Showing papers in "Photonics and Nanostructures: Fundamentals and Applications in 2013"

Nonlocal formalism for nanoplasmonics: Phenomenological and semi-classical considerations

Abstract: The plasmon response of metallic nanostructures is anticipated to exhibit nonlocal dynamics of the electron gas when exploring the true nanoscale. We extend the local-response approximation (based on Ohm's law) to account for a general short-range nonlocal response of the homogeneous electron gas. Without specifying further details of the underlying physical mechanism we show how this leads to a Laplacian correction term in the electromagnetic wave equation. Within the hydrodynamic model we demonstrate this explicitly and we identify the characteristic nonlocal range to be ξ NL ∼ v F / ω where v F is the Fermi velocity and ω is the optical angular frequency. For noble metals this gives significant corrections when characteristic device dimensions approach ∼1–10 nm, whereas at more macroscopic length scales plasmonic phenomena are well accounted for by the local Drude response.

Homogenization principles and effect of mixing on dielectric behavior

Abstract: This paper consists of two parts. First, a review of classical mixing principles lists the multitude of the various ways to characterize the effective permittivity of heterogeneous materials. Different connections between the various mixing formulas are underlined and the homogenization principles are classified into families of mixing rules. The second part emphasizes and analyzes the richness of the manner how the mixing process is able to create new types of dielectric behaviors, in particular with respect to enhancement of dielectric polarization, shifts of the dispersion parameters, and emergence of new effects in electrical response.

Regular ZnO nanopillar arrays by nanosphere photolithography

Abstract: Highly regular vertical ZnO nanopillar arrays were hydrothermally grown through a nucleation window pattern generated by nanosphere photolithography. The in-plane intensity modulation of the exposing ultraviolet light in the photoresist was performed by Stober silica or polystyrene nanospheres in the masking Langmuir–Blodgett monolayer. By comparing six different nanosphere diameters in the 180–700 nm range only those with diameter above the exposure wavelength of 405 nm generate a pattern in the thin photoresist layer. The pattern quality is improving with increasing diameter, therefore, the masking for nanopillar growth was demonstrated with 700 nm polystyrene nanospheres. The results of the nanosphere photolithography were supported by finite-difference time-domain calculations. This growth approach was shown to have the potential for low-cost, low-temperature, large area fabrication of ZnO pillars or nanowires enabling a precise engineering of geometry.

Homogenization analysis of complementary waveguide metamaterials

Abstract: We analyze the properties of complementary metamaterials as effective inclusions patterned into the conducting walls of metal waveguide structures. We show that guided wave metamaterials can be homogenized using the same retrieval techniques used for volumetric metamaterials, leading to a description in which a given complementary element is conceptually replaced by a block of material within the waveguide whose effective permittivity and permeability result in equivalent scattering characteristics. The use of effective constitutive parameters for waveguide materials provides an alternative point-of-view for the design of waveguide and microstrip based components, including planar lenses and filters, as well as devices with derived from a bulk material response. In addition to imparting effective constitutive properties to the waveguide, complementary metamaterials also couple energy from waveguide modes into radiation. Thus, complementary waveguide metamaterials can be used to modify and optimize a variety of antenna structures.

Perfect-absorber metamaterial based on flower-shaped structure

Abstract: We theoretically and experimetally investigated the narrow-band peak of perfect absorber (PA), which was realized with a metal–dielectric–metal scheme based on a flower-shaped structure (FSS). The PA slabs were designed and fabricated to work in the GHz range of electromagnetic radiation. The absorption is due to the magnetic influence and therefore, the resonance frequency can be easily controlled without affecting the efficiency of the absorption peak by changing the dimensional parameters of the FSS. In addition, the FSS also results in polarization independence of electromagnetic waves, as expected due to its geometry.

Homogenization of spatially dispersive metamaterial arrays in terms of generalized electric and magnetic polarizations

Abstract: An anisotropic homogenization theory for spatially dispersive periodic arrays is developed, based on the microscopic Maxwell equations, that yields causal, macroscopic permittivities, and inverse permeabilities for the fundamental Floquet modes of the arrays. (Macroscopic magnetoelectric coefficients are not required.) Reality conditions, reciprocity relations, passivity conditions, and causality relations are derived for these spatially dispersive macroscopic constitutive parameters. A significant feature of the formulation is that the macroscopic permittivities and permeabilities reduce to their anisotropic-continuum definitions in terms of ordinary macroscopic averages at the low spatial and temporal frequencies. In addition, diamagnetic metamaterial arrays require no special considerations or modifications to accommodate their unusual characteristics. A numerical example of a 2D array comprised of circular–cylinder inclusions is given that confirms the theoretical results for the computed electric and magnetic or diamagnetic macroscopic polarizations.

Liquid–crystal tunable waveguides for integrated plasmonic components

Abstract: A broad range of liquid–crystal tunable plasmonic waveguides, based on long-range, dielectric-loaded, and channel surface plasmon polaritons, are theoretically designed and investigated. Liquid–crystal switching is rigorously modeled by solving for the coupled elastic/electrostatic problem, whereas the optical studies are conducted via the finite-element method. Extensive tunability of key optical properties, such as modal index, propagation losses, and modal confinement is demonstrated for waveguides of different optical confinement scale. These highly functional waveguiding structures are proposed as building blocks for the design of functional components, e.g. optical attenuators, directional couplers and switches, in integrated plasmonic chips.

Probe absorption via two orthogonal standing-wave lasers in a semiconductor quantum well

Abstract: We investigate the two-dimensional (2D) probe absorption spectrum in a four-subband semiconductor quantum-well system driven by two orthogonal standing-wave lasers. It is found that the spatial distribution of 2D probe absorption spectrum can be significantly improved via adjusting the system parameters. The scheme shows the underlying probability for the formation of the 2D electron localization in a solid.

Recent advances on a DGTD method for time-domain electromagnetics

Abstract: During the last ten years, the discontinuous Galerkin time-domain (DGTD) method has progressively emerged as a viable alternative to well established finite-difference time-domain (FDTD) and finite-element time-domain (FETD) methods for the numerical simulation of electromagnetic wave propagation problems in the time-domain. In this paper, we review the historical development of the DGTD method and emphasize its recent adoption by the nanophotonic research community. In addition, we discuss about some of our recent efforts aiming at improving the accuracy, flexibility and efficiency of a non-dissipative order DGTD method, and also report on some preliminary works towards its extension to the numerical treatment of physical models and problems that are relevant to nanophotonics.

Effect of lateral shift of the light transmitted through a one-dimensional superconducting photonic crystal

Abstract: We investigate the lateral shift of the light transmitted through the ternary one-dimensional photonic crystal composed of two dielectric and one superconducting sublayers. The variations of the transmittivity spectra and the lateral shift of the light with the temperature have been investigated for both TE- and TM-polarized oblique incident light.

Near-field thermal emission from metamaterials

Abstract: A closed form expression for the local density of electromagnetic states (LDOS) due to a thermally emitting metamaterial bulk is derived from Maxwell's equations combined with fluctuational electrodynamics. The final form is the same as that for nonmagnetic materials, where the influence of the magnetic permeability is embedded in the Fresnel reflection coefficients. Spectral distributions of LDOS near metallic- and dielectric-based metamaterials are investigated. Results reveal that LDOS profiles are dominated by surface polaritons (SPs) in both TE and TM polarization states. A detailed discussion is provided on the necessary conditions for exciting TM- and TE-polarized SPs via a dispersion relation analysis that accounts for losses. Beyond the conventional conditions for excitation of SPs, the lossy dispersion relation analysis demonstrates mathematically that SPs exist when the imaginary parts of the permittivity or permeability, as well as n′n″, are close to zero, where n′ and n″ are the real and imaginary parts of the refractive index, respectively. An asymptotic expression for the extreme near field LDOS is derived, showing a Δ−3 power law relationship, as for nonmagnetic media, between LDOS and distance from the emitting bulk Δ. Results obtained from this study will assist in assessing material properties of arbitrarily electromagnetic materials in applications related to energy harvesting.

Effective medium representation and complex modes in 3D periodic metamaterials made of cubic resonators with large permittivity at mid-infrared frequencies

Abstract: We review some of the techniques that lead to the effective medium representation of a three-dimensional (3D) periodic metamaterial. We consider a 3D lattice of lead telluride cubic resonators at mid-infrared (MIR) frequencies. Each cubic resonator is modeled with both an electric and a magnetic dipole, through a method called the dual dipole approximation. The electric and magnetic polarizabilities of a cubic resonator are computed via full-wave simulations by mapping the resonator's scattered field under electric/magnetic excitation only to the field radiated by an equivalent electric/magnetic dipole. We then analyze the allowed modes in the lattice, with transverse polarization and complex wavenumber, highlighting the attenuation that each mode experiences after one free space wavelength. We observe the presence of two modes with low attenuation constant, dominant in different frequency ranges, able to propagate inside the lattice: this allows the treatment of the metamaterial as a homogeneous material with effective parameters, evaluated by using various techniques. We then show that the metamaterial under analysis allows for the generation of artificial magnetism (i.e., relative effective permeability different than unity, including negative permeability with low losses) at MIR frequencies.

Design, optimization and fabrication of 2D photonic crystals for solar cells

Abstract: This paper presents the optimization of 2D photonic crystals (PCs) onto Si wafers to improve the performance of c-Si PV cells. The objective is to find a structure capable of minimizing the reflectance of the Si wafer in the spectral range between 400 nm and 1000 nm. The study has been limited to PCs that can be fabricated and characterized with the tools and technology available and to dimensions in the same order as the visible light wavelength. PCs with different shapes and dimensions have been simulated and finally the optimum structure has been fabricated by a process based on laser interference lithography (LIL) and reactive ion etching (RIE). This optimized PC presents an average reflectance of 3.6% in the selected wavelength range, without any other material used as antireflective coating. This result means a drastic reduction in comparison with reflectance obtained out of the standard wet etch texturization used in current solar cell manufacturing lines.

Plasmonic finite-thickness metal–semiconductor–metal waveguide as ultra-compact modulator

Abstract: We propose a plasmonic waveguide with semiconductor gain material for optoelectronic integrated circuits. We analyze properties of a finite-thickness metal–semiconductor–metal (F-MSM) waveguide to be utilized as an ultra-compact and fast plasmonic modulator. The InP-based semiconductor core allows electrical control of signal propagation. By pumping the core we can vary the gain level and thus the transmittance of the whole system. The study of the device was made using both analytical approaches for planar two-dimensional case as well as numerical simulations for finite-width waveguides. We analyze the eigenmodes of the F-MSM waveguide, propagation constant, confinement factor, Purcell factor, absorption coefficient, and extinction ratio of the structure. We show that using thin metal layers instead of thick ones we can obtain higher extinction ratio of the device.

Layer homogenization of a 2D periodic array of scatterers

Abstract: The homogenization of a metamaterial made of a collection of scatterers periodically disposed is studied from an asymptotic theory and an optimization algorithm. Detailed numerical results are given for resonant scatterers and the spatial dispersion is investigated.

Surface plasmon–polariton modes of metallic single-walled carbon nanotubes

Abstract: Propagation of surface plasmon–polariton modes in metallic single-walled carbon nanotubes is investigated within the framework of the classical electrodynamics. Electronic excitations on the nanotube's surface are modeled by an infinitesimally thin layer of free-electron gas which is described by means of the linearized hydrodynamic theory. General expression of surface modes dispersion is obtained by solving Maxwell and hydrodynamic equations with appropriate boundary conditions. It is shown that the system generally disallows the separation of the transverse electric (TE) modes and transverse magnetic (TM) modes, except for the case of modes with no angular dependence.

Extraordinary wavelength dependence of self-collimation effect in photonic crystal with low structural symmetry

Abstract: We present the optical properties of a new type of photonic crystal (PC) named star-shaped PC (STAR-PC) with anomalous equi-frequency contours. Intentionally introducing low-symmetry in the primitive unit cell gives rise to progressively tilting flat contours, which are observed in the fifth band of the transverse magnetic mode. Due to the intrinsic dispersive feature of the proposed PCs, i.e. tilted self-collimation, the incident signal with different wavelengths can be successfully separated in a spatial domain without introducing any corrugations or complexities inside the structure. We show numerical investigations of wavelength selective characteristic of the proposed PC structure in both time and frequency domains. The STAR-PC approach can be considered a good candidate for the wavelength division applications in the design of compact photonic integrated circuits. For the purpose of wavelength separation implementations, the proposed structure may operate within the wavelength interval of 1484.5–1621.5 nm with a broad bandwidth of 8.82%. The corresponding inter-channel crosstalk value is as low as −19 dB and the calculated transmission efficiency is above 97%.

Cross-shaped metal–semiconductor–metal plasmonic crystal for terahertz modulator

Abstract: The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures The longitudinal modes disappear, while the horizontal mode moves to a higher frequency The former leads to an intensity modulation at 05 THz and 11 THz when the groove depth h = 60 μm, and the later leads to a band blue-shift from 1325 THz to 138 THz These results will be applied to THz modulation and tunable filtering

Broadband super-collimation with low-symmetric photonic crystal

Abstract: We investigate dispersive properties of two dimensional photonic crystal (PC) called star-shaped PC (STAR-PC) in order to succeed super-collimation over a broad bandwidth Both time- and frequency-domain numerical methods are conducted Due to introduced low-symmetry in the primitive cell, flat contours are observed at the fifth band for transverse magnetic mode The proposed structure supports a super-collimation effect over a broad wavelength range between 1443 nm and 1701 nm with a bandwidth of Δω = 1642% The intrinsic characteristic of STAR-PC provides in-plane beam propagation with a limited diffraction length of 120a, where a is the lattice constant By means of STAR-PC, one may realize super-collimation based single-mode optical devices with a low insertion loss, reduced dispersion and wide bandwidth

Corrections to the classical continuity boundary conditions at the interface of a composite medium

Abstract: Using the multiple-scales homogenization method, we derive generalized sheet transition conditions (GSTCs) for electromagnetic fields at the interface between two media, one of which is free-space and the other a certain type of composite material. The parameters in these new boundary conditions are interpreted as effective electric and magnetic surface susceptibilities, which themselves are related to the geometry of the scatterers that constitute the composite. We show that the effective tangential E and H fields are not continuous across the interface except in the limit when the lattice constant (the spacing between the scatterers—atoms, molecules or inclusions in the case of a composite material) of the composite medium is very small compared to a wavelength. We derive first-order corrections to the classical continuity conditions. For naturally occurring materials whose lattice constants are on an atomic scale, these effects are shown to be negligible for waves at optical frequencies or lower. However, once the lattice constant becomes a significant fraction of a wavelength (which is the case for many artificial dielectrics and metamaterials), the corrections can be important. In previous work we have alluded to the fact that such a GSTC is needed to correctly account for the surface effects when extracting the effective material properties of a metamaterial. The results of this current paper justify the assumptions made in that previous work. In general, these GSTCs will result in corrections to the classical Fresnel reflection and transmission coefficients (which are themselves merely zeroth-order approximations to the actual reflection and transmission coefficients), and in a separate publication we will use these GSTCs to address this issue.

Energy transfer channels at the diffraction-anomaly in transparent gratings and applications in sensors

Abstract: Diffraction anomaly corresponds to an energy re-distribution in the reflected and transmitted light beams and in different diffraction orders of a grating, which leads to sharp modulations on the transmission and reflection spectra. In gratings sitting on a transparent substrate, this portion of the energy is actually transferred to channels separated from the reflected and transmitted beams. These channels are based on multiple degenerated diffraction processes at the same wavelength as the diffraction anomaly. The spectroscopic response of these channels is sensitive to the change in the environmental refractive index and can be utilized in sensor devices.

Plasmonic waveguiding properties of the gap plasmon mode with a dielectric substrate

Abstract: The plasmonic waveguiding properties of the gap plasmon mode between two adjacent silver nanowires with a substrate are theoretically investigated using finite element method. The results show that there is a critical gap distance between two silver nanowires which approximately equals to the radius of the nanowires. When the gap distance is less than the critical distance, the influence of the substrate on the gap plasmon mode can be neglected. The gap plasmon mode has a combination of high confinement and long propagation length. Moreover, the plasmonic waveguiding properties of the gap plasmon mode are not sensitive to the wire-to-substrate distance between silver nanowires and the substrate.

Optical surface modes of photonic crystals for dual-polarization waveguide

Abstract: In this study, the design of a polarization-independent (dual-polarization) waveguide is presented by utilizing surface modes of photonic crystals. The waveguide structure operates in a frequency interval that is commonly shared by both transverse-electric (TE) and transverse-magnetic (TM) polarizations. The numerical calculations based on plane wave expansion and finite-difference time-domain methods are carried out to design and demonstrate a surface mode waveguide that provides confinement and guiding for both TE and TM modes. Once the relevant modes are properly excited, the high transmission efficiency of the photonic crystal surface waveguide is ensured. The demand to have polarization-insensitive devices makes our proposed design an important component for the photonic integrated circuit applications. Finally, we also propose a broadband surface mode photonic crystal waveguide with a bandwidth value of 28% for only TE polarization.

Multi-band left-handed metamaterial inspired by tree-shaped fractal geometry

Abstract: We report an alternative method of designing a new metamaterial with left handed (LH) characteristics over multi-band (MB) frequencies at microwave frequency regime. The resultant LH metamaterial (LHM) consisting of a single-sided tree-shaped fractal structure features triple magnetic resonances and one electric resonance apart from the lower metal plasma response, which is responsible for the three bands of negative refraction. The multi-resonant mechanism has been systematically studied to account for all electromagnetic behaviors, and capacitor–inductor circuit models are put forward for quantitative analysis. The LHM is balanced in the fundamental passband when only one layer is utilized, whereas the balanced condition is slightly broken when a collection of sub-wavelength cells are cascaded. The negative-zero-positive refraction of the fundamental LH band and the negative refraction of the higher LH band have been numerically validated by a prism-like LHM. For demonstration, a three-layer LHM slab sample is fabricated and measured. Consistent numerical and experimental results are observed. The method not requiring individual resonant particles and electrically continuous wires paves the way for a new route to compact MB LHM design.

Crossover from strong to weak exciton confinement and third-harmonic generation on one-dimensional quantum dots

Abstract: We study the effects of exciton confinement on the nonlinear optical susceptibility of one-dimensional quantum dots We use a direct numerical diagonalization to obtain the eigenenergies and eigenstates of the discretized Hamiltonian representing an electron–hole pair confined by a semiparabolic potential and interacting with each other via a Coulomb potential Density matrix perturbation theory is used to compute the nonlinear optical susceptibilities due to third-harmonic generation and the corresponding nonlinear corrections to the refractive index and absorption coefficient These quantities are analyzed as a function of ratio between the confinement length L and the exciton Bohr radius a 0 The Coulomb potential degrades the uniformity of the level separation We show that this effect promotes the emergence of multiple resonance peaks in the third-harmonic generation spectrum In the weak confinement regime β = L / a 0 ≫ 1, the third-order susceptibility is shown to decay as 1/ β 8 due to the prevalence of the hydrogenoid character of the exciton eigenstates

Improved plasmonic splitters and demultiplexers

Abstract: Analytical optimizations and numerical simulations are applied to enhance power transferring through a metal–insulator–metal (MIM) plasmonic junction. Employing the quasi static approximation for subwavelength devices, we derived a pure analytical model which complies very well with the simulation results. By inserting intermediate matching sections and stub structure at the MIM junctions, various matched plasmonic junctions and devices are designed. Both methods considerably improve the transmission spectra of the structure and enhance the bandwidth. The improved T-shaped splitters and demultiplexers of 50 nm width are designed with zero reflection at 1550 nm wavelength, 18% higher efficiency and broader bandwidth. Finite-difference time-domain simulations validate numerically our analysis and optimization results.

Nanostructured silicon geometries for directly bonded hybrid III-V-silicon active devices

Abstract: We discuss geometries with nanostructured cladding for active InP/silicon structures made by hetero-epitaxial bonding, which means that InP is directly bonded to silicon from a silicon-on-insulator without any intermediate layer. Such a cladding features low-index confinement and adds thermal sinking channels to those practised on the InP side. The first approach is a one-dimensional effective medium viewpoint, easily showing why grooves parallel to the waveguide are better. Then, two dimensional nanostructures are examined and found to perform better, given etching constraints. A more sophisticated geometry balancing thermal and optical confinement merits is then introduced thanks to a flip-flop algorithm.

High performance reflection-type 1D magnetophotonic crystals with flat-top responses

Abstract: In this paper, a theoretical study on the case of reflection-type one-dimensional magnetophotonic crystals (MPCs) has been carried out to establish high performance structures having concurrent high reflectance and large Kerr rotation with flat-top responses. The introduced MPCs are able to maintain their flat-top responses in a wide range of incident angle. For practical purposes, we have also inquired the influence of the error in the thickness of individual layers on the operational parameters of the MPCs. The reflectance flatness and bandwidth of the MPCs are appreciably stable against the imposed thickness errors.

Increasing the electromagnetic attenuation below a quasi-matched surface with use of passive hyperbolic metamaterials

Abstract: Electromagnetic attenuation effect within a short distance can be very useful in numerous devices and occasions. It is exploited for experimental (anechoic chambers in laboratories), military (anti-radar coatings of aircrafts and ships) and computational (realization of absorbing boundary conditions in software simulations) reasons. In this work, we compare the attenuation inflicted by a hyperbolic metamaterial with that occurred into an ordinary lossy dielectric. In order for the comparison to be fair, we use the same magnitude of permittivity and the same loss tangent in both cases; similarly, the reflection coefficient is kept low in all the regarded examples. The results indicate that the hyperbolic metamaterial vastly outperforms the commonly used dielectric and one can use these media in order to construct very thin and efficient attenuators or absorbers by considering moderate thermal losses.

Photonic crystal nanocavities in GaAs/AlGaAs with oxidised bottom cladding

Abstract: We present a solution to the difficult task of removing an oxide-based hard mask from a photonic crystal fabricated in the GaAs/AlGaAs system. We use a polymer backfill technique to seal the AlGaAs layer, thereby making it inaccessible to the wet-etch solution. This allows us to use a GaAs active layer for the photonic crystal placed on an oxidised AlGaAs layer which provides mechanical and thermal support. Using this technique, we fabricated GaAs-based photonic crystal cavities and measured respectable quality factors (Q ≈ 2200) despite the intrinsic asymmetry of the system. The technique presents a viable method for producing electrically injected photonic crystal cavities for operation on a mechanically stable and thermally conducting buffer layer.