New type high-index dielectric nanosensors based on the scattering intensity shift

Multi-band optical resonance of all-dielectric metasurfaces toward high-performance ultraviolet sensing

All-dielectric sensors featuring low-loss resonances have been proposed instead of plasmonic-based sensors. However, reported dielectric-based sensors generally work in the visible and near-infrared regions and detect the intensity variation of resonant modes because the electromagnetic energy is mainly confined inside dielectric nanoparticles. It is a challenge to adjust the hotspots from the inside to the surface of the all-dielectric metasurface. In this study, highly uniform Si₃N₄ all-dielectric metasurfaces have been successfully fabricated as sensing platforms by utilizing nanosphere self-assembly and plasma enhanced chemical vapor deposition techniques. Experimental and simulated results demonstrate that proposed Si₃N₄ all-dielectric metasurfaces exhibit multiple optical resonant modes in the ultraviolet and visible wavelength and present distinct field-confinement in the gaps of nanoparticles. The hotspots have been successfully adjusted to the surface of Si₃N₄ nanoparticles. Delightedly, Si₃N₄ all-dielectric metasurfaces show characteristic wavelength shifts with variation of the refractive index, and the sensitivity can reach 707 nm per RIU for trace detection as sensing substrates. Proposed Si₃N₄ all-dielectric metasurfaces are promising to act as high-sensitive sensing substrates in the ultraviolet and visible wavelength with the ease of high-throughput fabrication.

Fabrication and measurement of fiber optic localized surface plasmon resonance sensor based on gold nanoparticle dimer

Fiber optic localized surface plasmon resonance (FO LSPR) sensors capable of portable, real-time, and remote sensing are emerging with the progress of lab-on-fiber technology. However, the small area of the substrate by the optical fiber often restricts the sensitivity of the FO LSPR sensors. To improve the performance of the FO LSPR sensors, it is necessary to enhance the interactions between incident light and plasmonic nanostructures within a defined region. Dimer in which two nanoparticles are arranged with nanometer spacing can effectively increase the light-nanostructure interactions. It is well known that the nanogap made in the assembled nanoparticles significantly enhances the intensity of the electromagnetic field in the confined area by the hot spot effect. We fabricate the dimers of gold nanoparticles on the optical fiber with benzenethiol using a method that reduces the repulsive force between the nanoparticles. In the dimers, the strong plasmonic interaction between the two nanoparticles produces a longitudinal plasmon coupling band, which is compared to the transverse plasmon band by the monomer-based FO LSPR sensor with a similar density of gold nanoparticles. In the proposed sensor, the longitudinal band displays approximately 9.1 times improved sensitivity. When two types of sensors are applied to the biosensor application, the dimer-based FO LSPR sensor also proves an improved limit of detection of about 2.6 times. This method is expected to become a milestone in the field of measurement for small molecules and low concentration through the advancement of the yield and density of dimers.

Enhanced Refractive Index Sensitivity through Combining a Sol-Gel Adsorbate with a TiO2 Nanoimprinted Metasurface for Gas Sensing

We combine a gas-adsorbent microporous hybrid silica layer and a dense TiO2 Mie resonator array (metasurface), both obtained by sol-gel deposition and nanoimprint lithography, to form nanocomposite systems with high sensitivity for refractive index (RI) variations induced by gas adsorption. Using optical transduction based on direct specular reflection, we show spectral shifts of 4470 nm/RIU corresponding to 0.2 nm/ppm gas (air concentration) and reflection intensity changes of R* = 17 (R/RIU) and 0.55 × 10-3 R/ppm (air concentration). The metasurface is composed of hexagonally arranged TiO2 nanopillar arrays, whereas the surrounding sensitive material is a class II microporous hybrid silica, containing methyl and phenyl covalently bonded organic functions. This hybrid layer shows efficient adsorption capability of volatile organic molecules such as isopropanol, which is used to induce slight variations of RI around the TiO2 antennas. Specular reflectance variations at 45° incidence and refractive index measurements are performed using a spectroscopic ellipsometer. The presence of the titania metasurface enhances the signal by almost an order of magnitude with respect to the 2D counterpart (simulated as an effective medium approximation) and is attributed to the antenna effect, enhancing the interaction of the confined electromagnetic wave with the sensitive microporous medium. This sol-gel nanocomposite system presents many advantages such as high throughput and low-cost elaboration of elements and a high chemical, mechanical, and thermal resistance, ensuring high stability as a potential gas-sensitive nanocomposite layer for long periods. This work is a case study of improving the sensitivity of sol-gel gas-sensitive materials in optical transduction, which will be exploited in further works to develop artificial noses.

Rapid and sensitive detection of ochratoxin A using antibody-conjugated gold nanoparticles based on Localized Surface Plasmon Resonance

The regulation of tolerable levels of ochratoxin A in food for human and animal consumption has been defined in some countries. To meet these levels, simpler, more efficient, and faster analytical methods are being developed to facilitate the identification of this dangerous contaminant in food. Here, we combined gold nanoparticles (AuNPs) with anti-ochratoxin A (OTA) IgG to detect elementary levels of OTA based on Localized Surface Plasmon Resonance. AuNPs were prepared with trisodium citrate and characterized by UV-visible spectroscopy, X-ray, dynamic light scattering, and transmission electron microscopy. The conjugation of AuNPs to IgG anti-OTA was confirmed by bathochromic shift (UV-vis) and RAMAN spectroscopy. The sensitivity of the nanosensor was investigated by measuring LSPR band λ_max shifts. Our results suggest this assay is highly sensitive, with a lower detection limit of about 0.001 pg mL^-1. The LSPR nanosensor reduced detection limits by roughly 10 times compared to other methods. We demonstrated that the approach investigated here is a rapid and sensitive method for OTA detection.

Single Out-of-Resonance Dielectric Nanoparticles as Molecular Sensors

Light scattering from single nanoparticles and nanostructures is a commonly used readout method for nanosensors. Increasing the spectral sensitivity of resonant nanosensors to changes in their local surrounding has been the focus of many studies. Switching from spectral to intensity monitoring allows one to investigate nonresonant or out-of-resonance dielectric nanoparticles. Here, we systematically compared such dielectric silica nanoparticles with plasmonic gold nanorods by deriving analytical expressions and by performing experiments. The experiments show a similar sensitivity for the detection of an adsorbate layer for both particle types, which is in good agreement with theory. The flat spectral response of dielectric silica nanoparticles simplifies the choice of illumination wavelength. Furthermore, such dielectric nanoparticles can be made from many oxides, polymers, and even biological assemblies, broadening the choice of materials for the nanosensor.

Full-wave electromagnetic modes and hybridization in nanoparticle dimers

The plasmon hybridization theory is based on a quasi-electrostatic approximation of the Maxwell’s equations. It does not take into account magnetic interactions, retardation effects, and radiation losses. Magnetic interactions play a dominant role in the scattering from dielectric nanoparticles. The retardation effects play a fundamental role in the coupling of the modes with the incident radiation and in determining their radiative strength; their exclusion may lead to erroneous predictions of the excited modes and of the scattered power spectra. Radiation losses may lead to a significant broadening of the scattering resonances. We propose a hybridization theory for non-Hermitian composite systems based on the full-Maxwell equations that, overcoming all the limitations of the plasmon hybridization theory, unlocks the description of dielectric dimers. As an example, we decompose the scattered field from silicon and silver dimers, under different excitation conditions and gap-sizes, in terms of dimer modes, pinpointing the hybridizing isolated-sphere modes behind them.

Directional Fano Resonance in an Individual GaAs Nanospheroid

Fano resonance has been observed in a wide variety of nanophotonic structures such as photonic crystals, plasmonic structures, and metamaterials. It arises from the interference of discrete resonance states with broadband continuum states. As an emerging nanophotonic material, high-index all-dielectric nanomaterials provide a new platform to achieve Fano resonance by virtue of the simultaneous excited electric and magnetic resonances. However, to date, Fano resonance in the visible region has not been observed in individual high-index all-dielectric nanoparticles. Here, for the first time, the experimental observation of the directional Fano resonance is reported in an individual GaAs nanospheroid. The special geometry enables GaAs nanospheroids to generate spectrally overlapped electric and magnetic dipole resonances, which enhances their spectral coupling, giving rise to asymmetric-shaped backward scattering spectrum. This directional Fano resonance can be tuned by the aspect ratio of nanospheroids as well as excitation polarization. In addition, efficient directional light scattering is realized at the total scattering peak of the GaAs nanospheroid. The forward-to-backward scattering ratio can be largely enhanced due to Fano dip in the backward scattering spectrum. These findings suggest that high-index all-dielectric nanospheroid is a promising candidate for directional sources and optical switches.

Unravelling the Role of Electric and Magnetic Dipoles in Biosensing with Si Nanoresonators

High refractive index dielectric nanoresonators are attracting much attention due to their ability to control both electric and magnetic components of light. Due to the combination of confined modes with reduced absorption losses, they have recently been proposed as an alternative to nanoplasmonic biosensors. In this context, we study the use of semirandom silicon nanocylinder arrays, fabricated with simple and scalable colloidal lithography for the efficient and reliable detection of biomolecules in biological samples. Interestingly, electric and magnetic dipole resonances are associated with two different transduction mechanisms: extinction decrease and resonance red shift. By contrasting both observables, we identify clear advantages in tracking changes in the extinction magnitude. Our data demonstrate that, despite its simplicity, the proposed platform is able to detect prostate-specific antigen in human serum with limits of detection meeting clinical needs.

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.

The optical duality of tellurium nanoparticles for broadband solar energy harvesting and efficient photothermal conversion

Nanophotonic materials for solar energy harvesting and photothermal conversion are urgently needed to alleviate the global energy crisis. We demonstrate that a broadband absorber made of tellurium (Te) nanoparticles with a wide size distribution can absorb more than 85% solar radiation in the entire spectrum. Temperature of the absorber irradiated by sunlight can increase from 29° to 85°C within 100 s. By dispersing Te nanoparticles into water, the water evaporation rate is improved by three times under solar radiation of 78.9 mW/cm². This photothermal conversion surpasses that of plasmonic or all-dielectric nanoparticles reported before. We also establish that the unique permittivity of Te is responsible for the high performance. The real part of permittivity experiences a transition from negative to positive in the ultraviolet-visible-near-infrared region, which endows Te nanoparticles with the plasmonic-like and all-dielectric duality. The total absorption covers the entire spectrum of solar radiation due to the enhancement by both plasmonic-like and Mie-type resonances. It is the first reported material that simultaneously has plasmonic-like and all-dielectric properties in the solar radiation region. These findings suggest that the Te nanoparticle can be expected to be an advanced photothermal conversion material for solar-enabled water evaporation.

Resonant silicon nanoparticles with controllable crystalline states and nonlinear optical responses

High-throughput laser printing of resonant silicon nanoparticles has emerged as a novel tool for the fabrication of deeply subwavelength objects with various functionalities. The applications of resonant silicon nanoparticles crucially depend on their crystalline state. However, the ways to control the crystalline structure during laser printing remain unstudied. Here we demonstrate, both experimentally and theoretically, how the crystalline structure of silicon nanoparticles fabricated by a laser printing technique can be varied from almost amorphous to a polycrystalline state. In particular, we propose a method of crystalline structure control via changing the distance between the irradiated silicon film and the receiving substrate. This study allows the most optimal conditions for second harmonic generation to be revealed. We believe that the proposed method opens the door to fully controllable laser printing of functional nanoparticles and nanostructures.

Non-Plasmonic SERS with Silicon: Is It Really Safe? New Insights into the Optothermal Properties of Core/Shell Microbeads

Silicon is one of the most interesting candidates for plasmon-free surface-enhaced Raman scattering (SERS), because of its high-refractive index and thermal stability. However, here we demonstrate that the alleged thermal stability of silicon nanoshells irradiated by conventional Raman laser cannot be taken for granted. We investigated the opto-thermal behavior of SiO₂/Si core/shell microbeads (Si-rex) irradiated with three common Raman laser sources (λ = 532, 633, 785 nm) under real working conditions. We obtained an experimental proof of the critical role played by bead size and aggregation in heat and light management, demonstrating that, in the case of strong opto-thermal coupling, the temperature can exceed that of the melting points of both core and shell components. In addition, we also show that weakly coupled beads can be utilized as stable substrates for plasmon-free SERS experiments.

Electrically Controlled Scattering in a Hybrid Dielectric-Plasmonic Nanoantenna

Electrically tunable devices in nanophotonics offer an exciting opportunity to combine electrical and optical functions, opening up their applications in active photonic devices. Silicon as a kind of high refractive index dielectric material has shown comparable performances with plasmonic nanostructures in tailoring and modulating the electromagnetic waves. However, there are few studies on electrically tunable silicon nanoantennas. Here, for the first time we realize the spectral tailoring of an individual silicon nanoparticle in the visible range through changing the applied voltage. We observe that the plasmon-dielectric hybrid resonant peaks experience blue shift and obvious intensity attenuation with increasing the bias voltages from 0 to 1.5 V. A physical model has been established to explain how the applied voltage influences the carrier concentration and how carrier concentration modifies the permittivity of silicon and then the final scattering spectra. Our findings pave a new approach to build excellent tunable nanoantennas or other nanophotonics devices where the optical responses can be purposely controlled by electrical signals.

On-a-chip Biosensing Based on All-Dielectric Nanoresonators

Nanophotonics has become a key enabling technology in biomedicine with great promises in early diagnosis and less invasive therapies. In this context, the unique capability of plasmonic noble metal nanoparticles to concentrate light on the nanometer scale has widely contributed to biosensing and enhanced spectroscopy. Recently, high-refractive index dielectric nanostructures featuring low loss resonances have been proposed as a promising alternative to nanoplasmonics, potentially offering better sensing performances along with full compatibility with the microelectronics industry. In this letter we report the first demonstration of biosensing with silicon nanoresonators integrated in state-of-the-art microfluidics. Our lab-on-a-chip platform enables detecting Prostate Specific Antigen (PSA) cancer marker in human serum with a sensitivity that meets clinical needs. These performances are directly compared with its plasmonic counterpart based on gold nanorods. Our work opens new opportunities in the development of future point-of-care devices toward a more personalized healthcare.

Highly sensitive biosensors based on all-dielectric nanoresonators

Biosensing based on nanophotonic structures has shown a great potential for cost-efficient, high-speed and compact personal medical diagnostics. While plasmonic nanosensors offer high sensitivity, their intrinsically restricted resonance quality factors and strong heating due to metal absorption impose severe limitations on real life applications. Here, we demonstrate an all-dielectric sensing platform based on silicon nanodisks with strong optically-induced magnetic resonances, which are able to detect a concentration of streptavidin of as low as 10^-10 M (mol L^-1) or 5 ng mL^-1, thus pushing the current detection limit by at least two orders of magnitudes. Our study suggests a new direction in biosensing based on bio-compatible, non-toxic, robust and low-loss dielectric nanoresonators with potential applications in medicine, including disease diagnosis and drug detection.

Refractive Index Sensing Using Visible Electromagnetic Resonances of Supported Cu2O Particles

Plasmonic metal nanostructures, in colloidal or surface-supported forms, have been extensively studied in the context of metamaterials design and applications, in particular as refractometric sensing platforms. Recently, high refractive index (high-n) dielectric subwavelength structures have been experimentally shown to support strong Mie scattering resonances, predicted to exhibit analogous refractive index sensing capabilities. Here we present the first experimental demonstration of the use of supported high-n dielectric nano/microparticle ensembles as refractive index sensing platforms, using cuprous oxide as a model high-n material. Single-crystalline Cu₂O particles were deposited on transparent substrates using a chemical deposition scheme, showing well-defined electric and magnetic dipolar resonances (EDR and MDR, respectively) in the visible range, which change in intensity and wavelength upon changing the medium refractive index (n_m). The significant modulation of the MDR intensity when n_m is modified appears to be the most valuable empirical sensing parameter. The Mie scattering properties of Cu₂O particles, particularly the spectral dependence of the MDR on n_m, are theoretically modeled to support the experimental observations. MDR extinction changes (i.e., refractive index sensitivity) per particle are >100 times higher compared to localized surface plasmon resonance (LSPR) changes in supported Au nanoislands, encouraging the evaluation of Cu₂O and other high-n dielectric particles and sensing modes in order to improve the sensitivity in optical (bio)sensing applications.

Electromagnetic polarization-controlled perfect switching effect with high-refractive-index dimers and the beam-splitter configuration

Sub-wavelength particles made from high-index dielectrics, either individual or as ensembles, are ideal candidates for multifunctional elements in optical devices. Their directionality effects are traditionally analysed through forward and backward measurements, even if these directions are not convenient for in-plane scattering practical purposes. Here we present unambiguous experimental evidence in the microwave range that for a dimer of HRI spherical particles, a perfect switching effect is observed out of those directions as a consequence of the mutual particle electric/magnetic interaction. The binary state depends on the excitation polarization. Its analysis is performed through the linear polarization degree of scattered radiation at a detection direction perpendicular to the incident direction: the beam-splitter configuration. The scaling property of Maxwell's equations allows the generalization of our results to other frequency ranges and dimension scales, for instance, the visible and the nanometric scale.

Traditional metallic communication elements suffer from substantial losses in the visible and near-infrared. Here, Barreda et al. show in a proof of principle in the microwave regime that a pair of high-index dielectric spheres can operate as a perfect switch in a beam-splitter configuration.

Generating scattering dark states through the Fano interference between excitons and an individual silicon nanogroove

Effective interactions between excitons and resonating nanocavities are important for many emerging applications in nanophotonics. Although plasmonic nanocavities are considered promising substitutes for diffraction-limited dielectric microcavities, their practical applications are hindered by large ohmic loss and Joule heating. Other than plasmonic materials, high-refractive-index dielectric nanocavities is a new way to trap light in subwavelength scales. However, studies on the interaction between dielectric nanocavities and excitons are still scarce. Here, for the first time, we demonstrate that the Fano interference between molecular excitons and an individual silicon nanogroove can generate scattering dark modes. By placing J-aggregate excitons into a silicon nanogroove, the leaky magnetic resonant modes filling in the groove can tailor their scattering directivity and reduce the uncoupled radiation decay in a specific direction. This unidirectional 'dark state' brings a new approach to tailor the interaction between excitons and nanocavities without large near-field enhancement. By adjusting the resonant modes, the scattering spectra can change from a Fano asymmetric line shape to a significantly suppressed scattering dip. These findings indicate that silicon nanogrooves can provide a platform for integrated on-chip silicon-exciton hybrid optical systems in the future.

Exploiting the interaction between a semiconductor nanosphere and a thin metal film for nanoscale plasmonic devices

The interaction of silicon (Si) nanospheres (NSs) with a thin metal film is investigated numerically and experimentally by characterizing their forward scattering properties. A sharp resonant mode and a zero-scattering dip are found to be introduced in the forward scattering spectrum of a Si NS by putting it on a 50-nm-thick gold film. It is revealed that the sharp resonant mode arises from a new magnetic dipole induced by the electric dipole and its mirror image while the zero-scattering dip originates from the destructive interference between the new magnetic dipole and the original one together with its mirror image. A significant enhancement in both electric and magnetic fields is achieved at the contact point between the Si NS and the metal film. More interestingly, the use of a thin silver film can lead to vivid scattering light with different color indices. It is demonstrated that a small change in the surrounding environment of Si NSs results in the broadening of the resonant mode and the disappearance of the zero-scattering dip. Our findings indicate that dielectric-metal hybrid systems composed of semiconductor NSs and thin metal films act as attractive platforms on which novel nanoscale plasmonic devices can be realized.

Midrefractive Dielectric Modulator for Broadband Unidirectional Scattering and Effective Radiative Tailoring in the Visible Region

Nanoantennas have found many applications in ultrasmall sensors, single-molecule detection, and all-optical communication. Conventional nanoantennas are based on noble-metal plasmonic structures, but suffer from large ohmic loss and only possess dipolar plasmon modes. This has driven an intense search for all-dielectric materials beyond noble metals. Here, we propose midrefractive nanospheres as a novel all-dielectric material to realize broadband unidirectional radiation and effective radiative tailoring in the visible region. Midrefractive all-dielectric materials such as boron nanospheres possess broad and overlapping electric and magnetic dipole modes. The internal interaction between these two modes can route the radiation almost on the one side covering the whole visible range. Unlike the elaborate design in plasmonic nanostructures to obtain strong coupled broad and narrow modes, the bright mode in boron nanospheres is intrinsic, independent, and easily coupled with adjacent narrow modes. So the strong interaction in boron-based heterodimer is able to realize an independent and precise tailoring of the radiant and subradiant states by simply changing the particle sizes, respectively. Our findings imply midrefractivity materials like boron are excellent building blocks to support electromagnetic coupling operation in nanoscale devices, which will lead to a variety of emerging applications such as nanoantennas with directing exciton emission, ultrasensitive nanosensors, or even potential new construction of photonic metamaterials.

Fano resonant all-dielectric core/shell nanoparticles with ultrahigh scattering directionality in the visible region

We demonstrate Si-based single core/shell (Si/SiO₂) nanoparticles which exhibit the Fano resonance associated with ultrahigh scattering directionality. The SiO₂ shell plays a crucial role in achieving zero backscattering at the Fano resonance wavelength along with strongly-enhanced forward scattering. As a result, the front-to-back scattering-intensity ratio is five orders of magnitude greater than that of a Si nanoparticle. Furthermore, the Fano resonance wavelength is controlled over the entire visible region by changing the core diameter. The Fano spectra also show distinctive intensity modulations depending on the index of refraction of the surrounding medium. These unique features make Si/SiO₂ nanoparticles promising for the design of low-loss nano-antennas, metamaterials, and other nanophotonic devices.