Ole Albrektsen

Gap plasmon-based metasurfaces for total control of reflected light

In the quest to miniaturise photonics, it is of paramount importance to control light at the nanoscale. We reveal the main physical mechanism responsible for operation of gap plasmon-based gradient metasurfaces, comprising a periodic arrangement of metal nanobricks, and suggest that two degrees of freedom in the nanobrick geometry allow one to independently control the reflection phases of orthogonal light polarisations. We demonstrate, both theoretically and experimentally, how orthogonal linear polarisations of light at wavelengths close to 800 nm can be manipulated independently, efficiently and in a broad wavelength range by realising polarisation beam splitters and polarisation-independent beam steering, showing at the same time the robustness of metasurface designs towards fabrication tolerances. The presented approach establishes a new class of compact optical components, viz., plasmonic metasurfaces with controlled gradient birefringence, with no dielectric counterparts. It can straightforwardly be adapted to realise new optical components with hitherto inaccessible functionalities.

Efficient absorption of visible radiation by gap plasmon resonators

We demonstrate experimentally a periodic array of differently-sized and circularly-shaped gap plasmon resonators (GPRs) with the average absorption ~94% for unpolarized light in the entire visible wavelength range (400-750 nm). Finite-element simulations verify that the polarization insensitive broadband absorption originates from localized gap surface plasmons whose resonant excitations only weakly depend on the angle of incidence. Arrays of GPRs also exhibit enhanced local field intensities (~115) as revealed by scanning two-photon photoluminescence microscopy, that are spectrally correlated with the minima in corresponding linear reflection spectra.

Subwavelength Plasmonic Color Printing Protected for Ambient Use

We demonstrate plasmonic color printing with subwavelength resolution using circular gap-plasmon resonators (GPRs) arranged in 340 nm period arrays of square unit cells and fabricated with single-step electron-beam lithography. We develop a printing procedure resulting in correct single-pixel color reproduction, high color uniformity of colored areas, and high reproduction fidelity. Furthermore, we demonstrate that, due to inherent stability of GPRs with respect to surfactants, the fabricated color print can be protected with a transparent dielectric overlay for ambient use without destroying its coloring. Using finite-element simulations, we uncover the physical mechanisms responsible for color printing with GPR arrays and suggest the appropriate design procedure minimizing the influence of the protection layer.

Plasmonic metamaterial wave retarders in reflection by orthogonally oriented detuned electrical dipoles

We demonstrate that a pair of perpendicular electrical dipolar scatterers resonating at different frequencies can be used as a metamaterial unit cell to construct a nanometer-thin retarder in reflection, designing nanocross and nanobrick plasmonic configurations to function as reflecting quarter-wave plates at ~1520 and 770 nm, respectively. The design is corroborated experimentally with a monolayer of gold nanobricks, transforming linearly polarized incident radiation into circularly polarized radiation at ~780 nm.

Detuned Electrical Dipoles for Plasmonic Sensing

We demonstrate that a pair of electrical dipolar scatterers resonating at different frequencies, i.e., detuned electrical dipoles, can be advantageously employed for plasmonic sensing of the environment, both as an individual subwavelength-sized sensor and as a unit cell of a periodic array. It is shown that the usage of the ratio between the powers of light scattered into opposite directions (or into different diffraction orders), which peaks at the intermediate frequency, allows one to reach a sensitivity of ≈ 400 nm/RIU with record high levels of figure of merit exceeding 200. Qualitative considerations are supported with detailed simulations and proof-of-principle experiments using lithographically fabricated gold nanorods with resonances at 800 nm.

Long-range dielectric-loaded surface plasmon polariton waveguides operating at telecommunication wavelengths

We report on the realization of long-range dielectric-loaded surface plasmon polariton waveguides (LR-DLSPPWs) consisting of straight and bent subwavelength dielectric ridges deposited on thin and narrow metal stripes supported by a dielectric buffer layer covering a low-index substrate. Using imaging with a near-field optical microscope and end-fire coupling with a tapered fiber connected to a tunable laser at telecommunication wavelengths (1425-1545 nm), we demonstrate low-loss (propagation length ∼500 μm) and well-confined (mode width ∼1 μm) LR-DLSPPW mode guiding and determine the propagation and bend loss.

Continuous layer gap plasmon resonators.

We demonstrate both theoretically and experimentally that a gold nanostrip supported by a thin dielectric (silicon dioxide) film and a gold underlay forms an efficient (Fabry-Perot) resonator for gap surface plasmons. Periodic nanostrip arrays are shown to exhibit strong and narrow resonances with nearly complete absorption and quality factors of ~15-20 in the near-infrared. Two-photon luminescence microscopy measurements reveal intensity enhancement factors of ~120 in the 400-nm-period array of 85-nm-wide gold strips atop a 23-nm-thick silica film at the resonance wavelength of ~770 nm. Excellent resonant characteristics, the simplicity of tuning the resonance wavelength by adjusting the nanostrip width and/or the dielectric film thickness and the ease of fabrication with (only) one lithography step required make the considered plasmonic configuration very attractive for a wide variety of applications, ranging from surface sensing to photovoltaics.

Stimulated emission of surface plasmon polaritons by lead-sulphide quantum dots at near infra-red wavelengths.

Amplification of surface plasmon polaritons in planar metal-dielectric structure through stimulated emission is investigated using leakage-radiation microscopy configuration. The gain medium is a thin polymethylmethacrylate layer doped with lead-sulphide nanocrystals emitting at near-infrared wavelengths. We demonstrate an optical gain of ~200 cm(-1) for the mode under consideration, which corresponds to ~32% compensation of SPP loss.

Optical transparency by detuned electrical dipoles

We demonstrate that optical transparency can be realized with plasmonic metamaterials using unit cells consisting of detuned electrical dipoles (DED), thereby mimicking the dressed-state picture of the electromagnetically induced transparency (EIT) in atomic physics. Theoretically analyzing the DED cells with two and three different silver ellipsoids, we show the possibility of reaching a ?10 times decrease in group velocity and a propagation loss of ?1?dB per cell within the optical wavelength range of 625?640?nm. Similar configurations are realized with lithographically fabricated gold nanorods placed on a glass substrate and subsequently covered with a ~15-?m-thick polymer layer, featuring EIT-like transmission spectra with transparency windows at wavelengths of ~850?nm.

From plasmonic nanoantennas to split-ring resonators: tuning scattering strength

Evolution of the absorption and scattering cross sections, quality-factor (Q-factor), and field enhancement of three-dimensional retardation-based plasmonic resonators being transformed from straight gold nanorod antennas to split-ring resonators by bending is considered. The optical resonances are confirmed to be of plasmonic origin and are specifically shown to be related to the formation of standing waves of short-range surface plasmon polaritons supported by straight and bent nanorods. We verify that by bending nanoantennas it is possible to reduce and ultimately, in the split-ring resonator limit, practically eliminate their scattering at the fundamental resonance, resulting in a substantial increase in the corresponding Q-factors. The decrease in scattering by bending is connected with the attenuation of the electric-dipole response in favor of a magnetic-dipole one, leading to Q-factors exceeding the quasi-static limit by a factor of ∼1.7. Simultaneously, the structures exhibit local field enhancements of >50.