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In the framework of the Floquet theory of periodically driven quantum systems, it is demonstrated that irradiation of graphene by a circularly polarized electromagnetic field induces an attractive area in the core of repulsive potentials. Consequently, the quasi-stationary electron states bound by the repulsive potentials appear. The difference between such field-induced states in graphene and usual systems with the parabolic dispersion of electrons is discussed and possible manifestations of these states in electronic transport and optical spectra of graphene are considered., Comment: Published version

We formulate and analyze in detail the ground state quantum electrodynamical density functional theory (QEDFT) for a generalized Dicke model describing a collection of $N$ tight-binding dimers minimally coupled to a cavity photon mode. This model is aimed at capturing essential physics of molecules in quantum cavities in polaritonic chemistry, or quantum emitters embedded in mesoscopic resonators of the circuit QED, and, because of its simplicity, is expected to provide important insights regarding the general QEDFT. We adopt the adiabatic connection formalism and the diagrammatic many-body theory to regularly derive a sequence of explicit approximations for the exchange-correlation (xc) energy in the ground state QEDFT, and to compare their performance with the results of exact numerical diagonalization. Specifically, we analyze the earlier proposed one-photon optimized effective potential (OEP) scheme, its direct second order extensions, and a non-perturbative xc functional based on the photon random phase approximation (RPA). Our results demonstrate the excellent performance of RPA-QEDFT in the ultrastrong coupling regime, and for any number $N$ of Dicke molecules in the cavity. We study in detail the scaling of xc energy with $N$, and emphasize the importance for the ground state QEDFT of collective effects in the interaction of molecules with cavity photons. Finally, we discuss implications of our results for realistic systems.

We report the observation of band gaps for low loss exciton-polaritons propagating outside the light cone in GaAs-based planar waveguides patterned into two-dimensional photonic crystals. By etching square lattice arrays of shallow holes into the uppermost layer of our structure, we open gaps on the order of 10 meV in the photonic mode dispersion, whose size and light-matter composition can be tuned by proximity to the strongly coupled exciton resonance. We demonstrate gaps ranging from almost fully photonic to highly excitonic. Opening a gap in the exciton-dominated part of the polariton spectrum is a promising first step towards the realization of quantum-Hall-like states arising from topologically nontrivial hybridization of excitons and photons., Comment: published in Appl. Phys. Lett. in 2021

The existence of spontaneous magnetization that fingerprints a ground-state ferromagnetic order was recently observed in two-dimensional (2D) van der Waals materials. Despite progress in the fabrication and manipulation of the atom-thick magnets, investigation of nanoscale magnetization properties is still challenging due to the concomitant technical issues. We propose a promising approach for a direct visualization of the domain walls formed in 2D magnetic materials. By interfacing 2D magnet with a transition metal dichalcogenide (TMD) monolayer, the strong proximity effects enable pinning the TMD excitons on the domain wall. The emergent localization stems from the proximity-induced exchange mixing between spin-dark and spin-bright TMD excitons due to the local in-plane magnetization characteristic of the domain wall in the magnetic monolayer., Comment: 6 pages, 4 figures

Within the Floquet theory of periodically driven quantum systems, we demonstrate that an off-resonant high-frequency electromagnetic field can induce the Lifshitz phase transition in periodical structures described by the one-dimensional repulsive Hubbard model with the nearest and next-nearest-neighbor hopping. The transition changes the topology of electron energy spectrum at the Fermi level, transforming it from the two Fermi points to the four Fermi points, which facilitates the emergence of the superconducting fluctuations in the structure. Possible manifestations of the effect and conditions of its experimental observability are discussed., Comment: Published version

Light-matter coupling in a planar optical cavity substantially modifies the transport regimes in the system in presence of a short range excitonic disorder. Basing on Master equation for a resonantly coupled exciton-photon system, and treating disorder scattering in the Born-Markov approximation we demonstrate the onset of ballistic and diffusive transport regimes in the limits of weak and strong disorder respectively. We show that transport parameters governing the crossover between these two regimes strongly depend on the parameters characterizing light-matter coupling, in particular Rabi energy and detuning between excitonic and photonic modes. The presented theory agrees with recent experimental data on transport in disordered organic microcavities.

Deeply subwavelength lasers (or nanolasers) are highly demanded for compact on-chip bioimaging and sensing at the nanoscale. One of the main obstacles for the development of single-particle nanolasers with all three dimensions shorter than the emitting wavelength in the visible range is the high lasing thresholds and the resulting overheating. Here we exploit exciton-polariton condensation and mirror-image Mie modes in a cuboid CsPbBr$_3$ nanoparticle to achieve coherent emission at the visible wavelength of around 0.53~$\mu $m from its ultra-small ($\approx$0.007$\mu$m$^3$ or $\approx\lambda^3$/20) semiconductor nanocavity. The polaritonic nature of the emission from the nanocavity localized in all three dimensions is proven by direct comparison with corresponding one-dimensional and two-dimensional waveguiding systems with similar material parameters. Such a deeply subwavelength nanolaser is enabled not only by the high values for exciton binding energy ($\approx$35 meV), refractive index ($>$2.5 at low temperature), and luminescence quantum yield of CsPbBr$_3$, but also by the optimization of polaritons condensation on the Mie resonances. Moreover, the key parameters for optimal lasing conditions are intermode free spectral range and phonons spectrum in CsPbBr$_3$, which govern polaritons condensation path. Such chemically synthesized colloidal CsPbBr$_3$ nanolasers can be easily deposited on arbitrary surfaces, which makes them a versatile tool for integration with various on-chip systems., Comment: 29 pages, 19 Figures

Bright excitons in ferromagnetic monolayers CrI$_3$ efficiently interact with lattice magnetization, which makes possible all-optical resonant magnetization control in this material. Using the combination of ab-initio simulations within Bethe-Salpeter approach, semiconductor Bloch equations and Landau-Lifshitz equations, we construct a microscopic theory of this effect. Solving numerically the resulting system of the coupled equations describing the dynamics of atomic spins and spins of the excitons, we demonstrate the possibility of a tunable control of macroscopic magnetization of a sample.

Excitons in lead bromide perovskites exhibit high binding energy and high oscillator strength, allowing for a strong light-matter coupling regime in the perovskite-based cavities localizing photons at the nanoscale. This opens up the way for the realization of exciton-polariton Bose-Einstein condensation and polariton lasing at room temperature -- the inversion-free low-threshold stimulated emission. However, polariton lasing in perovskite planar photon cavities without Bragg mirrors has not yet been observed and proved experimentally. In this work, we employ perovskite metasurface, fabricated with nanoimprint lithography, supporting so-called exceptional points to demonstrate the room-temperature polariton lasing. The exceptional points in exciton-polariton dispersion of the metasurface appear upon optically pumping in the nonlinear regime in the spectral vicinity of a symmetry-protected bound state in the continuum providing high mode confinement with the enhanced local density of states beneficial for polariton condensation. The observed lasing emission possesses high directivity with a divergence angle of around 1$^\circ$ over one axis. The employed nanoimprinting approach for solution-processable large-scale polariton lasers is compatible with various planar photonic platforms suitable for on-chip integration., Comment: 12 pages, 3 figures

Systems supporting exciton-polaritons represent solid-state optical platforms with a strong built-in optical nonlinearity provided by exciton-exciton interactions. In conventional semiconductors with hydrogen-like excitons the nonlinearity rate demonstrates the inverse scaling with the binding energy. This makes excitons stable at room temperatures weakly interacting, which obviously limits the possibilities of practical applications of the corresponding materials for nonlinear photonics. We demonstrate experimentally and theoretically, that these limitations can be substantially softened in hybrid perovskites, such as MAPbBr3 due to the crucial role of the polaron effects mediating the inter-particle interactions. The resulting exciton-polaron-polaritons remain both stable and strongly interacting at room temperature, which is confirmed by large nonlinear blueshifts of lower polariton branch energy under resonant femtosecond laser pulse excitation. Our findings open novel perspectives for the management of the exciton-polariton nonlinearities in ambient conditions.

During the last years, giant optical anisotropy demonstrated its paramount importance for light manipulation which resulted in numerous applications ranging from subdiffraction light guiding to switchable nanolasers. In spite of recent advances in the field, achieving continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through chemical alteration of the ratio of halogen atoms (X = Br or Cl) in single-crystal CsPbX$_3$ halide perovskites. It turns out that the anisotropy originates from an excitonic resonance in the perovskite, which spectral position and strength are determined by the halogens composition. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range -- the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane, depending on perovskite shape -- rectangular and square. Hence, it can serve as an additional degree of freedom for anisotropy manipulation. As a practical demonstration, we created perovskite anisotropic nanowaveguides and show a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics., Comment: 18 pages, 3 figures

We developed a theory of electron scattering by a short-range repulsive potential in a cavity. In the regime of ultrastrong electron coupling to the cavity electromagnetic field, the vacuum fluctuations of the field result in the dynamical stabilization of a quasistationary polariton state confined in the core of the repulsive potential. When the energy of a free electron coincides with the energy of the confined state, the extremely efficient resonant nonelastic scattering of the electron accompanied by emission of a cavity photon appears. This effect is discussed as a basis for possible free-electron sources of nonclassical light., Comment: Published version

Recently reported large values of exciton-polariton nonlinearity of transition metal dichalcogenide (TMD) monolayers coupled to optically resonant structures approach the values characteristic for GaAs-based systems in the regime of strong light-matter coupling. Contrary to the latter, TMD-based polaritonic devices remain operational at ambient conditions and therefore have greater potential for practical nanophotonic applications. Here we present the study of the nonlinear properties of Ta$_2$O$_5$ slab waveguide coupled to a WSe$_2$ monolayer. We first confirm that the hybridization between waveguide photon mode and a 2D semiconductor exciton resonance gives rise to the formation of exciton-polaritons with Rabi splitting of 36 meV. By measuring transmission of ultrashort optical pulses through this TMD-based polaritonic waveguide, we demonstrate for the first time the strong nonlinear dependence of the output spectrum on the input pulse energy. Our theoretical model provides semi-quantitative agreement with experiment and gives insights into the dominating microscopic processes which determine the nonlinear pulse self-action: Coulomb inter-particle interaction and scattering to incoherent excitonic reservoir. We also confirm that at intermediate pump energies the system supports quasi-stationary solitonic regime of pulse propagation. Our results are essential for the development of nonlinear on-chip polaritonic devices based of 2D semiconductors.

Within the Floquet theory of periodically driven quantum systems, we demonstrate that a high-frequency electromagnetic field can be used as an effective tool to control excitonic properties of semiconductor quantum dots (QDs). It is shown, particularly, that the field both decreases the exciton binding energy and dynamically stabilizes the exciton, increasing its radiative lifetime. The developed theory can serve as a basis for the ultrafast method to tune spectral characteristics of the QD-based photon emitters by a high-frequency field., Comment: Published version. arXiv admin note: substantial text overlap with arXiv:2204.00787

Exciton-polaritons offer a versatile platform for realization of all-optical integrated logic gates due to the strong effective optical nonlinearity resulting from the exciton-exciton interactions. In most of the current excitonic materials there exists a direct connection between the exciton robustness to thermal fluctuations and the strength of exciton-exciton interaction, making materials with highest levels of exciton nonlinearity applicable at cryogenic temperatures only. Here, we show that strong polaronic effects, characteristic for perovskite materials, allow to overcome this limitation. Namely, we demonstrate the record-high value of the nonlinear optical response in nanostructured organic-inorganic halide perovskite MAPbI$_3$, experimentally detected as 19.7 meV blueshift of the polariton branch under femtosecond laser irradiation. This is substantially higher than characteristic values for the samples based on conventional semiconductors and monolayers of transition metal dichalcogenides. The observed strong polaron-enhanced nonlinearity exists for both tetragonal and orthorombic phases of MAPbI$_3$, and remains stable at elevated temperatures.

We present a quantitative study of the nonlinear optical response of layered perovskites placed inside planar photonic microcavities in the regime of strong light matter coupling, when excitonic and photonic modes hybridize and give rise to cavity polaritons. Two sources of nonlinearity are specified, the saturation of the excitonic transition with increase of the optical pump and Coulomb interaction between the excitons. It is demonstrated, that peculiar form of the interaction potential, specific to multilayer structure of organic perovskites, is responsible for substantial increase of the exciton binding energy and Rabi splitting with respect to conventional semiconductor systems. This results in dominant contribution of the Rabi splitting quench effect in the nonlinear optical response. Moreover, due to the tightly bound character of excitons, the density of Mott transition is essentially higher, allowing to reach extremely large polariton blueshifts of about 50 meV, which is order of magnitude higher than in conventional semiconductors.

We generalize the optimized effective potential (OEP) formalism in the quantum electrodynamical density functional theory (QEDFT) to the case of continuous distribution of photon modes, and study its applicability to dissipative dynamics of electron systems interacting with photons of lossy cavities. Specifically, we test whether this technique is capable of capturing the quantum features of electron-photon interaction related to spontaneous emission and the corresponding energy transfer from the electrons to cavity photons. For this purpose, we analyze a discrete three-site system with one electron coupled to photons of the cavity, which, in fact, is a minimal model allowing to eliminate classical radiation and the corresponding energy loss, but still have nontrivial density dynamics. By considering two typical spectral densities of photon modes, modeling (i) lossy cavity with Lorentzian broadening of photon peaks, and (ii) the Ohmic bath, and several representative dynamical regimes, we find that OEP-QEDFT demonstrates a good qualitative and quantitative performance, especially in the case when the disspation is dominated by one-photon processes.

We study theoretically the excitonic nonlinearity in hybrid organic-inorganic Ruddlesden-Popper perovskite thin films. The composite layered structure of these materials allows for flexible modulation of their excitonic response between the limiting cases of single atomic layer and wide quasi-two-dimensional quantum well. In particular, we demonstrate that transverse electric field leads to the spatial separation of charge carriers within the inorganic layer, giving rise to strongly interacting excitons possessing built-in dipole moment. Combined with exciton binding energy of the order of hundreds of meVs, this makes hybrid perovskites an optimal platform for tailoring of nonlinear optical response at reduced dimensionality.

Indirect quartic interaction of spins is suggested to play an important role in two dimensional magnets with trigonal prismatic symmetry such as Fe$_3$GeTe$_2$ monolayer. The proposed interaction is described by terms in micromagnetic energy that are linear in magnetization gradients. Such terms enable the stability of non-collinear magnetic textures. We investigate signatures of the quartic interaction in magnon spectra in the presence of anisotropy and external magnetic field. We also show how magnetic spirals, which are induced by the proposed interaction, can be manipulated by external field. Our analysis is based on symmetry considerations and can be used to quantify the quartic interaction strength in experiments with magnetic monolayers., Comment: 5 pages; 3 figures

We theoretically predict the formation of two-photon bound states in a two-dimensional waveguide network hosting a lattice of two-level atoms. The properties of these bound pairs and the exclusive domains of the parameter space where they emerge due to the interplay between the on-site photon blockade and peculiar shape of polariton dispersion resulting from the long-range radiative couplings between the qubits are investigated in detail. In addition, we analyze the effect of the finite-size system on localization characteristics of these excitations., Comment: 7 pages, 5 figures

Interfacing atomically thin van der Waals semiconductors with magnetic substrates enables additional control on their intrinsic valley degree of freedom and provides a promising platform for the development of novel valleytronic devices for information processing and storage. Here we study circularly polarized photoluminescence in heterostructures of monolayer MoSe$_2$ and thin films of ferrimagnetic bismuth iron garnet. We observe strong emission from charged excitons with negative valley polarization, which switches sign with increasing temperature, and demonstrate contrasting response to left and right circularly polarized excitation, associated with finite out-of-plane magnetization in the substrate. We propose a theoretical model accounting for magnetization-induced imbalance of charge carriers in the two valleys of MoSe$_2$, as well as for valley-switching scattering from B to A excitons and fast formation of trions with extended valley relaxation times, which shows excellent agreement with the experimental data. Our results provide new insights into valley physics in 2D semiconductors interfaced with magnetic substrates.

Efficient control of a magnetization without an application of the external magnetic fields is the ultimate goal of spintronics. We demonstrate, that in monolayers of $\text{CrI}_3$, magnetization can be switched all optically, by application of the resonant pulses of circularly polarized light. This happens because of the efficient coupling of the lattice magnetization with bright excitonic transition. $\text{CrI}_3$ is thus perspective functional material with high potential for applications in the domains of spintronics and ultra-fast magnetic memory.

Polaritonic lattices offer a unique testbed for studying nonlinear driven-dissipative physics. They show qualitative changes of a 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 (pseudo-spin) 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 the dataset, and extract the polaritonic phase diagram. Our work takes a step towards AI-enabled studies of polaritonic systems., Comment: to appear in Communications Physics

It is demonstrated theoretically that the circularly polarized irradiation of two-dimensional electron systems can induce the localized electron states which antiferromagnetically interact with conduction electrons, resulting in the Kondo effect. Conditions of experimental observation of the effect are discussed for semiconductor quantum wells., Comment: Published version

We analyze the many-particle correlations that affect the optical properties of two-dimensional semiconductors. These correlations manifest themselves through the specific optical resonances such as excitons, trions, etc. Starting from the generic electron-hole Hamiltonian and employing the microscopic Heisenberg equation of motion the infinite hierarchy of differential equations can be obtained. In order to decouple the system we address the cluster expansion technique which provides a regular procedure of consistent accounting of many-particle correlation contributions into the interband polarization dynamics. In particular, the partially taken into account three-particle correlations modify the behavior of absorption spectra with the emergence of a trion-like peak additional to excitonic ones. In contrast to many other approaches, the proposed one allows us to model the optical response of 2d semiconductors in the regime when the Fermi energies are of the order of the exciton and trion binding energies, thus allowing us to rigorously model the onset of the excitonic Mott transition, the regime being recently studied in various 2d semiconductors, such as transition metal dichalcogenides.

We consider a two-dimensional extension of the 1D waveguide quantum electrodynamics and investigate the nature of linear excitations in two-dimensional arrays of qubits coupled to networks of chiral waveguides. We show that the combined effects of chirality and long-range photon mediated qubit-qubit interactions lead to the emergence of the two-dimensional flat bands in the polaritonic spectrum, corresponding to slow strongly correlated light.

Variational quantum eigensolver (VQE) emerged as a first practical algorithm for near-term quantum computers. Its success largely relies on the chosen variational ansatz, corresponding to a quantum circuit that prepares an approximate ground state of a Hamiltonian. Typically, it either aims to achieve high representation accuracy (at the expense of circuit depth), or uses a shallow circuit sacrificing the convergence to the exact ground state energy. Here, we propose the approach which can combine both low depth and improved precision, capitalizing on a genetically-improved ansatz for hardware-efficient VQE. Our solution, the multiobjective genetic variational quantum eigensolver (MoG-VQE), relies on multiobjective Pareto optimization, where topology of the variational ansatz is optimized using the non-dominated sorting genetic algorithm (NSGA-II). For each circuit topology, we optimize angles of single-qubit rotations using covariance matrix adaptation evolution strategy (CMA-ES) -- a derivative-free approach known to perform well for noisy black-box optimization. Our protocol allows preparing circuits that simultaneously offer high performance in terms of obtained energy precision and the number of two-qubit gates, thus trying to reach Pareto-optimal solutions. Tested for various molecules (H$_2$, H$_4$, H$_6$, BeH$_2$, LiH), we observe nearly ten-fold reduction in the two-qubit gate counts as compared to the standard hardware-efficient ansatz. For 12-qubit LiH Hamiltonian this allows reaching chemical precision already at 12 CNOTs. Consequently, the algorithm shall lead to significant growth of the ground state fidelity for near-term devices., Comment: 10 pages, 1 supplementary circuit

Optical spectra of semiconductor quantum wells driven by an off-resonant oscillating field are studied theoretically. Due to the dynamical stabilization effect, the field induces the quasi-stationary electron states confined at repulsive scatterers and immersed into the continuum of states of conduction electrons. As a result, the Fano resonances in the spectra of interband optical transitions appear near the energies of the quasi-stationary states., Comment: Published version

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the role of the band structure and alignment of the constituent layers in the underlying dynamics remains largely unexplored. Here we study spin--valley relaxation dynamics in heterobilayers with different band structures engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modelling for Mo$_{1-x}$W$_{x}$Se$_2$/WSe$_2$ samples with different chemical compositions and stacking angles, we uncover the roles of interlayer exciton recombination and charge carrier spin depolarization in the overall valley dynamics. Our results provide insights into the microscopic spin--valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices.

We study a system of a transition metal dichalcogenide (TMD) monolayer placed in an optical resonator, where strong light-matter coupling between excitons and photons is achieved. We present quantitative theory of the nonlinear optical response for exciton-polaritons for the case of doped TMD monolayer, and analyze in detail two sources of nonlinearity. The first nonlinear response contribution stems from the Coulomb exchange interaction between excitons. The second contribution comes from the reduction of Rabi splitting that originates from phase space filling at increased exciton concentration and the composite nature of excitons. We demonstrate that both nonlinear contributions are enhanced in the presence of free electrons. As free electron concentration can be routinely controlled by an externally applied gate voltage, this opens a way of electrical tuning of the nonlinear optical response.

Optical bound states in the continuum (BICs) provide a way to engineer very narrow resonances in photonic crystals. The extended interaction time in such systems is particularly promising for enhancement of nonlinear optical processes and development of the next generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, we mix optical BIC in a photonic crystal slab with excitons in atomically thin semiconductor MoSe$_2$ to form nonlinear exciton-polaritons with a Rabi splitting of 27~meV, exhibiting large interaction-induced spectral blueshifts. The asymptotic BIC-like suppression of polariton radiation into far-field towards the BIC wavevector, in combination with effective reduction of excitonic disorder through motional narrowing, results in small polariton linewidths below 3~meV. Together with strongly wavevector-dependent Q-factor, this provides for enhancement and control of polariton--polariton interactions and resulting nonlinear optical effects, paving the way towards tunable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics.

Strain engineering is a powerful tool for tuning physical properties of 2D materials, including monolayer transition metal dichalcogenides (TMD) -- direct bandgap semiconductors with strong excitonic response. Here, we demonstrate an approach for local characterization of strain-induced modification of excitonic photoluminescence in TMD-based materials. We reversibly stress a monolayer of MoSe$_2$ with an AFM tip and perform spatio-spectral mapping of the excitonic photoluminescence in the vicinity of the indentation point. To fully reproduce the experimental data, we introduce the linear dependence of the exciton energy and corresponding photoluminescence intensity on the induced strain. Careful account for the optical resolution allows extracting these quantities with good agreement with the previous measurements, which involved macroscopic sample deformation. Our approach is a powerful tool for the study of local optomechanical properties of 2D direct bandgap semiconductors with strong excitonic response.

We theoretically explore nonresonantly pumped polaritonic graphene, a system consisting of a honeycomb lattice of micropillars in the regime of strong light-matter coupling. We demonstrate that, depending on the parameters of the structure, such as intensity of the pump and coupling strength between the pillars, the system shows rich variety of macroscopic ordering, including analogs of ferromagnetic, antiferromagnetic, and resonant valence bond phases. Transitions between these phases are associated with dramatic reshaping of the spectrum of the system connected with spontaneous appearance of topological order.

Anisotropic nanophotonic structures can couple the levels of a quantum emitter through the quantum interference effect. In this paper we study the coupling of quantum emitters excited states through the modes of a fully anisotropic structure: a structure for which all directions are physically nonequivalent. We consider an anisotropic metasurface as an illustrative example of such a structure. We point out a novel degree of freedom in controlling the temporal dynamics and spectral profiles of quantum emitters: namely, we show that a combination of the metasurface anisotropy and tilt of the emitter quantization axis with respect to the metasurface normal results in nonsymmetric dynamics between the transitions of electrons from left-circular state to the right-circular states and the inverse process. Our findings give an additional mechanism for control over the light emission by quantum systems and, vice versa, can be utilized for probing active transitions of quantum emitters.

A strong optical nonlinearity arises when coherent light is scattered by a semiconductor quantumdot (QD) coupled to a nano-photonic waveguide. We exploit the Fano effect in such a waveguide to control the phase of the quantum interference underpinning the nonlinearity, experimentally demonstrating a tunable quantum optical filter which converts a coherent input state into either a bunched, or antibunched non-classical output state. We show theoretically that the generation of non-classical light is predicated on the formation of a two-photon bound state due to the interaction of the input coherent state with the QD. Our model demonstrates that the tunable photon statistics arise from the dependence of the sign of two-photon interference (either constructive or destructive) on the detuning of the input relative to the Fano resonance.

We analyse nonequilibrium phase transitions in microcavity polariton condensates trapped in optically induced annular potentials. We develop an analytic model for annular optical traps, which gives an intuitive interpretation for recent experimental observations on the polariton spatial mode switching with variation of the trap size. In the vicinity of polariton lasing threshold we then develop a nonlinear mean-field model accounting for interactions and gain saturation, and identify several bifurcation scenarios leading to formation of high angular momentum quantum vortices. For experimentally relevant parameters we predict the emergence of spatially and temporally ordered polariton condensates (time crystals), which can be witnessed by frequency combs in the polariton lasing spectrum or by direct time-resolved optical emission measurements. In contrast to previous realizations, our polaritonic time crystal is spontaneously formed from an incoherent excitonic bath and does not inherit its frequency from any periodic driving field., Comment: 6 pages, 4 figures

Chern insulator phase is shown to emerge in two-dimensional arrays of polariton rings where time-reversal symmetry is broken due to the application of an out-of-plane magnetic field. The interplay of Zeeman splitting with the photonic analog of spin-orbit coupling (TE-TM splitting) inherently present in this system leads to the appearance of synthetic U(1) gauge field and the opening of topologically nontrivial spectral gaps. This results in the onset of topologically protected chiral edge states similar to those forming in quantum Hall effect. In one dimensional zigzag arrays of polariton rings edge states similar to those appearing in Su-SchriefferHeeger (SSH) model are formed.

Optical bound state in the continuum (BIC) is characterized by infinitely high quality factor resulting in drastic enhancement of light-matter interaction phenomena. We study the optical response of a one-dimensional photonic crystal slab with Kerr focusing nonlinearity in the vicinity of BIC analytically and numerically. We predict a strong nonlinear response including multistable behaviour, self-tuning of BIC to the frequency of incident wave, and breaking of symmetry protected BIC. We show that all of these phenomena can be observed in silicon photonic structure at the pump power of several $\mu$W/cm$^2$. We also analyze the modulation instability of the obtained solutions and the effect of the finite size of the structure on the stability. Our findings have strong implications for nonlinear photonics and integrated optical circuits.

Being motivated by recent achievements in the rapidly developing fields of optical bound states in the continuum (BICs) and excitons in monolayers of transition metal dichalcogenides, we analyze strong coupling between BICs in $\rm Ta_2O_5$ periodic photonic structures and excitons in $\rm WSe_2$ monolayers. We demonstrate that giant radiative lifetime of BICs allow to engineer the exciton-polariton lifetime enhancing it three orders of magnitude compared to a bare exciton. We show that maximal lifetime of hybrid light-matter state can be achieved at any point of $\mathbf{k}$-space by shaping the geometry of the photonic structure. Our findings open new route for the realization of the moving exciton-polariton condensates with non-resonant pump and without the Bragg mirrors which is of paramount importance for polaritonic devices., Comment: 7 pages, 5 figures

We developed the theory of electronic properties of semiconductor quantum rings with the Rashba spin-orbit interaction irradiated by an off-resonant high-frequency electromagnetic field (dressing field). Within the Floquet theory of periodically driven quantum systems, it is demonstrated that the dressing field drastically modifies all electronic characteristics of the rings, including spin-orbit coupling, effective electron mass and optical response. Particularly, the present effect paves the way to controlling the spin polarization of electrons with light in prospective ring-shaped spintronic devices., Comment: Published version

We demonstrate theoretically that a strong high-frequency circularly polarized electromagnetic field can turn a two-dimensional periodic array of interconnected quantum rings into a topological insulator. The elaborated approach is applicable to calculate and analyze the electron energy spectrum of the array, the energy spectrum of the edge states and the corresponding electronic densities. As a result, the present theory paves the way to optical control of the topological phases in ring-based mesoscopic structures., Comment: Published version

We study discrete nonlinear edge excitations of polaritonic kagome lattice. We show that when nontrivial topological phase of polaritons is realized, the kagome lattice permits propagation of bright solitons formed from topological edge states., Comment: 4 pages, 2 figures

It is shown theoretically that the renormalization of the electron energy spectrum of bilayer graphene with a strong high-frequency electromagnetic field (dressing field) results in the Lifshitz transition - the abrupt change in the topology of the Fermi surface near the band edge. This effect substantially depends on the polarization of the field: The linearly polarized dressing field induces the Lifshitz transition from the quadruply-connected Fermi surface to the doubly-connected one, whereas the circularly polarized field induces the multicritical point, where the four different Fermi topologies may coexist. As a consequence, the discussed phenomenon creates physical basis to control the electronic properties of bilayer graphene with light., Comment: Published version

We study theoretically the Coulomb interaction between excitons in transition metal dichalcogenide (TMD) monolayers. We calculate direct and exchange interaction for both ground and excited states of excitons. The screening of the Coulomb interaction, specific to monolayer structures, leads to the unique behavior of the exciton-exciton scattering for excited states, characterized by the non-monotonic dependence of the interaction as function of the transferred momentum. We find that the nontrivial screening enables the description of TMD exciton interaction strength by approximate formula which includes exciton binding parameters. The influence of screening and dielectric environment on the exciton-exciton interaction was studied, showing qualitatively different behavior for ground state and excited states of excitons. Furthermore, we consider exciton-electron interaction, which for the excited states is governed by the dominant attractive contribution of the exchange component, which increases with the excitation number. The results provide a quantitative description of the exciton-exciton and exciton-electron scattering in transition metal dichalcogenides, and are of interest for the design of perspective nonlinear optical devices based on TMD monolayers., Comment: 10 pages, 6 figures

We study exciton-polaritons in a two-dimensional Lieb lattice of micropillars. The energy spectrum of the system features two flat bands formed from $S$ and $P_{x,y}$ photonic orbitals, into which we trigger bosonic condensation under high power excitation. The symmetry of the orbital wave functions combined with photonic spin-orbit coupling gives rise to emission patterns with pseudospin texture in the flat band condensates. Our work shows the potential of polariton lattices for emulating flat band Hamiltonians with spin-orbit coupling, orbital degrees of freedom and interactions.

In our work, we study the dynamics of a single excitation in an one-dimensional array of two-level systems, which are chirally coupled through a single mode waveguide. The chirality is achieved owing to a strong optical spin-locking effect, which in an ideal case gives perfect unidirectional excitation transport. We obtain a simple analytical solution for a single excitation dynamics in the Markovian limit, which directly shows the tolerance of the system with respect to the fluctuations of emitters position. We also show that the Dicke state, which is well-known to be superradiant, has twice lower emission rate in the case of unidirectional quantum interaction. Our model is supported and verified with the numerical computations of quantum emmiters coupled via surface plasmon modes in a metalic nanowire. The obtained results are based on a very general model and can be applied to any chirally coupled system, that gives a new outlook on quantum transport in chiral nanophotonics., Comment: 5 pages, 5 figures

We demonstrate theoretically that the interaction of electrons in gapped Dirac materials (gapped graphene and transition-metal dichalchogenide monolayers) with a strong off-resonant electromagnetic field (dressing field) substantially renormalizes the band gaps and the spin-orbit splitting. Moreover, the renormalized electronic parameters drastically depend on the field polarization. Namely, a linearly polarized dressing field always decreases the band gap (and, particularly, can turn the gap into zero), whereas a circularly polarized field breaks the equivalence of valleys in different points of the Brillouin zone and can both increase and decrease corresponding band gaps. As a consequence, the dressing field can serve as an effective tool to control spin and valley properties of the materials and be potentially exploited in optoelectronic applications., Comment: Published version

We study a system of microcavity pillars arranged into a kagome lattice. We show that polarization-dependent tunnel coupling of microcavity pillars leads to the emergence of the effective spin-orbit interaction consisting of the Dresselhaus and Rashba terms, similar to the case of polaritonic graphene studied earlier. Appearance of the effective spin-orbit interaction combined with the time-reversal symmetry-breaking resulting from the application of the magnetic field leads to the nontrivial topological properties of the Bloch bundles of polaritonic wavefunction. These are manifested in opening of the gap in the band structure and topological edge states localized on the boundary. Such states are analogs of the edge states arising in topological insulators. Our study of polarization properties of the edge states clearly demonstrate that opening of the gap is associated with the band inversion in the region of the Dirac points of the Brillouin zone where the two bands corresponding to polaritons of opposite polarizations meet. For one particular type of boundary we observe a highly nonlinear energy dispersion of the edge state which makes polaritonic kagome lattice a promising system for observation of edge state solitons., Comment: 7 pages, 4 figures

Using the Feynman-Dyson diagram technique, we study nonlinear polariton-polariton scattering in a two-dimensional micropillar-based optical superlattice with hexagonal symmetry. We demonstrate that both the emerging polariton chirality and the loop Feynman diagrams up to infinite order should be strictly accounted for in the evaluation of the self-energy of the system. Further, we explicitly show that in such a design the time of polariton scattering towards the Dirac points can be drastically decreased which can be used, for instance, in engineering novel classes of polariton lasers with substantially reduced thresholds.