Experimental Techniques For Nonlinear Material Characterization: A Nonlinear Spectrometer Using A White-light Continuum Z-scan

The main goal of this dissertation is to introduce and demonstrate a new method for the rapid determination of the nonlinear absorption spectra and the dispersion of the nonlinear refraction of optical materials in the visible and near IR spectral regions. However, conventional methods like, white-light continuum pump-probe and Z-scan techniques were used to measure the peak 2PA cross-sections for a number of commercially available photoinitiators. In the new method mentioned above, a high energy, broadband femtosecond white-light continuum is used to replace the single wavelength source conventionally used in a Z-scan experiment. In a Z-scan experiment, the transmittance of a focused beam through a sample is monitored as the sample travels through the focus, in the Z direction, along the focused beam. Providing the sample exhibits nonlinear absorption and/or refraction, the detector monitors a change in transmittance and/or a change in the beam divergence (if the energy is partially collected through an aperture in front of the detector). Replacing the single wavelength source with a white-light continuum allows for a much faster way of measuring nonlinear absorption/refraction spectra. This could eliminate the need for using other tunable sources (e.g. Optical Parameter Generators/Amplifiers) for nonlinear measurements. These sources made nonlinear spectroscopy using Z-scan experiments a time consuming task. This new source/method allows for rapid and simultaneous measurement of the nonlinear absorption spectrum and the dispersion of the nonlinear refraction. We have confirmed the functionality of the continuum as a source for nonlinear optical characterization of materials by using it to perform Z-scans on the well characterized semiconductors ZnSe and ZnS and on solutions of organic dyes

Sub‐40 fs, 1060‐nm Yb‐fiber laser enhances penetration depth in nonlinear optical microscopy of human skin

Advancing the practical utility of nonlinear optical microscopy requires continued improvement in imaging depth and contrast. We evaluated second‐harmonic generation (SHG) and third‐harmonic generation images from ex vivo human skin and showed that a sub‐40 fs, 1060‐nm Yb‐fiber laser can enhance SHG penetration depth by up to 80% compared to a [Formula: see text] , 800 nm Ti:sapphire source. These results demonstrate the potential of fiber‐based laser systems to address a key performance limitation related to nonlinear optical microscopy (NLOM) technology while providing a low‐barrier‐to‐access alternative to Ti:sapphire sources that could help accelerate the movement of NLOM into clinical practice

Characterisation of a dipolar chromophore with third-harmonic generation applications in the near-IR

E-2-Tricyanovinyl-3-n-hexyl-5-[4-{bis(4-n-butylphenyl)amino}-2-methoxystyryl]-thiophene, 1, has previously been used to demonstrate applications relying on frequency tripling of 1.55 μm light. Here we report the synthesis and chemical characterisation of 1, along with quantum-chemical calculations and additional experimental investigations of its third-order nonlinear properties that give more insight into its frequency tripling properties. Although 1 can be processed into amorphous films, crystals can also be grown by slow evaporation of solutions; the crystal structure determined by X-ray diffraction shows evidence of significant contributions from zwitterionic resonance forms to the ground-state structure, and reveals centrosymmetric packing exhibiting π–π and C–H···N≡C interactions. Both solutions and films of 1 exhibit near-infrared two-photon absorption into the low-lying one-photon-allowed state with a peak two-photon cross-section of cɑ. 290 GM (measured using the white-light continuum method with a pump wavelength of 1800 nm) at a transition energy equivalent to degenerate two-photon absorption at cɑ. 1360 nm; two related chromophores are also found to show comparable near-IR two-photon cross-sections. Closed-aperture Z-scan measurements and quantum-chemical calculations indicate that the nonlinear refractive index and third-harmonic generation properties of 1 are strongly dependent on frequency in the telecommunications range, due the aforementioned two-photon resonance