Efficient excitation of surface plasmon polaritons (SPPs) remains one of the most challenging issues in areas of plasmonics related to information communication technologies. In particular, combining high SPP excitation efficiency and acceptance of any polarization of incident light appeared to be impossible to attain due to the polarized nature of SPPs. Here we demonstrate plasmonic couplers that represent arrays of gap SPP resonators producing upon reflection two orthogonal phase gradients in respective linear polarizations of incident radiation. These couplers are thereby capable of efficiently converting incident radiation with arbitrary polarization into SPPs that propagate in orthogonal directions dictated by the phase gradients. Fabricated couplers operate at telecom wavelengths and feature the coupling efficiency of ,25% for either of two linear polarizations of incident radiation and directivity of SPP excitation exceeding 100. We further demonstrate that an individual wavelength-sized unit cell, representing a meta-scatterer, can also be used for efficient and polarization sensitive SPP excitation in compact plasmonics circuits. Keywords: gap surface plasmons; metamaterials; metasurfaces; surface plasmon polaritons INTRODUCTION Surface plasmon polaritons (SPPs) are electromagnetic excitations, in which electromagnetic field in dielectric are coupled to collective electron oscillations in metal, which propagate along and are tightly bound to metal/dielectric interfaces. 1 Modern plasmonics, which embraces various phenomena associated with excitation, propagation and scattering of SPPs, became ubiquitous in extremely diverse areas, ranging from biochemical sensing, 2 quantum optics 3 and information communication technologies 4 to sustained energy sources. 5 SPPs are essentially transverse magnetic waves with the magnetic field oriented perpendicular to the propagation plane, a very important feature that dictates the polarization sensitivity of the SPP excitation efficiency by free propagating radiation. It is therefore understandable that various SPP couplers developed in the quest for the efficient and unidirectional SPP excitation operate with only one (linear) polarization of the incident light, 6-10 resulting thereby in the loss of light power carried by the orthogonal polarization.The recent progress in optical metasurfaces, which influence transmitted and reflected optical fields by imposing additional (surface) gradients onto their phases, 11,12 opened new possibilities for efficient coupling of propagating and surface waves. 13,14 Very recently, polarization-controlled tunable directional SPP coupling has been demonstrated using arrays of narrow (elongated) apertures in an otherwise opaque metal film, so that the direction of SPP excitation was dictated by the helicity of a circularly polarized incident beam. 15,16 Note that, in these configurations, the SPP excitation involves both the transmission through narrow apertures and the coupling of the transmitted
We perform advanced radiation leakage microscopy of routing dielectric-loaded plasmonic waveguiding structures. By direct plane imaging and momentum-space spectroscopy, we analyze the energy transfer between coupled waveguides as a function of gap distance and reveal the momentum distribution of curved geometries. Specifically, we observed a clear degeneracy lift of the effective indices for strongly interacting waveguides in agreement with coupled-mode theory. We use momentum-space representations to discuss the effect of curvature on dielectric-loaded waveguides. The experimental images are successfully reproduced by a numerical and an analytical model of the mode propagating in a curved plasmonic waveguide.
We report on photo-thermal modulation of thin film surface plasmon polaritons (SPP) excited at telecom wavelengths and traveling at a gold/air interface. By operating a modulated continuous-wave or a Q-switched nanosecond pump laser, we investigate the photo-thermally induced modulation of SPP propagation mediated by the temperature-dependent ohmic losses in the gold film. We use a fiber-to-fiber characterization set-up to measure accurately the modulation depth of the SPP signal under photo-thermal excitation. On the basis of these measurements, we extract the thermo-plasmonic coefficient of the SPP mode defined as the temperature derivative of the SPP damping constant. Next, we introduce a figure of merit which is relevant to characterize the impact of temperature onto the properties of bounded or weakly leaky SPP modes supported by a given metal at a given wavelength. By combining our measurements with tabulated values of the temperature-dependent imaginary part of gold dielectric function, we compute the thermo-optical coefficients (TOC) of gold at telecom wavelengths. Finally, we investigate a pulsed photo-thermal excitation of the SPP in the nanosecond regime. The experimental SPP depth of modulation obtained in this situation are found to be in fair agreement with the modulation depths computed by using our values of gold TOC.
A MHz-bandwidth thermo-optical (TO) plasmonic switch operating at telecommunication wavelengths and based on a hybrid solid-state silicon-loaded surface plasmon polariton waveguide design is demonstrated numerically. The nanosecond (ns) TO response of the switch is due to the high thermal conductivities of the employed materials and we demonstrate specifically a 10 dB extinction ratio in the time-dependent switch transmission which features a pulsed 1 ns rise time followed by a 25 ns fall time when the switch is photo-thermally activated by a ns pulse at 532 nm wavelength.
The thermo-optical dynamics of polymer loaded surface plasmon waveguide (PLSPPW) based devices photo-thermally excited in the nanosecond regime is investigated. We demonstrate thermo-absorption of PLSPPW modes mediated by the temperature-dependent ohmic losses of the metal and the thermally controlled field distribution of the plasmon mode within the metal. For a PLSPPW excited by sub-nanosecond long pulses, we find that the thermo-absorption process leads to modulation depths up to 50% and features an activation time around 2 ns whereas the relaxation time is around 800 ns, four-fold smaller than the cooling time of the metal film itself. Next, we observe the photo-thermal activation of PLSPPW racetrack shaped resonators at a time scale of 300 ns followed however by a long cooling time (18 μs) attributed to the poor heat diffusivity of the polymer. We conclude that nanosecond excitation combined to high thermal diffusivity materials opens the way to high speed thermo-optical plasmonic devices.
The production and characterization of ultradense, planarized, and organized silicon nanowire arrays with good crystalline and optical properties are reported. First, alumina templates are used to grow silicon nanowires whose height, diameter, and density are easily controlled by adjusting the structural parameters of the template. Then, post-processing using standard microelectronic techniques enables the production of high-density silicon nanowire matrices featuring a remarkably flat overall surface. Different geometries are then possible for various applications. Structural analysis using synchrotron X-ray diffraction reveals the good crystallinity of the nanowires and their long-range periodicity resulting from their high-density organization. Transmission electron microscopy also shows that the nanowires can grow on nonpreferential substrate, enabling the use of this technique with universal substrates. The good geometry control of the array also results in a strong optical absorption which is interesting for their use in nanowire-based optical sensors or similar devices.
A generic method for the growth of high density silicon nanowires in nano channels of alumina is presented. All fabrication steps are shown. Using nanoimprint lithography, fabricated porous alumina matrices present a perfect triangular array of vertical cylindrical pores. The gold catalyst was deposited at the bottom of the pores by an electrochemical way and catalytic growth of silicon nanowires was performed in a Chemical‐Vapour‐Deposition reactor. Scanning Electron Microscopy images are presented and detailed. We show that this system is the first step for the elaboration of a solar cell based on silicon core‐shell nanowires. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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