PROBING TWO-DIMENSIONAL SEMICONDUCTOR AND BIOLOGICAL TISSUE BY NONLINEAR OPTICAL MICROSPECTROSCOPY

Abstract

Two-dimensional materials, such as graphene and semiconductor transition metal dichalcogenides (TMDCs), exhibit remarkable optical properties which are of great potential for applications in modern electronics. The first part of this dissertation focuses on the dispersion of the second order resonant nonlinearity (χ(2)) in the single layer TMDC. We begin with the study of the nonlinear optical properties of monolayer TMDC, WSe2. We experimentally obtain the χ(2) dispersion data from the single layer sample of WSe2 by using broadband ultrashort pulse laser sources. The broadband pulse is generated by specially designed photonic crystal fiber (PCF). This PCF fiber is pumped by TiS mode-locked laser to generate continuum pulse that spans from visible to near-infrared. This continuum broadband pulse is used as a fundamental beam to generate signal at the second harmonic frequency in 2D semiconductor material. We detect the signal generated in the sample by using monochrometer and charge-coupled device (CCD), which provide the spectrum of the second harmonic signal that carries the signature of the materials. To get the images of these materials, we employ an optical parametric oscillator (OPO) tuning at reasonable wavelengths. Then we shine the beam on the sample, and after the signal has been generated in the sample, it gets reflected and this beam is then collected by photomultiplier (PMT) before angle scanned using galvo-mirror scanner to provide 200x200 µm2 imaging area. The dispersion obtained with better than 3 meV photon energy resolution showed peak value being within 6.3-8.4 x 10-19 m2/V range. We 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. In the second half of this work, we study two other materials. First, we show 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 our 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. Second, we demonstrate the quantitative spectroscopic characterization and imaging