Boosting magnetic field enhancement with radiative couplings of magnetic modes in dielectric nanostructures

Optical Cavity Design and Functionality for Molecular Strong Coupling

Optical cavity/molecule strong coupling offers attractive opportunities to modulate photochemical or photophysical processes. When atoms or molecules are placed in an optical cavity, they can coherently exchange photonic energy with optical cavity vacuum fields, entering the strong coupling interaction regime. Recent work suggests that the thermodynamic and kinetic properties of molecules can be significantly changed by strong coupling, resulting in the emergence of intriguing photochemical and photophysical phenomena. As more and more physico-chemical systems are studied under strong coupling conditions, optical cavities have also advanced in their sophistication, responsiveness, and (multi)functionality. In this review, we highlight some of these recent developments, particularly focusing on Fabry-Perot microcavities.

Control of light emission of quantum emitters coupled to silicon nanoantenna using cylindrical vector beams

Light emission of europium (Eu3+) ions placed in the vicinity of optically resonant nanoantennas is usually controlled by tailoring the local density of photon states (LDOS). We show that the polarization and shape of the excitation beam can also be used to manipulate light emission, as azimuthally or radially polarized cylindrical vector beam offers to spatially shape the electric and magnetic fields, in addition to the effect of silicon nanorings (Si-NRs) used as nanoantennas. The photoluminescence (PL) mappings of the Eu3+ transitions and the Si phonon mappings are strongly dependent of both the excitation beam and the Si-NR dimensions. The experimental results of Raman scattering and photoluminescence are confirmed by numerical simulations of the near-field intensity in the Si nanoantenna and in the Eu3+-doped film, respectively. The branching ratios obtained from the experimental PL maps also reveal a redistribution of the electric and magnetic emission channels. Our results show that it could be possible to spatially control both electric and magnetic dipolar emission of Eu3+ ions by switching the laser beam polarization, hence the near field at the excitation wavelength, and the electric and magnetic LDOS at the emission wavelength. This paves the way for optimized geometries taking advantage of both excitation and emission processes.

Giant localized electromagnetic field of highly doped silicon plasmonic nanoantennas

In this work, we present the analysis and design of an efficient nanoantenna sensor based on localized surface plasmon resonance (LSPR). A high refractive index dielectric nanostructure can exhibit strong radiation resonances with high electric field enhancement inside the gap. The use of silicon instead of metals as the material of choice in the design of such nanoantennas is advantageous since it allows the integration of nanoantenna-based structures into integrated-optoelectronics circuits manufactured using common fabrication methods in the electronic industry. It also allows the suggested devices to be mass-produced at a low cost. The proposed nanoantenna consists of a highly doped silicon nanorod and is placed on a dielectric substrate. Different shapes and different concentrations of doping for the nanoantenna structures that are resonant in the mid-infrared region are investigated and numerically analyzed. The wavelength of the enhancement peak as well as the enhancement level itself vary as the surrounding material changes. As a result, sensors may be designed to detect molecules via their characteristic vibrational transitions. The 3D FDTD approach via Lumerical software is used to obtain the numerical results. The suggested nanoantennas exhibit ultra-high local field enhancement inside the gap of the dipole structure.

Computational Investigation of Advanced Refractive Index Sensor Using 3-Dimensional Metamaterial Based Nanoantenna Array

Photonic researchers are increasingly exploiting nanotechnology due to the development of numerous prevalent nanosized manufacturing technologies, which has enabled novel shape-optimized nanostructures to be manufactured and investigated. Hybrid nanostructures that integrate dielectric resonators with plasmonic nanostructures are also offering new opportunities. In this work, we have explored a hybrid coupled nano-structured antenna with stacked multilayer lithium tantalate (LiTaO₃) and Aluminum oxide (Al₂O₃), operating at wavelength ranging from 400 nm to 2000 nm. Here, the sensitivity response has been explored of these nano-structured hybrid arrays. It shows a strong electromagnetic confinement in the separation gap (g) of the dimers due to strong surface plasmon resonance (SPR). The influences of the structural dimensions have been investigated to optimize the sensitivity. The designed hybrid coupled nanostructure with the combination of 10 layers of gold (Au) and Lithium tantalate (LiTaO₃) or Aluminum oxide (Al₂O₃) (five layers each) having height, h₁ = h₂ = 10 nm exhibits 730 and 660 nm/RIU sensitivity, respectively. The sensitivity of the proposed hybrid nanostructure has been compared with a single metallic (only gold) elliptical paired nanostructure. Depending on these findings, we demonstrated that a roughly two-fold increase in the sensitivity (S) can be obtained by utilizing a hybrid coupled nanostructure compared to an identical nanostructure, which competes with traditional sensors of the same height, (h). Our innovative novel plasmonic hybrid nanostructures provide a framework for developing plasmonic nanostructures for use in various sensing applications.

Magnetic toroidal dipole response in individual all-dielectric nanodisk clusters

Multipole electromagnetic resonances and their couplings are of crucial importance for both the fundamental understanding of light scattering by high-index all-dielectric nanostructures and lots of nanophotonic applications based on those nanostructures. Here, we show that magnetic dipole modes in a dielectric nanodisk cluster can easily form a magnetic toroidal dipole (MTD) mode. The cluster consists of five silicon nanodisks, where each nanodisk holds a magnetic dipole mode. These magnetic dipole modes can collectively couple with each other and form a MTD mode under suitable excitation. The MTD mode is confirmed by multipole expansion calculations and near field distributions, where two closed loops of magnetic field with opposite directions are seen. The response of the MTD is strong and comparable to that of a common electric dipole or magnetic dipole mode. It is also found that the MTD resonance is accompanied by an electric toroidal quadrupole mode in the cluster. The MTD mode is tunable by varying the geometries. We also fabricated silicon nanoparticle clusters and verified the MTD mode in the experiment. Our results illustrate the controllable excitation of strong high-order electromagnetic modes and these modes may open new opportunities for light manipulation at the nanoscale.

Resonant broadband unidirectional light scattering based on genetic algorithm

The spectrum overlapping of the radiative power between magnetic and electric dipole moments in nanoparticles can be used to realize unidirectional light scattering, which is promising for various kinds of applications. Nevertheless, it is still challenging to achieve such overlapping in a broadband manner. Herein, we propose that the combination of a genetic algorithm, Maxwell's equations, and electromagnetic multipole expansion can be used to design a nanoparticle that supports resonant broadband forward light scattering. Microwave experiments are performed to demonstrate our numerical results. The proposed method is quite general, and it can be straightforwardly generalized to design functional unidirectional scatters.

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

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

Manipulating the flow of light using Dirac-cone zero-index metamaterials

Metamaterials with a refractive index of zero exhibit properties that are important for integrated optics. Possessing an infinite effective wavelength and zero spatial phase change, zero-index metamaterials may be especially useful for routing on-chip photonic processes and reducing the footprint of nonlinear interactions. Zero-index has only been achieved recently in an integrated platform through a Dirac-cone dispersion, enabling some of these more exciting applications in an integrated platform. This paper presents an overview of Dirac-cone zero-index metamaterials, including the fundamental physics, history and demonstration in the optical regime, as well as current challenges and future directions.

Plasmonic nanoantenna-dielectric nanocavity hybrids for ultrahigh local electric field enhancement

A dielectric nanostructure with a high refractive index can exhibit strong optical resonances with considerable electric field enhancement around the entire structure volume. Here we show theoretically that a dielectric structure with this feature can boost the local electric field of a small plasmonic nanoantenna placed nearby. We construct a hybrid system of a plasmonic nanoantenna and a dielectric nanocavity, where the nanocavity is a concentric disk-ring structure with a lossless material n = 3.3 and the nanoantenna is a gold nanorod dimer. The resonant electric field enhancement at the gap center of the antenna in the hybrid structure reaches more than one order of magnitude higher than that of the individual antenna. The dielectric structure plays two roles in the hybrid system, namely the amplified excitation field and an environment causing the redshift of the antenna resonance. The hybrid configuration is applicable to the cases with various geometries and different materials of the hybrid system. Our results can find applications in enhanced nanoscale light-matter interactions such as surface-enhanced Raman scattering, nonlinear optics, and plasmon-exciton couplings.