Sergey I. Bozhevolnyi

Channel plasmon subwavelength waveguide components including interferometers and ring resonators

Photonic components are superior to electronic ones in terms of operational bandwidth, but the diffraction limit of light poses a significant challenge to the miniaturization and high-density integration of optical circuits. The main approach to circumvent this problem is to exploit the hybrid nature of surface plasmon polaritons (SPPs), which are light waves coupled to free electron oscillations in a metal that can be laterally confined below the diffraction limit using subwavelength metal structures. However, the simultaneous realization of strong confinement and a propagation loss sufficiently low for practical applications has long been out of reach. Channel SPP modes—channel plasmon polaritons (CPPs)—are electromagnetic waves that are bound to and propagate along the bottom of V-shaped grooves milled in a metal film. They are expected to exhibit useful subwavelength confinement, relatively low propagation loss, single-mode operation and efficient transmission around sharp bends. Our previous experiments showed that CPPs do exist and that they propagate over tens of micrometres along straight subwavelength grooves. Here we report the design, fabrication and characterization of CPP-based subwavelength waveguide components operating at telecom wavelengths: Y-splitters, Mach–Zehnder interferometers and waveguide–ring resonators. We demonstrate that CPP guides can indeed be used for large-angle bending and splitting of radiation, thereby enabling the realization of ultracompact plasmonic components and paving the way for a new class of integrated optical circuits.

Surface plasmon polariton based modulators and switches operating at telecom wavelengths

We report design, fabrication, and characterization of thermo-optic Mach–Zender interferometric modulators and directional-coupler switches whose operation utilizes the long-range surface-plasmon-polariton waveguiding along 15-nm-thin and 8-μm-wide gold stripes embedded in polymer and heated by electrical signal currents. The devices are characterized at the light wavelength of 1.55 μm, featuring low driving powers ( 30dB), moderate response times (∼1ms), and the total (fiber-to-fiber) insertion loss of ∼13dB (for modulators) and ∼11dB (for switches) when using single-mode fibers.

Surface-plasmon circuitry

Electromagnetic waves at the surface of a metal have the enormous bandwidth of a light pulse and can be channeled into circuit components smaller than the diffraction limit.

Demonstration of Magnetic Dipole Resonances of Dielectric Nanospheres in the Visible Region

Strong resonant light scattering by individual spherical Si nanoparticles is experimentally demonstrated, revealing pronounced resonances associated with the excitation of magnetic and electric modes in these nanoparticles. It is shown that the low-frequency resonance corresponds to the magnetic dipole excitation. Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in the visible region.

Efficient unidirectional nanoslit couplers for surface plasmons

The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components1,2. But in order that such potential can be realized, it is vital that the relatively poor light–SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3,4,5,6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system’s size. Back-side illumination of subwavelength apertures in optically thick metal films7,8,9,10,11,12,13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.

Integrated optical components utilizing long-range surface plasmon polaritons

New optical waveguide technology for integrated optics, based on propagation of long-range surface plasmon polaritons (LR-SPPs) along metal stripes embedded in dielectric, is presented. Guiding and routing of electromagnetic radiation along nanometer-thin and micrometer-wide gold stripes embedded in polymer via excitation of LR-SPPs is investigated in the wavelength range of 1250-1650 nm. LR-SPP guiding properties, such as the propagation loss and mode-field diameter, are investigated for different stripe widths and thicknesses. A propagation loss of /spl sim/6 dB/cm, a coupling loss of /spl sim/0.5 dB (per facet), and a bend loss of /spl sim/5 dB for a bend radius of 15 mm are evaluated for 15-nm-thick and 8-/spl mu/m-wide stripes at the wavelength of 1550 nm. LR-SPP-based 3-dB power Y-splitters, multimode interference waveguides, and directional couplers are demonstrated and investigated. At 1570 nm, coupling lengths of 1.9 and 0.8 mm are found for directional couplers with, respectively, 4- and 0-/spl mu/m-separated waveguides formed by 15-nm-thick and 8-/spl mu/m-wide gold stripes. LR-SPP-based waveguides and waveguide components are modeled using the effective-refractive-index method, and good agreement with experimental results is obtained.

Broadband Focusing Flat Mirrors Based on Plasmonic Gradient Metasurfaces

We demonstrate that metal-insulator-metal configurations, with the top metal layer consisting of a periodic arrangement of differently sized nanobricks, can be designed to function as broadband focusing flat mirrors. Using 50-nm-high gold nanobricks arranged in a 240-nm-period lattice on the top of a 50-nm-thick layer of silicon dioxide deposited on a continuous 100-nm-thick gold film, we realize a 17.3 × 17.3 μm(2) flat mirror that efficiently reflects (experiment: 14-27%; theory: 50-78%) and focuses a linearly polarized (along the direction of nanobrick size variation) incident beam in the plane of its polarization with the focal length, which changes from ~15 to 11 μm when tuning the light wavelength from 750 to 950 nm, respectively. Our approach can easily be extended to realize the radiation focusing in two dimensions as well as other optical functionalities by suitably controlling the phase distribution of reflected light.

Polymer-based surface-plasmon-polariton stripe waveguides at telecommunication wavelengths

Long-range surface-plasmon-polariton (LR–SPP) waveguiding along thin gold stripes embedded in polymer is investigated in the wavelength range of 1510–1620 nm. LR–SPP intensity distributions at the output are measured for different stripe widths and thicknesses. Coupling loss of ∼0.5 dB is achieved when exciting the fundamental LR–SPP mode along 10-nm-thick stripes of 6–10 μm width with a polarization maintaining fiber. LR–SPP propagation loss of 6–8 dB/cm is estimated (at 1550 nm) and attributed to scattering from inhomogeneities of the metal stripe and polymer cladding.

Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices

We demonstrate the realization of on-chip plasmonic analogue of electromagnetically induced transparency (EIT) in integrated plasmonic devices using detuned Fabry-Perot resonators aperture-side-coupled to a metal-insulator-metal (MIM) waveguide, with the transmission peak occurring at the intermediate wavelength. Strong MIM mode confinement along with localized side-coupling allows one to realize subwavelength photonic components with EIT-like transmission. Numerical results show that MIM components exhibiting pronounced EIT-like spectra in near infrared with the footprint of < 0.15 μm2 and group index of ~26 can be designed.

A generalized non-local optical response theory for plasmonic nanostructures

Metallic nanostructures exhibit a multitude of optical resonances associated with localized surface plasmon excitations. Recent observations of plasmonic phenomena at the sub-nanometre to atomic scale have stimulated the development of various sophisticated theoretical approaches for their description. Here instead we present a comparatively simple semiclassical generalized non-local optical response theory that unifies quantum pressure convection effects and induced charge diffusion kinetics, with a concomitant complex-valued generalized non-local optical response parameter. Our theory explains surprisingly well both the frequency shifts and size-dependent damping in individual metallic nanoparticles as well as the observed broadening of the crossover regime from bonding-dipole plasmons to charge-transfer plasmons in metal nanoparticle dimers, thus unravelling a classical broadening mechanism that even dominates the widely anticipated short circuiting by quantum tunnelling. We anticipate that our theory can be successfully applied in plasmonics to a wide class of conducting media, including doped semiconductors and low-dimensional materials such as graphene.N.A. Mortensen, S. Raza, M. Wubs, T. Søndergaard, & S. I. Bozhevolnyi Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg, Denmark Institute of Technology and Innovation, University of Southern Denmark, DK-5230 Odense, Denmark