Helicity-Preserving Optical Metafluids.

A colloidal suspension of photonic nanostructures exhibiting optical magnetism is dubbed an optical metafluid. A promising constituent of a metafluid is a nanosphere of high-refractive index dielectrics having the magnetic-type Mie resonances in the optical frequency. At the Kerker conditions, a dielectric nanosphere satisfies the electromagnetic duality symmetry condition and preserves the handedness of circularly polarized incident light. A metafluid of such dielectric nanospheres thus preserves the helicity of incident light. In the helicity-preserving metafluid, the local chiral fields around the constituent nanospheres are strongly enhanced, which improves the sensitivity of enantiomer-selective chiral molecular sensing. Here, we experimentally demonstrate that a solution of crystalline silicon nanospheres can be "dual" and "anti-dual" metafluids. We first theoretically address the electromagnetic duality symmetry of single silicon nanospheres. We then produce solutions of silicon nanospheres with narrow size distributions and experimentally demonstrate the "dual" and "anti-dual" behaviors.

Resonant Anti-Reflection Metasurfaces for Infrared Transmission Optics.

A fundamental capability needed for any transmissive optical component is anti-reflection, yet this capability can be challenging to achieve in a cost-effective manner over longer infrared wavelengths. We demonstrate that Mie-resonant photonic structures can enable high transmission through a high-index optical component, allowing it to function effectively over long-wavelength infrared wavelengths. Using silicon as a model system, we demonstrate a resonant metasurface that enables a window optic with transmission up to 40% greater than that of unpatterned Si. Imaging comparisons with unpatterned Si and off-the-shelf germanium optics are shown as well as modulation transfer function measurements, showing excellent performance and suitability for imaging applications. Our results show how resonant photonic structures can be used to improve optical transmission through high-index optical components and highlight their possible use in infrared imaging applications.

Optical Bistability in Nanosilicon with Record Low Q-Factor.

We demonstrated optical bistability in an amorphous silicon Mie resonator with a size of ∼100 nm and Q-factor as low as ∼4 by utilizing photothermal and thermo-optical effects. We not only experimentally confirmed the steep intensity transition and the hysteresis in the scattering response from silicon nanocuboids but also established a physical model to numerically explain the underlying mechanism based on temperature-dependent competition between photothermal heating and heat dissipation. The transition between the bistable states offered particularly steep superlinearity of scattering intensity, reaching an effective nonlinearity order of ∼100th power over excitation intensity, leading to the potential of advanced optical switching devices and super-resolution microscopy.

Color Printing and Encryption with Polarization-Switchable Structural Colors on All-Dielectric Metasurfaces.

Metasurface-based structural color with high resolution is promising for color printing and encryption. However, achieving tunable structural colors in practical applications is challenging owing to the immutability after the fabrication of metasurfaces. Herein, we proposed the polarization-switchable dielectric metasurfaces with full colors. The colorful images can be switched on/off by controlling the polarization of incident light. For the nanorods metasurfaces, all colors turned to black in the "off" mode because of the near-zero reflection, and the uniform black was advantageous for designing encryption applications. For the nanocrosses metasurfaces, colors reversed in two different "on" modes and images hidden in the "off" mode. With the polarization-sensitive metasurfaces, a fish-bird image, an overlapped dual-channel image, and a green-red heart image were obtained, respectively. The demonstrations can be applied to dynamic displays, optical cryptography, multichannel imaging, and optical data storage.

Efficient Second-Harmonic Generation from Dielectric Inter-subband Polaritonic Metasurfaces Coupled to Lattice Resonance.

Nonlinear optical metasurfaces offer a possibility to perform frequency mixing without the phase-matching constraints of bulk nonlinear crystals and with control of the local nonlinear response at a sub-wavelength scale. Nonlinear inter-subband polaritonic metasurfaces created by combining the semiconductor heterostructures with quantum-engineered inter-subband nonlinear response and electromagnetically engineered metal-clad nanoresonators offer by far the largest second-order nonlinear response of all condensed matter systems reported to date. However, the nonlinear optical response of these metasurfaces is limited by optical intensity saturation in the nanoresonator hot spots that prevented the achievement of power conversion efficiencies over 0.2% in three-wave mixing experiments. In this study, we propose and experimentally demonstrate dielectric inter-subband polaritonic metasurfaces for second-harmonic generation that achieve 0.37% power conversion efficiency. Our structure is created by a new design approach that combines dielectric resonators inducing Mie resonant modes with a lattice resonance to achieve a uniform and high field enhancement throughout the meta-atom volume.

Fluorophore-Decorated Mie Resonant Silicon Nanosphere for Scattering/Fluorescence Dual-Mode Imaging.

Inorganic nanoparticles with multiple functions have been attracting attention as multimodal nanoprobes in bioimaging, biomolecule detection, and medical diagnosis and treatment. A drawback of conventional metallic nanoparticle-based nanoprobes is the Ohmic losses that lead to fluorescence quenching of attached molecules and local heating under light irradiation. Here, metal-free nanoprobes capable of scattering/fluorescence dual-mode imaging are developed. The nanoprobes are composed of a silicon nanosphere core having efficient Mie scattering in the visible to near infrared range and a fluorophore doped silica shell. The dark-field scattering and photoluminescence images/spectra for nanoprobes made from different size silicon nanospheres and different kinds of fluorophores are studied by single particle spectroscopy. The fluorescence spectra are strongly modified by the Mie modes of a silicon nanosphere core. By comparing scattering and fluorescence spectra and calculated Purcell factors, the fluorescence enhancement factor is quantitatively discussed. In vitro scattering/fluorescence imaging studies on human cancer cells demonstrate that the developed nanoparticles work as scattering/fluorescence dual-mode imaging nanoprobes.

Dielectric metasurfaces for next-generation optical biosensing: a comparison with plasmonic sensing.

In the past decades, nanophotonic biosensors have been extended from the extensively studied plasmonic platforms to dielectric metasurfaces. Instead of plasmonic resonance, dielectric metasurfaces are based on Mie resonance, and provide comparable sensitivity with superior resonance bandwidth, Q factor, and figure-of-merit. Although the plasmonic photothermal effect is beneficial in many biomedical applications, it is a fundamental limitation for biosensing. Dielectric metasurfaces solve the ohmic loss and heating problems, providing better repeatability, stability, and biocompatibility. We review the high-Q resonances based on various physical phenomena tailored by meta-atom geometric designs, and compare dielectric and plasmonic metasurfaces in refractometric, surface-enhanced, and chiral sensing for various biomedical and diagnostic applications. Departing from conventional spectral shift measurement using spectrometers, imaging-based and spectrometer-less biosensing are highlighted, including single-wavelength refractometric barcoding, surface-enhanced molecular fingerprinting, and integrated visual reporting. These unique modalities enabled by dielectric metasurfaces point to two important research directions. On the one hand, hyperspectral imaging provides massive information for smart data processing, which not only achieve better biomolecular sensing performance than conventional ensemble averaging, but also enable real-time monitoring of cellular or microbial behaviour in physiological conditions. On the other hand, a single metasurface can integrate both functions of sensing and optical output engineering, using single-wavelength or broadband light sources, which provides simple, fast, compact, and cost-effective solutions. Finally, we provide perspectives in future development on metasurface nanofabrication, functionalization, material, configuration, and integration, towards next-generation optical biosensing for ultra-sensitive, portable/wearable, lab-on-a-chip, point-of-care, multiplexed, and scalable applications.

Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles.

Circularly polarized light carries spin angular momentum, so it can exert an optical torque on the polarization-anisotropic particle by the spin momentum transfer. Here, we show that giant positive and negative optical torques on Mie-resonant (gain) particles arise from the emergence of superhybrid modes with magnetic multipoles and electric toroidal moments, excited by linearly polarized beams. Anomalous positive and negative torques on particles (doped with judicious amount of dye molecules) are over 800 and 200 times larger than the ordinary lossy counterparts, respectively. Meanwhile, a rotational motor can be configured by switching the s- and p-polarized beams, exhibiting opposite optical torques. These giant and reversed optical torques are unveiled for the first time in the scattering spectrum, paving another avenue toward exploring unprecedented physics of hybrid and superhybrid multipoles in metaoptics and optical manipulations.

All-State Switching of the Mie Resonance of Conductive Polyaniline Nanospheres.

Polyaniline (PANI), a conductive polymer, is a promising active material for optical switching. In most studies, active switching has so far been realized only between two states, whereas PANI has a total of six states. The optical properties of nanoscale PANI in all six states have remained unclear. Herein we report on all-state switching of the Mie resonance on PANI nanospheres (NSs) and active plasmon switching on PANI-coated Au nanodisks (NDs). All-state switching of differently sized PANI NSs is achieved by proton doping/dedoping and electrochemical methods. Theoretical studies show that the scattering peaks of the individual PANI NSs originate from Mie resonances. All-state switching is further demonstrated on PANI-coated circular Au NDs, where an unprecedentedly large plasmon peak shift of ∼200 nm is realized. Our study not only provides a fundamental understanding of the optical properties of PANI but also opens the probability for developing high-performance dynamic media for active plasmonics.

Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation.

The Breakthrough Starshot Initiative aims to send a gram-scale probe to our nearest extrasolar neighbors using a laser-accelerated lightsail traveling at relativistic speeds. Thermal management is a key lightsail design objective because of the intense laser powers required but has generally been considered secondary to accelerative performance. Here, we demonstrate nanophotonic photonic crystal slab reflectors composed of 2H-phase molybdenum disulfide and crystalline silicon nitride, highlight the inverse relationship between the thermal band extinction coefficient and the lightsail's maximum temperature, and examine the trade-off between minimizing acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 23.3 Gm. We additionally demonstrate multiscale photonic structures featuring thermal-wavelength-scale Mie resonant geometries and characterize their broadband Mie resonance-driven emissivity enhancement and acceleration distance reduction. More broadly, our results highlight new possibilities for simultaneously controlling optical and thermal response over broad wavelength ranges in ultralight nanophotonic structures.

Resonance Couplings in Si@MoS2 Core-Shell Architectures.

Heterostructures of transition metal dichalcogenides and optical cavities that can couple to each other are rising candidates for advanced quantum optics and electronics. This is due to their enhanced light-matter interactions in the visible to near-infrared range. Core-shell structures are particularly valuable for their maximized interfacial area. Here, the chemical vapor deposition synthesis of Si@MoS2 core-shells and extensive structural characterization are presented. Compared with traditional plasmonic cores, the silicon dielectric Mie resonator core offers low Ohmic losses and a wider spectrum of optical modes. The magnetic dipole (MD) mode of the silicon core efficiently couples with MoS2 through its large tangential component at the core surface. Using transmission electron microscopy and correlative single-particle scattering spectroscopy, MD mode splitting is experimentally demonstrated in this unique Si@MoS2 core-shell structure. This is evidence for resonance coupling, which is limited to theoretical proposals in this particular system. A coupling constant of 39 meV is achieved, which is ≈1.5-fold higher than previous reports of particle-on-film geometries with a smaller interfacial area. Finally, higher-order systems with the potential to tune properties are demonstrated through a dimer system of Si@MoS2 , forming the basis for emerging architectures for optoelectronic and nanophotonic applications.

Mode Hybridization in Silicon Core-Gold Shell Nanosphere.

A dielectric core-metal shell nanosphere has attracted scientific and technological interests due to the unique optical resonances arising from the hybridization of surface plasmon modes and cavity modes. The previous studies focus on a low-index dielectric core without its own optical resonances. Here, optical resonances of a core-shell nanosphere with a high refractive index (n ≈ 4) core with the lowest order Mie resonances in the visible range are investigated theoretically and experimentally. Scattering and absorption spectra of a core-shell nanosphere for different values of the core refractive index are first analyzed, and there is a transition of the hybridization scheme around n ≈ 2. Above the value, a characteristic hybridized mode with strong absorption and weak scattering emerges in the near-infrared range. A core-shell nanosphere composed of a silicon core and a gold shell is prepared, and the resonance modes are studied by single particle scattering spectroscopy and electron energy loss spectroscopy (EELS) in a transmission electron microscope. The core-shell nanospheres exhibit the hybridized modes depending on the core diameter. The hybridized mode as well as the higher order one that is not observable in the scattering spectroscopy is observed in the EELS.

Silicon Nanoantenna Mix Arrays for a Trifecta of Quantum Emitter Enhancements.

Dielectric nanostructures have demonstrated optical antenna effects due to Mie resonances. Previous work has exhibited enhancements in absorption, emission rates and directionality with practical limitations. In this paper, we present a Si mix antenna array to achieve a trifecta enhancement of ∼1200-fold with a Purcell factor of ∼47. The antenna design incorporates ∼10 nm gaps, within which fluorescent molecules strongly absorb the pump laser energy through a resonant mode. In the emission process, the antenna array increases the radiative decay rates of the fluorescence molecules via a Purcell effect and provides directional emission through a separate mode. This work could lead to novel CMOS-compatible platforms to enhance fluorescence for biological and chemical applications.

Nonlinear Circular Dichroism in Mie-Resonant Nanoparticle Dimers.

We studied the nonlinear response of a dimer composed of two identical Mie-resonant dielectric nanoparticles illuminated normally by a circularly polarized light. We developed a general theory describing hybridization of multipolar modes of the coupled nanoparticles and reveal nonvanishing nonlinear circular dichroism (CD) in the second-harmonic generation (SHG) signal enhanced by the multipolar resonances in the dimer, provided its axis is oriented under an angle to the crystalline lattice of the dielectric material. We supported our multipolar hybridization theory by experimental results obtained for the AlGaAs dimers placed on an engineered substrate.

Mapping the Magnetic Field Intensity of Light with the Nonlinear Optical Emission of a Silicon Nanoparticle.

To detect the magnetic component of arbitrary unknown optical fields, a candidate probe must meet a list of demanding requirements, including a spatially isotropic magnetic response, suppressed electric effect, and wide operating bandwidth. Here, we show that a silicon nanoparticle satisfies all these requirements, and its optical magnetism driven multiphoton luminescence enables direct mapping of the magnetic field intensity distribution of a tightly focused femtosecond laser beam with varied polarization orientation and spatially overlapped electric and magnetic components. Our work establishes a powerful nonlinear optics paradigm for probing unknown optical magnetic fields of arbitrary electromagnetic structures, which is not only essential for realizing subwavelength-scale optical magnetometry but also facilitates nanophotonic research in the magnetic light-matter interaction regime.

Crystalline Silicon White Light Sources Driven by Optical Resonances.

Silicon (Si) is generally considered as a poor photon emitter, and various scenarios have been proposed to improve the photon emission efficiency of Si. Here, we report the observation of a burst of the hot electron luminescence from Si nanoparticles with diameters of 150-250 nm, which is triggered by the exponential increase of the carrier density at high temperatures. We show that the stable white light emission above the threshold can be realized by resonantly exciting either the mirror-image-induced magnetic dipole resonance of a Si nanoparticle placed on a thin silver film or the surface lattice resonance of a regular array of Si nanopillars with femtosecond laser pulses of only a few picojoules, where significant enhancements in two- and three-photon-induced absorption can be achieved. Our findings indicate the possibility of realizing all-Si-based nanolasers with manipulated emission wavelength, which can be easily incorporated into future integrated optical circuits.

Fluorescence Colour Control in Perylene-Labeled Polymer Chains Trapped by Nanotextured Silicon.

The ability to modulate, tune, and control fluorescence colour has attracted much attention in photonics-related research fields. Thus far, it has been impossible to achieve fluorescence colour control (FCC) for material with a fixed structure, size, surrounding medium, and concentration. Here, we propose a novel approach to FCC using optical tweezers. We demonstrate an optical trapping technique using nanotextured Si (black-Si) that can efficiently trap polymer chains. By increasing the laser intensity, the local concentration of perylene-labelled water-soluble polymer chains increased inside the trapping potential. Accordingly, the excimer fluorescence of perylene increased while the monomer fluorescence decreased, evidenced by a fluorescence colour change from blue to orange. Using nanostructure-assisted optical tweezing, we demonstrate control of the relative intensity ratio of fluorescence of the two fluorophores, thus showing remote and reversible FCC of the polymer assembly.

Colloidal Mie resonant silicon nanoparticles.

Nano- and microstructures of silicon (Si) exhibit electric and magnetic Mie resonances in the optical regime, providing a novel platform for controlling light at the nanoscale and enhancing light-matter interactions. In this Review, we present recent development of colloidal Si nanoparticles (NPs) that have wide range of applications in nanophotonics. Following brief summary of synthesis methods of amorphous and crystalline Si particles with high sphericity, optical responses of single Si particles placed on a substrate are overviewed. Then, the capability as a nanoantenna to control light-matter interactions is discussed in different systems. Finally, collective optical responses of Si NPs in solution are presented and the application potentials are discussed.

Colloidal Solutions of Silicon Nanospheres toward All-Dielectric Optical Metafluids.

A colloidal solution of nanophotonic structures exhibiting optical magnetism is dubbed a liquid-phase metamaterial or an optical metafluid. Over the decades, plasmonic nanoclusters have been explored as constituents of a metafluid. However, optical magnetism of plasmonic nanoclusters is usually much weaker than the electric responses; the highest reported intensity ratio of the magnetic-to-electric responses so far is 0.28. Here, we propose an all-dielectric metafluid composed of crystalline silicon nanospheres. First, we address the advantages of silicon as a constituent material of a metafluid among major dielectrics. Next, we experimentally demonstrate for the first time that a silicon nanosphere metafluid exhibits strong electric and magnetic dipolar Mie responses across the visible to near-infrared spectral range. The intensity ratio of the magnetic-to-electric responses reaches unity. Finally, we discuss the perspective to achieve unnaturally high (>3), low, and even near-zero (<1) refractive index in the metafluid.

All-Dielectric Silicon Nanoring Metasurface for Full-Color Printing.

Structural color has been particularly attractive as it provides a highly promising approach for next-generation color printing. Plasmonic nanostructures have been intensively investigated for color printing, while suffering from intrinsic loss that degrades the quality of the coloration. Dielectric materials have emerged as an alternative because of their high refractive index that enables highly confined optical modes within the nanostructure at the diffraction limit. Here, we demonstrate an all-dielectric nanoring metasurface for coloration. By harnessing the intrinsic nanoring structure design, vivid structural color has been achieved in the visible range. The color gamut is expected to occupy 115% of the standard color space (sRGB) on the chromaticity diagram of the International Commission on Illumination (CIE) 1931 in theory. Our structure can be applied to various complex devices and materials and find potential applications such as displays, information, and art works.