Lighting up silicon nanoparticles with Mie resonances

Directional surface plasmon polariton scattering using single magnetic nanoparticles

Directional surface plasmon polaritons (SPPs) are expected to promote the energy efficiency of plasmonic devices, via limiting the energy in a given spatial domain. The directional scattering of dielectric nanoparticles induced by the interference between electric and magnetic responses presents a potential candidate for directional SPPs. Magnetic nanoparticles can introduce permeability as an extra manipulation, whose directional scattered SPPs have not been investigated yet. In this work, we demonstrated the directional scattered SPPs by using single magnetic nanoparticles via simulation and experiment. By increasing the permeability and particle size, the high-order TEM modes are excited inside the particle and induce more forward directional SPPs. It indicated that the particle size manifests larger tuning range compared with the permeability. Experimentally, the maximum forward-to-backward (F-to-B) SPP scattering intensity ratio of 118.52:1 is visualized by using a single 1 μm Fe3O4 magnetic nanoparticle. The directional scattered SPPs of magnetic nanoparticles are hopeful to improve the efficiency of plasmonic devices and pave the way for plasmonic circuits on-chip.

Visualization Materials Using Silicon-Based Optical Nanodisks (ViSiON) for Enhanced NIR Imaging in Ophthalmology

ViSiON (visualization materials composed of silicon-based optical nanodisks) is presented, which offers a unique optical combination of near-infrared (NIR) optical properties and biodegradability. Initially, numerical simulations are conducted to calculate the total extinction and scattering effects of ViSiON by the diameter-to-thickness ratio, predicting precise control over its scattering properties in the NIR region. A top-down patterning technique is employed to synthesize ViSiON with accurate diameter and thickness control. ViSiON with a 50 nm thickness exhibits scattering properties over 400 times higher than that of 30 nm, rendering it suitable as a contrast agent for optical coherence tomography (OCT), especially in ophthalmic applications. Furthermore, ViSiON possesses inherent biodegradability in media, with ≈95% degradation occurring after 48 h, and the degradation rate can be finely tuned based on the quantity of protein coating applied to the surface. Subsequently, the OCT imaging capability is validated even within vessels smaller than 300 µm, simulating retinal vasculature using a retinal phantom. Then, using an ex ovo chick embryo model, it is demonstrated that ViSiON enhances the strength of protein membranes by 6.17 times, thereby presenting the potential for ViSiON as an OCT imaging probe capable of diagnosing retinal diseases.

Silicon eccentric shell nanoparticles fabricated by template-assisted deposition for Mie magnetic resonances enhanced light confinement

We report a structure of silicon eccentric shell particles array, fabricated by the SiO2particles monolayer array assisted deposition of amorphous Si, for high-efficiency light confinement. The SiO2particles monolayer array is tailored to regulate its interparticle distance, followed by silicon film deposition to obtain silicon eccentric shell arrays with positive and negative off-center distancee. We studied the Mie resonances of silicon solid sphere, concentric shell, eccentric shell and observed that the eccentric shell with positive off-centeresupports superior light confinement because of the enhanced Mie magnetic resonances. Spectroscopic measurements and finite difference time domain simulations were conducted to examine the optical performance of the eccentric shell particles array. Results show that the Mie magnetic resonance wavelength can be easily regulated by the size of the inner void of the silicon shell to realize tunable enhanced light confinement. It was found silicon shell withD= 460/520 nm offered high enhanced light absorption efficiency at wavelength ofλ= 830 nm, almost beyond the bandgap of the amorphous silicon.

Hexagonal-Ge Nanostructures with Direct-Bandgap Emissions in a Si-Based Light-Emitting Metasurface

Si-based emitters have been of great interest as an ideal light source for monolithic optical-electronic integrated circuits (MOEICs) on Si substrates. However, the general Si-based material is a diamond structure of cubic lattice with an indirect band gap, which cannot emit light efficiently. Here, hexagonal-Ge (H-Ge) nanostructures within a light-emitting metasurface consisting of a cubic-SiGe nanodisk array are reported. The H-Ge nanostructure is naturally formed within the cubic-Ge epitaxially grown on Si (001) substrates due to the strain-induced nanoscale crystal structure transformation assisted by far-from-equilibrium growth conditions. The direct-bandgap features of H-Ge nanostructures are observed and discussed, including a rather strong and linearly power-dependent photoluminescence (PL) peak around 1562 nm at room temperature and temperature-insensitive PL spectrum near room temperature. Given the direct-bandgap nature, the heterostructure of H-Ge/C-Ge, and the compatibility with the sophisticated Si technology, the H-Ge nanostructure has great potential for innovative light sources and other functional devices, particularly in Si-based MOEICs.

Unveiling Local Optical Properties Using Nanoimaging Phase Mapping in High-Index Topological Insulator Bi2Se3 Resonant Nanostructures

Topological insulators are materials characterized by an insulating bulk and high mobility topologically protected surface states, making them promising candidates for future optoelectronic and quantum devices. Although their electronic properties have been extensively studied, their mid-infrared (MIR) properties and prospective photonic capabilities have not been fully uncovered. Here, we use a combination of far-field and near-field nanoscale imaging and spectroscopy to study chemical vapor deposition-grown Bi2Se3 nanobeams (NBs). We extract the MIR optical constants of Bi2Se3, revealing refractive index values as high as n ∼ 6.4, and demonstrate that the NBs support Mie resonances across the MIR. Local near-field reflection phase mapping reveals domains of various phase shifts, providing information on the local optical properties of the NBs. We experimentally measure up to 2π phase-shift across the resonance, in excellent agreement with finite-difference time-domain simulations. This work highlights the potential of Bi2Se3 for quantum circuitry, nonlinear generation, high-Q metaphotonics, and photodetection.

Machine learning of all-dielectric core-shell nanostructures: the critical role of the objective function in inverse design

To integrate nanophotonics into light-based technologies, it is critical to elicit a desired optical response from its fundamental component, a nanoresonator. Because the optical resonance of a nanoresonator depends strongly on its base material and structural features, machine learning has been contemplated to enhance the design and optimization processes. However, its accuracy in searching the vast parameter space of nanophotonics still poses unresolved questions. Here, we show how the choice of objective functions, in combination with trained neural networks, can drastically change the optimization process-even for a simple nanophotonic structure. To assess how different objective functions select the correct structural parameters that generate a desired optical Mie response, we use a simple core-shell, all-dielectric nanostructure as the benchmark. By controlling the proportion of training data, which represents the "experience" level, we also quantify how the various objective functions perform in finding the ground-truth parameters. Our findings demonstrate that certain objective functions exhibit improved accuracy when used with highly "experienced" neural networks. Surprisingly, we also find other objective functions that perform better when paired with less "experienced" neural networks. Taken together, our results emphasize that it is critical to understand how neural networks are coupled to optimization schemes, as is evident even when a simple core-shell nanostructure is used.

Laser-Induced Graphene-based Flexible Substrate with Photothermal Conversion and Photoresponse Performance on Polyimide Film

Graphene-based flexible electronic devices are widely used in photoelectric components and photodetectors. However, it remains a huge challenge to fabricate graphene-based flexible devices efficiently and economically. Compared with the flexible electronic devices made by combining the flexible film with metal and semiconductor materials, the graphene-based flexible substrate (GFS) can be efficiently and conveniently induced by laser direct writing on the flexible film. In this paper, the GFS with a resistance of as low as 15 Ω was successfully induced by CO2 laser on a polyimide (PI) film in one step, and the GFS surface covered with carbon nanoparticles (GFSC) with a resistance of 25 Ω was further induced by femtosecond (fs) laser reprocessing. Benefiting from the laser-induced porous graphene structure, the absorptivity of GFS is up to 90% in the wavelength range of 200-2000 nm. The formation of carbon nanoparticles on the GFSC surface further improves the absorptivity to 97.5% in a wide spectral range. Under white light irradiation of 1 sun, the surface temperature of GFS reaches 65.7 °C and that of GFSC is up to 70.8 °C within 2 min. Under the irradiation of a light-emitting diode (LED) with a central wavelength of 365 nm, the highest photoresponsivity of GFS and GFSC was 8.8 and 1.3 mA/W, respectively. The response time and recovery time of GFS are 8 and 7.3 s, and those of GFSC are 8.3 and 6.7 s, respectively. Importantly, GFSC has a more stable photoresponse performance due to the better electron capture and transfer capability of carbon nanoparticles. It is believed that GFS and GFSC have great application potential in flexible photodetectors and sensors.

Dual Control of Enhanced Quasi-Bound States in the Continuum Emission from Resonant c-Si Metasurfaces

Optical bound states in the continuum (BICs) offer strong interactions with quantum emitters and have been extensively studied for manipulating spontaneous emission, lasing, and polariton Bose-Einstein condensation. However, the out-coupling efficiency of quasi-BIC emission, crucial for practical light-emitting devices, has received less attention. Here, we report an adaptable approach for enhancing quasi-BIC emission from a resonant monocrystalline silicon (c-Si) metasurface through lattice and multipolar engineering. We identify dual-BICs originating from electric quadrupoles (EQ) and out-of-plane magnetic dipoles, with EQ quasi-BICs exhibiting concentrated near-fields near the c-Si nanodisks. The enhanced fractional radiative local density of states of EQ quasi-BICs overlaps spatially with the emitters, promoting efficient out-coupling. Furthermore, coupling the EQ quasi-BICs with Rayleigh anomalies enhances directional emission intensity, and we observe inherent opposite topological charges in the multipolarly controlled dual-BICs. These findings provide valuable insights for developing efficient nanophotonic devices based on quasi-BICs.

Photonic Artifacts in Ratiometric Luminescence Nanothermometry

Ongoing developments in science and technology require temperature measurements at increasingly higher spatial resolutions. Nanocrystals with temperature-sensitive luminescence are a popular thermometer for these applications offering high precision and remote read-out. Here, we demonstrate that ratiometric luminescence thermometry experiments may suffer from systematic errors in nanostructured environments. We place lanthanide-based luminescent nanothermometers at controlled distances of up to 600 nm from a Au surface. Although this geometry supports no absorption or scattering resonances, distortion of the emission spectra of the thermometers due to the modified density of optical states results in temperature read-out errors of up to 250 K. Our simple analytical model explains the effects of thermometer emission frequencies, experimental equipment, and sample properties on the magnitude of the errors. We discuss the relevance of our findings in several experimental scenarios. Such errors do not always occur, but they are expected in measurements near reflecting interfaces or scattering objects.

Levitated Optomechanics with Meta-Atoms

We propose to introduce additional control in levitated optomechanics by trapping a meta-atom, i.e., a subwavelength and high-permittivity dielectric particle supporting Mie resonances. In particular, we theoretically demonstrate that optical levitation and center-of-mass ground-state cooling of silicon nanoparticles in vacuum is not only experimentally feasible but it offers enhanced performance over widely used silica particles in terms of trap frequency, trap depth, and optomechanical coupling rates. Moreover, we show that, by adjusting the detuning of the trapping laser with respect to the particle's resonance, the sign of the polarizability becomes negative, enabling levitation in the minimum of laser intensity, e.g., at the nodes of a standing wave. The latter opens the door to trapping nanoparticles in the optical near-field combining red and blue-detuned frequencies, in analogy to two-level atoms, which is of interest for generating strong coupling to photonic nanostructures and short-distance force sensing.

Low-Variance Surface-Enhanced Raman Spectroscopy Using Confined Gold Nanoparticles over Silicon Nanocones

Surface-enhanced Raman spectroscopy (SERS) substrates are of utmost interest in the analyte detection of biological and chemical diagnostics. This is primarily due to the ability of SERS to sensitively measure analytes present in localized hot spots of the SERS nanostructures. In this work, we present the formation of 67 ± 6 nm diameter gold nanoparticles supported by vertically aligned shell-insulated silicon nanocones for ultralow variance SERS. The nanoparticles are obtained through discrete rotation glancing angle deposition of gold in an e-beam evaporating system. The morphology is assessed through focused ion beam tomography, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The optical properties are discussed and evaluated through reflectance measurements and finite-difference time-domain simulations. Lastly, the SERS activity is measured by benzenethiol functionalization and subsequent Raman spectroscopy in the surface scanning mode. We report a homogeneous analytical enhancement factor of 2.2 ± 0.1 × 10⁷ (99% confidence interval for N = 400 grid spots) and made a comparison to other lithographically derived assemblies used in SERS. The strikingly low variance (4%) of our substrates facilitates its use for many potential SERS applications.

Multipolar silicon-based resonant meta-surface for electro-optical modulation and sensing

A multipolar silicon-based resonant meta-surface scheme is proposed and numerically presented via intercalating oblique slits into the silicon patches, leading to an ultra-sharp resonant spectrum via the excitation of electric and magnetic quadrupoles and their hybridization coupling. High-performance electro-optical modulator is demonstrated, showing a spectrally shifted modulation sensitivity up to 1.546 nm/V. Moreover, novel, to the best of our knowledge, optical sensing for ion solution concentration with the detection limitation down to 5.15 × 10^-3 is demonstrated as another application. These findings provide an impressive strategy for resonant silicon-based nano-photonics and opto-electronic devices.

Strong Plasmon-Mie Resonance in Si@Pd Core-Ω Shell Nanocavity

The surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) can be used to enhance the generation of the hot electrons in plasmon metal nanocavity. In this paper, Pd nanomembrane (NMB) is sputtered on the surface of Si nanosphere (NS) on glass substrate to form the Si@Pd core-Ω shell nanocavity. A plasmon-Mie resonance is induced in the nanocavity by coupling the plasmon resonance with the Mie resonance to control the optical property of Si NS. When this nanocavity is excited by near-infrared-1 (NIR-1, 650 nm-900 nm) femtosecond (fs) laser, the luminescence intensity of Si NS is dramatically enhanced due to the synergistic interaction of plasmon and Mie resonance. The generation of resonance coupling regulates resonant mode of the nanocavity to realize multi-dimensional nonlinear optical response, which can be utilized in the fields of biological imaging and nanoscale light source.

Resonance Coupling in Si@WS2Core-Ω Shell Nanostructure

Realizing strong laser-matter interaction in a heterostructure consisting of two-dimensional transition metal dichalcogenides (TMDCs) and an optical nanocavity is a potential strategy for novel photonic devices. In this paper, two core-Ω shell nanostructures, Si@WS₂ core-Ω shell nanostructure on glass/Si substrates, are briefly introduced. A strong laser-matter interaction occurred in the Si@WS₂ core-Ω shell nanostructure when it was excited by femtosecond (fs) laser in the near-infrared-1 region (NIR-1, 650 nm-950 nm), resulting in a resonance coupling between the electric dipole resonance (EDR) of the Si nanosphere (NS) and the exciton resonance of the WS₂ nanomembrane (NMB). The generation of resonance coupling regulates the resonant mode of the nanostructure to realize the multi-dimensional nonlinear optical response, which can be utilized in the fields of biological imaging and nanoscale light source.

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Substrate-Modulated Electric and Magnetic Resonances of Lithium Niobite Nanoparticles Illuminated by White Light

The manipulation of light at the nanoscale is important for nanophotonic research. Lithium niobite (LiNbO₃), as an ideal building block for metamaterials, has attracted great interest for its unique properties in the field of nonlinear optics. In this paper, we numerically studied the effect of different substrates on the optical resonances of a LiNbO₃ nanoparticle. The results show that the electric and magnetic resonances of such a system can be effectively adjusted by changing the substrate. Compared to the impact of dielectric substrate, the interaction between the LiNbO₃ nanoparticle and the Au film shows a fascinating phenomenon that a sharp resonance peak appears. The multipole decomposition of the scattering spectrum shows that the size, shape of the LiNbO₃ nanoparticle, and the thickness of the SiO₂ film between the particle and the Au film have a significant impact on the electromagnetic resonance of the LiNbO₃ nanoparticle. This work provides a new insight into LiNbO₃ nanoparticles, which may have potential use in the design of dielectric nanomaterials and devices.

Femtosecond Laser-Assisted Formation of Hybrid Nanoparticles from Bi-Layer Gold–Silicon Films for Microscale White-Light Source

It is very natural to use silicon as a primary material for microelectronics. However, silicon application in nanophotonics is limited due to the indirect gap of its energy band structure. To improve the silicon emission properties, it can be combined with a plasmonic part. The resulting metal–dielectric (hybrid) nanostructures have shown their excellence compared to simple metallic dielectric nanostructures. Still, in many cases, the fabrication of such structures is time consuming and quite difficult. Here, for the first time, we demonstrate a single-step and lithography-free laser-induced dewetting of bi-layer nanoscale-thickness gold–silicon films supported by a glass substrate to produce hybrid nanoparticles. For obtaining hybrid nanoparticles, we study nonlinear photoluminescence by mapping their optical response and morphology by scanning electron microscopy. This method can be used for the fabrication of arrays of hybrid nanoparticles providing white-light photoluminescence with a good control of their microscopic sizes and position. The developed approach can be useful for a wide range of photonic applications including the all-optical data processing and storage where miniaturization down to micro- and nanoscale together with an efficiency increase is of high demand.

Highly efficient nonlinear optical emission from a subwavelength crystalline silicon cuboid mediated by supercavity mode

The low quantum efficiency of silicon (Si) has been a long-standing challenge for scientists. Although improvement of quantum efficiency has been achieved in porous Si or Si quantum dots, highly efficient Si-based light sources prepared by using the current fabrication technooloy of Si chips are still being pursued. Here, we proposed a strategy, which exploits the intrinsic excitation of carriers at high temperatures, to modify the carrier dynamics in Si nanoparticles. We designed a Si/SiO2 cuboid supporting a quasi-bound state in the continuum (quasi-BIC) and demonstrated the injection of dense electron-hole plasma via two-photon-induced absorption by resonantly exciting the quasi-BIC with femtosecond laser pulses. We observed a significant improvement in quantum efficiency by six orders of magnitude to ~13%, which is manifested in the ultra-bright hot electron luminescence emitted from the Si/SiO2 cuboid. We revealed that femtosecond laser light with transverse electric polarization (i.e., the electric field perpendicular to the length of a Si/SiO2 cuboid) is more efficient for generating hot electron luminescence in Si/SiO2 cuboids as compared with that of transverse magnetic polarization (i.e., the magnetic field perpendicular to the length of a Si/SiO2 cuboid). Our findings pave the way for realizing on-chip nanoscale Si light sources for photonic integrated circuits and open a new avenue for manipulating the luminescence properties of semiconductors with indirect bandgaps.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Ultrasensitive Frequency Shifting of Dielectric Mie Resonance near Metallic Substrate

Dielectric resonators on metallic surface can enhance far-field scattering and boost near-field response having promising applications in nonlinear optics and reflection-type devices. However, the dependence of gap size between dielectric resonator and metallic surface on Mie resonant frequency is complex and desires a comprehensive physical interpretation. Here, we systematically study the effect of metallic substrate on the magnetic dipole (MD) resonant frequency at X-band by placing a high permittivity CaTiO₃ ceramic block on metallic substrate and regulating their gap size. The simulated and experimental results show that there are two physical mechanisms to codetermine the metallic substrate-induced MD frequency. The greatly enhanced electric field pair in the gap and the coupling of MD resonance with its mirror image are decisive for small and large gaps, respectively, making the MD resonant frequency present an exponential blue shift first and then a slight red shift with increasing gap size. Further, we use the two mechanisms to explain different frequency shifting properties of ceramic sphere near metallic substrate. Finally, taking advantage of the sharp frequency shifting to small gaps, the ceramic block is demonstrated to accurately estimate the thickness or permittivity of thin film on metallic substrate through a governing equation derived from the method of symbolic regression. We believe that our study will help to understand the resonant frequency shifting for dielectric particle near metallic substrate and give some prototypes of ultrasensitive detectors.

Greatly Enhanced Plasmon-Exciton Coupling in Si/WS2/Au Nanocavities

A strong light-matter interaction is highly desirable from the viewpoint of both fundamental research and practical application. Here, we propose a dielectric-metal hybrid nanocavity composed of a silicon (Si) nanoparticle and a thin gold (Au) film and investigate numerically and experimentally the coupling between the plasmons supported by the nanocavity and the excitons in an embedded tungsten disulfide (WS₂) monolayer. When a Si/WS₂/Au nanocavity is excited by the surface plasmon polariton generated on the surface of the Au film, greatly enhanced plasmon-exciton coupling originating from the hybridization of the surface plasmon polariton, the mirror-image-induced magnetic dipole, and the exciton modes is clearly revealed in the angle- or size-resolved scattering spectra. A Rabi splitting as large as ∼240 meV is extracted by fitting the experimental data with a coupled harmonic oscillator model containing three oscillators. Our findings open new horizons for constructing nanoscale photonic devices by exploiting dielectric-metal hybrid nanocavities.

Spontaneous Light Emission Assisted by Mie Resonances in Diamond Nanoparticles

Spontaneous light emission is known to be affected by the local density of states and enhanced when coupled to a resonant cavity. Here, we report on an experimental study of silicon-vacancy (SiV) color center fluorescence and spontaneous Raman scattering from subwavelength diamond particles supporting low-order Mie resonances in the visible range. For the first time to our knowledge, we have measured the size dependences of the SiV fluorescence emission rate and the Raman scattering intensity from individual diamond particles in the range from 200 to 450 nm. The obtained dependences reveal a sequence of peaks, which we explicitly associate with specific multipole resonances. The results are in agreement with our theoretical analysis and highlight the potential of intrinsic optical resonances for developing nanodiamond-based lasers and single-photon sources.

Enhanced Chiral Mie Scattering by a Dielectric Sphere within a Superchiral Light Field

A superchiral field, which can generate a larger chiral signal than circularly polarized light, is a promising mechanism to improve the capability to characterize chiral objects. In this paper, Mie scattering by a chiral sphere is analyzed based on the T-matrix method. The chiral signal by circularly polarized light can be obviously enhanced due to the Mie resonances. By employing superchiral light illumination, the chiral signal is further enhanced by 46.8% at the resonance frequency. The distribution of the light field inside the sphere is calculated to explain the enhancement mechanism. The study shows that a dielectric sphere can be used as an excellent platform to study the chiroptical effects at the nanoscale.

Broadband Raman scattering enhancement with reduced heat generation in a dielectric-metal hybrid nanocavity

The strongly localized electric field achieved in metallic nanoparticles (NPs) and nanostructures are commonly employed to realize surface-enhanced Raman scattering. However, the heat originating from the Ohmic loss of metals may lead to the damage of the analyzed molecules, which severely limits the practical applications of pure-metallic nanostructures. Here, we propose a dielectric-metallic hybrid nanocavity placing silicon (Si) NPs onto a gold (Au) film to realize broadband Raman scattering enhancement with significantly reduced heat generation. Our results reveal that the heat generation is dramatically reduced in the hybrid nanocavity as compared with its pure-metallic counterpart while a significantly enhanced electric field is maintained. We demonstrate numerically and experimentally that the optical resonances, which arise from the coherent coupling of the electric and magnetic dipoles excited inside the Si NP with their mirror images arisen from the Au film, can be employed to enhance the excitation and radiation of Raman signals, respectively. We find that the enhancement in the radiation of Raman signals plays a crucial role in enhancing the total Raman scattering. We also show that the hybrid nanocavity acts as a nano-antenna which effectively radiates Raman signals into the far-field. These findings indicate the advantages of such hybrid nanocavities in temperature-sensitive Raman scattering characterization and supply new strategies for designing nanoscale photonic devices of other functionalities with hybrid nanocavities.

Silicon Quantum Dot-Polymer Fabry-Pérot Resonators with Narrowed and Tunable Emissions

Luminescent silicon nanoparticles have been widely recognized as an alternative for metal-based quantum dots (QDs) for optoelectronics partly because of the high abundance and biocompatibility of silicon. To date, the broad photoluminescence line width (often >100 nm) of silicon QDs has been a hurdle to achieving competitive spectral purity and incorporating them into light-emitting devices. Herein we report fabrication and testing of straightforward configuration of Fabry-Pérot resonators that incorporates a thin layer of SiQD-polymer hybrid/blend between two reflective silver mirrors; remarkably these devices exhibit up-to-14-fold narrowing of SiQD emission and achieve a spectral bandwidth as narrow as ca. 9 nm. Our polymer-based, SiQD-containing Fabry-Pérot resonators also provide convenient spectral tunability, can be prepared using a variety of polymer hosts and substrates, and enable rigid as well as flexible devices.

All-Dielectric Nanostructures with a Thermoresponsible Dynamic Polymer Shell

We suggest a new strategy for creating stimuli-responsive bio-integrated optical nanostructures based on Mie-resonant silicon nanoparticles covered by an ensemble of similarity negatively charged polyelectrolytes (heparin and sodium polystyrene sulfonate). The dynamic tuning of the nanostructures' optical response is due to light-induced heating of the nanoparticles and swelling of the polyelectrolyte shell. The resulting hydrophilic/hydrophobic transitions significantly change the shell thickness and reversible shift of the scattering spectra for individual nanoparticles up to 60 nm. Our findings bring novel opportunities for the application of smart nanomaterials in nanomedicine and bio-integrated nanophotonics.

The Effect of Nitrogen Incorporation on the Optical Properties of Si-Rich a-SiCx Films Deposited by VHF PECVD

The influence of N incorporation on the optical properties of Si-rich a-SiC_x films deposited by very high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) was investigated. The increase in N content in the films was found to cause a remarkable enhancement in photoluminescence (PL). Relative to the sample without N incorporation, the sample incorporated with 33% N showed a 22-fold improvement in PL. As the N content increased, the PL band gradually blueshifted from the near-infrared to the blue region, and the optical bandgap increased from 2.3 eV to 5.0 eV. The enhancement of PL was suggested mainly from the effective passivation of N to the nonradiative recombination centers in the samples. Given the strong PL and wide bandgap of the N incorporated samples, they were used to further design an anti-counterfeiting label.

Fabry-Perot cavity enhanced three-photon luminescence of atomically thin platinum diselenide

Two-dimensional materials, such as transition metal dichalcogenides (TMDs), exhibit intriguing physical properties that lead to both fundamental research and technology development. The recently emerged platinum diselenide (PtSe2), as a new member of the TMDs, has attracted increasing attention because of its good air stability, large refractive index and high electron mobility. However, being atomically thin significantly hinders its interaction with light, severely limiting the spontaneous or stimulated linear and nonlinear emission. Particularly, its nonlinear up-converted emission has not been fully exploited yet. Here, we experimentally observed the distinct enhancement of nonlinear up-converted luminescence of CVD-grown PtSe2 atomic layers on a SiO2/Si substrate with the assistance of the Fabry-Perot cavity resonance. The laser irradiance dependent luminescence study reveals the three-photon process of this nonlinear emission for the first time. Compared with non-resonant excitation, the luminescence enhancement can be up to six times because of the optical interference induced local field enhancement at the excitation wavelength. Leveraging this three-photon luminescence, nonlinear optical imaging and encryption were demonstrated for exploring information security applications. These results will pave the way for integrating nonlinear optical devices with the PtSe2 2D material.

Femtosecond Laser Fabrication of Hybrid Metal-Dielectric Structures with Nonlinear Photoluminescence

Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric structures with strong nonlinear properties obtained by a single-step fabrication process. We determine the influence of several incident femtosecond pulses on the Au/Si bi-layer film on produced structure morphology. The created hybrid systems represent isolated nanoparticles with a height of 250–500 nm exceeding the total thickness of the Au-Si bi-layer. It is shown that fabricated hybrid nanostructures demonstrate enhancement of the SHG signal (up to two orders of magnitude) compared to the initial planar sample and a broadband photoluminescence signal (more than 200 nm in width) in the visible spectral region. We establish the correlation between nonlinear signal and phase composition provided by Raman scattering measurements. Such laser-induced structures have significant potential in optical sensing applications and can be used as components for different nanophotonic devices.

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.

Ultra-high quality graphene perfect absorbers for high performance switching manipulation

Wavelength-selective light absorption and the related switching operations are highly desired in optical devices. Herein, we report the approach for ultra-high quality (Q) graphene perfect optical absorption, which possesses impressive performance in switching manipulation. A record-breaking Q-factor (up to 10⁵) is observed, suggesting one or two orders of magnitude larger than that of the conventional graphene absorbers. The ultra-low external leakage loss rate of quasi-bound states in the continuum (BIC) resonator and the ultra-low intrinsic absorption loss rate in the resonant mode volume are the main contributions for the ultra-high Q perfect absorption. By introducing a Kerr nonlinear medium, spectral relative intensity can be changed from 0 to 100% when an ultra-low pump light with the intensity of only 5 kW cm^-2 is used. After a rather slight tuning of the refractive index (Δn = 5×10^-4) for the resonators, the absorption contrast ratio reaches 31 dB. The switching related spectral wavelength shift sensitivity is up to 915 nm/RIU and the figure of merit (FOM) is 50 833. These features confirm the ultra-high tunability and switching manipulation. It is believed that the ultra-high Q-factor absorption offered by all-dielectric configuration provides plentiful potential applications for graphene-based devices in the all-optical switch, modulator, notch filter, etc.

Stimulated Raman Scattering from Mie-Resonant Subwavelength Nanoparticles

Resonant dielectric structures have emerged recently as a new platform for subwavelength nonplasmonic photonics. It was suggested and demonstrated that magnetic and electric Mie resonances can enhance substantially many effects at the nanoscale including spontaneous Raman scattering. Here, we demonstrate stimulated Raman scattering (SRS) for isolated crystalline silicon (c-Si) nanoparticles and observe experimentally a transition from spontaneous to stimulated scattering manifested in a nonlinear growth of the signal intensity above a certain pump threshold. At the Mie resonance, the light gets confined into a low volume of the resonant mode with enhanced electromagnetic fields inside the c-Si nanoparticle due to its high refractive index, which leads to an overall strong SRS signal at low pump intensities. Our finding paves the way for the development of efficient Raman nanolasers for multifunctional photonic metadevices.

An Electrically Controlled Wavelength-Tunable Nanoribbon Laser

Nanoscale laser sources with downscaled device footprint, high energy efficiency, and high operation speed are pivotal for a wide array of optoelectronic and nanophotonic applications ranging from on-chip interconnects, nanospectroscopy, and sensing to optical communication. The capability of on-demand lasing output with reversible and continuous wavelength tunability over a broad spectral range enables key functionalities in wavelength-division multiplexing and finely controlled light-matter interaction, which remains an important subject under intense research. In this study, we demonstrate an electrically controlled wavelength-tunable laser based on a CdS nanoribbon (NR) structure. Typical "S"-shaped characteristics of pump power dependence were observed for dominant lasing lines, with concomitant line width narrowing. By applying an increased bias voltage across the NR device, the lasing resonance exhibits a continuous tuning from 510 to 520 nm for a bias field in the range 0-15.4 kV/cm. Systematic bias-dependent absorption and time-resolved photoluminescence (PL) measurements were performed, revealing a red-shifted band edge of gain medium and prolonged PL lifetime with increased electric field over the device. Both current-induced thermal reduction of the band gap and the Franz-Keldysh effect were identified to account for the modification of the lasing profile, with the former factor playing the leading role. Furthermore, dynamical switching of NR lasing was successfully demonstrated, yielding a modulation ratio up to ∼21 dB. The electrically tuned wavelength-reversible CdS NR laser in this work, therefore, presents an important step toward color-selective coherent emitters for future chip-based nanophotonic and optoelectronic circuitry.

Plasmonic nanosponges filled with silicon for enhanced white light emission

Plasmonic nanosponges are a powerful platform for various nanophotonic applications owing to extremely high local field enhancement in metallic nanopores. The filling of the nanopores with high-refractive index semiconductors (e.g. Si, Ge, GaP, etc.) opens up opportunities for the enhancement of nonlinear effects in these materials. However, this task remains challenging due to the lack of knowledge on the integration process of metal and high-index semiconductor components in such nanoobjects. Here, we investigate metal-dielectric nanoparticles fabricated from bilayer Si/Au films by the laser printing technique via a combination of theoretical and experimental methods. We reveal that these hybrid nanoparticles represent the Au sponge-like nanostructure filled with Si nanocrystallites. We also demonstrate that the Au net provides strong near-field enhancement in the Si grains increasing the white light photoluminescence in the hybrid nanostructures compared to uniform Si nanoparticles. These results pave the way for engineering the internal structure of the sponge-like hybrid nanoparticles possessing white light luminescence and control of their optical properties on demand.

Low-index dielectric metasurfaces supported by metallic substrates for efficient second-harmonic generation in the blue-ultraviolet range

Despite the great importance of high-index materials for dielectric nanophotonics, their optical functionalities are significantly limited for diverse photonic applications and thus, the usability of low-index materials should be explored. This work proposes the use of metallic substrates for low-index dielectric metasurfaces for significantly enhancing the local field and their optical responses. Plasmon-assisted dipole resonances mainly contribute to field enhancement in dielectric nanoparticles comprising the metasurfaces, where the intensity enhancement increases on decreasing the index of the nanoparticles when supported by metallic substrates. Another challenge with the current high-index materials is strong optical losses in the blue-ultraviolet range, which limit their practical applications such as harmonic generations in this spectral range. For a pump with a peak intensity of about 3.4 GW cm-2, a metasurface of lithium niobate nanodisk array supported by a gold substrate generates second harmonic at 400 nm with an efficiency of about 5 × 10-2%, which is one order of magnitude higher than the previously reported efficiency of harmonic generation in this range. The results presented in this work promise the significant extension of the current nonlinear nanophotonics, which is limited to high-index semiconducting materials.

Symmetry-broken square silicon patches for ultra-narrowband light absorption

The effect of ultra-narrowband light absorption enhancement is presented by using metamaterials with symmetry-broken square silicon patches (SSPs). The symmetry of the SSP can be broken by introducing a narrow slit deviating from its center. By breaking the symmetry of the SSPs, slit resonance mode with standing wave patterns can be excited, and the locations of the absorption peaks can be well estimated by using the Fabry-Pérot (F-P) cavity model. Although there is no excitation of surface plasmon resonance, ultra-narrowband light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. Good ultra-narrowband absorption features can be maintained as the parameters of the buffer layer and the SSPs are altered. When this type of symmetry-broken SSPs-based metamaterial is used in refractive-index sensors, it shows excellent sensing properties due to its stable ultra-narrowband absorption enhancement.

Si nano-cavity enabled surface-enhanced Raman scattering signal amplification

Surface-enhanced Raman scattering (SERS) detection technique has gained much attention as a powerful analytical tool in recent years. Nevertheless, the attention was mainly focused on the efficient scattering platform by structuring metals themselves, leading to more complex platforms and higher costs. Herein, a new and simple strategy to prepare large-area, low-cost, high-performance SERS substrate is introduced. Ultra-thin semiconductor silicon (Si) film is used as the functional layer for the metallic nano-particles based meta-surface. During the SERS sensing process, the emergence of a Si layer is observed to provide three key contributions: (1) to produce a maximal enhancement factor (EF) ∼470% compared to that of the bare meta-surface, (2) to keep a higher spectral stability for the Raman signal, and (3) to physically interdict the contact between the metal and the molecule. Moreover, the Si film's thickness is down to the scale of an electron's Bohr radius, indicating efficient electronic oscillations for the semiconductor material under electromagnetic excitation. The charge transfer behaviors between the molecules and the Si layer and metal nano-particles can also emerge. These findings could pave new insights on the surface-enhanced spectroscopy and lead to applications for the high-performance, large-area, low-cost SERS sensing process.

Trapping and filtering of light by single Si nanospheres in a GaAs nanocavity

The arbitrary manipulation of optical waves in the subwavelength dimension is a fundamental issue for the microminiaturization and integration of optic parts. In the past decade, major efforts were focused on the surface plasmon resonance mostly exhibited by metallic nanostructures, which could effectively capture and concentrate the visible light at the cost of high levels of intrinsic losses. However, the use of all-dielectric nanostructures can avoid the abovementioned problem due to lower intrinsic losses and the presence of abundant resonance modes. Herein, as a kind of building block for light manipulation, GaAs nanogrooves were fabricated and studied to obtain comprehensive information about the resonance modes in an individual all-dielectric nanogroove; by placing a single Si nanosphere in an isolated GaAs nanocavity, the nanogroove scattering could be controlled depending on the coupling strength of nanogrooves. The Lorentzian line approximation and harmonic oscillator coupling model were used to pursue the interactions among the resonance modes. Experimental and theoretical studies showed that this heterostructure could trap the broadband visible light in the back and filter the light with a specific wavelength in the front. These findings suggest that the proposed heterostructure can act as a light filter and an antenna on nanophotonic chips due to its unique optical properties.

Truncated titanium/semiconductor cones for wide-band solar absorbers

A truncated Ti and Si cones metasurface has been proposed for wide-band solar absorber (WSA), which produced a high average absorption of 94.7% in the spectral region from 500 to 4000 nm. A maximal enhancement factor of 166.0% was achieved by the WSA in comparison with the absorption of Ti/Si cylinder resonators based absorber. Under the standard solar radiance, a high full-spectrum solar absorption efficiency of 96.1% was obtained for the WSA in the energy range from 0.28 to 4.0 eV. The spectral bandwidth with absorption above 90% is up to 3.402 μm, which shows an enhancement factor of 165.0% than that of the WSA intercalated by the SiO₂. Other semiconductors such as Ge, GaAs have been utilized to form the WSA, which also maintained the near-unity absorption in the wide-band spectrum. The plasmonic resonant response of the Ti material and the strong electromagnetic coupling capability of the Si resonator, and the plasmonic near-field coupling by the adjacent truncated cones were the main contributions for the impressive absorption behaviors. These findings pave a new way for achieving full-spectrum solar absorber via combining the Ti material and semiconductors, which could open potential approaches for active optoelectronic devices such as photo-detectors, hot-electron related modulators, and solar cells, etc.

Enhanced excitation and emission from 2D transition metal dichalcogenides with all-dielectric nanoantennas

The recently emerged concept of all-dielectric nanophotonics based on optical Mie resonances in high-index dielectric nanoparticles has proven to be a promising pathway to boost light-matter interactions at the nanoscale. In this work, we discuss the opportunities enabled by the interaction of dielectric nanoresonators with 2D transition metal dichalcogenides (2D TMDCs), leading to weak and strong coupling regimes. We perform a comprehensive analysis of bright exciton photoluminescence (PL) enhancement from various 2D TMDCs, including WS₂, MoS₂, WSe₂, and MoSe₂ via their coupling to Mie resonances of a silicon nanoparticle. For each case, we find the system parameters corresponding to maximal PL enhancement taking into account excitation rate, Purcell factor and radiation efficiency. We demonstrate numerically that all-dielectric Si nanoantennas can significantly enhance the PL intensity from 2D TMDC by a factor of hundred through precise optimization of the geometrical and material parameters. Our results may be useful for high-efficiency 2D TMDC-based optoelectronic, nanophotonic, and quantum optical devices.

Ideal magnetic dipole resonances with metal-dielectric-metal hybridized nanodisks

Magnetic resonances generated with nonmagnetic nanostructures have been widely used to design various functional nanophotonic devices, and it is important to realize pure magnetic dipole scattering for the unambiguous study of magnetic light-matter interactions. However, the magnetic responses often spectrally overlapping with other multipoles, which is the main obstacle to achieve ideal magnetic dipole resonances. This study proposes and theoretically demonstrates that an ideal magnetic dipole resonance can be excited with metal-dielectric-metal hybridized nanodisks. It is shown that although the generated magnetic dipole scattering around the bonding resonance of the hybridized nanodisk is spectrally overlapping with strong electric dipole and electric quadrupole contributions, an almost perfect current loop can be generated by adjusting the geometry parameters and the refractive index of the dielectric layer, thereby leading to the suppressing of the overlapping multipoles and the formation of an ideal magnetic dipole scattering. What's more important is that both electric and magnetic near-fields are enhanced simultaneously with the increasing of the refractive index of the dielectric layer, which makes the hybridized nanodisk a promising platform for enhanced magnetic light-matter interactions.

Cast-and-Use Super Black Coating Based on Polymer-Derived Hierarchical Porous Carbon Spheres

Super black materials with extremely low reflectance and high absorption of incident light are crucial in various applications with strict optical requirements. However, the existing super black materials suffer from technical complexity in practical use whereas the underlying physics of their ultralow reflectance is still unclear. Herein, hierarchical porous carbon spheres are prepared using poly(vinylidene chloride- co-vinyl chloride) as a precursor. Dehydrochlorination treatment has proved to be very crucial for the morphology robustness of the porous polymer spheres during carbonization process. Coatings cast from the dispersion of these carbon spheres exhibit a hemispherical reflectance of <0.2% in the visible region and <0.3% in the range of 300-2000 nm, and the lowest reflectance of 0.14% can be reached. This super black coating has great advantages in its easy availability of starting materials, low equipment requirement, and high adaptability onto various substrates. Besides, a theoretical investigation suggests that a small scatterer size and low volume-filling ratio are the two most essential factors in realizing ultralow reflectance, which also offers an instructive guidance for the rational design of super black materials.

Beyond quantum confinement: excitonic nonlocality in halide perovskite nanoparticles with Mie resonances

Halide perovskite nanoparticles have demonstrated pronounced quantum confinement properties for nanometer-scale sizes and strong Mie resonances for 102 nm sizes. Here we studied the intermediate sizes where the nonlocal response of the exciton affects the spectral properties of Mie modes. The mechanism of this effect is associated with the fact that excitons in nanoparticles have an additional kinetic energy that is proportional to k2, where k is the wavenumber. Therefore, they possess higher energy than in the case of static excitons. The obtained experimental and theoretical results for MAPbBr3 nanoparticles of various sizes (2-200 nm) show that for particle radii comparable with the Bohr radius of the exciton (a few nanometers in perovskites), the blue-shift of the photoluminescence, scattering, and absorption cross-section peaks related to quantum confinement should be dominating due to the weakness of Mie resonances for such small sizes. On the other hand, for larger sizes (more than 50-100 nm), the influence of Mie modes increases, and the blue shift remains despite the fact that the effect of quantum confinement becomes much weaker.

Active tuning of the Fano resonance from a Si nanosphere dimer by the substrate effect

All-dielectric materials have aroused great interest for their unique light scattering and lower losses compared with plasmonics. Generally, optical properties made by all-dielectric materials can be passively controlled by varying the geometry, size and refractive index at the design stage. Therefore, the realization of active tuning in the field of nanophotonics is important to improve the practicality and achieve light-on-chip technology in the future. Herein, we combine the high refractive index of Si and the phase transition of VO₂ to form an active tuning hybrid nanostructure with higher quality factor by depositing Si nanospheres on the VO₂ layer with an Al₂O₃ substrate. As the temperature goes up, the refractive index of the VO₂ layer switches from high to low. The scattering intensity of the magnetic dipole resonance of Si nanospheres decreases differently depending on their size, while the intensity of the electric dipole resonance remains almost unchanged. Meanwhile, Fano resonances are observed in the Si nanosphere dimers with a continuous variable Fano lineshape when adjusting the temperature. Mie theory and substrate-induced resonant magneto-electric effects are used to analyze and explain these phenomena. Tuning of the Fano resonance is attributed to the substrate effect from the interaction between Si nanospheres and phase transition of the VO₂ layer with temperature. These light scattering properties of such a hybrid nanostructure make it promising for temperature sensing or as a light source at the nanometer scale.

Metal Substrate-Induced Line Width Compression in the Magnetic Dipole Resonance of a Silicon Nanosphere Illuminated by a Focused Azimuthally Polarized Beam

We investigate the modification of the magnetic dipole resonance of a silicon nanosphere, which is illuminated by a focused azimuthally polarized beam, induced by a metal substrate. It is found that the magnetic dipole of the silicon nanosphere excited by the focused azimuthally polarized beam and its image dipole induced by the metal substrate are out of phase. The interference of these two anti-parallel dipoles leads to a dramatic line width compression in the magnetic dipole resonance, manifested directly in the scattering spectrum of the silicon nanosphere. The quality factor of the modified magnetic dipole resonance is enhanced by a factor of ∼ 2.5 from ∼ 14.62 to ∼ 37.25 as compared with that of the silicon nanosphere in free space. Our findings are helpful for understanding the mode hybridization in the silicon nanosphere placed on a metal substrate and illuminated by a focused azimuthally polarized beam and useful for designing photonic functional devices such as nanoscale sensors and color displayers.