S. Lepeshov, A. Gorodetsky, A. Krasnok, E. Rafailov, and P. Belov, Enhancement of Terahertz Photoconductive Antenna Operation by Optical Nanoantennas, Laser Photonics Rev. 11, 1600199 (2017)

Photoconductive antennas are promising sources of widely used terahertz radiation for spectroscopy, imaging, and detection. Recent advancements in nanophotonics have improved their energy conversion efficiency, using optical nanoantennas to enhance absorption, reduce carrier lifetime, and improve thermal efficiency. This Review summarizes key findings, compares implementation approaches, and explores future prospects, including the use of all-dielectric nanoantennas.

D. G. Baranov, D. A. Zuev, S. I. Lepeshov, O. V. Kotov, A. E. Krasnok, A. B. Evlyukhin, and B. N. Chichkov, All-Dielectric Nanophotonics: The Quest for Better Materials and Fabrication Techniques, Optica 4, 814 (2017)

We have summarized high-index materials and fabrication techniques for all-dielectric nanostructures. Our evaluation of materials in visible and IR ranges focuses on scattering efficiency and Q factors for magnetic Mie resonance. Advantages and drawbacks of fabrication methods are discussed. We anticipate advancements in higher refractive index materials and low-cost manufacturing methods, benefiting nanophotonics and resonant all-dielectric nanostructure design.

A. Krasnok, M. Tymchenko, and A. Alù, Nonlinear Metasurfaces: A Paradigm Shift in Nonlinear Optics, Mater. Today 21, 8 (2018)

We present a comprehensive review of recent advancements in small-scale nonlinear optics, focusing on high-harmonic generation from ultrathin metasurfaces. Our work highlights the use of plasmonic, high-index dielectric, and semiconductor-loaded plasmonic resonators. Additionally, we discuss recent progress in wavefront control using metasurfaces for nonlinear wave generation. We compare approaches to enhance nonlinearities in ultrathin metasurfaces and provide insights into the future development of this promising research field.

A. Krasnok, S. Li, S. Lepeshov, R. Savelev, D. G. Baranov, and A. Alú, All-Optical Switching and Unidirectional Plasmon Launching with Nonlinear Dielectric Nanoantennas, Phys. Rev. Appl. 9, 14015 (2018)

We have developed a highly tunable dielectric nanoantenna with silicon particles and a dipole emitter. The nanoantenna's slow group-velocity guided modes, related to the Van Hove singularity, yield a large Purcell factor and high sensitivity to nanoparticle permittivity. By exciting the electron-hole plasma with ultrafast laser pumping, we achieved significant variations in radiation patterns and Purcell factor using low intensities. Furthermore, we demonstrated the launch of unidirectional surface-plasmon polaritons on an Ag substrate through EHP excitation in the nanoantenna.

L. Lin, S. Lepeshov, A. Krasnok, T. Jiang, X. Peng, B. A. Korgel, A. Alù, and Y. Zheng, All-Optical Reconfigurable Chiral Meta-Molecules, Mater. Today 25, 10 (2019)

We have experimentally achieved all-optical reconfigurable chiral meta-molecules using metallic and dielectric colloidal particles as artificial atoms. These meta-molecules serve two purposes: enhancing optical chirality for surface-enhanced chiroptical spectroscopy and acting as active components in optofluidic and nanophotonic devices. Additionally, they provide microscopic models to understand chirality in atomic and molecular systems.

S. Lepeshov, A. Krasnok, and A. Alù, Nonscattering-to-Superscattering Switch with Phase-Change Materials, ACS Photonics 6, 2126 (2019).

We have explored hybrid metal-semiconductor core-shell nanoantennas with phase change materials (PCMs). By matching Mie resonances and utilizing anapole states, we achieve large scattering and cloaking effects. Varying PCM crystallinity enables significant changes in scattering, including a switch from Kerker to anti-Kerker regimes. This holds promise for low-intensity nonlinear photonics.

D. G. Baranov, Y. Xiao, I. A. Nechepurenko, A. Krasnok, A. Alù, and M. A. Kats, Nanophotonic Engineering of Far-Field Thermal Emitters, Nat. Mater. 18, 920 (2019)

Thermal emission, the fundamental process of objects radiating energy, is now engineered with nanostructured materials to control spectrum, direction, polarization, and temporal response. This Review outlines its physics, nanophotonic approaches, and applications in energy harvesting, lighting, and radiative cooling.

S. Abdollahramezani, O. Hemmatyar, H. Taghinejad, A. Krasnok, Y. Kiarashinejad, M. Zandehshahvar, A. Alù, A. Alù, and A. Adibi, Tunable Nanophotonics Enabled by Chalcogenide Phase-Change Materials, Nanophotonics 9, 1189 (2020).

We have reviewed the latest advancements and emerging trends in tunable MSs and PICs employing chalcogenide PCMs. We highlight the distinct material properties, structural transformations, and thermo-optic effects of established chalcogenide PCM classes. Furthermore, we discuss the use of deep learning techniques for optimizing reconfigurable MSs and analyzing light-matter interactions. Finally, we address the current challenges and offer insights into future developments in this promising field of adjustable nanophotonics.

A. Krasnok, Metalenses Go Atomically Thick and Tunable, Nat. Photonics 14, 409 (2020)

By utilizing exciton resonances in atomically thick semiconductors, researchers have now demonstrated the ultimate down scaling of optical lenses and reported on their efficacious electrical tunability.

Z. Zhang, M. Kang, X. Zhang, X. Feng, Y. Xu, X. Chen, H. Zhang, Q. Xu, Z. Tian, W. Zhang, A. Krasnok, J. Han, and A. Alù, Coherent Perfect Diffraction in Metagratings, Adv. Mater. 32, 2002341 (2020).

We have demonstrated how coherent control of metagratings using multiple wave excitations enables highly reconfigurable, broadband metasurfaces with large diffraction efficiency. By tuning the relative phase difference between two waves, energy distribution between diffraction orders is continuously adjustable, enhancing efficiency and bandwidth. Our work extends the feasibility of thin electric metagratings in practical scenarios, overcoming limitations of single-layer ultrathin metastructures.

A. Krasnok, Photonic Rashba Effect, Nat. Nanotechnol. 15, 893 (2020).

The intercalation of an antennae array with a geometric Pancharatnam–Berry phase into a defective two-dimensional photonic crystal slab enables a spin-dependent splitting of directional emission in momentum space, that is, a Rashba effect for photons.

S. V. Kutsaev, A. Krasnok, S. N. Romanenko, A. Y. Smirnov, K. Taletski, and V. P. Yakovlev, Up‐And‐Coming Advances in Optical and Microwave Nonreciprocity: From Classical to Quantum Realm, Adv. Photonics Res. 2, 2000104 (2021).

Herein, the up-and-coming advances in optical nonreciprocity, including new materials (Weyl semimetals, topological insulators, metasurfaces), active structures, time-modulation, parity-time (PT)-symmetry breaking, nonlinearity combined with a structural asymmetry, quantum nonlinearity, unidirectional gain and loss, chiral quantum states and valley polarization are overviewed. A general description of nonreciprocal systems is provided and the pros and cons of the mentioned approaches to nonreciprocity are discussed.

Sajjad Abdollahramezani, Omid Hemmatyar, Mohammad Taghinejad, Hossein Taghinejad, Alex Krasnok, Ali A. Eftekhar, Christian Teichrib, Sanchit Deshmukh, Mostafa El-Sayed, Eric Pop, Matthias Wuttig, Andrea Alu, Wenshan Cai, Ali Adibi, Electrically driven programmable phase-change meta-switch reaching 80% efficiency, Nature Communications (in press) (2022)

We experimentally demonstrate a fast, reversible, and highly efficient in situ electrically-driven metasurface using a phase-change chalcogenide alloy, Ge2Sb2Te5 (GST). Our optimized design incorporates a resistive microheater to enable non-volatile, multilevel optical modulation. The hybrid plasmonic-PCM meta-switch achieves a record-breaking 11-fold reflectance modulation, 250 nm spectral tuning, and potentially kHz-level switching speed. This work paves the way for fully integrable dynamic metasurfaces with great potential for beamforming applicationsю

Z. Deng, T. Shi, A. Krasnok, X. Li, and A. Alù, Observation of Topologically Robust Localized Magnetic Plasmon Skyrmions, Nature Communications (in press) (2022)

We have successfully constructed topologically robust plasmonic skyrmions in a tailored meta-structure. Our work demonstrates magnetic localized spoof plasmons (LSPs) squeezed down to λ^3/10^6, independent of external interference. Through direct measurement of vectorial magnetic fields, we reveal multi-resonant LSP eigen-modes corresponding to pi-twist skyrmion configurations. The meta-structure's flexibility confirms real-space topological robustness, enabling versatile skyrmionic textures. These magnetic LSP skyrmions open doors to ultra-compact and durable plasmonic devices, including sensors, wearable electronics, and compact antennas.

S. Lepeshov and A. Krasnok, Tunable Phase-Change Metasurfaces, Nat. Nanotechnol. 16, 615 (2022).

The use of phase-change materials makes metasurfaces and nanoantennas electrically tunable and switchable, bringing their functionality to the next level.

 A. Krasnok and A. Alù, Low-Symmetry Nanophotonics, ACS Photonics acsphotonics.1c00968 (2022).

This Perspective has explored the profound impact of symmetry violations on nanophotonics. By exploiting broken symmetries, we have harnessed wave interactions in nanostructures to achieve diverse functionalities. Specifically, we have investigated localized surface polaritons through broken geometrical symmetries, moiré photonics, in-plane inversion symmetry breaking for valleytronics and nonradiative state control, time-reversal symmetry breaking for optical nonreciprocity, and parity-time symmetry breaking. Through this comprehensive analysis, our Perspective underscores the pivotal role of low and broken symmetry in governing nanoscale light, offering extensive applications in optical technologies.

Functional Metadevices

Omid Hemmatyar, Sajjad Abdollahramezani, Ioannis Zeimpekis, Sergey Lepeshov, Alex Krasnok, Asir Intisar Khan, Kathryn M Neilson, Christian Teichrib, Tyler Brown, Eric Pop, Daniel W Hewak, Matthias Wuttig, Andrea Alu, Otto L Muskens, Ali Adibi, Enhanced meta-displays using advanced phase-change materials, Nature Communications (in press) (2022).

We have achieved remarkable results by utilizing tunable all-dielectric reflective metasurfaces, incorporating advanced low-loss optical phase change materials (PCMs) such as antimony trisulphide (Sb2S3) and antimony triselenide (Sb2Se3). Our work has resulted in the development of switchable, high-saturation, high-efficiency, and high-resolution dynamic meta-pixels. By leveraging polarization-sensitive building blocks, our meta-pixels can generate two different colors when illuminated by orthogonally polarized incident beams. This unique combination of material phase and polarization state allows a single reconfigurable metasurface with fixed geometrical parameters to produce four distinct wide-gamut colors