Novel Optical Applications Based on Photon-phonon Interactions

Abstract

The phenomena of photon-phonon interactions can be found in all forms of matters including gases, plasma, liquids and solids. The applications based on such interactions, including Raman scattering, Bragg Scattering, polariton resonance, phonon-assisted Antistoke photoluminescence, etc. has been intensively investigated. In this dissertation, we present our study of three novel applications in the field of THz generation, hot phonons in transistors, and optical refrigeration. In Chapter 1, we studied the backward propagating Terahertz (THz) generation using optical rectification in periodically poled LiNbO3 and LiTaO3 samples with ultrafast laser pulse excitation. With the LiNbO3 sample, we have generated the highest frequency at 4.8 THz at the poling period of 7.1 µm, corresponding to an output wavelength of 62.5 µm. We have observed an enhancement factor as large as 61 in the output power comparing to that generated from bulk LiNbO3, which was attributed to the phonon polariton resonance-enhanced nonlinear optical coefficients. For the LiTaO3 samples, we have reached the highest output power of nearly 100 µW. Based on our study, the effective second-order nonlinear coefficient of LiTaO3 are enhance by factors of from 3.7 to 23, leading to the enhancement of THz output powers. The enhancement is rooted in a polariton resonance at the frequency of 127 cm-1, which can be induced by the nonlinear mixing of two transverse-optical phonons due to strong anharmonicity of LiTaO3. We also designed a second wafer with significantly shorter poling periods, and indeed we have observed the entire resonant peak. In Chapter 2, we studied the hot phonon behavior of GaN high electron mobility transistors (HEMT). We mainly investigated our effort on two methods utilizing Raman scattering to measure the phonon temperature, i.e. the hot phonon population of GaN HEMT device under operation. The ultimate goal was to employ these methods on the study of isotope disorder introduced GaN device and verify whether its phonon behavior is optimized than that in normal devices. The first method extracts phonon temperatures from the ratio of Antistokes and Stokes Raman signal intensities, which requires complex experimental procedures and tendency to wrong temperature deductions. The second method is based on the fitting of phonon temperature to the shift of Stokes Raman peak model, which leads to simple and fast measurement while sophisticated analysis with strong dependence to sample material properties. Comparing two methods, we believe the second one is advantageous due to our limited experimental condition, and it can be improved with proper calibration of the model. In Chapter 3, we studied the upconverstion of photoluminescence (PL) from both a free-standing bulk GaN sample and a GaN nanowire sample. When the excitation energy is in the tail of bandgap edge, the PL upconverstion can be attributed to phonon-assisted Antistokes photoluminescence (ASPL). We explored the potential of laser cooling based on such a phenomena with the analysis of PL intensity trending with pump power, excitation wavelength, and temperature. Such analysis proves the fact that the ASPL we measured is originated in single photon process assisted by phonons