High-index Dielectric Nanoantennas Research Articles

Multimode hybrid gold-silicon nanoantennas for tailored nanoscale optical confinement.

High-index dielectric nanoantennas, which provide an interplay between electric and magnetic modes, have been widely used as building blocks for a variety of devices and metasurfaces, both in linear and nonlinear regimes. Here, we investigate hybrid metal-semiconductor nanoantennas, consisting of a multimode silicon nanopillar core coated with a gold layer, that offer an enhanced degree of control over the mode selection and confinement, and emission of light on the nanoscale exploiting high-order electric and magnetic resonances. Cathodoluminescence spectra revealed a multitude of resonant modes supported by the nanoantennas due to hybridization of the Mie resonances of the core and the plasmonic resonances of the shell. Eigenmode analysis revealed the modes that exhibit enhanced field localization at the gold interface, together with high confinement within the nanopillar volume. Consequently, this architecture provides a flexible means of engineering nanoscale components with tailored optical modes and field confinement for a plethora of applications, including sensing, hot-electron photodetection and nanophotonics with cylindrical vector beams.

Large-scale controlled coupling of single-photon emitters to high-index dielectric nanoantennas by AFM nanoxerography.

Improving the brightness of single-photon sources by means of optically resonant nanoantennas is a major stake for the development of efficient nanodevices for quantum communications. We demonstrate that nanoxerography by atomic force microscopy makes possible the fast, robust and repeatable positioning of model quantum nanoemitters (nitrogen-vacancy NV centers in nanodiamonds) on a large-scale in the gap of silicon nanoantennas with a dimer geometry. By tuning the parameters of the nanoxerography process, we can statistically control the number of deposited nanodiamonds, yielding configurations down to a unique single photon emitter coupled to these high index dielectric nanoantennas, with high selectivity and enhanced brightness induced by a near-field Purcell effect. Numerical simulations are in very good quantitative agreement with time-resolved photoluminescence experiments. A multipolar analysis reveals in particular all the aspects of the coupling between the dipolar single emitter and the Mie resonances hosted by these simple nanoantennas. This proof of principle opens a path to a genuine and large-scale spatial control of the coupling of punctual quantum nanoemitters to arrays of optimized optically resonant nanoantennas. It paves the way for future fundamental studies in quantum nano-optics and toward integrated photonics applications for quantum technologies.

(Invited) High Index Dielectric Nanoantennas for Light Management, Controlled Photoluminescence of Quantum Emitters, and Semiconductor-Based Plasmonics

Recently, an alternative to plasmonics emerged with high-refractive index dielectric nanostructures (e.g. silicon, n≈4 in the visible range), which offer the same kind of applications as nanostructures of noble metals, but with interesting advantages and despite lower field enhancement and confinement volume. Thus, they can be used to enhance and control light scattering efficiency, nonlinear emission, Purcell effect, etc. Furthermore, silicon nanostructures offer several key advantages: absorption losses are far weaker than in metals for wavelengths longer than the direct band gap, presence of strong magnetic resonances much more difficult to obtain in metals, and access to semiconductor (CMOS) technology for reproducible large-scale nanostructure fabrication.The first mean for controlling optical properties is given by manipulating Mie resonances instead of local surface plasmon resonances (LSPR). The Mie resonances can be adjusted by modifying the size, shape, and material of those nanostructures.The other mean is obtained by doping the nanostructures generating free carriers, thus leading to tunable LSPRs as function of dopant concentration (contrary to metals which have a fixed free electron concentrations).We present two examples describing the interest of using silicon nanoantennas, which are: (i) emission rate modification of quantum emitters placed in the silicon nanoantenna near-field (we focus also on the important role of both electric and magnetic components of light), and (ii) tunable Si-based plasmonics in the MIR to NIR spectral range by tuning dopant concentration and structural parameters of Si nanostructures.We show the effect of a Si nanoantenna on the spontaneous emission of quantum emitters placed in its vicinity. The emission rate corresponds to the local density of photonic states (LDOS), which is modified by the nanostructure. Thus, it can be controlled (enhancement or quenching) by properly designing the nanoantenna. For application in field-enhanced spectroscopy and single molecule detection, the goal is to obtain the highest enhancement. We discuss results of photoluminescence enhancement of various emitters placed around simple Si nanoantennas, with a special focus on the photoluminescence of Eu3+-doped thin films deposited on the Si nanoantennas. These rare earth ions exhibit both electric dipole (ED) and magnetic dipole (MD) transitions of nearly equal strength. We show that these ED and MD transitions are very sensitive to the electric and magnetic LDOS, respectively. This can be evidenced using Si nanoantennas, which support both electric and magnetic resonances of comparable strength. Due to the very high index, they also give the unique opportunity to separate and redistribute in the nearfield the energy of the magnetic and electric parts of the electromagnetic field, otherwise inextricably connected in the far field.In a second part, we present theoretical results about doped Si nanostructures based on the Green Dyadic Method initially used for studying optical properties in metal nanostructures. We show that cuboid Si nanostructures of a few tenths of nm in each dimension, with carrier concentration around 2.6 1021 cm-3, give LSPR close to the telecom wavelength at 1.54 μm. Experimentally, an original strategy has been developed to incorporate and active dopant at concentrations above 1020 cm-3. Such strategy involves out-of-equilibrium annealing by using nanosecond laser pulses and liquid phase epitaxy. We can thus expect tuning the LSPR frequency in the MIR to NIR range depending on the dopant concentration and the nanostructure shape. Preliminary experimental results are shown and compared to theory.

Active and Tunable Nanophotonics With Dielectric Nanoantennas

In recent years, high-index dielectric nanoantennas supporting Mie resonances have emerged as a promising platform to create low-loss nanophotonic devices. The multipolar resonances naturally supported by this kind of nanoantennas allow designing systems in which the out-coupling radiation, and in particular the directivity, can be tailored at will. Moreover, unlike plasmonic counterparts, local electromagnetic fields in resonant dielectric nanoparticles concentrate inside the structures rather than outside. This allows enhancing light–matter interactions in their volume and boosting their linear and nonlinear optical properties by choosing suitable semiconductors as the material platform to build them. In this article, we will discuss recent results on active and tunable nanophotonics based on resonant dielectric and semiconductor nanostructures. Specifically, we will focus on two main topics: active tuning of dielectric nanoantennas and enhancement and spectral shaping of light emission using dielectric nanoantennas, including lasing. Finally, future research directions and proposed practical applications will also be discussed.

Coherent Multiphoton Control of Gallium Phosphide Nanodisk Resonances

High-index dielectric nanoantennas have recently emerged as promising alternatives to plasmonic nanoantennas for concentrating and manipulating light at the nanoscale. For example, gallium phosphid...

Plasmonic versus All-Dielectric Nanoantennas for Refractometric Sensing: A Direct Comparison

In comparison to nanoplasmonic structures, resonant high-index dielectric nanoantennas hold several advantages that may benefit nanophotonic applications, including CMOS compatibility and low ohmic losses. One such application area might be label-free refractometric sensing, where changes in individual antenna resonance properties are used to quantify changes in the surrounding refractive index, for example, due to biomolecular binding. Here, we analyze and compare the sensing performance of silicon and gold nanodisks using a common and unbiased testing framework. We find that the all-dielectric system is fully capable of effectively monitoring small changes in bulk refractive index and biomolecular coverage, but the sensitivity is five to ten times lower than the plasmonic counterpart. However, this drawback is partly compensated for by a more linear response to adsorbate layer thickness changes and an approximately four times smaller susceptibility to photothermal heating. Finally, dielectric sensors may show promise if certain strategies are employed to improve their performance, which could thus bridge the gap between the two systems.

Broadband forward scattering from dielectric cubic nanoantenna in lossless media.

Dielectric photonics platform provides unique possibilities to control light scattering via utilizing high-index dielectric nanoantennas with peculiar optical signatures. Despite the intensively growing field of all-dielectric nanophotonics, it is still unclear how surrounding media affect scattering properties of a nanoantenna with complex multipole response. Here, we report on light scattering by a silicon cubic nanoparticle embedded in lossless media, supporting optical resonant response. We show that significant changes in the scattering process are governed by the electro-magnetic multipole resonances, which experience spectral red-shift and broadening over the whole visible and near-infrared spectra as the indices of media increase. Most interestingly, the considered nanoantenna exhibits the broadband forward scattering in the visible and near-infrared spectral ranges due to the Kerker-effect in high-index media. The revealed effect of broadband forward scattering is essential for highly demanding applications in which the influence of the media is crucial such as health-care, e.g., sensing, treatment efficiency monitoring, and diagnostics. In addition, the insights from this study are expected to pave the way toward engineering the nanophotonic systems including but not limited to Huygens-metasurfaces in media within a single framework.

Compact and high-efficient wavelength demultiplexing coupler based on high-index dielectric nanoantennas**Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0205700 and 2015CB932403) and the National Natural Science Foundation of China (Grant Nos. 11174062, 51472057, and 21790364).

Wavelength demultiplexing waveguide couplers have important applications in integrated nanophotonic devices. Two of the most important indicators of the quality of a wavelength demultiplexing coupler are coupling efficiency and splitting ratio. In this study, we utilize two asymmetric high-index dielectric nanoantennas directly positioned on top of a silicon-on insulator waveguide to realize a compact wavelength demultiplexing coupler in a communication band, which is based on the interference of the waveguide modes coupled by the two nanoantennas. We add a Au substrate for further increasing the coupling efficiency. This has constructive and destructive influences on the antenna’s in-coupling efficiency owing to the Fabry–Perot (FP) resonance in the SiO2 layer. Therefore, we can realize a wavelength demultiplexing coupler with compact size and high coupling efficiency. This coupler has widespread applications in the areas of wavelength filters, on-chip signal processing, and integrated nanophotonic circuits.

(Invited) High-Index Dielectric Nanoantennas for Light Management, Nonlinear Optics, and Controlled Photoluminescence of Quantum Emitters

Over the last twenty years, nanoscale optics was mainly driven by plasmonics since noble metal nanostructures, thanks to localized surface plasmon resonances (LSPR), can be used to manipulate, concentrate or redirect visible light. Such properties have led to numerous actual or potential applications in integrated optics, sensors, nonlinear optics, field-enhanced spectroscopies, or photovoltaics. Recently, an alternative to plasmonics emerged with high refractive index dielectric nanostructures (e.g. silicon, n≈4 in the visible range), which offer the same range of applications as plasmonics by manipulating Mie optical resonances instead of LSPRs. The Mie resonances can be adjusted by modifying the size, shape, and material of those nanostructures. Furthermore, high index dielectric nanostructures offer several key advantages: absorption losses are far weaker than in metals for wavelengths longer than the direct band gap (near UV to near IR for Si), presence of strong magnetic resonances much more difficult to obtain in metals, and access to semiconductor (CMOS) technology for nanostructure fabrication (essential for easier transfer towards applications). We present three examples describing the interest of using silicon nanoantennas: enhancement and directionality control of light scattering, enhancement and polarization control of second harmonic generation (SHG), and emission rate modification of quantum emitters placed in the silicon nanoantenna near-field. We focus also on the important role of both electric and magnetic components of light. First, we show that silicon nanospheres and nanowires are able to scatter and/or absorb very efficiently visible light. Furthermore, due to a kind of Fano process involving the interference between a broad resonance and a sharp higher mode simultaneously excited, the direction of light scattering can be very different (either dominating forward or backward scattering). Second, we investigate the nonlinear emission of silicon nanowires (Si-NWs). We show that SHG is strongly increased if a Mie optical mode corresponding to the fundamental (laser) wavelength is supported by the nanowire. According to nonlinear scattering theory and the so-called Miller's rule, the highest SHG is predicted for an optimum overlap of resonant modes and linear susceptibilities, at both the fundamental and harmonic wavelengths, respectively. Contrary, no SHG can be detected in absence of any resonance. This behavior, and experimental SHG mappings of individual Si-NWs, are well reproduced by numerical simulations (Green Dyadic Method - GDM). We also show that, depending on the Si-NW diameter, different surface of bulk-like contributions to the SHG are addressed. This also results in different polarizations of SHG as function of both the Si-NW diameter and incident light polarization. Third, we show the effect of a high index dielectric nanoantenna on the spontaneous emission of quantum emitters placed in its vicinity. The emission rate corresponds to the local density of photonic states (LDOS), which is modified by the nanostructure. Thus, it can be controlled (enhancement or quenching) by properly designing the nanoantenna. For application in field-enhanced spectroscopy and single molecule detection, the goal is to obtain the highest enhancement. We discuss results of photoluminescence enhancement of molecules or quantum dots placed around simple Si nanoantennas (nanowires, nanodisk dimers). We also present preliminary results of the photoluminescence of Eu3+-doped clusters deposited on top of Si nanoantennas. These rare earth ions exhibit both electric dipole (ED) and magnetic dipole (MD) transitions of nearly equal strength. We show that these ED and MD transitions are very sensitive to the electric and magnetic LDOS, respectively. This can be evidenced using Si nanoantennas, which support both electric and magnetic resonances of comparable strength. Due to the very high index, they also give the unique opportunity to separate and redistribute in the nearfield the energy of the magnetic and electric parts of the electromagnetic field, otherwise inextricably connected in the farfield.

Dual-band unidirectional forward scattering with all-dielectric hollow nanodisk in the visible.

High-index dielectric nanoantennas have become an emerging branch of optical nanoantennas, essentially due to their low loss. These types of nanoantennas can achieve both forward and backward unidirectional scattering, enabled by electric dipole and magnetic dipole interaction. Here, we show that the scattering directionality can be further enhanced if higher-order moments are properly balanced and reach the generalized Kerker condition at two different wavelengths in an all-dielectric hollow nanodisk. Moreover, putting the nanodisks in an array of transverse configuration can enhance the unidirectionality to be needle-like, with the main lobe angular beam width α<15°. Finally, we show that such unidirectional radiation properties can be maintained for a local electric dipole source.

Efficient Third Harmonic Generation from Metal-Dielectric Hybrid Nanoantennas.

High refractive index dielectric nanoantennas are expected to become key elements for nonlinear nano-optics applications due to their large nonlinearities, low energy losses, and ability to produce high electric field enhancements in relatively large nanoscale volumes. In this work, we show that the nonlinear response from a high-index dielectric nanoantenna can be significantly improved by adding a metallic component to build a metal-dielectric hybrid nanostructure. We demonstrate that the plasmonic resonance of a Au nanoring can boost the anapole mode supported by a Si nanodisk, strongly enhancing the electric field inside the large third-order susceptibility dielectric. As a result, a high third harmonic conversion efficiency, which reaches 0.007% at a third harmonic wavelength of 440 nm, is obtained. In addition, by suitably modifying geometrical parameters of the hybrid nanoantenna, we tune the enhanced third harmonic emission throughout the optical regime. Coupling metallic and dielectric nanoantennas to expand the potential of subwavelength structures opens new paths for efficient nonlinear optical effects in the visible range on the nanoscale.