UCLA

With the rapid development of mid-infrared laser sources with high peak power and ultrashort pulse durations, it is essential to understand the nonlinear optical properties of materials used for optical elements and photonic devices in the intensity regimes that are now accessible. Further progress in the field demands a new database for nonresonant nonlinear characteristics of widely used transparent mid-infrared materials. In this dissertation, we experimentally characterize the mid-infrared nonlinear optical response of semiconductor materials far from band gap resonances using orders of magnitude higher intensity and shorter pulse durations compared to previous studies with nanosecond laser pulses.This is first done using 200 ps, 10.6 μm CO2 laser pulses at intensities between 1–10 GW/cm2. The nonlinear refractive indices of mid-infrared transparent semiconductors GaAs, n-Ge, and ZnSe are determined, and observations of beat-wave nonlinearity enhancement in GaAs are attributed to a controllable free carrier nonlinearity. Unexpectedly high nonresonant nonlinear absorption is measured, which is a result of accumulated free carrier absorption effects over the course of the intense, picosecond pulses. Measurements of the same materials’ nonlinear optical response at similar intensities are made using 220 fs laser pulses around 10 μm produced via difference-frequency generation. Nonlinear refraction is shown to be nearly constant over this broad parameter range. However, nonlinear absorption of femtosecond pulses is found begin at much higher intensity and exhibit stronger intensity scaling than with picosecond pulses. Strong-field photoionization is discussed in the context of the Keldysh theory. Additionally, measurements of the third-order nonlinearity of the remarkable semiconductor Tellurium are made for the first time, demonstrating a giant nonlinear refractive index ranking among the largest known in a bulk material. Three-photon absorption is observed, and the interplay between strong nonlinear optical and propagation effects are investigated numerically by solving the two-dimensional generalized nonlinear Schrödinger equation. Finally, mid-infrared photonic applications are demonstrated. The first measurements of second harmonic generation using 3 ps CO2 laser pulses are made, showing promise for a future platform delivering high-power, high-energy laser pulses around 5 μm or a two-color mid-infrared source suitable for THz generation in air-plasma filaments. All-optical semiconductor switching is studied on femtosecond timescales, employing different materials and wavelengths, for future ultrafast pulse switching and modulation applications.