arXiv subjects

Martina Hentschel

Publications and source records attributed to Martina Hentschel.

At least 19 recordsLinked to original sources

arXiv2026

Localization in microcavities revealed by phase-space non-Hermitian skin effect

Contrary to the semiclassical expectation for fully chaotic systems, localization of resonances is found to be a common feature in open microcavities. In spiral-shaped dielectric microcavities, a substantial fraction of resonances localize on polygonal patterns in real space, are chiral, and their momentum distributions accumulate near the critical line for total internal reflection. Despite the extensive investigation, the physical mechanism responsible for their remarkable abundance has remained a long-standing question. Addressing this, we reveal a physical correspondence between an inhomogeneous-loss Hatano-Nelson model and the dielectric phase space of a spiral microcavity. We show that the combination of geometry-induced momentum drift and refractive escape yields a generalized non-Hermitian skin effect in the phase space momentum. We identify this mechanism as the origin of the critical-line localization of resonances in open chaotic spiral microcavities, extending the skin-effect concept beyond nonreciprocal lattices to phase space and to open chaotic wave systems.

physics.optics↗

arXiv2026

Spectral and spatial filtering of whispering gallery modes in precision-engineered microbubble resonators

Similar to microspheres, thin-walled microbubble resonators support whispering gallery modes (WGMs) that combine ultrahigh Q-factors and small effective mode volumes. In contrast, their hollow nature enables enhanced interactions with encapsulated materials and lower spectral mode density due to the tight radial confinement of the optical modes. However, the existence of a high axial-mode density still leads to significant mode mixing and modal interference that can complicate spectral shift measurements, thereby limiting sensing applications. To address this limitation, we have fabricated geometric filters directly on the surface of microbubbles using focused ion beam (FIB) milling. Based on numerical calculations, we first designed and then fabricated large tapered patterns, such as circular surface dips or holes, that could effectively filter modes while minimizing optical scattering losses. Local lateral mode confinement and partial recovery of high Q-factors were experimentally achieved by adding shallow slit patterns. Using few-mode engineered microbubble resonators, we subsequently demonstrated pressure sensing and wide spectral tuning of WGMs free from mode-mixing artifacts. This precision engineering approach promises improved mode isolation, tunable directional emission, and ultrasensitive measurements in microbubble resonator devices.

physics.optics↗

arXiv2026

Quantum Chaos in Phase Space

Mesoscopic devices, with system sizes in the range of several to several dozens wavelengths, represent paradigmatic model systems for the observation of quantum chaotic behaviour based on semiclassical concepts. Those electronic and photonic billiard cavities are small enough for interference effects not to be ignored. Nonetheless, the classical ray or particle tracing picture can often provide a substantial understanding of the dynamics of the system along the lines of classical-quantum, or ray-wave correspondence. This well-established principle turns out to be particularly useful when applied not only in real space, but by extending it to phase space such that both location and momentum information can contribute to a deeper and more comprehensive understanding of the dynamical behaviour.

quant-ph↗

arXiv2025

Exceptional-point-controlled mode interaction in three-dimensional microcavities represented by generalized Husimi functions

Non-Hermitian photonics has attracted significant interest and influences several key areas such as optical metamaterials, laser physics, and nonlinear optics. While non-Hermitian effects have been widely addressed in two-dimensional systems, we focus on realistic three-dimensional devices. To this end we generalize established phase space methods from mesoscopic optics and introduce Husimi functions for three-dimensional systems that deepen the insight and access to the mode morphology and their dynamics. We illustrate that four-dimensional Husimi functions can be represented using a specific projection in two dimensions and illustrate it for (conical) cylindrical cavities. The non-Hermitian character of the intrinsically open photonic systems is in particular revealed when examining the TE and TM polarization character of the resonance modes. Unlike the 2D case, polarization is not conserved in three-dimensional cavities, and we use generalized Husimi function to represent the interaction of polarization modes. We find their dynamics to be ruled by a network of exceptional points in the parameter space spanned by the refractive index and the cavity geometry tilt angle. This approach not only enhances our understanding of cavity modes but also aids in the design of more efficient photonic devices and systems.

physics.optics↗

arXiv2024

Complex dynamics in circular and deformed bilayer graphene inspired billiards with anisotropy and strain

While billiard systems of various shapes have been used as paradigmatic model systems in the fields of nonlinear dynamics and quantum chaos, few studies have investigated anisotropic billiards. Motivated by the tremendous advances in using and controlling electronic and optical mesoscopic systems with bilayer graphene representing an easily accessible anisotropic material for electrons when trigonal warping is present, we investigate billiards of various anisotropies and geometries using a trajectory tracing approach founded in the concept of ray-wave correspondence. We find that the presence of anisotropy can render the billiards' dynamics dramatically from its isotropic counterpart. It may induce chaotic and mixed dynamics in otherwise integrable systems, and may stabilize originally unstable trajectories. We characterize the dynamics of anisotropic billiards in real and phase space using Lyapunov exponents and the Poincaré surface of section as phase space representation.

physics.optics↗

arXiv2024

Steering internal and outgoing electron dynamics in bilayer graphene cavities by cavity design

Ballistic, gate-defined devices in two-dimensional materials offer a platform for electron optics phenomena influenced by the material's properties and gate control. We study the ray trajectory dynamics of all-electronic, gate-defined cavities in bilayer graphene to establish how distinct regimes of the internal and outgoing charge carrier dynamics can be tuned and optimized by the cavity shape, symmetry, and parameter choice, e.g., the band gap and the cavity orientation. In particular, we compare the dynamics of two cavity shapes, o'nigiri, and Limaçon cavities, which fall into different symmetry classes. We demonstrate that for stabilising regular, internal cavity modes, such as periodic and whispering gallery orbits, it is beneficial to match the cavity shape to the bilayer graphene Fermi line contour. Conversely, a cavity of a different symmetry than the material dispersion allows one to determine preferred emission directionalities in the emitted far-field.

cond-mat.mes-hall↗

arXiv2023

Coupled deformed microdisk cavities featuring non-Hermitian properties

Coupled cavities are of interest as they expose qualitatively new effects, such as non-Hermitian properties, that are beyond the possibilitie of individual cavities. Here, we investigate the coupling between two dielectric two-dimensional microdisk cavities and compare circular vs. deformed (limaçon) resonator shapes as a function of their distance and address the effect of coupling on the far-field emission properties. We find that the asymmetric coupling characteristic for non-circular, deformed cavities induces non-Hermitian properties prominently evident in a mode-dependent chirality of the coupled cavity modes. We use an analytical model to explain our findings and reveal the direct connection between coupling asymmetry and the resulting sense of rotation of the coupled modes. While the overall far-field directionality remains robust for intercavity distances larger than two wavelengths, we observe enhanced and reversed emission for smaller distances even for only two coupled cavities. Our findings could prove useful for future applications such as far-field emission control and sensing .

physics.optics↗

arXiv2023

Gate-tunable regular and chaotic electron dynamics in ballistic bilayer graphene cavities

The dispersion of any given material is crucial for its charge carriers' dynamics. For all-electronic, gate-defined cavities in gapped bilayer graphene, we developed a trajectory-tracing algorithm aware of the material's electronic properties and details of the confinement. We show how the anisotropic dispersion of bilayer graphene induces chaotic and regular dynamics depending on the gate voltage, despite the high symmetry of the circular cavity. Our results demonstrate the emergence of non-standard fermion optics solely due to anisotropic material characteristics.

cond-mat.mes-hall↗

arXiv2023

Quantum transport in graphene nanoribbon networks: complexity reduction by a network decimation algorithm

We study electronic quantum transport in graphene nanoribbon (GNR) networks on mesoscopic length scales. We focus on zigzag GNRs and investigate the conductance properties of statistical networks. To this end we use a density-functional-based tight-binding model to determine the electronic structure and quantum transport theory to calculate electronic transport properties. We then introduce a new efficient network decimation algorithm that reduces the complexity in generic three-diemnsional GNR networks. We compare our results to semi-classical calculations based on the nodal analysis approach and discuss the dependence of the conductance on network density and network size. We show that a nodal analysis model cannot reproduce the quantum transport results nor their dependence on model parameters well. Thus, solving the quantum network by our efficient approach is mandatory for accurate modelling the electron transport through GNR networks.

cond-mat.mes-hall↗

arXiv2022

Universality and beyond in optical microcavity billiards with source-induced dynamics

Optical microcavity billiards are a paradigm of a mesoscopic model system for quantum chaos. We demonstrate the action and origin of ray-wave correspondence in real and phase space using far field emission characteristics and Husimi functions. Whereas universality induced by the invariant-measure dominated far field emission is known to be a feature shaping the properties of many lasing optical microcavities, the situation changes in the presence of sources that we discuss here. We investigate the source-induced dynamics and the resulting limits of universality while we find ray-picture results to remain a useful tool in order to understand the wave behaviour of optical microcavities with sources. We demonstrate the source-induced dynamics in phase space from the source ignition until a stationary regime is reached comparing results from ray, ray-with-phase, and wave simulations and explore ray-wave correpondence.

physics.optics↗

arXiv2022

Mesoscopic Möbius ladder lattices as non-Hermitian model systems

While classic quantum chaos originated from the idea to set into context nonlinear physics and Hermitian quantum mechanics, non-Hermitian models have enhanced the field in recent years. At the same time, low-dimensional effective matrix models have proven to be a powerful tool in accessing the physical properties of a system in a semiquantitative manner. Here, we focus on two realizations of non-Hermitian physics in mesoscopic systems. First, we consider spiral optical microcavities in which the asymmetric scattering between whispering gallery modes induces the non-Hermitian behaviour. Second, for parity-time (PT) symmetric ladder lattices we compare circular and Möbius geometries. We find the effective coupling between even and odd parity modes to be symmetric but complex in a microscopically derived 2 x 2 matrix model, resulting in non-Hermitian behaviour as well. Most importantly, the Möbius topology acts like a scatterer that induces a qualitatively new form of (avoided) level crossing - a PT-broken phase terminated by exceptional points - resulting from the symmetric but non-Hermitian coupling.

quant-ph↗

arXiv2021

Dirac fermion optics and directed emission from single- and bilayer graphene cavities

High-mobility graphene hosting massless charge carriers with linear dispersion provides a promising platform for electron optics phenomena. Inspired by the physics of dielectric optical micro-cavities where the photon emission characteristics can be efficiently tuned via the cavity shape, we study corresponding mechanisms for trapped Dirac fermionic resonant states in deformed micro-disk graphene billiards and directed emission from those. In such graphene devices a back-gate voltage provides an additional tunable parameter to mimic different effective refractive indices and thereby the corresponding Fresnel laws at the boundaries. Moreover, cavities based on single-layer and double-layer graphene exhibit Klein- and anti-Klein tunneling, respectively, leading to distinct differences with respect to dwell times and resulting emission profiles of the cavity states. Moreover, we find a variety of different emission characteristics depending on the position of the source where charge carriers are fed into the cavites. Combining quantum mechanical simulations with optical ray tracing and a corresponding phase-space analysis, we demonstrate strong confinement of the emitted charge carriers in the mid field of single-layer graphene systems and can relate this to a lensing effect. For bilayer graphene, trapping of the resonant states is more efficient and the emission characteristics do less depend on the source position.

cond-mat.mes-hall↗

arXiv2021

Husimi Functions for Coupled Optical Resonators

Phase-space analysis has been widely used in the past for the study of optical resonant systems. While it is usually employed to analyze the far-field behaviour of resonant systems we focus here on its applicability to coupling problems. By looking at the phase-space description of both the resonant mode and the exciting source it is possible to understand the coupling mechanisms as well as to gain insights and approximate the coupling behaviour with reduced computational efforts. In this work we develop the framework for this idea and apply it to a system of an asymmetric dielectric resonator coupled to a waveguide.

physics.optics↗

arXiv2020

Non-orientability induced PT phase transition in Moebius ladder lattices

We study parity-time (PT) phase transitions in the energy spectra of ladder lattices caused by the interplay between non-orientability and non-Hermitian PT symmetry. The energy spectra show level crossings in circular ladder lattices with increasing on-site energy gain-loss because of the orientability of a normal strip. However, the energy levels show PT phase transitions in PT-symmetric Moebius ladder lattices due to the non-orientability of a Moebius strip. In order to understand the level crossings of PT symmetric phases, we generalize the rotational transformation using a complex rotation angle. We also study the modification of resonant tunneling induced by a sharply twisted interface in PT-symmetric ladder lattices. Finally, we find that the perfect transmissions at the zero energy are recovered at the exceptional points of the PT-symmetric system due to the self-orthogonal states.

quant-ph↗

arXiv2020

Spin-orbit interaction of light in three-dimensional microcavities

We investigate the spin-orbit coupling of light in three-dimensional cylindrical and tube-like whispering gallery mode resonators. We show that its origin is the transverse confinement of light in the resonator walls, even in the absence of inhomogeneities or anisotropies. The spin-orbit interaction results in elliptical far-field polarization (spin) states and causes spatial separation of polarization handedness in the far field. The ellipticity and spatial separation are enhanced for whispering gallery modes with higher excitation numbers along the resonator height. We analyze the asymmetry of the ellipticity and the tilt of the polarization orientation in the far field of cone-like microcavities. Furthermore, we find a direct relationship between the tilt of the polarization orientation in the far field and the local inclination of the resonator wall. Our findings are based on FDTD-simulations and are supported by three-dimensional diffraction theory.

physics.optics↗

arXiv2019

Nonhermitian defect states from lifetime differences

Nonhermitian systems provide new avenues to create topological defect states. An unresolved general question is how much the formation of these states depends on asymmetric backscattering, be it nonreciprocal as in the nonhermitian skin effect or reciprocal as encountered between the internal states of asymmetric microresonators. Here, we demonstrate in a concrete, practically accessible setting of a lossy coupled-resonator optical waveguide that nonhermitian defect states can exist in open optical systems due to lifetime differences, without the need for asymmetric backscattering within or between the individual resonators. We apply our findings to a finite system of coupled circular resonators perturbed by nanoparticles, following the concept of creating an interface by inverting the position of the nanoparticles in half of the chain. We compare a coupled-mode tight-binding approximation to full-wave numerical simulations, showing that spectrally isolated defect states can indeed be implemented in this simple nonhermitian photonic device.

physics.optics↗

arXiv2019

Super-directional light emission and emission reversal from micro cavity arrays

Optical microdisk cavities with certain asymmetric shapes are known to possess unidirectional far-field emission properties. Here, we investigate arrays of these dielectric microresonators with respect to their emission properties resulting from the coherent behaviour of the coupled constituents. This approach is inspired by electronic mesoscopic physics where the additional interference effects are known to enhance the properties of the individual system. As an example we study the linear arrangement of nominally identical Limaçon-shaped cavities and find mostly an increase of the portion of directional emitted light while its angular spread is largely diminished from 20 degrees for the single cavity to about 3 degrees for a linear array of 10 Limaçon resonators, in fair agreement with a simple array model. Moreover, by varying the inter-cavity distance we observe windows of reversion of the emission directionality and super-directionality that can be interesting for applications. We introduce a generalized array factor model that takes the coupling into account.

physics.optics↗

arXiv2019

Analytical Fresnel laws at generic curved interfaces

Fresnel laws, the quantitative information of the amount of light that is reflected from a planar interface in dependence on its angle of incidence, are at the core of ray optics. However, these formulae do not hold at curved interfaces and deviations are appreciable when wavelength and radius of curvature are comparable. This is of particular interest for optical microcavities that play an important role in many modern research fields and applications such as microlasers. Their convexly curved interfaces modify Fresnel's law in a characteristic manner: the onset of total internal reflection is shifted to angles larger than the critical angle. Here, we derive the missing Fresnel laws for concavely curved refractive index boundaries, enabling the analytical description of light in complex mesoscopic optical structures that will be important in future nano- and microphotonic applications.

physics.optics↗