arXiv subjects

Stefanie Kroker

Publications and source records attributed to Stefanie Kroker.

At least 19 recordsLinked to original sources

arXiv2026

Fabrication-Aware Design of a Hybrid Metasurface-Bragg Mirror for Low-Noise Precision Optics

High-reflectivity coatings for precision interferometry must simultaneously minimize optical losses and thermally driven displacement noise. Dielectric Bragg mirrors provide robust high reflectance but rely on thick multilayer coatings, whereas metasurface mirrors provide high reflectance and low noise but are sensitive to fabrication-induced deviations. A fabrication-aware hybrid mirror concept is introduced that combines a resonant single-layer metasurface, an etch-stop layer, an antiresonant spacer, and a reduced Bragg reflector. Fabrication effects, including geometric tolerances and line-edge roughness, are explicitly considered in the optical design. Full-wave electromagnetic simulations are combined with a truncated-Gaussian Monte Carlo analysis to determine the performance distribution under the assumed fabrication conditions. The ideal metasurface design achieves a modeled non-reflected power of $1-R<10^{-5}$. After roughness-aware reoptimization, the modeled non-reflected power remains below $1.04\cdot10^{-4}$ at $80\%$ fabrication yield. For the cryogenic ETpathfinder gravitational-wave testbed, two Bragg layer pairs reduce the non-reflected power of the complete stack to approximately $6.6\,\mathrm{ppm}$. The estimated thermal displacement-noise amplitude spectral density is $1.97\cdot10^{-20}\,\mathrm{m/\sqrt{Hz}}$ at $100\,\mathrm{Hz}$. The architecture connects fabrication robustness, optical performance, and thermal-noise reduction within a single design framework.

physics.optics↗

arXiv2026

Benchmarking Dual-Polarization Silicon Nitride Photonic Integrated Circuits for Trapped-Ion Quantum Technologies

Trapped ions are one of the most advanced platforms for quantum technologies, with applications ranging from quantum computing to precision timekeeping. A crucial step towards more compact and scalable systems involves integrating photonic integrated circuits (PICs) into surface ion traps to enable on-chip light delivery and optical addressing of individual ions. Currently, most implementations rely solely on transverse-electric (TE) mode grating couplers, where the emitted light is polarized in the plane of the chip. In this work, we design, fabricate and characterize silicon nitride (Si$_3$N$_4$) PIC components, including incoupling structures, splitters, and grating couplers that support both TE and transverse-magnetic (TM) modes with comparable optical losses. We benchmark the PIC at 760\,nm, which is a typical wavelength for Yb$^{+}$-applications. The fabricated grating couplers enable the outcoupling of collimated free-space beams for both polarizations, exhibiting distinct emission angles. This dual-polarization capability gives more flexibility in polarization control and expands the accessible optical design space for trapped-ion quantum technologies.

physics.optics↗

arXiv2026

High-Beam-Quality Meta-Grating Couplers for Large Collimated Free-Space Beams on Silicon-on-Insulator

Photonic integrated circuits on the silicon-on-insulator (SOI) platform typically interface with free space via grating couplers, but scaling these to collimated beams with diameters beyond 100 $μ$m requires a fundamentally different regime of extremely weak, spatially distributed coupling. While such large-area couplers have been demonstrated, their beam quality has remained largely uncharacterized, even though applications such as coupling into high-finesse resonators or trapping of cold atoms require both a large aperture and a near-Gaussian profile. This article presents an SOI meta-grating coupler that emits collimated, near-Gaussian beams of approximately 300 $μ$m waist diameter. The design synthesizes the required emission profile from a spatially tailored coupling strength, realized by locally varying a sub-wavelength unit cell while independently setting the local emission angle. This approach achieves the very low coupling strengths required for large beams and yields a measured beam quality of $M^2 \leq 1.10$. The scheme extends directly to other target profiles, such as flat-top or higher-order modes, rendering meta-grating couplers a practical chip-to-free-space interface for mode-matching-sensitive applications.

physics.optics↗

arXiv2026

Cordierite-based optical resonators with extremely low thermal expansion

Applications for ultra-stable lasers outside controlled laboratory environments require compact and robust optical resonators with reduced sensitivity to temperature fluctuations. The low thermal expansion coefficient (CTE) and the high stiffness make cordierite-based ceramics, such as NEXCERA, attractive for vibration insensitive room-temperature resonators. We revisit the effective CTE of resonators with spacers and mirrors made of different materials and use finite element simulations to analyze the impact of a CTE mismatch in a cordierite-based resonator with mirrors made of ultra-low expansion (ULE) glass or fused silica (FS). This enabled us to determine the CTE of a cordierite spacer from the measured effective CTE of a resonator. We confirm a six-fold larger CTE slope of cordierite around the zero-crossing temperature than in ULE glass. The steep CTE slope, in combination with the large stiffness, makes cordierite-based resonators far less sensitive to CTE mismatch with FS mirrors, thereby eliminating the need for additional compensation rings. We further consider the so far neglected case, where the CTE of the spacer is larger than that of the mirror, and propose resonator designs in which the thermal length change of the spacer is fully or partially compensated by the deflection of the mirrors. This results in a cordierite-based resonator with ULE mirrors whose effective CTE can be close to zero over a temperature range of several tens of Kelvin. We are extending our concept to resonators based on crystalline materials with high stiffness and low isothermal length change, such as silicon, enabling compact and robust room-temperature resonators for terrestrial and space-born applications.

physics.optics↗

arXiv2026

Beyond Bragg-Mirrors for Gravitational Wave Telescopes: A Fabrication Tolerant Hybrid Metasurface-Bragg Mirror Design

Coating thermal noise in high-reflectivity test-mass mirrors is a major limitation for future gravitational-wave detectors, especially in the 10--300 Hz band. ET-Pathfinder therefore requires mirror coatings that combine very high reflectance at 1.55 micrometer with low thermal noise under cryogenic conditions. Conventional dielectric Bragg mirrors provide high reflectance but require thick coatings, whereas metasurface mirrors can reduce coating-related noise but are limited by fabrication tolerances and line-edge roughness. We present a hybrid metasurface--Bragg mirror concept tailored to ET-Pathfinder. The design combines a fabrication-tolerant one-layer metasurface, an anti-resonant Fabry--Perot spacer, and a reduced dielectric Bragg stack. Optical performance is evaluated using full-wave electromagnetic simulations, while fabrication robustness is assessed with a truncated-Gaussian Monte Carlo analysis. Line-edge roughness is included as a systematic edge-smoothing effect. The resulting reflectance distributions are used to determine the minimum Bragg-stack support required to meet system-level specifications. The ideal metasurface exceeds 99.999% reflectance. When fabrication uncertainties and line-edge roughness are included, the metasurface reflectance is limited to about 99.9% at the 95% yield level. The remaining transmission can be compensated by a supporting Bragg stack with as few as seven layer pairs. For this configuration, the hybrid mirror achieves a total thermal displacement noise about one order of magnitude below the projected ET-Pathfinder coating-noise budget. These results show that fabrication-limited metasurface reflectance can be compensated within a hybrid architecture, enabling reduced coating thickness and thermal noise for next-generation gravitational-wave detectors.

astro-ph.IM↗

arXiv2025

Mechanical loss and stability analysis of NEXCERA in ultra-stable optical cavities

NEXCERA has emerged as a ceramic-based material for spacers in ultra-stable optical cavities, with a coefficient of thermal expansion that crosses zero near room temperature. In such cavities, frequency stability is ultimately limited by Brownian thermal noise in the cavity components. A key parameter in this context is the mechanical loss, which has remained unknown for NEXCERA. In this work, we investigate the mechanical loss of NEXCERA N117B at room temperature for various resonances using the gentle nodal suspension technique. We measure a promising minimum mechanical loss of $ϕ= 1.89\times 10^{-5}$, indicating the suitability of NEXCERA for low-noise optical cavities. Using this value, we calculate the thermal noise of a cavity with a NEXCERA spacer and compare its performance to established materials such as ULE and Zerodur, taking into account different mirror substrate options. Our analysis shows that NEXCERA is a strong candidate for ultra-stable cavities due to its low thermal noise. Combined with its previously reported low linear drift, it offers a highly attractive option for long-term stable optical frequency references.

physics.optics↗

arXiv2025

Guidelines for designs for ultrastable laser with $\mathbf{10^{-17}}$ fractional frequency instability

Lasers with long coherence time and narrow linewidth are an essential tool for quantum sensors and clocks. Ultrastable cavities and laser systems are now commercially available with fractional frequency instabilities in the mid $10^{-16}$ range. This document aims to provide technical guidance for researchers starting in the field of ultrastable lasers and to give an outlook toward the next generation of improved ultrastable lasers. These guidelines have arisen from the scope of the EMPIR project ``Next generation ultrastable lasers'' ( https://www.ptb.de/empir2021/nextlasers ) with contributions from the European project partners.

physics.optics↗

arXiv2024

Temperature-dependent mechanical losses of Eu$^{3+}$:Y$_{2}$SiO$_{5}$ for spectral hole burning laser stabilization

We investigate the mechanical loss characteristics of Eu$^{3+}$:Y$_2$SiO$_5$$\unicode{x2013}$a promising candidate for ultra-low-noise frequency stabilization through the spectral hole burning technique. Three different mechanical oscillators with varying surface-to-volume ratios and crystal orientations are evaluated. In this context, we perform mechanical ringdown and spectral measurements spanning temperatures from room temperature down to $15\,\mathrm{K}$. By doing so, we measure a maximum mechanical quality factor of $Q=3676$, corresponding to a loss angle of $ϕ=2.72\times 10^{-4}$. For a spectral hole burning laser stabilization experiment at $300\,\mathrm{mK}$, we can estimate the Allan deviation of the fractional frequency instability due to Brownian thermal noise to be below $σ_{δν/ν_0} = 2.5\times 10^{-18}$, a value lower than the estimated thermal-noise limit of any current cavity-referenced ultra-stable laser experiment.

physics.optics↗

arXiv2021

Two-color grating magneto-optical trap for narrow-line laser cooling

We demonstrate for the first time the two-color cooling and trapping of alkaline-earth atoms in a grating magneto-optical trap (gMOT). The trap is formed by a single incident laser beam together with four secondary beams that are generated via diffraction from a nanostructured wafer. A grating structure for a gMOT operating with strontium atoms is optimized and fabricated. We trap $10^6$ $^{88}$Sr atoms on the $^1$S$_0$ $\rightarrow$ $^1$P$_1$ transition at $461\;\mathrm{nm}$ and transfer $25\;\%$ of these atoms to the second cooling stage on the narrower $^1$S$_0$ $\rightarrow$ $^3$P$_1$ intercombination transition at $689\;\mathrm{nm}$, preparing a sample of $2.5\times 10^5$ atoms at $5\;μ$K. These results demonstrate for the first time the applicability of the gMOT technology in conjunction with two widely differing wavelengths and enable the continued miniaturization of alkaline-earth based quantum technologies like optical atomic clocks.

physics.atom-ph↗

arXiv2021

Impact of Earth's gravity on Gaussian beam propagation in hemispherical cavities

We theoretically investigate the influence of gravity on laser light in a hemispherical optical cavity, operating on Earth. The propagation of light in such a cavity is modeled by a Gaussian beam, affected by the Earth's gravitational field. On laboratory scale, this field is described by the spacetime of homogeneous gravity, known as Rindler spacetime. In that spacetime, the beam is bent downwards and acquires a height dependent phase shift. As a consequence the phase fronts of the laser light differ from those of a usual Gaussian beam. Assuming that the initial beam enters the cavity along its symmetry axis, these gravitational effects cause variations of the beam phase with every cavity round trip. Detailed calculations are performed to investigate how these phase variations depend on the beam parameters and the cavity setup. Moreover, we discuss the implications of our findings for cavity calibration techniques and cavity-based laser stabilization procedures.

gr-qc↗

arXiv2020

Gravitational light deflection in Earth-based laser cavity experiments

As known from Einstein's theory of general relativity, the propagation of light in the presence of a massive object is affected by gravity. In this work, we discuss whether the effect of gravitational light bending can be observed in Earth-based experiments, using high-finesse optical cavities. In order to do this, we theoretically investigate the dynamics of electromagnetic waves in the spacetime of a homogeneous gravitational field and give an analytical expression for the resulting modifications to Gaussian beam propagation. This theoretical framework is used to calculate the intensity profile at the output of a Fabry-Pérot cavity and to estimate the imprints of Earth's gravity on the cavity output signal. In particular, we found that gravity causes an asymmetry of the output intensity profile. Based on that, we discuss a measurement scheme, that could be realized in facilities like the GEO600 gravitational wave detector and the AEI 10 m detector prototype.

gr-qc↗

arXiv2019

Modular Hybrid Plasmonic Integrated Circuit -- Rotation, Nanofocusing and Nonlinear Enhancement

We introduce a modular approach for efficiently interfacing photonic integrated circuits with deep-sub-wavelength hybrid plasmonic functionality. We demonstrate that an off-the-shelf silicon-on-insulator waveguide can be post-processed into an integrated hybrid plasmonic circuit by evaporating a silica and gold nanolayer. The circuit consists of a plasmonic rotator and a nanofocusser module, which together result in nano-scale, nonlinear wavelength conversion. We experimentally characterize each module, and demonstrate an intensity enhancement of $>200$ in a calculated mode area of $50\,{\rm nm}^2$ at $λ= 1320\,{\rm nm}$ using second harmonic generation. This work opens the door to customized plasmonic functionalities on industry-standard waveguides, bridging conventional integrated photonic circuits with hybrid plasmonic devices. This approach promises convenient access to nanometre-scale quantum information processing, nonlinear plasmonics, and single-molecule sensing.

physics.optics↗

arXiv2019

Key signal contributions in photothermal deflection spectroscopy

We report on key signal contributions in photothermal deflection spectroscopy (PDS) of semiconductors at photon energies below the bandgap energy and show how to extract the actual absorption properties from the measurement data. To this end, we establish a rigorous computation scheme for the deflection signal including semi-analytic raytracing to analyze the underlying physical effects. The computation takes into account linear and nonlinear absorption processes affecting the refractive index and thus leading to a deflection of the probe beam. We find that beside the linear mirage effect, nonlinear absorption mechanisms make a substantial contribution to the signal for strongly focussed pump beams and sample materials with high two-photon absorption coefficients. For example, the measured quadratic absorption contribution exceeds 5% at a pump beam intensity of about ${1.3}\times{10^{5}}\;{W}/{cm^{2}}$ in Si and at ${5}\times{10^{4}}\;{W}/{cm^{2}}$ in GaAs. In addition, our method also includes thermal expansion effects as well as spatial gradients of the attenuation properties. We demonstrate that these effects result in an additional deflection contribution which substantially depends on the distance of the photodetector from the readout point. This distance dependent contribution enhances the surface related PDS signal up to two orders of magnitude and may be misinterpreted as surface absorption if not corrected in the analysis of the measurement data. We verify these findings by PDS measurements on crystalline silicon at a wavelength of 1550 nm and provide guidelines how to extract the actual attenuation coefficient from the PDS signal.

cond-mat.mtrl-sci↗

arXiv2018

Thermal noise of beam splitters in laser gravitational wave detectors

We present the calculation of thermal noise in interferometric gravitational-wave detectors due to the thermal fluctuations of the beam splitter (BS). This work makes use of a recently developed method of the analysis of thermal noise in mirrors from first principles, based on the fluctuation dissipation theorem. The evaluation of BS thermal noise is carried out for the two different grav- itational wave observatories, GEO600 and the Advanced Laser Interferometer Gravitational Wave Observatory (aLIGO). The analysis evaluates thermal noise from both the substrate and the optical reflective and antireflective stacks located on the BS surface. We demonstrate that the fluctuations of both reflecting and anti-reflecting surfaces significantly contribute to the total thermal noise of the BS. The oscillating intensity pattern couples small-scale distortions of the surface to the overall phase readout, and therefore increases the overall thermal noise. In the case of aLIGO, the BS contribution is with $0.3\%$ negligibly small. At a frequency of 500Hz, the BS causes about $10\%$ of GEO600's sensitivity limit. BS noise impairs the feasible sensitivity of the GEO-HF design proposal by about $50\%$.

astro-ph.IM↗

arXiv2018

Highly reflective low-noise etalon-based meta-mirror

We present a concept of a mirror for the application in high-reflectivity low-noise instruments such as interferometers. The concept is based on an etalon with a metasurface (meta-etalon) on the front and a conventional multilayer stack on the rear surface. The etalon in combination with the metasurface enables a dedicated spatial weighing of the relevant thermal noise processes and by this a substantial reduction of the overall read out thermal noise. We exemplary illustrate the benefit of the proposed etalon for thermal noise in two applications: The test masses of the Einstein Telescope gravitational wave detector and a single-crystalline cavity for laser frequency stabilization. In the Einstein Telescope the thermal noise of the etalon even at room temperature outperforms existing concepts for operation temperatures at 10K. For the laser stabilization cavity, a reduction of the modified Allan deviation of an order of magnitude is predicted.

physics.ins-det↗

arXiv2018

Thermal noise in complex systems

We present a method to calculate the power spectral density of Brownian noise in complex optomechanical systems using Levin's approach of virtual pressure and present first mechanical loss measurements for high-purity GaAs over a wide temperature range from 7 K to 250 K. The loss reveals three Debye loss peaks. Each peak corresponds to an Arrhenius-like relaxation process with activation energies of 17.9 meV, 65.4 meV and 123 meV respectively. Additional light induced damping was observed for photon energies below and above the fundamental gap of GaAs in contrast to observations by Okamoto et al.

physics.app-ph↗

arXiv2016

Materials Pushing the Application Limits of Wire Grid Polarizers further into the Deep Ultraviolet Spectral Range

Wire grid polarizers (WGPs), periodic nano-optical meta-surfaces, are convenient polarizing elements for many optical applications. However, they are still inadequate in the deep ultraviolet spectral range. We show that to achieve high performance ultraviolet WGPs a material with large absolute value of the complex permittivity and extinction coefficient at the wavelength of interest has to be utilized. This requirement is compared to refractive index models considering intraband and interband absorption processes. We elucidate why the extinction ratio of metallic WGPs intrinsically humble in the deep ultraviolet, whereas wide bandgap semiconductors are superior material candidates in this spectral range. To demonstrate this, we present the design, fabrication and optical characterization of a titanium dioxide WGP. At a wavelength of 193 nm an unprecedented extinction ratio of 384 and a transmittance of 10 % is achieved.

physics.optics↗

arXiv2015

Cancellation of lateral displacement noise of 3-port gratings for coupling light to cavities

Reflection gratings enable light coupling to optical cavities without transmission through substrates. Gratings that have three ports and are mounted in second-order Littrow configuration even allow the coupling to high-finesse cavities using low diffraction efficiencies. In contrast to conventional transmissive cavity couplers, however, the phase of the diffracted light depends on the lateral position of the grating, which introduces an additional noise coupling. Here we experimentally demonstrate that this kind of noise cancels out once both diffracted output ports of the grating are combined. We achieve the same signal-to-shot-noise ratio as for a conventional coupler. From this perspective, 3-port grating couplers in second-order Littrow configuration remain a valuable approach to reducing optical absorption of cavity coupler substrates in future gravitational wave detectors.

physics.optics↗