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.

Thickness-Dependent Drude Plasma Frequency in Transdimensional Plasmonic TiN.

Plasmonic transdimensional materials (TDMs), which are atomically thin metals of precisely controlled thickness, are expected to exhibit large tailorability and dynamic tunability of their optical response as well as strong light confinement and nonlocal effects. Using spectroscopic ellipsometry, we characterize the complex permittivity of ultrathin films of passivated plasmonic titanium nitride with thicknesses ranging from 1 to 10 nm. By measuring passivated TiN, we experimentally distinguish between the contributions of an oxide layer and thickness to the optical properties. A decrease in the Drude plasma frequency and increase in the damping in thinner films is observed due to spatial confinement. We explain the experimental trends using a nonlocal Drude dielectric response theory based on the Keldysh-Rytova (KR) potential that predicts the thickness-dependent optical properties caused by electron confinement in plasmonic TDMs. Our experimental findings are consistent with the KR model and demonstrate quantum-confinement-induced optical properties in plasmonic transdimensional TiN.

Determining plasmonic hot-carrier energy distributions via single-molecule transport measurements.

Hot carriers in plasmonic nanostructures, generated via plasmon decay, play key roles in applications such as photocatalysis and in photodetectors that circumvent bandgap limitations. However, direct experimental quantification of steady-state energy distributions of hot carriers in nanostructures has so far been lacking. We present transport measurements from single-molecule junctions, created by trapping suitably chosen single molecules between an ultrathin gold film supporting surface plasmon polaritons and a scanning probe tip, that can provide quantification of plasmonic hot-carrier distributions. Our results show that Landau damping is the dominant physical mechanism of hot-carrier generation in nanoscale systems with strong confinement. The technique developed in this work will enable quantification of plasmonic hot-carrier distributions in nanophotonic and plasmonic devices.

Solar Thermoplasmonic Nanofurnace for High-Temperature Heterogeneous Catalysis.

Most of existing solar thermal technologies require highly concentrated solar power to operate in the temperature range 300-600 °C. Here, thin films of refractory plasmonic TiN cylindrical nanocavities manufactured via flexible and scalable process are presented. The fabricated TiN films show polarization-insensitive 95% broadband absorption in the visible and near-infrared spectral ranges and act as plasmonic "nanofurnaces" capable of reaching temperatures above 600 °C under moderately concentrated solar irradiation (∼20 Suns). The demonstrated structures can be used to control nanometer-scale chemistry with zeptoliter (10-21 L) volumetric precision, catalyzing C-C bond formation and melting inorganic deposits. Also shown is the possibility to perform solar thermal CO oxidation at rates of 16 mol h-1 m-2 and with a solar-to-heat thermoplasmonic efficiency of 63%. Access to scalable, cost-effective refractory plasmonic nanofurnaces opens the way to the development of modular solar thermal devices for sustainable catalytic processes.

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.

Experimental Demonstration of Anomalous Field Enhancement in All-Dielectric Transition Magnetic Metamaterials.

Anomalous field enhancement accompanied by resonant absorption phenomenon was originally discussed in the context of plasma physics and in applications related to radio-communications between the ground and spacecraft returning to Earth. Indeed, there is a critical period of time when all communications are lost due to the reflection/absorption of electromagnetic waves by the sheath of plasma created by a high speed vehicle re-entering the atmosphere. While detailed experimental studies of these phenomena in space are challenging, the emergence of electromagnetic metamaterials enables researchers exceptional flexibility to study them in the laboratory environment. Here, we experimentally demonstrated the strong localized field enhancement of magnetic field for an electromagnetic wave propagating in Mie-resonance-based inhomogeneous metamaterials with magnetic permeability gradually changing from positive to negative values. Although these experiments were performed in the microwave frequency range, the proposed all-dielectric approach to transition metamaterials can be extended to terahertz, infrared, and visible frequencies. We anticipate that these results, besides most basic science aspects, hold the potential for numerous applications, including low-intensity nonlinear transformation optics, topological photonics, and the broader area of surface and interface science.

Spinning light on the nanoscale.

Light beams with orbital angular momentum have significant potential to transform many areas of modern photonics from imaging to classical and quantum communication systems. We design and experimentally demonstrate an ultracompact array of nanowaveguides with a circular graded distribution of channel diameters that coverts a conventional laser beam into a vortex with an orbital angular momentum. The proposed nanoscale beam converter is likely to enable a new generation of on-chip or all-fiber structured light applications.

Twisted light in a nonlinear mirror.

Opposite directionality of the Poynting vector and the wave vector, an inherent property of negative index metamaterials (NIMs), was predicted to enable backward phase-matching condition for a second harmonic generation (SHG) process. As a result, such a nonlinear negative index slab acts as a nonlinear mirror. In this Letter, we predict that SHG with structured light carrying orbital angular momentum (OAM) and propagating in NIMs results in a possibility of generating a backward propagating beam with simultaneously doubled frequency, OAM, and reversed rotation direction of the wavefront. These results may find applications in high-dimensional communication systems, quantum information processing, and optical manipulation on a nanoscale.

Virtual hyperbolic metamaterials for manipulating radar signals in air.

Microwave beam transmission and manipulation in the atmosphere is an important but difficult task. One of the major challenges in transmitting and routing microwaves in air is unavoidable divergence because of diffraction. Here we introduce and design virtual hyperbolic metamaterials (VHMMs) formed by an array of plasma channels in air as a result of self-focusing of an intense laser pulse, and show that such structure can be used to manipulate microwave beams in air. Hyperbolic, or indefinite, metamaterials are photonic structures that possess permittivity and/or permeability tensor elements of opposite sign with respect to one another along principal axes, resulting in a strong anisotropy. Our proof-of-concept results confirm that the proposed virtual hyperbolic metamaterial structure can be used for efficient beam collimation and for guiding radar signals around obstacles, opening a new paradigm for electromagnetic wave manipulation in air.