Nonlinear optics refers to the interaction of intense light with matter, where the optical response of a material scales nonlinearly with the amplitude of the electric field. Nonlinear effects include multiphoton absorption phenomena and harmonic generation. In this talk, I will describe two applications of nonlinear optics for the characterization of laser ablation plasmas and for the microscopic analysis of solid substrates.
Generation of odd harmonics of a fundamental driving laser in centrosymmetric media, with pulses of nanosecond to femtosecond duration, yields coherent light in the in the vacuum ultraviolet and extreme ultraviolet regions of the spectrum. Recent research has revealed the advantages offered by laser ablation plasmas as nonlinear media, both in terms of increased conversion efficiency and extension of the harmonic spectrum towards shorter wavelengths. In the talk I will illustrate how the nonlinear response of laser ablation plasmas of solid targets, assists in differentiating distinct populations of species and enables the in-situ diagnosis of the complex ablation plasma environment. I will give examples covering the perturbative (low-order harmonics) and non-perturbative high harmonic generation regimes, with emphasis in the case of nanoparticle-containing ablation plasmas that lead to efficient conversion towards the short-wavelength range through high-harmonic generation.
The second application concerns the use of non‐linear optical microscopy (NLOM) for the three dimensional mapping and profiling of multi-layer structures. In NLOM imaging, near infrared, ultrafast femtosecond laser excitation exploits several nonlinear optical effects for high contrast imaging of solid samples with micrometric resolution. The imaging modes of Multi‐Photon Excited Fluorescence, and Second or Third Harmonic Generation provide non‐destructive accurate determination of thickness within multi‐layer, multi-component samples and objects and of the composition as a function of depth. Examples of applications of NLOM for characterization of solid substrates will be given highlighting the case of substrates of interest in Cultural Heritage.