Nikita Nefedkin,Michele Cotrufo,Alex Krasnok,Andrea Alù, Dark-State Induced Quantum Nonreciprocity (invited), Advanced Quantum Technologies, 2100112 (2022)

The emerging field of quantum computing has been rapidly growing and has shown interesting opportunities to overcome the limitations of classical computers for many currently unfeasible problems. A key technology required for quantum computation devices is the unidirectional signal propagation and routing, whereby electromagnetic radiation propagates asymmetrically between two points. Conventional isolators based on the magneto-optical effect in ferrite materials are expensive, barely tunable, bulky, and incompatible with planar technologies. This work presents our recent results on the isolation effect obtained by a suitable combination of quantum nonlinearities and symmetry breaking. Using an example of a two-qubit system, we show that the dark state and its properties are crucial to establishing large nonreciprocity in this class of systems.

D. G. Baranov, R. S. Savelev, S. V. Li, A. E. Krasnok, and A. Alù, Modifying Magnetic Dipole Spontaneous Emission with Nanophotonic Structures, Laser Photon. Rev. 11, 1600268 (2017)Here, we review the state-of-the-art advances in the field of spontaneous emission enhancement of magnetic dipole quantum emitters with the use of various nanophotonics systems. We provide the general theory describing the Purcell effect of magnetic emitters, overview realizations of specific nanophotonics structures allowing for the enhanced magnetic dipole spontaneous emission, and give an outlook on the challenges in this field, which remain open to future research.

S. Lepeshov, M. Wang, A. Krasnok, O. Kotov, T. Zhang, H. Liu, T. Jiang, B. Korgel, M. Terrones, Y. Zheng, and A. Alú, Tunable Resonance Coupling in Single Si Nanoparticle–Monolayer WS 2 Structures, ACS Appl. Mater. Interfaces 10, 16690 (2018)

Here, we address the issue of resonance coupling in hybrid exciton–polariton structures based on single Si nanoparticles (NPs) coupled to monolayer (1L)-WS2. We predict a strong coupling regime with a Rabi splitting energy exceeding 110 meV for a Si NP covered by 1L-WS2 at the magnetic optical Mie resonance because of the symmetry of the mode. Further, we achieve a large enhancement in the Rabi splitting energy up to 208 meV by changing the surrounding dielectric material from air to water. The prediction is based on the experimental estimation of TMDC dipole moment variation obtained from the measured photoluminescence spectra of 1L-WS2 in different solvents. An ability of such a system to tune the resonance coupling is realized experimentally for optically resonant spherical Si NPs placed on 1L-WS2. The Rabi splitting energy obtained for this scenario increases from 49.6 to 86.6 meV after replacing air by water. Our findings pave the way to develop high-efficiency optoelectronic, nanophotonic, and quantum optical devices.

A. Krasnok, S. Lepeshov, and A. Alú, Nanophotonics with 2D Transition Metal Dichalcogenides [Invited], Opt. Express 26, 15972 (2018)

Here, we review the state-of-the-art advances of hybrid exciton-polariton structures based on monolayer TMDCs coupled to plasmonic and dielectric nanocavities. We discuss the optical properties of 2D WS2, WSe2, MoS2 and MoSe2 materials, paying special attention to their energy bands, photoluminescence/absorption spectra, excitonic fine structure, and to the dynamics of exciton formation and valley depolarization. We also discuss light-matter interactions in such hybrid exciton-polariton structures. Finally, we focus on weak and strong coupling regimes in monolayer TMDCs-based exciton-polariton systems, envisioning research directions and future opportunities for this material platform.

L. Sun, C.-Y. Wang, A. Krasnok, J. Choi, J. Shi, J. S. Gomez-Diaz, A. Zepeda, S. Gwo, C.-K. Shih, A. Alù, and X. Li, Separation of Valley Excitons in a MoS2 Monolayer Using a Subwavelength Asymmetric Groove Array, Nat. Photonics 13, 180 (2019)

Here, we demonstrate that valley-polarized excitons can be sorted and spatially separated at room temperature by coupling a MoS2 monolayer to a subwavelength asymmetric groove array. In addition to separation of valley excitons in real space, emission from valley excitons is also separated in photon momentum-space; that is, the helicity of photons determines a preferential emission direction. Our work demonstrates that metasurfaces can facilitate valley transport and establish an interface between valleytronic and photonic devices, thus addressing outstanding challenges in the field of valleytronics.

M. Wang, Z. Wu, A. Krasnok, T. Zhang, M. Liu, H. Liu, L. Scarabelli, J. Fang, L. M. Liz‐Marzán, M. Terrones, A. Alù, and Y. Zheng, Dark‐Exciton‐Mediated Fano Resonance from a Single Gold Nanostructure on Monolayer WS 2 at Room Temperature, Small 15, 1900982 (2019).

Herein, the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles and monolayer WS2 is explored. A narrow Fano resonance is observed when the hybrid system is surrounded by water, and the narrowing of the spectral Fano linewidth is attributed to the plasmon-enhanced decay of dark K-K excitons. These results reveal that dark excitons in monolayer WS2 can strongly modify Fano resonances in hybrid plasmon–exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.

Guangwei Hu, Alex Krasnok, Yarden Mazor, Cheng-Wei Qiu, and Andrea Alù, Moiré Hyperbolic Metasurfaces, Nano Lett. 20, 3217 (2020)

Here, we study moiré hyperbolic plasmons in pairs of hyperbolic metasurfaces (HMTSs), unveiling analogous phenomena at the mesoscopic scale. HMTSs are known to support confined surface waves collimated toward specific directions determined by the metasurface dispersion. By rotating two evanescently coupled HMTSs with respect to one another, we unveil rich dispersion engineering, topological transitions at magic angles, broadband field canalization, and plasmon spin-Hall phenomena. These findings open remarkable opportunities to advance metasurface optics, enriching it with moiré physics and twistronic concepts.

[A. Krasnok and A. Alu, Active Nanophotonics, Proc. IEEE 108, 628 (2020)

Active nanophotonics, which combines the latest advances in nanotechnology with gain materials, has recently become a vital area of optics research, both from the physics, material science, and engineering standpoint. In this article, we review recent efforts in enabling active nanodevices for lasing and optical sources, loss compensation, and to realize new optical functionalities, like PT-symmetry, exceptional points, and nontrivial lasing based on suitably engineered distributions of gain and loss in nanostructures.

G. Hu, Q. Ou, G. Si, Y. Wu, J. Wu, Z. Dai, A. Krasnok, Y. Mazor, Q. Zhang, Q. Bao, C.-W. Qiu, and A. Alù, Topological Polaritons and Photonic Magic Angles in Twisted α-MoO3 Bilayers, Nature 582, 209 (2020).

Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0 is the free-space wavelength. Our findings extend twistronics10 and moiré physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.

M. Wang, A. Krasnok, S. Lepeshov, G. Hu, T. Jiang, J. Fang, B. A. Korgel, A. Alù, and Y. Zheng, Suppressing Material Loss in the Visible and Near-Infrared Range for Functional Nanophotonics Using Bandgap Engineering, Nat. Commun. 11, 5055 (2020).

Here, we employ bandgap engineering to synthesize hydrogenated amorphous Si nanoparticles (a-Si:H NPs) offering ideal features for functional nanophotonics. We observe significant material loss suppression in a-Si:H NPs in the visible range caused by hydrogenation-induced bandgap renormalization, producing strong higher-order resonant modes in single NPs with Q factors up to ~100 in the visible and near-IR range. We also realize highly tunable all-dielectric meta-atoms by coupling a-Si:H NPs to photochromic spiropyran molecules. ~70% reversible all-optical tuning of light scattering at the higher-order resonant mode under a low incident light intensity is demonstrated. Our results promote the development of high-efficiency visible nanophotonic devices.

L. Huang, A. Krasnok, A. Alu, Y. Yu, D. Neshev, and A. Miroshnichenko, Enhanced Light-Matter Interaction in Two-Dimensional Transition Metal Dichalcogenides, Reports Prog. Phys. (2022).

Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. We continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. 

J. W. Choong, N. Nefedkin, and A. Krasnok, Collectively Driven Optical Nanoantennas, Phys. Rev. A 103, 043714 (2021).

Herein, using full-wave numerical simulations backed by a quantum model, we unveil that coherent excitation allows controlling antenna multipoles, on-demand excitation of nonradiative states, enhanced directivity, and improving antenna radiation efficiency. This collective excitation corresponds to the states with nonzero dipole moment in the quantum picture, where the quantum phase is well defined. The results of this work bring another degree of freedom––the collective phase of an ensemble of quantum emitters––to control optical nanoantennas and, as such, pave the way to the use of collective excitations for nanophotonic devices with superb performance. To make the discussion independent of the frequency range, we consider the all-dielectric design and use dimensionless units.

J. Fang, M. Wang, K. Yao, T. Zhang, A. Krasnok, T. Jiang, J. Choi, E. Kahn, B. A. Korgel, M. Terrones, X. Li, A. Alù, and Y. Zheng, Directional Modulation of Exciton Emission Using Single Dielectric Nanospheres, Adv. Mater. 33, 2007236 (2021).

Herein, a highly miniaturized platform is explored for the control of emission based on individual subwavelength Si nanospheres (SiNSs) to modulate the directional excitation and exciton emission of 2D transition metal dichalcogenides (2D TMDs). A modified Mie theory for dipole–sphere hybrid systems is derived to instruct the optimal design for desirable modulation performance. Controllable forward-to-backward intensity ratios are experimentally validated in 532 nm laser excitation and 635 nm exciton emission from a monolayer WS2. Versatile light emission control is achieved for different emitters and excitation wavelengths, benefiting from the facile size control and isotropic shape of SiNSs.

Q. Zhang, G. Hu, W. Ma, P. Li, A. Krasnok, R. Hillenbrand, A. Alù, and C.-W. Qiu, Interface Nano-Optics with van Der Waals Polaritons, Nature 597, 187 (2021).

Polaritons are hybrid excitations of matter and photons. In recent years, polaritons in van der Waals nanomaterials—known as van der Waals polaritons—have shown great promise to guide the flow of light at the nanoscale over spectral regions ranging from the visible to the terahertz. A vibrant research field based on manipulating strong light–matter interactions in the form of polaritons, supported by these atomically thin van der Waals nanomaterials, is emerging for advanced nanophotonic and opto-electronic applications. Here we provide an overview of the state of the art of exploiting interface optics—such as refractive optics, meta-optics and moiré engineering—for the control of van der Waals polaritons. This enhanced control over van der Waals polaritons at the nanoscale has not only unveiled many new phenomena, but has also inspired valuable applications—including new avenues for nano-imaging, sensing, on-chip optical circuitry, and potentially many others in the years to come.

Sergey Lepeshov, Andrey Vyshnevyy, Alex Krasnok, Cloaking a nanolaser, preprint arXiv:2110.14077 (2022).

In this work, we propose a nanolaser design based on a semiconductor nanoparticle with gain coated by a phase transition material (Sb2S3), switchable between lasing and cloaking (nonscattering) states at the same operating frequency without change in pumping. The operation characteristics of the nanolaser are rigorously investigated. The designed nanolaser can operate with optical or electric pumping and possesses attributes of a thresholdless laser due to the high beta-factor and strong Purcell enhancement in the strongly confined Mie resonance mode. We design a reconfigurable metasurface composed of lasing-cloaking metaatoms that can switch from lasing to a nonscattering state in a reversible manner.

Quantum materials