Wonseok Shin

Elements for Plasmonic Nanocircuits with Three‐Dimensional Slot Waveguides

Over the last decade, the fi eld of plasmonics has received sig-nifi cant attention for its ability to utilize engineered metallic nanostructures to manipulate the fl ow of light down to the deep subwavelength scale (see, for example, recent reviews [ 1–4 ]). Extended metal structures can serve as compact optical waveguides that can transport information in the form of surface plasmon-polaritons (SPPs). Among the wide variety of plasmonic waveguiding geometries being investigated, planar structures consisting of one or more fl at metal-dielectric interfaces (supporting single-interface SPPs, metal-dielectric-metal gap plasmons, long-range surface plasmons, etc) have received the most intensive study; [ 5–7 ] their popularity arises from their fundamental physical importance as well as the relative ease of analyzing and modeling such two-dimensional (2D) structures. We note, however, that chip-scale photonic nanocir-cuits require that fl exible routing of light occurs within a 2D platform, which implies that waveguides used for such on-chip optical links need to be three-dimensional (3D). Among an expanding family of 3D plasmonic waveguides including nanoparticle arrays, [ 8 ] metallic nanowires [ 9 ] and V-shaped grooves, [ 10 ] metallic slot waveguides, which consist of subwavelength slots in thin metallic fi lms, represent a promising candidate for chip-scale nanocircuitry because of their tight mode confi nement, reasonable propagating length and intrinsic broadband nature. This waveguide geometry also naturally lends itself to a wide variety of applications which require combined electrical and optical functions. For example, an electro-optic modulator based on a slot or gap plasmon waveguide can conveniently use the same metals to defi ne an optical guide and the electrical contacts that generate an electric fi eld across a switching medium. [ 11 ] Although the general properties of such plasmonic slot waveguides have been investigated to a certain extent, [ 12–16 ] a systematic study is still lacking for the basic building blocks necessary for the routing and manipulation of SPPs in 3D slot waveguides. In this work, we aim to analyze and optimize several fundamental elements for 3D plasmonic interconnects, such as mirrors, bends, and splitters. We start our investigation with the mode characteristics of straight slots, including the mode index, loss factor, energy confi nement as well as the spatial distributions of all the fi eld components. Without loss of generality, in all numerical studies we use a representative wavelength of λ 0 = 850 nm that falls between the visible …

Dislocated Double-Layer Metal Gratings: An Efficient Unidirectional Coupler

We propose theoretically and demonstrate experimentally a dislocated double-layer metal grating structure, which operates as a unidirectional coupler capable of launching surface plasmon polaritons in a desired direction under normal illumination. The structure consists of a slanted dielectric grating sandwiched between two gold gratings. The upper gold grating has a nonzero lateral relative displacement with respect to the lower one. Numerical simulations show that a grating structure with 7 periods can convert 49% of normally incident light into surface plasmons with a contrast ratio of 78 between the powers of the surface plasmons launched in two opposite directions. We explain the unidirectional coupling phenomenon by the dislocation-induced interference of the diffracted waves from the upper and lower gold gratings. Furthermore, we developed a simple and cost-effective technique to fabricate the structure via tilted two-beam interference lithography and subsequent shadow deposition of gold. The experimental results demonstrate a coupling efficiency of 36% and a contrast ratio of 43. The relatively simple periodic nature of our structure lends itself to large-scale low-cost fabrication and simple theoretical analysis. Also, unlike the previous unidirectional couplers based on aperiodic structures, the design parameters of our unidirectional coupler can be determined analytically. Therefore, this structure can be an important component for surface-plasmon-based nanophotonic circuits by providing an efficient interface between free-space and surface plasmon waves.

Choice of the perfectly matched layer boundary condition for frequency-domain Maxwell's equations solvers

We show that the performance of frequency-domain solvers of Maxwells equations is greatly affected by the kind of the perfectly matched layer (PML) used. In particular, we demonstrate that using the stretched-coordinate PML (SC-PML) results in significantly faster convergence speed than using the uniaxial PML (UPML). Such a difference in convergence behavior is explained by an analysis of the condition number of the coefficient matrices. Additionally, we develop a diagonal preconditioning scheme that significantly improves solver performance when UPML is used.

Broadband Sharp 90-degree Bends and T-Splitters in Plasmonic Coaxial Waveguides

We demonstrate numerically that sharp 90° bends and T-splitters can be designed in plasmonic coaxial waveguides at deep-subwavelength scale to operate without reflection and radiation over a broad range of wavelengths, including the telecommunication wavelength of 1.55 μm. We explain the principles of the operation using a transmission line model of the waveguide in the quasi-static limit. The compact bends and T-splitters open up a new avenue for the design of densely integrated optical circuits with minimal crosstalk.

Plasmonic coaxial waveguide-cavity devices.

We theoretically investigate three-dimensional plasmonic waveguide-cavity structures, built by side-coupling stub resonators that consist of plasmonic coaxial waveguides of finite length, to a plasmonic coaxial waveguide. The resonators are terminated either in a short or an open circuit. We show that the properties of these waveguide-cavity systems can be accurately described using a single-mode scattering matrix theory. We also show that, with proper choice of their design parameters, three-dimensional plasmonic coaxial waveguide-cavity devices and two-dimensional metal-dielectric-metal devices can have nearly identical transmission spectra. Thus, three-dimensional plasmonic coaxial waveguides offer a platform for practical implementation of two-dimensional metal-dielectric-metal device designs.

Accelerated solution of the frequency-domain Maxwell’s equations by engineering the eigenvalue distribution of the operator

We introduce a simple method to accelerate the convergence of iterative solvers of the frequency-domain Maxwells equations for deep-subwavelength structures. Using the continuity equation, the method eliminates the high multiplicity of near-zero eigenvalues of the operator while leaving the operator nearly positive-definite. The impact of the modified eigenvalue distribution on the accelerated convergence is explained by visualizing residual vectors and residual polynomials.

Instantaneous electric energy and electric power dissipation in dispersive media

We derive the instantaneous densities of electric energy and electric power dissipation in lossless and lossy dispersive media for a time-harmonic electric field. The instantaneous quantities are decomposed into DC and AC components, some of which are shown to be independent of the dispersion of dielectric constants. The AC component of the instantaneous energy density can be used to visualize propagation of electromagnetic waves through complex 3D structures.

Spectral light separator based on deep-subwavelength resonant apertures in a metallic film

We propose to funnel, select, and collect light spectrally by exploiting the unique properties of deep-subwavelength resonant apertures in a metallic film. In our approach, each aperture has an electromagnetic cross section that is much larger than its physical size while the frequency of the collected light is controlled by its height through the Fabry-Perot resonance mechanism. The electromagnetic crosstalk between apertures remains low despite physical separations in the deep-subwavelength range. The resulting device enables an extremely efficient, subwavelength way to decompose light into its spectral components without the loss of photons and spatial coregistration errors. As a specific example, we show a subwavelength-size structure with three deep-subwavelength slits in a metallic film designed to operate in the mid-wave infrared range between 3 and 5.5 μm.

Multi-frequency finite-difference frequency-domain algorithm for active nanophotonic device simulations

We introduce a multi-frequency finite-difference frequency-domain algorithm for active nanophotonic devices simulations from first principles. This algorithm overcomes large time-scale differences between optical and modulation frequencies and efficiently simulates performances of modulated devices.

Unified picture of modal loss rates from microwave to optical frequencies in deep- subwavelength metallic structures: A case study with slot waveguides

The behavior of the modal loss rate in deep-subwavelength metallic structures depends strongly on frequency: as the mode size decreases, at optical frequencies, the modal loss rate always increases to the theoretical upper bound Γ/2, whereas at microwave frequencies, it remains far lower than Γ/2, where Γ is the electron collision frequency of the metal. By analyzing the metallic slot waveguide as a model system, we show that these significantly different behaviors of the modal loss rate at optical and microwave frequencies are actually two extreme cases of a single universal behavior. Specifically, we show that as the mode size decreases, the loss rate always plateaus first and then increases to Γ/2, regardless of frequency. The only difference between frequencies is the properties of the plateau: at optical frequencies, the plateau is narrow, allowing the loss rate to reach Γ/2 at a relatively large mode size, whereas at microwave frequencies, the plateau is wide and formed at 13ω, defining a practicall...