arXiv subjects

Stefan Nolte

Publications and source records attributed to Stefan Nolte.

At least 19 recordsLinked to original sources

arXiv2026

High-performance silicon-metal laser welding resisting extreme conditions

Reliable material joining is essential for countless industrial applications. While femtosecond laser welding provides a route beyond conventional bonding methods, demonstrations of silicon-metal joints are rare due to nonlinear propagation effects and have so far been limited to shear joining strengths of a few MPa. Here, we demonstrate high-strength silicon-Kovar laser welding using sub-nanosecond pulses. By optimizing the focal position, the welding pattern, the laser polarization, and the metal roughness, remarkable shear joining strengths up to 15.8 MPa are achieved. The silicon-metal joints withstand temperatures of up to 500 °C and are hermetically sealed. Together with the remarkable strength values, the resistance to harsh environments underpins the applicability of silicon-metal welding in various fields including aerospace, nuclear science, and metallurgy.

physics.optics↗

arXiv2026

Comparative analysis of fiber Bragg grating filter losses inscribed by continuous wave UV and femtosecond-IR lasers for astrophotonics

Fiber Bragg grating (FBG) filters have been demonstrated as promising components in astrophotonic instrumentation for near-infrared ground-based observations. Given the photon-starved nature of astronomical applications, it is critical to minimize insertion losses across astrophotonic components. In addition to the insertion loss (IL) introduced by specialty fibers and inscription techniques, FBGs exhibit cladding mode (CM) losses. In this work, we studied the loss characteristics of five filter lines in three photosensitive fibers, i.e., a low-numerical-aperture (NA) fiber, a high-NA bend-insensitive fiber, and a cladding-mode-suppressed (CMS) fiber, and in a non-photosensitive fiber, SMF-28. The filters were inscribed using two phase mask-based illumination methods: a continuous wave ultraviolet (UV) laser with a complex phase mask allowing for multi-channel filters, and a femtosecond infrared (fs-IR) laser with phase mask integrated shaping apertures for spectral profile control. Our results show that UV-inscribed gratings in high-NA bend-insensitive fiber yield the lowest CM losses ($\approx$ 0.5 dB) among photosensitive fibers, but exhibit the highest IL (4.6 dB), and FBGs in non-photosensitive SMF-28 fiber, inscribed with fs-IR, achieve the lowest IL (< 0.05 dB) with a comparatively higher CM loss (0.93 dB). To reduce the high IL in high-NA fiber, we explored tapering and bridging methods and report that bridging reduces IL by $\sim$ 3 dB. We show that both filter platforms remain viable for integration into an astrophotonic system, with IL below 1 dB. Finally, we propose a compact bridge-fiber scheme with the potential to further reduce IL to below 0.5 dB while reducing the number of bridging fibers and, consequently, the number of splice junctions by 50%.

astro-ph.IM↗

arXiv2026

Using the Ehrenfest theorem for determining the self-focusing and self-trapping of nonlinear beams

We discuss how to generalize the Ehrenfest theorem for the computation of the width of nonlinear waves obeying the nonlinear Schrodinger equation. To do that, we model the nonlinear potential as a quantum harmonic oscillator (QHO) whose strength depends on the power and on the wavefunction width. We apply the model to different types of nonlinear responses, eventually comparing the results with numerical simulations. Our model has the advantage of explaining the main properties of nonlinear confined waves, such as stability and breathing, in a relatively simple and intuitive manner.

nlin.PS↗

arXiv2026

Asynchronous Multi-photon Interference for Quantum Networks

Advanced quantum communication protocols require high-visibility quantum interference between photons generated at distant nodes, which places stringent demands on optical synchronization. Conventionally, synchronization of optical wave packets relies on pulsed sources and precise optical path stabilization. An alternative approach employs continuous-wave (CW) photon-pair sources, where temporal indistinguishability is enforced by post-selecting detection events within a coincidence window $τ_w$ shorter than the photon coherence time $T_c$. Despite its conceptual simplicity, the quantitative relation between relevant time scales, achievable interference visibility, and usable multi-photon rates has remained unclear. Here, we develop in detail and experimentally validate a theoretical framework that quantitatively describes time-resolved multi-photon interference in the CW regime. We explicitly incorporate detector timing jitter, photon coherence time, and temporal post-selection. The model is verified using four-photon Hong-Ou-Mandel interference measurements. Based on this validated framework, we determine the coincidence window that maximizes usable four-photon rates for a target visibility. Finally, we compare CW and pulsed SPDC sources under equivalent indistinguishability constraints and show that CW operation can achieve comparable rates while relaxing optical synchronization requirements.

quant-ph↗

arXiv2026

Systematic Study of Amorphous ABC Heterostructures at the Atomic Scale as a Second-Order Nonlinear Optical Metamaterial

Systematic exploration of amorphous ABC heterostructures revealed that nanoscale morphological modifications markedly improved their artificial bulk second-order susceptibility. These amorphous birefringent heterostructures were fabricated through plasma-enhanced atomic layer deposition of three oxides, effectively breaking the centrosymmetry on the nanoscale. We observe a dependence of the optical nonlinearity on the thickness variation of three constituent materials, SiO$_2$, TiO$_2$, and Al$_2$O$_3$, ranging from tens of nanometers to the atomic scale, and these thin films exhibit second-order susceptibility at their interfaces. Our findings reveal that the enhancement of nonlinear optical properties is strongly correlated with a high density of layers and superior interface quality, where the interface second-order nonlinearity transitions to bulk-like second-harmonic generation. An effective bulk second-order susceptibility of $χ_{zzz} = 2.0 \pm 0.2$~pm/V at the wavelength of 1032~nm is achieved, comparable to typical values for conventional monocrystalline nonlinear materials.

physics.app-ph↗

arXiv2025

Light in Slices: How to Enable Precise Laser Processing?

Ultrashort (femtosecond, fs) laser pulses have fascinating properties as they allow to confine optical energy on extreme scales in space and time. Such fs-laser pulsed beams can be seen as spatially thin slices of intense light that are radially and axially constrained to the micrometer scale, while simultaneously propagating at the extremely high speed of light. Their high peak intensities and their short time lapse makes such laser pulses unique tools for materials processing, as their duration is shorter than the time required to transfer absorbed optical energy, via electron-phonon coupling, from the electronic system of the solid to its lattice. Hence, the laser pulse energy remains localized during the interaction and does not spread via diffusion into the area surrounding the irradiated region. As one consequence, the fs-laser thus offers increased precision for material modification or ablation accompanied by a reduced heat-affected zone of only a few hundred nanometers. On the other hand, the high laser peak intensities can enable nonlinear material interactions that are rendering unique material excitation and relaxation pathways possible. In this chapter, we briefly review the reasons for the enormous success of ultrashort pulse lasers in materials processing - both for the processing of the surface or in the bulk of solids. We identify the underlying fundamental processes that can limit the precision or the up-scaling of the laser processing towards large volumes, areas, or processing rates. Strategies to overcome such limitations will be outlined and questions on the ultimate limits of laser material processing will be answered.

physics.optics↗

arXiv2025

Linear and Nonlinear Optical Properties of SiO$_2$/TiO$_2$ Heterostructures Grown by Plasma Enhanced Atomic Layer Deposition

Second harmonic (SH) radiation can only be generated in non-centrosymmetric bulk crystals under the electric-dipole approximation. Nonlinear thin films made from bulk crystals are technologically challenging because of complex and high temperature fabrication processes. In this work, heterostructures made of amorphous materials SiO$_2$ and TiO$_2$ were prepared by a CMOS-compatible technique named plasma enhanced atomic layer deposition (PEALD) with deposition temperature at 100 °C. By using the uniaxial dispersion model, we characterized the form-birefringence properties, which can enable the phase matching condition in waveguides or other nonlinear optical applications. By applying a fringe-based technique, we determined the largest diagonal component of the effective second-order bulk susceptibility $χ_{zzz}^{(2)}$ = 1.30$\pm$0.13 pm/V at a wavelength of 1032 nm. Noteworthy, we observed strong SH signals from two-component nanolaminates, which are several orders of magnitude larger than from single layers. The SH signals from our samples only require the broken inversion symmetry at the interface. Here optical properties of nanocomposites can be precisely tuned by the promising PEALD technology.

cond-mat.mtrl-sci↗

arXiv2025

How to measure laser chirp rate at single-emitter excitation energies

We present a simple and direct method for measuring laser chirp rate, i.e., group delay dispersion (GDD) of ultrashort laser pulses at power levels compatible with single-quantum-emitter excitation. Traditional pulse characterization techniques rely on nonlinear optical processes that require high peak powers, making them unsuitable for the attojoule-to-femtojoule regime relevant to quantum photonics. Our approach utilizes a wavelength-to-time mapping method in which the arrival times of spectrally filtered components of a broadband pulse are recorded using a superconducting nanowire single-photon detector and correlated via a high-resolution time-tagging system. The resulting linear relationship between wavelength and arrival time directly yields the dispersion parameter and, subsequently, the GDD. Beyond single-emitter excitation, this technique can be applied in areas such as single-photon spectroscopy, ultralow-power optical communications, and time-domain quantum control, where linear and non-destructive dispersion characterization is essential.

physics.optics↗

arXiv2025

Deep learning-driven adaptive optics for laser wavefront correction

{We report on an intensity-only and deep-learning based method for laser beam characterization that allows to predict the underlying optical field within milliseconds. A simple near-field / far-field camera setup enables online control of an adaptive optics to optimize beam quality. The robustness and precision of the method is enhanced by applying the concept of phase diversity based on spiral phase plates.

physics.optics↗

arXiv2025

Spin-dependent routing of optical beams in the bulk of twisted anisotropic media

We theoretically discuss a new kind of photonic spin-Hall effect (PSHE) for optical beams propagating inside an inhomogeneously twisted anisotropic material. The rotation angle plays the role of an effective gauge field. When the twisting distribution is odd symmetric, the optical beams move along mirror-symmetric trajectories according to their helicity. Connection of this volumetric PSHE with the geometric phase and the Kapitza effect is elucidated.

physics.optics↗

arXiv2025

The universality of filamentation-caused challenges of ultrafast laser energy deposition in semiconductors

Light propagation in semiconductors is the cornerstone of emerging disruptive technologies holding considerable potential to revolutionize telecommunications, sensors, quantum engineering, healthcare, and artificial intelligence. Sky-high optical nonlinearities make these materials ideal platforms for photonic integrated circuits. The fabrication of such complex devices could greatly benefit from in-volume ultrafast laser writing for monolithic and contactless integration. Ironically, as exemplified for Si, nonlinearities act as an efficient immune system self-protecting the material from internal permanent modifications that ultrashort laser pulses could potentially produce. While nonlinear propagation of high-intensity ultrashort laser pulses has been extensively investigated in Si, other semiconductors remain uncharted. In this work, we demonstrate that filamentation universally dictates ultrashort laser pulse propagation in various semiconductors. The effective key nonlinear parameters obtained strongly differ from standard measurements with low-intensity pulses. Furthermore, the temporal scaling laws for these key parameters are extracted. Temporal-spectral shaping is finally proposed to optimize energy deposition inside semiconductors. The whole set of results lays the foundations for future improvements, up to the point where semiconductors can be selectively tailored internally by ultrafast laser writing, thus leading to countless applications for in-chip processing and functionalization, and opening new markets in various sectors including technology, photonics, and semiconductors.

physics.optics↗

arXiv2024

Robust Single-Photon Generation for Quantum Information Enabled by Stimulated Adiabatic Rapid Passage

The generation of single photons using solid-state quantum emitters is pivotal for advancing photonic quantum technologies, particularly in quantum communication. As the field continuously advances towards practical use cases and beyond shielded laboratory environments, specific demands are placed on the robustness of quantum light sources during operation. In this context, the robustness of the quantum light generation process against intrinsic and extrinsic effects is a major challenge. Here, we present a robust scheme for the coherent generation of indistinguishable single-photon states with very low photon number coherence (PNC) using a three-level system in a semiconductor quantum dot. Our novel approach combines the advantages of adiabatic rapid passage (ARP) and stimulated two-photon excitation (sTPE). We demonstrate robust quantum light generation while maintaining the prime quantum-optical quality of the emitted light state. Moreover, we highlight the immediate advantages for the implementation of various quantum cryptographic protocols.

quant-ph↗

arXiv2024

Transverse Inscription of Silicon Waveguides by Picosecond Laser Pulses

In this paper, picosecond laser inscription of segmented waveguides in crystalline silicon based on a deterministic single-pulse modification process is demonstrated.Pulses of 43 ps duration at 1.55 $μ$m wavelength are used to transversely inscribe periodic structures with a pulse-to-pulse pitch of around 2 $μ$m. Infrared shadowgraphy images and Raman spectroscopy measurements indicate that the modifications exhibit a spherical shape. Characterization of waveguide performance at 1.55 $μ$m for various pulse energies and periods is carried out. Direct comparison with numerical simulations confirms the presence of graded index waveguides, encompassing a micrometer core size and a maximum refractive index change of around $7\times 10^{-3}$. This short-pulse inscription approach can pave the way for three-dimensional integrated photonic devices in the bulk of silicon.

physics.optics↗

arXiv2024

Phase retrieval algorithm applied to high-energy ultrafast lasers

A standardized phase retrieval algorithm is presented and applied to an industry-grade high-energy ultrashort pulsed laser to uncover its spatial phase distribution. We describe in detail how to modify the well-known algorithm in order to characterize particularly strong light sources from intensity measurements only. With complete information about the optical field of the unknown light source at hand, virtual back propagation can reveal weak points in the light path such as apertures or damaged components.

physics.optics↗

arXiv2023

Application of the Green function formalism to the interplay between avalanche and multiphoton ionization induced by optical pulses

A fundamental brick of light-matter interaction at large optical intensities is the generation of a plasma. The optically-induced plasma in turn plays a fundamental role in determining the optical propagation. The plasma generation is a result of the interplay between multi-photon, tunnel and avalanche ionization. Here we use the basic rate equations to discuss an analytical model for the interaction between these physical effects. After defining a nonlinear impulse response for the system, we describe how the interplay depends on the features of the optical pulses. Our approach strongly simplifies the modelling of the propagation of ultrashort-pulses, paving the way to a much easier and faster interpretation of experimental observations, with potential impact on the broad fields of ultrafast light-matter interaction and laser micro-machining.

physics.optics↗

arXiv2023

Compact Chirped Fiber Bragg Gratings for Single-Photon Generation from Quantum Dots

A scalable source of single photons is a key constituent of an efficient quantum photonic architecture. To realize this, it is beneficial to have an ensemble of quantum emitters that can be collectively excited with high efficiency. Semiconductor quantum dots hold great potential in this context, due to their excellent photophysical properties. Spectral variability of quantum dots is commonly regarded as a drawback introduced by the fabrication method. However, this is beneficial to realize a frequency-multiplexed single-photon platform. Chirped pulse excitation, relying on the so-called adiabatic rapid passage, is the most efficient scheme to excite a quantum dot ensemble due to its immunity to individual quantum dot parameters. Yet, the existing methods of generating chirped laser pulses to excite a quantum emitter are bulky, lossy, and mechanically unstable, which severely hampers the prospects of a quantum dot photon source. Here, we present a compact, robust, and high-efficiency alternative for chirped pulse excitation of solid-state quantum emitters. Our simple plug-and-play module consists of chirped fiber Bragg gratings (CFBGs), fabricated via femtosecond inscription, to provide high values of dispersion in the near-infrared spectral range, where the quantum dots emit. We characterize and benchmark the performance of our method via chirped excitation of a GaAs quantum dot, establishing high-fidelity single-photon generation. Our highly versatile chirping module coupled to a photon source is a significant milestone toward realizing practical quantum photonic devices.

physics.optics↗

arXiv2023

QUICK$^3$ -- Design of a satellite-based quantum light source for quantum communication and extended physical theory tests in space

Modern quantum technologies have matured such that they can now be used in space applications, e.g., long-distance quantum communication. Here, we present the design of a compact true single photon source that can enhance the secure data rates in satellite-based quantum key distribution scenarios compared to conventional laser-based light sources. Our quantum light source is a fluorescent color center in hexagonal boron nitride. The emitter is off-resonantly excited by a diode laser and directly coupled to an integrated photonic processor that routes the photons to different experiments performed directly on-chip: (i) the characterization of the single photon source and (ii) testing a fundamental postulate of quantum mechanics, namely the relation of the probability density and the wave function (known as Born's rule). The described payload is currently being integrated into a 3U CubeSat and scheduled for launch in 2024 into low Earth orbit. We can therefore evaluate the feasibility of true single photon sources and reconfigurable photonic circuits in space. This provides a promising route toward a high-speed quantum network.

quant-ph↗

arXiv2023

Enhanced Surface Second Harmonic Generation in Nanolaminates

Second-harmonic generation (SHG) is a second-order nonlinear optical process that is not allowed in media with inversion sym-metry. However, due to the broken symmetry at the surface, surface SHG still occurs, but is generally small. We experimentally investi-gate the surface SHG in periodic stacks of alternating, subwave-length dielectric layers, which have a large number of surfaces, thus enhancing surface SHG considerably. To this end, multilayer stacks of SiO2/TiO2 were grown by Plasma Enhanced Atomic Layer Deposition (PEALD) on fused silica substrates. With this technique individual layers of a thickness of less than 2 nm can be fabricated. We experimentally show that under large angles of incidence (> 20 degrees) there is substantial SHG, well beyond the level, which can be observed from simple interfaces. We perform this experiment for samples with different periods and thickness of SiO2/TiO2 and our results are in agreement with theoretical calculations.

physics.optics↗