Nonlinear Optical Effects in the Picosecond Regime for Chemical Sensing and Polycrystalline Semiconductor Frequency Conversion

Abstract

In this work, we employ nonlinear optical spectroscopic techniques using picosecond visible, near-IR, and mid-IR laser pulses to resolve knowledge gaps in light-matter interactions of organic molecules and semiconductors. Nonlinear optics provides an array of different and unique capabilities compared to linear optics including dual-detection of IR-active vibrational modes and Raman-active modes of biomolecules and the generation of broadband IR continuum using high harmonic generation in polycrystalline semiconductors like zinc selenide.

Sections 2-4 utilize hyper-Raman scattering to study biomolecules in solution. In Section 2, we outline the design of a hyper-Raman microscope and present early work on dual detection of Raman and hyper-Raman scattered light from biomolecules. In Section 3, the hyper-Raman microscope system was used to explore the hyper-Raman allowed transitions of organic molecules in solution, and polarization resolved spectra of biomolecules was reported. In Section 4, we utilize the sensitivity of hyper-Raman to water librations to study water solvation chemistry in a mixed solution of water with dimethyl sulfoxide.

Sections 5-6 study high harmonic generation (HHG) in poly-crystalline Zinc Selenide (poly-ZnSe). Sections 5 and 6 contrast each other by performing the same experiment in different laser excitation regimes. In Section 5/6, the experiment is performed using high power (TW/cm2)/lower power (GW/cm2), broadband/narrowband, femtosecond (100 fs)/picosecond (30 ps), mid-IR laser pulses. In Section 5, we observe efficient frequency conversion via HHG resulting in a broad supercontinuum spanning the near-IR and visible region, a result that has been replicated by other research groups. However, understanding this efficient conversion is complicated by numerous nonlinear effects simultaneously occurring in the material. In Section 6, we approach frequency conversion in poly-ZnSe at lower power at the threshold of HHG generation to study the different nonlinear effects that contribute to broadband continuum generation. We find that random quasi-phase matching is an essential ingredient to the efficient frequency conversion in disordered, polycrystalline semiconductors like poly-ZnSe.