L Maillé

S Le Ber

M A Dourges

R Pailler

A Guette

J Roger

Manufacturing of ceramic matrix composite using a hybrid process combining TiSi 2 active filler infiltration and preceramic impregnation and pyrolysis

Keywords: CMC, active filler, slurry impregnation, flexural strength, nitridation

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Manufacturing of ceramic matrix composite using a hybrid process combining TiSi2 active filler infiltration and preceramic impregnation and pyrolysis

Infiltration and Hot Processing (SI-HP) or using several other techniques [START_REF] Naslain | Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview[END_REF][START_REF] Naslain | SiC-matrix composites: Nonbrittle ceramics for thermo-structural application[END_REF][START_REF] Rosso | Ceramic and metal matrix composites: Routes and properties[END_REF][START_REF] Magnant | Carbon fiber/reaction-bonded carbide matrix for composite materials -Manufacture and characterization[END_REF][START_REF] Magnant | Fiber-reinforced ceramic matrix composites processed by a hybrid technique based on chemical vapor infiltration, slurry impregnation and spark plasma sintering[END_REF][START_REF] Gonon | Comparison of two processes for manufacturing ceramic matrix composites from organometallic precursors[END_REF] . In order to be competitive on the civil aeronautics market, low cost CMC processing such as liquid phase routes including polymer impregnation/pyrolysis are particularly developed. Using complementary methods of densification such as slurry impregnation with filler powder and liquid polymer impregnation enables to obtain an effective process with a low price/performance ratio. Adding fillers to the polymer allows modulating certain properties of the final ceramic, such as mechanical behaviour, electrical or thermal properties. However, an inherent shrinkage is observed after pyrolysis of the polymer, even when inert powders are inserted in the matrix. The repetition of numerous impregnation and pyrolysis cycles is then necessary to obtain a dense material [START_REF] Jones | Polymer derived ceramic matrix composites[END_REF] . P. Greil suggested overcoming this problem with the addition of active fillers, which react during pyrolysis under reactive atmosphere to form oxides, carbides or nitrides leading to significant volume expansions [START_REF] Greil | Active-filler-controlled pyrolysis of preceramic polymers[END_REF][START_REF] Greil | Near net shape manufacturing of polymer derived ceramics[END_REF][START_REF] Suttor | Fiber-reinforced ceramic-matrix composites with a polysiloxane/boron-derived matrix[END_REF][START_REF] Greil | Near net shape manufacturing of ceramics[END_REF][START_REF] Greil | Pyrolysis of Active and Passive Filler-loaded Preceramic Polymers, Handbook of Advanced Ceramics[END_REF][START_REF] Erny | Microstructure development of oxycarbide composites during active-filler-controlled polymer[END_REF][START_REF] Ma | Processing and characterization of particles reinforced Si-O-C composites via pyrolysis of polysiloxane with SiC or/and Al fillers[END_REF][START_REF] Akkas | Silicon oxycarbide-based composites produced from pyrolysis of polysiloxanes with active Ti filler[END_REF][START_REF] Xie | Active filler (aluminum-aluminum nitride) controlled polycarbosilane pyrolysis[END_REF][START_REF] Ade | Electroconductive ceramic composites with low-to-zero shrinkage during sintering[END_REF][START_REF] Torrey | Composite polymer derived ceramic system for oxidizing environments[END_REF] . These reactions occur with a volume expansion that can compensate for the polymer shrinkage. However, most active fillers react only at high temperatures (T > 1400°C). This can be a major drawback if the fibres are damaged during the heating at high temperatures.

Titanium disilicide powder (TiSi 2 , density = 4.01 g/cm [START_REF] Rosso | Ceramic and metal matrix composites: Routes and properties[END_REF] ) is identified as an interesting active filler [START_REF] Vojtech | High temperature oxidation of titanium-silicon alloys[END_REF] . Under nitrogen atmosphere, the nitridation of TiSi 2 starts around 1000°C, and leads to the formation of TiN (d = 5.43 g/cm 3 ) and Si 3 N 4 (d = 3.19 g/cm [START_REF] Rosso | Ceramic and metal matrix composites: Routes and properties[END_REF] ) with a 57 volume percent increase when the reaction is complete. It is well-known that the powder size can influence the reaction rate; therefore several studies were performed to prepare ceramic composites with a small size powder obtained by ball-milling [START_REF] Ber | An accurate approach of the nitridation process of TiSi 2 Powder[END_REF][START_REF] Ber | Ceramic matrix composites densification by active filler impregnation followed by a P.I.P. process[END_REF][START_REF] Maillé | Study of the nitridation process of TiSi 2 powder[END_REF] . To control the process, the nitridation of TiSi 2 must be well understood. In the first part of this paper, we explore the influence of temperature and time on the nitridation rate of micron and submicron TiSi 2 powders. The preparation of the composites and the mechanical behaviour (3-point bending tests) of CMC containing powders of TiSi 2 or TiSi 2 nitrided within their matrices are then presented.

Materials and experimental procedure

A high purity micrometer-sized TiSi 2 powder (C-54 stable phase, 99.95% in purity, ~ 45 µm, Neyco) is used during this work. A study of XRD patterns by Rietveld method (fullprof 2K [START_REF] Rodriguez-Carvajal | Recent advances in magnetic structure determination by neutron powder diffraction[END_REF] ) shows the presence of 8.6%wt of free silicon and of 91.4%wt of TiSi 2 phase. This raw powder is milled with a planetary ball mill (Retsch PM200 Composites are fabricated from 2D fibre preforms (~ 2 mm thickness) made of woven Nicalon fibres (Nippon Carbon Co.) and they are covered by PyC interphase. These SiCbased fibres are unstable at high temperatures because of the silicon oxycarbide phase they contain, which decomposes beyond 1150 °C; the manufacturing of CMC is therefore limited to this maximum temperature when using these fibres.

To prepare the CMC, the fibre preforms are first consolidated by one PIP cycle with a phenylcontaining polysiloxane (resin 1 -Table 1), then impregnated with a slurry containing the TiSi 2 powder.

The active filler powders are mechanically mixed with ethanol in order to obtain slurry. The is calculated according to the equation ( 1):

σ R = 3 F L / 2 w t² ( 1
)
where F is the maximal applied force, L the support span (50 mm), w the width (~ 10 mm) and t the thickness (~ 2 mm) of the specimen. We used a universal testing machine (Instron 5860) at a cross-head speed of 0.5 mm/min at room temperature.

Results and discussion

Nitridation process

3.1.a. Commercial TiSi 2 powder

According to the Ti-Si-N phase diagram [START_REF] Sambasivan | Phase chemistry in the Ti-Si-N system -Thermochemical stability diagrams[END_REF][START_REF] Paulasto | Interfacial reactions in Ti/Si 3 N 4 and TiN/Si diffusion couples[END_REF][START_REF] Ma | The thermodynamic assessment of the Ti-Si-N system and the interfacial reaction analysis[END_REF][START_REF] Beyers | Phase-equilibria in thin-film metallizations[END_REF] , the nitridation of TiSi 2 is described by equations 1 and 2 depending whether it is partial or complete.

2 TiSi 2 (s) + N 2 (g) = 2 TiN (s) + 4 Si (s) W/Wo = 13.5% (Equation 1)

6 TiSi 2 (s) + 11 N 2 (g) = 6 TiN (s) + 4 Si 3 N 4 (s) W/Wo = 49.4% (Equation 2)

In order to study the phenomenon of nitridation as a function of temperature, a non-isothermal nitridation of as received TiSi 2 powder (d 50 ~ 10 µm -Table 2) is first performed. The sample is heated from 20 to 1300°C at a low rate of 1°C/min in pure flowing nitrogen gas. The weight gain and its derivative are plotted as a function of the temperature in Figure 1.

From those measurements, it appears that no significant weight gain is obtained until the temperature reaches 900°C. In Figure 1, it is possible to observe two phenomena on the curve of thermogravimetric analysis. The first began at 1130°C until 1250°C. It is attributed to the transformation of TiSi 2 into free silicon and TiN according the Equation 1 (partial nitridation), into the surface and inside the grains. The mass gain corresponds to around 14 % that is expected [START_REF] Maillé | Study of the nitridation process of TiSi 2 powder[END_REF] .

The second phenomenon corresponds to the transformation of free silicon on Si 3 N 4 according the Equation 2 (complete nitridation) as it is described by Cordoba et al. [START_REF] Cordoba | Nitriding study of titanium silicide intermetallics obtained by mechanical alloying[END_REF] .

Our studies are oriented on CMC prepared with Nicalon fibre for aeronautic applications.

These SiC based fibres are unstable at high temperature (>1150°C). The manufacturing of CMC is therefore limited to temperature of 1100°C.

Sections of nitrided TiSi 2 grains are observed by SEM (Figure 2). EDX of TiSi 2 nitrided during 5 hours at 1100°C reveals the formation of TiN and Si when TiSi 2 reacts with nitrogen (Figure 2a). XRD patterns confirm the presence of TiN (JCPDS 38-1420) and Si (JCPDS 27-1402) phases, and the absence of Si 3 N 4 . Only the 3 µm in diameter grains would completely react with nitrogen according to the equation 1. In the middle of the grain, the TiSi 2 phase is still present, with the expected stoichiometry. The process of nitridation explored at 1100°C during 5 hours is in good agreement both with the equation 1, and the data published previously [START_REF] Maillé | Study of the nitridation process of TiSi 2 powder[END_REF] , inducing a maximum weight gain of 13.5% (ΔW/W 0 ) and a maximum volume gain of 36.9% (ΔV/V 0 ). Some thermodynamic studies in the Ti-Si-N ternary system indicate that TiSi 2 can be nitrided into TiN and Si 3 N 4 at 1100°C. [START_REF] Paulasto | Interfacial reactions in Ti/Si 3 N 4 and TiN/Si diffusion couples[END_REF][START_REF] Ma | The thermodynamic assessment of the Ti-Si-N system and the interfacial reaction analysis[END_REF][START_REF] Beyers | Phase-equilibria in thin-film metallizations[END_REF] This assertion is not confirmed by our first experiments, even after 8 hours of nitridation at 1100°C. In order to verify the existence of the Si 3 N 4 phase, a TiSi 2 powder was nitrided at 1100°C during 50 h. The Figure 2b presents transverse sections of a TiSi 2 grain observed by SEM after such a treatment. EDX confirms the presence of the TiN phase and demonstrates the formation of a slight quantity of the Si 3 N 4 phase. XRD pattern confirms this assertion. The reaction kinetics of free silicon with nitrogen to form Si 3 N 4 is very low.

It was decided to mill the starting powder with the intention to increase the specific area and to promote the reaction mechanism corresponding to the equation 2.

3.1.b. Influence of milling conditions on nitridation

A high-energy planetary milling with ethanol as solvent is performed in order to increase the efficiency of the nitridation reaction [START_REF] Suryanarayana | Mechanical alloying and milling[END_REF] . Milling conditions and characteristics of sub-micron powders obtained are reported in Table 2.

Thermogravimetric analyses are performed on milled powders at 1100°C (Figure 3).

As expected, the nitridation of planetary milled powders begins clearly at a lower temperature of 650°C, with much steeper slopes during the heating stage. Furthermore, the weight gain exceeds 29.5 weight percent after 5 h at 1100°C. Such a high value implies the formation of Si 3 N 4 .

After 5 h at 1100°C in N 2 , XRD patterns of all nitrided powders indicated the presence of TiN and Si (Figure 4). Only raw nitrided powders still exhibits peaks of TiSi 2 . The XRD pattern of the TiSi 2 powder nitrided after planetary milling included also small peaks corresponding to the β-Si 3 N 4 phase.

Flexural strength of composites

Using infiltration of the milled TiSi 2 powder (25 ± 4 weight %), nitridation and PIP with the resin 2 (27 ± 5 weight % for the first impregnation), and different conditions of thermal treatments, various composites are produced and their mechanical properties are determined (Table 3). More particularly, the influence of (i) the presence of TiSi 2 powder, (ii) the nitridation conditions and (iii) the repetition of the impregnation and pyrolysis cycles are studied. The flexural properties are obtained using 3-point bending tests. The strengths are distributed between 35 and 160 MPa with only one PIP cycle and reach 215 MPa with three PIP cycles. The same range of the higher flexural strength value is reported in literature for CMCs processed by liquid routes. Authors often choose to multiply PIP cycles [START_REF] Ziegler | Fiber-reinforced composites with polymer-derived matrix: processing, matrix formation and properties[END_REF][START_REF] Dong | Microstructural evolution and mechanical performances of SiC/SiC composites by polymer impregnation/microwave pyrolysis (PIMP) process[END_REF][START_REF] Lu | Fabrication of a Nicalon (TM) fiber/Si 3 N 4 -based ceramic-matrix composite by the polymer pyrolysis method[END_REF][START_REF] Shin | Low-temperature processing of ceramic woven fabric ceramicmatrix composites[END_REF] , and sometimes introduce passive or active fillers [START_REF] Nakano | Fabrication and characterization of 3-dimentional carbon-fiber-reinforced silicon-carbide and siliconnitride composites[END_REF][START_REF] Stantschev | Long fibre reinforced ceramics with active fillers and a modified intra-matrix bond based on the LPI process[END_REF] . As it was expected, the values of strengths depend particularly on the nature of the initial powder, the degree of advancement (progression) of in-situ reaction of nitridation and the number of PIP cycles. The last point can be explained regarding the final porosity of CMC measured as 15.5% for CMC2 and 7.3% for CMC5. Results are presented in more detail in the following paragraph. The ultimate flexural strength of CMC5 corresponds to the higher value, and it is obtained when 3 PIP cycles are performed.

3.2.b. Impact of the completion of the nitridation reaction

In order to show the influence of the total nitridation of TiSi 2 powder on the mechanical properties of CMC, carbon fibres guipex preform (ex-PAN) were used instead of Nicalon ones because carbon fibres are stable under heat treatments; filled preforms can be treated up to 1300°C without degradation of carbon fibres. The same dimensions of samples are used to study the mechanical properties. CMC6 and CMC7 were fabricated in the same way than the others composites with only one PIP cycle and thermal treatment up to 1100°C and 1300°C respectively. Figure 7 shows the importance of completing the nitridation of the TiSi 2 according to mechanism corresponding to the Equation 2, leading to the presence of TiN and Si 3 N 4 . The completion of the nitridation reaction obtained after treatment up to 1300°C improves both the stiffness and the ultimate flexural strength of the composite CMC7 compared to CMC6.

Figure 8 shows the overall microstructure of a CMC with totally nitride TiSi 2 (CMC7). It can be observed that with nitridation at 1300°C, the grains fill the inter-bundle porosity efficiently.

These results show the influence of complete nitridation of TiSi 2 powder filled in a preform. It can be proposed that this step in the preparation of composites by a hybrid process is of major importance since that can improve the mechanical behavior of CMC.

Conclusion

A new hybrid process of CMC manufacturing involving a first step of infiltration and nitridation of TiSi 2 powders under nitrogen atmosphere was performed. First the influence of sized of active filler has been studied. The increase of the specific surface area of the TiSi 2 powder improves the nitridation kinetics and leads to facilitate the formation of Si 3 N 4 at low temperature (1100°C). Secondly, relatively dense composites are produced by a process combining (i) the active filler impregnation, (ii) the in-situ nitridation process of this active filler, (iii) the impregnation and pyrolysis of a preceramic polymer. Choosing this liquid route and SiC Nicalon fibre preforms have enabled to develop a low-cost process performed at temperatures below 1100°C. The flexural strength of prepared composites depends on the composition of fillers. The better mechanical properties are obtained when the filler is all active. The presence of a larger proportion of Si 3 N 4 in the matrix is associated with an increase of the ultimate flexural strength and of the Young's modulus as it was proved using C fibres reinforcement and at 1300°C as treatment temperature.

  Maillé, Simon Le Ber, Marie-Anne A Dourges, René Pailler, Alain Guette, Jérôme Roger To cite this version: Laurence Maillé, Simon Le Ber, Marie-Anne A Dourges, René Pailler, Alain Guette, et al.. Manufacturing of ceramic matrix composite using a hybrid process combining TiSi2 active filler infiltration and preceramic impregnation and pyrolysis. Journal of the European Ceramic Society, 2014, 34 (2), pp.189-195. 10.1016/j.jeurceramsoc.2013.08.031. hal-01844660 1. Introduction In the preparation of Ceramic Matrix Composites (CMC), densification of fibre preforms can be performed via different routes, such as Chemical Vapour Infiltration (CVI), Polymer Impregnation and Pyrolysis (PIP), sol-gel route, Reactive Melt Infiltration (RMI) or Slurry

Figures 5 Figures 6

 56 Figures 5 show the distribution of different constituents inside the composite CMC5. The

3. 2 .Figure 6

 26 Figure 6 shows stress-displacement curves of the bending tests for composites obtained with

  of active filler powder could be obtained with an optimum concentration of 2.5 mg of PEI per square meter of powder. The fibre preforms are immersed in a beaker of suspension and the impregnation is performed under vacuum for one hour. The samples are then removed from the suspension, and the solvent is evaporated by heating at 100°C under vacuum for one hour. The composites impregnated with the active filler are then nitrided under a flow of nitrogen gas at a ramping rate of 10°C/min up to 1100°C, and maintained at this temperature for 5 h. It is important to notice that the nitridation is performed before impregnation of polymer. To suppress the contact with oxygen during pyrolysis an oxygen scavenger is used. Treatments are carried out in a furnace using alumina crucibles. A methyl-polysiloxane (resin 2 -Table1) requiring no solvent is chosen as preceramic polymer to perform the final PIP process. Impregnation is carried out in a beaker

addition of poly-ethylene imine (PEI) enabled to stabilize the colloidal suspension. A concentration of 15%V under vacuum during one hour. The samples are cured by a thermal treatment of 1 h at 60°C under vacuum and pyrolysis is achieved by heat treatment up to 1000°C. The mechanical behaviour of CMC samples is explored using bending test. The ultimate flexural strength (σ R )

Acknowledgements

This work was supported by Herakles (Safran Group) and by the French national project NaCoMat.


Alex Krasnok
email: akrasnok@utexas.edu

Martín Caldarola

Dr Nicolas Bonod

Andrea Alú

Dr A Krasnok

Dr A Alù

Spectroscopy and Biosensing with Optically Resonant Dielectric Nanostructures

Keywords: Resonant dielectric nanoparticles, nanophotonics, fluorescence spectroscopy, surfaceenhanced Raman scattering, biosensing, lab-on-a-chip technology, Purcell effect, strong coupling, hybrid exciton-polariton systems

Resonant dielectric nanoparticles (RDNs) made of materials with large positive dielectric permittivity, such as Si, GaP, GaAs, have become a powerful platform for modern light science, enabling various fascinating applications in nanophotonics and quantum optics. In addition to light localization at the nanoscale, dielectric nanostructures provide electric and magnetic resonant responses throughout the visible and infrared spectrum, low dissipative losses and optical heating, low doping effect and absence of quenching, which are interesting 2 for spectroscopy and biosensing applications. In this review, we present state-of-the-art applications of optically resonant high-index dielectric nanostructures as a multifunctional platform for light-matter interactions. Nanoscale control of quantum emitters and applications for enhanced spectroscopy including fluorescence spectroscopy, surface-enhanced Raman scattering (SERS), biosensing, and lab-on-a-chip technology are surveyed. We describe the theoretical background underlying these effects, overview realizations of specific resonant dielectric nanostructures and hybrid excitonic systems, and outlook the challenges in this field, which remain open to future research.

Introduction

The operation principles of plasmonic nanoantennas and nanostructures are based on the optical properties of metals (e.g., Au and Ag). They have a profound impact on nanophotonics, as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale [START_REF] Bharadwaj | Optical Antennas[END_REF][START_REF] Novotny | Antennas for Light[END_REF][3][START_REF] Liu | Individual Nanoantennas Loaded with Three-Dimensional Optical Nanocircuits[END_REF][START_REF] Alu | Modeling and Features of Optical Nanoantennas[END_REF][START_REF] Monticone | Metamaterials and Plasmonics: From Nanoparticles to Nanoantenna Arrays, Metasurfaces, and Metamaterials[END_REF][START_REF] Koenderink | Nanophotonics: Shrinking Light-Based Technology[END_REF][START_REF] Monticone | Plasmonic and Nanophotonic Devices[END_REF][START_REF] Koenderink | Single-Photon Nanoantennas[END_REF][START_REF] Dionne | Localized Fields, Global Impact: Industrial Applications of Resonant Plasmonic Materials[END_REF] . These nanostructures can significantly enhance the interactions between quantum emitters (e.g., quantum dots, defect centers in solid, molecules) and their surrounding photonic environment [START_REF] Anger | Enhancement and Quenching of Single-Molecule Fluorescence[END_REF][START_REF] Kühn | Enhancement of Single-Molecule Fluorescence Using a Gold Nanoparticle as an Optical Nanoantenna[END_REF][START_REF] Biagioni | Nanoantennas for Visible and Infrared Radiation[END_REF][START_REF] Marquier | Revisiting Quantum Optics with Surface Plasmons and Plasmonic Resonators[END_REF] , leading to giant luminescence enhancement 9,15-24 , ultrafast emission in the picosecond range [START_REF] Akselrod | Probing the Mechanisms of Large Purcell Enhancement in Plasmonic Nanoantennas[END_REF][START_REF] Straubel | Plasmonic Nanoantenna Based Triggered Single-Photon Source[END_REF][START_REF] Hoang | Ultrafast Room-Temperature Single Photon Emission from Quantum Dots Coupled to Plasmonic Nanocavities[END_REF] , strong coupling [START_REF] Chikkaraddy | Single-Molecule Strong Coupling at Room Temperature in Plasmonic Nanocavities[END_REF][START_REF] Baranov | Novel Nanostructures and Materials for Strong Light-Matter Interactions[END_REF] , surface enhanced Raman scattering (SERS) [START_REF] Dykman | Gold Nanoparticles in Biomedical Applications: Recent Advances and Perspectives[END_REF][START_REF] Huang | SERS-Enabled Lab-on-a-Chip Systems[END_REF][START_REF] Gwo | Plasmonic Metasurfaces for Nonlinear Optics and Quantitative SERS[END_REF] , optical interconnections [START_REF] Alù | Wireless at the Nanoscale: Optical Interconnects Using Matched Nanoantennas[END_REF][START_REF] Alù | Input Impedance, Nanocircuit Loading, and Radiation Tuning of Optical Nanoantennas[END_REF] , and control of emission patterns [START_REF] Curto | Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna[END_REF][START_REF] Aouani | Bright Unidirectional Fluorescence Emission of Molecules in a Nanoaperture with Plasmonic Corrugations[END_REF][START_REF] Moiseev | Light Outcoupling from Quantum Dot-Based Microdisk Laser via Plasmonic Nanoantenna[END_REF][START_REF] Alù | Tuning the Scattering Response of Optical Nanoantennas with Nanocircuit Loads[END_REF] .

Because of these advantages, plasmonic nanostructures are broadly used in many applications, including near-field microscopy [START_REF] Eghlidi | Resolution and Enhancement in Nanoantenna-Based Fluorescence Microscopy[END_REF][START_REF] Van Zanten | Imaging Individual Proteins and Nanodomains on Intact Cell Membranes with a Probe-Based Optical Antenna[END_REF][START_REF] Mivelle | Hybrid Photonic Antennas for Subnanometer Multicolor Localization and Nanoimaging of Single Molecules[END_REF] , biosensors [START_REF] Zhang | Substrate-Induced Fano Resonances of a Plasmonic Nanocube: A Route to Increased-Sensitivity Localized Surface Plasmon Resonance Sensors Revealed[END_REF][START_REF] Pellegrotti | Plasmonic Photothermal Fluorescence Modulation for Homogeneous Biosensing[END_REF] , photovoltaics [START_REF] Atwater | Plasmonics for Improved Photovoltaic Devices[END_REF][START_REF] Spinelli | Plasmonic Light Trapping in Thin-Film Si Solar Cells[END_REF] , photodetection [START_REF] Knight | Photodetection with Active Optical Antennas[END_REF] , and medicine [START_REF] Hirsch | Nanoshell-Mediated near-Infrared Thermal Therapy of Tumors under Magnetic Resonance Guidance[END_REF][START_REF] Kim | Tumor-Homing Multifunctional Nanoparticles for Cancer Theragnosis: Simultaneous Diagnosis, Drug Delivery, and Therapeutic Monitoring[END_REF] . However, the typical plasmonic materials, gold and silver, have finite conductivities at optical frequencies, leading to inherent dissipation of the electromagnetic energy. This dissipation is caused by interband transitions of valence electrons to the Fermi surface or the electrons near the Fermi surface to the next unoccupied states in the conduction band [START_REF] Naik | Alternative Plasmonic Materials: Beyond Gold and Silver[END_REF] , which leads to Joule heating of the structure and its local environment. For many applications, heat generation in the nanostructure and its surroundings are detrimental, since the behavior of the molecules under study can be modified dramatically with temperature.

Moreover, elevated temperatures may lead to reshaping and even complete destruction of the nanostructures. In addition, a quantum emitter placed in the nanometer proximity of a metallic nanostructure will be quenched due to the dominant non-radiative decay channels, leading to the necessity of various dielectric spacers that reduce the overall enhancement effects.

The issue of optical losses in plasmonic nanostructures has been addressed in many recent studies [START_REF] Naik | Alternative Plasmonic Materials: Beyond Gold and Silver[END_REF][START_REF] West | Searching for Better Plasmonic Materials[END_REF][START_REF] Boltasseva | Low-Loss Plasmonic Metamaterials[END_REF][START_REF] Tassin | A Comparison of Graphene, Superconductors and Metals as Conductors for Metamaterials and Plasmonics[END_REF][START_REF] Khurgin | How to Deal with the Loss in Plasmonics and Metamaterials[END_REF][START_REF] Khurgin | Ultimate Limit of Field Confinement by Surface Plasmon Polaritons[END_REF][START_REF] Khurgin | Replacing Noble Metals with Alternative Materials in Plasmonics and Metamaterials: How Good an Idea?[END_REF] and it is still an object of discussion. It has been shown that plasmonic resonators are always accompanied by losses because of their nature: in order to achieve the resonant behavior for subwavelength plasmonic resonators, part of the optical energy is stored in the kinetic energy of electrons [START_REF] Khurgin | How to Deal with the Loss in Plasmonics and Metamaterials[END_REF] . Studies show that even high-Tc superconductors cannot be a good alternative to noble metals because they would have to operate at energies less than the superconductive gap, that is, in the THz or far-IR [START_REF] Tassin | A Comparison of Graphene, Superconductors and Metals as Conductors for Metamaterials and Plasmonics[END_REF] . Using highly doped semiconductors in place of metals is also not a panacea, because the Fermi level decline necessarily means a decrease of the plasma frequency, thus impacting surface and localized plasmonic resonances [START_REF] Khurgin | How to Deal with the Loss in Plasmonics and Metamaterials[END_REF] , at which noble metals demonstrate superior properties. We also note that there is a limit to the overall electric field enhancement in metal nanostructures because of Landau damping [START_REF] Khurgin | How to Deal with the Loss in Plasmonics and Metamaterials[END_REF][START_REF] Khurgin | Ultimate Limit of Field Confinement by Surface Plasmon Polaritons[END_REF] , which is also caused by high densities of free electrons.

To circumvent the issues of loss and heating, the use of high refractive index dielectrics such as Si, Ge or GaP has been proposed [START_REF] Zhao | Mie Resonance-Based Dielectric Metamaterials[END_REF][START_REF] Evlyukhin | Optical Response Features of Si-Nanoparticle Arrays[END_REF][START_REF] Evlyukhin | Demonstration of Magnetic Dipole Resonances of Dielectric Nanospheres in the Visible Region[END_REF][59][START_REF] Zywietz | Laser Printing of Silicon Nanoparticles with Resonant Optical Electric and Magnetic Responses[END_REF][START_REF] Evlyukhin | Optical Spectroscopy of Single Si Nanocylinders with Magnetic and Electric Resonances[END_REF][START_REF] Krasnok | Towards All-Dielectric Metamaterials and Nanophotonics[END_REF][START_REF] Savelev | From Optical Magnetic Resonance to Dielectric Nanophotonics (A Review)[END_REF][START_REF] Kuznetsov | Optically Resonant Dielectric Nanostructures[END_REF][START_REF] Decker | Resonant Dielectric Nanostructures: A Low-Loss Platform for Functional Nanophotonics[END_REF][START_REF] Staude | Metamaterial-Inspired Silicon Nanophotonics[END_REF][START_REF] Krasnok | Nonlinear Metasurfaces: A Paradigm Shift in Nonlinear Optics[END_REF][START_REF] Baranov | All-Dielectric Nanophotonics: The Quest for Better Materials and Fabrication Techniques[END_REF] . When compared to metallic nanostructures, the main difference resides in the significantly reduced absorption in these materials: while optical resonances of metallic nanoparticles (NPs) are based on conduction electron oscillations (plasmons), the optical properties of RDNs are based on polarization charges and thus present inherently lower Ohmic losses. Note that an equivalence between the electromagnetic fields yielded by plasmonic and Mie resonators was recently established theoretically in Ref. [START_REF] Devilez | Mimicking Localized Surface Plasmons with Dielectric Particles[END_REF] . It has been demonstrated that NPs and nanostructures made of such dielectric materials provide low dissipative losses, resonant electric and magnetic optical response, moderate light localization at the nanoscale, and strong inherent Raman scattering, which are not typical for plasmonic counterparts [START_REF] Kuznetsov | Optically Resonant Dielectric Nanostructures[END_REF][START_REF] Staude | Metamaterial-Inspired Silicon Nanophotonics[END_REF][START_REF] Baranov | All-Dielectric Nanophotonics: The Quest for Better Materials and Fabrication Techniques[END_REF][START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF] . Because of their distinct properties, RDNs have been proposed for high-harmonic generation [START_REF] Krasnok | Nonlinear Metasurfaces: A Paradigm Shift in Nonlinear Optics[END_REF][START_REF] Shcherbakov | Nonlinear Interference and Tailorable Third-Harmonic Generation from Dielectric Oligomers[END_REF][START_REF] Makarov | Self-Adjusted All-Dielectric Metasurfaces for Deep Ultraviolet Femtosecond Pulse Generation[END_REF][START_REF] Smirnova | Multipolar Nonlinear Nanophotonics[END_REF][START_REF] Camacho-Morales | Nonlinear Generation of Vector Beams from AlGaAs Nanoantennas[END_REF][START_REF] Carletti | Shaping the Radiation Pattern of Second-Harmonic Generation from AlGaAs Dielectric Nanoantennas[END_REF][START_REF] Shibanuma | Efficient Third Harmonic Generation from Metal-Dielectric Hybrid Nanoantennas[END_REF][START_REF] Grinblat | Enhanced Third Harmonic Generation in Single Germanium Nanodisks Excited at the Anapole Mode[END_REF][START_REF] Kruk | Nonlinear Optical Magnetism Revealed by Second-Harmonic Generation in Nanoantennas[END_REF] , photonic topological insulators [START_REF] Slobozhanyuk | Three-Dimensional All-Dielectric Photonic Topological Insulator[END_REF][START_REF] Kruk | Edge States and Topological Phase Transitions in Chains of Dielectric Nanoparticles[END_REF] , boosting the luminescence from quantum emitters such as NV-centers in nanodiamonds [START_REF] Krasnok | Enhanced Emission Extraction and Selective Excitation of NV Centers with All-Dielectric Nanoantennas[END_REF] , quantum dots [START_REF] Staude | Shaping Photoluminescence Spectra with Magnetoelectric Resonances in All-Dielectric Nanoparticles[END_REF][START_REF] Peter | Directional Emission from Dielectric Leaky-Wave Nanoantennas[END_REF][START_REF] Rutckaia | Quantum Dot Emission Driven by Mie Resonances in Silicon Nanostructures[END_REF][START_REF] Yuan | Strong Photoluminescence Enhancement in All-Dielectric Fano Metasurface with High Quality Factor[END_REF] , perovskites [START_REF] Tiguntseva | Resonant Silicon Nanoparticles for Enhancement of Light Absorption and Photoluminescence from Hybrid Perovskite Films and Metasurfaces[END_REF] , dye molecules [START_REF] Albella | Electric and Magnetic Field Enhancement with Ultralow Heat Radiation Dielectric Nanoantennas: Considerations for Surface-Enhanced Spectroscopies[END_REF][START_REF] Regmi | All-Dielectric Silicon Nanogap Antennas to Enhance the Fluorescence of Single Molecules[END_REF][START_REF] Cambiasso | Bridging the Gap between Dielectric Nanophotonics and the Visible Regime with Effectively Lossless Gallium Phosphide Antennas[END_REF][START_REF] Sugimoto | Colloidal Dispersion of Subquarter Micrometer Silicon Spheres for Low-Loss Antenna in Visible Regime[END_REF] , and carbon nanotubes [START_REF] Ma | Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces[END_REF] .

In addition, resonant dielectric nanostructures have been used for scattering engineering [START_REF] Krasnok | Experimental Demonstration of Superdirective Dielectric Antenna[END_REF][START_REF] Zuev | Fabrication of Hybrid Nanostructures via Nanoscale Laser-Induced Reshaping for Advanced Light Manipulation[END_REF][START_REF] Markovich | Enhancement of Artificial Magnetism via Resonant Bianisotropy[END_REF][START_REF] Li | All-Dielectric Antenna Wavelength Router with Bidirectional Scattering of Visible Light[END_REF][START_REF] Shibanuma | Experimental Demonstration of Tunable Directional Scattering of Visible Light from All-Dielectric Asymmetric Dimers[END_REF][START_REF] Garcia-Etxarri | Optical Polarization Möbius Strips on All-Dielectric Optical Scatterers[END_REF][START_REF] Khaidarov | Asymmetric Nanoantennas for Ultrahigh Angle Broadband Visible Light Bending[END_REF] , ultrafast switchers and modulators [START_REF] Noskov | Nonlinear Metal-Dielectric Nanoantennas for Light Switching and Routing[END_REF][100][101][102][103][104][105] , optical interconnections on a chip [106][107][108][109] , light trapping structures [110][111][112][113][114] , colored metasurfaces [115][116][117][118] , enhanced Raman scattering [119][120][121][122] .

Interesting photonic phenomena such as Fano resonances [123][124][125][126][127] , Purcell effect [128][129][130][131][132] , and strong coupling 133 have been shown in dielectric nanostructures as well.

In recent times, the vanguard of this highly developing research area has been shifting to spectroscopy and biosensing applications. It has been demonstrated that, unlike high-Q microcavities, RDNs have a broadband response and thus low-quality factors that are compensated by nanoscale field confinement that allows for high fluorescence enhancement.

Moreover, the low absorption of these materials reduces quenching effects that can be detrimental for fluorescence enhancement applications. In this work, we review the state-ofthe-art applications of optically resonant high-index dielectric nanoparticles as a multifunctional platform for light-matter interactions with a special emphasis on optical spectroscopy (Figure 1, left) and biosensing (Figure 1, right). We focus on nanoscale control of light emitters and its applications for enhanced spectroscopy, such as surface-enhanced Raman scattering (SERS), fluorescence spectroscopy, biosensing, including the state-of-theart technology. In Section A., we summarize the main outcomes of Mie theory to describe light scattering on high-index RDNs. Then, we give a theoretical background in the areas of spontaneous emission, especially focusing on weak and strong coupling regimes, mode volume, and Purcell effect. Section B. is devoted to state-of-art studies on high-index nanoantennas and their application for quantum source light emission. Section C. surveys optical spectroscopy with RDNs, including surface-enhanced Raman scattering (SERS), many and single-molecule fluorescence. Section D is devoted to biosensors based on RDNs. Finally, we summarize these findings and present an outlook for the other effects like in vivo imaging, thermometry at the nanoscale, and single-molecule strong coupling achievable with highindex RDNs.

A. Theoretical background

Mie theory and nanoparticle heating. The optical response of spherical metallic and dielectric NPs of arbitrary radius R can be calculated with the multipolar Mie theory developed by Gustav Mie in 1908 134 . Below, we give some general conclusions that follow from this theory. The field of an incident plane wave with electric field strength E0 scattered by a spherical particle made of a material with dielectric permittivity 2 n   ( n is the refractive index of the material) can be written as a sum of partial spherical waves with coefficients l a and l b that are electric and magnetic scattering amplitudes.

    22 2 1 2 1 2 (2 1) | | | | , () 2 (2 1) Re , () , sct l l l ext l l l abs ext sct Q l a b kR Q l a b kR Q Q Q              (1)
where l defines the order of partial wave, k is the wavenumber

( ) ( ) ( ) ( ) ,, ab ll ll a a b b l l l l RR ab R iT R iT   (2)
where l R and l T can be expressed as , usually known as the cavity decay rate, which determines the half-width of resonance line at half maximum 138 . The resonance is usually described by its quality factor, which is defined as cav /2

() () () () ( ) ( ) ( ) ( ), ( ) ( ) ( ) ( ), ( ) ( ) ( ) ( ), ( ) ( ) ( ) ( ), a l l l l l a l l l l l b l l l l l b l l l l l R n kR nkR nkR kR T n kR nkR nkR kR R n nkR kR kR nkR T n kR nkR nkR kR                         (3) Here, 1/2 ( ) ( ), 2 ll x x J x     1/2 ( ) ( ), 2 ll x x N x     1/ 2 () l Jx 
Q   .
In general, the cavity decay rate includes both radiative and absorptive parts:

rad loss cav cav cav   
, such that the quality factor can be expressed as 1

1 1 rad loss Q Q Q     139 .
Using Eqs. ( 1)-( 3), we perform a comparative analysis of the scattering and absorption cross sections of plasmonic and high-index dielectric NPs. For that purpose, we use silver (Ag) which has the lowest absorption levels in the optical range among other noble metals 140 .

We assume that the RDN is made of crystalline silicon (c-Si), which also exhibits the best performance in the visible region 141 . The calculation results are shown in Figure 2. The Ag NP features only electric-type resonances, including electric dipole (ed), electric quadrupole (eq), and electric octupole (eo), whose dispersion with increasing radius (R) is schematically shown by white dashed lines. On the contrary, the spectrum of resonant modes of Si NP is richer and it includes the magnetic dipole (md) moment, as well as magnetic quadrupole (mq) and higher order resonant modes. We note that although more sophisticated plasmonic structures can support modes of magnetic nature 142,143 , they suffer from dissipative losses and complexity in fabrication. shows that the Si NP has significantly less dissipative losses in the entire range of parameters.

In addition, the dissipative losses of the Si NP rapidly fall as the wavelength increases, which is explained by the lower losses of Si away from the main absorption band. Note that gold (Au) NPs have even greater dissipative losses than Ag, while the former are more stable in ambient conditions 144 .

The emitted or scattered electromagnetic energy in a given direction is described by the directivity D 3,[START_REF] Alu | Modeling and Features of Optical Nanoantennas[END_REF]145 , whose standard definition is given by tot 4 ( , ) ( , )

,

P D P      (4)
where

2 tot sct P R IQ  
is the total radiated or scattered power into the far-field, i.e., the integral of the angular distribution of the radiated power over the spherical surface, ( , )

 are the standard angular coordinates of the spherical coordinate system. The directivity quantifies the modification that must be experienced by the emitted power to obtain in the same direction the same Poynting vector modulus yielded by an isotropic source at the same point. This expression is normalized to 4 , thus the directivity of an isotropic source equals unity, and the one of a small dipole is 1.

sct l l l l l l l l l l Q i i kR l i a b ll l i a b ll                           (5)
where [START_REF] Bharadwaj | Optical Antennas[END_REF] (cos( )) ( ) , sin( )

l l P     (1) (cos( )) ( ) , l l dP d    
and [START_REF] Bharadwaj | Optical Antennas[END_REF] l P is the associated Legendre polynomial. Usually NPs are deposited on a dielectric substrate, providing them with an optically inhomogeneous environment. Such dielectric substrate can break the symmetry and, thus, a dependence of the scattered signal on ( ) may appear.

The absorption cross section

For a discussion of this expression and its scope of applicability, see Ref. 147 . Thus, dielectric nanostructures located in a medium with low thermal conductivity ( 2  ) may be heated, which has been already proven to be an effective way for the creation of tunable nanophotonic devices 148 . Moreover, it has been demonstrated that, when optical fields are concentrated into nanoscale volumes, the speed of thermo-optical effects can approach the picosecond scale paving the way to ultrafast and ultrastrong nonlinear effects 149 .

Figure 3 shows the temperature increase after heating of (a) Ag and (b) Si spherical NPs in air ( 2 0.025

  W/m K  150 )
, as a function of their radius R and wavelength. The light intensity is assumed constant 0.1 I  2 mW/μm . At wavelengths below 400 nm, both Ag and Si NPs exhibit temperature increase up to 250 K. However, the temperature increase for the magnetic dipole resonance in Si NP is significantly reduced as the wavelength increases. At wavelengths larger 700 nm, the heating of Si NP is vanishingly small, whereas the Ag NP loses its resonant properties. For experimentally measured temperature increase in plasmonic and dielectric nanoantennas see Section C. Generally, a Fano resonance emerges from the interference between a discrete and a continuum state, and leads to an asymmetric spectral lineshape due to the constructive and destructive interference of radiation from these two states. The Fano resonance in photonic nanostructures has been extensively studied in various systems [160][161][162][163][164] . In nanocavity-molecule structures the Fano resonance is caused by the coupling between at least one broad plasmonic mode and one narrow excitonic resonance [START_REF] Monticone | Metamaterials and Plasmonics: From Nanoparticles to Nanoantenna Arrays, Metasurfaces, and Metamaterials[END_REF][165][166][167][168] It has been shown that the Fano resonance can induce extra narrow peaks and dips in the optical spectra and it has a great potential for biosensing due to its high sensitivity to environmental changes.

Incoherent effects include (i) altering the emission rate and its spectra, and (ii) different nonradiative decay processes through energy and/or charge transfer 169 . Below we focus on the first effect, while the interested reader may refer to Refs. [170][171][172][173][174][175] for the second effect. In addition, a plasmonic nanocavity may initiate different chemical reactions 176 , which are also out of the scope of this Review.

Altering the excitation rate relies on the ability of a nanocavity to localize light at subwavelength scales, overcoming the diffraction and single molecule detection limits 177 . In other words, an optical nanocavity changes the electric and magnetic fields in its vicinity.

Since electric dipole transitions are usually much stronger than magnetic ones in small optical emitters, we consider a molecule with an induced electric dipole moment. Below, we will also

|| , ||

d d     nE nE (8)
where 0

 is the molecule absorption cross-section in the absence of a nanocavity, d n is the orientation of dipole (it is assumed that the nanocavity does not change the molecule structure). Thus, since nanocavities may enhance the near-field distribution in a certain space region, the absorption cross-section of a molecule placed in this region is also enlarged. This

gives rise to molecule excitation and overall luminescence intensity enhancement.

From the perspective of enhancement by individual NPs, the absence of size cut-off frequency for dipole plasmon resonance of plasmonic NPs results in much stronger field enhancement in their vicinity as compared to dielectric ones with same sizes in the sub-100nm range. However, larger Si NPs, possessing a magnetic Mie-type resonance at the optical frequencies yield comparable near-field enhancement [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF]178 . This effect was proved in bulk SERS experiments, where resonant Si NPs produce larger SERS effect as compared to Au ones of the same sizes 179 , see Section C.

The coupling Hamiltonian of a hybrid system of coupled nanocavity (plasmonic or dielectric) and exciton subsystem in the Jaynes-Cummings formalism 158 is defined by the matrix element v0 ( )

, ij Hg    d E r (9)
where || e j i  dr is the transition dipole moment of a two-level exciton subsystem, e is the elementary charge, r is the radius-vector operator, and v0 () Er is the vacuum electric field of the resonator mode at the molecule position, is the reduced Planck constant. The value g is called coupling constant and has a special importance for the theory of coupled nanophotonic systems.

There are two characteristic regimes of spontaneous emission of a molecule coupled to a dielectric or plasmonic nanocavity. Namely, if the coupling constant is less than the total

decay rate cav mol     of a system, ( g   )
, the system is in the so-called weak-coupling regime. In this regime, there is no coherent energy exchange between the molecule and nanocavity, and the variation of spontaneous emission can be described by the Purcell factor, which will be defined below. If the opposite condition is satisfied ( g   ), the system becomes strongly coupled, which leads to a splitting of the scattering spectrum, which is known as Rabi splitting 29,180 . In the strong-coupling regime, the system demonstrates energy exchange between the molecule and the resonator (Rabi oscillations), resulting in a more complex behavior of the spontaneous emission, which becomes non-exponential.

The strong-coupling regime is very attractive for quantum optics applications, including the realization of hybrid electron-photon quasiparticles 181 , Bose-Einstein condensates 182,183 , single-photon switches 184,185 , all-optical logic 186,187 , and control of chemical reactions 188,189 . Moreover, the strong coupling regime has a special interest in quantum optics and nanophotonics if the system can be tuned at time scales less than the characteristic time of energy transfer between the cavity mode and the molecule, which is described by the inverse coupling constant (1/g).

How can the strong coupling be achieved in an arbitrary system? Since the coupling constant is proportional to the vacuum electric field of the cavity mode as

eff 0 2 d g V  
 [START_REF] Marquier | Revisiting Quantum Optics with Surface Plasmons and Plasmonic Resonators[END_REF]190 , in order to increase g one can increase the dipole moment of the exciton subsystem and/or reduce the effective mode volume eff V , which can be defined in the low loss limit as [START_REF] Marquier | Revisiting Quantum Optics with Surface Plasmons and Plasmonic Resonators[END_REF]191  

2 eff 2 ( ) | ( ) | . max ( ) | ( ) | r E r dV V r E r     (10)
Usually, the effective mode volume is much less than the physical volume of the resonator.

For example, plasmonic nanocavities demonstrate unprecedented effective mode shrinking, up to values in the order of 34 /10  138 . This equation ( 10) is not always applicable to nanocavities (especially to plasmonic ones) because they often demonstrate high dissipative rates 192 . This issue has been addressed many times in the literature 138,[192][193][194][195][196][197] . In particular, a detailed analysis for relatively small plasmonic nanocavities without gain has been presented in Ref. 138 , leading to the concept of complex mode volume.

In the scattering spectra, the strong coupling regime appears as an anticrossing at the frequency of excitonic resonance. To simulate the coherent scattering spectra of the system the coupled harmonic oscillator model can be applied [198][199][200][201] . This model considers a cavity driven by an external field, which is coupled to the molecule (exciton resonance). According to this theory, in the harmonic oscillator approximation the scattering cross section of the hybrid system with two coupled oscillators is given by is the value of density of states on resonance assuming that the nanocavity mode has a Lorentzian profile of width cav 2 . Using Eq. ( 9), this equation can be expressed as

            (12)
Thus, the Purcell effect results in a modification of the spontaneous emission rate mol  of a molecule induced by its interaction with an inhomogeneous environment (nanocavity), and is quantitatively expressed by the Purcell factor, Eq. ( 12) 128,129,138,[203][204][205] . This modification is significant if the environment is a resonator tuned to the molecule emission frequency. Open nanocavities such as plasmonic or RDN nanoantennas can change the spontaneous emission lifetime of a single quantum emitter, that is very useful in spectroscopy and microscopy of single NV centers in nanodiamonds 206 , Eu 3+ -doped nanocrystals 207 , optical sensing 138,208 , and bioimaging [START_REF] Acuna | Fluorescence Enhancement at Docking Sites of DNA-Directed Self-Assembled Nanoantennas[END_REF] .

In general, the Purcell effect gives rise to enhancement of emission and dissipation rates. Taking into account that usually one is interested in the radiative part of spontaneous emission, it is useful to distinguish the radiative ( rad The optimal performance of the Yagi-Uda nanoantenna has been achieved for directors (particles 2-5) radii corresponding to the magnetic resonance (70 nm), and the radius of reflector (particle 1) corresponding to an electric resonance (75 nm) at a given frequency. This solution ensures that the directors and reflector operate in opposite reactive regimes (inductive and capacitive), consistent with antenna theory [224][225][226][227] . In Figure 4(b) the directivity of alldielectric Yagi-Uda nanoantennas as a function of wavelength for a separation distance between particles of 70 nm is presented. The insets demonstrate the 3D radiation patterns at particular wavelengths. A strong maximum of the nanoantenna directivity (see Eq. ( 4))

reaching 12 at the wavelength of 500 nm has been achieved. By comparing plasmonic and alldielectric Yagi-Uda nanoantennas, it has been demonstrated that the all-dielectric ones may exhibit better radiation efficiency, also allowing more compact design. A comprehensive analysis of such nanoantennas has been presented in Ref. 228 .

In Ref. 229 it has been shown that more complex core-shell structures consisting of Ag core and Si shell may be adjusted to satisfy the resonant Kerker condition, possessing both resonant and unidirectional scattering at the same wavelength. The directionality of the structure can be further improved by arranging the core-shell NPs in an array.

These ideas were developed in a series of subsequent works 128,228,[230][231][232][233][234][235] . It has been demonstrated that the unique optical properties and low dissipative losses make RDNs perfect candidates for the design of high-performance nanoantennas, low-loss metamaterials, and other novel all-dielectric nanophotonic devices. The key to such novel functionalities of highindex dielectric nanophotonic elements is the ability of subwavelength dielectric NPs to support simultaneously both electric and magnetic resonances, which can be controlled by their shape 236,237 . Moreover, the electric and magnetic field localization in the vicinity of alldielectric nanoantennas has been theoretically predicted 178 and experimentally measured in the microwave 238 , terahertz 239 and visible 240 ranges that has paved the way to applications of such systems for spectroscopy and sensing.

The existence of magnetic modes in RDNs provides a very powerful tool for the creation of highly directive nanoscale light sources. In Refs. [START_REF] Krasnok | Experimental Demonstration of Superdirective Dielectric Antenna[END_REF]241 the concept of so-called superdirective nanoantennas based on the excitation of higher-order magnetic modes in RDNs of radius 90 nm has been proposed. The superdirective regime (highly directive radiation by an antenna of subwavelength dimensions) is achieved by placing an emitter within a small notch created on the nanosphere surface. It turns out that such a small modification of the sphere allows for efficient excitation of higher-order modes. Then, it has been demonstrated that this effect can be applied for emission extraction enhancement and to simplify the initialization of a single emitter in an assemble with a subwavelength separation among individual emitters [START_REF] Krasnok | Enhanced Emission Extraction and Selective Excitation of NV Centers with All-Dielectric Nanoantennas[END_REF] . In Ref. 242 it has been demonstrated that mixing of dipolar and quadrupolar modes may also lead to highly directive emission to perform an efficient switch of the emission directivity with respect to the wavelength.

Mie resonances in dielectric particles can also increase the Purcell effect, associated with either electric or magnetic transition rates nearby quantum emitters. Their large quality factors compensate the low field confinement as compared to the plasmon resonances of metallic nanostructures, for which nonradiative decay channels dominate. In Ref. 230 where the Purcell factor enhancement up to ~200 times using RDNs is reported. Further increase of the Purcell factor (up to several hundreds) can be achieved by relying on the Van Hove singularity of a chain of high-index nanoparticles 129 . Such approach allows to substantially enhance the local density of optical states at the location of a quantum source and thus achieve high values of the Purcell factor with relatively small dielectric nanostructures, while leaving unaltered all their other advantages.

Recently, it has been theoretically demonstrated that hybrid metal-dielectric nanoantennas based on dielectric particles on metallic films yield high Purcell factor (>5000), high quantum efficiency (>90%) and highly directional emission at visible and infrared wavelengths 132 . A similar design of highly efficient metal-dielectric patch antennas for singlephoton emission has been proposed in Ref. 243 . Such hybrid nanostructures may benefit from the use of both approaches, plasmonic and dielectric, and currently are a subject of intense studies.

In order to get a strong local field enhancement, plasmonic dimers [START_REF] Kinkhabwala | Large Single-Molecule Fluorescence Enhancements Produced by a Bowtie Nanoantenna[END_REF][START_REF] Acuna | Fluorescence Enhancement at Docking Sites of DNA-Directed Self-Assembled Nanoantennas[END_REF][START_REF] Busson | Accelerated Single Photon Emission from Dye Molecule-Driven Nanoantennas Assembled on DNA[END_REF][START_REF] Punj | Self-Assembled Nanoparticle Dimer Antennas for Plasmonic-Enhanced Single-Molecule Fluorescence Detection at Micromolar Concentrations[END_REF] or oligomers can be used [244][245][246][247][248] . The same approach is also possible for dielectrics, 125,240,249,250 when local field enhancement factor in the gap of a Si dimer may be more than one order of magnitude 240 . Such enhancement was applied to achieve high SERS and enhanced luminescence effects [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF][START_REF] Albella | Electric and Magnetic Field Enhancement with Ultralow Heat Radiation Dielectric Nanoantennas: Considerations for Surface-Enhanced Spectroscopies[END_REF][START_REF] Cambiasso | Bridging the Gap between Dielectric Nanophotonics and the Visible Regime with Effectively Lossless Gallium Phosphide Antennas[END_REF]178 . Thus, high-index RDNs also provide field enhancement and Purcell effects that are large enough for detection of a few or even single molecules [START_REF] Regmi | All-Dielectric Silicon Nanogap Antennas to Enhance the Fluorescence of Single Molecules[END_REF] (see Section C).

As we already mentioned above, high-index dielectric NPs possess electric and magnetic resonant optical responses. Therefore, they provide a unique opportunity to enhance the photoluminescence intensity from naturally weak magnetic dipole transitions, giving an additional spectroscopy tool 130,[251][252][253][254][255] . Moreover, it has been shown that there are materials with forbidden dipole transitions to the first order, e.g., some transition metals such as Cr 3+

and lanthanide series ions such as Eu 3+ and Er 3+ . Thus, the combination of such materials with [START_REF] Zywietz | Laser Printing of Silicon Nanoparticles with Resonant Optical Electric and Magnetic Responses[END_REF]125 and chemical means 125 , single Ge disks (Figure 5(d)) and Si disk dimers (Figure 5(e)), fabricated by electron beam lithography [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF]257 . Recently, disk dimers have been also realized in form of split GaAs nanoposts 258 . More complex RDNs, including all-dielectric oligomers (Figure 5(c)) 259 and metasurfaces (Figure 5(f)) 260 , have also been produced by electron beam lithography.

Moreover, different laser assisted methods have been also developed [START_REF] Zywietz | Laser Printing of Silicon Nanoparticles with Resonant Optical Electric and Magnetic Responses[END_REF]261,262 . For a comprehensive review of available materials and fabrication techniques, readers can be addressed to Ref. [START_REF] Baranov | All-Dielectric Nanophotonics: The Quest for Better Materials and Fabrication Techniques[END_REF] .

Hybrid dielectric nanocavity-excitonic systems. Nanostructures, which provide resonant optical modes, are the first constituting part of hybrid nanocavity-excitonic systems.

The second part is an excitonic system, which support resonant electron transitions between two or even more well-defined quantum levels. Here we summarize all realized different hybrid RDN-excitonic systems, including dye molecules, J-aggregates, perovskites and transition metal dichalcogenides, Figure 6. 11)).

It has been also shown that Si NPs can increase the PL emission from MAPbI3 perovskites [START_REF] Tiguntseva | Resonant Silicon Nanoparticles for Enhancement of Light Absorption and Photoluminescence from Hybrid Perovskite Films and Metasurfaces[END_REF] . Figure 6(c A monolayer of transition metal dichalcogenide (TMDC) [263][264][265] is formed by a hexagonal layer of transition metal atoms (Mo, W) hosted between two hexagonal lattices of chalcogenide atoms (S, Se). Electronically, TMDCs behave as two-dimensional semiconductors with their bandgaps lying in the visible range. In the atomic monolayer limit these materials are particularly interesting, when their bandgap becomes direct enabling enhanced interaction of the dipole transition with light 266 . Due to their monolayer composition, high oscillator strength, and capabilities for tuning, these materials are a very promising class of materials in the context of various optoelectronic applications, such as photodetection and light harvesting 267 , phototransistors and modulation 268 , light-emitting diodes 269 , and lasers 270 .

In Ref. 271

C. Spectroscopy with resonant dielectric nanoantennas

Resonant NPs and nanostructures have been proven to be powerful tools for enhancing light-matter interactions due to their ability to enhance and localize optical energy at the nanoscale, boosting PL spectra through the Purcell effect and increasing the molecules excitation rates (see Section A). Two spectroscopy techniques based on these effects have been proposed: surface enhanced Raman scattering (SERS) 154,[272][273][274][275][276][277][278] and enhanced fluorescence spectroscopy [START_REF] Punj | Self-Assembled Nanoparticle Dimer Antennas for Plasmonic-Enhanced Single-Molecule Fluorescence Detection at Micromolar Concentrations[END_REF]169,[279][280][281] .

Despite plasmonics has demonstrated tremendous success in these phenomena 282,283 , the concept of all-dielectric nanophotonics can also serve as a platform for high-effective detection scheme, even with single-molecule sensitivity. First of all, as we already mentioned in the previous section, low Ohmic losses in RDNs prevent parasitic heating of the analyzed objects [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF][START_REF] Albella | Electric and Magnetic Field Enhancement with Ultralow Heat Radiation Dielectric Nanoantennas: Considerations for Surface-Enhanced Spectroscopies[END_REF]178 , and second, high radiative part of the Purcell factor and directivity improve signal extraction [START_REF] Krasnok | Enhanced Emission Extraction and Selective Excitation of NV Centers with All-Dielectric Nanoantennas[END_REF] . Figures 7(a)-(d) show the enhancement of radiative decay rate and quantum efficiency (see Eq. ( 13)) of an electric dipolar emitter positioned between two Si (a), (b) and Au (c), (d) nanospheres for longitudinal and transverse dipole orientations. It can be seen that the Si nanoantennas have the quantum efficiency exceeding that of the Au nanoantennas 178 . Moreover, this good enhancement performance is achieved in combination with low heat generation. Figures 7(e)-(g) show the measured temperature increase on Si and Au nanoantennas when excited at the dipolar resonance. Notably, Si nanoantennas show a nearly constant temperature while the Au nanoantennas lead to a temperature increase of more than 100 o C in the nanogap [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF] . This allows to conduct spectroscopy measurements near nanoantennas at a constant temperature over a wide range of excitation powers. Thanks to this near-field enhancement at a nanoscale volume, this all-dielectric nanoantenna can enhance the Raman scattering of a polymer thin film by a factor of 10 3 , in excellent agreement with the theoretical expectation, as shown in Figure 8(c). Moreover, the authors performed molecular thermometry measurements to demonstrate that the dielectric nanoantennas produce ultra-low heating when illuminating at their resonance wavelength (results shown in Figures 7(e)-(g)), thus overcoming one of the main drawbacks of traditional plasmonic materials such as gold.

In addition to the SERS capabilities combined with low Ohmic losses, all-dielectric nanoantennas can be used efficiently for fluorescence enhancement. Importantly, due to the absence of quenching and of free carriers, which are inherent to metallic NPs, the photoluminescence experiments with dielectric NPs do not require any dielectric spacer. In Ref. [START_REF] Sugimoto | Colloidal Dispersion of Subquarter Micrometer Silicon Spheres for Low-Loss Antenna in Visible Regime[END_REF] the enhancement in photoluminescence signal from Rhodamine B molecules up to 200 times with just single subwavelength dielectric (Si) NPs without any dielectric spacers has been demonstrated [see Figure 6(a)].

As in plasmonics, higher photoluminescence enhancement factors require more complicated structures like dimers, trimers and oligomers, which have been theoretically and experimentally studied recently [START_REF] Regmi | All-Dielectric Silicon Nanogap Antennas to Enhance the Fluorescence of Single Molecules[END_REF]123,240,256,259,[284][285][286] . In Figure 9 we summarize the results obtained so far in many molecule fluorescence enhancement using all-dielectric dimer nanoantennas, from Refs. [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF][START_REF] Cambiasso | Bridging the Gap between Dielectric Nanophotonics and the Visible Regime with Effectively Lossless Gallium Phosphide Antennas[END_REF] . In both cases, the experimental scheme is similar: dielectric disk dimers with a nanometric gap are fabricated on a substrate and then covered with a polymer thin-film with embedded fluorescent molecules, allowing a controlled and uniform distribution of molecules on the nanoantennas, Figure 9(a). To excite and detect the enhanced fluorescence signal, an epi-confocal configuration was used. The main difference in the samples is the material used: while the first uses silicon (Si) dimers, the second uses gallium phosphide (GaP) dimers. Note that in both cases the molecules can be in direct contact with the nanostructure (no spacers were used) possible due to the reduced quenching effect of dielectric nanoantennas. The reported maximum enhancement factor for the Si dimers is  while for the GaP dimers is 3 3.6 10  . The reported higher enhancement can be explained by the higher near-field intensity in the gap for the GaP nanoantennas and the better emission enhancement due to the nearly zero losses in the visible for this material 89 . To prove this fact further, the authors measured a fluorescence lifetime reduce of at least 22 times, limited by the experimental setup.

Recently, fluorescence enhancement using dielectric nanoantennas was pushed to the single-molecule limit [START_REF] Regmi | All-Dielectric Silicon Nanogap Antennas to Enhance the Fluorescence of Single Molecules[END_REF] . A dimer of silicon discs with a 20-nm gap and a broad dipolar resonance at the visible was used to obtain a x270 fluorescence brightness increase for single molecules of crystal violet, over performing gold structures with similar gaps. The experimental approach for this experiment is illustrated in

D. Biosensing with resonant dielectric nanoparticles

The study of biological analytes through optical and nanophotonic structures (mainly metallic-based ones) has been an actively developing research area for a few decades [288][289][290] .

The operation principle of the vast majority of proposed biosensors is based on the dependence of the plasmonic resonance of metal NPs on their environment, where the NPs are hosted 246,[291][292][293] . Namely, the plasmonic resonance of a metal NP shifts if the refractive index of the surrounding dielectric changes (see Figure 2(e)). Therefore, if one activates the NPs by specific linker molecules that are targeted on a certain substance that should be detected, it will cause the measurable shift of the resonance when the substance appears in the NP environment. As we already mentioned in Section A., the figure of merit (FoM) of plasmonic sensors is commonly defined as the resonance shift (S) upon a change in the refractive index of the surrounding dielectric normalized by the resonance line width (   ), see Eq. ( 7). Thus, narrow resonances improve the FoM of plasmonic sensors 288 . Specific applications of SPRbased biosensing that can be found in the literature include detection of contaminants in food and water 294 , immunoassays 295 , DNA-protein interactions 296 , and detection and monitoring of antigen-antibody binding events in real time 297 .

However, the sensing performance of plasmonic nanostructures is limited by their conductive and absorptive nature due to free electrons. The highly absorptive nature of metals inducing undesirable local heating 147,298 , which can lead to denaturation of molecules and hinder the development for in vivo sensing 299 . Very recently, it has been proven that RDNs may become a powerful platform for biological and medical applications including optical biosensors 260,300,301 . On the one hand, high-index RDNs possess low dissipative losses, and their heating at ordinary intensities of laser excitation is sufficiently lower [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF]178 . On the other hand, RDNs exhibit both electric and magnetic dipole Mie resonances, which offers more ability to detect the changes in ambient conditions --altering of scattering pattern. It allows one to use them in biosensing applications by similar way as it is done for plasmon sensors.

In Ref. 300 both sensing principles, namely (i) the intensity variations in the transmitted or backscattered radiation by the RDNs at the magnetic dipole resonance and (ii) the changes in the radiation pattern at the frequency that satisfies Kerker condition (see Section B.) of near-zero forward radiation have been theoretically proposed. This idea has been experimentally demonstrated by Bontempi et al. in Ref. 157 in the system of 2D array of Si nanoparticles (metasurface) sensitive to streptavidin. To achieve this functionality, the Si metasurface, previously fabricated by e-beam lithography, has been activated generating hydroxyl radicals (--OH) on the surface of nanodisks. This surface activation process was possible because silicon has a thin layer of native oxide after fabrication [START_REF] Caldarola | Non-Plasmonic Nanoantennas for Surface Enhanced Spectroscopies with Ultra-Low Heat Conversion[END_REF]302 . Then, to perform linker functionalization, the silane-polyethylene glycol (PEG)-biotin molecules as the linking medium between the Si nanodisks and streptavidin have been used, Figure 11(a).

When the streptavidin bounds to the biotin on the linker molecule using the binding sites it changes the metasurface transmittance spectra, approach. The results demonstrate the streptavidin detection limit of 10 -10 , which is two orders of magnitudes better than existing nanosensors 303 . Similar approach has been proposed in Ref. 304 for the periodic silicon nanowire-based arrays. It has been demonstrated that by functionalizing the surface of Si RDNs with a graphene monolayer, such structures can be used to optically detect low-concentration surface adsorption events. The specific label-free detection limit using immunoglobulin G protein (IgG) is found to be approximately 300 pM with an extracted maximum sensor resonance shift of 5.42 nm.

The RDNs provide wide opportunities to design more complex systems possessing Fano resonances [START_REF] Decker | Resonant Dielectric Nanostructures: A Low-Loss Platform for Functional Nanophotonics[END_REF]123,125,259,284 (see Section A). It has been demonstrated that at the Fano resonance frequency the near-field of RDNs increases to levels even larger than of a single constituting element, providing an additional powerful tool to enhancing light-matter interaction effects. Therefore, the Fano resonance can be utilized to improve the sensor FoM, which has been realized in Ref. 260 , where a FoM of 103 for Si metasurface has been reported.

We note that this value is significantly higher than the best FoMs obtained with Fano resonant plasmonic sensors, that are on the order of 23. 305 The application of high-index RDNs for biosensing has been further developed in Ref. 306 , where it has been combined with a microfluidics system, thus realizing the lab-on-achip platform enables detecting low concentrations of prostate specific antigen (PSA) cancer marker in human serum.

Summary and Outlook

In this paper, we have reviewed the state-of-the-art applications of optically resonant high-index dielectric nanostructures as a multifunctional platform for light-matter interactions, especially focusing on spectroscopy and biosensing. We have given a comprehensive survey of surface enhanced Raman scattering, fluorescence spectroscopy, biosensing and lab-on-chip technologies based on RDNs. We have accompanied the work by a theoretical background underlying the operation principles of these techniques, with an overview of specific resonant dielectric nanostructure and hybrid excitonic systems.

To summarize all results presented in the Review, we may conclude that resonant dielectric nanostructures have many advantages compared to plasmonic ones, including low energy dissipation into heat, resonant enhancement of magnetic fields in dielectric nanoparticles and bring novel functionalities in both spectroscopy and sensing applications.

Table 1 summarizes the presented results with sensitivity and FoM for different nanostrucutres used for biosensing. More details about plasmonic-based biosensing can be found in the literature 307 , here we reproduced only a small subset for comparison with RDNs.

We may conclude saying that the most sensitive sensor, from the point of view of FoM, presented to date has been proposed in Ref. 260 . This sensor consists of a Si metasurface with Fano-resonant double-gap split-ring elements. The Fano resonance provides a narrow peak in the transmittance spectra, which is sensitive to the surrounding dielectric variations. The sensitivity of the proposed RDNs does not exceed the sensitivity of the best plasmonic devices realized up to date. However, the trend allows one to expect the emergence of more sensitive RDN-based devices in the near future.

Thus, despite impressive and encouraging results in this area, consensus about the best performing designs has not been formed yet. This is a young research area where more studies and experiments are needed. We envision several future research directions in this context.

First, it has been recently demonstrated that second-harmonic generation from dielectric nanoparticles made of bio-compatible hybrid perovskites (barium titanate, BaTiO3) can be significantly enhanced at the magnetic Mie resonance 308 . This opens a way to use such particles for different kind of bioimaging applications, where they can be efficiently used as second-harmonic imaging probes in in vitro and in vivo tissues 309,310 . Moreover, an additional enhancement of second-harmonic generation from BaTiO3 nanoparticles has been achieved by additional coupling to a surface plasmon resonance of a gold nanoparticle at the pumping frequency 311 .

Second, the concept of all-dielectric nanophotonics can be a new approach to provide effective detection of molecular events beyond the diffraction limit, while revealing analyte conformation and its local temperature in real time. It has been shown that silicon nanoparticles possess resonantly enhanced inherent Raman response 121 . This effect can be potentially utilized for simultaneous detection of a nanoscale molecular event (protein unfolding) and control of the local temperature in real time. The high sensitivity and subdiffraction spatial resolution can be achieved due to resonant properties of silicon nanoantennas, whereas precise nanothermometry is caused by the effect of heating on the Raman scattering signal.

Finally, as discussed above, increasing of the resonance quality factor may make the sensor more sensitive. In this regard, it is useful to note that almost non-radiating resonant states called bound states in the continuum (BICs) have been recently discovered in photonics [312][313][314][315] . These are localized states in the continuous spectrum of an electromagnetically open resonant (nano)structure. An existence of these states allows designing photonic resonators with diverging and potentially infinite lifetimes and quality factors, leading to many vital applications, including biosensors. It has been recently demonstrated that RDNs are a powerful platform for designing BIC states 316,317 which can be explored for advanced spectroscopy and biosensor applications.



  of the electric and magnetic eigenmodes of the particle, respectively. These eigenmodes manifest themselves at real frequencies as resonant enhancement of scattering and extinction cross section spectra. In the general case, these expressions are satisfied energy leaks out of the particle and is dissipated into absorption, the modes possess a finite lifetime cav 1/ Im[ ] l  

Figures 2 (

 2 b), (d) show the calculated normalized absorption cross sections spectra of (b) the Ag NP and (d) the Si NP. A comparative analysis of these results

FigureFigures 2 ( 1 h

 21 Figure of merit for sensors. The dependence of the plasmonic and Mie resonance

  mention a special type of materials with forbidden electric dipole transitions. The electric field enhancement factor 0 | / | EE , with E and 0 E being the electric field strength near a nanocavity and the impinging one, defines the absorption cross-section of a molecule (excitation rate), which can be written out in

  the Purcell factor ( p F ), written in two forms, as a function of the coupling constant g and of the quality factor Q 14,202 :

1 

 1 In order to characterize the fraction of energy emitted into photons one also defines the quantum yield (or quantum efficiency) , which reflects the fraction of energy emitted by an isolated molecule, the resulting quantum yield  ( fluorescence enhancement factor fl  is jointly affected by both the excitation enhancement, and the emission modification can be expressed as light, and by introducing the collection efficiency coll  we take into account that only a part of emitted photons can reach a photodetector.Concluding this Section we note that different combinations of the quality factor (Q)and effective mode volume ( eff V ) define not only the Purcell effect (  ) but also many other optical processes, including optical bistability and single-photon Kerr nonlinearity 209 . This makes resonant nanostructures with designable Q and eff V vital for many applications.B. Resonant dielectric nanostructures and hybrid excitonic systemsResonant dielectric nanoantennas and nanostructures. Dielectric particles made of titania were proposed in 2010 to design highly directive lens antennas 210 . The authors suggested to couple dielectric materials to yield high directivity and plasmonic particles to yield high decay rates. Low refractive index microspheres were proposed a few years earlier to probe fluorescent signals of molecules[211][212][213] and high resolution microscopy 214 . Then, in 2011 it has been shown that one Si NP can act as a Huygens element in the optical range (Figure4(a))215 , demonstrating so-called Kerker effect[216][217][218][219][220][221][222] . It has been demonstrated that such nanoantennas are able to switch the radiation pattern between forward and backward directions due to the presence of electric and magnetic resonant modes (see Section A.). Then, in Ref.223 the high directionality of Yagi-Uda nanoantenna 145 composed of Si NPs has been shown, Figure 4(b).

  it has been shown theoretically that near-infrared quadrupolar magnetic resonances in Si NPs can preferentially promote magnetic versus electric radiative exciton in trivalent erbium ions at 1.54 μm (see Figure4(c,d)). The distance-dependent interaction between magnetic (electric) dipole emitters and induced magnetic or electric dipoles and quadrupoles has been derived analytically and compared to full-field calculations based on Mie theory. Single Si NP can give rise to emission enhancement of up to 90 times for longitudinal magnetic dipole emitters and up to 15 times for transverse electric emitters. For simulated Purcell factor enhancement of dielectric dimer nanoantennas and their comparison with plasmonic ones see Section C.,

  RDNs may pave the way to fundamentally novel optical phenomena. Dielectric structures supporting strong magnetic field enhancement 0 | / | HH factors (up to 20 and even more) have been proposed 130,256 , with || H and 0 || H being the magnetic field strength near a nanostructure and the one of the impinging wave. Progress in dielectric nanoantenna research is accompanied by developments in their fabrication methods. Figure 5 displays various realized dielectric nanoantennas and resonant nanostructures fabricated by different methods, including single Si spheres (Figure 5(a)) and Si dimers (Figure 5(b)), fabricated by femtosecond-laser ablation

Figure 6 (

 6 Figure 6(a) shows PL enhancement factors in hybrid Si NP & dye molecules system as a function of corresponding scattering intensities obtained for nine single Si spheres 90 . It

  ) demonstrates the PL intensity spectra from hybrid Si NP & MAPbI3 perovskite system (red curve) and from reference MAPbI3 perovskite (black curve). 50% enhancement of PL intensity from the perovskite layer with Si RDNs and 200% enhancement for a nanoimprinted metasurface with Si NPs on top have been experimentally achieved owing to reduced non-radiative channels for energy dissipation in the presence of Si NPs and the Purcell effect.

  the issue of resonance coupling in single Si NP-monolayer TMDC (WS2) heterostructures has been addressed, Figure 6(d). A transition from weak to strong coupling regimes with coupling constant exceeding 200 meV for a Si NP covered by a monolayer WS2 at the magnetic optical Mie resonance has been demonstrated. This effect has been achieved by changing the surrounding dielectric material from air to water. The ability of such system to tunable resonance coupling has been experimentally realized for spherical Si NPs arranged on a WS2 flake. The coupling constant increases from 24.8 meV up to 43.4 meV by replacing air by water. A variety of RDNs have been recently proposed and investigated for multiple applications. Their ability to localize light at the nanoscale and to boost light-matter interaction within different excitonic systems in both weak and strong coupling regimes have been demonstrated. The next two Sections are devoted to a review of the spectroscopy and biosensing applications of the proposed RDNs.

Figure 8 (

 8 Figure 8(a) shows experimental results of all-dielectric nanoantennas for surface

Figure 10 (Figure 10

 1010 Figure 10(f).

Figure 11 (

 11 b). The collective resonance (dip around 1.5 μm ) is very sensitive to the streptavidin in the biological analyte and even 10 -5 M concentrations cause its red-shifting by 10 nm, which can be resolved by standard optical means. After the removal of the streptavidin (purple curve), the resonance position returns to the position prior to the application of streptavidin (the biotin-coated case). Figure11(c) demonstrates the color map of the transmittance spectra of nanodisk arrays measured with vertically polarized light for different streptavidin concentrations. The resonance at around 1488 nm red shifts as the concentration of streptavidin increases and can be seen by the ascending blue area denoting the transmittance dip.Figure 11(d) shows the sensitivity of this

Figure 11 (

 11 e) shows the picture of a microfluidic system with eight channels, which include optically resonant silicon nanodisk arrays of different dimensions and lattice parameter. The operation principle of the system is also based on the dependence of RDN collective resonances on the environment conditions. Namely, the increased effective refractive index at the sensor vicinity resulting from the binding steps causes the resonances of the nanodisk arrays to redshift enabling an accurate monitoring of the adsorption and binding kinetics of the molecules, Figure11(f). Moreover, it has been demonstrated by direct comparison that the operation performance of RDN-based devices far exceeds its Au counterpart, including higher sensitivity and higher stability in solution.

Figure 1 .

 1 Figure 1. Schematic representation of topics addressed in this review: optical Spectroscopy

Figure 2 . 1 h

 21 Figure 2. Optical properties of metallic (Ag) and dielectric (Si) nanoparticles. (a),(c)

Figure 3 .

 3 Figure 3. Temperature increase calculated by Eq. (6) for heating of (a) silver and (b) silicon

Figure 6 .

 6 Figure 6. Different realized hybrid dielectric nanoparticle-excitonic systems. (a) PL

  2

	The quantities sct P , ext P and abs P denote respectively the scattering, extinction and absorption
	power,	I is the excitation intensity (	h I n c 	2 00 | | /2 E	), c is the speed of light, and 0  is the
	dielectric constant. Electric and magnetic scattering amplitudes are given by
		( )		( )
					hh n n  c   k   	,  is the
	wavelength in vacuum and

2

hh n   is the dielectric permittivity of the surrounding medium.

Table 1 .

 1 The sample (100 ng/ml PSA) and control (no PSA) experiments are in grey and purple, respectively. Figure adapted with permission from: (a--d)--Ref.157 , (e, f)--Ref.306 . Comparison of the proposed spectroscopy and biosensor systems based on resonant dielectric and metallic nanostructures.

	Material Nanostructure	Type	Sensitivity,	FoM	Reference
				nm/RIU
	Au	Sphere	ensemble	90	1.5	Underwood (1994) 318
	Au	Rod	ensemble	239	1.8	Paulo (2017) 319
	Au	Bipyramid	ensemble	352	4.5	Burgin (2008) 320
	Au	Rod-shape holes	metasurface	588	3.8	Liu 2010 321
		on film
	Au	Patch Nanohole metasurface	313	23.3	Liu (2010) 305
	Si	Disks	ensemble	-	-	Bontempi (2017) 157
	Si	Disks	ensemble	227	4.5	Yavas (2017) 306
	Si	Double-gap	metasurface	289	103	Yang (2014) 260
		split-ring

Acknowledgements

This work was also partially supported by the Air Force Office of Scientific Research and by the Welch Foundation with grant No, F-1802.

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Authors information

The authors declare no conflict of interest.