Confined States Tailoring Of Exciton-polaritons In A ZnO Whispering Gallery Mode Microcavity

As half-light half-matter quasiparticles formed by the strong coupling between excitons and photons in semiconductor microcavities,exciton-polaritons have been attracting a great deal of attention for many decades.Due to their inherited photonic components,Exciton-polaritons have extremely small effective masses,which are on the order of 10^–5 m_e(m_e:mass of electrons in vacuum).Because of such small effective masses,exciton-polaritons are ideal testing beds for experimental studies of macroscopic quantum coherence,such as Bose-Einstein condensation(BEC)at room temperature,superfluidity,and quamtum vortex.On the other hand,because of their extremely small effective masses,the de Broglie wavelength of exciton-polaritons are several orders of magnitude longer than those of electrons/atoms.As a result,tailoring of the energy states for exciton-polaritons is much easier than real atoms or electrons.For example,one can efficiently control the energy states of exciton-polaritons by confing them by a micrometer scale trap.Thanks to the rapid progresses made on the fundamental physics of exciton-polaritons in the past decades,a variety of polariton-based optoelectronic device prototypes have been developed,such as:exciton-polariton superlattices,polariton routers,polariton light emitting diodes,and polariton transistors.However,before commercial application of polariton-based devices,an efficient control of the energy states for exciton-polaritons is a must.Although lots of studies have already been carried out on this topic in the past decades,it is noteworthy that most of these pioneering work is essentially limited to the Ga As quantum well system,which is operable only at cryogenic temperature.However,from the point of view of commercial applications,devices that can be run at room temperature are much more favorable.In this aspect,wide band-gap semiconductors,such as Zn O,are promising candidates.Following these ideas,we carried out systematic studies on the tailoring of energy states of exciton-polaritons using one-dimensional Zn O microrods.Our main findings in this thesis are summarized in the following:Firstly,we realized an all-optically controlled dynamic superlattice for exciton-polaritons.In this work,we exploit the half-light half-matter nature of exciton-polaritons and successfully injected periodic potential onto the one-dimensional Zn O microwire for exciton-polaritons.Due to their inherited excitonic component,exciton-polaritons can interact with other charge carriers via repulsive Coulomb forces.However,as the effective masses of exciton-polaritons are extremely small,the exciton reservoir,which is usually injected by non-resonant optical pumping,behaves as an effective potential barrier for polaritons.Following this idea,we generate a spatially periodic laser spot and project it onto the Zn O microrod through our home-built angle-resolved micro-photoluminescence system.In this way,a periodic potential,i.e.a superlattice,is successfully introduced for exciton-polaritons.Moreover,as lifetime of excitons is limited(typically several hundreds of picoseconds),the superlattice built in this way is essentially dynamic.What's more,the periodicity of such optical superlattice can be adjusted in real time.In our experiment,we studied the optical properties of such exciton-polariton dynamic superlattice systematically.Theoretically,we carried out numerical simulations using the well-known Kronig-Penney model.The theoretical simulations are found to be in excellent agreement with our experimental results.Secondly,we performed systematic studies on the optical properties and dyamics of exciton-polaritons in a Fabry-Perot type microcavity.Due to the smooth facets of Zn O microrods grown by chemical vapor deposition method,the hexagonal cross-sections of the as-grown Zn O microrods form naturally whispering gallery microcavities.The strong coupling between the excitonic states and the whispering gallery cavity modes lead to the formation of exciton-polaritons.On the other hand,the facets on the two ends on Zn O microrod can form an additional Fabry-Perot type microcavity.This Fabry-Perot microcavity also have important effects on the optical properties,as well as dynamics,of the exciton-polaritons.In this work,we developed a modified Young's double-slit measurement system,which is suitable for semiconductor nanostructures,based on our home-built angle-resolved photoluminescence system.With this system,we carried out systematic studies on the optical properties and dynamics of exciton-polariton condensate at room temperature.We revealed the parities of the Fabry-Perot microcavity directly.Moreover,thorough power-dependent studies,we observed clearly mode competition behaviors for the exciton-polariton condensate.Theoretically,we developed a five-level rate equation mode and simulated the observed mode competition behaviors successfully.Finally,we realized a wavelength-tunable single-mode polariton laser with ultralow threshold.As bosonic quasi-particles,exciton-polaritons condense via a stimulated scattering process.The efficiency of this process depends sensitively on the population of both excitons and polaritons.Exploiting this property,we tried to achieve the spatial matching between the excitonic reservoir and the polariton wavefunction.By doing so,we successfully realized polariton lasing with ultralow threshod.Meanwhile,by tailoring the energy states of exciton-polaritons via a static trap,which is introduced by defects,we were able to selectively inject polaritons into one specific state and therefore generate single-mode polariton lasing.What's more,by introducing asymmetric traps,single-mode polariton lasing can be realized even at excited states.Our findings in this work can therefore serve as a guide for the design of high-efficiency polariton-based coherent light sources.