arXiv subjects

Saga Westerberg

Publications and source records attributed to Saga Westerberg.

8 recordsLinked to original sources

arXiv2026

Phase-locked beam splitting with continuously tunable intensity ratio using stochastic phase masks

We introduce a common-path beam splitter based on a randomly partitioned phase mask displayed on a spatial light modulator (SLM). The SLM aperture is divided into square patches, each randomly assigned to one of three phase ramps that steer light to three independent foci: two phase-locked beams and a dump that takes up the surplus power. We observe the beams in the focal plane of a lens. The fractions of patches assigned to each ramp set the focal intensities, with the focus positions controlled by the ramp slopes.We first demonstrate experimentally that the intensity ratio between the two foci can be tuned from 0 to 1 without distorting either of the spatial modes, and with their relative phase constant to within a few tens of mrad. We then derive a closed-form expression for the ensemble-averaged focal intensity, which decomposes into coherent foci of the desired pattern and a speckle background whose shape and amplitude are set by the patch size. The model is validated against experiment and against numerical Fraunhofer propagation, with quantitative agreement over about three orders of magnitude in intensity. We also identify the patch size as the central design parameter and characterize its experimental bounds.

physics.optics↗

arXiv2026

Phase-matching in high-order harmonic generation with a plasma-reshaped driving field

The transition from titanium-sapphire to ytterbium-based laser technology enables turnkey implementations of high-order harmonic generation (HHG) setups at high repetition rates, opening up new applications, e.g., in semiconductor metrology. This, however, requires exploring and understanding the impact of new experimental conditions. Here, we study HHG in neon over the spectral range 8-17 nm with high spectral resolution, using a post-compressed ytterbium laser. Our measurements reveal a large pulse-energy- and pressure-dependent spectral shift, spanning more than two harmonic orders. The femto-chirp of the harmonic emission combined with the blueshift of the fundamental field allows us to relate the observed spectral shifts to the temporal window of harmonic generation, which is determined by phase-matching. To understand these dynamics, it is essential to consider the reshaping of the driving laser field inside the plasma, despite the low driving intensity and long pulse duration. We introduce an on-axis phase-matching description that captures these effects and reproduces the experimental and simulated trends. These results demonstrate the central role of plasma-induced laser reshaping in phase-matching of HHG, which should be accounted for when optimizing sources in a multi-parameter landscape.

physics.optics↗

arXiv2026

Near single-cycle pulse generation using a cascaded all-bulk multi-pass cell compression system

We experimentally demonstrate the generation of sub-two-cycle optical pulses using a cascaded post-compression scheme based on two bulk multi-pass cells. Starting from 220 fs, 100 $μ$J pulses around 1030 nm, sequential spectral broadening and pulse compression reduce the pulse duration first to 50 fs and ultimately to 5.5 fs (1.6 optical cycles) at a central wavelength of 1058 nm. The ultra-broadband spectrum is enabled by a dispersion-controlled cavity. Comprehensive spectral, temporal, and spatial characterization confirms excellent pulse quality. Despite operating the second multi-pass cell at peak powers several hundred times above the critical power for self-focusing in fused silica, no significant spatio-spectral or spatio-temporal distortions are observed, enabling direct use of the compressed 40 $μ$J pulses in further experiments. Numerical simulations of the nonlinear spectral broadening show excellent agreement with the experimental results, supporting the underlying physical picture. These findings establish bulk multi-pass cells as an efficient, compact, and robust platform for generating few-cycle pulses with excellent beam quality, offering considerable potential for strong-field and ultrafast applications, including extreme-ultraviolet generation and isolated attosecond pulse production.

physics.optics↗

arXiv2026

Enhanced dynamic range spatio-spectral metrology of few-cycle laser pulses

Accurate spatio-temporal and spatio-spectral metrology is critical to the characterization and use of ultra-short, high-power lasers. The emergence of few cycle pulses, with bandwidths of tens or hundreds of nanometers, poses a significant challenge to existing metrology techniques. This is due both to large discrepancies in the sensitivities of the measurements at different wavelengths and to variation in the spectral intensity at those wavelengths. In this paper, the authors propose spectral filtering and stitching of the measurements as a robust, simple solution that enhances the dynamic range of the measurements, allowing accurate few-cycle pulse reconstruction. This enhancement is demonstrated using INSIGHT -- the most commonly used spatio-spectral measurement device -- as well as using IMPALA and spatially resolved Fourier transform spectrometry.

physics.optics↗

arXiv2025

Compact, intense attosecond sources driven by hollow Gaussian beams

High-order harmonic generation (HHG) enables the up-conversion of intense infrared or visible femtosecond laser pulses into extreme-ultraviolet attosecond pulses. However, the highly nonlinear nature of the process results in low conversion efficiency, which can be a limitation for applications requiring substantial pulse energy, such as nonlinear attosecond time-resolved spectroscopy or single-shot diffractive imaging. Refocusing of the attosecond pulses is also essential to achieve a high intensity, but difficult in practice due to strong chromatic aberrations. In this work, we address both the generation and the refocusing of attosecond pulses by sculpting the driving beam into a ring-shaped intensity profile with no spatial phase variations, referred to as a Hollow Gaussian beam (HGB). Our experimental and theoretical results reveal that HGBs efficiently redistribute the driving laser energy in the focus, where the harmonics are generated on a ring with low divergence, which furthermore decreases with increasing order. Although generated as a ring, the attosecond pulses can be refocused with greatly reduced chromatic spread, therefore reaching higher intensity. This approach enhances the intensity of refocused attosecond pulses and enables significantly higher energy to be delivered in the driving beam without altering the focusing conditions. These combined advantages open pathways for compact, powerful, tabletop, laser-driven attosecond light sources.

physics.optics↗

arXiv2025

Influence of the laser pulse duration in high-order harmonic generation

High-order harmonic generation (HHG) in gases has been studied for almost 40 years in many different conditions, varying the laser wavelength, intensity, focusing geometry, target design, gas species, etc. However, no systematic investigation of the effect of the pulse duration has been performed in spite of its expected impact on phase-matching of the high-order harmonics. Here, we develop a compact post-compression method based on a bulk multi-pass cell enabling tunable Fourier-limited pulse durations. We examine the HHG yield as a function of the pulse duration, ranging from 42 fs to 180 fs, while maintaining identical focusing conditions and generating medium. Our findings reveal that, for a given intensity, there exists an optimum pulse duration - not necessarily the shortest - that maximizes conversion efficiency. This optimum pulse duration increases as the intensity decreases. The experimental results are corroborated by numerical simulations, which show the dependence of HHG yield on the duration and peak intensity of the driving laser and underscore the importance of the interplay between light-matter interaction and phase-matching in the non-linear medium. Our conclusion explains why HHG could be demonstrated in 1988 with pulses as long as 40 ps and intensities of just a few $10^{13}$ W/cm${^2}$.

physics.optics↗

arXiv2024

Energy scaling in a compact bulk multi-pass cell enabled by Laguerre-Gaussian single-vortex beams

We report pulse energy scaling enabled by the use of Laguerre-Gaussian single-vortex ($\text{LG}_{0,l}$) beams for spectral broadening in a sub-40 cm long Herriott-type bulk multi-pass cell. Beams with orders ${l= 1-3}$ are generated by a spatial light modulator, which facilitates rapid and precise reconfiguration of the experimental conditions. 180 fs pulses with 610 uJ pulse energy are post-compressed to 44 fs using an $\text{LG}_{0,3}$ beam, boosting the peak power of an Ytterbium laser system from 2.5 GW to 9.1 GW. The spatial homogeneity of the output $\text{LG}_{0,l}$ beams is quantified and the topological charge is spectrally-resolved and shown to be conserved after compression by employing a custom spatio-temporal coupling measurement setup.

physics.optics↗

arXiv2024

Compact, folded multi-pass cells for energy scaling of post-compression

Combining high peak 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 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 mJ 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 mJ with a setup size reaching 2.5 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.

physics.optics↗