arXiv subjects

Ulf Peschel

Publications and source records attributed to Ulf Peschel.

At least 19 recordsLinked to original sources

arXiv2026

Optical extinction and near-field properties of plasmonic dimers: Role of particle shape and separation

Localized surface plasmons (LSPs) supported by metallic nanostructures exhibit a great tunability of their resonance frequencies and associated spatial field distributions. In particular, nanoscale gaps and sharp corners exhibit promising near-field enhancements for applications such as biochemical sensing and nanoantennas. In this work, we investigate the optical properties of bowtie nanowire dimers excited by linearly polarized light, including polarization orthogonal to the dimer axis. To distinguish dimer-separation-induced and monomer-shape-induced effects, we compare bowtie, circular-cylindrical dimer and triangular wires. We emphasize the advantages of a B\'ezier-type corner parametrization with tunable curvature and employ the discontinuous-Galerkin time-domain finite-element method for numerical calculations. This enables the identification of hybrid resonances and the systematic analysis of the role of geometrical parameters. For all investigated geometries, we compare the spatially local Drude model with the spatially nonlocal Halevi model. For the specific case of silver, we demonstrate quantitative curvature-dependent LSP line shifts arising from the longitudinal nonlocality captured by the Halevi model. For higher-order LSPs, this nonlocality gives rise to a sequence of resonances that may be exploited for light-harvesting applications. Furthermore, we investigate the field enhancement associated with LSP resonances and, motivated by the occurrence of both hot and cold spots, introduce geometry-dependent measures for assessing the ability of nanostructures to enhance nonlinear optical effects, such as through the optimal positioning of SERS-active molecules. To facilitate the simultaneous study these measures, we propose the use of suitable radar charts.

physics.optics↗

arXiv2026

Optimized near-field optical response via adaptive tip illumination

The performance of tip-enhanced optical microscopy is often limited by inefficient coupling of the excitation field to the plasmonic tip apex, as well as by thermal drift and optical aberrations. Here, we demonstrate that adaptive wavefront shaping based on Zernike mode provides a practical approach to achieving robust near-field optimisation at the tip apex. Using a sequential feedback algorithm, initially using the near-field signal, we narrow the illumination point-spread function and suppress sidelobes. This demonstrates that Zernike-mode control can be used for both aberration correction and field engineering. In tip-enhanced Raman measurements of a Janus MoSSe monolayer, conventional near-field optimisation increases the signal intensity by around 1.4 fold. A second optimisation step based directly on the Raman-band intensity yields a further 5 to 15 fold enhancement, depending on the specific tips used. These results establish a systematic, optics-based strategy for optimising tip fields, providing a transferable framework for improving tip-enhanced and related near-field spectroscopies.

physics.optics↗

arXiv2026

Self-consistent evaluation of the Berry connection for Wannier functions

The Berry connection is a gauge-dependent quantity frequently used to describe the optical response of solids. Its evaluation requires a k-derivative with respect to the cell periodic-part of the Bloch-functions and is commonly calculated in the Wannier basis by using overlap matrices of cell-periodic parts of Bloch-functions at neighboring k-points. So far, all proposed interpolation schemes for the Berry connection do not account for the matrix structure of the overlap matrices explicitly but treat the matrix elements as independent, or only distinguish between diagonal and off-diagonal entries. In this work, we propose a self-consistent interpolation scheme based on the matrix logarithm resulting in a strongly improved accuracy. Furthermore, we discuss how the basis set incompleteness of the bands used in the ab-initio calculation imposes constraints on the accuracy. We quantify the basis incompleteness based on the singular values of the overlap matrices and relate it to the invariant part of the spread functional $\Omega_\mathrm{I}$ of the Wannier functions. Numerical calculations for monolayer MoS$_2$ and bulk Si demonstrate that the proposed interpolation scheme is much less sensitive to the Wannierization details and leads to an improved quality of the velocity matrix and the optical conductivity.

cond-mat.mtrl-sci↗

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↗

arXiv2025

Modeling high-order harmonic generation in quantum dots using a real-space tight-binding approach

Recently, the size-dependence of high-order harmonic generation (HHG) in quantum dots has been investigated experimentally. In particular, for longer driving wavelengths and QDs smaller than 3\,nm, HHG was strongly suppressed, however, there is no computational model capable of describing the strong-field response of such systems. In this work, we introduce a computationally efficient three-dimensional real-space tight-binding model specifically designed for the simulation of HHG in confined systems. The model parameters are meticulously derived from density functional theory (DFT) calculations for the semiconductor bulk, followed by a process of Wannierization. Our findings demonstrate that the proposed model accurately captures the observed dependency of the HHG yield on the quantum dot size. Additionally, we simulate the HHG yield for elliptically polarized pulses for different QD-sizes and driving wavelengths up to $5\,μ{\mathrm{m}}$. The herein proposed model fills the theoretical void in simulating HHG within medium-sized nanostructures, which cannot be described by methods applied for periodic solids or small molecules or atoms.

physics.optics↗

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

Universal Routing of Light via Optical Thermodynamics

Understanding and exploiting the dynamics of complex nonlinear systems is nowadays at the core of a broad range of scientific and technological endeavors. Within the optical domain, light evolution in a nonlinear multimode environment presents a formidable problem, as its chaotic evolution often hinders predictive insights. Recently, an optical thermodynamic framework has been put forward that, in a systematic manner, can not only predict but also harness the intricate behavior of these systems. In this work, by deploying entropic principles, we demonstrate a counterintuitive optical process in which light, launched into any input port of a judiciously designed nonlinear array, universally channels into a tightly localized ground state, a response that is completely unattainable in linear conservative arrangements. This phenomenon arises from the interplay between lattice structure and the way the kinetic and nonlinear Hamiltonian components unfold, leading to two optical thermal processes: a Joule-Thomson-like expansion followed by mode thermalization. Experimentally, this effect is demonstrated in properly configured nonlinear time-synthetic mesh lattices, where the optical temperature approaches near zero, causing light to condense at a single spot, regardless of the initial excitation position. The effect demonstrated here opens new avenues for applying the principles of optical thermodynamics in realizing novel optical functionalities, such as all-optical beam steering, multiplexing, and nonlinear beam shaping in high-power regimes, while also offering a greater understanding of the remarkable physics of light-matter interactions in multimode nonlinear systems.

physics.optics↗

arXiv2025

Semiconductor Bloch equations in Wannier gauge with well-behaved dephasing

The semiconductor Bloch equations (SBEs) with a dephasing operator for the microscopic polarizations are a well established approach to simulate high-harmonic spectra in solids. We discuss the impact of the dephasing operator on the stability of the numerical integration of the SBEs in the Wannier gauge. It is shown that the standard approach to apply dephasing is ill-defined in the presence of band crossings and leads to artifacts in the carrier distribution. They are caused by rapid changes of the dephasing operator matrix elements in the Wannier gauge, which render the convergence of the simulation in the stationary basis infeasible. In the comoving basis, also called Houston basis, these rapid changes can be resolved, but only at the cost of a largely increased computation time. As a remedy, we propose a modification of the dephasing operator with reduced magnitude in energetically close subspaces. This approach removes the artifacts in the carrier distribution and significantly speeds up the calculations, while affecting the high-harmonic spectrum only marginally. To foster further development, we provide our parallelized source code.

physics.optics↗

arXiv2025

Ultrafast Coherent Bandgap Modulation Probed by Parametric Nonlinear Optics

Light-matter interactions in crystals are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non-invasive spectroscopy. While we often assume that the border between the two regimes of modulation and detection is sharp and well-defined, there are experiments where the boundaries fade. The study of these transition regions allows us to identify the real potentials and inherent limitations of the most commonly used optical spectroscopy techniques. Here, we measure and explain the co-existence between bandgap modulation and non-invasive spectroscopy in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor. We report a clear deviation from the typical quadratic power scaling of second-harmonic generation near an exciton resonance, and we explain this unusual result based on all-optical modulation driven by the intensity-dependent optical Stark and Bloch-Siegert shifts in the $\pm$K valleys of the Brillouin zone. Our experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which we extract the resonant transition dipole moments and dephasing times of the used sample. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light-matter interactions can modify the band structure of a crystal, even in the weak-field regime. Furthermore, our results strengthen the understanding of ultrafast all-optical control of electronic states in two-dimensional materials, with potential applications in valleytronics, Floquet engineering, and light-wave electronics.

physics.optics↗

arXiv2025

Real-time simulations of laser-induced electron excitations in crystalline ZnO

We investigate non-equilibrium electron dynamics in crystalline ZnO induced by ultrashort, relatively intense, infrared laser pulses. Our focus is on understanding the mechanism that facilitates efficient conduction band population in ZnO to enable optically pumped lasing. We consider two different pulse frequencies (in the near-infrared and mid-infrared) for which experimental data are available, and we calculate the electronic response of a ZnO crystal for a wide range of pulse intensities. We apply and compare three complementary theoretical approaches: the analytical Keldysh model, the numerical solution of the semiconductor Bloch equations, and real-time time-dependent density functional theory. We conclude that time-dependent density functional theory is a valid \textit{ab initio} approach for predicting conduction band population, that offers an accurate enough description of static and transient optical properties of solids and provides physics insight into the intermediate excitation regime, where electronic excitations are determined by the interplay of intraband tunneling, a consequence of band bending, and interband multi-photon absorption.

cond-mat.mtrl-sci↗

arXiv2025

Detecting high-dimensional time-bin entanglement in fiber-loop systems

Many quantum communication protocols rely on the distribution of entanglement between the different participating parties. One example is quantum key distribution (QKD), an application that has matured to commercial use in recent years. However, difficulties remain, especially with noise resilience and channel capacity in long-distance communication. One way to overcome these problems is to use high-dimensional entanglement, which has been shown to be more robust to noise and enables higher secret-key rates. It is therefore important to have access to certifiable high-dimensional entanglement sources to confidently implement these advanced QKD protocols. Here, we develop a method for certifying high-dimensional time-bin entanglement in fiber-loop systems. In these systems, entanglement creation and detection can utilize the same physical components, and the number of time bins, and thus the entanglement dimension, can be adapted without making physical changes to the setup. Our certification method builds on previous proposals for the certification of angular-momentum entanglement in photon pairs. In particular, measurements in only two experimentally accessible bases are sufficient to obtain a lower bound on the entanglement dimension for both two- and multiphoton quantum states. Numerical simulations show that the method is robust against typical experimental noise effects and works well even with limited measurement statistics, thus establishing time-bin encoded photons as a promising platform for high-dimensional quantum-communication protocols.

quant-ph↗

arXiv2024

A hybrid source of quantum light for generation of frequency tunable Fock states

We propose a scheme for quantum-light generation in a nonlinear cavity hybridized with a 2-level system and theoretically show that, when excited by a series of controlled pump pulses, the hybrid source generates Fock states with high probabilities. E.g., 1- and 2-photon states can be generated near-on-demand, and Fock states with up to $7$ photons with a probability above $50\%$. The tailorable nature of the nonlinear cavity allows for generating Fock states with arbitrary frequencies, even with a fixed 2-level system, creating fundamentally new opportunities in all areas of quantum technologies.

quant-ph↗

arXiv2024

Topologically tunable polaritons based on two-dimensional crystals in a photonic lattice

Topological photonics is an emergent research discipline which interlinks fundamental aspects of photonics, information processing and solid-state physics. Exciton-polaritons are a specifically interesting platform to study topological phenomena, since the coherent light matter coupling enables new degrees of freedom such as tunable non-linearities, chiralities and dissipation. Room-temperature operation of such exciton-polaritons relies on materials comprising both, large exciton binding energies and oscillator strength. We harness widely spectrally tunable, room temperature exciton-polaritons based on a WS2 monolayer in an open optical cavity to realize a polariton potential landscape which emulates the Su-Schrieffer-Heeger (SSH) Hamiltonian. It comprises a domain boundary hosting a topological, exponentially localized mode at the interface between two lattices characterized by different Zak-phases which features a spectral tunability over a range as large as 80 meV. Moreover, we utilize the unique tilt-tunability of our implementation, to transform the SSH-lattice into a Stark-ladder. This transformation couples the topologically protected defect mode to propagating lattice modes, and effectively changes the symmetry of the system. Furthermore, it allows us to directly quantify the Zak-phase difference $Δ_{Zak}=(1.13\pm 0.11)π$ between the two topological phases. Our work comprises an important step towards in-situ tuning topological lattices to control and guide light on non-linear chips.

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

Theory of hydrodynamic phenomena in optical mesh lattices

Signatures of superfluid-like behaviour have recently been observed experimentally in a nonlinear optical mesh lattice, where the arrival time of optical pulses propagating in a pair of coupled optical fiber loops is interpreted as a synthetic spatial dimension. Here, we develop a general theory of the fluid of light in such optical mesh lattices. On the one hand, this theory provides a solid framework for an analytical and numerical interpretation of the experimental observations. On the other hand it anticipates new physical effects stemming from the specific spatio-temporally periodic geometry of our set-up. Our work opens the way towards the full exploitation of optical mesh lattices system as a promising platform for studies of hydrodynamics phenomena in fluids of light in novel configurations.

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↗

arXiv2024

Unveiling the Role of Electron-Phonon Scattering in Dephasing High-Order Harmonics in Solids

High-order harmonic generation (HHG) in solids is profoundly influenced by the dephasing of the coherent electron-hole motion driven by an external laser field. The exact physical mechanisms underlying this dephasing, crucial for accurately understanding and modelling HHG spectra, have remained elusive and controversial, often regarded more as an empirical observation than a firmly established principle. In this work, we present comprehensive experimental findings on the wavelength-dependency of HHG in both single-atomic-layer and bulk semiconductors. These findings are further corroborated by rigorous numerical simulations, employing ab initio real-time, real-space time-dependent density functional theory and semiconductor Bloch equations. Our experimental observations necessitate the introduction of a novel concept: a momentum-dependent dephasing time in HHG. Through detailed analysis, we pinpoint momentum-dependent electron-phonon scattering as the predominant mechanism driving dephasing. This insight significantly advances the understanding of dephasing phenomena in solids, addressing a long-standing debate in the field. Furthermore, our findings pave the way for a novel, all-optical measurement technique to determine electron-phonon scattering rates and establish fundamental limits to the efficiency of HHG in condensed matter.

physics.optics↗

arXiv2022

Nonclassical light generation and control from laser-driven semiconductor intraband excitations

We investigate the generation of higher-order harmonics from a quantum optics perspective via the interaction of a semiconductor with a coherent pump field focusing on the regime where strong-field intraband excitations dominate. The related Schrödinger equation for the system is solved approximately and the classical and quantum characteristics of the fundamental light mode as well as the higher frequency modes are analyzed. We find intricate but sufficiently mild modifications of the fundamental mode and coherent displacements depending on the position quadrature component of the driving laser field for the harmonic modes within our approximations due to the intricate induced nonlinear interactions. Similar to high-harmonic generation in atoms, all radiation field modes are entangled, allowing for potential novel protocols for quantum information processing with high photon numbers over a large range of frequencies.

quant-ph↗