Optically induced topological states on the surface of mercury telluride

This is the final version. Available from the American Physical Society via the DOI in this recordWe developed the theory which describes the Floquet engineering of surface electronic modes in bulk mercury telluride (HgTe) by a circularly polarized electromagnetic field. The analysis shows that the field results in the appearance of the surface states which arise from the mixing of conduction and valence bands of HgTe. Their branches lie near the center of the Brillouin zone and have the Dirac dispersion characteristic for topological states. Besides them, the irradiation induces the gap between the conduction and valence bands of HgTe. Thus, the irradiation can turn mercury telluride into a topological insulator from a gapless semiconductor. It is demonstrated that the optically induced states differ substantially from the nontopological surface states existing in HgTe without irradiation. The structure of the found states is studied both analytically and numerically in the broad range of their parameters.European Union Horizon 2020RannísRussian Foundation for Basic ResearchMinistry of Education and Science of Russian FederationGovernment of the Russian Federatio

Nonlinear Quantum Optics with Trion Polaritons in 2D Monolayers: Conventional and Unconventional Photon Blockade

This is the final version. Available from the American Physical Society via the DOI in this recordWe study a 2D system of trion-polaritons at the quantum level and demonstrate that for monolayer semiconductors they can exhibit a strongly nonlinear optical response. The effect is due to the composite nature of trion-based excitations resulting in their nontrivial quantum statistical properties, and enhanced phase space filling effects. We present the full quantum theory to describe the statistics of trion-polaritons, and demonstrate that the associated nonlinearity persists at the level of few quanta, where two qualitatively different regimes of photon antibunching are present for weak and strong single photon-trion coupling. We find that single photon emission from trion-polaritons becomes experimentally feasible in state-of-the-art transition metal dichalcogenide (TMD) setups. This can foster the development of quantum polaritonics using 2D monolayers as a material platform.Government of the Russian FederationITMO Fellowship and Professorship ProgramRussian Science FoundationEngineering and Physical Sciences Research Council (EPSRC

Spinning polariton vortices with magnetic field

This is an accepted manuscript of an article published by American Physical Society in Physical Review B on 12/03/2020, available online: https://doi.org/10.1103/PhysRevB.101.104308 The accepted version of the publication may differ from the final published version.We study the formation dynamics of spinor polariton condensates trapped in ring-shaped confining potentials created by excitonic reservoirs. We consider in detail the interplay of the effective spin-orbit interaction provided by transverse electric and transverse magnetic photonic modes splitting (TE-TM splitting) and exciton Zeeman splitting provided by an external magnetic field. We demonstrate that tuning of the trap size obtained by shaping of the external nonresonant and depolarized pumping allows formation of pairs of half-vortices of topological charges ±1/2 in both spin components. Further, we show that the probabilities of the realizations of four possible vortex configurations strongly depend on the value of the magnetic field. For certain values of the field, the probability of the formation of a vortex with desired topological charge reaches 90%, which opens the possibility of on-demand control of angular momentum of quantum fluids of light with a magnetic field.Published versio

Ab-initio study of electronic properties of a two-dimensional array of carbon nanotubes

This is the final version. Available from IOP Publishing via the DOI in this record. The equilibrium geometry and electronic band structure of a planar array of carbon nanotubes are studied with the use of the Quantum Espresso code - a plane-wave realisation of the density functional theory (DFT). The many-electron correlations and van der Waals corrections are taken into account. The optimal distance between nanotubes in the array corresponding to the minimum of the total energy of the system is found. A strongly anisotropic hyperbolic dispersion is demonstrated for low-energy charge carriers in an array of quasi-metallic (15,0) carbon nanotubes with the optimal inter-tube separation governed by van der Waals forces.Icelandic Research FundEuropean Union Horizon 2020Ministry of Education and Science of Russian FederationGovernment of Russian Federatio

Stimulated emission of terahertz radiation by exciton-polariton lasers

We show that planar semiconductor microcavities in the strong coupling regime can be used as sources of stimulated terahertz radiation. Emitted terahertz photons would have a frequency equal to the splitting of the cavity polariton modes. The optical transition between upper and lower polariton branches is allowed due to mixing of the upper polariton state with one of the excited exciton states and is stimulated in the polariton laser regime

Robust polaritons in magnetic monolayers of CrI3

This is the final version. Available from the American Physical Society via the DOI in this recordWe show that the regime of strong light-matter coupling with remarkable magnetic properties can be implemented in systems based on monolayers of chromium triiodide (CrI3). This two-dimensional material combines the presence of strongly bound excitonic complexes with ferromagnetic ordering below the Curie temperature. Using microscopic first-principles calculations, we reveal a rich spectrum of optical transitions corresponding to both Wannier- and Frenkel-type excitons, including those containing electrons with an effective negative mass. We show that excitons of different polarizations efficiently hybridize with a photonic mode of a planar microcavity. Due to the peculiar selection rules, polariton modes become well resolved in circular polarizations. The strong optical oscillator strength of excitons and cavity confinement leads to large values of the Rabi splitting, reaching 35 meV for a single monolayer and giant Zeeman splitting between polariton modes of up to 20 meV. This makes CrI3 an excellent platform for magnetopolaritonic applications.HSE UniversityEngineering and Physical Sciences Research Council (EPSRC)NATOIcelandic Research FundMinistry of Science and Education of Russian Federatio

Driven-dissipative spin chain model based on exciton-polariton condensates

An infinite chain of driven-dissipative condensate spins with uniform nearest-neighbor coherent coupling is solved analytically and investigated numerically. Above a critical occupation threshold the condensates undergo spontaneous spin bifurcation (becoming magnetized) forming a binary chain of spin-up or spin-down states. Minimization of the bifurcation threshold determines the magnetic order as a function of the coupling strength. This allows control of multiple magnetic orders via adiabatic (slow ramping of) pumping. In addition to ferromagnetic and anti-ferromagnetic ordered states we show the formation of a paired-spin ordered state $\left|\dots \uparrow \uparrow \downarrow \downarrow \dots \right. \rangle$ as a consequence of the phase degree of freedom between condensates

Observation of inversion, hysteresis, and collapse of spin in optically trapped polariton condensates

The spin and intensity of optically trapped polariton condensates are studied under steady-state elliptically-polarised nonresonant pumping. Three distinct effects are observed: (1) spin inversion where condensation occurs in the opposite handedness from the pump, (2) spin/intensity hysteresis as the pump power is scanned, and (3) a sharp ‘spin collapse’ transition in the condensate spin as a function of the pump ellipticity. We show these effects are strongly dependent on trap size and sample position and are linked to small counterintuitive energy differences between the condensate spin components. Our results, which fail to be fully described within the commonly used nonlinear equations for polariton condensates, show that a more accurate microscopic picture is needed to unify these phenomena in a two-dimensional condensate theory

Microscopic theory of exciton and trion polaritons in doped monolayers of transition metal dichalcogenides

This is the final version. Available on open access from Nature Research via the DOI in this recordData availability: The data that support the findings of this study are available from the corresponding author upon reasonable request.Code availability: All the codes were implemented using home-built programs, which use standard open-source software packages.We study a doped transition metal dichalcogenide (TMDC) monolayer in an optical microcavity. Using the microscopic theory, we simulate spectra of quasiparticles emerging due to the interaction of material excitations and a high-finesse optical mode, providing a comprehensive analysis of optical spectra as a function of Fermi energy and predicting several modes in the strong light-matter coupling regime. In addition to exciton-polaritons and trion-polaritons, we report polaritonic modes that become bright due to the interaction of excitons with free carriers. At large doping, we reveal strongly coupled modes corresponding to excited trions that hybridize with a cavity mode. We also demonstrate that the increase of carrier concentration can change the nature of the system’s ground state from the dark to the bright one. Our results offer a unified description of polaritonic modes in a wide range of free electron densities.Deutsche Forschungsgemeinschaft (DFG)Russian Science FoundationIcelandic Research FundRussian Foundation for Basic Research (RFBR)Engineering and Physical Sciences Research Council (EPSRC)University at BuffaloSUNY Research Seed Grant Progra

Machine learning of phase transitions in nonlinear polariton lattices

This is the final version. Available from Nature Research via the DOI in this record. The data that support the findings of this study are available from the corresponding author upon reasonable request.The code for the analysis is available from the corresponding author upon reasonable request.Polaritonic lattices offer a unique testbed for studying nonlinear driven-dissipative physics. They show qualitative changes of their steady state as a function of system parameters, which resemble non-equilibrium phase transitions. Unlike their equilibrium counterparts, these transitions cannot be characterised by conventional statistical physics methods. Here, we study a lattice of square-arranged polariton condensates with nearest-neighbour coupling, and simulate the polarisation (pseudospin) dynamics of the polariton lattice, observing regions with distinct steady-state polarisation patterns. We classify these patterns using machine learning methods and determine the boundaries separating different regions. First, we use unsupervised data mining techniques to sketch the boundaries of phase transitions. We then apply learning by confusion, a neural network-based method for learning labels in a dataset, and extract the polaritonic phase diagram. Our work takes a step towards AI-enabled studies of polaritonic systems.Engineering and Physical Sciences Research Council (EPSRC)Russian Foundation for Basic ResearchNATOIcelandic Research FundIcelandic Research Fun