Matthias Stegmaier

Mixed-Mode Operation of Hybrid Phase-Change Nanophotonic Circuits

Phase change materials (PCMs) are highly attractive for nonvolatile electrical and all-optical memory applications because of unique features such as ultrafast and reversible phase transitions, long-term endurance, and high scalability to nanoscale dimensions. Understanding their transient characteristics upon phase transition in both the electrical and the optical domains is essential for using PCMs in future multifunctional optoelectronic circuits. Here, we use a PCM nanowire embedded into a nanophotonic circuit to study switching dynamics in mixed-mode operation. Evanescent coupling between light traveling along waveguides and a phase-change nanowire enables reversible phase transition between amorphous and crystalline states. We perform time-resolved measurements of the transient change in both the optical transmission and resistance of the nanowire and show reversible switching operations in both the optical and the electrical domains. Our results pave the way toward on-chip multifunctional optoelectronic integrated devices, waveguide integrated memories, and hybrid processing applications.

Aluminum nitride nanophotonic circuits operating at ultraviolet wavelengths

Aluminum nitride (AlN) has recently emerged as a promising material for integrated photonics due to a large bandgap and attractive optical properties. Exploiting the wideband transparency, we demonstrate waveguiding in AlN-on-Insulator circuits from near-infrared to ultraviolet wavelengths using nanophotonic components with dimensions down to 40 nm. By measuring the propagation loss over a wide spectral range, we conclude that both scattering and absorption of AlN-intrinsic defects contribute to strong attenuation at short wavelengths, thus providing guidelines for future improvements in thin-film deposition and circuit fabrication.

The open graph archive: a community-driven effort

A graphbase, a term coined by Knuth [7], is a database of graphs and computer programs that generate, analyze, manipulate, and visualize graphs. The terms graphlibrary and grapharchive are often used as synonyms for this term. Our vision is to provide an infrastructure and quality standards for a public graphbase, named the Open Graph Archive, that is accessible to researchers and other interested parties around the world via the worldwide web. This paper describes the current work undertaken towards this goal; the paper is also intended to be a call for participation since this will be a community-driven effort where most of the content will be provided by users of the system.

Broadband directional coupling in aluminum nitride nanophotonic circuits

Aluminum nitride (AlN)-on-insulator has emerged as a promising platform for the realization of linear and non-linear integrated photonic circuits. In order to efficiently route optical signals on-chip, precise control over the interaction and polarization of evanescently coupled waveguide modes is required. Here we employ nanophotonic AlN waveguides to realize directional couplers with a broad coupling bandwidth and low insertion loss. We achieve uniform splitting of incoming modes, confirmed by high extinction-ratio exceeding 33dB in integrated Mach-Zehnder Interferometers. Optimized three-waveguide couplers furthermore allow for extending the coupling bandwidth over traditional side-coupled devices by almost an order of magnitude, with variable splitting ratio. Our work illustrates the potential of AlN circuits for coupled waveguide optics, DWDM applications and integrated polarization diversity schemes.

Mode control and mode conversion in nonlinear aluminum nitride waveguides

While single-mode waveguides are commonly used in integrated photonic circuits, emerging applications in nonlinear and quantum optics rely fundamentally on interactions between modes of different order. Here we propose several methods to evaluate the modal composition of both externally and device-internally excited guided waves and discuss a technique for efficient excitation of arbitrary modes. The applicability of these methods is verified in photonic circuits based on aluminum nitride. We control modal excitation through suitably engineered grating couplers and are able to perform a detailed study of waveguide-internal second harmonic generation. Efficient and broadband power conversion between orthogonal polarizations is realized within an asymmetric directional coupler to demonstrate selective excitation of arbitrary higher-order modes. Our approach holds promise for applications in nonlinear optics and frequency up/down-mixing in a chipscale framework.

All-optical signal processing using phase-change nanophotonics

Photonic data storage would dramatically improve performance in existing computing architectures by avoiding time and energy consuming electro-optical conversion. To date, photonic memories have been predominantly volatile and lose their content if the input power is switched off. We exploit hybrid photonic-phase-change materials for realizing non-volatile, all-photonic memories and computing structures. By using optical near-field coupling we realize bit storage of up to ten levels in a single device that readily switches between intermediate states. We show that individual phase-change elements can be addressed through two-pulse encoding in waveguide arrays. Such multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and point towards a new paradigm in all-photonic memory and non-conventional computing.

Selective excitation of guided modes in integrated aluminum nitride photonic circuits

We demonstrate methods to identify the modal composition of light guided through integrated multimode waveguides and discuss a scheme how arbitrary higher-order modes can selectively be excited. Exemplary, we show efficient and broadband polarization conversion.