Role of surface plasmon polaritons and other waves in the radiation of resonant optical dipole antennas

Spontaneous emission enhancement by rotationally-symmetric optical nanoantennas: impact of radially and axially propagating surface plasmon polaritons

The excitation and radiation properties of rotationally-symmetric optical nanoantennas are independent of the azimuth angle, which enables great convenience and superior performances in practical applications. However, for rotationally-symmetric nanoantennas, the physical mechanisms behind their resonance properties remain to be clarified. In this paper, firstly, for a simple single-nanocylinder-on-mirror antenna (S-antenna), we establish a first-principles-based semianalytical model of surface plasmon polariton (SPP) by considering an intuitive multiple-scattering process of the radially-propagating gap surface plasmon (RGSP) in the nanogap and the axially-propagating surface plasmon (ASP) on the nanocylinder. The model can comprehensively reproduce all the radiation properties of the S-antenna such as the total and radiative emission rates, SPP excitation rates, and far-field radiation pattern. The model indicates that when the antenna radius is small (respectively, large), the enhancement of spontaneous emission mainly results from the resonance of ASP (respectively, RGSP). To show the wide applicability of the SPP model along with its unveiled decisive role of the RGSP and ASP in the spontaneous emission enhancement for other rotationally-symmetric nanoantennas of cylindrical shapes, we extend the SPP model to a more complex ring-nanocylinder-on-mirror antenna (R-antenna) that supports two ASPs. Moreover, to provide an explicit explanation of the resonance properties of the R-antenna, we further establish a semianalytical model for the resonant modes (called quasinormal modes, QNMs) supported by the R-antenna based on the SPP model, which quantitatively reveals the role of the RGSP and ASP in forming the antenna resonant modes and the resultant enhancement of spontaneous emission.

Spontaneous Emission Enhancement by a Rectangular-Aperture Optical Nanoantenna: An Intuitive Semi-Analytical Model of Surface Plasmon Polaritons

The spontaneous-emission enhancement effect of a single metallic rectangular-aperture optical nanoantenna on a SiO2 substrate was investigated theoretically. By considering the excitation and multiple scattering of surface plasmon polaritons (SPPs) in the aperture, an intuitive and comprehensive SPP model was established. The model can comprehensively predict the total spontaneous emission rate, the radiative emission rate and the angular distribution of the far-field emission of a point source in the aperture. Two phase-matching conditions are derived from the model for predicting the resonance and show that the spontaneous-emission enhancement by the antenna comes from the Fabry–Perot resonance of the SPP in the aperture. In addition, when scanning the position of the point source and the aperture length, the SPP model does not need to repeatedly solve the Maxwell’s equations, which shows a superior computational efficiency compared to the full-wave numerical method.

Smart optical cross dipole nanoantenna with multibeam pattern

In this paper, an optical smart multibeam cross dipole nano-antenna has been proposed by combining the absorption characteristic of graphene and applying different arrangements of directors. By introducing a cross dipole nano-antenna with two V-shaped coupled elements, the maximum directivity of 8.79 dBi has been obtained for unidirectional radiation pattern. Also, by applying various arrangements of circular sectors as director, different types of radiation pattern such as bi- and quad-directional have been attained with directivities of 8.63 and 8.42 dBi, respectively, at the wavelength of 1550 nm. The maximum absorption power of graphene can be tuned by choosing an appropriate chemical potential. Therefore, the radiation beam of the proposed multibeam cross dipole nano-antenna has been controlled dynamically by applying a monolayer graphene. By choosing a suitable chemical potential of graphene for each arm of the suggested cross dipole nano-antenna without the director, the unidirectional radiation pattern shifts ± 13° at the wavelength of 1550 nm. Also, for the multibeam nano-antenna with different arrangements of directors, the bi- and quad-directional radiation patterns have been smartly modified to uni- and bi-directional ones with the directivities of 10.1 and 9.54 dBi, respectively. It is because of the graphene performance as an absorptive or transparent element for different chemical potentials. This feature helps us to create a multipath wireless link with the capability to control the accessibility of each receiver.

Tunable Plasmonic Resonances in TiN Nanorod Arrays

In this work, titanium nitride (TiN) nanorod arrays were fabricated using glancing angle deposition in a magnetron sputtering system. The deposition parameters, including the bias on the substrate and the flow rate of nitrogen, were varied to deposit various TiN nanorod arrays. Before glancing angle deposition was conducted, uniform TiN films were deposited and their permittivity spectra, for various deposition parameters, were obtained. The effect of the deposition parameters on the morphology of the nanorods is analyzed here. The polarization-dependent extinctance spectra of TiN nanorod arrays were measured and compared. Extinction, which corresponds to the longitudinal mode of localized surface plasmon resonance, can be significantly changed by tuning the N2 flow rate and substrate bias voltage during deposition.

Excitation and analyzation of different surface plasmon modes on a suspended Ag nanowire

Silver nanowires (AgNWs), as one of the most important plasmonic waveguides, can support several different plasmonic modes. These surface plasmon polariton (SPP) modes have different electric field distributions, effective mode areas, propagation lengths and losses and thus can be used for different applications, from efficiently collecting single photons to carrying quantum entanglement. Therefore, the excitation and analysis of these different SPP modes are of pivotal importance for the development of subwavelength optical devices. In this work, we investigate different SPP modes on a suspended AgNW adhered to a fiber taper. Theoretical simulations and experimental results show that the desired SPP modes can be selectively excited by adjusting either the polarization of the excitation light or the coupling length between the fiber taper and the AgNW. Moreover, fundamental and higher-order SPP modes can be distinguished by means of a far-field method. Our results not only enable convenient and controllable excitation of the desired SPP modes but also provide unique insight into the optical properties of plasmonic waveguides.

Semianalytical model for the electromagnetic enhancement by a rectangular nanowire optical antenna on metallic substrate

The electromagnetic enhancement by a metallic nanowire optical antenna on metallic substrate is investigated theoretically. By considering the excitation and multiple scattering of surface plasmon polaritons in the nanogap between the antenna and the substrate, we build up an intuitive and comprehensive model that provides semianalytical expressions for the electromagnetic field in the nanogap to achieve an understanding of the mechanism of electromagnetic enhancement. Our results show that antennas with short lengths that support the lowest order of resonance can achieve a high electric-field enhancement factor over a large range of incidence angles. Two phase-matching conditions are derived from the model for predicting the antenna lengths at resonance. Excitation of symmetric or antisymmetric localized surface plasmon resonance is further explained with the model. The model also shows superior computational efficiency compared to the full-wave numerical method when scanning the antenna length, the incidence angle, or the wavelength.

Surface enhancement of THz wave by coupling a subwavelength LiNbO3 slab waveguide with a composite antenna structure

Highly intense terahertz electromagnetic field and efficiently surface localized terahertz field in subwavelength volumes are of vital importance for terahertz photonics integration, also will greatly accelerate the development for integrated applications in biochemical sensing, imaging, terahertz spectroscopy, enhancement of nonlinear effects and even quantum research. In this paper, we achieved large terahertz field enhancement and surface field localization through depositing a pair of Au composite antennas on a LiNbO₃ subwavelength slab waveguide, which can serve as an excellent on-chip platform for terahertz research and application. The antennas consist of two opposing tip-to-tip triangles separated by a gap, and each triangle combines with a strip antenna. Time-resolved imaging and finite-difference time-domain method were used to resolve the characteristics of the designed antennas experimentally and simulatively. Through these methods, we demonstrated outstanding abilities of the platform: leading to a large electric field enhancement, concentrating almost full terahertz energy on the waveguide’s surface when they are resonant with the terahertz waves and tunable resonant frequency. These abilities make the subwavelength waveguide coupling with the composite antennas be able to sever as a good integrated device to identify terahertz-sensitive small objects, or an excellent platform to terahertz spectroscopy and quantum research.

Efficient Wideband Numerical Simulations for Nanostructures Employing a Drude-Critical Points (DCP) Dispersive Model

A highly efficient numerical approach for simulating the wideband optical response of nano-architectures comprised of Drude-Critical Points (DCP) media (e.g., gold and silver) is proposed and validated through comparing with commercial computational software. The kernel of this algorithm is the subdomain level discontinuous Galerkin time domain (DGTD) method, which can be viewed as a hybrid of the spectral-element time-domain method (SETD) and the finite-element time-domain (FETD) method. An hp-refinement technique is applied to decrease the Degrees-of-Freedom (DoFs) and computational requirements. The collocated E-J scheme facilitates solving the auxiliary equations by converting the inversions of matrices to simpler vector manipulations. A new hybrid time stepping approach, which couples the Runge-Kutta and Newmark methods, is proposed to solve the temporal auxiliary differential equations (ADEs) with a high degree of efficiency. The advantages of this new approach, in terms of computational resource overhead and accuracy, are validated through comparison with well-known commercial software for three diverse cases, which cover both near-field and far-field properties with plane wave and lumped port sources. The presented work provides the missing link between DCP dispersive models and FETD and/or SETD based algorithms. It is a competitive candidate for numerically studying the wideband plasmonic properties of DCP media.

Tuning the transmission of surface plasmon polaritons across nano and micro gaps in gold stripes

We applied a far-field technique to measure the surface plasmon propagation over a wide range of gap sizes in thin gold stripes. This is realized with a grating technique which allows the excitation and out coupling of surface plasmon polaritons (SPPs). With this method the intensity can be monitored before and after the gap. The observations show that the SPPs can transmit over gaps with a width of 1μm with a probability of about 40% for Au stripe-waveguides (7 µm width) at a wavelength of 780 nm. The transmission decays exponentially above a gap size of 1 µm. The results also demonstrate that the transmission has non-monotonic behavior for gap sizes smaller than 1 µm that we attribute to excitation of Fabry-Perot modes and resonant localized plasmons within the gap. The experimental results are supported by numerical simulations using a Finite-Difference Time-Domain (FDTD) approach.

Manipulating surface-plasmon-polariton launching with quasi-cylindrical waves

Launching the free-space light to the surface plasmon polaritons (SPPs) in a broad bandwidth is of importance for the future plasmonic circuits. Based on the interference of the pure SPP component, the bandwidths of the unidirectional SPP launching is difficult to be further broadened. By greatly manipulating the SPP intensities with the quasi-cylindrical waves (Quasi-CWs), an ultra-broadband unidirectional SPP launcher is experimentally realized in a submicron asymmetric slit. In the nano-groove of the asymmetric slit, the excited Quasi-CWs are not totally damped, and they can be scattered into the SPPs along the metal surface. This brings additional interference and thus greatly manipulates the SPP launching. Consequently, a broadband unidirectional SPP launcher is realized in the asymmetric slit. More importantly, it is found that this principle can be extended to the three-dimensional subwavelength plasmonic waveguide, in which the excited Quasi-CWs in the aperture could be effectively converted to the tightly guided SPP mode along the subwavelength plasmonic waveguide. In the large wavelength range from about 600 nm to 1300 nm, the SPP mode mainly propagates to one direction along the plasmonic waveguide, revealing an ultra-broad (about 700 nm) operation bandwidth of the unidirectional SPP launching.