arXiv subjects

Andreas Knorr

Publications and source records attributed to Andreas Knorr.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Diamagnetic Interactions in Ultrastrongly Coupled 2D Electrons

The quantum Hopfield model is widely used to describe ultrastrong light--matter coupling between cavity photons and collective bosonic excitations in solids, where the diamagnetic interaction is conventionally assumed to be a constant. We experimentally demonstrate that the diamagnetic response of Landau polaritons is reduced under strong terahertz field excitation. We show that this behavior originates from field-driven redistribution of electrons into the nonparabolic regime of the conduction band of GaAs, which reduces the plasma frequency and consequently the diamagnetic interaction strength. A microscopic hot-electron model reproduces the observed nonlinear response. Motivated by this microscopic picture, we propose a nonlinear extension of the Hopfield model with a Kerr-like interaction. Our results establish a route toward nonlinear cavity quantum electrodynamics and driven ultrastrong light--matter coupling beyond the conventional linear Hopfield description, which is capable of creating uniquely quantum optical effects such as squeezed light generation.

quant-ph↗

arXiv2026

Probing the nonlocality of Landau levels in GaAs quantum wells through modified Purcell factors, Lamb shifts and dipole emitted spectra

In a two-dimensional electron gas, a strong perpendicular magnetic field confines electrons to quantized cyclotron orbits, giving rise to Landau levels with discrete orbit radii. Even the smallest Landau orbit, set by the magnetic length, spans tens of nanometers for fields of a few Tesla, imposing an intrinsic nonlocal response to electromagnetic excitations. From a microscopic theory of the nonlocal susceptibility, we derive the Green's function, the central quantity governing all electromagnetic interactions, and evaluate Purcell factors, Lamb shifts, and emission spectra from a proximal dipole emitter beyond the Markov and rotating-wave approximations. Significant nonlocal effects resulting from spatial dispersion of the Landau level response modify the response for experimentally relevant situations up to distances of hundreds of nanometers and, in particular, brighten locally dipole-forbidden transitions due to near-field gradients at multiples of the cyclotron frequency. The relevant length scales are typical of state-of-the-art nanostructured terahertz architectures, and some of our nonlocal features are consistent with recent experiments using Landau level polaritons.

cond-mat.mes-hall↗

arXiv2026

Electron- and Lattice-Temperature Dependence of the Optical Response of Gold Nanoparticles

Transient absorption spectroscopy is routinely used to study the electron dynamics in plasmonic gold nanoparticles. Typically, the transient absorption bleach is analyzed as measure for the electron temperature. However, the implicitly assumed linear dependence between bleach intensity and temperature has not been systematically studied. Similarly, the influence of lattice heating also lacks a detailed analysis. Here, we solve momentum-resolved metal Boltzmann-Bloch equations for a semi-analytic access to the temperature-dependent gold nanoparticle absorption. We confirm the theory with steady state and transient absorption experiments, define regions of linear correlation between transient absorption bleach intensity and electron temperature and reveal a strong impact of the lattice temperature on the TA bleach intensity.

cond-mat.mes-hall↗

arXiv2026

Theory of x-ray scattering from optically pumped excitons in atomically thin semiconductors

We propose a framework to explore the internal charge distribution of mesoscopic quasiparticles by inelastic x-ray scattering, while also accounting for the conventional scattering from electrons. Specifically, we investigate a new contribution of intrinsic and optically pumped excitons (bound electron-hole pairs) to the x-ray scattering spectrum of transition metal dichalcogenides (TMDCs). The optical excitation leads to the creation of Wannier exciton populations, adding new quasi-elastic processes beyond the conventional electronic features to the x-ray scattering spectra. Differential spectra (with and without optical pumping) can be used to isolate and identify the internal charge distribution of the optically pumped excitons in the scattering response, potentially offering insights into many-body interactions and quasi-particle dynamics in 2D systems.

cond-mat.mes-hall↗

arXiv2026

Boltzmann-Bloch Equation Approach to the Theory of the Optical Inter- and Intraband Response in Noble Metals

In this paper we introduce momentum-resolved metal Boltzmann-Bloch equations (MBBE) for the combined description of electronic intra- and interband processes in noble metals. This microscopic framework incorporates a full treatment of many-body electron-electron and electron-phonon interactions, relevant for relaxation and dephasing processes after optical excitation. For the example of gold, we calculate the linear optical response for near-infrared and visible energies. This provides insight into the interplay of microscopic processes hidden in phenomenological Drude-Lorentz models. The complex geometry of the Fermi surface is treated by an anisotropic electronic dispersion model, which is necessary to explain the temperature dependent spectrum over the whole frequency range of intra- and interband transitions.

cond-mat.mes-hall↗

arXiv2026

Probing Interface-Driven Mechanisms of Non-Classical Light in van der Waals Heterostructures

Single-photon emitters in two-dimensional semiconductors offer a versatile platform for integrated quantum photonics, yet their performance is strongly influenced by local dielectric environments and substrate-induced disorder. Here, we examine SPEs in monolayer WSe$_2$ incorporated into hBN/WSe$_2$/Clinochlore van der Waals heterostructures and assess how interface-mediated dielectric modulation governs their optical and quantum characteristics. Low-temperature micro-photoluminescence reveals narrow emission lines (100 - 300 $μ$eV) and robust non-classical behavior, with $g^{(2)}(0) = 0.13 \pm 0.02$ on SiO$_2$ and $0.54 \pm 0.02$ for emitters directly coupled to Clinochlore. Magneto-optical measurements yield effective g-factors near -8, consistent with defect states hybridized with dark excitons. WSe$_2$ on Clinochlore exhibits up to a fivefold enhancement in emission intensity, attributed to coupling with Fe-related substrate states that introduce resonant absorption near 1.75 eV. Kelvin probe force microscopy confirms strong dielectric contrast across thin and thick Clinochlore regions. Time-resolved photoluminescence shows that emitters on SiO$_2$ display a single $\approx 4$ ns lifetime, whereas those on Clinochlore exhibit biexponential dynamics with sub-nanosecond and tens-of-nanoseconds decay components. A phenomenological model incorporating coupling to bright and dark Fe-related states in Clinochlore accounts for modified excitation pathways. These results establish interface dielectric engineering in vdW heterostructures as an effective strategy for tailoring the radiative dynamics and brightness of quantum emitters in atomically thin materials.

cond-mat.mtrl-sci↗

arXiv2026

Many-Body Rashba Spin-Orbit Interaction and Exciton Spin Relaxation in Atomically Thin Semiconductor Structures

We propose a pair spin-orbit interaction (PSOI) mechanism by establishing a mesoscopic many-particle Rashba Hamiltonian. In lowest order, this Hamiltonian self-consistently describes exciton spin relaxation in monolayer transition metal dichalcogenides (TMDC) due to local electric fields caused by spatial asymmetries in the dielectric environment. For a monolayer MoSe$_2$ on a SiO$_2$ substrate above 77$\,$K showing a bright-dark splitting in the meV range, the local electric field causes fast intravalley spin relaxation on a sub-picosecond timescale, whereas it is negligible for other TMDCs with larger bright-dark splitting.

cond-mat.mes-hall↗

arXiv2026

Excitonic Theory of the Ultrafast Optical Response of 2D-Quantum-Confined Semiconductors at Elevated Densities

An excitonic approach to the ultrafast optical response of confined semiconductors at elevated densities below the Mott transition is presented. The theory is valid from the coherent regime, where coherent excitonic transitions and biexcitons dominate, to the incoherent regime, where excitonic occupations dominate. Numerical simulations of the $1s$ exciton dynamics during intense circularly polarized pump pulses in two different Coulomb-interaction regimes are performed for two-dimensional semiconductors: Moderate Coulomb interaction is compared with dominating Coulomb interaction with respect to the light-matter interaction strength. The different many-body contributions are disentangled and it is found, that excitonic Rabi oscillations in the Coulomb-dominated regime are considerably less strong. By also comparing circular and linear excitation in a MoSe$_2$ monolayer, it is found, that linear excitation creates a regime, where excitonic Rabi oscillations are almost completely suppressed.

cond-mat.mes-hall↗

arXiv2026

Coulomb Interaction in Atomically Thin Semiconductors and Density-Independent Exciton-Scattering Processes

In quantum-kinetic approaches to the dynamics of Coulomb-bound many-body correlations such as excitons, trions, biexcitons or higher-order correlations, a detailed knowledge of the many-body Coulomb Hamiltonian serving as a starting point is important. In this manuscript, the second-quantized description of the Coulomb interaction between Bloch electrons in a Heisenberg-equation-of-motion approach in atomically thin semiconductors is derived and reviewed. Emphasis is put on a discussion of Umklapp processes and the dielectric screening including all local-field effects. A link between \textit{ab initio} methods of screening and few-band models in effective-mass approximations for the quantum kinetics is established and all important Coulomb scattering processes contributing to the exciton energy landscape and density-independent exciton scattering are discussed.

cond-mat.mes-hall↗

arXiv2026

Theory of In-Plane-Magnetic-Field-Dependent Excitonic Spectra in Atomically Thin Semiconductors

The linear absorption spectrum of excitons in TMDC monolayers under the influence of an in-plane magnetic field is theoretically studied. We demonstrate that in-plane magnetic fields induce a hybridization between spin-bright and spin-dark exciton transitions, resulting in a brightening of spin-dark excitons. We analytically investigate spectral features including resonance energy shifts, broadening and amplitudes ratios. In particular, for a MoSe$_2$ monolayer with radiatively-limited linewidth, we find a complex interplay of dark-bright splitting and linewidth difference of both involved spin-bright and spin-dark excitons.

cond-mat.mes-hall↗

arXiv2025

Dissecting intervalley coupling mechanisms in monolayer transition metal dichalcogenides

Monolayer (1L) transition metal dichalcogenides (TMDs) provide a unique opportunity to control the valley degree of freedom of optically excited charge carriers due to the spin-valley locking effect. Despite extensive studies of the valley-contrasting physics, stimulated by perspective valleytronic applications, a unified picture of competing intervalley coupling processes in 1L-TMDs is lacking. Here, we apply broadband helicity-resolved transient absorption to explore exciton valley polarization dynamics in 1L-WSe${}_2$. By combining experimental results with microscopic simulations, we dissect individual intervalley coupling mechanisms and reveal the crucial role of phonon-assisted scattering in the fast decay of the A exciton circular dichroism and the formation of the dichroism of opposite polarity for the B exciton. We further provide a consistent description of the valley depolarization driven by the combined action of Coulomb scattering processes and indicate the presence of efficient single spin-flip mechanisms. Our study brings us closer to a complete understanding of exciton dynamics in TMDs.

cond-mat.mes-hall↗

arXiv2025

Ultrafast transition from coherent to incoherent polariton nonlinearities in a hybrid 1L-WS2/plasmon structure

Exciton polaritons based on atomically thin semiconductors are essential building blocks of quantum optoelectronic devices. Their properties are governed by an ultrafast and oscillatory energy transfer between their excitonic and photonic constituents, resulting in the formation of polaritonic quasiparticles with pronounced nonlinearities induced by the excitonic component. In metallic nanoresonators, dissipation phenomena limit the polariton lifetime to a few ten femtoseconds, so short that the role of these polaritons for the nonlinearities of such hybrids is yet unexplored. Here, we use ultrafast two-dimensional electronic spectroscopy (2DES) to uncover coherent polariton dynamics in a hybrid monolayer (1L) WS2/plasmonic nanostructure. With respect to an uncoupled WS2 flake, we observe an over 20-fold, polarization-dependent enhancement of the optical nonlinearity and a rapid evolution of the 2DES spectra within ~70 fs. We relate these dynamics to a transition from coherent polaritons to incoherent excitations, unravel the microscopic optical nonlinearities, and show the potential of coherent polaritons for ultrafast all-optical switching.

cond-mat.mes-hall↗

arXiv2025

Theory of Non-Linear Electron Relaxation in Thin Gold Films and Their Signatures in Optical Observables

Based on the momentum-resolved Boltzmann equation, we provide self-consistent numerical calculations of the dynamics of conduction electrons in thin noble metal films after linear and non-linear optical excitations with infrared and terahertz frequencies. Focusing exclusively on electron-phonon interaction, orientational relaxation is introduced and acts as dephasing of the optical excitation on a scale of tens of fs. In the linear regime, our numerical results agree with the experimental fits to a Drude model and predicts for non-linear excitations a field strength dependency of the orientational relaxation rate. In the THz regime, where the orientational relaxation proceeds faster than the oscillation cycle of the excitation THz field, a new high order dissipative Kerr-type non-linearity is predicted. This non-linearity originates from the Pauli blocking included in the electron-phonon scattering and results in a non-linearly increasing transmission of the film, detectable in experiments.

cond-mat.mes-hall↗

arXiv2025

Ultrafast Electrons in Noble Metals: Orientational Relaxation, Thermalization and Cooling in Terms of Electron-Phonon Interaction

We investigate the momentum-resolved dynamics of conduction electrons in noble metals following ultrashort optical excitation with linearly polarized light. Using a momentum-resolved Boltzmann equation approach for electron-phonon interaction, we solve for the combined effects of orientational relaxation, thermalization, and cooling. We introduce momentum orientational relaxation as the initial step in the equilibration of an optically excited non-equilibrium electron gas by highlighting its importance for the optical response of noble metals and the dephasing of plasmonic excitations. Our numerical results for gold reveal that orientational relaxation exists independently of the absorbed optical energy and dominates on time scales on the first tens of fs after excitation. Incorporating also thermalization and cooling on times up to a few ps, our approach provides a simultaneous description of optical and thermal properties of noble metals under initial non-equilibrium conditions.

cond-mat.mes-hall↗

arXiv2025

Ab initio Maxwell-Bloch Approach for X-Ray Excitations in Two-Dimensional Materials

The combination of Maxwell and X-ray Bloch equations forms an appropriate framework to describe ultrafast time-resolved X-ray experiments on attosecond time scale in crystalline solids. However, broadband experiments such as X-ray absorption near edge spectroscopy or resonant inelastic X-ray scattering require a detailed knowledge of the electronic structure and transition matrix elements. Here, we show how to fill this gap by combining the Maxwell-X-ray Bloch formalism with first-principles calculations treating explicitly the core states. The resulting X-ray absorption spectrum recovers key spectral signatures which were missing in our previous work relying on a semi-empirical tight-binding approach.

cond-mat.mtrl-sci↗

arXiv2024

Laterally Extended States of Interlayer Excitons in Reconstructed MoSe$_2$/WSe$_2$ Heterostructures

Heterostructures made from 2D transition-metal dichalcogenides are known as ideal platforms to explore excitonic phenomena ranging from correlated moiré excitons to degenerate interlayer exciton ensembles. So far, it is assumed that the atomic reconstruction appearing in some of the heterostructures gives rise to a dominating localization of the exciton states. We demonstrate that excitonic states in reconstructed MoSe$_2$/WSe$_2$ heterostructures can extend well beyond the moiré periodicity of the investigated heterostructures. The results are based on real-space calculations yielding a lateral potential map for interlayer excitons within the strain-relaxed heterostructures and corresponding real-space excitonic wavefunctions. We combine the theoretical results with cryogenic photoluminescence experiments, which support the computed level structure and relaxation characteristics of the interlayer excitons.

cond-mat.mes-hall↗

arXiv2024

Microscopic theory for a minimal oscillator model of exciton-plasmon coupling in hybrids of 2d semiconductors and metal nanoparticles

The common model to describe exciton-plasmon interaction phenomenologically is the coupled oscillator model. Originally developed for atomic systems rather than solid-state matter, this model treats both excitons and plasmons as single harmonic oscillators coupled via a constant which can be fitted to experiments. In this work, we present a modified coupled oscillator model specifically designed for exciton-plasmon interactions in hybrids composed of two-dimensional excitons, such as in a transition metal dichalcogenide (TMDC) monolayers and metal nanoparticles while maintaining the simplicity of the commonly applied coupled oscillator models. Our approach is based on a microscopic perspective and Maxwell's equations, allowing to analytically derive an effective exciton-plasmon coupling constant. Our findings highlight the importance of the spatial dispersion, i.e., the delocalized nature of TMDC excitons, necessitating the distinction between bright and momentum-dark excitons. Both types of excitons occur at different resonance energies and exhibit a qualitatively different coupling with localized plasmons. We find a strong coupling between the plasmon and momentum-dark excitons, while a weakly coupled bright exciton manifests as an additional, third peak in the spectrum. Consequently, we propose a realistic modeling of the primary spectral features in experiments incorporating three harmonic oscillator equations instead of the conventional two. However, we also shed light on the limitations of the three coupled oscillator model in describing the line shape of extinction and scattering cross section spectra.

cond-mat.mes-hall↗

arXiv2024

Dynamical spectra from one and two-photon Fock state pulses exciting a single chiral qubit in a waveguide

We study the dynamical light emission from few-photon Fock states in waveguide-QED with a chiral two-level system. We first investigate the time dynamics of the system by calculating the emitter population and illustrate the breakdown of the weak excitation approximation, for both 1-photon and 2-photon excitation. We show how a 1-photon pulse yields a transmitted long-time spectrum that is identical to the input pulse, despite significant population effects. However, the dynamical spectra and spectral intensity show rich population effects. We also show the differences between 1-photon and 2-photon excitation, where the latter shows clear signatures of nonlinear saturation effects. Analytical and numerically exact matrix product state solutions are shown.

quant-ph↗