Studies On Coulomb Interactions And Relaxation Oscillations In Polariton Systems

Exciton-polaritons are quasi-particles formed by the strong coupling between excitons in semiconductors and photons confined in microcavities.They have been attracting lots of attention due to the rich physics they carry,such as Bose-Einstein condensation at room temperature,superfluidity,quantized vortices and solitons.Their unique characteristics also make them promising candidate for the development of new opto-electronic devices.The unique properties of exciton-polaritons stem essentially from their half-light half-matter nature.As a result of their excitonic component,exciton-polaritons can interact via Coulomb forces with each other or interact with other charge carriers.Though relatively weak,researches in the past decades,both theoretical and experimental,have shown that such Coulomb interactions play a critical role in the optical properties and dynamics of exciton-polaritons.Much of the fascinating polariton physics reported so far is closely related to such polaritonic interactions.On the other hand,as a result of their inherited photonic component,exciton-polaritons have extremely small effective masses(typically on the order of 10^-5 m_e,with m_e being free electron mass)and very short lifetime（typically a few picoseconds）.The extremely small effective masses render exciton-polaritons very large de Broglie wavelength,making them capable of Bose-Einstein condensing at room temperature or even higher.Their short lifetime,on the other hand,make them a nonequilibrium open-dissipative system with very rich dynamics.Thus,it is also very critical to reveal the dynamics,as well as the controlling mechanism,of the nonlinear polaritonic effects such as Bose-Einstein condensation and parametric scattering.Concerning these,in this thesis,we carry out a series of researches focusing on the polaritonic interactions,condensate and the relaxation dynamics.Our work can be summarized as the following:（1）Precise measurements on the polariton-polariton interaction constant.In our work,we managed to measure the polariton-polariton interaction constant with high precision in the following way:（1）We isolate polaritons from hot excitons and plasma via the ballistic transport of exciton-polaritons;（2）We calibrated the absolute polaritonic density by using the Bose-Einstein condensation of polaritons as a benchmark.Based on these strategies,we finally obtain the exact value of polariton-polariton interactions and verified that it is larger than theoretical prediction by two orders of magnitude.（2）Room temperature Bose-Einstein condensation of exciton-polaritons and their parametric scatterings in one-dimensional（1D）whispering gallery（WG）microcavities.Thanks to the relatively large excitonic binding energies and high spatial overlap between the gain material and optical resonant mode in 1D ZnO WG microcavities,we realized successfully the Bose-Einstein condensation of exciton-polaritons at room temperature.Then we further studied the effects of excitonic fraction on the condensation of polaritons.Based on these,we continued to study the parametric scatterings as well as ballistic transport of exciton-polaritons.We verified that signal of parametric scatterings comes from the optical pumping region while that of ballistic transport stems from area outside the pumping region.These results provide us a new strategy to generate parametric scattering signals with high quality.（3）Studies on the relaxation oscillations of exciton-polaritons driven by parametric scattering.Our work in this part focuses on this topic.By growing high quality ZnO microrods and developing angle-resolved spectroscopic technique with spatial filtering,we managed to observe polariton-polariton parametric scattering signals with very high signal-to-noise ratio.What’s more,we observed interference patterns in the source and signal/idler beams for pumping power above threshold.By means of theoretical simulations using the open-dissipative Gross-Pitaevskii equations,we verified that the observed interference patterns stem from the relaxation oscillations of exciton-polaritons driven by parametric scattering processes.This is a completely new mechanism for relaxation oscillations that has never been explored before.It represents a breakthrough in the understanding of light-matter condensation and could find application in a variety of polariton-based quantum devices.Moreover,in this work we show that by utilizing the polariton-reservoir interactions,polariton dynamics can be projected from the time domain to the energy space.This produces an alternative way for experimental studies of polariton dynamics with greatly reduced complexity.