Enhanced Optical Cross Section via Collective Coupling of Atomic Dipoles in a 2D Array

Strongly Interacting Photons in 2D Waveguide QED

One-dimensional confinement in waveguide quantum electrodynamics (QED) plays a crucial role to enhance light-matter interactions and to induce a strong quantum nonlinear optical response. In two or higher-dimensional settings, this response is reduced since photons can be emitted within a larger phase space, opening the question whether strong photon-photon interaction can be still achieved. In this study, we positively answer this question for the case of a 2D square array of atoms coupled to the light confined into a two-dimensional waveguide. More specifically, we demonstrate the occurrence of long-lived two-photon repulsive and bound states with genuine 2D features. Furthermore, we observe signatures of these effects also in free-space atomic arrays in the form of weakly subradiant in-band scattering resonances. Our findings provide a paradigmatic signature of the presence of strong photon-photon interactions in 2D waveguide QED.

Metasurface-Based Hybrid Optical Cavities for Chiral Sensing

Quantum metasurfaces, i.e., two-dimensional subwavelength arrays of quantum emitters, can be employed as mirrors towards the design of hybrid cavities, where the optical response is given by the interplay of a cavity-confined field and the surface modes supported by the arrays. We show that stacked layers of quantum metasurfaces with orthogonal dipole orientation can serve as helicity-preserving cavities. These structures exhibit ultranarrow resonances and can enhance the intensity of the incoming field by orders of magnitude, while simultaneously preserving the handedness of the field circulating inside the resonator, as opposed to conventional cavities. The rapid phase shift in the cavity transmission around the resonance can be exploited for the sensitive detection of chiral scatterers passing through the cavity. We discuss possible applications of these resonators as sensors for the discrimination of chiral molecules. Our approach describes a new way of chiral sensing via the measurement of particle-induced phase shifts.

Trapping and Imaging Single Dysprosium Atoms in Optical Tweezer Arrays

We report the preparation and observation of single atoms of dysprosium in arrays of optical tweezers with a wavelength of 532 nm, imaged on the intercombination line at 626 nm. We use the anisotropic light shift specific to lanthanides and in particular a large difference in tensor and vector polarizabilities between the ground and excited states to tune the differential light shift and produce tweezers in near-magic or magic polarization. This allows us to find a regime where single atoms can be trapped and imaged. Using the tweezer array toolbox to manipulate lanthanides will open new research directions for quantum physics studies by taking advantage of their rich spectrum, large spin, and magnetic dipole moment.

Optical Properties of Concentric Nanorings of Quantum Emitters

A ring of sub-wavelength spaced dipole-coupled quantum emitters features extraordinary optical properties when compared to a one-dimensional chain or a random collection of emitters. One finds the emergence of extremely subradiant collective eigenmodes similar to an optical resonator, which features strong 3D sub-wavelength field confinement near the ring. Motivated by structures commonly appearing in natural light-harvesting complexes (LHCs), we extend these studies to stacked multi-ring geometries. We predict that using double rings allows us to engineer significantly darker and better confined collective excitations over a broader energy band compared to the single-ring case. These enhance weak field absorption and low-loss excitation energy transport. For the specific geometry of the three rings appearing in the natural LH2 light-harvesting antenna, we show that the coupling between the lower double-ring structure and the higher energy blue-shifted single ring is very close to a critical value for the actual size of the molecule. This creates collective excitations with contributions from all three rings, which is a vital ingredient for efficient and fast coherent inter-ring transport. This geometry thus should also prove useful for the design of sub-wavelength weak field antennae.

Linear optical elements based on cooperative subwavelength emitter arrays

We describe applications of two-dimensional subwavelength quantum emitter arrays as efficient optical elements in the linear regime. For normally incident light, the cooperative optical response, stemming from emitter-emitter dipole exchanges, allows the control of the array's transmission, its resonance frequency, and bandwidth. Operations on fully polarized incident light, such as generic linear and circular polarizers as well as phase retarders can be engineered and described in terms of Jones matrices. Our analytical approach and accompanying numerical simulations identify optimal regimes for such operations and reveal the importance of adjusting the array geometry and of the careful tuning of the external magnetic fields amplitude and direction.

Magneto-Optical Chirality in a Coherently Coupled Exciton-Plasmon System

Chirality is a fundamental asymmetry phenomenon, with chiral optical elements exhibiting asymmetric response in reflection or absorption of circularly polarized light. Recent realizations of such elements include nanoplasmonic systems with broken-mirror symmetry and polarization-contrasting optical absorption known as circular dichroism. An alternative route to circular dichroism is provided by spin-valley polarized excitons in atomically thin semiconductors. In the presence of magnetic fields, they exhibit an imbalanced coupling to circularly polarized photons and thus circular dichroism. Here, we demonstrate that polarization-contrasting optical transitions associated with excitons in monolayer WSe₂ can be transferred to proximal plasmonic nanodisks by coherent coupling. The coupled exciton-plasmon system exhibits magneto-induced circular dichroism in a spectrally narrow window of Fano interference, which we model in a master equation framework. Our work motivates the use of exciton-plasmon interfaces as building blocks of chiral metasurfaces for applications in information processing, nonlinear optics, and sensing.

Lattice Resonances Excited by Finite-Width Light Beams

Periodic arrays of metallic nanostructures support collective lattice resonances, which give rise to optical responses that are, at the same time, stronger and more spectrally narrow than those of the localized plasmons of the individual nanostructures. Despite the extensive research effort devoted to investigating the optical properties of lattice resonances, the majority of theoretical studies have analyzed them under plane-wave excitation conditions. Such analysis not only constitutes an approximation to realistic experimental conditions, which require the use of finite-width light beams, but also misses a rich variety of interesting behaviors. Here, we provide a comprehensive study of the response of periodic arrays of metallic nanostructures when excited by finite-width light beams under both paraxial and nonparaxial conditions. We show how as the width of the light beam increases, the response of the array becomes more collective and converges to the plane-wave limit. Furthermore, we analyze the spatial extent of the lattice resonance and identify the optimum values of the light beam width to achieve the strongest optical responses. We also investigate the impact that the combination of finite-size effects in the array and the finite width of the light beam has on the response of the system. Our results provide a solid theoretical framework to understand the excitation of lattice resonances by finite-width light beams and uncover a set of behaviors that do not take place under plane-wave excitation.

Efficient nano-photonic antennas based on dark states in quantum emitter rings

Nanoscopic arrays of quantum emitters can feature highly sub-radiant collective excitations with a lifetime exponentially growing with emitter number. Adding an absorptive impurity as an energy dump in the center of a ring shaped polygon allows to exploit this feature to create highly efficient single photon antennas. Here among regular polygons with an identical center absorbing emitter, a nonagon exhibits a distinct optimum of the absorption efficiency. This special enhancement originates from the unique emergence of a subradiant eigenstate with dominant center occupation. Only for nine emitters the sum of coupling strengths of each emitter to all others matches the center to the ring coupling. Analogous to a parabolic mirror the antenna ring then concentrates incoming radiation at its center without being significantly excited itself. Similar large efficiency enhancements, which even prevail for broadband excitation, can also be engineered for other antenna sizes by tailoring the frequency and magnitude of the central absorber. Interestingly, for very small structures a quantum treatment predicts an even stronger enhancement for the single photon absorption enhancement than a classical dipole model. As natural light harvesting structures are often based on ring shaped structures, the underlying principle might be exploited there as well.

Tunable Directional Emission and Collective Dissipation with Quantum Metasurfaces

Subwavelength atomic arrays, recently labeled as quantum metamaterials, have emerged as an exciting platform for obtaining novel quantum optical phenomena. The strong interference effects in these systems generate subradiant excitations that propagate through the atomic array with very long lifetimes. Here, we demonstrate that one can harness these excitations to obtain tunable directional emission patterns and collective dissipative couplings when placing judiciously additional atoms nearby the atomic array. For doing that, we first characterize the optimal square array geometry to obtain directional emission patterns. Then, we characterize the best atomic positions to couple efficiently to the subradiant metasurface excitations and provide several improvement strategies based on entangled atomic clusters or bilayers. Afterward, we also show how the directionality of the emission pattern can be controlled through the relative dipole orientation between the auxiliary atoms and the one of the array. Finally, we benchmark how these directional emission patterns translate into to collective, anisotropic dissipative couplings between the auxiliary atoms by studying the lifetime modification of atomic entangled states.

Moiré graphene nanoribbons: nearly perfect absorptions and highly efficient reflections with wide angles

The strong absorption and reflection from atomically thin graphene nanoribbons has been demonstrated over the past decade. However, due to the significant band dispersion of graphene nanoribbons, the angle of incident wave has remained limited to a very narrow range. Obtaining strong absorption and reflection with a wide range of incident angles from atomically thin graphene layers has remained an unsolvable problem. Here, we construct a tunable moiré superlattice composed of a pair of graphene nanoribbon arrays to achieve this goal. By designing the interlayer coupling between two graphene nanoribbon arrays with mismatched periods, the moiré flat bands and the localization of their eigen-fields was realized. Based on the moiré flat bands of graphene nanoribbons, highly efficient reflection and nearly perfect absorption was achieved with a wide range of incident angles. Even more interesting, is how these novel phenomena can be tuned through the adjustment of the graphene's Fermi energy, either electrostatically or chemically. Our designed moiré graphene nanoribbons suggest a promising platform to engineer moiré physics with tunable behaviors, and may have potential applications in the field of wide-angle absorbers and reflectors in the mid-infrared region.

Quantum Nonlinear Optics Based on Two-Dimensional Rydberg Atom Arrays

We propose the combination of subwavelength, two-dimensional atomic arrays and Rydberg interactions as a powerful platform to realize strong, coherent interactions between individual photons with high fidelity. The atomic spatial ordering guarantees efficient atom-light interactions without the possibility of scattering light into unwanted directions, allowing the array to act as a perfect mirror for individual photons. In turn, Rydberg interactions enable single photons to alter the optical response of the array within a potentially large blockade radius R_{b}, which can effectively punch a large "hole" for subsequent photons. We show that such a system enables a coherent photon-photon gate or switch, with a significantly better error scaling (∼R_{b}^{-4}) than in a disordered ensemble. We also investigate the optical properties of the system in the limit of strong input intensities and show that this many-body quantum driven dissipative system can be modeled well by a semiclassical model based on holes punched in a classical mirror.

Observation of Pauli blocking in light scattering from quantum degenerate fermions

[Figure: see text].

Creating Quantum Emitters in Hexagonal Boron Nitride Deterministically on Chip-Compatible Substrates

Two-dimensional hexagonal boron nitride (hBN) that hosts room-temperature single-photon emitters (SPEs) is promising for quantum information applications. An important step toward the practical application of hBN is the on-demand, position-controlled generation of SPEs. Strategies reported for deterministic creation of hBN SPEs either rely on substrate nanopatterning that is not compatible with integrated photonics or utilize radiation sources that might introduce unpredictable damage or contamination to hBN. Here, we report a radiation- and lithography-free route to deterministically activate hBN SPEs by nanoindentation with atomic force microscopy (AFM). The method applies to hBN flakes on flat silicon dioxide-silicon substrates that can be readily integrated into on-chip photonic devices. The achieved SPE yields are above 30% for multiple indent sizes, and a maximum yield of 36% is demonstrated for indents around 400 nm. Our results mark an important step toward the deterministic creation and integration of hBN SPEs with photonic and plasmonic devices.

Controlling Interactions between Quantum Emitters Using Atom Arrays

We investigate the potential for two-dimensional atom arrays to modify the radiation and interaction of individual quantum emitters. Specifically, we demonstrate that control over the emission linewidths, resonant frequency shifts, and local driving field enhancement in impurity atoms is possible due to strong dipole-dipole interactions within ordered, subwavelength atom array configurations. We demonstrate that these effects can be used to dramatically enhance coherent dipole-dipole interactions between distant impurity atoms within an atom array. Possible experimental realizations and potential applications are discussed.

Strongly Correlated States of Light and Repulsive Photons in Chiral Chains of Three-Level Quantum Emitters

We study the correlated transport of photons through a chain of three-level emitters that are coupled chirally to a photonic mode of a waveguide. It is found that this system can transfer a weak classical input into a strongly correlated state of light in a unitary manner. Our analysis reveals two-photon scattering eigenstates, that are akin to Fano resonances or shape resonances in particle collisions and facilitate the emergence of antibunched light with long-range correlations upon crossing a critical length of the chain. By operating close to conditions of electromagnetically induced transparency of the three-level medium, a high degree of antibunching and photon transmission can be maintained in the presence of moderate losses. These features suggest a promising mechanism for single-photon generation and may open the door to exploring correlated quantum many-body states of light with repulsively interacting photons.

Many-Body Signatures of Collective Decay in Atomic Chains

Fully inverted atoms placed at exactly the same location synchronize as they deexcite, and light is emitted in a burst (known as "Dicke's superradiance"). We investigate the role of finite interatomic separation on correlated decay in mesoscopic chains and provide an understanding in terms of collective jump operators. We show that the superradiant burst survives at small distances, despite Hamiltonian dipole-dipole interactions. However, for larger separations, competition between different jump operators leads to dephasing, suppressing superradiance. Collective effects are still significant for arrays with lattice constants of the order of a wavelength, and lead to a photon emission rate that decays nonexponentially in time. We calculate the two-photon correlation function and demonstrate that emission is correlated and directional, as well as sensitive to small changes in the interatomic distance. These features can be measured in current experimental setups, and are robust to realistic imperfections.

Geometric Control of Collective Spontaneous Emission

Dipole spin-wave states of atomic ensembles with wave vector k(ω) mismatched from the dispersion relation of light are difficult to access by far-field excitation but may support rich phenomena beyond the traditional phase-matched scenario in quantum optics. We propose and demonstrate an optical technique to efficiently access these states. In particular, subnanosecond laser pulses shaped by a home-developed wideband modulation method are applied to shift the spin wave in k space with state-dependent geometric phase patterning, in an error-resilient fashion and on timescales much faster than spontaneous emission. We verify this control through the redirection, switch off, and recall of collectively enhanced emission from a ^{87}Rb gas with ∼75% single-step efficiency. Our work represents a first step toward efficient control of electric dipole spin waves for studying many-body dissipative dynamics of excited gases, as well as for numerous quantum optical applications.

Optical Magnetism and Huygens' Surfaces in Arrays of Atoms Induced by Cooperative Responses

By utilizing strong optical resonant interactions in arrays of atoms with electric dipole transitions, we show how to synthesize collective optical responses that correspond to those formed by arrays of magnetic dipoles and other multipoles. Optically active magnetism with the strength comparable with that of electric dipole transitions is achieved in collective excitation eigenmodes of the array. By controlling the atomic level shifts, an array of spectrally overlapping, crossed electric and magnetic dipoles can be excited, providing a physical realization of a nearly reflectionless quantum Huygens' surface with the full 2π phase control of the transmitted light that allows for extreme wavefront engineering even at a single photon level. We illustrate this by creating a superposition of two different orbital angular momentum states of light from an ordinary input state that has no orbital angular momentum.

Quantum Metamaterials with Magnetic Response at Optical Frequencies

We propose novel quantum antennas and metamaterials with a strong magnetic response at optical frequencies. Our design is based on the arrangement of natural quantum emitters with only electric dipole transition moments at distances smaller than a wavelength of light but much larger than their physical size. In particular, we show that an atomic dimer can serve as a magnetic antenna at its antisymmetric mode to enhance the decay rate of a magnetic transition in its vicinity by several orders of magnitude. Furthermore, we study metasurfaces composed of atomic bilayers with and without cavities and show that they can fully reflect the electric and magnetic fields of light, thus, forming nearly perfect electric or magnetic mirrors. The proposed metamaterials will embody the intrinsic quantum functionalities of natural emitters such as atoms, ions, color center, or molecules and can be fabricated with available state-of-the-art technologies, promising several applications both in classical optics and quantum engineering.

Collective Shift in Resonant Light Scattering by a One-Dimensional Atomic Chain

We experimentally study resonant light scattering by a one-dimensional randomly filled chain of cold two-level atoms. By a local measurement of the light scattered along the chain, we observe constructive interferences in light-induced dipole-dipole interactions between the atoms. They lead to a shift of the collective resonance despite the average interatomic distance being larger than the wavelength of the light. This result demonstrates that strong collective effects can be enhanced by structuring the geometrical arrangement of the ensemble. We also explore the high intensity regime where atoms cannot be described classically. We compare our measurement to a mean-field, nonlinear coupled-dipole model accounting for the saturation of the response of a single atom.

Light reflection and transmission in planar lattices of cold atoms

Manipulation of light using atoms plays a fundamental and important role in emerging technologies such as integrated photonics, information storage, and quantum sensors. Specifically, there have been intense theoretical efforts involving large samples of cold neutral atoms for coherent control of light. Here we present a theoretical scheme that enables efficient computation of collective optical responses of mono- and bi-layer planar square lattices of dense, cold two-level atoms using classical electrodynamics of coupled dipoles in the limit of low laser intensity. The steady-state transmissivity and reflectivity are obtained at a field point far away from the atomic lattices in the regime with no Bragg reflection. While our earlier method was based on exact solution of the electrodynamics for a small-scale lattice, here we calculate the dipole moments assuming that they are the same at all lattice sites, as for an infinite lattice. Atomic lattices with effectively over one hundred times more sites than in our earlier exact computations can then be simulated numerically with fewer computational resources. We have implemented an automatic selection of the number of sites under the given convergence criteria. We compare the numerical results from both computational schemes. We also find similarities and differences of a stack of two atomic lattices from a two-atom sample. Such aspects may be exploited to engineer a stack for potential applications.

Near-Resonant Light Scattering by a Subwavelength Ensemble of Identical Atoms

We study theoretically the scattering of light by an ensemble of N resonant atoms in a subwavelength volume. We consider the low intensity regime so that each atom responds linearly to the field. While N noninteracting atoms would scatter N^{2} more than a single atom, we find that N interacting atoms scatter less than a single atom near resonance. In addition, the scattered power presents strong fluctuations, either from one realization to another or when varying the excitation frequency. We analyze this counterintuitive behavior in terms of collective modes resulting from the light-induced dipole-dipole interactions. We find that for small samples and sufficiently large atom number, their properties are governed only by their volume.

Analysis of the Limits of the Near-Field Produced by Nanoparticle Arrays

Periodic arrays are an exceptionally interesting arrangement for metallic nanostructures because of their ability to support collective lattice resonances. These modes, which arise from the coherent multiple scattering enabled by the lattice periodicity, give rise to very strong and spectrally narrow optical responses. Here, we investigate the enhancement of the near-field produced by the lattice resonances of arrays of metallic nanoparticles when illuminated with a plane wave. We find that, for infinite arrays, this enhancement can be made arbitrarily large by appropriately designing the geometrical characteristics of the array. On the other hand, in the case of finite arrays, the near-field enhancement is limited by the number of elements of the array that interact coherently. Furthermore, we show that, as the near-field enhancement increases, the length scale over which it extends above and below the array becomes larger and its spectral linewidth narrows. We also analyze the impact that material losses have on these behaviors. As a direct application of our results, we investigate the interaction between a nanoparticle array and a dielectric slab placed a certain distance above it and show that the extraordinary near-field enhancement produced by the lattice resonance can lead to very strong interactions, even at significantly large separations. This work provides a detailed characterization of the limits of the near-field produced by lattice resonances and, therefore, advances our knowledge of the optical response of periodic arrays of nanostructures, which can be used to design and develop applications exploiting the extraordinary properties of these systems.

Cavity Quantum Electrodynamics with Frequency-Dependent Reflectors

We present a general framework for cavity quantum electrodynamics with strongly frequency-dependent mirrors. The method is applicable to a variety of reflectors exhibiting sharp internal resonances as can be realized, for example, with photonic-crystal mirrors or with two-dimensional atomic arrays around subradiant points. Our approach is based on a modification of the standard input-output formalism to explicitly include the dynamics of the mirror's internal resonance. We show how to directly extract the interaction parameters from the comparison with classical transfer matrix theory and how to treat the non-Markovian dynamics of the cavity field mode introduced by the mirror's internal resonance. As an application within optomechanics, we illustrate how a non-Markovian Fano-resonance cavity with a flexible photonic-crystal mirror can provide both sideband resolution as well as strong heating suppression in optomechanical cooling. This approach, amenable to a wide range of systems, opens up possibilities for using hybrid frequency-dependent reflectors in cavity quantum electrodynamics for engineering novel forms of light-matter interactions.

Subradiant Bell States in Distant Atomic Arrays

We study collective "free-space" radiation properties of two distant single-layer arrays of quantum emitters as two-level atoms. We show that this system can support a long-lived Bell superposition state of atomic excitations exhibiting strong subradiance, which corresponds to a nonlocal excitation of the two arrays. We describe the preparation of these states and their application in quantum information as a resource of nonlocal entanglement, including deterministic quantum state transfer with high fidelity between the arrays representing quantum memories. We discuss experimental realizations using cold atoms in optical trap arrays with subwavelength spacing, and analyze the role of imperfections.

Lasing and Amplification from Two-Dimensional Atom Arrays

We explore the ability of two-dimensional periodic atom arrays to produce light amplification and generate laser emission when gain is introduced through external optical pumping. Specifically, we predict that lasing can take place for arbitrarily weak atomic scatterers assisted by cooperative interaction among atoms in a 2D lattice. We base this conclusion on analytical theory for three-level scatterers, which additionally reveals a rich interplay between lattice and atomic resonances. Our results provide a general background to understand light amplification and lasing in periodic atomic arrays, with promising applications in the generation, manipulation, and control of coherent photon states at the nanoscale.

Quantum Nonlinear Optics in Atomically Thin Materials

We show that a nonlinear optical response associated with a resonant, atomically thin material can be dramatically enhanced by placing it in front of a partially reflecting mirror, rendering otherwise weakly nonlinear systems suitable for experiments and applications involving quantum nonlinear optics. Our approach exploits the nonlinear response of long-lived polariton resonances that arise at particular distances between the material and the mirror. The scheme is entirely based on free-space optics, eliminating the need for cavities or complex nanophotonic structures. We analyze a specific implementation based on exciton-polariton resonances in two-dimensional semiconductors and discuss the role of imperfections and loss.

Collective Lamb Shift of a Nanoscale Atomic Vapor Layer within a Sapphire Cavity

We measure the near-resonant transmission of light through a dense medium of potassium vapor confined in a cell with nanometer thickness in order to investigate the origin and validity of the collective Lamb shift. A complete model including the multiple reflections in the nanocell reproduces accurately the observed line shape. It allows the extraction of a density-dependent shift and width of the bulk atomic medium resonance, deconvolved from the cavity effect. We observe an additional, unexpected dependence of the shift with the thickness of the medium. This extra dependence demands further experimental and theoretical investigations.

Hybridization of Lattice Resonances

Plasmon hybridization, the electromagnetic analog of molecular orbital theory, provides a simple and intuitive method to describe the plasmonic response of complex nanostructures from the combination of the responses of their individual constituents. Here, we follow this approach to investigate the optical properties of periodic arrays of plasmonic nanoparticles with multiparticle unit cells. These systems support strong collective lattice resonances, arising from the coherent multiple scattering enabled by the lattice periodicity. Due to the extended nature of these modes, the interaction between them is very different from that among localized surface plasmons supported by individual nanoparticles. This leads to a much richer hybridization scenario, which we exploit here to design periodic arrays with engineered properties. These include arrays with two-particle unit cells, in which the interaction between the individual lattice resonances can be canceled or maximized by controlling the relative position of the particles within the unit cell, as well as arrays whose response can be made either invariant to the polarization of the incident light or strongly dependent on it. Moreover, we explore systems with three- and four-particle unit cells and show that they can be designed to support lattice resonances with complex hybridization patterns in which different groups of particles in the unit cell can be selectively excited. The results of this work serve to advance our understanding of periodic arrays of nanostructures and provide a methodology to design periodic structures with engineered properties for applications in nanophotonics.

Cooperative Effects in Closely Packed Quantum Emitters with Collective Dephasing

In a closely packed ensemble of quantum emitters, cooperative effects are typically suppressed due to the dephasing induced by the dipole-dipole interactions. Here, we show that by adding sufficiently strong collective dephasing, cooperative effects can be restored. Specifically, we show that the dipole force on a closely packed ensemble of strongly driven two-level quantum emitters, which collectively dephase, is enhanced in comparison to the dipole force on an independent noninteracting ensemble. Our results are relevant to solid-state systems with embedded quantum emitters such as color centers in diamond and superconducting qubits in microwave cavities and waveguides.

Realization of an Electrically Tunable Narrow-Bandwidth Atomically Thin Mirror Using Monolayer MoSe_{2}

The advent of two-dimensional semiconductors, such as van der Waals heterostructures, propels new research directions in condensed matter physics and enables development of novel devices with unique functionalities. Here, we show experimentally that a monolayer of MoSe_{2} embedded in a charge controlled heterostructure can be used to realize an electrically tunable atomically thin mirror, which effects 87% extinction of an incident field that is resonant with its exciton transition. The corresponding maximum reflection coefficient of 41% is only limited by the ratio of the radiative decay rate to the nonradiative linewidth of exciton transition and is independent of incident light intensity up to 400  W/cm^{2}. We demonstrate that the reflectivity of the mirror can be drastically modified by applying a gate voltage that modifies the monolayer charge density. Our findings could find applications ranging from fast programable spatial light modulators to suspended ultralight mirrors for optomechanical devices.

Large Excitonic Reflectivity of Monolayer MoSe_{2} Encapsulated in Hexagonal Boron Nitride

We demonstrate that a single layer of MoSe_{2} encapsulated by hexagonal boron nitride can act as an electrically switchable mirror at cryogenic temperatures, reflecting up to 85% of incident light at the excitonic resonance. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor. We show that the MoSe_{2} monolayer exhibits power-and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used.

Optimization of photon storage fidelity in ordered atomic arrays

A major application for atomic ensembles consists of a quantum memory for light, in which an optical state can be reversibly converted to a collective atomic excitation on demand. There exists a well-known fundamental bound on the storage error, when the ensemble is describable by a continuous medium governed by the Maxwell-Bloch equations. However, these equations are semi-phenomenological, as they treat emission of the atoms into other directions other than the mode of interest as being independent. On the other hand, in systems such as dense, ordered atomic arrays, atoms interact with each other strongly and spatial interference of the emitted light might be exploited to suppress emission into unwanted directions, thereby enabling improved error bounds. Here, we develop a general formalism that fully accounts for spatial interference, and which finds the maximum storage efficiency for a single photon with known spatial input mode into a collection of atoms with discrete, known positions. As an example, we apply this technique to study a finite two-dimensional square array of atoms. We show that such a system enables a storage error that scales with atom number N a like ∼ ( log N a ) 2 ∕ N a 2 , and that, remarkably, an array of just 4 × 4 atoms in principle allows for an error of less than 1%, which is comparable to a disordered ensemble with an optical depth of around 600.

Cavity Antiresonance Spectroscopy of Dipole Coupled Subradiant Arrays

An array of N closely spaced dipole coupled quantum emitters exhibits super- and subradiance with characteristic tailorable spatial radiation patterns. Optimizing the emitter geometry and distance with respect to the spatial profile of a near resonant optical cavity mode allows us to increase the ratio between light scattering into the cavity mode and free space emission by several orders of magnitude. This leads to distinct scaling of the collective coherent emitter-field coupling vs the free space decay as a function of the emitter number. In particular, for subradiant states, the effective cooperativity increases much faster than the typical linear ∝N scaling for independent emitters. This extraordinary collective enhancement is manifested both in the amplitude and the phase profile of narrow collective antiresonances appearing at the cavity output port in transmission spectroscopy.

Design of metasurface polarizers based on two-dimensional cold atomic arrays

Engineering light-matter interaction using cold atomic arrays is one of the central topics in modern optics. Here we have demonstrated the capability of two-dimensional asymmetric cold atomic arrays as microscopic metasurfaces for controlling polarization states of light. The designed linear polarizer can lead to an extinction ratio over 20dB as well as a high transmittance over 0.8 for the permitted polarization at zero detuning. For detuned driving light, changing lattice constants can also achieve high performance linear polarizers. We have also accomplished a circular polarizer by manipulating the phases of transmitted light. A theoretical analysis based on Bloch theorem shows the underlying mechanism for this performance is actually attributed to cooperative effects in periodic lattices. Finally, we discuss in detail the effects of system size, lattice imperfection and nonzero driving light linewidth in practical implementation. The present study paves a way to design extremely miniaturized metasurfaces using cold atoms and other two-level systems, showing great potential in quantum information and quantum metrology sciences as well as the fundamental physics of light-matter interaction.

Cooperative Resonances in Light Scattering from Two-Dimensional Atomic Arrays

We consider light scattering off a two-dimensional (2D) dipolar array and show how it can be tailored by properly choosing the lattice constant of the order of the incident wavelength. In particular, we demonstrate that such arrays can operate as a nearly perfect mirror for a wide range of incident angles and frequencies, and shape the emission pattern from an individual quantum emitter into a well-defined, collimated beam. These results can be understood in terms of the cooperative resonances of the surface modes supported by the 2D array. Experimental realizations are discussed, using ultracold arrays of trapped atoms and excitons in 2D semiconductor materials, as well as potential applications ranging from atomically thin metasurfaces to single photon nonlinear optics and nanomechanics.

Storing Light with Subradiant Correlations in Arrays of Atoms

We show how strong light-mediated resonant dipole-dipole interactions between atoms can be utilized in a control and storage of light. The method is based on a high-fidelity preparation of a collective atomic excitation in a single correlated subradiant eigenmode in a lattice. We demonstrate how a simple phenomenological model captures the qualitative features of the dynamics and sharp transmission resonances that may find applications in sensing.

Emergence of correlated optics in one-dimensional waveguides for classical and quantum atomic gases

We analyze the emergence of correlated optical phenomena in the transmission of light through a waveguide that confines classical or ultracold quantum degenerate atomic ensembles. The conditions of the correlated collective response are identified in terms of atom density, thermal broadening, and photon losses by using stochastic Monte Carlo simulations and transfer matrix methods of transport theory. We also calculate the "cooperative Lamb shift" for the waveguide transmission resonance, and discuss line shifts that are specific to effectively one-dimensional waveguide systems.