Pritha Mondal, Subham Kumar Saha, Awadhesh Narayan, Anshu Pandey
We describe the process of electron condensation into a localized state in a structureless jellium that is coupled to a finite cavity. It is shown that there exists a temperature T_0 below which electrons within the jellium localize. This process is driven by enhancement of correlations between the electrons that are coupled to the cavity.
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In spite of decades of work it has remained unclear whether or not superradiant quantum phases, referred to here as photon condensates, can occur in equilibrium. In this Letter, we first show that when a non-relativistic quantum many-body system is coupled to a cavity field, gauge invariance forbids photon condensation. We then present a microscopic theory of the cavity quantum electrodynamics of an extended Falicov-Kimball model, showing that, in agreement with the general theorem, its insulating ferroelectric and exciton condensate phases are not altered by the cavity and do not support phot…
A system of magnetic molecules coupled to microwave cavities ($LC$ resonators) undergoes the equilibrium superradiant phase transition. The transition is experimentally observable. The effect of the coupling is first illustrated by the vacuum-induced ferromagnetic order in a quantum Ising model and then by the modification of the magnetic phase diagram of ${\rm Fe_8}$ dipolar crystals, exemplifying the cooperation between intrinsic and photon-induced spin-spin interactions. Finally, a transmission experiment is shown to resolve the transition, measuring the quantum electrodynamical control of …
We develop a quantum model for non-equilibrium Bose-Einstein condensation of photons and polaritons in planar microcavity devices. The model builds upon laser theory and includes the spatial dynamics of the cavity field, a saturation mechanism and some frequency-dependence of the gain: quantum Langevin equations are written for a cavity field coupled to a continuous distribution of externally pumped two-level emitters with a well-defined frequency. As a an example of application, the method is used to study the linearised quantum fluctuations around a steady-state condensed state. In the good-…
Theory of photon condensation in an arbitrary gauge condensed matter cavity model
推导了电磁场中凝聚物质转变为光子凝聚相的任意规范判据，证明其规范不变性，并揭示光子凝聚可在空间均匀场中发生且其磁电特性依赖于规范选择。
A model for direct two band excitons in a disordered quantum well coupled to light in a cavity is investigated. In the limit in which the exciton density is high, we assess the impact of weak `pair-breaking' disorder on the feasibility of condensation of cavity polaritons. The mean-field phase diagram shows a `lower density' region, where the condensate is dominated by electronic excitations and where disorder tends to close the condensate and quench coherence. Increasing the density of excitations in the system, partially due to the screening of Coulomb interaction, the excitations contributi…
We propose a way to increase the lifetime of two-dimensional direct excitons and show the possibility to observe their macroscopically coherent state at high temperatures. For a single GaAs quantum well embedded in photonic layered heterostructures with subwavelength period, we predict the exciton radiative decay to be strongly suppressed. Quantum hydrodynamic approach is used to study the Berezinskii-Kosterlitz-Thouless crossover in a finite exciton system with intermediate densities. Below the estimated critical temperatures, drastic growth of the correlation length is shown to be accompanie…
The realization of equilibrium superradiant quantum phases (photon condensates) in a spatially-uniform quantum cavity field is forbidden by a "no-go" theorem stemming from gauge invariance. We here show that the no-go theorem does not apply to spatially-varying quantum cavity fields. We find a criterion for its occurrence that depends solely on the static, non-local orbital magnetic susceptibility $χ_{\rm orb}(q)$, of the electronic system (ES) evaluated at a cavity photon momentum $\hbar q$. Only 3DESs satisfying the Condon inequality $χ_{\rm orb}(q)>1/(4π)$ can harbor photon condensation. Fo…
Raman-phonon-polariton condensation in a transversely pumped cavity
本文提出利用横向泵浦拉曼方案实现可调声子-光子耦合的声子极化激元凝聚，并通过Floquet稳定性分析给出超辐射态和激光态的相图，为工程电子关联态提供新工具。
Room-temperature cavity exciton-polariton condensation in perovskite quantum dots
研究在室温下利用钙钛矿量子点实现极子凝聚，通过可调谐光学谐振腔中的Gaussian形变形势阱，展示了极子凝聚的超线性强度依赖性和其他特性，为极子器件开发奠定基础。
The Bose-Einstein condensation (BEC) of magnetoexcitonic polaritons in two-dimensional (2D) electron-hole system embedded in a semiconductor microcavity in a high magnetic field $B$ is predicted. There are two physical realizations of 2D electron-hole system under consideration: a graphene layer and quantum well (QW). A 2D gas of magnetoexcitonic polaritons is considered in a planar harmonic potential trap. Two possible physical realizations of this trapping potential are assumed: inhomogeneous local stress or harmonic electric field potential applied to excitons and a parabolic shape of the s…
Photon condensation, Van Vleck paramagnetism, and chiral cavities
本文提出了一个平面方形分子与量子化空间变化腔电磁矢势耦合的规范不变模型，发现在时间手征腔中，分子处于Van Vleck顺磁区域时会发生向光子凝聚态的平衡量子相变。
We create exciton-polaritons in a zero-dimensional (0D) microcavity filled with organic ladder-type conjugated polymer in the strong light-matter interaction regime. Photonic confinement at wavelength scale is realized in the longitudinal direction by two dielectric Bragg mirrors and laterally by a sub-micron Gaussian-shaped defect. The cavity is separated into two parts allowing nanometer position control and enabling tuning of exciton and photon fractions of the polariton wavefunction. Polariton condensation is achieved with non-resonant, picosecond optical excitation at ambient conditions a…
The Bose-Einstein condensation (BEC) of the two-dimensional (2D) magnetoexciton-polaritons in microcavity, when the Landau quantization of the electron and hole states accompanied by the Rashba spin-orbit coupling plays the main role, were investigated. The Landau quantization levels of the 2D heavy holes with nonparabolic dispersion law and third order chirality terms both induced by the external electric field perpendicular to the semiconductor quantum well as the strong magnetic field B gives rise to the nonmonotous dependence on B of the magnetoexciton energy levels and of the polariton en…
We consider polariton condensation in a generalized Dicke model, describing a single-mode cavity containing quantum dots, and extend our previous mean-field theory to allow for finite-size fluctuations. Within the fluctuation-dominated regime the correlation functions differ from their (trivial) mean-field values. We argue that the low-energy physics of the model, which determines the photon statistics in this fluctuation-dominated crossover regime, is that of the (quantum) anharmonic oscillator. The photon statistics at the crossover are different in the high- and low- temperature limits. Whe…
We examine the Bose-Einstein condensation of exciton-polaritons in a semiconductor microcavity via an electrical current. We propose that by embedding a quantum dot p-i-n junction inside the cavity, the tunneling current through the device can reveal features of condensation due to a one-to-one correspondence of the photons to the condensate polaritons. Such a device can also be used to observe the phase interference of the order parameters from two condensates.
First-order photon condensation in magnetic cavities: A two-leg ladder model
本文提出通过 Peierls 代换耦合非均匀腔模的双腿自由费米子梯模型，揭示方形与三角形几何中的一阶光子凝聚相变，并以 DMRG 验证平均场解耦的局限及光-物质纠缠的作用。
In this thesis, we study two different aspects of many-particle physics. In the first part, we study the Bose-Einstein condensation of microcavity exciton-polaritons in different artificial lattices. Bose-Einstein condensation is a quantum phase transition, which allows the system to macroscopically occupy its ground state and develop coherence spontaneously. Often studied in microcavities, which are optical cavities that trap light at specific wavelengths, exciton-polaritons are a kind of quasiparticle arising from the strong coupling between quantum well excitons and cavity photons. By perio…
Dynamics and condensation of polaritons in an optical nanocavity coupled to two-dimensional materials
本研究通过激子反应坐标变换推导马尔可夫主方程，探究耦合二维材料的光学纳米腔中强耦合区的极化激元动力学，揭示了腔模宽度、温度等对极化激元凝聚的影响及极限温度的关键作用。
Interplay between photon condensation and electron-electron interactions in molecular systems
研究方形平面晶格分子模型中，通过顺磁Van Vleck机制产生的光子凝聚，发现除半填充和单电子情况外，电子-电子相互作用可导致一级相变。
The optical hybridization of the electronic states in strongly coupled molecule-cavity systems have revealed unique properties such as lasing, room temperature polariton condensation, and the modification of excited electronic landscapes involved in molecular isomerization. Here we show that molecular vibrational modes of the electronic ground state can also be coherently coupled with a micro-cavity mode at room temperature, given the low vibrational thermal occupation factors associated with molecular vibrations, and the collective coupling of a large ensemble of molecules immersed within the…
We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This is achieved through a careful balance between strong light-trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with moderate dielectric constant. This enables the formation of long-lived, dense 10~$μ$m - 1~cm scale cloud of exciton-polaritons with vacuum Rabi splitting (VRS) that is roughly 7\% of the bare exciton recombination energy. We introduce a w…
There is growing experimental interest in coupling cavity photons to the cyclotron resonance excitations of electron liquids in high-mobility semiconductor quantum wells or graphene sheets. These media offer unique platforms to carry out fundamental studies of exciton-polariton condensation and cavity quantum electrodynamics in a regime in which electron-electron interactions are expected to play a pivotal role. Focusing on graphene, we present a theoretical study of the impact of electron-electron interactions on a quantum Hall polariton fluid, that is a fluid of magneto-excitons resonantly c…
Absence of a Superradiant Phase Transition in Dirac Landau Polaritons
通过太赫兹光谱测量，发现石墨烯朗道极化子在超强耦合下未出现超辐射相变，排除了No-Go定理规避的可能性。
Cavity-polaritons in semiconductor microstructures have emerged as a promising system for exploring nonequilibrium dynamics of many-body systems. Key advances in this field, including the observation of polariton condensation, superfluidity, realization of topological photonic bands, and dissipative phase transitions, generically allow for a description based on a mean-field Gross-Pitaevskii formalism. While observation of polariton intensity squeezing and decoherence of a polarization entangled photon pair by a polariton condensate provide counter-examples, quantum effects in these experiment…
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