ULTRAFAST COHERENT SPECTROSCOPIES AND LASER SOURCES BASED ON THIRD-ORDER NONLINEAR OPTICAL PHENOMENA

This document is composed of four different research efforts. In the first chapter of this dissertation, I present my experimental work on seeded Yb-doped Q-switched fiber laser based on stimulated Brillouin scattering (SBS). I began by experimenting with different types of fiber laser cavity configurations in order to find an ideal cavity that would successfully generate the free-running stochastic nonlinear SBS mirror as well as to reinforce the injected light mode. Experimentally, the ideal configuration involved a diffraction grating as the cavity back-mirror in the Littrow configuration with added cavity losses to enhance the SBS effect. Single SBS pulses exhibited the cascaded SBS effect and pulse width (FWHM) typically less than 2 ns. Continuous sine wave modulated light, continuous triangular wave modulated light and a dynamic pulsed halfsine modulated light injections successfully synchronized the SBS repetition rate, and to some degree the pulse-to-pulse intensity, to the waveform frequency. The injected waveforms effectively seeded the SBS Q-switching mechanism and stabilized the laser repetition rate in a novel way. Output spectrum showed the injected waveforms nearly, but not completely, suppressed the free-running cavity modes due to fiber terminations and the output pulse wavelength was slightly blue-shifted compared to the injection spectrum. This was a curious result and is under further investigation. In the second part of this work, I show experimentation regarding the resolution of fine spectral features within several Raman active vibrational modes in potassium titanyl phosphate (KTP) crystal. Measurements are performed using a femtosecond time-domain coherent anti-Stokes Raman scattering spectroscopy technique that is capable of delivering equivalent spectral resolution of 0.1 cm-1. The Raman spectra retrieved from my measurements show several spectral components corresponding to vibrations of different symmetry with distinctly different damping rates. In particular, linewidths for unassigned optical phonon mode triplet centered at around 820 cm-1 are found to be 7.5±0.2 cm-1, 9.1±0.3 cm-1, and 11.2±0.3 cm-1. Results of these experiments will ultimately help to design an all-solid-state source for sub-optical-wavelength waveform generation that is based on stimulated Raman scattering. In the third section of this document, I demonstrate and analyze a series of experiments in traditional and soft condensed matter using coherent optical spectroscopy and microscopy with ultrafast time resolution. I will show capabilities of resolving both real and imaginary parts of the third-order nonlinearity in the vicinity of Raman resonances from a medium probed within microscopic volumes with an equivalent spectral resolution of better than 0.1 cm-1. I can differentiate between vibrations of various types within unit cells of crystals, as well as perform targeted probes of areas within biological tissue. Vibrations within TiO6 octahedron and the ones for the Ti-O-P intergroup were studied in potassium titanyl phosphate crystal to reveal multiline structure within targeted phonon modes with closely spaced vibrations having distinctly different damping rates (~0.5 ps-1 versus ~1.1 ps-1). I also detected 1.7-2.6 ps-1 decay of C-C stretching vibrations in fat tissue and compared that with the corresponding vibration in oil. I lastly demonstrate an effective microspectroscopy technique by tracing the dispersion of second order nonlinear susceptibility (χ(2)) in a monolayer tungsten diselenide (WSe2). The χ(2) dispersion obtained with better than 3 meV photon energy resolution showed peak value being within 6.3-8.4×10-19 m2/V range. I estimate the fundamental bandgap to be at 2.2 eV. Sub-structure in the χ(2) dispersion reveals a contribution to the nonlinearity due to exciton transitions with exciton binding energy estimated to be at 0.7 eV