Nanoparticles of high refractive index materials can possess strong magnetic polarizabilities and give rise to artificial magnetism in the optical spectral range. While the response of individual dielectric or metal spherical particles can be described analytically via multipole decomposition in the Mie series, the influence of substrates, in many cases present in experimental observations, requires different approaches. Here, the comprehensive numerical studies of the influence of a substrate on the spectral response of high-index dielectric nanoparticles were performed. In particular, glass, perfect electric conductor, gold, and hyperbolic metamaterial substrates were investigated. Optical properties of nanoparticles were characterized via scattering cross-section spectra, electric field profiles, and induced electric and magnetic moments. The presence of substrates was shown to have significant impact on particle's magnetic resonances and resonant scattering cross-sections. Variation of substrate material provides an additional degree of freedom in tailoring optical properties of magnetic multipoles, important in many applications.

Magnetic dipole radiation tailored by substrates: numerical investigation

A graphene-based cylindrical hybrid surface plasmon polariton waveguide, composed of a silicon nanowire core surrounded by a silica layer and then a graphene layer, is investigated using the finite-difference time-domain method. The analytical solutions and the numerical simulation show that an ultra-small mode area and a large propagation length can be achieved with this waveguide. Utilizing the perturbation theory of coupled mode, we demonstrate that the six lowest-order coupling modes originate from the coupling of the three lowest-order single-waveguide modes, and the m = 1 order yy-coupling mode possesses the maximum coupling length and the minimum crosstalk. This waveguide can be used for photonic integrated circuits in the mid-infrared range.

Analysis of Mid-Infrared Surface Plasmon Modes in a Graphene-Based Cylindrical Hybrid Waveguide

The goal of confining light at the deep-subwavelength scale while retaining moderate attenuation has been pur- sued for years in the field of plasmonics. However, few fea- sible configurations at present are excellent at balancing the tradeoff between confinement and loss. This work proposes to overcome the above limitation by using hybrid wedge structures, which consist of triangular metal wedges loaded with nanometric low/high-index dielectric claddings. Owing to the superior guiding properties of wedge plasmons in con- junction with high refractive index contrast near wedge tips, the modal sizes can be squeezed into significantly smaller spaces than those of their conventional wedge and planar hybrid counterparts, while simultaneously featuring propaga- tion distances over tens of micrometers at telecommunica- tion wavelengths. Studies on the evolution from a single metallic wedge to semiconductor-insulator-metal wedge(s) re- veal strategies for continuous improvement of the optical per- formance. Discussions concerning practical issues including crosstalk and mode excitation have further elucidated their po- tential in building high-performance nanophotonic components.

Deep‐subwavelength light confinement and transport in hybrid dielectric‐loaded metal wedges

A graphene-based long-range surface plasmon polariton (LRSPP) hybrid waveguide, which is composed of two identical outer graphene nanoribbons and two identical inner silica layers symmetrically placed on both sides of a silicon layer, is investigated using the finite-difference time-domain method. By combining the simulated results with the coupled mode perturbation theory, we demonstrate that the LRSPP and short-range SPP (SRSPP) modes originate from the coupling of the same modes of the two graphene nanoribbons. For the LRSPP mode, an ultra-long propagation length (~10 μm) and an ultra-small mode area (~10-7A0, where A0 is the diffraction-limited mode area) can be simultaneously achieved. This waveguide can be used for future photonic integrated circuits functional in the mid-infrared range.

Graphene-based long-range SPP hybrid waveguide with ultra-long propagation length in mid-infrared range.

We examine the question of coherent perfect absorption (CPA) of single photons, and more generally, of the quantum fields by a macroscopic medium. We show the CPA of path entangled single photons in a Fabry-Perot interferometer containing an absorptive medium. The frequency of perfect absorption can be controlled by changing the interferometer parameters like the reflectivity and the complex dielectric constant of the material. We exhibit similar results for path entangled photons in micro-ring resonators. For entangled fields like the ones produced by a down converter the CPA aspect is evident in phase sensitive detection schemes such as in measurements of the squeezing spectrum.

Coherent perfect absorption of path entangled single photons

We study the surface plasmon modes in a silver double-nanowire system by employing the eigenmode analysis approach based on the finite element method. Calculated dispersion relations, surface charge distributions, field patterns and propagation lengths of ten lowest energy plasmon modes in the system are presented. These ten modes are categorized into three groups because they are found to originate from the monopole-monopole, dipole-dipole and quadrupole-quadrupole hybridizations between the two wires, respectively. Interestingly, in addition to the well studied gap mode (mode 1), the other mode from group 1 which is a symmetrically coupled charge mode (mode 2) is found to have a larger group velocity and a longer propagation length than mode 1, suggesting mode 2 to be another potential signal transporter for plasmonic circuits. Scenarios to efficiently excite (inject) group 1 modes in the two-wire system and also to convert mode 2 (mode 1) to mode 1 (mode 2) are demonstrated by numerical simulations.

Dispersion relation, propagation length and mode conversion of surface plasmon polaritons in silver double-nanowire systems.

Dispersion relation, propagation length and mode conversion of surface plasmon polaritons in silver double-nanowire systems

We propose to use double negative (DNG) metamaterial slabs to build effective super-absorbers and perfect nanodetectors for single divergent sources. We demonstrate by numerical simulations that an absorbing nanoparticle properly placed inside a DNG slab back-covered with a perfect electric conductor or perfect magnetic conductor mirror can absorb up to 100% radiation energy of a single dipole source placed outside the slab. Furthermore, we also show that even the simple DNG slab without any absorbing nanoparticle could be used as a perfect absorber for both plane and divergent beams. The proposed systems may focus the radiation in nanoscale and thus have applications in optical nanodevices for a variety of different purposes.

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Wei-Jin Zheng

Papers

Dispersion relation, propagation length and mode conversion of surface plasmon polaritons in silver double-nanowire systems.

Dispersion relation, propagation length and mode conversion of surface plasmon polaritons in silver double-nanowire systems

Metamaterial slab-based super-absorbers and perfect nanodetectors for single dipole sources.