Roadmap for Optical Metasurfaces.

Metasurfaces have recently risen to prominence in optical research, providing unique functionalities that can be used for imaging, beam forming, holography, polarimetry, and many more, while keeping device dimensions small. Despite the fact that a vast range of basic metasurface designs has already been thoroughly studied in the literature, the number of metasurface-related papers is still growing at a rapid pace, as metasurface research is now spreading to adjacent fields, including computational imaging, augmented and virtual reality, automotive, display, biosensing, nonlinear, quantum and topological optics, optical computing, and more. At the same time, the ability of metasurfaces to perform optical functions in much more compact optical systems has triggered strong and constantly growing interest from various industries that greatly benefit from the availability of miniaturized, highly functional, and efficient optical components that can be integrated in optoelectronic systems at low cost. This creates a truly unique opportunity for the field of metasurfaces to make both a scientific and an industrial impact. The goal of this Roadmap is to mark this "golden age" of metasurface research and define future directions to encourage scientists and engineers to drive research and development in the field of metasurfaces toward both scientific excellence and broad industrial adoption.

Topological electromagnetic waves in dispersive and lossy plasma crystals.

Topological photonic crystals, which offer topologically protected and back-scattering-immune transport channels, have recently gained significant attention for both scientific and practical reasons. Although most current studies focus on dielectric materials with weak dispersions, this study focuses on topological phases in dispersive materials and presents a numerical study of Chern insulators in gaseous-phase plasma cylinder cells. We develop a numerical framework to address the complex material dispersion arising from the plasma medium and external magnetic fields and identify Chern insulator phases that are experimentally achievable. Using this numerical tool, we also explain the flat bands commonly observed in periodic plasmonic structures, via local resonances, and how edge states change as the edge termination is periodically modified. This work opens up opportunities for exploring band topology in new materials with non-trivial dispersions and has potential radio frequency (RF) applications, ranging from plasma-based lighting to plasma propulsion engines.

Engineering the temporal dynamics of all-optical switching with fast and slow materials.

All-optical switches control the amplitude, phase, and polarization of light using optical control pulses. They can operate at ultrafast timescales - essential for technology-driven applications like optical computing, and fundamental studies like time-reflection. Conventional all-optical switches have a fixed switching time, but this work demonstrates that the response-time can be controlled by selectively controlling the light-matter-interaction in so-called fast and slow materials. The bi-material switch has a nanosecond response when the probe interacts strongly with titanium nitride near its epsilon-near-zero (ENZ) wavelength. The response-time speeds up over two orders of magnitude with increasing probe-wavelength, as light's interaction with the faster Aluminum-doped zinc oxide (AZO) increases, eventually reaching the picosecond-scale near AZO's ENZ-regime. This scheme provides several additional degrees of freedom for switching time control, such as probe-polarization and incident angle, and the pump-wavelength. This approach could lead to new functionalities within key applications in multiband transmission, optical computing, and nonlinear optics.

Machine learning assisted quantum super-resolution microscopy.

One of the main characteristics of optical imaging systems is spatial resolution, which is restricted by the diffraction limit to approximately half the wavelength of the incident light. Along with the recently developed classical super-resolution techniques, which aim at breaking the diffraction limit in classical systems, there is a class of quantum super-resolution techniques which leverage the non-classical nature of the optical signals radiated by quantum emitters, the so-called antibunching super-resolution microscopy. This approach can ensure a factor of [Formula: see text] improvement in the spatial resolution by measuring the n -th order autocorrelation function. The main bottleneck of the antibunching super-resolution microscopy is the time-consuming acquisition of multi-photon event histograms. We present a machine learning-assisted approach for the realization of rapid antibunching super-resolution imaging and demonstrate 12 times speed-up compared to conventional, fitting-based autocorrelation measurements. The developed framework paves the way to the practical realization of scalable quantum super-resolution imaging devices that can be compatible with various types of quantum emitters.

Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity.

The negatively charged boron vacancy (VB-) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, VB- preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of VB- defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of VB- with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled VB- defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single VB- defects.

Enhancing the graphene photocurrent using surface plasmons and a p-n junction.

The recently proposed concept of graphene photodetectors offers remarkable properties such as unprecedented compactness, ultrabroadband detection, and an ultrafast response speed. However, owing to the low optical absorption of pristine monolayer graphene, the intrinsically low responsivity of graphene photodetectors significantly hinders the development of practical devices. To address this issue, numerous efforts have thus far been made to enhance the light-graphene interaction using plasmonic structures. These approaches, however, can be significantly advanced by leveraging the other critical aspect of graphene photoresponsivity enhancement-electrical junction control. It has been reported that the dominant photocarrier generation mechanism in graphene is the photothermoelectric (PTE) effect. Thus, the two energy conversion mechanisms involved in the graphene photodetection process are light-to-heat and heat-to-electricity conversions. In this work, we propose a meticulously designed device architecture to simultaneously enhance the two conversion efficiencies. Specifically, a gap plasmon structure is used to absorb a major portion of the incident light to induce localized heating, and a pair of split gates is used to produce a p-n junction in graphene to augment the PTE current generation. The gap plasmon structure and the split gates are designed to share common key components so that the proposed device architecture concurrently realizes both optical and electrical enhancements. We experimentally demonstrate the dominance of the PTE effect in graphene photocurrent generation and observe a 25-fold increase in the generated photocurrent compared to the un-enhanced cases. While further photocurrent enhancement can be achieved by applying a DC bias, the proposed device concept shows vast potential for practical applications.

Ten years of spasers and plasmonic nanolasers.

Ten years ago, three teams experimentally demonstrated the first spasers, or plasmonic nanolasers, after the spaser concept was first proposed theoretically in 2003. An overview of the significant progress achieved over the last 10 years is presented here, together with the original context of and motivations for this research. After a general introduction, we first summarize the fundamental properties of spasers and discuss the major motivations that led to the first demonstrations of spasers and nanolasers. This is followed by an overview of crucial technological progress, including lasing threshold reduction, dynamic modulation, room-temperature operation, electrical injection, the control and improvement of spasers, the array operation of spasers, and selected applications of single-particle spasers. Research prospects are presented in relation to several directions of development, including further miniaturization, the relationship with Bose-Einstein condensation, novel spaser-based interconnects, and other features of spasers and plasmonic lasers that have yet to be realized or challenges that are still to be overcome.

Modulating phase by metasurfaces with gated ultra-thin TiN films.

Active control over the flow of light is highly desirable because of its applicability to information processing, telecommunication, and spectroscopic imaging. In this paper, by employing the tunability of carrier density in a 1 nm titanium nitride (TiN) film, we numerically demonstrate deep phase modulation (PM) in an electrically tunable gold strip/TiN film hybrid metasurface. A 337° PM is achieved at 1.550 µm with a 3% carrier density change in the TiN film. We also demonstrate that a continuous 180° PM can be realized at 1.537 µm by applying a realistic experiment-based gate voltage bias and continuously changing the carrier density in the TiN film. The proposed design of active metasurfaces capable of deep PM near the wavelength of 1.550 µm has considerable potential in active beam steering, dynamic hologram generation, and flat photonic devices with reconfigurable functionalities.

Spatial and Temporal Nanoscale Plasmonic Heating Quantified by Thermoreflectance.

The field of thermoplasmonics has thrived in the past decades because it uniquely provides remotely controllable nanometer-scale heat sources that have augmented numerous technologies. Despite the extensive studies on steady-state plasmonic heating, the dynamic behavior of the plasmonic heaters in the nanosecond regime has remained largely unexplored, yet such a time scale is indeed essential for a broad range of applications such as photocatalysis, optical modulators, and detectors. Here, we use two distinct techniques based on the temperature-dependent surface reflectivity of materials, optical thermoreflectance imaging (OTI) and time-domain thermoreflectance (TDTR), to comprehensively investigate plasmonic heating in both spatial and temporal domains. Specifically, OTI enables the rapid visualization of plasmonic heating with sub-micron resolution, outperforming a standard thermal camera, and allows us to establish the connection between the optical absorptance and heating efficiency as well as to analyze plasmonic heating dynamics on the millisecond scale. Using the TDTR technique, we, for the first time, study the optical resonance-dependent heat-transfer dynamics of a nanometer-scale plasmonic structure in the nanosecond regime and use a detailed computational model to extract the impulse response and thermal interface conductance of a multilayer plasmonic structure. The study reveals a quantitative relationship between the dimensions of the nanopatterned structure and its spatiotemporal thermal response to the light pulse excitation, a thermoplasmonic effect resulting from the spatial distribution of the absorbed electromagnetic energy. We also conclude that the two thermoreflectance techniques provide necessary feedback to nanoscale thermoplasmonic heat management, for which optimization in either heating power or temperature decay speed is needed.

Enhanced absorption and photoluminescence from dye-containing thin polymer film on plasmonic array.

Thin films containing light emitters act as light-to-light converters that absorb the incident light and emit luminescence. This well-known phenomenon is photoluminescence (PL). When a photoluminescent film is notably thinner than the absorption length of emitters, it exhibits weak absorption of incident light. The absorption can be increased by depositing the thin film on a plasmonic array of metallic nanocylinders arranged with a specific periodicity. The array couples the incident light into the thin film, facilitating the plasmon-enhanced absorption by the emitters in the film. In this study, we demonstrate both experimentally and numerically the plasmon-enhanced absorption of a rhodamine 6G-containing film that is thinner than its absorption length using a periodic array of Al nanocylinders. The experimental results demonstrate that the spectrally integrated PL intensity is increased up to 3.78 times. In addition to enhanced absorption, the array is also found to diffract the PL into a direction determined by the periodicity, thereby facilitating the multiplied enhancement of PL. The combination of the two factors yields a PL intensity enhanced up to 10 times at a specific angle and wavelength. Numerical simulations combining the carrier kinetics with full-wave electromagnetics in the time-domain support the experimental observations.

Continuous-discontinuous Galerkin time domain (CDGTD) method with generalized dispersive material (GDM) model for computational photonics.

The discontinuous Galerkin time domain (DGTD) method and its recent flavor, the continuous-discontinuous Galerkin time domain (CDGTD) method, have been extensively applied to simulations in the radio frequency (RF) and microwave (MW) regimes due to their inherent ability to efficiently model multiscale problems. We propose to extend the CDGTD method to nanophotonics while exploiting its advantages which have already been established in the RF and MW regimes, such as domain decomposition, non-conformal meshing, high-order elements, and hp-refinement. However, at optical frequencies many materials are highly dispersive, so the modeling of nanophotonic devices requires accurate handling of different dielectric functions, including those of plasmonic elements, dielectrics, and tunable materials. In this paper, we propose a CDGTD method that incorporates a generalized dispersive material (GDM) model which is an efficient way to implement a wide range of optical dispersion models with a universal analytic function. Physics-based dispersion models, such as the Drude, Debye, Lorentz, and critical points as well as more complicated behavior founded on ab-initio principles can all be obtained as special cases of the universal GDM approach. The accuracy and convergence of this GDM-incorporated CDGTD are verified by numerical examples. The CDGTD method, equipped with the GDM model, paves the way to the efficient design and optimization of large scale photonic devices with a diverse range of optical dispersive materials.

Plasmonic metasurfaces for subtractive color filtering: optimized nonlinear regression models.

We develop and explore a nonlinear regression modeling approach to designing subtractive color filters (SCFs) based on plasmonic metasurfaces. The approach opens up the possibility of rapidly choosing a desired optimized SCF design with high color saturation and brightness using an analytical expression. In this Letter, colors are produced by absorbing the light of specific wavelengths and reflecting the remaining spectrum with silver gap-plasmon nanoantennas deposited on a silver film. First, we design three different SCFs-yellow, magenta, and cyan. Then, by adjusting the design parameters of the nanoantennas, we optimize their high absorption resonance peaks (reflections dips), which are tunable over the visible spectrum. Finally, by using nonlinear regression analysis, we fit our results to a cubic regression model. Accordingly, a SCF for a color of choice can be designed in a straightforward way. This is a promising technique that provides a methodology to design preoptimized filters for practical applications such as color printing, high-resolution chromatic displays, and multi-spectral imaging.

Power Balance and Temperature in Optically Pumped Spasers and Nanolasers.

Spasers and nanolasers produce a significant amount of heat, which impedes their realizability. We numerically investigate the farfield emission and thermal load in optically pumped spasers with a coupled electromagnetic/thermal model, including additional temperature discontinuities due to interfacial Kapitza resistance. This approach allows to explore multiple combinations of constitutive materials suitable for robust manufacturable spasers. Three main channels of heat generation are quantified: metal absorption at pumping and spasing wavelengths and nonradiative relaxations in the gain material. Out-radiated power becomes comparable with absorption for spasers of realistic dimensions. Two optimized spaser configurations emitting light near 520 nm are compared in detail: a prolate metal-core/gain-shell and an oblate gain-core/metal-shell. The metal-shell design, which with the increasing size transforms into a metal-clad nanolaser, achieves an internal light-extraction efficiency of 22.4%, and stably operates up to several hundred picoseconds, an order of magnitude longer than the metal-core spaser.

Lead Halide Perovskite Nanostructures for Dynamic Color Display.

Nanoprint-based color display using either extrinsic structural colors or intrinsic emission colors is a rapidly emerging research field for high-density information storage. Nevertheless, advanced applications, e. g., dynamic full-color display and secure information encryption, call for demanding requirements on in situ color change, nonvacuum operation, prompt response, and favorable reusability. By transplanting the concept of electrical/chemical doping in the semiconductor industry, we demonstrate an in situ reversible color nanoprinting paradigm via photon doping, triggered by the interplay of structural colors and photon emission of lead halide perovskite gratings. It solves the aforementioned challenges at one go. By controlling the pumping light, the synergy between interlaced mechanisms enables color tuning over a large range with a transition time on the nanosecond scale in a nonvacuum environment. Our design presents a promising realization of in situ dynamic color nanoprinting and will empower the advances in structural color and classified nanoprinting.

Ultrabright Room-Temperature Sub-Nanosecond Emission from Single Nitrogen-Vacancy Centers Coupled to Nanopatch Antennas.

Solid-state quantum emitters are in high demand for emerging technologies such as advanced sensing and quantum information processing. Generally, these emitters are not sufficiently bright for practical applications, and a promising solution consists in coupling them to plasmonic nanostructures. Plasmonic nanostructures support broadband modes, making it possible to speed up the fluorescence emission in room-temperature emitters by several orders of magnitude. However, one has not yet achieved such a fluorescence lifetime shortening without a substantial loss in emission efficiency, largely because of strong absorption in metals and emitter bleaching. Here, we demonstrate ultrabright single-photon emission from photostable nitrogen-vacancy (NV) centers in nanodiamonds coupled to plasmonic nanocavities made of low-loss single-crystalline silver. We observe a 70-fold difference between the average fluorescence lifetimes and a 90-fold increase in the average detected saturated intensity. The nanocavity-coupled NVs produce up to 35 million photon counts per second, several times more than the previously reported rates from room-temperature quantum emitters.

Ultrathin and multicolour optical cavities with embedded metasurfaces.

Over the past years, photonic metasurfaces have demonstrated their remarkable and diverse capabilities in advanced control over light propagation. Here, we demonstrate that these artificial films of deeply subwavelength thickness also offer new unparalleled capabilities in decreasing the overall dimensions of integrated optical systems. We propose an original approach of embedding a metasurface inside an optical cavity-one of the most fundamental optical elements-to drastically scale-down its thickness. By modifying the Fabry-Pérot interferometric principle, this methodology is shown to reduce the metasurface-based nanocavity thickness below the conventional λ/(2n) minimum. In addition, the nanocavities with embedded metasurfaces can support independently tunable resonances at multiple bands. As a proof-of-concept, using nanostructured metasurfaces within 100-nm nanocavities, we experimentally demonstrate high spatial resolution colour filtering and spectral imaging. The proposed approach can be extrapolated to compact integrated optical systems on-a-chip such as VCSEL's, high-resolution spatial light modulators, imaging spectroscopy systems, and bio-sensors.

High-Resolution Large-Ensemble Nanoparticle Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface.

The intrinsic loss in a plasmonic metasurface is usually considered to be detrimental for device applications. Using plasmonic loss to our advantage, we introduce a thermoplasmonic metasurface that enables high-throughput large-ensemble nanoparticle assembly in a lab-on-a-chip platform. In our work, an array of subwavelength nanoholes in a metal film is used as a plasmonic metasurface that supports the excitation of localized surface plasmon and Bloch surface plasmon polariton waves upon optical illumination and provides a platform for molding both optical and thermal landscapes to achieve a tunable many-particle assembling process. The demonstrated many-particle trapping occurs against gravity in an inverted configuration where the light beam first passes through the nanoparticle suspension before illuminating the thermoplasmonic metasurface, a feat previously thought to be impossible. We also report an extraordinarily enhanced electrothermoplasmonic flow in the region of the thermoplasmonic nanohole metasurface, with comparatively larger transport velocities in comparison to the unpatterned region. This thermoplasmonic metasurface could enable possibilities for myriad applications in molecular analysis, quantum photonics, and self-assembly and creates a versatile platform for exploring nonequilibrium physics.

Engineered nonlinear materials using gold nanoantenna array.

Gold dipole nanoantennas embedded in an organic molecular film provide strong local electromagnetic fields to enhance both the nonlinear refractive index (n2) and two-photon absorption (2PA) of the molecules. An enhancement of 53× for 2PA and 140× for nonlinear refraction is observed for BDPAS (4,4'-bis(diphenylamino)stilbene) at 600 nm with only 3.7% of gold volume fraction. The complex value of the third-order susceptibility enhancement results in a sign change of n2 for the effective composite material relative to the pure BDPAS film. This complex nature of the enhancement and the tunability of the nanoantenna resonance allow for engineering the effective nonlinear response of the composite film.

Dynamic Control of Nanocavities with Tunable Metal Oxides.

Fabry-Pérot metal-insulator-metal (MIM) nanocavities are widely used in nanophotonic applications due to their extraordinary electromagnetic properties and deeply subwavelength dimensions. However, the spectral response of nanocavities is usually controlled by the spatial separation between the two reflecting mirrors and the spacer's refractive index. Here, we demonstrate static and dynamic control of Fabry-Pérot nanocavities by inserting a plasmonic metasurface, as a passive element, and a gallium doped-zinc oxide (Ga:ZnO) layer as a dynamically tunable component within the nanocavities' spacer. Specifically, by changing the design of the silver (Ag) metasurface one can "statically" tailor the nanocavity response, tuning the resonance up to 200 nm. To achieve the dynamic tuning, we utilize the large nonlinear response of the Ga:ZnO layer near the epsilon near zero wavelength to enable effective subpicosecond (<400 fs) optical modulation (80%) at reasonably low pump fluence levels (9 mJ/cm2). We demonstrate a 15 nm red shift of a near-infrared Fabry-Pérot resonance (λ ≅ 1.16 µm) by using a degenerate pump probe technique. We also study the carrier dynamics of Ga:ZnO under intraband photoexcitation via the electronic band structure calculated from first-principles density functional method. This work provides a versatile approach to design metal nanocavities by utilizing both the phase variation with plasmonic metasurfaces and the strong nonlinear response of metal oxides. Tailorable and dynamically controlled nanocavities could pave the way to the development of the next generation of ultrafast nanophotonic devices.