Speaker: Qihua Xiong (School of Physical and Mathematical Sciences Nanyang Technological University, Singapore 637371)

Time: September 21, 2017 14:00

Place: 物理系理科楼C302报告厅

Abstract:The interaction of light with matter gives rises to a wide range of linear and nonlinear phenomena that we are familiar with, such as absorption and scattering, spontaneous or stimulated emission, and second harmonic generation. In polar semiconductors, the electromagnetic field of light polarizes the matter leading to the formation of elemental excitations such as excitons and exciton polaritons, due to long-range dipolar force as well as additional coupling to the optical fields. In this talk, I will first introduce the background of exciton, exciton polaritons and the electron/exciton-longitudinal optical phonon (LOP) interactions in semiconductors. I will then present the first fluorescence laser cooling of semiconductors based on cadmium sulfide nanoribbons, enabled by enhanced strong exciton-LOP coupling at nanoscale. In zinc telluride nanoribbons, resolved-sideband Raman cooling of LOPs can be realized with a similar physical picture of cavity optomechanics, in which the excitonic mode was utilized to assist the photoelastic Raman scattering from the LOPs. By detuning the laser pumping, the dipole oscillation of the LOPs is photoelastically attenuated (enhanced) to a colder (hotter) state, corresponding to the laser cooling of amplification of the LOPs respectively. Finally, I will introduce our recent work on strong light-matter coupling in all-inorganic perovskite crystals embedded in optical microcavities. Those prototype semiconductors have exceptionally large exciton binding energy, strong oscillator strength and facile epitaxy-free growth. Room temperature exciton polariton lasing can be realized, which was unambiguously evidenced by a superlinear power dependence, macroscopic ground state occupation, blueshift of ground state emission, and the build-up of long-range spatial coherence, suggesting considerable promise of room temperature polariton devices and coherent light sources extending from the ultraviolet to near infrared range. Future work will be briefly discussed, with a particular interest on transient spectroscopy investigations of semiconductors.

Biography: Qihua Xiong received his B.S. degree in physics from Wuhan University in 1997, and then finished three years graduate studies at the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. He went to the United States in 2000 and received Ph.D. degree under the supervision of Prof. Peter C. Eklund from The Pennsylvania State University in 2006. After three years postdoctoral experience in Prof. Charles M. Lieber’s group at Harvard University, he joined Nanyang Technological University as an assistant professor in 2009 and promoted to Nanyang Associate Professor in 2014. He was promoted to full Professor in 2016 recently. He is a Fellow of Singapore National Research Foundation awarded in 2009 and the inaugural NRF Investigatorship Award by Singapore National Research Foundation in 2014. He is the recipient of IPS Nanotechnology Physics Award (2015) and Nanyang Award for Research Excellence of NTU (2014). Prof. Xiong’s research focuses on light-matter interactions of emergent quantum matter by optical spectroscopy approaches. He recently ventured into the field of 2D layered materials and laser cooling of solids.