important steps towards the successful treatment of diseases such as cancer. Therefore, an increasing effort is put into the analysis and detection of potential biomarkers indicative of the cancer state of the patient. Optical methods, such as infrared absorption or Raman spectroscopy, have already found widespread use. The main challenges thereby are posed by the reliable detection of biomarkers on the few molecule level and the unequivocal attribution of spectral features to certain chemical species. While the former condition is met by the use of surface-enhanced Raman scattering (SERS), the latter is especially demanding as typical blood or saliva samples usually contain several thousand different molecules with overlapping spectral features. The aim of this project is to assess the possible use of a combination of surface-enhanced and femtosecond stimulated Raman spectroscopy (FSRS) for the detection of biomarkers with increased sensitivity and specificity. Due to the stimulated nature of the nonlinear process, the surface enhancement can be increased by several orders of magnitude compared to the enhancement usually found in SERS. This will give a dramatic increase in sensitivity. Further, the unprecedented temporal resolution of FSRS (<100fs) adds an additional criterion for discriminating molecules that otherwise exhibit similar Raman spectra. This is based on the idea that different molecules will respond differently to optical excitation, for example, to impulsive Raman scattering. Apart from the practical aspect, the study of surface-enhanced FSRS opens the door to track the dynamics of chemical reactions on the single molecule level. This will have a great impact on the study of catalytic reactions on surfaces and nanostructures.