arXiv subjects

Carsten Rockstuhl

Publications and source records attributed to Carsten Rockstuhl.

At least 37 records · Page 2Linked to original sources

arXiv2026

Lattice-induced sound trapping in biperiodic metasurfaces of acoustic resonators

A referential example of a physical system that supports bound states in the continuum (BICs) with an infinite quality factor ($Q$ factor) is a metasurface of discrete scatterers (resonators), whose response can be significantly modified by exploiting lattice interactions. In this work, we explore the multipole-interference mechanism for realizing accidental acoustic BICs (trapped modes) at $Γ$-point (in-plane Bloch wave vector $\mathbf{k}_{\parallel} = \mathbf{0}$) of biperiodic metasurfaces of acoustic resonators with one resonator per unit cell. To do so, we expand the pressure field from the metasurface into a series of scalar zonal ($m = 0$) spherical multipoles, carried by a normally incident plane wave, and formulate analytical conditions on the resonator multipole moments under which an eigenmode becomes a BIC. The conditions enable us to determine the lattice constant and frequency values that facilitate the formation of an axisymmetric BIC with a specific parity, resulting from destructive interference between zonal multipoles of the same parity, despite each moment radiating individually. By employing the T-matrix method for acoustic metasurfaces, we numerically investigate the BIC resonance in various structures, including finite arrays, and also the transformation of such resonances into high-$Q$ quasi-BIC regimes, which can be excited by a plane wave at normal incidence.

physics.class-ph↗

arXiv2026

Modal Analysis of Gyrotropic Waveguides

We report on the modal analysis of open gyrotropic waveguides (GWs). The GWs consist of a non-circular gyrotropic (i.e., gyroelectric and gyromagnetic) core and an infinitely extending isotropic cladding. To solve this problem, we develop two independent full-wave methods. The first technique is an extended integral equation (EIE) method, an extension of a previously developed EIE used to calculate the propagation constants in composite dielectric-isotropic waveguides. The second technique is a Chebyshev expansion method (CEM). In both implementations, the electric and magnetic fields in the gyrotropic core are expanded in superpotential-based cylindrical vector wave functions (SUPER-CVWFs), recently developed for the problem of oblique multiple scattering by gyrotropic cylinders. Both techniques allow us to calculate the propagation constants in the general case, without approximations. Various non-circular gyrotropic waveguides are considered. The EIE and CEM results are validated against a commercial finite element solver, and the accuracy and computational performance of both methods are benchmarked. A microwave application is presented where the complex propagation constants of ferrite rods are computed in the presence of an external magnetic flux density bias. Our work advances the theory of propagation in open waveguides, from dielectric/isotropic to gyrotropic ones, and enables the design of contemporary waveguiding structures.

physics.optics↗

arXiv2025

Ultrasonic metamaterial at MHz frequencies using microstructured glass

Acoustic metamaterials enhance traditional material properties through microstructure engineering, providing new opportunities to shape sound fields in applications ranging from biomedical imaging, clinical therapy to non-destructive testing. However, at the MHz frequency ranges, only a few metamaterial architectures exist. They are often highly attenuating or difficult to manufacture, and generally provide limited 3D control over sound propagation. Here, we introduce a MHz-frequency ultrasonic metamaterial based on laser-engraved glass. By structuring meta-voxels with different engraving patterns, we define a fully-3D, anisotropic metamaterial exhibiting local variations in the sound speed of up to 20% compared to unstructured glass, and losses 100x lower than in comparable 3D printed metamaterials. We use this metamaterial to define a library of standard elements that can be modularly combined to create and shape complex-patterned ultrasonic fields. Our experiments are supported by a theoretical model, which provides additional insights into the microstructural origin of the metamaterial behavior and opens the door to designing tailored ultrasound fields and responses.

physics.app-ph↗

arXiv2025

Towards a fully differentiable digital twin for solar cells

Maximizing energy yield (EY) - the total electric energy generated by a solar cell within a year at a specific location - is crucial in photovoltaics (PV), especially for emerging technologies. Computational methods provide the necessary insights and guidance for future research. However, existing simulations typically focus on only isolated aspects of solar cells. This lack of consistency highlights the need for a framework unifying all computational levels, from material to cell properties, for accurate prediction and optimization of EY prediction. To address this challenge, a differentiable digital twin, Sol(Di)$^2$T, is introduced to enable comprehensive end-to-end optimization of solar cells. The workflow starts with material properties and morphological processing parameters, followed by optical and electrical simulations. Finally, climatic conditions and geographic location are incorporated to predict the EY. Each step is either intrinsically differentiable or replaced with a machine-learned surrogate model, enabling not only accurate EY prediction but also gradient-based optimization with respect to input parameters. Consequently, Sol(Di)$^2$T extends EY predictions to previously unexplored conditions. Demonstrated for an organic solar cell, the proposed framework marks a significant step towards tailoring solar cells for specific applications while ensuring maximal performance.

physics.comp-ph↗

arXiv2025

Unified Evolution of Electromagnetic Sources in Homogeneous Fields

In this work, we study the behavior of elementary electromagnetic sources, i.e., point-like electric charges and intrinsic magnetic dipoles, in the presence of homogeneous electromagnetic fields in a classical and covariant setting. We show that the respective evolution equations for both kinds of sources can be formulated using a single Lorentz-like transformation with suitably adjusted parameters, thereby unifying the fundamental behavior of these intrinsic particle properties. We arrive at this description by expressing the evolution of the electric sources, governed by the Lorentz force, as a series of infinitesimal boosts and rotations acting on them, with the electric and magnetic field as the corresponding parameters. Upon suitable adjustments, we find a new effective Lorentz-like transformation, applicable to both electric and magnetic sources. We provide the results using both the tensorial and pseudovector representation of the magnetic sources. Finally, we obtain a non-relativistic limit of the evolution equation for magnetic sources.

physics.class-ph↗

arXiv2025

Efficient computation of thermal radiation from biperiodic layered systems using the T-matrix method

Metasurfaces are becoming important tools for the control of thermal radiation. Understanding their functional possibilities on computational grounds requires evaluating the response of the biperiodic layered system for many degrees of freedom, including several radiation directions and polarisations, while varying lattice spacing, thicknesses, and/or materials of homogeneous layers, over a range of frequencies. The diverse set of cases that need to be considered in simulations prompts for efficient numerical tools to handle them. To respond to this need, we present a method for computing the thermal radiation from metasurfaces that combines the directional Kirchhoff law with efficient T-matrix based calculations. We show that such a method can accurately reproduce experimental data from a metasurface made of platinum square plates. Additionally, we predict highly circularly polarised emissivity from a chiral metasurface. When comparing CPU-times, the method outperforms other approaches such as rigorous coupled wave analysis already at the modest number of 61 cases per frequency.

physics.optics↗

arXiv2025

Uncovering hidden resonances in non-Hermitian systems with scattering thresholds

The points where diffraction orders emerge or vanish in the propagating spectrum of periodic non-Hermitian systems are referred to as scattering thresholds. Close to these branch points, resonances from different Riemann sheets can tremendously impact the optical response. However, these resonances are so far elusive for two reasons. First, their contribution to the signal is partially obscured, and second, they are inaccessible for standard computational methods. Here, the interplay of scattering thresholds with resonances is explored and a multi-valued rational approximation is introduced to access the hidden resonances. The theoretical and numerical approach is used to analyze the resonances of a plasmonic line grating. This work elegantly explains the occurrence of pronounced spectral features at scattering thresholds applicable to many nanophotonic systems of contemporary and future interest.

physics.optics↗

arXiv2025

Inverse-designed dispersive time-varying nanostructures

Time-varying nanostructures allow us to control the spatial and temporal properties of light. The temporal modulation of the nanostructures constitutes an additional degree of freedom to control their scattering properties on demand and in a reconfigurable manner. However, these additional parameters create a vast design space, raising challenges in identifying optimal designs. Therefore, tools from the field of photonic inverse design must be used to optimize the degrees of freedom of the system to facilitate predefined optical responses. To further develop this field, here we introduce a differentiable transition (T-) matrix-based inverse design framework for dispersive time-varying nanostructures. The electron density of the material of the nanostructures is modulated non-adiabatically as a generic periodic function of time. Using the inverse design framework, the temporal shape of the electron density can be manipulated to reach the target functionality. Our computational framework is exploited, exemplarily, in two instances. First, the decay rate enhancement of oscillating dipoles near time-varying spheres is controlled on demand. Second, using spatiotemporal metasurfaces, a system supporting asymmetric transmission of light at visible frequencies is designed. Our work paves the way toward programmable spatiotemporal metasurfaces and space-time crystals for a future generation of reconfigurable functional photonic devices.

physics.optics↗

arXiv2025

Chiral cavities made from lattices of highly electromagnetically-chiral scatterers

The infamous weakness of molecular chiroptical responses challenges the all-optical realization of crucial applications such as enantio-selective sorting of chiral molecules, or biasing chiral chemical reactions. Chiral optical cavities are a natural choice for confronting this challenge. Ideally, the dissymmetry between the two helicities inside such cavities is maximized. In here, we propose a chiral infrared optical cavity formed by planar mirrors made of diffracting lattices of silver helices with almost maximum electromagnetic chirality. It combines the strong helicity selectivity of the helices with the helicity-preserving reflectivity that planar systems show at large incidence angles. For the manifold of cavity modes which have a component with zero in-plane momentum, we demonstrate an unprecedented dissymmetry of 95 % inside the cavity at the target frequency, making it a compelling candidate for enantio-selective applications.

physics.optics↗

arXiv2025

Oblique Multiple Scattering by Gyrotropic Cylinders

In this work, we develop a full-wave vectorial solution for the 2.5-dimensional (2.5-D), i.e., at oblique plane wave incidence, electromagnetic (EM) multiple scattering (MS) by a collection of gyrotropic cylinders. All cylinders are infinitely long and share a common $z$-axis. However, each cylinder can have a different cross-section with an arbitrary shape and different gyrotropic material properties, i.e., both gyroelectric and gyromagnetic anisotropies are considered. The solution to the problem combines the following three elements: (i) development of a superpotentials-based cylindrical vector wave function (CVWF) expansion to express the EM field in the gyrotropic region; (ii) utilization of the extended boundary condition method (EBCM) to account for non-circular cylinders; (iii) use of Graf's formulas, specifically adapted for the CVWFs, to apply the EBCM at each cylinder. The developed theory allows us to calculate various scattering characteristics, including the scattering and extinction cross-sections and the multipole decomposition, enabling the design and in-depth investigation of various contemporary engineering and physics applications. The method is exhaustively validated with analytical techniques and COMSOL Multiphysics. The computational performance is also discussed. Finally, we study a potential microwave application of the MS by ferrite configurations, and demonstrate broadband forward scattering by introducing oblique incidence and anisotropy. Our method may be used to analyze, design, and optimize contemporary microwave, optical, and photonic applications by beneficially tailoring the scattering properties via oblique incidence and anisotropy.

physics.optics↗

arXiv2025

Predicting the optical properties of organometallic nanoparticles with a scale-bridging method: The importance of the embedding

It remains a prime question of how to describe the optical properties of large molecular clusters accurately. Quantum chemical methods capture essential electronic details but are infeasible for entire clusters, while optical simulations handle cluster-scale effects but miss crucial quantum effects. To overcome such limitations, we apply here a multi-scale modeling approach, combining precise quantum chemistry calculations with Maxwell scattering simulations, to study the linear and nonlinear optical response of finite-size supramolecular gold-cysteine nanoparticles dispersed in water. In this approach, every molecular unit that forms the cluster is represented by a polarizability and a hyperpolarizability, and the overall response is obtained from solving an optical multiple scattering problem. We particularly demonstrate how important it is to accurately consider the environment of the individual molecular units when computing their polarizability and hyperpolarizability. In our quantum chemical simulations, we do so at the level of a static partial charge field that represents the presence of other molecular units. Without correctly considering these effects of the embedding, predictions would deviate from experimental observations even qualitatively. Our findings pave the way for more accurate predictions of the optical response of complex molecular systems, which is crucial for advancing applications in nanophotonics, biosensing, and molecular optoelectronics.

physics.optics↗

arXiv2025

Studying thermal radiation with T-matrices

We introduce a basic formalism for computing thermal radiation by combining Waterman's T-matrix method with an algebraic approach to light-matter interactions. The formalism applies to nano-particles, clusters thereof, and also molecules. In exemplary applications, we explore how a chiral structure can induce an imbalance in the circular polarization of thermal radiation. While the imbalance is rather small for a chiral molecule such as R-BINOL, a much larger imbalance is observed for an optimized silver helix of approximately 200 nm in size. Besides the directional Kirchhoff law used in this article, the formalism is suitable for implementing more nuanced theories, and it provides a straightforward path to the computation of thermal radiation spectra of astronomical objects moving at relativistic speeds with respect to the measurement devices.

physics.optics↗

arXiv2025

Optimal multipole center for subwavelength acoustic scatterers

The multipole expansion is a powerful framework for analyzing how subwavelength-size objects scatter waves in optics or acoustics. The calculation of multipole moments traditionally uses the scatterer's center of mass as the reference point. The theoretical foundation of this heuristic convention remains an open question. Here, we challenge this convention by demonstrating that a different, optimal multipole center can yield superior results. The optimal center is crucial -- it allows us to accurately express the scattering response while retaining a minimum number of multipole moments. Our analytical technique for finding the optimal multipole centers of individual scatterers, both in isolation and within finite arrays, is validated through numerical simulations. Our findings reveal that such an optimized positioning significantly reduces quadrupole contributions, enabling more accurate monopole-dipole approximations in acoustic calculations. Our approach also improves the computational efficiency of the T-matrix method, offering practical benefits for metamaterial design and analysis.

physics.class-ph↗

arXiv2025

Multiphoton Quantum Interference at Ultracompact Inverse-Designed Multiport Beam Splitter

Photonic quantum technologies enter a new phase when realized in photonic integrated circuits, leading to a great advance in practical applications. In the pursuit of high integration density and low circuit complexity, ultracompact devices delivered by topology optimization offer a promising solution to miniaturize these photonic systems even further. However, their potential for quantum experiments has not yet been fully explored despite the constant development. In this work, we demonstrate multiphoton quantum interference using a topology-optimized tritter with a size of 8.0 $\rmμ$m $\times$ 4.5 $\rmμ$m. We characterize the tritter and reconstruct its transfer matrix by means of single- and two-photon statistics. We also perform heralded three-photon quantum interference with the tritter. The measured four-fold coincidence features a peak with visibility of (-47.9$\pm$ 8.6)%, which is in fair agreement with the prediction of -55.8% estimated from the reconstructed transfer matrix. Our work confirms successful multiphoton quantum interference at an ultracompact interferometer and demonstrates the possibility of utilizing topology-optimized multiport interferometers in various fields of quantum technologies.

quant-ph↗

arXiv2025

Inverse Design of 3D Nanophotonic Devices with Structural Integrity Using Auxiliary Thermal Solvers

3D additive manufacturing enables the fabrication of nanophotonic structures with subwavelength features that control light across macroscopic scales. Gradient-based optimization offers an efficient approach to design these complex and non-intuitive structures. However, expanding this methodology from 2D to 3D introduces complexities, such as the need for structural integrity and connectivity. This work introduces a multi-objective optimization method to address these challenges in 3D nanophotonic designs. Our method combines electromagnetic simulations with an auxiliary heat-diffusion solver to ensure continuous material and void connectivity. By modeling material regions as heat sources and boundaries as heat sinks, we optimize the structure to minimize the total temperature, thereby penalizing disconnected regions that cannot dissipate thermal loads. Alongside the optical response, this heat metric becomes part of our objective function. We demonstrate the utility of our algorithm by designing two 3D nanophotonic devices. The first is a focusing element. The second is a waveguide junction, which connects two incoming waveguides for two different wavelengths into two outgoing waveguides, which are rotated by 90° to the incoming waveguides. Our approach offers a design pipeline that generates digital blueprints for fabricable nanophotonic materials, paving the way for practical 3D nanoprinting applications.

physics.optics↗

arXiv2025

A scalar product for the radiation of resonant modes

We introduce the conformally-invariant scalar product, originally devised for radiation fields, to the study of the modes of optical resonators. This scalar product allows one to normalize and compare resonant modes using their corresponding radiation fields. Such fields are polychromatic fields free of divergences, which are determined from the complex frequencies and the modal fields on the surface of the resonator. The scalar product is expressed as surface integrals involving the modal fields, multiplied by closed-form factors incorporating the complex frequencies. In a practical application, we study the modes of disk-shaped whispering gallery resonators, and show that the proposed scalar product accurately predicts the geometry-dependent crossings and anti-crossings between modes.

physics.optics↗

arXiv2025

Electromagnetic Multipole Theory for Two-dimensional Photonics

We develop a full-wave electromagnetic (EM) theory for calculating the multipole decomposition in two-dimensional (2-D) structures consisting of isolated, arbitrarily shaped, inhomogeneous, anisotropic cylinders or a collection of such. To derive the multipole decomposition, we first solve the scattering problem by expanding the scattered electric field in divergenceless cylindrical vector wave functions (CVWF) with unknown expansion coefficients that characterize the multipole response. These expansion coefficients are then expressed via contour integrals of the vectorial components of the scattered electric field evaluated via an electric field volume integral equation (EFVIE). The kernels of the EFVIE are the products of the tensorial 2-D Green's function (GF) expansion and the equivalent 2-D volumetric electric and magnetic current densities. We validate the theory using the commercial finite element solver COMSOL Multiphysics. In the validation, we compute the multipole decomposition of the fields scattered from various 2-D structures and compare the results with alternative formulations. Finally, we demonstrate the applicability of the theory to study an emerging photonics application on oligomers-based highly directional switching using active media. This analysis addresses a critical gap in current literature, where multipole theories exist primarily for three-dimensional (3-D) particles of isotropic materials. Our work enhances the understanding and utilization of the optical properties of 2-D, inhomogeneous, and anisotropic cylindrical structures, contributing to advancements in photonic and meta-optics technologies.

physics.optics↗

arXiv2024

A Tensor Product Space for Studying the Interaction of Bipartite States of Light with Nanostructures

Pairs of entangled photons are important for applications in quantum nanophotonics, where their theoretical description must accommodate their bipartite character. Such character is shared at the other end of the intensity range by, for example, the two degenerate instances of the pump field involved in second-harmonic generation. The description and numerical simulation of the interaction of nanophotonic structures with bipartite states of light is challenging regardless of their intensity, and has important technological applications. To address such a challenge, we develop here a theoretical and computational framework for studying the interaction of material structures with bipartite states of light. The theory of the framework rests on the symmetrized tensor product space of two copies of an electromagnetic Hilbert space. For the computational side, the convenient T-matrix method is extended to the tensor product space. When the response of the object to one part of the state is independent of the other part, the T-matrix for bipartite states is a simple function of the typical T-matrix of the single Hilbert space. Such separable material response is relevant, for example, in the interaction of entangled biphoton states with nanostructures. Non-separable operators are identified as the adequate objects to fully integrate non-linear effects such as sum frequency generation or parametric down-conversion. As an example of application, we derive selection rules for second-order non-linear processes in objects with rotational and/or mirror symmetries, and verify them numerically in two different MoS$_2$ clusters.

physics.optics↗