arXiv subjects

Lukas Rebholz

Publications and source records attributed to Lukas Rebholz.

8 recordsLinked to original sources

arXiv2026

Broken-symmetry phenomena enhanced by quasi-bound states in the continuum

Many of the most powerful and elegant models in physics are grounded in symmetries. In electrodynamics, for example, geometric symmetries govern the observable effects of light-matter interactions. However, for man-made objects, exact symmetries are rarely met and tiny deviations are common. Nonetheless, even approximate symmetries keep many symmetry-derived rules effectively intact. However, as we will show here, this is not universally true. We demonstrate that an incremental violation of the symmetry of a carefully designed system can produce an optical response maximally different from the unbroken symmetry case. To do so, we exploit symmetry-protected quasi-bound states in the continuum (qBICs). Specifically, we design a four-fold rotationally symmetric metasurface composed of nearly dual-symmetric meta-atoms that supports a pair of spectrally aligned electric and magnetic qBICs. At normal incidence, symmetry forbids helicity-preserving reflection. However, for arbitrarily small deviations from normal incidence, the strong resonant enhancement associated with the qBICs overcomes the near-symmetry suppression and enables perfect helicity-preserving reflection. This rapidly emerging violation of symmetry-rules reveals a fundamental intricacy when it comes to treating near-symmetric systems. At the same time, our work opens the door to novel applications in metrology and sensing.

physics.optics↗

arXiv2026

Pole-Expansion of the T-Matrix Based on a Matrix-Valued AAA-Algorithm

The transition matrix (T-matrix) is a complete description of an object's linear scattering response. As such, it has found wide adoption for the theoretical and computational description of multiple-scattering phenomena. In its original form, the T-matrix describes the interaction of a scatterer with a monochromatic source. In practice, however, information about the T-matrix is usually needed in an extended spectral domain. To access the frequency-dispersion, one might naively sample T-matrices over a finely resolved set of discrete frequencies and store one T-matrix per frequency. This approach has multiple drawbacks: it is computationally expensive, requires excessive memory, and it disregards the physical origin of the spectral features, weakening physical interpretability. To overcome these major limitations, we leverage a pole-expansion technique to represent the T-matrix with arbitrary frequency resolution within a selected frequency domain via a set of resonant contributions. A matrix-valued variant of the recently established adaptive Antoulas-Anderson (AAA) algorithm for rational approximation enables us to compute the pole-expansion at minimal computational cost using only a small number of direct evaluations. We demonstrate the benefits of such a representation with examples ranging from semi-analytically accessible scatterers to quasi-dual bound states in the continuum. To allow the wider community to capitalize on these findings, we provide open-source tools to perform the presented pole-expansion of the T-matrix.

physics.optics↗

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

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

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

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↗

arXiv2024

Analyzing the acceleration time and reflectance of light sails made from homogeneous and core-shell spheres

Deciding on appropriate materials and designs for use in light sails, like the one proposed in the Breakthrough Starshot Initiative, is a topic that requires much care and forethought. Here, we offer a feasible option in the form of metasurfaces made of periodically arranged homogeneous and core-shell spheres. Using the re-normalized T-matrix from Mie theory, we explore the reflectance, absorptance, and acceleration time of such metasurfaces. We focus on spheres made from aluminum, silicon, silicon dioxide, and combinations thereof. Since the light sails are foreseen to be accelerated using Earth-based laser arrays to 20% of the speed of light, one needs to account for relativistic effects. As a result, a high broadband reflectance is essential for effective propulsion. We identify metasurfaces that offer such properties combined with a low absorptance to reduce heating and deformation. We highlight a promising extension to the case of a metasurface made from homogeneous silicon spheres, as already discussed in the literature, by adding a layer of silicon dioxide. The high broadband reflectance of the silicon and silicon dioxide combination is explained by the favorable interference of the multipolar contributions of the outgoing field up to quadrupolar order. We also consider the impact of an embedding material characterized by different refractive indices. Refractive indices up to 1.13 maintain over 90% reflectance without re-optimizing the light sail.

physics.optics↗

arXiv2024

Separating the Material and Geometry Contribution to the Circular Dichroism of Chiral Objects Made from Chiral Media

The chirality of an object can be studied by measuring the circular dichroism, that is, the difference in absorption of light with different helicity. The chiral optical response of an object, however, can have two different origins. On the one hand, it can be linked to the chiral geometry of the object. On the other hand, it can be linked to the chiral material from which the object is made. Whereas previously, no distinction between the two contributions could be made, we report here a computational approach that allows us to separate these two contributions to the circular dichroism of an object. We consider separately the cases where geometry-related resonances affect the optical response and where they are absent. In both cases, we find the circular dichroism to be easily decomposable if a geometrically achiral object has a similar absorption spectrum to the chiral object under investigation. Furthermore, in the non-resonant case, the contribution attributed to the material can be obtained without taking any geometry into account. Besides being of fundamental importance, the possibility of disentangling both contributions will be important for guiding the future design of chiral objects and devices.

physics.optics↗