David Sonnenberg

Guided neuronal growth on arrays of biofunctionalized GaAs/InGaAs semiconductor microtubes

We demonstrate embedded growth of cortical mouse neurons in dense arrays of semiconductor microtubes. The microtubes, fabricated from a strained GaAs/InGaAs heterostructure, guide axon growth through them and potentially enable electrical and optical probing of propagating action potentials. The coaxial nature of the microtubes—similar to myelin—is expected to enhance the signal transduction along the axon. We present a technique of suppressing arsenic toxicity and prove the success of this technique by overgrowing neuronal mouse cells.

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.

Local Droplet Etching: Self-assembled Nanoholes for Quantum Dots and Nanopillars

We review the mechanism and recent applications of the self-organized patterning of semiconductor surfaces by local droplet etching (LDE). LDE is a nanofabrication technique that is applicable in situ during molecular beam epitaxy (MBE) and fully compatible with state-of-the-art MBE systems. Most importantly, as a local etching technique that works with a number of different materials, it adds a new degree of freedom to established self-assembling techniques. During LDE, metallic droplets drill nanoholes into a semiconductor surface with structural parameters adjustable over a wide range by the process conditions. In subsequent overgrowth steps the holes are filled for the formation of nanostructures like, e.g., quantum dots (QDs)Quantum dots or quantum pillars. In comparison to other QD systems, the LDE dots have the key advantages that they are strain-free, highly uniform, and that their size is precisely adjustable. In addition, vertically stacked quantum dot molecules have been realized. Crystalline nanopillars are created by a combination of in situ LDE with ex situ selective etching that are highly attractive for studies of ballistic phonon and electron transport, e.g., in the field of thermoelectrics.

Guided Growth and Electrical Probing of Neurons on Arrays of Biofunctionalized GaAs/InGaAs Semiconductor Microtubes

We demonstrate embedded growth of cortical mouse neurons in dense arrays of semiconductor microtubes (see Figure (a,b)). The microtubes, fabricated from a strained GaAs/InGaAs heterostructure, guide axon growth through them and thus, enable the outgrowth of complex, artificial neuronal networks (see Figure (c)). At the same time, in situ electrical sensing is made possible. We present methods of stimulating and sensing action potentials, where electrodes are embedded inside the microtubes (see Figure d)). The wrapping of these electrodes around the axon greatly increases the contact area, and, with the fabrication of multiple electrodes along the tube length allow for the measurement of action potential propagation along single axons. The coaxial nature of the microtubes - similar to myelin - is expected to enhance the signal transduction along the axon.Our choice of GaAs, an optical III-V semiconductor, offers a variety of advantages over Si despite its toxicity: Its electron velocity and mobility is generally higher than, resulting in lower noise levels of possible electronic devices. We present a technique of suppressing arsenic toxicity and prove its efficiency by the results of neuronal cell culture.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Tunable Plasmonic Nanoantennas in Rolled-up Microtubes Coupled to Integrated Quantum Wells

We propose and realize a tunable plasmonic nanoantenna design consisting of two stacked Ag cuboids that are integrated into a rolled-up semiconductor microtube. The antenna’s resonance is tuned by varying the cuboid’s distance to match the photoluminescence emission of an embedded GaAs quantum well. Spatially, spectrally and temporally resolved photoluminescence measurements reveal a redshift and a reduction in lifetime of the quantum-well emission as signatures for the coupling to the antenna system. By means of finite-element electromagnetic simulations, we assign the coupling to an excitation of a high-order plasmonic mode inside the Ag cuboids.

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.

Synthetic neuronal circuits: Optically active semiconductor microtubes as remotely accessible sensors for action potentials

In this work an advanced method of optical readout of action-potentials using optically active semiconductor microtubes (MTs) as resonators and potential tool for optogenetic circuits and 3D environment is presented. The microtubes are fabricated using a well-established method of lattice mismatched layers. GaAs quantum wells (QW) embedded in the multilayers act as optically active sensors for action potentials.