Exciton And Exciton-polariton Condensation And Related Josephson Effect

Exciton (electron-hole pair) is an elementary excitation, which forms by the Coulomb attractive interaction between an electron in the conduction band and a hole in the valence band, and has important influence on the optical properties of semiconduc-tor. In the low density regime, excitons can be considered as bosons, and condense under certain conditions. Exciton polariton is a new elementary excitation, and is born in the strong coupling between exciton and photon. It has some properties of exciton and pho-ton simultaneously. Usually, semiconductor microcavity is a good platform to investigate exciton polaritons, and gives many interesting physical phenomena. Exciton polaritons are bosonic when the density is low, and their critical temperature of condensation is high because of the very small effective mass.In this dissertation, we study exciton and exciton-polariton condensation and related Josephson effect. To obtain higher critical temperature of condensation, we investigate exciton condensation and related properties in the spatially separated graphene double layer. On this basis, thermal transport is studied in the exciton Josephson junction. At the same time, the condensation and Josephson effect of exciton polaritons are discussed in the planar semiconductor microcavity, and different methods are put forward to modulate this effect. Comparing the exciton-polariton Josephson effect with that in superconduct-ing tunnel junctions, we find that there are some similarities as well as differences, when the interactions between exciton polaritons is fully taken into account. In detail, all the work is organized as follows: In chapter one, we first briefly introduce the two kinds of elementary excitations, i.e., exciton and exciton polariton, and related properties. Then the experimental devices are given to investigate these excitations. At last, the major theoretical methods we used are summarized.In chapter two, we study the exciton condensation and Coulomb drag effect in a spa-tially separated graphene double layer. First we give the critical density of particles for condensation to start at zero temperature, and it is related with the permittivity and thick-ness of the dielectric film between the two sheets of graphene. In this structure, excitons condense when the density of particles is low and distance between two graphene mono-layers is small. By introducing the ground-state fidelity, we obtain the phase diagram of exciton condensate, which accurately differentiates the Bose-Einstein condensate (BEC), Bardeen-Cooper-Schrieffer state (BCS), and their crossover. For a given gate voltage, we find the superfluid portion lowers the drag conductivity. As the gate voltage increases, there exists a minimum drag conductivity, which comes from the combined effect of the longitudinal conductivity in each graphene layer and superfluid density. This result is applicable to detect the exciton condensate experimentally.In chapter three, we take advantage of the high critical temperature of exciton con-densation to construct an exciton Josephson junction in graphene double layer and inves-tigate the thermal transport. The total heat current, including the quasiparticle current and interference current, can be controlled by the temperatures of the two condensates. When a temperature is higher than the critical value, the interference current vanishes, but the quasiparticle current still exists. Importantly, we find that there are two physical effects indispensable in designing thermal devices:One is the thermal rectifier with the rectification ratio up to3.3×104%as appropriate parameters are used; the other is the thermal logic gate, and its on and off states are operated easily. These results are feasible to experimentally verify under the present conditions.In chapter four, we discuss the polarizations and phases in an exciton-polariton am-plifier based on a semiconductor microcavity, and find the signal amplification is greatly affected by the polarizations and relative phase of the signal, pump, and idler excitation lasers. When the intensities of three lasers are fixed, the signal amplification is largest at certain relative phase and circular polarization degrees. Interestingly, the light from the signal state can be modulated continuously from the left to the right circular polarization or vice versa with the relative phase. This result can be used to provide a stable source of polarized light in the spin-dependent optoelectronic devices.In chapter five, we introduce a double-trap geometry in the planar semiconductor microcavity, and consider the exciton-polariton Josephson effect. We find there are two different methods to modulate this effect effectively. One is an external magnetic field normal to the plane of the microcavity. We obtain a critical magnetic field, below which only the dc Josephson effect exists. When the magnetic field is larger than the critical value, the ac Josephson effect occurs, and the Josephson frequency increases with the magnetic field linearly. These phenomena reflect the remarkable competition between the Zeeman energy and the interactions of exciton polaritons. Besides, the spontaneous polarization separation and macroscopic quantum self-trapping are realized by regulating the magnetic field. The other method is using a uniaxial stress. When the stress increases, we get two critical transition stresses, which correspond to the0-π and dc-ac transition, respectively. Importantly, the Shapiro steps still emerge in the system with interaction, but some steps may be suppressed by the interaction, which is an obvious difference with the superconducting Josephson effect.In the last chapter, we give a summary of our work and make some expectation for the future investigation.