COMPRESSIVE‐SENSING SUM‐FREQUENCY GENERATION MICROSCOPY OF PATTERN FORMATION IN THE DIP‐COATING PROCESS

In this dissertation, a novel surface vibrational spectral-microscope via the compressive-sampling (CS) technique and the broadband infrared-visible sum-frequency generation (SFG) has been established as a powerful and useful tool to investigate the morphology and structures of different chemical systems involving the dynamic patterning process. The new microscope is designed, constructed, and optimized for a femtosecond laser system with the CS technique. The unique microscope takes the advantages of both the broadband pulse of a femtosecond laser and the high repetition rate, which can help to reduce the spectra sampling time. CS technique provides the SFG microscope the flexible imaging conditions in variable FOV and resolutions, which can be adapted for different imaging purposes. Inspired by the structural illumination concept, the new geometry is utilized to optimize and enhanced the signal level by 12 times compared to the original design. The CS-SFG microscope is utilized successfully to investigate the structure and the distribution of a lubricant dip-coated Au surface. Taking the advantage of SFG intrinsic surface specificity, the formation of the coated film is revealed through the spectra and SFG images. The well-ordered dotriacontane (DTC) molecules display both flat-on and edge-on geometries. The anisotropy of the spectra provides the evidence of the adsorption symmetries and the well-organized structure. The SFG images shows the morphology composed by the stripes in different directions which, together with the spectra, suggests both Marangoni flow and crystallization effect influence the formation of the films. Comparing to the linear image, the interfacial DTCs are acting as a template in the dip-coating process for the high coverage deposition at the boundary that the thick films grows with the accumulation of more DTC molecules on the site where the bottom stripes located. As a comparison, the dip-coated films at the DTC-glass interfaces are also studied with the microscope. The structure of the interfacial molecules is specifically resolved through spectroscopy and microscopy for the first time. From the in-plane azimuthal anisotropic SFG spectra of the interfacial DTC molecules, the symmetry and the possible configuration of the assembled DTC molecules are discussed.