We present a physics-informed neural network for optical coating design. We demonstrate state-of-the-art chirped mirror optimization and introduce a generalist model with parameter-efficient fine-tuning to rapidly generate high-precision designs for arbitrary targets.

Optimizing the transient response of air-coupled ultrasonic transducers is essential for applications such as ultrasonic welding, non-destructive testing, acoustic levitation, and emerging fields like gas-phase sono-photonics, where fast activation and deactivation directly improve process efficiency, modulation speed, and overall system performance. While transient response optimization is also relevant for pulse-echo-based systems in imaging, sensing and time-of-flight (ToF) ranging/positioning, the methods presented in this work address a critical bottleneck in industrial and scientific settings, where the transducer’s mechanical response often limits system speed rather than front-end electronics. In this work, we minimize both the activation and deactivation times of such transducers by implementing voltage boosting and resistive damping. We derive analytical solutions using the 4-element Butterworth-Van Dyke model, enabling accurate predictions of transient behavior during both activation and deactivation. Activation transients are captured by fully analytical expressions, while deactivation is treated semi-analytically: symbolic eigenvalue formulas are derived and then evaluated numerically. Activation time is minimized by applying a boost voltage, reducing it by up to 91.94%. A metal-oxide semiconductor field-effect transistor (MOSFET) relay is incorporated into the model, allowing electrical disconnection of the load and enabling the transducer to deactivate under open-circuit conditions. Deactivation time is reduced by up to 98.44% using a parallel resistor matched to each transducer. These solutions are validated through experimental measurements on multiple transducers (28 kHz to 490 kHz), and the results show strong agreement between our predictions and measured data, demonstrating the effectiveness of the proposed optimization strategies. To support direct adoption in both research and industrial environments, we provide an open-source Python framework implementing these methods.

Second-harmonic generation (SHG) is a widely used nonlinear optical process that has been optimized for high frequency conversion efficiency, broad bandwidths, and optimum spatial and temporal quality of the generated light. However, limitations of the generation efficiency arise due to temporal dispersion and pulse walk-off effects introduced by the nonlinear crystal. To circumvent these limitations, quasi-waveguide schemes, in particular multi-pass cells (MPCs), can be employed, providing flexible phase-tuning capabilities. In this work, we demonstrate for the first time broadband and efficient SHG in an MPC. SHG efficiencies of 72

Multi-pass cells (MPCs) have emerged as versatile systems for nonlinear pulse compression supporting large single-stage compression factors. However, their performance is limited at extreme B-integrals by the onset of secondary nonlinear effects, which lead to the appearance of parasitic spectral peaks. In this Letter, we report the experimental observation of vector modulation instability (VMI) as a primary limiting mechanism in gas-filled MPCs operating at B-integrals >30 radians. The VMI signal is polarized orthogonally to the input beam and distinct from spectral features arising due to quasi-phase-matched four-wave mixing (QPM-FWM). We experimentally show that the VMI gain scales with accumulated B-integral, rather than with only gas density or peak power. Crucially, employing a circularly polarized input pulse suppresses VMI. This mitigation strategy, combined with dispersion management, is confirmed via simulations as a way for single-stage compression factors to exceed 40.

Q-switched lasers are compact, cost-effective, and highly pulse energy-scalable sources for nanosecond-scale laser pulses. The technology has been developed for many decades and is widely used in scientific, industrial and medical applications. However, their inherently narrow bandwidth imposes a lower limit on pulse duration-typically in the few-hundred-picosecond range-limiting the applicability of Q-switched technology in fields that require ultrafast laser pulses in the few-picosecond or femtosecond regime. In contrast, mode-locked lasers can produce broad-band, ultrafast ( <1 ps) pulses, but are complex, expensive, and typically require a large footprint. To bridge the parameter gap between these two laser platforms-in terms of pulse duration and achievable peak power-we here propose a Herriott-type multi-pass cell (MPC) based post-compression scheme for shortening the pulse durations of Q-switched lasers down to the ultrafast, picosecond regime. We experimentally demonstrate post-compression of 0.5 ns, 1 mJ pulses from a Q-switched laser to 24 ps using a compact glass-rod MPC for spectral broadening. We verify this result numerically and show that compression down to a few picoseconds is possible using the nanosecond MPC (nMPC). Through spectral filtering approaches, the nMPC suppresses detrimental nonlinear processes such as stimulated Raman scattering, which have set severe limitations for fiber-based post-compression of Q-switched lasers until today. Our results pave the way to cost-efficient and compact ultrafast laser platforms based on Q-switched laser technology.

A novel class of optical waveguiding systems use acoustically modulated gas-phase media to guide light over long distances while largely avoiding material-induced damage limitations [1]. From an acoustics perspective, two key challenges arise: empirical characterization of the acoustic field in a highly confined geometry on the order of a few millimeters, and the nonstandard excitation scheme in duct acoustics in which acoustic energy is coupled transversely to the symmetry axis of a waveguide. Consequently, numerical simulation is essential for understanding the acoustic behavior and for deriving design guidelines for systematic exploration of optical waveguides. In this work, the acoustic field is characterized by finite-element analysis using a 2D model for axially invariant configurations and a 3D model to assess geometric effects and crosstalk between acoustic sources. The numerical study is guided by optical requirements: (1) maximum acoustic pressure along the axial waveguide center, (2) rotational symmetry of the cross-sectional field, and (3) minimal axial pressure fluctuations. Acoustic losses are included via a wide-duct approximation. Partial model validation is obtained by re-evaluating published measurement results for a comparable, larger-scale resonator configuration. The results show that stable operation requires axial segment lengths below the acoustic wavelength and that robust field formation is achieved with at least four acoustic sources per segment. These findings establish a foundation for future coupled acousto-optic simulations aimed at predicting light confinement and propagation in gas-phase optical waveguides.

Ultrafast lasers with simultaneously high average and peak power have become indispensable for driving a multitude of applications, including high-harmonic generation, strong-field physics, and particle source applications. Both parametric amplifiers and post-compressed Ytterbium lasers have emerged as prime platforms to meet these demands. While multi-pass cell (MPC) based post-compression offers broadband output with high beam quality, it provides limited wavelength tunability and suffers from temporal contrast degradation. Conversely, optical parametric amplifiers (OPAs) provide spectral tunability and high temporal contrast but they are limited by low pump-to-signal conversion efficiency and spatial beam inhomogeneities. Here, we introduce the Optical Parametric Multi-Pass Cell Amplifier (OPMPC), a hybrid architecture that overcomes the limitations of both schemes. Our approach utilizes two non-collinearly intersecting MPCs providing broadband parametric amplification of the seed pulses and complete idler removal after each pass through the crystal, thereby suppressing back-conversion. We experimentally demonstrate a record pump-to-signal power conversion efficiency of 43

The accurate characterization of air-coupled high-power (sound pressure level SPLmin > 120 dB) ultrasonic fields is essential for advancing the development and control of ultrasonic transducer arrays. Conventional characteri zation methods often fail to provide both contactless visualization and a quantitative sound pressure distribution without the need for time-consuming point-by-point scanning. Acoustic pressure variations modulate the refrac tive index of the medium, e.g., ambient air, and thereby change the optical path length, which can be measured interferometrically. In this paper, Fizeau interferometry is systematically compared with Schlieren imaging and optical microphone measurements. The results show strong agreement with these reference methods and con firm the reliability of the interferometric approach. The key contribution of this study is the demonstration that Fizeau interferometry enables simultaneous contactless two-dimensional field visualization and rapid, quantita tive sound pressure mapping. In contrast to conventional methods, the proposed approach combines a full-field (& empty;(sample )= 2 '') acquisition within 40 ms without scanning at a high spatial resolution of 23 & micro;m and quantitative ca pability. Consequently, it provides a rapid tool for characterizing ultrasonic fields in applications such as acoustic levitation or sono-photonic systems.

The goal to control short-wavelength radiation for the investigation and manipulation of ultrafast dynamics in quantum systems coevolves with the growing availability of extreme-ultraviolet (XUV) and x-ray sources from high-harmonic generation and free-electron lasers. Here, we present an XUV spatio-spectral phase modulator based on an intense XUV laser beam propagating through an optically thick resonant target, introducing dispersion profile variations around the resonance both perpendicular to and along the laser propagation direction. The resulting dipole radiation gets spectrally reshaped and becomes more divergent as compared to the original beam in the far field. As an experimental demonstration, the intense-XUV-induced double-peak off-axis structure in the far-field spectrum obtained at the Free-Electron Laser in Hamburg (FLASH) shows indications of the underlying XUV-driven Rabi dynamics and resonant pulse propagation effects. The presented work highlights a ubiquitous phenomenon occurring when an intense laser beam passes through a resonant medium.

Abstract As photonic systems grow more complex, it becomes increasingly difficult to capture their behaviour within the conventional four dimensions of space and time, particularly for systems operating at the nanometer scale, where strong confinement effects, near-field interactions, and subwavelength structuring introduce additional layers of complexity. The concept of 5D photonics reflects this shift by incorporating additional physical, material, computational, adaptive, and quantum degrees of freedom as active components in design, control, and function. Rather than defining a single extra coordinate, higher-dimensional photonics is about operating photonic systems within expanded, dynamically accessible state spaces where multiple dimensions can interact and evolve together. This roadmap brings together perspectives ranging from modeling and design concepts to experimental platforms, materials, components, and system-level implementations. It covers a wide spectrum of synthetic and structured dimensions, nonlinear and strong-field regimes, adaptive and reconfigurable architectures, cyber-physical and engineering approaches, as well as inherently high-dimensional quantum and excitonic systems. Across all these areas, higher-dimensional thinking emerges not as an abstract construct but as a practical tool for enabling new functionalities, overcoming conventional design limitations, and bridging physical systems with digital and AI-driven layers. By framing these diverse developments within a shared higher-dimensional perspective, the roadmap aims to provide orientation in a rapidly expanding field, reveal conceptual connections between traditionally separate areas of photonics, and highlight common challenges and opportunities. In doing so, it positions higher-dimensional photonics as a central paradigm for developing future photonic technologies that are increasingly adaptive, intelligent, and integrated across physical and virtual domains.

Optical thin-film coatings are integral to modern photonics and in particular to ultrafast lasers, providing precise control of dispersion and reflectivity, thus enabling tailored pulse shaping. Designing these coatings represents an inverse problem, requiring the mapping of desired optical properties to physical designs, a task that poses major challenges for traditional heuristic methods, which often rely on time-consuming, expert-guided iteration and can be constrained by the choice of an initial starting point. Here, we present an artificial intelligence (AI) framework for optical thin-film coating design that accelerates the design process, achieving excellent performance characteristics without expert intervention. We discuss our AI approach and demonstrate the capabilities of our algorithm by designing a complex broadband high-reflectivity mirror with state-of-the-art performance characteristics including a -200fs2 group delay dispersion covering a spectral range of 940 nm to 1120 nm.

We measured the spectral flux of the Class 5 Moonlander high harmonic generation source for two driver systems using similar pulse parameters, but at different central wavelength: a solid state multipass cell compressed Yb laser and an optical parametric amplifier system. Both laser systems provide 17 W average power at 100 kHz repetition rate at comparable pulse durations, 29 fs vs 20 fs FWHM. Main difference is the central wavelength of 1030 nm for the MPC system versus the OPCPA system with a central wavelength of 800 nm. We performed high harmonic generation in with both laser systems in Argon and Krypton gas media producing broad-band XUV radiation ranging from 20 to 60 eV photon energy. Using a spectrometer and an XUV diode the spectral photon flux after filtering was determined. For both systems we could reach state-of-the-art performance with photon flux of 1011 photons/s/eV to 1013 photons/s/eV after filtering at the output of the light source. We will discuss the advantages and disadvantages of the different drivers for the high harmonic generation.

Acoustically modulated media offer a versatile platform for dynamic control of laser light, but conventional implementations in solid or liquid phases are limited by optical damage thresholds and restricted spectral ranges. A gas-phase medium can overcome these limitations. However, this approach requires the generation of large acoustic pressure amplitudes to induce significant refractive index changes. This work investigates air-coupled acoustic waveguides designed to generate high pressure amplitude and radially symmetric acoustic fields in a gas-phase resonator. A radial resonance condition for cylindrical resonators is derived using an asymptotic approximation of the Bessel function, enabling resonator diameters to be tailored to specific transducer frequencies. Two circular waveguides are fabricated. One follows the derived resonance condition and the other one adheres to a lambda/2 inter-element spacing rule as a reference. Both are characterized using a laser Doppler vibrometer, yielding the pressure induced changes in the refractive index as changes in the optical path length (Delta OPL) of the measurement laser. The resonant design yields a maximal peak-to-peak.OPL of 140.17nm compared to 64.22nm for the lambda/2 configuration, with only minor differences in radial symmetry. These findings show that resonance tuning in cylindrical air-coupled waveguides enables a controlled increase of refractive index modulation, providing a practical route to scalable acousto-optic devices in gaseous media.

In the context of ultrafast spectroscopy, the availability of few-femtosecond UV pulses is key to disclose the role of electron dynamics in photo-activated biochemically relevant processes. Here, we present an optical setup for the generation of UV pulses spectrally tunable between 270 and 350 nm with transform limited durations of 3.0 and 2.9 fs, respectively, and repetition rates up to 50 kHz, using resonant dispersive wave (RDW) emission in an argon-filled hollow-core fiber. The RDW emission is driven by 1030 nm sub-20 fs pulses produced by post-compressing an Yb-based laser with a dispersion-engineered multi-pass cell (MPC). The combination of an MPC with a capillary constitutes a compact source to deliver few-femtosecond UV pulses at high-repetition rates, which is ideal for statistically demanding experiments in molecular physics.

Ultrafast molecular phenomena, such as inter- and intramolecular energy transfer, play a pivotal role in determining the final functionality of photoactive biochemically-relevant systems. Photosynthesis is a notable example [1]. Understanding how the natural environment influences the light-induced ultrafast dynamics requires to study the molecules in solution [2]. Our goal is to experimentally examine molecules embedded in water clusters on the femtosecond timescale through time resolved XUV photoelectron spectroscopy [3]. However, the diluted nature of the target samples, leading to low statistics, calls for XUV sources operating in the multi-kHz regime.

Multi-pass cell (MPC)-based post-compression has emerged as a key technique for generating high-power, few-femtosecond laser pulses [1]. However, due to spectral modulations intrinsic to self-phase modulation (SPM), the compressed output pulse is accompanied with pre- and post-pulses deteriorating the temporal contrast with increasing compression factor [2]. This phenomenon does not only reduce the energy of the main pulse but may be detrimental to applications where pre-pulses can initiate undesirable interactions. A prominent approach to enhance the temporal contrast is nonlinear polarization ellipse rotation (NER), which can effectively suppresses weak signals outside the temporal window of the main pulse through intensity-dependent polarization rotation [3]. Another efficient method involves operating the MPC in a positive dispersion regime, known as enhanced frequency chirping (EFC) which reduces the spectral modulations caused by SPM [4].

>Combining high peak power and high average power has long been a key challenge of ultrafast laser technology,crucial for applications such as laser-plasma acceleration and strong-field physics. A promising solution lies in post-compressed ytterbium lasers, but scaling these to high pulse energies presents a major bottleneck. Post-compression techniques, particularly Herriott-type multi-pass cells (MPCs), have enabled large peak power boosts at high average powers but their pulse energy acceptance reaches practical limits defined by setup size and coating damage threshold. In this work, we address this challenge and demonstrate, to our knowledge, a novel type of compact, energy-scalable MPC (CMPC). By employing a novel MPC configuration and folding the beam path,the CMPC introduces a new degree of freedom for downsizing the setup length, enabling compact setups even for large pulse energies. We experimentally and numerically verify the CMPC approach, demonstrating post-compression of 8 m J pulses from 1 ps down to 51 fs in atmospheric air using a cell roughly 45 cm in length at low fluence values. Additionally, we discuss the potential for energy scaling up to 200 m J with a setup size reaching2.5 m. Our work presents a new approach to high-energy post-compression, with up-scaling potential far beyond the demonstrated parameters. This opens new routes for achieving the high peak and average powers necessary for demanding applications of ultrafast lasers.

Ultrafast laser systems critically depend on optical thin film coatings to control dispersion and light propagation, enabling precise shaping of light. Optical thin film coating design represents a complex inverse problem traditionally relying on computationally intensive numerical optimization methods. These conventional approaches, exemplified by the widely used Needle Algorithm [1], require significant computational resources and often rely on expert intervention [2]. We here propose a new approach to these challenges and present a physics-informed machine learning framework based on an autoencoder architecture and use it to design an ultra-broadband dispersive mirror.

The diffraction-based deflection of a laser beam in air requires the use of intense ultrasound fields generated by ultrasound transducers. These transducers can be arranged as programmable transducer phased arrays for standing waves (when using a reflector) or propagating waves. A qualitative visualization method of these acoustic fields is required for the optimization of ultrasound field arrangements. We present an optical measurement system for a contactless and single-shot visualization of high-power ultrasound fields based on an interferometer. Thus, small changes in refractive index induced by the intense ultrasound waves are quantitatively detectable. The interferometric method enables two-dimensional imaging of the sound pressure distributions of individually shaped propagating or standing ultrasound waves generated by the transducer phased array.