Kevin F. MacDonald

Metamaterial electro-optic switch of nanoscale thickness

NanoBioScience Lab, Istituto Italiano di Tecnologia, Via Morego 30, I16163 Genova, Italy andBIONEM Lab, University of Magna Graecia, viale Europa, I88100 Catanzaro, Italy(Dated: December 21, 2009)Combining metamaterials with functional media brings a new dimension to their performance.Here we demonstrate substantial resonance frequency tuning in a photonic metamaterial hybridizedwith an electrically/optically switchable chalcogenide glass. The transition between amorphousand crystalline forms brings about a 10% shift in the near-infrared resonance wavelength of anasymmetric split-ring array, providing transmission modulation functionality with a contrast ratioof 4:1 in a device of sub-wavelength thickness.We demonstrate an innovative concept for nanoscale electro-optic switching. It exploits the frequency shift of a narrow-band Fano resonance mode in a plasmonic planar metamaterial induced by a change in the dielectric properties of an adjacent chalcogenide glass layer. An electrically stimulated transition between amorphous and crystalline forms of the glass brings about a 150 nm shift in the near-infrared resonance providing transmission modulation with a contrast ratio of 4:1 in a device of subwavelength thickness.

An All‐Optical, Non‐volatile, Bidirectional, Phase‐Change Meta‐Switch

Non-volatile, bidirectional, all-optical switching in a phase-change metamaterial delivers high-contrast transmission and reflection modulation at near- to mid-infrared wavelengths in device structures down to ≈1/27 of a wavelength thick.

Plasmon Spectroscopy and Imaging of Individual Gold Nanodecahedra: A Combined Optical Microscopy, Cathodoluminescence, and Electron Energy-Loss Spectroscopy Study

Imaging localized plasmon modes in noble-metal nanoparticles is of fundamental importance for applications such as ultrasensitive molecular detection. Here, we demonstrate the combined use of optical dark-field microscopy (DFM), cathodoluminescence (CL), and electron energy-loss spectroscopy (EELS) to study localized surface plasmons on individual gold nanodecahedra. By exciting surface plasmons with either external light or an electron beam, we experimentally resolve a prominent dipole-active plasmon band in the far-field radiation acquired via DFM and CL, whereas EELS reveals an additional plasmon mode associated with a weak dipole moment. We present measured spectra and intensity maps of plasmon modes in individual nanodecahedra in excellent agreement with boundary-element method simulations, including the effect of the substrate. A simple tight-binding model is formulated to successfully explain the rich plasmon structure in these particles encompasing bright and dark modes, which we predict to be fully observable in less lossy silver decahedra. Our work provides useful insight into the complex nature of plasmon resonances in nanoparticles with pentagonal symmetry.

Nanostructured Plasmonic Medium for Terahertz Bandwidth All-Optical Switching

Periodic nanostructuring can enhance the optical nonlinearity of plasmonic metals by several orders of magnitude. By patterning a gold film, the largest sub-100 femtosecond nonlinearity is achieved, which is suitable for terahertz rate all-optical data processing as well as ultrafast optical limiters and saturable absorbers.

Ultrafast all-optical switching via coherent modulation of metamaterial absorption

We report on the demonstration of a femtosecond all-optical modulator providing, without nonlinearity and therefore at arbitrarily low intensity, ultrafast light-by-light control. The device engages the coherent interaction of optical waves on a metamaterial nanostructure only 30 nm thick to efficiently control absorption of near-infrared (750–1040 nm) femtosecond pulses, providing switching contrast ratios approaching 3:1 with a modulation bandwidth in excess of 2 THz. The functional paradigm illustrated here opens the path to a broad family of meta-devices for ultrafast optical data processing in coherent networks.

High-contrast modulation of light with light by control of surface plasmon polariton wave coupling

We have demonstrated a mechanism for modulating light with light by controlling the efficiency with which light is coupled into a plasmon polariton wave. An optical fluence of 15mJ∕cm2 in the control channel is sufficient to achieve nearly a ten-fold intensity modulation of the signal beam reflected from a Glass ∕MgF2∕Ga structure. The mechanism depends on a nanoscale light-induced structural transformation in the gallium layer and has transient switching times of the order of a few tens of nanoseconds. It offers high modulation contrast for signals in the visible and near infrared spectral ranges.

Controlling light with light using coherent metadevices: all-optical transistor, summator and invertor

An all-optical device based on a planar plasmonic metamaterial is proposed that has summator, inverter and small-signal amplifier functions. Optical processing of optical data signals is strongly needed to overcome the ‘electronic bottleneck’ in current optical communication systems. Now, researchers at the University of Southampton in the UK and Nanyang Technological University in Singapore have theoretically demonstrated the feasibility of exploiting the coherent interaction of light beams in an ultrathin (substantially subwavelength) plasmonic metamaterial to achieve this. As the proposed device does not use nonlinear optical media, it should be possible to operate it at very low power levels. The energy redistribution between the four ports of the device can provide nonlinear input-output signal dependencies and may be controlled at very low intensity levels with multi-terahertz bandwidth and without distorting the signal.

Coherent control of Snell's law at metasurfaces

It was recently demonstrated that the well-known Snells law must be corrected for phase gradient metasurfaces to account for their spatially varying phase, leading to normal and anomalous transmission and reflection of light on such metasurfaces. Here we show that the efficiency of normal and anomalous transmission and reflection of light can be controlled by the intensity or phase of a second coherent wave. The phenomenon is illustrated using gradient metasurfaces based on V-shaped and rectangular apertures in a metal film. This coherent control effect can be exploited for wave front shaping and signal routing.

Continuous metal plasmonic frequency selective surfaces

The fabrication of indented (‘intaglio’) or raised (‘bas-relief’) sub-wavelength metamaterial patterns on a metal surface provides a mechanism for changing and controlling the colour of the metal without employing any form of chemical surface modification, thin-film coating or diffraction effects. We show that a broad range of colours can be achieved by varying the structural parameters of metamaterial designs to tune absorption resonances. This novel approach to the ‘structural colouring’ of pure metals offers great versatility and scalability for both aesthetic (e.g. jewellery design) and functional (e.g. sensors, optical modulators) applications. We focus here on visible colour but the concept can equally be applied to the engineering of metallic spectral response in other electromagnetic domains.

Hyperspectral imaging of plasmonic nanostructures with nanoscale resolution

We report on the first realization of a hyperspectral imaging technique for surface plasmon polaritons on metallic nanostructures. The technique uses a scanning electron beam and allows for simple visualization of light emission from decoupled plasmons, providing information on decay lengths and feature sizes with nanometer resolution.