Mahesh Vasudevan Namboodiri

• The vibrations of nuclei, formation and breaking of chemical bonds and biological transformations that happen on a time scale of picoseconds to femtoseconds can be captured using nonlinear spectroscopic techniques employing ultrafast laser pulses. The high peak power of the femtosecond pulses can induce a nonlinear polarization efficiently, which in turn acts as the source of the signal, which contains dynamical information. The combination of multiple laser pulses can be used to monitor the evolution of the molecules. The thesis discusses the pump-probe technique and the four-wave mixing process, coherent anti-Stokes Raman scattering (CARS). These techniques are applied to monitor exciton dynamics in organic semiconductor thin films and vibrational dynamics. Time resolved CARS experiments in Bromine vapor is presented, which show the potential of this technique to probe vibrational dynamics in both ground and excited state potential energy surfaces. The dynamical information obtained could be used to generate chemical specific contrast for non-invasive imaging. The investigation of the influence of the nanostructuring of materials on the ultrafast photo-induced dynamics is of considerable interest. The scanning near-field imaging technique can be integrated with nonlinear spectroscopic techniques to improve spatial and axial resolution of the images. Additionally, the dynamics can be probed down to nano-scale dimension. The thesis shows the proof of principle examples for this combination, in order to obtain exciton map and vibrational contrast images with sub-wavelength resolution.