arXiv subjects

Philipp Gagel

Publications and source records attributed to Philipp Gagel.

13 recordsLinked to original sources

arXiv2025

Probing Local Topology in a Disordered Higher-Order Topological Insulator

Higher-order topology is prized for its ability to realize lower-dimensional boundary states which are stable beyond fine-tuning. However, disorder presents a failure mechanism that can destroy topological in-gap states. Here, we investigate a disordered two-dimensional polariton lattice and employ the spectral localizer framework to define a real-space topological index rooted in crystalline spatial symmetries. This framework enables direct real-space mapping of topology beyond conventional momentum-space classifications, confirming the presence of corner and edge modes in this generalized Su-Schrieffer-Heeger model. Furthermore, it can directly quantify topological protection of a state. We leverage the versatility of our platform to experimentally realize normally distributed, random disorder and find that the corner states persist until the spectral gap closes. Experimentally, this corresponds to a disorder strength of approximately one quarter of the spectral gap. The spectral localizer accurately identifies the disorder strength at which the bandgap closes, establishing the framework as a predictive tool for every finite size system. Our results broaden the design principles for higher-order topological insulators and open the way towards imple menting disorder-resilient devices for robust lasing, light-routing, and quantum computation.

physics.optics↗

arXiv2025

Artificial Gauge Fields and Dimensions in a Polariton Hofstadter Ladder

Artificial gauge fields allow uncharged particles to mimic the behavior of charged particles subjected to magnetic fields, providing a powerful platform for exploring topological physics. Neutral particles, like photons, are typically unaffected by real magnetic fields. However, it is possible to introduce artificial gauge fields that control the effective dynamics of these neutral particles. Topological exciton-polariton lasers have attracted considerable interest, in part due to the wide range of tunable system parameters, but often require strong magnetic fields to realise propagating topological edge states. Here we show, that by using an artificial gauge field the topological Hall effect in a micron-scale micropillar chain is experimentally realised, exploiting the circular polarisation of polaritons as an artificial dimension. By careful rotational alignment of elliptical micropillars, we introduce an effective plaquette phase that induces strictly polarisation-dependent edge-state propagation, demonstrating non-reciprocal transport of the polariton pseudospins. Our results demonstrate that the dimensionality limitation of topological interface states as well as requirements for strong external magnetic fields in coupled topological laser arrays can be overcome by utilizing polarisation effects and careful engineering of the potential landscape. Our results open new ways towards the implementation of topological polariton lattices and related optically active devices with additional artificial dimension.

physics.optics↗

arXiv2025

Hybrid confinement techniques for polariton simulators

Exciton-polariton III-V semiconductor microcavities provide a robust platform for emulating complex Hamiltonians, enabling topological photonics and quantum simulation for advanced photonic functionalities. Here, we introduce two novel fabrication techniques - etch-and-oversputter and deposit-and-oversputter - that overcome limitations of traditional photonic confinement. Both use structured, locally elongated semiconductor cavities to create deep, highly controllable potentials, while leveraging high-quality GaAs-based materials, which achieve excellent Q-factors. A sputtered all-dielectric top mirror introduces an innovative hybrid approach, simplifying fabrication while maintaining quality compared to deep ion etching. Utilizing a Kagome lattice as a benchmark, we show high-quality optical band structures previously inaccessible with deep etching. Furthermore, we study a two-dimensional breathing Kagome lattice and demonstrate polariton lasing from a zero-dimensional corner mode, confirming precise control over couplings and tight polariton localization. These methods enable fabrication of intricate lattices, including higher-order topological insulators, or on-chip quantum regimes utilizing the polariton blockade mechanism due to tight photonic confinement.

physics.optics↗

arXiv2024

Topological Optical Waveguiding of Exciton-Polariton Condensates

One-dimensional models with topological non-trivial band structures are a simple and effective way to study novel and exciting concepts in topological photonics. In this work we are studying the propagation of light-matter quasi-particles, so called exciton-polaritons, in waveguide arrays. Specifically, we are investigating topological states at the interface between dimer chains, characterized by a non-zero winding number. In order to exercise precise control over the polariton propagation, we study non-resonant laser excitation as well as resonant excitation in transmission geometry. The results highlight a new platform for the study of quantum fluids of light and non-linear optical propagation effects in coupled semiconductor waveguides.

physics.optics↗

arXiv2024

An electrically pumped topological polariton laser

With a seminal work of Raghu and Haldane in 2008, concepts of topology have been successfully introduced in a wide range of optical systems, emulating specific lattice Hamiltonians. Certainly, one of the most promising routes to an application of topological photonics in an actual device are topological lasers, where efficient and highly coherent lasing from a topologically non-trivial mode is achieved. While some attempts have been made to excite such structures electrically, the majority of published fundamental experiments use a form of laser excitation. In this paper, we use a lattice of vertical resonator polariton micropillars to form an exponentially localized topological Su-Schrieffer-Heeger defect. Upon electrical excitation of the p-i-n doped structure, the system unequivocally shows polariton lasing from the topological defect using a carefully placed gold contact. Despite the presence of doping and electrical contacts, the polariton band structure clearly preserves its topological properties. At high excitation power the Mott density is exceeded leading to highly efficient lasing in the weak coupling regime. This work is an important step towards applied topological lasers using vertical resonator microcavity structures.

physics.optics↗

arXiv2021

Accuracy Evaluation of Touch Tasks in Commodity Virtual and Augmented Reality Head-Mounted Displays

An increasing number of consumer-oriented head-mounted displays (HMD) for augmented and virtual reality (AR/VR) are capable of finger and hand tracking. We report on the accuracy of off-the-shelf VR and AR HMDs when used for touch-based tasks such as pointing or drawing. Specifically, we report on the finger tracking accuracy of the VR head-mounted displays Oculus Quest, Vive Pro and the Leap Motion controller, when attached to a VR HMD, as well as the finger tracking accuracy of the AR head-mounted displays Microsoft HoloLens 2 and Magic Leap. We present the results of two experiments in which we compare the accuracy for absolute and relative pointing tasks using both human participants and a robot. The results suggest that HTC Vive has a lower spatial accuracy than the Oculus Quest and Leap Motion and that the Microsoft HoloLens 2 provides higher spatial accuracy than Magic Leap One. These findings can serve as decision support for researchers and practitioners in choosing which systems to use in the future.

cs.HC↗

arXiv2020

Bloch Oscillations of Hybrid Light-Matter Particles in a Waveguide Array

Bloch oscillations are a phenomenon well known from quantum mechanics where electrons in a lattice experience an oscillatory motion in the presence of an electric field gradient. Here, we report on Bloch oscillations of hybrid light-matter particles, called exciton-polaritons, being confined in an array of coupled microcavity waveguides. To this end, we carefully design the waveguides, widths and their mutual couplings such that a constant energy gradient is induced perpendicular to the direction of motion of the propagating exciton-polaritons. This technique allows us to directly observe and study Bloch oscillations in real- and momentum-space. Furthermore, we support our experimental findings by numerical simulations based on a modified Gross-Pitaevskii approach. Our work provides an important transfer of basic concepts of quantum mechanics to integrated solid state devices, using quantum fluids of light.

cond-mat.mes-hall↗

arXiv2020

Kagome Flatbands for Coherent Exciton-Polariton Lasing

Kagome lattices supporting Dirac cone and flatband dispersions are well known as a highly frustrated, two-dimensional lattice system. Particularly the flatbands therein are attracting continuous interest based on their link to topological order, correlations and frustration. In this work, we realize coupled microcavity implementations of Kagome lattices hosting exciton-polariton quantum fluids of light. We demonstrate precise control over the dispersiveness of the flatband as well as selective condensation of exciton-polaritons into the flatband. Subsequently, we focus on the spatial and temporal coherence properties of the laser-like emission from these polariton condensates that are closely connected to the flatband nature of the system. Notably, we find a drastic increase in coherence time due to the localization of flatband condensates. Our work illustrates the outstanding suitability of the exciton-polariton system for detailed studies of flatband states as a platform for microlaser arrays in compact localized states, including strong interactions, topology and non-linearity.

physics.optics↗

arXiv2020

Pen-based Interaction with Spreadsheets in Mobile Virtual Reality

Virtual Reality (VR) can enhance the display and interaction of mobile knowledge work and in particular, spreadsheet applications. While spreadsheets are widely used yet are challenging to interact with, especially on mobile devices, using them in VR has not been explored in depth. A special uniqueness of the domain is the contrast between the immersive and large display space afforded by VR, contrasted by the very limited interaction space that may be afforded for the information worker on the go, such as an airplane seat or a small work-space. To close this gap, we present a tool-set for enhancing spreadsheet interaction on tablets using immersive VR headsets and pen-based input. This combination opens up many possibilities for enhancing the productivity for spreadsheet interaction. We propose to use the space around and in front of the tablet for enhanced visualization of spreadsheet data and meta-data. For example, extending sheet display beyond the bounds of the physical screen, or easier debugging by uncovering hidden dependencies between sheet's cells. Combining the precise on-screen input of a pen with spatial sensing around the tablet, we propose tools for the efficient creation and editing of spreadsheets functions such as off-the-screen layered menus, visualization of sheets dependencies, and gaze-and-touch-based switching between spreadsheet tabs. We study the feasibility of the proposed tool-set using a video-based online survey and an expert-based assessment of indicative human performance potential.

cs.HC↗

arXiv2020

Coherent topological polariton laser

Topological concepts have been applied to a wide range of fields in order to successfully describe the emergence of robust edge modes that are unaffected by scattering or disorder. In photonics, indications of lasing from topologically protected modes with improved overall laser characteristics were observed. Here, we study exciton-polariton microcavity traps that are arranged in a one-dimensional Su-Schrieffer-Heeger lattice and form a topological defect mode from which we unequivocally observe highly coherent polariton lasing. Additionally, we confirm the excitonic contribution to the polariton lasing by applying an external magnetic field. These systematic experimental findings of robust lasing and high temporal coherence are meticulously reproduced by a combination of a generalized Gross-Pitaevskii model and a Lindblad master equation model. Thus, by using the comparatively simple SSH geometry, we are able to describe and control the exciton-polariton topological lasing, allowing for a deeper understanding of topological effects on microlasers.

cond-mat.mes-hall↗

arXiv2020

Exciton-polaritons in flatland: Controlling flatband properties in a Lieb lattice

In recent years, novel two-dimensional materials such as graphene, bismuthene and transition-metal dichalcogenides have attracted considerable interest due to their unique physical properties. A range of physical effects can be transferred to the realms of photonics by creating artificial photonic lattices emulating these two-dimensional materials. Here, exciton-polaritons in semiconductor microcavities offer an exciting opportunity to study a part-light, part-matter quantum fluid of light in a complex lattice potential. In this paper, we study exciton-polaritons in a two-dimensional Lieb lattice of buried optical traps. The $S$ and $P_{xy}$ photonic orbitals of such a Lieb lattice give rise to the formation of two flatbands which are of greatest interest for the distortion-free storage of compact localized states. By using a well controlled etch-and-overgrowth technique, we manage to control the trapping as well as the site couplings with great precision. This allows us to spectroscopically monitor the flatness of the flatbands across the full Brillouin zone. Furthermore, we demonstrate experimentally that these flatbands can be directly populated by condensation under non-resonant laser excitation. Finally, using this advanced device approach we demonstrate resonant and deterministic excitation of flatband modes in transmission geometry. Our findings establish the exciton-polariton systems as a highly controllable, optical many-body system to study flatband effects and for distortion-free storage of compact localized states.

cond-mat.mes-hall↗

arXiv2019

ReconViguRation: Reconfiguring Physical Keyboards in Virtual Reality

Physical keyboards are common peripherals for personal computers and are efficient standard text entry devices. Recent research has investigated how physical keyboards can be used in immersive head-mounted display-based Virtual Reality (VR). So far, the physical layout of keyboards has typically been transplanted into VR for replicating typing experiences in a standard desktop environment. In this paper, we explore how to fully leverage the immersiveness of VR to change the input and output characteristics of physical keyboard interaction within a VR environment. This allows individual physical keys to be reconfigured to the same or different actions and visual output to be distributed in various ways across the VR representation of the keyboard. We explore a set of input and output mappings for reconfiguring the virtual presentation of physical keyboards and probe the resulting design space by specifically designing, implementing and evaluating nine VR-relevant applications: emojis, languages and special characters, application shortcuts, virtual text processing macros, a window manager, a photo browser, a whack-a-mole game, secure password entry and a virtual touch bar. We investigate the feasibility of the applications in a user study with 20 participants and find that, among other things, they are usable in VR. We discuss the limitations and possibilities of remapping the input and output characteristics of physical keyboards in VR based on empirical findings and analysis and suggest future research directions in this area.

cs.HC↗

arXiv2018

Efficient Pose Tracking from Natural Features in Standard Web Browsers

Computer Vision-based natural feature tracking is at the core of modern Augmented Reality applications. Still, Web-based Augmented Reality typically relies on location-based sensing (using GPS and orientation sensors) or marker-based approaches to solve the pose estimation problem. We present an implementation and evaluation of an efficient natural feature tracking pipeline for standard Web browsers using HTML5 and WebAssembly. Our system can track image targets at real-time frame rates tablet PCs (up to 60 Hz) and smartphones (up to 25 Hz).

cs.CV↗