Browsing by Subject "Polariton"
Now showing 1 - 3 of 3
- Results Per Page
- Sort Options
Item Direct electrical control of exciton and polariton condensates(2017-08-11) Xie, Ming, 1988-; MacDonald, Allan H.; Demkov, Alexander A; Fiete, Gregory A; Tutuc, Emanuel; Zhou, JianshiExcitons are among the most ubiquitous bosonic quasiparticle excitations in solid state systems whose lifetime can be extended such that quasi-thermal equilibrium states can be reached. They have thus been promising subjects for the study of Bose-Einstein condensation (BEC) physics during the last few decades. Indeed, convincing evidence for BEC of excitons and exciton polaritons, which are coupled state of quantum-well excitons and cavity photons, have been reported. In this dissertation, we study the property of two dimensional exciton and exciton-polariton condensates driven by external electric bias and discuss possible electro-optic device applications. In the first chapter, we give a brief introduction to the basic concept of excitons, exciton-polaritons and their condensation, each followed by a short survey of the past and ongoing researches in their respective fields. The second chapter presents our study on the electrically controlled coherent excitonic steady state in two dimensional transition-metal dichalcogenide (TMD) double-layer system. We point out that a pair of electrical contacts with chemical potentials separated by less than the spatially indirect band gap of a 2D double-layer can be used to establish an exciton reservoir with an electrically controlled chemical potential. Equilibration between the exciton fluid and the contacts proceeds via a process involving virtual intermediate states in which an unpaired electron or hole occupies a free carrier state in one of the 2D layers. We relate the equilibration rate between excitons and contacts to the tunneling conductances between the contacts and the individual 2D layers when the contact chemical potentials align with the free-carrier bands. As an initial exploration of how spatially indirect 2D exciton systems can be studied and manipulated electrically, we explain how dc and ac voltage variation can be used to measure thermodynamic properties of the exciton fluid and to characterize the strength of its coupling to the reservoir. In the third chapter, we study the thermo-electric transport properties of excitons. Based on the two-particle tunneling process discussed in previous chapter, we propose a possible realization of a thermo-electric cooling device in which indirect excitons act as the heat carriers. Excitons are injected from the cold side of the device and flow downhill a chemical potential gradient, which is maintained by the bias voltage difference set up between the cold and the hot side, to the hot side where they break into individual electrons and holes exiting to the electrodes. In the fourth chapter, we present a theory of exciton or exciton-polariton supercurrent generation. We describe a mechanism by which an electrical bias voltage applied across a unipolar semiconductor quantum well can drive an exciton or polariton supercurrent. The mechanism depends on the properties of electronic quasiparticles in quantum wells or two-dimensional materials when they are dressed by interactions with the coherent exciton field of an exciton condensate or the coherent exciton and photon fields of a polariton condensate, and on approximate conservation of the sum of the total photon and exciton numbers. We propose experiments that can be performed to realize this new light-matter coupling effect, and discuss possible applications. Assuming the dominant process for electron transfer between conduction and valence bands is by scattering off the condensate, electrical bias voltages can be used to control the condensate. We study the in-plane transport properties of electrical current through the unipolar system, and show how the coherent condensate fields respond to the current flow. The possibility of tailoring light via electrical current and vice versa simultaneously might lead to interesting new applications.Item Interactions and topology in two-dimensional semiconductor and semimetal(2018-09-14) Xue, Fei, 1990-; MacDonald, Allan H.; Chelikowsky, James R; Fiete, Gregory A; Register, Lenoard FThis dissertation presents our study of interaction and topology effects in two-dimensional semiconductor and semimetal. In the first part, we explore the ground state thermodynamic phases in various systems using a mean-field theory including long-range Coulomb interaction. In particular, we have discovered two distinct phases with different number of exciton condensate flavors in a bilayer system of two-dimensional semiconductors with spin/valley degree of freedoms. The exciton condensate phase is characterized by spontaneous inter-layer phase coherence and counterflow superfluidity. We have also studied the phase diagram of a model quantum spin Hall system as a function of band inversion and band-coupling strength, demonstrating that when band hybridization is weak, an interaction-induced nematic insulator state emerges over a wide range of band inversion. We also develop a fully microscopic theory of equilibrium polariton condensates that treats the two-dimensional quantum well band states explicitly and goes beyond the commonly used model in which bare excitons are treated as Bose particles that are coupled via flip-flop interactions with cavity photons. In the second part, we present our study of collective excitations in an exciton condensate using a time-dependent mean-field theory. By constructing a fluctuation Lagrangian based on a variational wave-function capturing exciton density and phase fluctuations, we are able to get the collective mode spectra and demonstrate the stability of exciton condensate phase. Moreover, we apply a linear response theory to identify a number of collective modes with a strong electron-hole pairing amplitude(Higgs-like) component. Next, we generalize our time-dependent mean-field theory to finite temperature to address the long debate question that whether a CDW phase of a semimetal is due to Coulomb interaction or electron-phonon interaction. Our theory includes both intraband and interband excitations and direct and exchange interactions, and allows for the formation of exciton condensates at low temperatures.Item Novel tools for ultrafast spectroscopy(2011-12) Jarvis, Thomas William; Li, Elaine; Fink, Manfred; Keto, John; Lim, Sang-Hyun; Shih, Chih-Kang; Sitz, GregExciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described.