<p>The thesis consists of two main parts of nonlinear optical instrumentation development. </p> <p>Fluorescence-detected mid-infrared photothermal (F-PTIR) microscopy is demonstrated for sub-diffraction limited mid-infrared microspectroscopy of model systems and applied to probe phase transformations in amorphous solid dispersions. To overcome the diffraction limit in infrared imaging, a highly localized temperature-dependent photothermal effect is an attractive alternative indicator to infrared absorption. Photothermal atomic force microscopy infrared spectroscopy (AFM-IR) achieves nanometer resolution by monitoring heat caused expansion but only restricted on the surface. For 3D imaging, optically detected photothermal infrared (O-PTIR) combines an infrared laser with a visible probe source with to transduce photothermal refractive index changes (e.g., from changes in beam divergence or scattering). The sensitivity of O-PTIR is ultimately limited by the relatively weak dependence of refractive index with temperature, exhibiting changes of ~0.01% per oC. Fluorescence-detected photothermal mid-infrared (F-PTIR) spectroscopy (Fig. 1) is demonstrated herein to support 3D imaging with improved photothermal sensitivity. In F-FTIR, the sensitivity of fluorescence quantum efficiency to temperature change (~1-2% per oC) is used to transduce transient heat flux from localized IR absorption. The infrared spatial resolution of F-FTIR is defined by fluorescence microscopy and the thermal diffusivity of the sample instead of infrared wavelength. Initial F-PTIR proof of concept studies are described for microparticle assemblies of silica gel and polyethylene glycol, followed by applications of F-PTIR for analysis of localized composition within phase-separated domains induced by water vapor exposure of an amorphous solid dispersion (ritonavir in copovidone).</p> <p>Fluorescence recovery while photobleaching (FRWP) is demonstrated as a method for quantitative measurements of rapid diffusion mapping over the microsecond to millisecond time scale. Diffusion measurements are critical for molecular mobility assessment in cell biology, materials science and pharmacology. Fluorescence recovery after photobleaching (FRAP) is a well-known noninvasive optical microscopy method for measuring diffusion coefficients of macromolecules, such as proteins in cells and viscous solutions. However, conventional point-bleach FRAP is challenging to implement with multi-photon excitation and typically only supports diffusion analysis over millisecond time scales due to camera frame rate limitations. FRWP with patterned illumination addresses these limitations of FRAP by probing the fluorescence intensity changes while bleaching a comb pattern within a field of view (FoV). Fast-scanning of an ultrafast excitation beam distributes heat rapidly over multiple adjacent pixels, minimizing local heating effects that could complicate analogous diffusion measurements by point-bleach FRAP with multiphoton excitation. In FRWP, time-scales of the probed diffusion events are defined by a single line-pass time of a resonant scanning-mirror with a period of 125  s. In FRWP, the bleach pattern spans locations across the whole FoV, enabling diffusion mapping through image segmentation. More than a hundred bleaching and recovery events can be recorded during a single 10s measurement. Normal and anomalous diffusion of rhodamine-labeled bovine serum albumin (BSA) molecules was studied as a model system, with applications targeting rapid assessment of therapeutic macromolecule mobility within heterogeneous biological environments.</p>