High-index Dielectric Nanoparticles Research Articles

Universal Click-Chemistry Approach for the DNA Functionalization of Nanoparticles.

Nanotechnology has revolutionized the fabrication of hybrid species with tailored functionalities. A milestone in this field is the deoxyribonucleic acid (DNA) conjugation of nanoparticles, introduced almost 30 years ago, which typically exploits the affinity between thiol groups and metallic surfaces. Over the last decades, developments in colloidal research have enabled the synthesis of an assortment of nonmetallic structures, such as high-index dielectric nanoparticles, with unique properties not previously accessible with traditional metallic nanoparticles. However, to stabilize, integrate, and provide further functionality to nonmetallic nanoparticles, reliable techniques for their functionalization with DNA will be crucial. Here, we combine well-established dibenzylcyclooctyne-azide click-chemistry with a simple freeze-thaw method to achieve the functionalization of silica and silicon nanoparticles, which form exceptionally stable colloids with a high DNA surface density of ∼0.2 molecules/nm2. Furthermore, we demonstrate that these functionalized colloids can be self-assembled into high-index dielectric dimers with a yield of over 50% via the use of DNA origami. Finally, we extend this method to functionalize other important nanomaterials, including oxides, polymers, core-shell, and metal nanostructures. Our results indicate that the method presented herein serves as a crucial complement to conventional thiol functionalization chemistry and thus greatly expands the toolbox of DNA-functionalized nanoparticles currently available.

Self-hybridisation between interband transitions and Mie modes in dielectric nanoparticles

Abstract We discuss the possibility of self-hybridisation in high-index dielectric nanoparticles, where Mie modes of electric or magnetic type can couple to the interband transitions of the material, leading to spectral anticrossings. Starting with an idealised system described by moderately high constant permittivity with a narrow Lorentzian, in which self-hybridisation is visible for both plane-wave and electron-beam excitation, we embark on a quest for realistic systems where this effect should be visible. We explore a variety of spherical particles made of traditional semiconductors such as Si, GaAs, and GaP. With the effect hardly discernible, we identify two major causes hindering observation of self-hybridisation: the very broad spectral fingerprints of interband transitions in most candidate materials, and the significant overlap between electric and magnetic Mie modes in nanospheres. We thus depart from the spherical shape, and show that interband–Mie hybridisation is indeed feasible in the example of GaAs cylinders, even with a simple plane-wave source. This so-far unreported kind of polariton has to be considered when interpreting experimental spectra of Mie-resonant nanoparticles and assigning modal characters to specific features. On the other hand, it has the potential to be useful for the characterisation of the optical properties of dielectric materials, through control of the hybridisation strength via nanoparticle size and shape, and for applications that exploit Mie resonances in metamaterials, highly-directional antennas, or photovoltaics.

Controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force of femtosecond laser pulses

Nanoparticles made of different materials usually support optical resonances in the visible to near infrared spectral range, such as the localized surface plasmons observed in metallic nanoparticles and the Mie resonances observed in dielectric ones. Such optical resonances, which are important for practical applications, depend strongly on the morphologies of nanoparticles. Laser irradiation is a simple but effective way to modify such optical resonances through the change in the morphology of a nanoparticle. Although laser-induced shaping of metallic nanoparticles has been successfully demonstrated, it remains a big challenge for dielectric nanoparticles due to their larger Young’s modulus and smaller thermal conductivities. Here, we proposed and demonstrated a strategy for realizing controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force induced by femtosecond laser pulses. It was found that both Si and Ge nanoparticles can be lit up by resonantly exciting the optical resonances with femtosecond laser pulses, leading to the luminescence burst when the laser power exceeds a threshold. In addition, the morphologies of Si and Ge nanoparticles can be modified by utilizing the giant absorption force exerted on them and the reduced Young’s modulus at high temperatures. The shape transformation from sphere to ellipsoid can be realized by laser irradiation, leading to the blueshifts of the optical resonances. It was found that Si and Ge nanoparticles were generally elongated along the direction parallel to the polarization of the laser light. Controllable shaping of Si and Ge can be achieved by deliberately adjusting the excitation wavelength and the laser power. Our findings are helpful for understanding the giant absorption force of femtosecond laser light and are useful for designing nanoscale photonic devices based on shaped high-index nanoparticles.

Anisotropic Purcell Effect and Quantum Interference in Fractal Aggregates of Nanoparticles

We study theoretically the emergence of an anisotropic Purcell factor in random two-dimensional fractal aggregates of nanoparticles. These nanoparticles can either be metallic nanoparticles made of silver, which exhibit surface plasmon resonances, or high-index dielectric nanoparticles like silicon, which possess optical Mie resonances. To calculate the spontaneous emission rates of a quantum emitter, we utilize the electromagnetic Green’s tensor within the framework of the coupled-dipole method. Our findings reveal that the Purcell factor exhibits spatial variations, with certain regions, referred to as hot spots, displaying high values for dipoles oriented within the plane of the fractal aggregate, while dipoles oriented vertically to the aggregate have values close to unity. This anisotropy in the Purcell factor leads to significant quantum interference effects in the spontaneous emission paths of multi-level quantum emitters. As a consequence of this quantum interference, we demonstrate the occurrence of population trapping in a V-type quantum emitter embedded within a fractal aggregate of nanoparticles which cannot otherwise take place if the emitter is placed in vacuum.

Directional surface plasmon polariton scattering by single low-index dielectric nanoparticles: simulation and experiment

Directionally scattered surface plasmon polaritons (SPPs) promote the efficiency of plasmonic devices by limiting the energy within a given spatial domain, which is one of the key issues to plasmonic devices. Benefitting from the magnetic response induced in high-index dielectric nanoparticles, unidirectionally scattered SPPs have been achieved via interference between electric and magnetic resonances excited in the particles. Yet, as the magnetic response in low-index dielectric nanoparticles is too weak, the directionally scattered SPPs are hard to detect. In this work, we demonstrate forward scattered SPPs in single low-index polystyrene (PS) nanospheres. We numerically illustrate the excitation mechanism of plasmonic induced electric and magnetic multipole modes, as well as their contributions to forward SPP scattering of single PS nanospheres. We also simulate the SPP scattering field distribution obtaining a forward-to-backward scattering intensity ratio of 50.26:1 with 1 μm PS particle. Then the forward scattered SPPs are experimentally visualized by Fourier transforming the real-space plasmonic imaging to k-space imaging. The forward scattered SPPs from low-index dielectric nanoparticles pave the way for SPP direction manipulation by all types of nanomaterials.

Controlling the magnetic and electric responses of dielectric nanoparticles via near-field coupling

Among the two materials families used in nanophotonics, the fundamental mode for metal nanostructures is electric, while that for dielectric nanostructures is magnetic. Consequently, the optical properties of hybrid dimers that incorporate both materials have prompted significant research. Here, we study the optical response of such hybrid dimers with the coupled electric and magnetic dipoles method, and demonstrate how their electromagnetic interactions can be used to modulate the electric and magnetic responses. The conditions for the complete suppression of the magnetic dipole in the visible range for high refractive index dielectric nanoparticles are described. The electric and magnetic responses can be enhanced or reduced by positioning the dielectric particle in the nodes of the standing wave formed by the metallic particle. This controlled near-field interaction provides a handle on the near- and far-field responses of the system, with possible applications such as sensing and optical switches.

All-dielectric resonant metaphotonics: opinion

Nanophotonics underpins the future technologies for creating reconfigurable optical circuitry for high-performing optical devices, ultrafast computers, and very compact efficient biosensors integrated on optics-driven chips with densely packed components. To localize light on the subwavelength scales, plasmonics was suggested as the only available platform. However, the recently emerged field of Mie resonant metaphotonics (or Mie-tronics) provides novel opportunities for subwavelength optics employing resonances in high-index dielectric nanoparticles and structured surfaces. Here we present our view on this rapidly developing area of research and discuss recent advances and future trends in a design of all-dielectric structures with high quality factor (Q factor) resonances for efficient spatial and temporal control of light.

Multifunctional and Transformative Metaphotonics with Emerging Materials.

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

Nonradiating sources for efficient wireless power transfer

Abstract Nonradiating sources of energy realized under a wave scattering on high-index dielectric nanoparticles have attracted a lot of attention in nano-optics and nanophotonics. They do not emit energy to the far-field, but simultaneously provides strong near-field energy confinement. Near-field wireless power transfer technologies suffer from low efficiency and short operation distance. The key factor to improve efficiency is to reduce the radiation loss of the resonators included in the transmitter and receiver. In this paper, we develop a wireless power transfer system based on nonradiating sources implemented using colossal permittivity dielectric disk resonator and a subwavelength metal loop. We demonstrate that this nonradiating nature is due to the hybrid anapole state originated by destructive interference of the fields generated by multipole moments of different parts of the nonradiating source, without a contribution of toroidal moments. We experimentally investigate a wireless power transfer system prototype and demonstrate that higher efficiency can be achieved when operating on the nonradiating hybrid anapole state compared to the systems operating on magnetic dipole and magnetic quadrupole modes due to the radiation loss suppression.

High-Q Fano Terahertz resonance based on bound states in the continuum in All-dielectric metasurface

In recent years, the concept of bound states in the continuum (BICs) in metasurfaces has become a research focus, providing an important physical mechanism for engineering high-quality-factor (Q-factor) optical resonances in high-index dielectric nanoparticles and all-dielectric metasurfaces. In this paper, the basic concept of BICs is first introduced, and then, a sampling of relevant literature on the application of BICs in terahertz (THz) metasurfaces is summarized. An approach that allows the effective control of the appearance of sharp resonances in metasurfaces associated with the BICs is discussed. A novel structure is then proposed, and an interesting attribute of BICs with a sharp resonance dip is demonstrated. High Q factors can manifest due to high confinement of light and near-field enhancement of the physical mechanism, where BICs transform into quasi-BICs due to the width adjustment of the crystals. Moreover, the transmission is stable, despite the variations of the structural parameters. When this metasurface is used as a refractive-index sensor, a sensitivity of 465.74 GHz/RIU and a figure of merit of 32,984 are obtained, indicating excellent performance.

Light absorption enhancement in ultrathin perovskite solar cells using light scattering of high-index dielectric nanospheres.

Arrays of high-index dielectric nanoparticles supporting both electrical and magnetic resonances have gained increasing attention for their excellent light-trapping (LT) effects, thus greatly improving the performance of ultrathin solar cells. This work explores front-located, high-index dielectric subwavelength nanosphere arrays as an efficient and broadband LT structure patterned on top of an ultrathin perovskite solar cell (PSC) for a greatly enhanced absorption. Combined strong light scattering and anti-reflection properties achieved by optimized geometrical parameters of the LT structure lead to a broadband absorption enhancement in the ultrathin thickness of a photoactive layer (100 nm) yielding the short-circuit current density (Jsc) of 18.7 mA/cm2, which is 31.7% higher than that of a planar counterpart. Moreover, effects of the LT structure on far-field radiation patterns, scattering cross-sections, multipoles' contributions, and asymmetry parameters along with the incidence angle and polarization dependence are investigated. The present strategy could be applied to diverse applications, such as other ultrathin or semitransparent solar cells, absorbers and photodetectors.

Metal, dielectric and hybrid nanoantennas for enhancing the emission of single quantum dots: A comparative study

The confinement of electromagnetic energy to subwavelength volumes through nanoscale antennas can be used to enhance the spontaneous emission of quantum emitters. With this aim, different configurations of metallic and high refractive index dielectric nanoparticles have been explored. Here, we carry out a comparative analysis of planar metallic, high refractive index dielectric, and hybrid nanoantennas considering three different parameters: the Purcell factor enhancement, radiation efficiency, and directionality properties. We focus our study on different geometries and material combinations of a dimer of cylinders. A dimer made of two gold nanocylinders is the most promising candidate for improving the spontaneous emission. While most previous works have paid attention to the redirection of the scattered emission in the nanoparticle plane, our proposed nanostructure of two large gold cylinders (R=λ/4) emits most of the radiation upwards. This effect is due to the strong quadrupolar electric contribution to the resonant mode. With the aim to further improve the directionality properties, additional silicon nanocylinders are used as directors of the scattered radiation, increasing the directivity by a factor of 2.4 with respect to the gold dimer without directors. All in all, a hybrid structure composed of a gold dimer and silicon nanoparticles is proposed to enhance the spontaneous emission of a single quantum dot and to govern its emission pattern. The results shown in this work could be useful in fluorescence enhancement or in quantum photonics. They are particularly interesting for the development of single-photon sources based on quantum dots and other nanoscale emitters.

Novel non-plasmonic nanolasers empowered by topology and interference effects

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

Broadband Unidirectional Forward Scattering with High Refractive Index Nanostructures: Application in Solar Cells.

High refractive index dielectric (HRID) nanoparticles are a clear alternative to metals in nanophotonic applications due to their low losses and directional scattering properties. It has been demonstrated that HRID dimers are more efficient scattering units than single nanoparticles in redirecting the incident radiation towards the forward direction. This effect was recently reported and is known as the “near zero-backward” scattering condition, attained when nanoparticles forming dimers strongly interact with each other. Here, we analyzed the electromagnetic response of HRID isolated nanoparticles and aggregates when deposited on monolayer and graded-index multilayer dielectric substrates. In particular, we studied the fraction of radiation that is scattered towards a substrate with known optical properties when the nanoparticles are located on its surface. We demonstrated that HRID dimers can increase the radiation emitted towards the substrate compared to that of isolated nanoparticles. However, this effect was only present for low values of the substrate refractive index. With the aim of observing the same effect for silicon substrates, we show that it is necessary to use a multilayer antireflection coating. We conclude that dimers of HRID nanoparticles on a graded-index multilayer substrate can increase the radiation scattered into a silicon photovoltaic wafer. The results in this work can be applied to the design of novel solar cells.

Colloidal Mie Resonators for All‐Dielectric Metaoptics

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

Constructive and destructive interference of Kerker-type scattering in an ultrathin silicon Huygens metasurface

High refractive index dielectric nanoparticles have provided a new platform for exotic light manipulation through the interference of multipole modes. The Kerker effect is one example of a Huygens source design. Rather than exploiting interference between the electric dipole and magnetic dipole, as in many conventional Huygens source designs, we explore Kerker-type suppressed backward scattering mediated by the dominant electric dipole, toroidal dipole and magnetic quadrupole. These modes are provided by a designed and fabricated CMOS compatible ultra-thin Silicon nanodisk metasurface with a suppressed magnetic dipole contribution, and verified through multipole decomposition. The non-trivial substrate effect is considered using a semi-analytical transfer matrix model. The model successfully predicts the observed reflection dip. By applying a general criterion for constructive and destructive interference, it is shown that while constructive interference occurs between the electric and toroidal dipole contributions, the experimentally observed suppressed backward Kerker-type scattering arises from the destructive interference between backward scattered contributions due to the total electric dipole and the magnetic quadrupole. Our study paves the way towards new types of Huygens sources or metasurface design, such as for peculiar transverse Kerker scattering.

High-Q Dielectric Mie-Resonant Nanostructures (Brief Review)

Future technologies underpinning high-performance optical communications, ultrafast computations and compact biosensing will rely on densely packed reconfigurable optical circuitry based on nanophotonics. For many years, plasmonics was considered as the only available platform for nanoscale optics, but the recently emerged novel field of Mie resonant metaphotonics provides more practical alternatives for nanoscale optics by employing resonances in high-index dielectric nanoparticles and structures. In this mini-review we highlight some recent trends in the physics of dielectric Mie-resonant nanostructures with high quality factor (Q factor) for efficient spatial and temporal control of light by employing multipolar resonances and the bound states in the continuum. We discuss a few applications of these concepts to nonlinear optics, nanolasers, subwavelength waveguiding, and sensing.

Strong second-harmonic generation in dielectric optical nanoantennas resulting from the hybridization of magnetic dipoles and lattice resonances

Magnetic dipole resonances supported by high refractive index dielectric nanoparticles have appeared recently as a new alternative to plasmonic nanostructures to manipulate light–matter interactions. The volume field enhancement within the particles significantly boosts some nonlinear processes, mitigating the need for conventional phase-matching conditions. In this paper, we show that by using an AlGaAs nanocylinder array whose lattice resonance spectrally matches the magnetic resonance of individual particles, the two resonances will couple with each other, leading to their strong hybridization and helping achieve a stronger circulation of the electric fields within the particles. As a result of this mode hybridization, at a low pump laser power intensity of 0. 053 G W / c m 2 , the efficiency of second-harmonic generation can be further enhanced by more than 2 orders of magnitude compared to that from an individual nanocylinder.

Multipole analysis of substrate-supported dielectric nanoresonator metasurfaces via the T -matrix method

Substrates, and layered media in general, are ubiquitous and affect the properties of any object in their vicinity. However, their influence is, in an arbitrary framework, challenging to quantify analytically, especially for large arrays which additionally escape explicit numerical treatment due to the computational burden. In this work, we utilize a versatile $T$-matrix-based framework to generalize the coupled multipole model toward arbitrarily high multipole orders and substrate-supported arrays. We then employ it to study substrate-supported random/amorphous arrays of high index dielectric nanoparticles which are of wide interest due to relatively low losses and a highly tunable optical response, making them promising elements for nanophotonic devices. We discuss how multipole coupling rules evolve in the presence of a substrate in amorphous arrays for three interaction mechanisms: direct coupling between particles, substrate-mediated interparticle coupling, and substrate-mediated self-coupling. We show how the interplay of array density, distance from the substrate, and the latter's refractive index determine the optical response of an array. As an example, we use this framework to analyze refractometric sensing with substrate-supported arrays and demonstrate that the substrate plays a crucial role in determining the array sensitivity.

Bound States in the Continuum in Anisotropic Plasmonic Metasurfaces.

The concept of optical bound states in the continuum (BICs) currently drives the field of dielectric resonant nanophotonics, providing an important physical mechanism for engineering high-quality (high-Q) optical resonances in high-index dielectric nanoparticles and structured dielectric metasurfaces. For structured metallic metasurfaces, realization of BICs remains a challenge associated with strong dissipative losses of plasmonic materials. Here, we suggest and realize experimentally anisotropic plasmonic metasurfaces supporting high-Q resonances governed by quasi-BIC collective resonant modes. Our metasurfaces are composed of arrays of vertically oriented double-pillar meta-molecules covered by a thin layer of gold. We engineer quasi-BIC modes and observe experimentally sharp resonances in mid-IR reflectance spectra. Our work suggests a direct route to boost the resonant field enhancement in plasmonic metasurfaces via combining a small effective mode volume of plasmonic systems with engineered high-Q resonances provided by the BIC physics, with multiple applications to enhance light-matter interaction for nano-optics and quantum photonics.