Femtosecond Dynamics Of Exciton-polariton Condensation At Room Temperature

Due to the strong coupling between light and matter,exciton polaritons have attracted much attention in both fundamental physics and optoelectronic applications because of their unique half-light and half-matter properties.The photon component of exciton polariton endows it with a very small effective mass(equivalent to 10^-5 times the mass of electrons),thus exhibits much novel physical phenomena.It is the ideal carrier to explore the Bose-Einstein condensate（BEC）,superfluidity,vortex and other macro quantum state.The exciton component reflectes in the interaction between particles,resulting in many nonlinear effects,such as parametric scattering,parametric amplification.The measurement of ultrafast dynamical behavior of exciton polaritons will further deepen the understanding of their physical mechanisms.The dynamics of exciton polaritons at room temperature occurs on a sub-picosecond time scale,revealing ultrafast dynamics with femtosecond time precision is of great significance for promoting frontier research on polaritons and the development of functional devices.Based on the self-developed femtosecond angle-resolved spectroscopic imaging（FARSI）technique,the femtosecond ultrafast dynamics of exciton polariton condensation in a one-dimensional Zinc Oxide（Zn O）whispering gallery microcavity has been measured.In our experiment,it is found that higher pump power leads to faster buildup dynamics of condensation,which is manifested in faster generation and earlier relaxation of exciton polariton condensation.Under the higher power excitation,the angular distribution contraction of momentum can be used as an additional criterion for the coherence of condensation.By measuring the second-order time dependence of the macroscopic quantum state of the exciton-polariton condensate at room temperature,this thesis confirms that the superbunching is the character of the exciton-polariton Bose cascade process.The power dependence experiment of the second-order time correlation shows that when there is only a single mode of condensation in the system,superbunching cannot be produced,while when there are two or more than two modes of condensation,the system exhibits the Bose cascade effect,accompanying with superbunching emission.The exciton component of exciton polariton makes it strong nonlinear effect.We characterize the interband parametric scattering behavior based on our FARSI technique,revealing that there is a hundred femtoseconds time-delay between the scattering source and the scattering signal in this process.In addition,this thesis proposed and demonstrated the exciton polariton ultrafast optical switch based on the photon parametric process,the high-speed regulation of exciton polariton BEC at room temperature has been realized.The photon component of exciton polariton can undergo parametric processes with photons of different frequency under the condition of energy and momentum conservation.The photons of exciton polaritons are scattered parametrically,resulting in the annihilation of the exciton polariton,showing that the exciton polaritons are quickly dissipated,and the condensed signal is truncated to realize the switching function.The switching speed reaches hundreds of femtoseconds,corresponding to the terahertz frequency response,the extinction ratio exceeds 60d B.By using femtosecond laser to construct parabolic potential well,two-color light field,multi-light field and other control methods to realize the ultra-fast control of exciton polariton.By using two spatially separated femtosecond pumps to jointly excite one-dimensional Zn O microrods,the resonant and non-resonant macroscopic quantum states can be realized by changing the power,the beam spacing,and the timedelay between laser pulses,all these changes are reflected in the width,depth and envelope shape of the potential well.In addition,based on the nonlinearity of the exciton material composition and the ultrafast properties of the photon composition,the room temperature exciton polariton temporal transistor is demonstrated,and the prototype devices such as temporal logic gate,temporal amplifier,multistage amplifier and ultrafast optical switch are realized.