Matthias Kraft

Graphene as a Tunable Anisotropic or Isotropic Plasmonic Metasurface.

We demonstrate a tunable plasmonic metasurface by considering a graphene sheet subject to a periodically patterned doping level. The unique optical properties of graphene result in electrically tunable plasmons that allow for extreme confinement of electromagnetic energy in the technologically significant regime of THz frequencies. Here, we add an extra degree of freedom by using graphene as a metasurface, proposing to dope it with an electrical gate patterned in the micron or submicron scale. By extracting the effective conductivity of the sheet, we characterize metasurfaces periodically modulated along one or two directions. In the first case, and making use of the analytical insight provided by transformation optics, we show an efficient control of THz radiation for one polarization. In the second case, we demonstrate a metasurface with an isotropic response that is independent of wave polarization and orientation.

Graphene, plasmons and transformation optics

Here we study subwavelength gratings for coupling into graphene plasmons by means of an an- alytical model based on transformation optics that is not limited to very shallow gratings. We consider gratings that consist of a periodic modulation of the charge density in the graphene sheet, and gratings formed by this conductivity modulation together with a dielectric grating placed in close vicinity of the graphene. Explicit expressions for the dispersion relation of the plasmon po- laritons supported by the system, and reflectance and transmittance under plane wave illumination are given. We discuss the conditions for maximising the coupling between incident radiation and plasmons in the graphene, finding the optimal modulation strength for a conductivity grating.

Transformation Optics: A Time- and Frequency-Domain Analysis of Electron-Energy Loss Spectroscopy.

Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) play a pivotal role in many of the cutting edge experiments in plasmonics. EELS and CL experiments are usually supported by numerical simulations, which-though accurate-may not provide as much physical insight as analytical calculations do. Fully analytical solutions to EELS and CL systems in plasmonics are rare and difficult to obtain. This paper aims to narrow this gap by introducing a new method based on transformation optics that allows to calculate the quasistatic frequency- and time-domain response of plasmonic particles under electron beam excitation. We study a nonconcentric annulus (and ellipse in the Supporting Information ) as an example.

Landau-zener Transitions In the Presence of Harmonic Noise

We study the influence of off-diagonal harmonic noise on transitions in a Landau–Zener model. We demonstrate that the harmonic noise can change the transition probabilities substantially and that its impact depends strongly on the characteristic frequency of the noise. In the underdamped regime of the noise process, its effect is compared with the one of a deterministic sinusoidally oscillating function. While altering the properties of the noise process allows one to engineer the transitions probabilities, driving the system with a deterministic sinusoidal function can result in larger and more controlled changes of the transition probability. This may be relevant for realistic implementations of our model with Bose–Einstein condensates in noise-driven optical lattices.

Bianisotropy and magnetism in plasmonic gratings

We present a simple design to achieve bianisotropy at visible wavelengths: an ultrathin plasmonic grating made of a gold grating covered by a thin flat layer of gold. We show experimentally and through simulations that the grating exhibits magnetoelectric coupling and features asymmetric reflection and absorption, all that with a device thickness of a tenth of the operating wavelength. We compared the spectral results and retrieved the effective material parameters of different polarizations and devices. We show that both asymmetry and strong coupling between the incoming light and the optically interacting surfaces are required for obtaining asymmetric optical behavior in metasurfaces.

Hidden symmetries in plasmonic gratings

Plasmonic gratings constitute a paradigmatic instance of the wide range of applications enabled by plasmonics. While subwavelength metal gratings find applications in optical biosensing and photovoltaics, atomically thin gratings achieved by periodically doping a graphene monolayer perform as metasurfaces for the control of terahertz radiation. In this paper we show how these two instances of plasmonic gratings inherit their spectral properties from an underlying slab with translational symmetry.We develop an analytical formalism to accurately derive the mode spectrum of the gratings that provides a great physical insight.

Graphene as a tunable plasmonic metasurface with transformation optics

Graphene can be biased by electrical gating or by chemical doping, which modifies its Fermi level and enables the existence of surface plasmons propagating along graphene. These surface plasmons, because of the two-dimensional (2D) nature of this material, have very short wavelengths and extreme out-of-plane confinement. Graphene plasmons feature in the THz regime with relatively low losses, where the plasmonic response of metals is weak. These facts, added to the important ingredient of tunability with external bias, make graphene a very suitable platform for the design of plasmonic metasurfaces in the THz regime. Metasurfaces, which are the 2D counterpart of metamaterials, consist of a planar arrangement of resonant subwavelengthsize building blocks. By appropriately them, metasurfaces provide an ultrathin platform for manipulating electromagnetic waves, and they have shown novel phenomena and applications as from broadband light bending and anomalous reflection and refraction. In this contribution we demonstrate a tunable meta-surface by inducing a periodic conductivity modulation in a graphene sheet. The conductivity modulation acts as a subwavelength grating that allows for external terahertz radiation to couple into the surface plasmons supported by the graphene layer. The coupling to highly confined plasmons provides a dramatic enhancement of the interaction of electromagnetic radiation with graphene. Making use of transformation optics we derive analytical expressions for the optical response of the system, and find the optimal configuration for coupling into plasmons. These graphene conductivity gratings are tunable plasmonic metasurfaces characterized by an effective surface conductivity, which is derived from a sheet retrieval method. In addition, for the case of conductivity modulations along two directions we find that the graphene metasurface responds isotropically under electromagnetic fields incident at any angle. Thus, the modulated graphene can be used for fast switching and tuning of THz radiation of arbitrary polarisation and orientation.

Transformation optics and EELS, a frequency- and time-domain analysis

Electron energy loss spectroscopy is one of the most important and versatile tools for experiments at the frontier of plasmonics research. In this short paper, we present a new method based on Transformation optics to analyze the electromagnetic response of plasmonic particles under electron beam excitation. We present analytical results for the electron energy loss and photon scattering spectra of a 2-d crescent and 3-d dimer, as well as their time-domain response. We believe this method can support and guide future EELS experiments in plasmonics and beyond.