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Researcher profile

Christian Wolff

Christian Wolff contributes to research discovery and scholarly infrastructure.

ResearcherAffiliation not importedOpen to collaborate
cond-mat.mes-hallphysics.opticsComputation and Languagephysics.app-phquant-ph

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Published work

13 published item(s)

preprint2026arXiv

Annotation Quality in Aspect-Based Sentiment Analysis: A Case Study Comparing Experts, Students, Crowdworkers, and Large Language Model

Aspect-Based Sentiment Analysis (ABSA) enables fine-grained opinion analysis by identifying sentiments toward specific aspects or targets within a text. While ABSA has been widely studied for English, research on other languages such as German remains limited, largely due to the lack of high-quality annotated datasets. This paper examines how different annotation sources influence the development of German ABSA. To this end, an existing dataset is re-annotated by experts to establish a ground truth, which serves as a reference for evaluating annotations produced by students, crowdworkers, Large Language Models (LLMs), and experts. Annotation quality is compared using Inter-Annotator Agreement (IAA) and its impact on downstream model performance for different ABSA subtasks. The evaluation focuses on Aspect Category Sentiment Analysis (ACSA) and Target Aspect Sentiment Detection (TASD). We apply State-of-the-Art (SOTA) methods for ABSA, including BERT-, T5-, and LLaMA-based approaches to assess performance differences, spanning fine-tuning and in-context learning with instruction prompts. The findings provide practical insights into trade-offs between annotation reliability and efficiency, offering guidance for dataset construction in under-resourced Natural Language Processing (NLP) scenarios.

0 citations0 reviewsNo code linkedOpen access
preprint2026arXiv

Zero-Shot to Full-Resource: Cross-lingual Transfer Strategies for Aspect-Based Sentiment Analysis

Aspect-based Sentiment Analysis (ABSA) extracts fine-grained opinions toward specific aspects within text but remains largely English-focused despite major advances in transformer-based and instruction-tuned models. This work presents a multilingual evaluation of state-of-the-art ABSA approaches across seven languages (English, German, French, Dutch, Russian, Spanish, and Czech) and four subtasks (ACD, ACSA, TASD, ASQP). We systematically compare different transformer architectures under zero-resource, data-only, and full-resource settings, using cross-lingual transfer, code-switching and machine translation. Fine-tuned Large Language Models (LLMs) achieve the highest overall scores, particularly in complex generative tasks, while few-shot counterparts approach this performance in simpler setups, where smaller encoder models also remain competitive. Cross-lingual training on multiple non-target languages yields the strongest transfer for fine-tuned LLMs, while smaller encoder or seq-to-seq models benefit most from code-switching, highlighting architecture-specific strategies for multilingual ABSA. We further contribute two new German datasets, an adapted GERestaurant and the first German ASQP dataset (GERest), to encourage multilingual ABSA research beyond English.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Disentangling cathodoluminescence spectra in nanophotonics: particle eigenmodes vs transition radiation

Cathodoluminescence spectroscopy performed in an electron microscope has proven a versatile tool for analysing the near- and far-field optical response of plasmonic and dielectric nanostructures. Nevertheless, the transition radiation produced by electron impact is often disregarded in the interpretation of the spectra recorded from resonant nanoparticles. Here we show, experimentally and theoretically, that transition radiation can by itself generate distinct resonances which, depending on the time of flight of the electron beam inside the particle, can result from constructive or destructive interference in time. Superimposed on the eigenmodes of the investigated structures, these resonances can distort the recorded spectrum and lead to potentially erroneous assignment of modal characters to the spectral features. We develop an intuitive analogy that helps distinguish between the two contributions. As an example, we focus on the case of silicon nanospheres, and show that our analysis facilitates the unambiguous interpretation of experimental measurements on Mie-resonant nanoparticles.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Fundamental issues with light propagation through $\mathcal{PT}$-symmetric systems

We analyse the emergence of unphysical superluminal group velocities in Su--Schrieffer--Heeger (SSH) parity-time ($\mathcal{PT}$) symmetric chains, and explore the origins of such a behaviour. By comparing the band structure of an infinite loss-gain SSH chain with that of a one-dimensional Bragg stack, we first exclude insufficient coupling consideration in the tight-binding description as the cause of group-velocity divergence. We then focus on material dispersion, and show that indeed, restoring causality in the description of both the lossy and the gain components resolves the problem and recovers finite group velocities, whose real part can only exceed the speed of light in vacuum when accompanied by a significant imaginary part. Our analysis introduces thus the required practical limits in the performance of common $\mathcal{PT}$-symmetric systems.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Nonreciprocal vortex isolator by stimulated Brillouin scattering in chiral photonic crystal fibre

Optical non-reciprocity, which breaks the symmetry between forward and backward propagating optical waves, has become vital in photonic systems and enables many key devices, such as optical isolators, circulators and optical routers. Most conventional optical isolators involve magneto-optic materials, but devices based on optical nonlinearities, optomechanically induced transparency and stimulated Brillouin scattering (SBS) have also been demonstrated. So far, however, they have only been implemented for linearly or randomly polarized LP01-like fundamental modes. Here we report a light-driven nonreciprocal isolator for optical vortex modes, based on topology-selective SBS in chiral photonic crystal fibre. The device can be reconfigured as an amplifier or an isolator by adjusting the frequency of the control signal. The experimental results show vortex isolation of 22 dB, which is at the state-of-the-art in fundamental mode isolators using SBS. This unique device may find applications in optical communications, fibre lasers, quantum information processing and optical tweezers.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Quantum coherent control in pulsed waveguide optomechanics

Coherent control of traveling acoustic excitations in a waveguide system is an interesting way to manipulate and transduce classical and quantum information. So far, these interactions, often based on optomechanical resonators or Brillouin scattering, have been studied in the steady-state regime using continuous waves. However, waveguide experiments are often based on optical pump pulses which require treatment in a dynamic framework. In this paper, we present an effective Hamiltonian formalism in the dynamic regime using optical pulses that links waveguide optomechanics and cavity optomechanics, which can be used in the classical and quantum regime including quantum noise. Based on our formalism, a closed solution for coupled-mode equation under the undepleted assumption is provided and we found that the strong coupling regime is already accessible in current Brillouin waveguides by using pulses. We further investigate several possible experiments within waveguide optomechanics, including Brillouin-based coherent transfer, Brillouin cooling, and optoacoustic entanglement.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

Extremely confined gap plasmon modes: when nonlocality matters

Historically, the field of plasmonics has been relying on the framework of classical electrodynamics, with the local-response approximation of material response being applied even when dealing with nanoscale metallic structures. However, when approaching the atomic-scale confinement of the electromagnetic radiation, mesoscopic effects are anticipated to become observable, e.g., those associated with the nonlocal electrodynamic surface response of the electron gas. We investigate nonlocal effects in propagating gap surface plasmon modes in ultrathin metal--dielectric--metal planar waveguides, exploiting monocrystalline gold flakes separated by atomic-layer-deposited aluminum oxide. We use scanning near-field optical microscopy to directly access the near-field of such confined gap plasmon modes and measure their dispersion relation (via their complex-valued propagation constants). We compare our experimental findings with the predictions of the generalized nonlocal optical response theory to unveil signatures of nonlocal damping, which becomes appreciable for smaller dielectric gaps.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Anisotropic Second Harmonic Generation From Monocrystalline Gold Flakes

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Apart from suggesting an approach for directional enhancement of nonlinear response in plasmonic systems, we anticipate that our findings can be used as a rapid and non-destructive method for characterization of crystal quality and orientation that may be of significant importance in future applications.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Role of diffusive surface scattering in nonlocal plasmonics

The recent generalised nonlocal optical response (GNOR) theory for plasmonics is analysed, and its main input parameter, namely the complex hydrodynamic convection-diffusion constant, is quantified in terms of enhanced Landau damping due to diffusive surface scattering of electrons at the surface of the metal. GNOR has been successful in describing plasmon damping effects, in addition to the frequency shifts originating from induced-charge screening, through a phenomenological electron diffusion term implemented into the traditional hydrodynamic Drude model of nonlocal plasmonics. Nevertheless, its microscopic derivation and justification is still missing. Here we discuss how the inclusion of a diffusion-like term in standard hydrodynamics can serve as an efficient vehicle to describe Landau damping without resorting to computationally demanding quantum-mechanical calculations, and establish a direct link between this term and the Feibelman $d$ parameter for the centroid of charge. Our approach provides a recipe to connect the phenomenological fundamental GNOR parameter to a frequency-dependent microscopic surface-response function. We therefore tackle one of the principal limitations of the model, and further elucidate its range of validity and limitations, thus facilitating its proper application in the framework of nonclassical plasmonics.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Stimulated plasmon polariton scattering

The plasmon and phonon polaritons of two-dimensional (2d) and van-der-Waals materials have recently gained substantial interest. Unfortunately, they are notoriously hard to observe in linear response because of their strong confinement, low frequency and longitudinal mode symmetry. Here, we propose a fundamentally new approach of harnessing nonlinear resonant scattering that we call stimulated plasmon polariton scattering (SPPS) in analogy to the opto-acoustic stimulated Brillouin scattering (SBS). We show that SPS allows to excite, amplify and detect 2d plasmon and phonon polaritons all across the THz-range while requiring only optical components in the near-IR or visible range. We present a coupled-mode theory framework for SPS and based on this find that SPS power gains exceed the very top gains observed in on-chip SBS by at least an order of magnitude. This opens exciting new possibilities to fundamental studies of 2d materials and will help closing the THz gap in spectrocopy and information technology.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

The importance of substrates for the visibility of "dark" plasmonic modes

Dark plasmonic modes have interesting properties, such as a longer lifetime and a narrower linewidth than their radiative counterpart, as well as little to no radiative losses. However, they have not been extensively studied yet due to their optical inaccessibility. Using electron-energy loss (EEL) and cathodoluminescence (CL) spectroscopy, the dark radial breathing modes (RBMs) in thin, monochrystalline gold nanodisks are systematically investigated in this work. It is found that the RBMs can be detected in a CL set-up despite only collecting the far-field. Their visibility in CL is attributed to the breaking of the mirror symmetry by the high-index substrate, creating an effective dipole moment. The outcoupling into the far-field is demonstrated to be enhanced by a factor of 4 by increasing the thickness of the supporting SiN membrane from 5 to 50 nm due to the increased net electric dipole moment in the substrate. Furthermore, it is shown that the resonance energy of RBMs can be easily tuned by varying the diameter of the nanodisk, making them promising candidates for nanophotonic applications.

0 citations0 reviewsNo code linkedOpen access
preprint2019arXiv

Magnetic and Electric Mie-Exciton Polaritons in Silicon Nanodisks

Light-matter interactions at the nanoscale constitute a fundamental ingredient for engineering applications in nanophotonics and quantum optics. To this regard electromagnetic Mie resonances excited in high-refractive index dielectric nanoparticles have recently attracted interest because of their lower losses and better control over the scattering patterns compared to their plasmonic metallic counterparts. The emergence of several resonances in those systems results in an overall high complexity, where the electric and magnetic dipoles have significant overlap in the case of spherical symmetry, thus concealing the contributions of each resonance separately. Here we show, experimentally and theoretically, the emergence of strong light-matter coupling between the magnetic and electric-dipole resonances of individual silicon nanodisks coupled to a J-aggregated organic semiconductor resonating at optical frequencies, evidencing how the different properties of the two resonances results in two different coupling strengths. The energy splittings observed are of the same order of magnitude as in similar plasmonic systems, thus confirming dielectric nanoparticles as promising alternatives for localized strong coupling studies. The coupling of both the electric and magnetic dipole resonances can offer interesting possibilities for the control of directional light scattering in the strong-coupling regime and the dynamic tuning of nanoscale light-matter coupled states by external fields.

0 citations0 reviewsNo code linkedOpen access
preprint2019arXiv

Plasmonic monolithic lithium niobate directional coupler switches

From the onset of high-speed optical communications, lithium niobite (LN) has been the material of choice for electro-optic modulators owing to its large electro-optic response, wide transparent window, excellent thermal stability and long-term material reliability. Conventional LN electro-optic modulators while continue to be the workhorse of the optoelectronic industry become progressively too bulky, expensive and power hungry to fully serve the needs of this industry rapidly progressing towards highly integrated, cost-effective and energy efficient components and circuits. Recently developed monolithic LN nanophotonic platform enables the realization of electro-optic modulators that are significantly improved in terms of compactness, bandwidth and energy efficiency, while still demanding relatively long, on the mm-scale, interaction lengths. Here we successfully deal with this challenge and demonstrate plasmonic electro-optic directional coupler switches consisting of two closely spaced nm-thin gold nanostripes monolithically fabricated on LN substrates that guide both coupled electromagnetic modes and electrical signals influencing their coupling and thereby enabling ultra-compact switching and modulatiofunctionalities. The extreme confinement of both slow-plasmon modes and electrostatic fields created by two nanostripes along with their nearly perfect spatial overlap allowed us to achieve a 90% modulation depth with 20-$μ$m-long switches characterized by a electro-optic modulation efficiency of 0.3 Vcm. Our monolithic LN plasmonic platform enables ultra-dense integration of high-performance active photonic components, enabling a wide range of cost-effective optical communication applications demanding $μ$m-scale footprints, ultrafast operation, robust design and high environmental stability.

0 citations0 reviewsNo code linkedOpen access