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)

Resonance coupling in hybrid exciton-polariton structures with single Si nanoparticles coupled to 1L-WS2 shows strong coupling and a Rabi splitting energy exceeding 110 meV. By changing the surrounding dielectric from air to water, the Rabi splitting energy is enhanced to 208 meV. Experimental evidence demonstrates tunable resonance coupling, increasing Rabi splitting energy from 49.6 to 86.6 meV. These findings have implications for 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)

We comprehensively examine hybrid exciton-polariton structures, featuring monolayer TMDCs coupled to plasmonic and dielectric nanocavities. Highlighting the optical properties, including energy bands, photoluminescence/absorption spectra, excitonic fine structure, and exciton dynamics, we explore light-matter interactions. Emphasizing weak and strong coupling regimes, we envision future prospects 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)

We show room-temperature sorting of valley-polarized excitons using a MoS2 monolayer and subwavelength asymmetric groove array. Valley excitons are separated in real space and photon momentum-space, enabling efficient valley transport and bridging valleytronics with photonics. A significant advancement for 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).

This study investigates surface plasmon interaction with dark excitons in hybrid systems of gold nanotriangles and monolayer WS2. Water surrounding the system induces a narrow Fano resonance, attributed to plasmon-enhanced decay of dark K-K excitons. These findings demonstrate the significant impact of dark excitons on Fano resonances in plasmon-exciton systems, offering opportunities for advanced 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)

We uncover significant phenomena in moiré hyperbolic plasmons within pairs of hyperbolic metasurfaces (HMTSs). Rotating evanescently coupled HMTSs reveals dispersion engineering, magic angle transitions, broadband field canalization, and plasmon spin-Hall effects. These findings revolutionize metasurface optics, integrating moiré physics and twistronics.

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).

We demonstrate control and manipulation of phonon polariton dispersion in van der Waals bilayers, observing tunable topological transitions from open to closed dispersion contours at a photonic magic twist angle. These transitions enable low-loss tunable polariton canalization and diffractionless propagation with subwavelength resolution. Our findings expand twistronics and moiré physics to nanophotonics and polaritonics, with implications for 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).

Bandgap engineering enables the synthesis of hydrogenated amorphous Si nanoparticles (a-Si:H NPs) with exceptional properties for nanophotonics. Hydrogenation-induced bandgap renormalization in a-Si:H NPs reduces material loss, resulting in robust resonant modes with Q factors up to ~100 in visible and near-IR ranges. By coupling a-Si:H NPs with photochromic spiropyran molecules, we achieve highly adjustable all-dielectric meta-atoms. We demonstrate ~70% reversible all-optical tuning of light scattering at higher-order resonant modes, even under low incident light intensity. These findings drive advancements in efficient 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 enhance light-matter interactions in 2D TMDCs. We overview excitons and cavity-enhanced emission, discuss progress in light emission, strong coupling, and valleytronics. We review tunable photonic devices, enhanced light absorption, and engineered light scattering in TMDCs.

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

Using simulations and a quantum model, we discover that coherent excitation enables control of antenna multipoles, on-demand excitation of nonradiative states, enhanced directivity, and improved radiation efficiency. These findings introduce a new degree of freedom for controlling optical nanoantennas, paving the way for high-performance nanophotonic devices.

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).

We present a highly miniaturized platform using subwavelength Si nanospheres (SiNSs) to control emission in 2D transition metal dichalcogenides (2D TMDs). Our modified Mie theory guides optimal design for modulation performance. Experimental validation demonstrates controllable forward-to-backward intensity ratios in laser excitation and exciton emission from monolayer WS2. Versatile light emission control is achieved with different emitters and excitation wavelengths, facilitated by the 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).

Van der Waals polaritons, hybrid excitations of matter and photons, have emerged as a powerful tool for guiding light at the nanoscale across various spectral regions. Manipulating these polaritons through interface optics, such as refractive optics, meta-optics, and moiré engineering, offers unprecedented control. This control has led to the discovery of new phenomena and holds immense potential for nano-imaging, sensing, on-chip optical circuitry, and future applications. Overall, van der Waals polaritons pave the way for advanced nanophotonics and opto-electronic technologies.

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