Multiscale Supercrystal Meta-atoms.

Meta-atoms are the building blocks of metamaterials, which are employed to control both generation and propagation of light as well as provide novel functionalities of localization and directivity of electromagnetic radiation. In many cases, simple dielectric or metallic resonators are employed as meta-atoms to create different types of electromagnetic metamaterials. Here, we fabricate and study supercrystal meta-atoms composed of coupled perovskite quantum dots. We reveal that these multiscale structures exhibit specific emission properties, such as spectrum splitting and polaritonic effects. We believe that such multiscale supercrystal meta-atoms will provide novel functionalities in the design of many novel types of active metamaterials and metasurfaces.

Electrostatic Control of Nonlinear Photonic-Crystal Polaritons in a Monolayer Semiconductor.

Integration of 2D semiconductors with photonic crystal slabs provides an attractive approach to achieving strong light-matter coupling and exciton-polariton formation in a chip-compatible geometry. However, for the development of practical devices, it is crucial that polariton excitations are easily tunable and exhibit a strong nonlinear response. Here we study neutral and charged exciton-polaritons in an electrostatically gated photonic crystal slab with an embedded monolayer semiconductor MoSe2 and experimentally demonstrate a novel approach to optical control based on polariton nonlinearity. We show that spatial modulation of the dielectric environment within the photonic crystal unit cell results in the formation of two distinct excitonic species with significantly different nonlinear responses of the corresponding charged exciton-polaritons under optical pumping. This behavior enables optical switching with ultrashort laser pulses and can be sensitively controlled via an electrostatic gate voltage. Our results open new avenues toward the development of active polaritonic devices in a compact chip-compatible implementation.

Probing and Control of Guided Exciton-Polaritons in a 2D Semiconductor-Integrated Slab Waveguide.

Guided 2D exciton-polaritons, resulting from the strong coupling of excitons in semiconductors with nonradiating waveguide modes, provide an attractive approach toward developing novel on-chip optical devices. These quasiparticles are characterized by long propagation distances and efficient nonlinear interactions but cannot be directly accessed from the free space. Here we demonstrate a powerful approach for probing and manipulating guided polaritons in a Ta2O5 slab integrated with a WS2 monolayer using evanescent coupling through a high-index solid immersion lens. Tuning the nanoscale lens-sample gap allows for extracting all of the intrinsic parameters of the system. We also demonstrate the transition from weak to strong coupling accompanied by the onset of the motional narrowing effect: with the increase of exciton-photon coupling strength, the inhomogeneous contribution to polariton line width, inherited from the exciton resonance, becomes fully lifted. Our results enable the development of integrated optics employing room-temperature exciton-polaritons in 2D semiconductor-based structures.

Giant and Tunable Excitonic Optical Anisotropy in Single-Crystal Halide Perovskites.

During the last years, giant optical anisotropy has demonstrated its paramount importance for light manipulation. In spite of recent advances in the field, the achievement of continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through the chemical alteration of halogen atoms in single-crystal halide perovskites. 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. As a practical demonstration, we have created perovskite anisotropic nanowaveguides and shown 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.

Frequency Combs with Parity-Protected Cross-Correlations and Entanglement from Dynamically Modulated Qubit Arrays.

We develop a general theoretical framework to dynamically engineer quantum correlations and entanglement in the frequency-comb emission from an array of superconducting qubits in a waveguide, rigorously accounting for the temporal modulation of the qubit resonance frequencies. We demonstrate that when the resonance frequencies of the two qubits are periodically modulated with a π phase shift, it is possible to realize simultaneous bunching and antibunching in cross-correlations as well as Bell states of the scattered photons from different sidebands. Our approach, based on the dynamical conversion between the quantum excitations with different parity symmetry, is quite universal. It can be used to control multiparticle correlations in generic dynamically modulated dissipative quantum systems.

Polaron-Enhanced Polariton Nonlinearity in Lead Halide Perovskites.

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 the exciton-exciton interaction, making materials with the highest levels of exciton nonlinearity applicable at cryogenic temperatures only. Here, we show that strong polaronic effects, characteristic for perovskite materials, allow overcoming this limitation. Namely, we demonstrate a record-high value of the nonlinear optical response in the nanostructured organic-inorganic halide perovskite MAPbI3, experimentally detected as a 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 orthorhombic phases of MAPbI3 and remains stable at elevated temperatures.

Waveguide Quantum Optomechanics: Parity-Time Phase Transitions in Ultrastrong Coupling Regime.

We develop a rigorous theoretical framework for interaction-induced phenomena in the waveguide quantum electrodynamics (QED) driven by mechanical oscillations of the qubits. Specifically, we predict that the simplest setup of two qubits, harmonically trapped over an optical waveguide, enables the ultrastrong coupling regime of the quantum optomechanical interaction. Moreover, the combination of the inherent open nature of the system and the strong optomechanical coupling leads to emerging parity-time (PT) symmetry, quite unexpected for a purely quantum system without artificially engineered gain and loss. The PT phase transition drives long-living subradiant states, observable in the state-of-the-art waveguide QED setups.

Room-Temperature Spin Polariton Diode Laser.

A spin-polarized laser offers inherent control of the output circular polarization. We have investigated the output polarization characteristics of a bulk GaN-based microcavity polariton diode laser at room temperature with electrical injection of spin-polarized electrons via a FeCo/MgO spin injector. Polariton laser operation with a spin-polarized current is characterized by a threshold of ∼69 A/cm^{2} in the light-current characteristics, a significant reduction of the electroluminescence linewidth and blueshift of the emission peak. A degree of output circular polarization of ∼25% is recorded under remanent magnetization. A second threshold, due to conventional photon lasing, is observed at an injection of ∼7.2 kA/cm^{2}. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed with the carrier and exciton rate equations and the Gross-Pitaevskii equations for the condensate and there is good agreement between measured and calculated data.

Microcavity with saturable nonlinearity under simultaneous resonant and nonresonant pumping: multistability, Hopf bifurcations and chaotic behaviour.

We studied optical response of microcavity non-equilibrium exciton-polariton Bose-Einstein condensate with saturable nonlinearity under simultaneous resonant and non-resonant pumping. We demonstrated the emergence of multistabile behavior due to the saturation of the excitonic absorption. Stable periodic Rabi-type oscillations of the excitonic and photonic condensate components in the regime of the stationary pump and their transition to the chaotic dynamics through the cascade of Hopf bifurcations by tuning of the electrical pump are revealed.

Generation of Photon-Plasmon Quantum States in Nonlinear Hyperbolic Metamaterials.

We develop a general theoretical framework of integrated paired photon-plasmon generation through spontaneous wave mixing in nonlinear plasmonic and metamaterial nanostructures, rigorously accounting for material dispersion and losses in the quantum regime through the electromagnetic Green function. We identify photon-plasmon correlations in layered metal-dielectric structures with 70% internal heralding quantum efficiency and reveal a novel mechanism of broadband generation enhancement due to topological transition in hyperbolic metamaterials.

Near-field coupling and resonant cavity modes in plasmonic nanorod metamaterials.

Plasmonic resonant cavities are capable of confining light at the nanoscale, resulting in both enhanced local electromagnetic fields and lower mode volumes. However, conventional plasmonic resonant cavities possess large Ohmic losses at metal-dielectric interfaces. Plasmonic near-field coupling plays a key role in a design of photonic components based on the resonant cavities because of the possibility to reduce losses. Here, we study the plasmonic near-field coupling in the silver nanorod metamaterials treated as resonant nanostructured optical cavities. Reflectance measurements reveal the existence of multiple resonance modes of the nanorod metamaterials, which is consistent with our theoretical analysis. Furthermore, our numerical simulations show that the electric field at the longitudinal resonances forms standing waves in the nanocavities due to the near-field coupling between the adjacent nanorods, and a new hybrid mode emerges due to a coupling between nanorods and a gold-film substrate. We demonstrate that this coupling can be controlled by changing the gap between the silver nanorod array and gold substrate.

Controlling electromagnetic scattering with wire metamaterial resonators.

Manipulation of radiation is required for enabling a span of electromagnetic applications. Since properties of antennas and scatterers are very sensitive to the surrounding environment, macroscopic artificially created materials are good candidates for shaping their characteristics. In particular, metamaterials enable controlling both dispersion and density of electromagnetic states, available for scattering from an object. As a result, properly designed electromagnetic environments could govern wave phenomena and tailor various characteristics. Here electromagnetic properties of scattering dipoles, situated inside a wire medium (metamaterial), are analyzed both numerically and experimentally. The effect of the metamaterial geometry, dipole arrangement inside the medium, and frequency of the incident radiation on the scattering phenomena is studied in detail. It is shown that the resonance of the dipole hybridizes with Fabry-Perot modes of the metamaterial, giving rise to a complete reshaping of electromagnetic properties. Regimes of controlled scattering suppression and super-scattering are experimentally observed. Numerical analysis is in agreement with the experiment, performed at the GHz spectral range. The reported approach to scattering control with metamaterials could be directly mapped into optical and infrared spectral ranges by employing scalability properties of Maxwell's equations.

Compton-like polariton scattering in hyperbolic metamaterials.

We study the scattering of polaritons by free electrons in hyperbolic photonic media and demonstrate that the unconventional dispersion and high local density of states of electromagnetic modes in composite media with hyperbolic dispersion can lead to a giant Compton-like shift and dramatic enhancement of the scattering cross section. We develop a universal approach to study multiphoton processes in nanostructured media and derive the intensity spectrum of the scattered radiation for realistic metamaterial structures.

Complex band structure of nanostructured metal-dielectric metamaterials.

We study complex eigenmodes of layered metal-dielectric metamaterials. Varying losses from weak to realistic, we analyze band structure of the metamaterial and clarify effect of losses on its intrinsic electromagnetic properties. The structure operates in a regime with infinite numbers of eigenmodes, whereas we analyze dominant ones.

Photoluminescence imaging of single photon emitters within nanoscale strain profiles in monolayer WSe2.

Local deformation of atomically thin van der Waals materials provides a powerful approach to create site-controlled chip-compatible single-photon emitters (SPEs). However, the microscopic mechanisms underlying the formation of such strain-induced SPEs are still not fully clear, which hinders further efforts in their deterministic integration with nanophotonic structures for developing practical on-chip sources of quantum light. Here we investigate SPEs with single-photon purity up to 98% created in monolayer WSe2 via nanoindentation. Using photoluminescence imaging in combination with atomic force microscopy, we locate single-photon emitting sites on a deep sub-wavelength spatial scale and reconstruct the details of the surrounding local strain potential. The obtained results suggest that the origin of the observed single-photon emission is likely related to strain-induced spectral shift of dark excitonic states and their hybridization with localized states of individual defects.

Brightening of a dark monolayer semiconductor via strong light-matter coupling in a cavity.

Engineering the properties of quantum materials via strong light-matter coupling is a compelling research direction with a multiplicity of modern applications. Those range from modifying charge transport in organic molecules, steering particle correlation and interactions, and even controlling chemical reactions. Here, we study the modification of the material properties via strong coupling and demonstrate an effective inversion of the excitonic band-ordering in a monolayer of WSe2 with spin-forbidden, optically dark ground state. In our experiments, we harness the strong light-matter coupling between cavity photon and the high energy, spin-allowed bright exciton, and thus creating two bright polaritonic modes in the optical bandgap with the lower polariton mode pushed below the WSe2 dark state. We demonstrate that in this regime the commonly observed luminescence quenching stemming from the fast relaxation to the dark ground state is prevented, which results in the brightening of this intrinsically dark material. We probe this effective brightening by temperature-dependent photoluminescence, and we find an excellent agreement with a theoretical model accounting for the inversion of the band ordering and phonon-assisted polariton relaxation.

Bosonic Hofstadter butterflies in synthetic antiferromagnetic patterns.

The emergence of Hofstadter butterflies for bosons in synthetic-gauge-field antiferromagnetic (AFM) patterns is theoretically studied. We report on a specific tight-binding model of artificial AFM structures incorporating both nearest and next-to-nearest neighbour tunnelings and allowing for the formation of the fractal spectra even with the vanishing gauge field flux through the lattice. The model is applied to square and honeycomb lattices. Possible experimental realization is suggested for the lattices of microring resonators connected by waveguides. Finally, the structure of the butterflies is analyzed for different points in the magnetic Brillouin zone for both the ferromagnetic and AFM patterns.

Nonlinear polaritons in a monolayer semiconductor coupled to optical bound states in the continuum.

Optical bound states in the continuum (BICs) provide a way to engineer very narrow resonances in photonic crystals. The extended interaction time in these systems is particularly promising for the enhancement of nonlinear optical processes and the 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 the optical BIC in a photonic crystal slab with excitons in the atomically thin semiconductor MoSe2 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 the far field toward the BIC wavevector, in combination with effective reduction of the excitonic disorder through motional narrowing, results in small polariton linewidths below 3 meV. Together with a strongly wavevector-dependent Q-factor, this provides for the enhancement and control of polariton-polariton interactions and the resulting nonlinear optical effects, paving the way toward tuneable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics.

Resonant spin wave excitation in magnetoplasmonic bilayers using short laser pulses.

In magnetically ordered solids a static magnetic field can be generated by virtue of the transverse magneto-optical Kerr effect (TMOKE). Moreover, the latter was shown to be dramatically enhanced due to the optical excitation of surface plasmons in nanostructures with relatively small optical losses. In this paper we suggest a new method for resonant optical excitation in a prototypical bilayer composed of a noble metal (Au) with grating and a ferromagnetic thin film of yttrium iron garnet (YIG) via a frequency comb. Based on magnetization dynamics simulations we show that for a frequency comb with certain parameters, chosen to be resonant with the spin-wave excitations of YIG, the TMOKE is drastically enhanced, hinting at possible technological applications in optical control of spintronics systems.

Effective surface conductivity of optical hyperbolic metasurfaces: from far-field characterization to surface wave analysis.

Metasurfaces offer great potential to control near- and far-fields through engineering optical properties of elementary cells or meta-atoms. Such perspective opens a route to efficient manipulation of the optical signals both at nanoscale and in photonics applications. In this paper we show that a local surface conductivity tensor well describes optical properties of a resonant plasmonic hyperbolic metasurface both in the far-field and in the near-field regimes, where spatial dispersion usually plays a crucial role. We retrieve the effective surface conductivity tensor from the comparative analysis of experimental and numerical reflectance spectra of a metasurface composed of elliptical gold nanoparticles. Afterwards, the restored conductivities are validated by semi-analytic parameters obtained with the nonlocal discrete dipole model with and without interaction contribution between meta-atoms. The effective parameters are further used for the dispersion analysis of surface plasmons localized at the metasurface. The obtained effective conductivity describes correctly the dispersion law of both quasi-TE and quasi-TM plasmons in a wide range of optical frequencies as well as the peculiarities of their propagation regimes, in particular, topological transition from the elliptical to hyperbolic regime with eligible accuracy. The analysis in question offers a simple practical way to describe properties of metasurfaces including ones in the near-field zone with effective conductivity tensor extracting from the convenient far-field characterization.