Third-harmonic generation for efficient frequency conversion and microscopic imaging.

摘要

Third-harmonic (TH) generation microscopy is a highly versatile technique with a broad range of applications that derives its contrast from inhomogeneities in the linear and nonlinear optical properties of materials. The work presented in this thesis centers on the use of TH generation for microscopic imaging of dielectric thin film coatings and air plasmas, and analyzes stacks of thin films for efficient TH generation. We analyzed theoretically and experimentally the influence of spherical aberration in TH generation and determined the conditions under which this effect can be neglected. These results are relevant for applications involving material characterization, such as the determination of third-order nonlinear susceptibilities. We developed a model for TH generation in stacked materials that takes into account interference effects and nonlinear frequency conversion simultaneously. Unlike previous models, we extend this approach to include the contribution from a substrate for focused beams and for illumination with ultrashort light pulses. The model was applied to determine the nonlinear susceptibilities of several oxide single-layer films. This model was also applied to design novel structures, consisting of an optimized sequence of films, for efficient TH generation. Considerable TH signal enhancement is predicted from such structures, a consequence of fundamental field enhancements and periodic corrections of the phase mismatch. Efficiencies are predicted to reach 20-30%, exceeding that of previous methods. We also demonstrated the value and sensitivity of TH microscopy (THM) to detect localized material anisotropies in transparent optical thin films. The effect of annealing and of different deposition techniques was investigated. The results suggest the potential use of THM as a tool for monitoring thin films during their manufacturing. THM was also applied to characterize laser damage morphology. We studied the TH signal generated by a weak probe beam intersecting transversely a pump laser-induced plasma in air and developed a model for the generation of such signal. Unlike in previous experiments, the intensity of the probe pulse was chosen small enough such that filamentation was avoided. We investigated the functional dependence of the TH signal with respect to the electron density, retrieved the nonlinear susceptibility of the plasma, and measured electron density distributions with sub picosecond time resolution.