Andreas Rottler

Rolled-up nanotechnology for the fabrication of three-dimensional fishnet-type GaAs-metal metamaterials with negative refractive index at near-infrared frequencies

We propose and demonstrate the fabrication of a three-dimensional fishnet metamaterial by utilizing rolled-up nanotechnology. It consists of 6 alternating layers of silver and (In)GaAs with an array of subwavelength holes “drilled” by focused ion beams. By means of finite-integration technique simulations, we show that the fabricated structure is a single-negative material possessing a negative real part of the refractive index in the near-infrared regime. We show that the fabricatedmaterial can be made double negative by slightly changing the size of the holes.

Rolled-Up Metamaterials

In this paper we review metamaterials fabricated from self-rolling strained metal-semiconductor layer systems. These systems relax their strain upon release from the substrate by rolling up into microtubes with a cross-section similar to a rolled-up carpet. We show that the walls of these microtubes represent three-dimensional optical metamaterials which so far could be used, for example, for the realization of broadband hyperlenses, fishnet metamaterials, or optically active three-dimensional metamaterials utilizing the unique possibility to stack optically active semiconductor heterostructures and metallic nanostructures. Furthermore, we discuss THz metamaterials based on arrays of rolled-up metal semiconductor microtubes and helices.

Tailoring of high- Q -factor surface plasmon modes on silver microtubes

We investigate the excitation of surface plasmon polaritons on silver tubes with finite-difference time-domain simulations. These surface plasmon polaritons exhibit azimuthal whispering gallery modes with quality factors in the hundreds. We show that the high quality factors arise from the coupling of the surface plasmon modes to photonic modes inside the tube. We examine the influence of a gain material on the quality factors and find that for material data of rhodamine 6G, the quality factors are enhanced significantly up to values of 3000.

Controlling the Spontaneous Emission Rate of Quantum Wells in Rolled-Up Hyperbolic Metamaterials

We experimentally demonstrate the enhancement of the spontaneous emission rate of GaAs quantum wells (QWs) embedded in rolled-up metamaterials. We investigate the spontaneous emission lifetime of semiconductor QW heterostructures which are directly integrated inside rolled-up microtubes (RMTs) with hyperbolic dispersion. RMTs are prepared by self-rolling of strained metal-semiconductor layer systems [1]. In the rolling-up process, multiple functional layers are closely stacked on top of each other, forming a three dimensional radial metamaterial which operates at optical frequencies. Rolling up metal-semiconductor systems is a unique and elegant route to incorporate optically active QW structures directly inside a multilayer system and results in a high quality active metamaterial composed of identical functional layers. Within this approach, the variable metal and semiconductor layer thicknesses allow to control the effective permittivity tensor of the metamaterial according to an effective medium approach. Thereby, the topology of the dispersions iso-frequency surface can be changed from a closed ellipsoidal iso-frequency surface to an open hyperboloidal surface corresponding to a so called hyperbolic metamaterial. We show by means of time resolved low temperature photoluminescence (PL) spectroscopy, that the spontaneous emission rate of the embedded QWs is enhanced by a factor of 2 as a signature of the topological transition to a hyperbolic medium. Our work is the first to prove spontaneous emission enhancement of robust QW structures in a hyperbolic metamaterial and at the same time opens-up the pathway to use this ansatz for novel integrated quantum light sources with tailored emission properties.

Gain in three-dimensional metamaterials utilizing semiconductor quantum structures

We demonstrate gain in a three-dimensional metal/semiconductor metamaterial by the integration of optically active semiconductor quantum structures. The rolling-up of a metallic structure on top of strained semiconductor layers containing a quantum well allows us to achieve a three-dimensional superlattice consisting of alternating layers of lossy metallic and amplifying gain material. We show that the transmission through the superlattice can be enhanced by exciting the quantum well optically under both pulsed or continuous wave excitation. This points out that our structures can be used as a starting point for arbitrary three-dimensional metamaterials including gain.

Terahertz metamaterials based on arrays of rolled-up gold/(In)GaAs tubes

We investigate metamaterials based on arrays of rolled-up gold/(In)GaAs microrolls. By finite-integration-technique simulations we show that these arrays interact resonantly with the magnetic component of an electromagnetic field and exhibit a negative effective permeability at terahertz frequencies. We find a strong dependence of the resonance frequency on small variations in the winding number n. We show that this dependence can be removed, if desired, by applying an additional slit into the metal layer of the tube.

Controlling the spontaneous emission rate of quantum wells in rolled-up hyperbolic metamaterials

We experimentally demonstrate the enhancement of the spontaneous emission rate of GaAs quantum wells (QWs) embedded in rolled-up metamaterials. We investigate the spontaneous emission lifetime of semiconductor QW heterostructures which are directly integrated inside rolled-up microtubes (RMTs) with hyperbolic dispersion. RMTs are prepared by self-rolling of strained metal-semiconductor layer systems [1]. In the rolling-up process, multiple functional layers are closely stacked on top of each other, forming a three dimensional radial metamaterial which operates at optical frequencies. Rolling up metal-semiconductor systems is a unique and elegant route to incorporate optically active QW structures directly inside a multilayer system and results in a high quality active metamaterial composed of identical functional layers. Within this approach, the variable metal and semiconductor layer thicknesses allow to control the effective permittivity tensor of the metamaterial according to an effective medium approach. Thereby, the topology of the dispersions iso-frequency surface can be changed from a closed ellipsoidal iso-frequency surface to an open hyperboloidal surface corresponding to a so called hyperbolic metamaterial. We show by means of time resolved low temperature photoluminescence (PL) spectroscopy, that the spontaneous emission rate of the embedded QWs is enhanced by a factor of 2 as a signature of the topological transition to a hyperbolic medium. Our work is the first to prove spontaneous emission enhancement of robust QW structures in a hyperbolic metamaterial and at the same time opens-up the pathway to use this ansatz for novel integrated quantum light sources with tailored emission properties.