Chemically selective microspectroscopy with broadband shaped femtosecond laser pulses
This doctoral thesis presents a new, unified approach to nonlinear microspectroscopy employing tailored broadband femtosecond laser radiation. The key concept is to functionalize the femtosecond excitation in order to implement a series of multiphoton spectroscopy techniques, especially for microsc...
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|Summary:||This doctoral thesis presents a new, unified approach to nonlinear microspectroscopy employing tailored broadband femtosecond laser radiation. The key concept is to functionalize the femtosecond excitation in order to implement a series of multiphoton spectroscopy techniques, especially for microscopic imaging. The most important application is coherent anti-Stokes Raman scattering (CARS) spectroscopy, which allows chemical identification of untreated samples in microscopy due to their characteristic vibrational spectra. The presented approach allows huge experimental simplifications of CARS, schemes for very rapid spectral acquisition and determination of the chemical composition (based on the quantitative analysis of entangled multiplex spectra by evolutionary algorithm fitting), as well as new methods for microscopic CARS measurements in the time-domain, resolving molecular vibrations temporally. This is possible, because coherent control of the signal generation is applied, manipulating the quantum mechanical processes of the underlying light-matter interaction by shaping the excitation light field in phase, amplitude and polarization. Thus, spectroscopic function and even molecular control is imprinted on the excitation pulses. It is shown that this idea of functional “photonic integration” can be pursued even further by incorporating an interferometric detection scheme in the same pulses without any additional optical elements in the experimental setup, drastically improving the measurement sensitivity by more than three orders of magnitude. In addition to these novel conceptual findings, new technological developments have been invented and pushed forward. These include the generation of ultrabroadband femtosecond radiation in microstructured optical fibres and its precise phase measurement and management, which is a prerequisite for coherent control. In this context, a new pulse-shaper enabled variant of SPIDER was developed, allowing very rapid compression in collinear beam geometry in the microscope. Employing the developed set of tools and concepts, application examples are given ranging from quantitative chemical imaging of polymer blend samples, to the chemical identification of potentially hazardous powdery substances and the microanalytical sensing of the chemical composition in a microfluidic device.|