Spin-wave interference in three-dimensional rolled-up ferromagnetic microtubes

We have investigated spin-wave excitations in rolled-up Permalloy microtubes using microwave absorption spectroscopy. We find a series of quantized azimuthal modes which arise from the constructive interference of Damon-Eshbach type spin waves propagating around the circumference of the microtubes, forming a spin-wave resonator. The mode spectrum can be tailored by the tube's radius and number of rolled-up layers.Comment: 12 pages, 4 figure

Enhanced Transmission in Rolled-up Hyperlenses utilizing Fabry-Pe\'rot Resonances

We experimentally demonstrate that the transmission though rolled-up metal/semiconductor hyperlenses can be enhanced at desired frequencies utilizing Fabry-P\'erot resonances. By means of finite difference time domain simulations we prove that hyperlensing occurs at frequencies of high transmission.Comment: 3 pages, 3 figure

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

Field- and geometry-controlled avoided crossings of spin-wave modes in reprogrammable magnonic crystals

We study the spin dynamics in arrays of densely packed submicron Ni80Fe20 wires which form one-dimensional magnonic crystals. They are subject to an in-plane magnetic field H being collinear with the wires. In the case when neighboring wires are magnetized antiparallel, broadband spin-wave spectroscopy reveals a mode repulsion behavior around a certain field Hmr. We attribute this to dipolar coupling and avoided crossing of resonant modes of individual wires. The modes are found to hybridize across the array and form acoustic and optical modes. When an array of alternating-width wires is considered, Hmr is found to vary characteristically as a function of the width difference Δw of neighboring wires. Interestingly, the sign of Hmr reflects the orientation of the wires’ magnetization. For our devices we find experimentally frequency splittings δf on the order of 1 GHz between the acoustic and optical mode. We use micromagnetic modeling to analyze spin precession profiles and investigate the hybridization of modes. The simulated splitting is larger than the observed one. We attribute the discrepancy to a reduced dipolar coupling in the real samples. Using a theoretical model which considers the reduced dipolar coupling we analyze δf for different geometrical parameters such as the edge-to-edge separation a and the width difference Δw. Though relevant for Hmr, Δw is not decisive for δf. Instead, a is key for the frequency splitting. The results are relevant in order to tailor the dynamic response and band structure of magnonic crystals

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 embedded in rolled-up metamaterials. We fabricate microtubes whose walls consist of alternating Ag and (In)(Al) GaAs layers with incorporated active GaAs quantum-well structures. By variation of the layer thickness ratio of the Ag and (In)(Al) GaAs layers we control the effective permittivity tensor of the metamaterial according to an effective medium approach. Thereby, we can design samples with elliptic or hyperbolic dispersion. Time-resolved low temperature photoluminescence spectroscopy supported by finite-difference time-domain simulations reveal a decrease of the quantum well's spontaneous emission lifetime in our metamaterials as a signature of the crossover from elliptic to hyperbolic dispersion