Multimodal microscopy in mid-infrared via flexible pulse shaping

Abstract

In this thesis, mid-infrared (MIR) pulses with arbitrary temporal and spectral shape are generated via a difference-frequency process for application in a non-linear Raman microscope. Solely by shaping the sub 10 fs driving pulses, the broadband spectra of the MIR pulses are switched to narrowband and tuneable ones. In MIR transmission spectroscopy, these narrowband MIR spectra allow for investigating molecular vibrations from 1250 to 3250 cm-1 with spectral resolutions below 20 cm-1. Furthermore, MIR transmission microspectroscopy is combined with coherent-anti-Stokes Raman scattering (CARS) to provide a direct comparison of spectra and images obtained in one spot of the sample. Sum-frequency (SF) microspectroscopy is an additional technique, which complements the toolbox of this non-linear Raman microscope with the potential to investigate non-centrosymmetric systems. The flexibility of the pulse shaper allows for implementing two different SF-methods. Whereas the heterodyne multiplex method acquires the whole SF spectrum by imprinting only three different phase functions, the homodyne MIR-scanning method generates a high SF intensity directly linked to one vibrational mode. In all applications, the phase of MIR pulses must be well-known. This phase is determined in the focal plane of the microscope over more than 1000 cm-1 via two methods based on the dispersion-scan.