Polaron Polaritons in the Integer and Fractional Quantum Hall Regimes

Abstract

Elementary quasi-particles in a two dimensional electron system can be described as exciton-polarons since electron-exciton interactions ensures dressing of excitons by Fermi-sea electron-hole pair excitations. A relevant open question is the modification of this description when the electrons occupy flat-bands and electron-electron interactions become prominent. Here, we perform cavity spectroscopy of a two dimensional electron system in the strong-coupling regime where polariton resonances carry signatures of strongly correlated quantum Hall phases. By measuring the evolution of the polariton splitting under an external magnetic field, we demonstrate the modification of electron-exciton interactions that we associate with phase space filling at integer filling factors and polaron dressing at fractional filling factors. The observed non-linear behavior shows great promise for enhancing polariton-polariton interactions. PACS numbers: 71.36.+c, 73.43.Fj Strong coupling of excitons in a semiconductor quantum well (QW) to a microcavity mode leads to formation of quasiparticles called cavity exciton polaritons [1]. Polaritons have played a central role in the investigation of nonequilibrium condensation and superfluidity of photonic excitations [2,3]. While polaritons acquire a finite nonlinearity due to their exciton character, interactions between polaritons in undoped QWs are not strong enough for realizing strongly interacting photonic systems [4]. Two-dimensional electron systems (2DES) evolving in large magnetic fields, in contrast, are a fertile ground for many-body physics due to prominence of electron-electron interactions. Formation of skyrmion excitations in the vicinity of filling factor ν = 1 is a consequence of such interactions. More spectacularly, electron correlations lead to the formation of fractional quantum Hall (FQH) states where the ground state exhibits topological order [5][6][7]. Moreover, it has been proposed that a sub-class of FQH states exhibit nonabelian quasi-particles which can be used to implement topological quantum computation [8]. The nature of optical excitations of a 2DES have also recently generated lots of interest: experimental and theoretical [9, 10] studies in transition metal dichalcogenide (TMD) monolayers have established that these excitations should be described in the framework of the Fermi polaron problem, as a collective excitation resulting from exciton-electron interactions [11][12][13][14]. In this context, optical excitation of an exciton leads to generation of an electron screening cloud that results in formation of a lower energy attractive exciton-polaron [9,10]. In this work, we report corresponding signatures in GaAs, where energy scales are known to differ significantly compared to TMD monolayers [15,16], due to a particularly small binding energy of the bound-molecular trion state. More importantly, our work presents experimental signatures of polaron formation at nonzero magnetic fields where electrons are confined to the lowest Landau level.It has r...

Nonlinear optics in the fractional quantum Hall regime

“…A consistent description of the new excitonic excitations that emerge in the presence of a 2DES was first provided by R. Suris [40], and were later termed exciton-polarons [8][9][10]. Strong coupling to an optical mode in turn results in the formation of polaron-polaritons as the elementary excitations of the 2DES-cavity system [9,11]. Under an external magnetic field B orthogonal to the 2DES surface, discrete Landau levels LLn e (LLn hh ) form out of the conduction (heavy-hole valence) band.…”

“…1b). Saturation could also be a consequence of (slow) light-induced modifications of n e , which may start to play a role at the highest powers investigated [11].…”

“…We observe generic strong dispersion of the polariton energies with magnetic field around integer and fractional values of ν. This striking behavior of the linear optical spectrum stems from strong modification of electron-exciton interactions in and around gapped quantum Hall states [11], which in turn leads to a ν-dependent modification of the cavity-polaron coupling strength. Especially striking are the energy shifts experienced by the lower polariton in σ − polarization (LP σ − ) at filling factors ν = 1, 2/3 and 2/5.…”

Accelerating Polaritons with External Electric and Magnetic Fields

“…This effect, well known in 2DEG transport [52,53], results from a trade-off in our device geometry between optimizing the optical properties and retaining good transport characteristics. It can be suppressed by thinning down the doping QWs, at the expense of an increased lightsensitivity of the device [36].…”

“…This QW is remotely doped from both sides by Si impurities embedded in thin GaAs QWs (10 nm thickness) located at the nodes of the cavity field. The doping QWs thus provide electrons to the optically active QW where a 2DEG is formed with nominal electron density of n e = 0.33 · 10 11 cm −2 and mobility µ = 1.6 · 10 6 cm 2 V −1 s −1 (see [36] for further details on the sample fabrication). To study the interplay between polariton transport and 2DEG physics, the sample is etched in the form of a Hall bar with annealed electrical contacts to the 2DEG, as depicted in Fig.…”