Philipp Schau

Design of high-transmission metallic meander stacks with different grating periodicities for subwavelength-imaging applications

When replacing a bulk negative index material (NIM) with two resonant surfaces that allow for surface plasmon polariton (SPP) propagation it is possible to recreate the same near-field imaging effects as with Pendrys perfect lens. We show that a metallic meander structure is perfectly suited as such a resonant surface due to the tunability of the short (SRSPP) and long range surface plasmon (LRSPP) frequencies by means of geometrical variation. Furthermore, the Fano-type pass band between the SRSPP and LRSPP frequencies of a single meander sheet retains its dominant role when being stacked. Hence, the pass band frequency position, which is determined by the meander geometry, controls also the pass band of a meander stack. When building up stacks with different periodicities the pass band shifts in frequency for each sheet in a different way. We rigorously calculate the spectra of various meander designs and show that this shift can be compensated by changing the remaining geometrical parameters of each single sheet. We also present a basic idea how high- transmission stacks with different periodicities can be created to enable energy transfer at low loss over practically arbitrary distances inside such a stack. The possibility to stack meander sheets of varying periodicity might be the key to far field superlenses since a controlled transformation of evanescent modes to traveling wave modes of higher diffraction order could be enabled.

Polarization scramblers with plasmonic meander-type metamaterials

Due to plasmonic excitations, metallic meander structures exhibit an extraordinarily high transmission within a well-defined pass band. Within this frequency range, they behave like almost ideal linear polarizers, can induce large phase retardation between s- and p-polarized light and show a high polarization conversion efficiency. Due to these properties, meander structures can interact very effectively with polarized light. In this report, we suggest a novel polarization scrambler design using spatially distributed metallic meander structures with random angular orientations. The whole device has an optical response averaged over all pixel orientations within the incident beam diameter. We characterize the depolarizing properties of the suggested polarization scrambler with the Mueller matrix and investigate both single layer and stacked meander structures at different frequencies. The presented polarization scrambler can be flexibly designed to work at any wavelength in the visible range with a bandwidth of up to 100 THz. With our preliminary design, we achieve depolarization rates larger than 50% for arbitrarily polarized monochromatic and narrow-band light. Circularly polarized light could be depolarized by up to 95% at 600 THz.

Coupling between surface plasmons and Fabry-Pérot modes in metallic double meander structures

The excitation and transfer of evanescent electromagnetic waves appears as key challenge for the realization of optical imaging devices with super resolution. In this process surface plasmon polaritons (SPP) overtake the role as indispensable mediators between source fields and propagating fields. Therefore, the interaction between SPPs and the vacuum field in a double meander structure (DMS) is investigated. The occurrence of Fabry-Pérot (FP) modes within such a cavity and the SPP modes of the meander structure is analyzed to understand the interaction of both mode systems in the combined double meander structure. We show that the known Fano-type passband of single meander structures keeps its dominant role in the DMS and demonstrate the frequency selective role of meander mirrors within this meander cavity. The meander geometry determined passband frequency position also controls nearly solely the passband of the DMS. For far field superlenses (FSL) the energy transfer at low loss over practically arbitrary distances inside the structure is a key property. A resonant amplitude transfer can be obtained between resonantly coupled meander surfaces for unlimited distances in practical cases. This property enables a controlled transformation of evanescent modes to traveling wave modes of higher diffraction order useful for superlens operation.

Metamaterials for optical and photonic applications for space : preliminary results

The European Space Agency (ESA) in the frame of its General Study Program (GSP) has started to investigate the opportunity of using metamaterials in space applications. In that context, ESA has initiated two GSP activities which main objectives are 1) to identify the metamaterials and associated optical properties which could be used to improve in the future the performances of optical payloads in space missions, 2) to design metamaterial based devices addressing specific needs in space applications. The range of functions for metamaterials to be investigated is wide (spectral dispersion, polarisation control, light absorption, straylight control...) and so is the required spectral range, from 0.4μm to 15μm. In the frame of these activities several applications have been selected and the designs of metamaterial based devices are proposed and their performances assessed by simulations.

Rigorous modeling of meander-type metamaterials for sub-lambda imaging

It has been shown that surface plasmon polaritons (SPPs) have a dominant influence on the unique properties of negative index materials (NIMs). Consequently, one could replace bulk NIMs by resonantly coupled surfaces that allow the propagation of SPPs. We show that a metallic meander structure is perfectly suited as such a resonant surface due to the tunability of the short range SPP (SRSPP) and long range SPP (LRSPP) frequencies by means of geometrical variation. Furthermore, the pass band between the SRSPP and LRSPP frequencies of a single meander sheet, induced by two Fanotype resonances, retains its dominant role when being stacked. In this report we demonstrate how a stack consisting of two meander structures can mimic perfect imaging known from Pendrys lens within this pass band region. On the other hand, to observe sub-wavelength features in the far-field more than (perfect) near-field imaging is necessary. We propose a stack of meander structures with successively increasing periodicity capable to decrease the lateral wave vector until near-field to far-field transformation is achieved. When stacking multiple meander structures with different periodicities, the pass band shifts in frequency for each sheet in a different way. We rigorously calculate the spectra of various meander designs and show that this shift can be compensated by changing other geometrical parameters of each single sheet. Such meander stacks can transfer energy resonantly over large distances with a high transmission and might enable sub-wavelength imaging.

Resolving the depth of fluorescent light by structured illumination and shearing interferometry

A method for the depth-sensitive detection of fluorescent light is presented. It relies on a structured illumination restricting the excitation volume and on an interferometric detection of the wave front curvature. The illumination with two intersecting beams of a white-light laser separated in a Sagnac interferometer coupled to the microscope provides a coarse confinement in lateral and axial direction. The depth reconstruction is carried out by evaluating shearing interferograms produced with a Michelson interferometer. This setup can also be used with spatially and temporally incoherent light as emitted by fluorophores. A simulation workflow of the method was developed using a combination of a solution of Maxwells equations with the Monte Carlo method. These simulations showed the principal feasibility of the method. The method is validated by measurements at reference samples with characterized material properties, locations and sizes of fluorescent regions. It is demonstrated that sufficient signal quality can be obtained for materials with scattering properties comparable to dental enamel while maintaining moderate illumination powers in the milliwatt range. The depth reconstruction is demonstrated for a range of distances and penetration depths of several hundred micrometers.

Depolarization of a randomly distributed plasmonic meander metasurface characterized by Mueller matrix spectroscopic ellipsometry

Metallic nanostructures offer efficient solutions in polarization control with a very low thickness. In this report, we investigate the optical properties of a nano-fabricated plasmonic pseudo-depolarizer using Mueller matrix spectroscopic ellipsometry in transmission configuration. The depolarizer is composed of 256 square cells, each containing a periodically corrugated metallic film with random orientation. The full Mueller matrix was analyzed as a function of incident angle in a range between 0 and 20° and over the whole rotation angle range. Depolarization could be achieved in two visible wavelength regions around the short-range and long-range surface plasmon polariton frequencies, respectively. Furthermore, depolarization for circularly polarized light was 2.5 times stronger than that for linearly polarized light. Our results could work as a guidance for realizing a broadband high efficiency dielectric metasurface depolarizers.

Experimental demonstration of dispersion engineering through mode interactions in plasmonic microcavities

Plasmonic microcavities are compact systems having the capability to confine light in an extremely small volume. Light matter interactions can therefore be mediated very effectively by them. In this report we demonstrate experimentally that dispersion of photonic cavity modes can be tuned to a large degree in a plasmonic microcavity with two identical corrugated metallic films as resonant mirrors. The modification of the dispersion is induced by interactions between the photonic and plasmonic modes. Additionally, the excited surface waves are strongly enhanced by the gratings, which is important for coupling and enhancing evanescent fields. To realize such a cavity, we employed self-assembled monolayer nanosphere crystals as a prepatterned substrate. Metal/dielectric/metal films were subsequently deposited on it. The cavity length was used to tune the interaction strength. As a result, the original positively dispersive FP mode, i.e., the resonance frequency is increased with the incident angle, becomes independent or even negatively dependent on the incident angle. Due to the hexagonal textured corrugation of the metal film and the existence of some line defects in a large area, the optical response is isotropic and independent of the specific polarization. This behavior can have potential applications for light emission devices, plasmonic color filters and subwavelength imaging.

Sub-wavelength imaging using stacks of metallic meander structures with different periodicities

Two resonant surfaces, which allow the propagation of surface plasmon polaritons (SPPs) can mimic negative index materials (NIM). Hence, it is possible to recreate the near-field imaging effects known from Pendrys perfect lens. The metallic meander structure is a well-suited candidate for such a resonant surface due to the excitation and tunability of the short (SRSPP) and long range surface plasmon (LRSPP) frequencies. Furthermore, the Fano-type pass band between the SRSPP and LRSPP frequencies of a single meander sheet retains its dominant role when being stacked. We demonstrate how a stack consisting of two meander structures can perfectly image within this pass band region and propose a stack of meander structures with successively increasing periodicity. Such a stack might be capable to decrease the lateral wave vector until near-field to far-field transformation is achieved. The frequency shift of the pass band for each sheet can be compensated by changing other geometrical parameters. We rigorously calculate the spectra of various meander designs and show that meander stacks transfer energy resonantly over large distances with a high transmission.

Retrieving the axial position of fluorescent light emitting spots by shearing interferometry

Abstract. A method for the depth-resolved detection of fluorescent radiation based on imaging of an interference pattern of two intersecting beams and shearing interferometry is presented. The illumination setup provides the local addressing of the excitation of fluorescence and a coarse confinement of the excitation volume in axial and lateral directions. The reconstruction of the depth relies on the measurement of the phase of the fluorescent wave fronts. Their curvature is directly related to the distance of a source to the focus of the imaging system. Access to the phase information is enabled by a lateral shearing interferometer based on a Michelson setup. This allows the evaluation of interference signals even for spatially and temporally incoherent light such as emitted by fluorophors. An analytical signal model is presented and the relations for obtaining the depth information are derived. Measurements of reference samples with different concentrations and spatial distributions of fluorophors and scatterers prove the experimental feasibility of the method. In a setup optimized for flexibility and operating in the visible range, sufficiently large interference signals are recorded for scatterers placed in depths in the range of hundred micrometers below the surface in a material with scattering properties comparable to dental enamel.