High-index Dielectric Nanostructures Research Articles

Thermometries for Single Nanoparticles Heated with Light.

The development of efficient nanoscale photon absorbers, such as plasmonic or high-index dielectric nanostructures, allows the remotely controlled release of heat on the nanoscale using light. These photothermal nanomaterials have found applications in various research and technological fields, ranging from materials science to biology. However, measuring the nanoscale thermal fields remains an open challenge, hindering full comprehension and control of nanoscale photothermal phenomena. Here, we review and discuss existent thermometries suitable for single nanoparticles heated under illumination. These methods are classified in four categories according to the region where they assess temperature: (1) the average temperature within a diffraction-limited volume, (2) the average temperature at the immediate vicinity of the nanoparticle surface, (3) the temperature of the nanoparticle itself, and (4) a map of the temperature around the nanoparticle with nanoscale spatial resolution. In the latter, because it is the most challenging and informative type of method, we also envisage new combinations of technologies that could be helpful in retrieving nanoscale temperature maps. Finally, we analyze and provide examples of strategies to validate the results obtained using different thermometry methods.

Resonant coupling of molecular excitons and optical anapoles in silicon nanosphere-J-aggregate heterostructures under vector beam illumination.

Resonant excitation of high-index dielectric nanostructures and their coupling with molecular excitons provide great opportunities for engineering adaptable platforms for hybrid functional optical devices. Here, we numerically calculate resonance coupling of nonradiating anapole states to molecular excitons within silicon nanosphere-J-aggregate heterostructures under illumination with radially polarized cylindrical vector beams. The results show that the resonance coupling is accompanied by a scattering peak around the exciton transition frequency, and the anapole state splits into a pair of anticrossing eigenmodes with a mode splitting energy of ≈200m e V. We also investigate the resonance coupling as a function of the J-aggregate parameters, such as thickness, exciton transition linewidth, and oscillator strength. Resonant coupling of the anapole states and J-aggregate heterostructures could be a promising platform for future nanophotonic applications such as in information processing and sensing.

Local versus bulk circular dichroism enhancement by achiral all-dielectric nanoresonators

Abstract Large optical chirality in the vicinity of achiral high-index dielectric nanostructures has been recently demonstrated as useful means of enhancing molecular circular dichroism. We theoretically study the spatial dependence of optical chirality enhancement in the vicinity of high-index dielectric nanodisks and highlight its importance for the design of nanophotonic platforms for circular dichroism enhancement. Using a T-matrix framework, we demonstrate that, depending on the disk aspect ratio, chirality is enhanced preferentially along different directions. We employ various statistical procedures, including surface, volume and orientation averaging, to predict enhancement of chiroptical effects and show that optimal properties of a nanostructure depend substantially on whether spatial maximum or average chirality enhancement is sought after. The results indicate that at times it is beneficial to sacrifice helicity preservation for a larger field enhancement. Similarly, the optimal choice of the nanostructure is influenced by presence of a substrate, which limits the space available to be occupied by analyte molecules and impacts the optical chirality in the vicinity of the nanostructure.

Enhanced Thermo-optical Response by Means of Anapole Excitation.

High refractive index (HRI) dielectric nanostructures offer a versatile platform to control the light–matter interaction at the nanoscale as they can easily support electric and magnetic modes with low losses. An additional property that makes them extraordinary is that they can support low radiative modes, so-called anapole modes. In this work, we propose a spectrally tunable anapole nanoheater based on the use of a dielectric anapole resonator. We show that a gold ring nanostructure, a priori nonresonant, can be turned into a resonant unit by just filling its hole with an HRI material supporting anapole modes, resulting in a more efficient nanoheater able to amplify the photothermal response of the bare nanoring. As proof of concept, we perform a detailed study of the thermoplasmonic response of a gold nanoring used as heating source and a silicon disk, designed to support anapole modes, located in its center acting as an anapolar resonator. Furthermore, we utilize the anapole excitation to easily shift the thermal response of these structures from the shortwave infrared range to the near-infrared range.

Magnetic and electric Purcell factor control through geometry optimization of high index dielectric nanostructures.

We design planar silicon antennas for controlling the emission rate of magnetic or electric dipolar emitters. Evolutionary algorithms coupled to the Green Dyadic Method lead to different optimized geometries which depend on the nature and orientation of the dipoles. We discuss the physical origin of the obtained configurations thanks to modal analysis but also emphasize the role of nanoscale design of the LDOS. We complete our study using finite element method and demonstrate an enhancement up to 2 × 103 of the magnetic Purcell factor in europium ions. Our work brings together random optimizations to explore geometric parameters without constraint, a first order deterministic approach to understand the optimized designs and a modal analysis which clarifies the physical origin of the exaltation of the magnetic Purcell effect.

Lasing action in a strongly coupled silicon nanowire pair.

High-index dielectric nanostructures are of particular interest for nanoscale lasing due to their low absorption losses. However, the relatively weak near-field restricts the isolated dielectric cavities as low-threshold integrated on-chip laser sources. Here, we demonstrate lasing action in a silicon nanowire pair with 32 nm gap coated with dye-doped shell on the silicon-on-insulator platform. It is found that the quality factor Q is dominated by the coupling of the silicon nanowire pair, which depends on the gap size, the nanowire width, and the dye thickness. A lasing peak at the wavelength of 529 nm with FWHM of 0.6 nm is experimentally realized by the Si nanowire pair width, and the corresponding pumping power threshold is ∼34 µW/cm2. The proposed strategy, based on the well-established Si planar process, lays the groundwork for practical integrated nanolasers that have potential applications in photonic circuits.

Applications of Hybrid Metal‐Dielectric Nanostructures: State of the Art

Enhancing the light‐matter interactions is important for many different applications like sensing, surface enhanced spectroscopies, solar energy harvesting, and for quantum effects such as nonlinear frequency generation or spontaneous and stimulated emission. Hybrid metal‐dielectric nanostructures have shown extraordinary performance in this respect, demonstrating their superiority with respect to bare metallic or high refractive index dielectric nanostructures. Such hybrid nanostructures can combine the best of two worlds: strong confinement of the electromagnetic energy by metallic structures and high scattering directivity and low losses of the dielectric ones. In this review, following a general overview of the properties of metal‐dielectric nanostructures, some of their most relevant applications including directional scattering, sensing, surface enhanced Raman spectroscopy, absorption enhancement, fluorescence and quantum dot emission enhancement, nonlinear effects, as well as lasing, are summarized.

Novel non-plasmonic nanolasers empowered by topology and interference effects

Abstract Historically, nanophotonics deals with a control of light at the nanoscale being closely connected with the rapid advances in plasmonics – the physics of surface plasmon polaritons supported by metal–dielectric interfaces. Properly engineered nanostructures allow the subwavelength propagation of light and its strong confinement in nanowaveguides and nanocavities, making possible the field enhancement and lasing. Spaser was suggested as a special type of nanolaser with a very small footprint that can be modulated quickly thus becoming a good candidate for on-chip optical data processing. However, recent developments in the physics of high-index dielectric nanoparticles and resonant dielectric metasurfaces allowed to advance the field of nanophotonics and introduce novel nonplasmonic nanostructures and nanolasers empowered by topology and interference effects. Here we present first some examples of experimentally realized spasers, and then discuss the recent developments in the cutting-edge high-index dielectric nanostructures employed for nonplasmonic nanolasers based on Mie resonances, anapole states, bound states in the continuum, and the physics of topological phases.

Optical Anapole Modes in Gallium Phosphide Nanodisk with Forked Slits for Electric Field Enhancement

High refractive index dielectric nanostructures represent a new frontier in nanophotonics, and the unique semiconductor characteristics of dielectric systems make it possible to enhance electric fields by exploiting this fundamental physical phenomenon. In this work, the scattered radiation spectral features and field-enhanced interactions of gallium phosphide disks with forked slits at anapole modes are investigated systematically by numerical and multipole decomposition analyses. Additional enhancement of the electric field is achieved by opening the forked slits to create high-intensity hot spots inside the disk, and nearby molecules can access these hot spots directly. The results reveal a novel approach for near-field engineering such as electric field localization, nonlinear optics, and optical detection.

Frequency Tripling via Sum-Frequency Generation at the Nanoscale

High refractive index dielectric nanostructures can be exploited to enhance nonlinear optical processes via the strong light confinement by their resonant modes. The sensitive dependence of these modes on the geometry and material composition offers ample opportunities for tailoring the optical response of the system. Here we report sum-frequency generation (SFG) by individual AlGaAs nanocylinders, pumped by two pulsed beams at a telecommunication frequency, ω, and its duplicate, 2ω. Under such a scheme, the SFG at 3ω = ω + 2ω is degenerate with the third-harmonic generation (THG) seeded by the input at ω, thereby tripling its frequency via the second-order permittivity χ(2), rather than via χ(3) as in THG. The concurrent detection of THG and SFG enables us to determine χ(3) = 7 × 10–20 m2/V2 by comparing experiments to numerical simulations and based on the known value of χ(2). We observe a rich size- and polarization-dependent behavior that gives appeal to the studied system as a key component of miniaturized photonic devices.

Colloidal Mie Resonators for All‐Dielectric Metaoptics

High refractive index dielectric nanostructures exhibiting electric and magnetic Mie resonances offer a powerful platform for realizing unprecedented control of electromagnetic waves at the nanoscale and enhanced light–matter interactions. Dielectric metasurfaces with a variety of functionalities are realized due to the recent developments of nanofabrication technologies. In addition, colloidal dispersions of high refractive index dielectric nanoparticles have emerged as a special form of metamaterials that leads to a wider range of applications in photonics. This review focuses on the recent developments of colloidal Mie resonators made of moderate (2.5–3.5) to high (>3.5) refractive index dielectrics operating in the visible to near‐infrared range. Starting from the brief summary of fundamental properties of spherical dielectric nanoparticles, fabrication methods of colloidal Mie resonators composed of different materials and optical functionalities such as the enhanced light–matter interactions, the chirality, and the structural color generation are overviewed.

From Fano to Quasi-BIC Resonances in Individual Dielectric Nanoantennas.

Sharp optical resonances in high-index dielectric nanostructures have recently attracted significant attention for their promising applications in nanophotonics. Fano resonances, as well as resonances associated with bound states in the continuum (BIC), have independently shown a great potential for applications in nanoscale lasers, sensors, and nonlinear optical devices. Here, we demonstrate experimentally a close connection between Fano and quasi-BIC resonances excited in individual dielectric nanoantennas. We analyze systematically the resonant response of AlGaAs nanoantennas pumped with a structured light in the near-infrared range. We trace a variation of the scattering spectrum that fully agrees with an analytical expression governed by a Fano parameter and observe directly a transition to a quasi-BIC resonance. Our results suggest a unified approach toward the analysis of sharp resonances in subwavelength nanostructures originating from strong coupling of optical modes that can provide high energy localization for enhanced light-matter interactions.

Создание метаповерхностей на основе гибридных перовскитов

Recent developments in the physics of high-index resonant dielectric nanostructures suggest alternative mechanisms for subwavelength light control driven by Mie resonances with strong magnetic response that can be employed for a design of novel optical metasurfaces. Here we demonstrate metasurfaces based on nanoimprinted perovskite films optimized by alloying the organic cation part of perovskites. We reveal that such metasurfaces can exhibit a significant enhancement of both linear and nonlinear photoluminescence (up to 70 times) combined with advanced stability. Our results suggest a cost-effective approach based on nanoimprint lithography and combined with simple chemical reactions for creating a new generation of functional metasurfaces which may pave a way towards highly efficient planar optoelectronic metadevices.

Fabrication of Ge2Sb2Te5 crystal micro/nanostructures through single-shot Gaussian-shape femtosecond laser pulse irradiation.

Femtosecond (fs) laser-thin film interaction is one of the most practical methods for fabricating functional nanostructures. However, the details of the interaction mechanism remain unclear. In this study, we demonstrate an abnormal ablation effect on nanofilms by using a tightly focused single fs laser pulse. After the irradiation of a single Gaussian-shaped femtosecond laser pulse, a molten micro/nanopatch at the irradiated central high-power zone is isolated from the surrounding film. The confined localized threshold effect is proposed as the main mechanism for the phase isolation. With this effect, the high refractive index dielectric Ge2Sb2Te5 crystal nanostructures can be fabricated by directed dewetting of the isolated molten micro/nanopatch on Si substrates. After the laser irradiation, the central isolated liquid through an amorphous GST film is transformed into a crystalline state after resolidification. The isolated central micro/nanopatch size can be controlled by the focused spot size and pulse energy, so that the morphologies (size, geometrical morphology, and distribution) of GST nanostructures can be flexibly modulated. Furthermore, separated solid and liquid phase states detected using spatial-temporal-resolved microscopy validates the crucial role of the confined-localized threshold effect in the dewetting effect based on the separated liquid phase.

Anapole mediated giant photothermal nonlinearity in nanostructured silicon

Featured with a plethora of electric and magnetic Mie resonances, high index dielectric nanostructures offer a versatile platform to concentrate light-matter interactions at the nanoscale. By integrating unique features of far-field scattering control and near-field concentration from radiationless anapole states, here, we demonstrate a giant photothermal nonlinearity in single subwavelength-sized silicon nanodisks. The nanoscale energy concentration and consequent near-field enhancements mediated by the anapole mode yield a reversible nonlinear scattering with a large modulation depth and a broad dynamic range, unveiling a record-high nonlinear index change up to 0.5 at mild incident light intensities on the order of MW/cm2. The observed photothermal nonlinearity showcases three orders of magnitude enhancement compared with that of unstructured bulk silicon, as well as nearly one order of magnitude higher than that through the radiative electric dipolar mode. Such nonlinear scattering can empower distinctive point spread functions in confocal reflectance imaging, offering the potential for far-field localization of nanostructured Si with an accuracy approaching 40 nm. Our findings shed new light on active silicon photonics based on optical anapoles.

Second-harmonic generation with metal/dielectric/metal hybridized nanoantennas: enhanced efficiency, reduced mode volume and ideal magnetic/electric dipole scattering

Although nonlinear optical responses with high refractive index dielectric nanostructures benefit from the large mode volume of the Mie resonances, it is an obstacle to achieve ultra-compact devices compared with plasmonic nanostructures. Besides that, the nonlinear emissions for dielectric nanoparticles often possess complex multipolar contributions, which is not favorable for nonlinear far-field manipulating such as beam steering. This study numerically demonstrates that with the formation of mode-matching condition, the efficiency of second-harmonic generation with an Au/AlGaAs/Au hybridized sandwich nanodisk can be enhanced by two orders compared with a same size AlGaAs nanoparticle, and the mode volume is reduced by about two orders at the same time. What is more interesting is that the nonlinear sources and far-field scattering are tailored by the hybridized resonances, where an ideal magnetic (or electric) dipole scattering around a broadband spectral range is observed for the second-harmonic generation, which provides a reliable way to generate pure magnetic light and to manipulate the nonlinear far-field emissions. The realization of enhanced nonlinear efficiency, the reduced mode volume, the background-free responses, and the ideal magnetic/electric dipole scattering make the sandwich hybridized nanoparticle a promising platform for nonlinear nanophotonics.

Enhanced second-harmonic generation from L-shaped AlGaAs nanoantennas

Nanostructured materials offer many advantages over bulk materials in nonlinear optical phenomena. For example, the subwavelength scale allows the neglect of phase-matching issues. The ability to control tight confinement and large resonant enhancement of electromagnetic fields also results in higher nonlinear frequency conversion efficiencies than bulk materials. Compared with plasmonic nanostructures, high-index dielectric nanostructures render larger mode volumes and show great promise for enhanced nonlinear optical processes. In this work, we present resonantly enhanced second-harmonic generation from L-shaped AlGaAs nanoantennas. By tuning the nanostructure thickness, arm length, and width, the overlap of resonances at fundamental and second-harmonic wavelengths can be achieved using any pump wavelength in a broad near-infrared spectral region. We predict SHG efficiency exceeding 4 × 10 − 3 with a pump intensity of 1 G W / c m 2 at two representative pump wavelengths of 1675 and 1800 nm. Our findings are valuable for applications involving all-dielectric nanoantennas, such as efficient sensing and single biomolecule tracking microscopy.

Toroidal dipole and magnetic multipole excitations from the same nanostructure with different direction of electric dipole emitters

The toroidal dipole (TD) and magnetic multipole with optical response from high-index dielectric nanostructures are emerging optical nanostructures especially in the visible spectrum. However, interference of the involved multipoles is generally overlooked in the ultraviolet region. In this work, a novel nanostructure which can achieve excitations of TD, magnetic quadrupole and magnetic dipole is demonstrated by simply changing the direction of the electric dipole emitters in the wavelength range between 230 and 430 nm. Near perfect TD and magnetic excitations from the same structure are accomplished and our results also provide insights into the design and application of light nanoantennas, spectroscopy, and ultrafast light emitting devices.

Selective excitation and enhancement of multipolar resonances in dielectric nanospheres using cylindrical vector beams

Resonant excitation and manipulation of complex interactions among two or more resonances in high-index dielectric nanostructures provide great opportunities for engineering novel optical phenomena and applications. However, difficulties often arise when interpreting the observed spectra because of the overlap of the broad resonances contributed by many factors such as particle size, shape, and background index. Therefore, selective excitation of resonances that spectrally overlap with each other provides a gateway towards an improved understanding of the complex interactions. Here, we demonstrate selective excitation and enhancement of multipolar resonances of silicon nanospheres using cylindrical vector beams (CVBs) with different diameters of nanospheres and numerical apertures (NAs) of the excitations. By combining single particle spectroscopy and electrodynamic simulations, we show that the radially polarized beam can selectively excite the electric multipoles, whereas the azimuthally polarized beam can selectively excite the magnetic multipoles even though multipolar resonances are convoluted together due to their spectral overlap. Moreover, focusing the CVBs with high NA can lead to a dominant longitudinal polarization of the electric or magnetic field. We show that the enhanced longitudinal polarization with increasing NA of the radially and azimuthally polarized beams can selectively enhance the electric and magnetic multipolar resonances, respectively. Our approach can be used as a spectroscopy tool to enhance and identify multipolar resonances leading to a better understanding of light-matter interactions in other dielectric nanostructures as well as serve as a first step toward excitation of dark mode and Fano resonances in dielectric oligomers by breaking the symmetry of the nanostructures.

Strong field enhancement in individual Φ-shaped dielectric nanostructures based on anapole mode resonances.

Due to their ability to produce high electric field enhancements in relatively large nanoscale volumes with minimum absorption and nonradiating properties, anapole modes excited in high index dielectric nanostructures have attracted considerable attentions in these years. We propose a design strategy to simultaneously excite the anapole mode efficiently and maintain its resonant wavelength, which has been remained as a challenge in the conventional dielectric nanostructures. Based on analyzing the relationship between the field enhancement factor and scattering intensity of the electric and toroidal dipoles, we introduce two and four nanocuboids into the nil field intensity areas in the silicon disk system, respectively. The geometric volume of the system can be increased effectively and the electric field enhancement is boosted to be 190% and 250% while the resonant wavelength of the anapole mode is almost maintained constant. The systems combined with a slot in the strongest field intensity area also follow the same law, revealing that the design strategy can be easily extended to other geometric, material and frequency systems. Different from the design strategy to add new components into the areas with strong field intensity, the incorporations occurring at the minimum intensity area is another design scheme to engineer the properties of the resonant systems and can find broad applications in nano-device designs.