Ultrafast Mechanisms of Nonlinear Refraction and Two-photon Photochromism

Derived from a material\u27s third-order nonlinearity, nonlinear refraction (NLR) occurs at any wavelength in any material, and may exhibit noninstantaneous dynamics depending on its physical origins. The main subject of this dissertation is to investigate the underlying mechanisms responsible for the NLR response in different phases of matter, e.g. liquids, gases, and semiconductors, by extensively using our recently developed ultrafast Beam Deflection (BD) technique. An additional subject includes the characterization of a novel two-photon photochromic molecule. In molecular liquids, the major nonlinear optical (NLO) response can be decomposed into a nearly instantaneous bound-electronic NLR (Kerr effect), originating from the real part the electronic second hyperpolarizability, ?, and noninstantaneous mechanisms due to nuclear motions. By adopting the methodology previously developed for carbon disulfide (CS2), we have measured the NLO response functions of 23 common organic solvents, providing a database of magnitudes and temporal dynamics of each mechanism, which can be used for predicting the outcomes of any other NLR related experiments such as Z-scan. Also, these results provide insight to relate solvent nonlinearities with their molecular structures as well as linear polarizability tensors. In the measurements of air and gaseous CS2, coherent Raman excitation of many rotational states manifests as revivals in the transient NLR, from which we identify N2, O2 and two isotopologues of CS2, and unambiguously determine the dephasing rate, and rotational and centrifugal constants of each constituent. Using the revival signal as a self-reference, ? is directly measured for CS2 molecules in gas phase, which coincides with the ? determined from liquid phase measurements when including the Lorentz-Lorenz local field correction. In semiconductors, the Kerr effect dominates the NLR in the sub-gap regime. Here, we primarily focus on investigating the dispersion of nondegenerate (ND) NLR, namely the refractive index change at frequency ?_a due to the presence of a beam at frequency ?_b. The magnitude and sign of the ND-NLR coefficient n_2 (?_a;?_b ) are determined for ZnO, ZnSe and CdS over a broad spectral range for different values of nondegeneracy, which closely follows our earlier predictions based on nonlinear Kramers-Kronig relations. In the extremely nondegenerate case, n_2 (?_a;?_b ) is positively enhanced near the two-photon absorption (2PA) edge, suggesting applications for nondegenerate all-optical switching. Additionally, n_2 (?_a;?_b ) exhibits a strong anomalous nonlinear dispersion within the ND-2PA spectral region, providing a large phase modulation of a femtosecond pulse with bandwidth centered near the zero-crossing frequency. Another subject of this dissertation is the characterization of a spiro-type two-photon photochromic molecule, in which Förster resonance energy transfer (FRET) is utilized to activate the ring-opening effect from a 2PA-donor chromophore. Evidence of energy transfer is observed via fluorescence measurements of the quantum yield, excitation spectra and anisotropy. The absorption and lifetime of the open form are measured in a dye-doped sol-gel matrix. Transient absorption measurements indicate both ring opening and closing occurs on a several picosecond time scale along with multiple transient photoproducts, from which a high FRET efficiency is measured in agreement with theoretical predictions. This efficient 2PA-FRET photochrome may be implemented into photonic devices such as optical memories. However, with a relatively small open-form absorption cross section and significant ring closing, the photochrome may not be viable for enhancing nonlinear absorption in applications such as optical limiting