Excited-state Condensation And Dynamics Of Exciton-polaritons In Optically-controlled Traps

Exciton-polaritons are half-light half-matter quasi-particles formed by the strong coupling between semiconductor excitons and microcavity photons.It has been continuously attracting lots of attention in the past decades due to the rich physics they carry and their potential application in optoelectronics.The effective mass of a typical polariton is only 10^-5 me（me:free electron mass in vacuum）,elevating the critical-temperature of Bose-Einstein condensation up to room temperature or even higher.On the other hand,the inherited excitonic component of polaritons leads to strong inter-particle Coulomb interactions,which generates a wide variety of nonlinear effects.Exciton-polariton is an ideal system for the study of many novel physics（such as Bose-Einstein condensation（BEC）,superfluidity,vortex,etc.）.In order to move forward to the wide application of exciton-polaritons in high-performance opto-electronic devices,such as all-optical devices,effective manipulation of the polariton condensate and its flow is essential.To this end,one efficient way is to confine them in potential traps.In recent years,people have introduced static traps which are usually realized by mechanically patterning the samples.The introduction of these static traps has led to a variety of interesting findings,such as evaporative cooling,binding and antibinding polariton states.However,such mechanically introduced traps have the disadvantages that the manufacturing process is complicated,and that the parameters cannot be changed once set.The generation and control of the reconfigurable and flexible potentials is a subject of intense study in recent years.In this thesis,we create dynamically adjustable potential wells in one-dimensional（1D）Zn O whispering gallery microcavities for exciton-polaritons by all-optical means.The condensation effect of exciton-polaritons and the behavior of pure quantum state transitions in such optically-controlled traps have been studied.Details of this thesis are listed as following:（1）Firstly,we grew one-dimensional Zn O whispering gallery microcavities with smooth surface and regular hexagonal cross-section by chemical vapor deposition method.Then we inject Gaussian potential barriers into these 1D Zn O microrods by means of non-resonant optical pumping.（2）Adjustable parabolic traps were created by our two-beam excitation scheme.By means of spatial confinement via these optical traps,the continuum polariton state was successfully quantized into harmonic oscillator states.The condensation characteristics of exciton-polariton in these harmonic traps were studied in detail by means of angle-resolved microscopic spectroscopy.（3）We report experimental studies on the dynamics of excited-state condensate for exciton-polaritons confined in an optically generated trap.Experimentally,we characterized the confined polariton condensate by varying the trap width and the barrier height.Theoretically,we calculated the spatial overlap between the polariton wavefunction and the excitonic reservoir.Direct comparison of these results verified that such polariton-reservoir overlap was responsible for the observed excitedstate polariton condensate.