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G. Ramer:
"Development and Characterization of a Near-Field Infrared Microscope by the Coupling of AFM and QCL Spectroscopy";
Betreuer/in(nen), Begutachter/in(nen): B. Lendl, C. Huck, G. Friedbacher; Chemische Technologien und Analytik, 2016; Rigorosum: 28.04.2016.

Infrared spectroscopic imaging is a non-destructive, molecular specific technique for chemical analysis. It has found wide application in material science, bio-medicine and quality control. However, infrared imaging has one major limitation: its spatial
resolution is Rayleigh limited to the low micrometer range. To
overcome this limitation and improve the spatial resolution of infrared imaging scanning probe based near-field imaging techniques can be used. Resonance enhanced - photothermal induced resonance (RE-PTIR) is a promising near-field imaging technique for the mid-infrared spectral range. It combines a tunable pulsed infrared laser and an atomic force microscope (AFM) to achieve infrared imaging at a resolution better than 50 nm. In this work a RE-PTIR setup was designed and assembled from a commercially available AFM and a pulsed, broadly tunable external cavity - quantum cascade laser (EC-QCL). The laser tuning behavior was characterized by performing Fourier transform infrared (FTIR) step scan measurements. To optimize the laser spot size two different methods of focusing the laser were implemented and tested. Using physical optics propagation simulation the spot size of the laser beam on the sample was optimized. As the need for tracking the contact resonance frequency to achieve stable measurements became apparent, a controller for tracking the resonance was designed and constructed. The controller generated trigger pulses for the EC-QCL and digitized the resulting cantilever deflection signal. To allow flexible, parallel digital signal processing, the calculations needed for resonance tracking were performed on a field programmable gate array (FPGA). After evaluation, the results were output to the AFM controller via digital analog converters (DACs) to enable the acquisition of location specific photothermal induced resonance (PTIR) signals. The FPGA programming to control RE-PTIR measurements was developed in tandem with a test bench that simulated the photo-expansion signal for a given laser pulse train. The finished controller used the modulus of the cantilever deflection signal to determine the amplitude of the cantilever and swept a range of frequencies to ensure that the maximum amplitude of a resonance mode was detected. In its implementation at the end of this work, the RE-PTIR setup covered a spectral range of 1039.6 cm−1 by using a source that combined the output of four EC-QCLs . The controller electronics allowed to track shifts of the contact resonance across jumps >10 kHz while generating updated infrared near-field measurements fast enough to allow contact mode imaging at usual speeds of approx. 1 line per second. Update rates of 350 Hz of the PTIR were easily possible without distorting the shape of the resonance curve. Each update included a full sweep of the selected range of the resonance, detection of the maximum amplitude and output of the amplitude value. The system also allowed to acquire single point spectra across the whole range of the EC-QCL source at a spectral resolution sufficient for solid state spectroscopy (approx. 1 cm−1). Using this setup, spectra of polymer films down to a thickness of 60nm were collected. These were in excellent compliance with far field infrared reference spectra. For a polymerfilm as thin as 8nm detection of strong absorption bands was demonstrated to be still possible. A ten fold improvement in the signal to noise ratio was achieved in comparison to a lock-in detector by using the controller. Time resolved infrared near-field measurements recording the change in secondary structure of a poly-L-lysine polypeptide film were demonstrated using the PTIR setup. In order to detect the changes in the secondary structure PTIR spectra across the peptide amide I band had to be acquired, as the change manifests as a band shift in the infrared spectrum. The controller has been designed in an open and flexible way, using open electronics and freely available software. This will allow facile reconfiguration for future improvements and replication by others.