arXiv subjects

Christian Brahms

Publications and source records attributed to Christian Brahms.

At least 19 recordsLinked to original sources

arXiv2026

Ultra-broadband transient absorption down to 200 nm enabled by soliton dynamics in gas-filled hollow capillary fibers

The deep ultraviolet (DUV) window (200-300 nm) is essential for the characterization of (bio)chemical and material systems through the UV signatures of nucleobases, amino acids, peptide bonds, many organic moieties, and wide bandgap transitions. However, extending ultrafast spectroscopy to the DUV to access the associated electronic and structural dynamics has largely remained elusive, due to the limited bandwidth and efficiency of common femtosecond DUV pulse sources. We now close this gap and demonstrate ultra-broadband femtosecond transient absorption (TA) spanning 200-800 nm, achieving unprecedented coverage of the entire DUV window. We generate supercontinuum probe pulses through soliton self-compression in a helium-filled hollow capillary fiber at a repetition rate of 20 kHz and fully suppress their high intrinsic intensity fluctuations via a correlation matrix referencing scheme. We thus achieve detector-noise-limited TA measurements with an exceptional resolution of 9 $\mu$OD in one second, and demonstrate these novel capabilities by resolving the ultrafast spin-crossover dynamics of a Fe(II) complex in the DUV. This work opens the path to unravel previously inaccessible photophysical and photochemical dynamics encoded in the DUV.

physics.optics↗

arXiv2026

Roadmap on Attosecond Science

Twenty-five years have passed since the first experimental demonstration of attosecond pulses, marking the advent of our ability to resolve and control electron motion in real time. What began as a technological breakthrough - generating the shortest flashes ever produced - has evolved into a powerful approach for probing and steering electronic dynamics in atoms, molecules, and solids. This roadmap, authored by leading experts in the field, surveys the recent rapid progress in the generation and characterization of attosecond pulses, emerging attosecond measurement and control techniques, and their expanding range of applications. It reviews current and future developments in attosecond light sources, including novel laser technologies, waveform synthesizers, new schemes for high-order harmonic generation, attosecond pulse generation at free-electron lasers, and structured light. Advances in attosecond measurement methodologies are also discussed, encompassing all-attosecond pump-probe spectroscopy, attosecond four-wave mixing, attosecond microscopy, spectroscopy with light transients, and attosecond interferometry. Furthermore, the roadmap addresses applications of attosecond spectroscopy to reveal electron dynamics in molecules and condensed matter systems from both theoretical and experimental perspectives, and highlights emerging directions at the interface with quantum optics and quantum entanglement. Overall, this work aims to serve as a comprehensive resource for navigating the evolving landscape of attosecond science.

physics.optics↗

arXiv2024

Supercontinuum generation in methane-filled hollow-core antiresonant fiber

We report the generation of a multi-octave supercontinuum spanning from 350 nm to 1700 nm with exceptional spectral flatness and high conversion efficiency to both the visible and near infrared region, by pumping a methane-filled hollow-core antiresonant fiber with 1030 nm laser pulses. The dynamics exhibited signs of both modulational instability and stimulated Raman scattering. Fiber lengths ranging from 15 to 200~cm were investigated along with gas pressures up to 50 bar and pump pulse durations from 220~fs up to 10~ps. The best supercontinuum, in terms of spectral width and flatness, was achieved with 220~fs pulses, 25~bar filling pressure, and 60~cm propagation length. Comparison with argon-filled fiber with matched nonlinearity and dispersion showed that the Raman contribution enhances the supercontinuum generation process compared to a pure modulational instability-based process. The average power was scaled up by increasing the pulse repetition rate to 50~kHz, but further scaling was hindered by linear and nonlinear absorption leading to fiber damage.

physics.optics↗

arXiv2024

HISOL: high-energy soliton dynamics enable ultrafast far-ultraviolet laser sources

Ultrafast laser sources in the far ultraviolet (100 nm to 300 nm) have been the subject of intense experimental efforts for several decades, driven primarily by the requirements of advanced experiments in ultrafast science. Resonant dispersive wave emission from high-energy laser pulses undergoing soliton self-compression in a gas-filled hollow capillary fibre promises to meet several of these requirements for the first time, most importantly by combining wide-ranging wavelength tuneability with the generation of extremely short pulses. In this Perspective, we give an overview of this approach to ultrafast far-ultraviolet sources, including its historical origin and underlying physical mechanism, the state of the art and current challenges, and our view of potential applications both within and beyond ultrafast science.

physics.optics↗

arXiv2024

Decoupled few-femtosecond phase transitions in vanadium dioxide

The nature of the insulator-to-metal phase transition in vanadium dioxide (VO2) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily driven by the electronic or structural degree of freedom, but measurements to date have been stymied by their sensitivity to only one of these components and/or their limited temporal resolution. Here we use ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the electronic and structural phase transitions in VO2 at their fundamental time scales. We find that the system transforms into a bad-metallic phase within 10 fs after photoexcitation, but requires another 100 fs to complete the transition, during which we observe electronic oscillations and a partial re-opening of the bandgap, signalling a transient semi-metallic state. Comparisons with tensor-network simulations and density-functional theory calculations show these features originate from oscillations around the equilibrium high-symmetry atomic positions during an unprecedentedly fast structural transition, in which the vanadium dimers separate and untwist with two different timescales. Our results resolve the complete structural and electronic nature of the light-induced phase transition in VO2 and establish ultra-broadband few-femtosecond spectroscopy as a powerful new tool for studying quantum materials out of equilibrium.

cond-mat.str-el↗

arXiv2023

Robust isolated attosecond pulse generation with self-compressed sub-cycle drivers from hollow capillary fibers

High-order harmonic generation (HHG) arising from the non-perturbative interaction of intense light fields with matter constitutes a well-established tabletop source of coherent extreme-ultraviolet and soft X-ray radiation, which is typically emitted as attosecond pulse trains. However, ultrafast applications increasingly demand isolated attosecond pulses (IAPs), which offer great promise for advancing precision control of electron dynamics. Yet, the direct generation of IAPs typically requires the synthesis of near-single-cycle intense driving fields, which is technologically challenging. In this work, we theoretically demonstrate a novel scheme for the straightforward and compact generation of IAPs from multi-cycle infrared drivers using hollow capillary fibers (HCFs). Starting from a standard, intense multi-cycle infrared pulse, a light transient is generated by extreme soliton self-compression in a HCF with decreasing pressure, and is subsequently used to drive HHG in a gas target. Owing to the sub-cycle confinement of the HHG process, high-contrast IAPs are continuously emitted almost independently of the carrier-envelope phase (CEP) of the optimally self-compressed drivers. This results in a CEP-robust scheme which is also stable under macroscopic propagation of the high harmonics in a gas target. Our results open the way to a new generation of integrated all-fiber IAP sources, overcoming the efficiency limitations of usual gating techniques for multi-cycle drivers.

physics.optics↗

arXiv2023

On-target delivery of intense ultrafast laser pulses through hollow-core anti-resonant fibers

We report the flexible on-target delivery of 800 nm wavelength, 5 GW peak power, 40 fs duration laser pulses through an evacuated and tightly coiled 10 m long hollow-core nested anti-resonant fiber by positively chirping the input pulses to compensate for the anomalous dispersion of the fiber. Near-transform-limited output pulses with high beam quality and a guided peak intensity of 3 PW/cm2 were achieved by suppressing plasma effects in the residual gas by pre-pumping the fiber after evacuation. This appears to cause a long-term removal of molecules from the fiber core. Identifying the fluence at the fiber core-wall interface as the damage origin, we scaled the coupled energy to 2.1 mJ using a short piece of larger-core fiber to obtain 20 GW at the fiber output. This scheme can pave the way towards the integration of anti-resonant fibers in mJ-level nonlinear optical experiments and laser-source development.

physics.optics↗

arXiv2023

The effect of rotational Raman response on ultra-flat supercontinuum generation in gas-filled hollow-core photonic crystal fibers

We experimentally and numerically investigate flat supercontinuum generation in gas-filled anti-resonant guiding hollow-core photonic crystal fiber. By comparing results obtained with either argon or nitrogen we determine the role of the rotational Raman response on the supercontinuum formation. When using argon, a supercontinuum extending from 350 nm to 2 {\mu}m is generated through modulational instability. Although argon and nitrogen exhibit similar Kerr nonlinearity and dispersion, we find that the energy density of the continuum in the normal dispersion region is significantly lower when using nitrogen. Using numerical simulations, we find that due to the closely spaced rotational lines in nitrogen, gain suppression in the fundamental mode causes part of the pump pulse to be coupled into higher-order modes which reduces the energy transfer to wavelengths shorter than the pump.

physics.optics↗

arXiv2023

Dispersion-tuning of nonlinear optical pulse dynamics in gas-filled hollow capillary fibers

We experimentally investigate the nonlinear optical pulse dynamics of ultrashort laser pulses propagating in gas-filled hollow capillary fibers in different dispersion regimes, which are achieved by tuning the gas pressure. When the pulse propagates in the anomalous dispersion regime we observe soliton dynamics accompanied with soliton-plasma effects, such as self-compression, resonant dispersive-wave emission in the fundamental as well as in higher-order modes, soliton blue-shifting and ionization-induced pulse splitting. Propagation of the pulse in the vicinity of the zero-dispersion wavelength results in pulse splitting and subsequent cross-phase modulation leading to the generation of an additional frequency-shifted band and a 3-octave broad supercontinuum. In the case of pulses propagating in normal dispersion we observe the generation of a broad and flat supercontinuum. In this regime, the experimental results are less well described by simulations that consider only the propagation dynamics inside the fiber. Free-space simulations of the beam propagation in the bulk gas, before the capillary entrance, suggest that this discrepancy is caused by self-focusing and ionization altering the pulse spatial and temporal shape, affecting both the coupling efficiency and the subsequent propagation inside the capillary.

physics.optics↗

arXiv2023

Efficient and compact source of tuneable ultrafast deep ultraviolet laser pulses at 50 kHz repetition rate

Deep ultraviolet (DUV) laser pulses with tuneable wavelength and very short duration are a key enabling technology for next-generation technology and ultrafast science. Their generation has been the subject of extensive experimental effort, but no technique demonstrated thus far has been able to meet all requirements in one light source. Here we demonstrate a bright, efficient, and compact source of tuneable deep ultraviolet ultrafast laser pulses based on resonant dispersive wave emission in hollow capillary fibre. In a total footprint of only 120 x 75 cm, including the ytterbium-based drive laser, we generate pulses between 208 nm and 363 nm at 50 kHz repetition rate with a total efficiency of up to 3.6%. Down-scaling of the DUV generation reduces the required energy sufficiently to enable the generation of two-colour few-femtosecond DUV pulses.

physics.optics↗

arXiv2022

Generation and characterization of frequency tuneable sub-15 fs pulses in a gas-filled hollow-core fiber pumped by a Yb:KGW laser

We investigate soliton self-compression and photoionization effects in an argon-filled antiresonant hollow-core photonic crystal fiber pumped with a commercial Yb:KGW laser. Before the onset of photoionization, we demonstrate self-compression of our 220~fs pump laser to 13~fs in a single and compact stage. By using the plasma driven soliton self-frequency blueshift, we also demonstrate a tuneable source from 1030 to ~700 nm. We fully characterize the compressed pulses using sum-frequency generation time-domain ptychography, experimentally revealing the full time-frequency plasma-soliton dynamics in hollow-core fibre for the first time.

physics.optics↗

arXiv2022

The effect of nonlinear lensing on the coupling of ultrafast laser pulses to hollow-core waveguides

Gas-filled hollow-core fibres are a flexible platform for the manipulation of ultrafast laser pulses through a variety of nonlinear optical effects. Efficient high-fidelity coupling of the initial pulses is very important for system performance. Here we study the effect of self-focusing in gas-cell windows on the coupling of ultrafast laser pulses into hollow-core fibres using (2+1)-dimensional numerical simulations. As expected, we find that the coupling efficiency is degraded and the duration of the coupled pulses changed when the entrance window is too close to the fibre entrance. The interplay of nonlinear spatio-temporal reshaping and the linear dispersion of the window create different results depending on the window material, pulse duration, and pulse wavelength, with longer-wavelength beams more tolerant of high intensity in the window. While shifting the nominal focus to compensate can restore some of the lost coupling efficiency, it improves the pulse duration only marginally. From our simulations we derive a simple expression for the minimum distance between the window and the HCF entrance facet. Our results have implications for the often space-constrained design of hollow-core-fibre systems, especially where the input energy is not constant.

physics.optics↗

arXiv2022

From Raman frequency combs to supercontinuum generation in nitrogen-filled hollow-core anti-resonant fiber

We demonstrate a route to supercontinuum generation in gas-filled hollow-core anti-resonant fibers through the creation of a broad vibrational Raman frequency comb followed by continuous broadening and merging of the comb lines through either rotational Raman scattering or the optical Kerr effect. Our demonstration experiments, utilizing a single pump pulse with 20 ps duration at 532 nm in a nitrogen-filled fiber, produce a supercontinuum spanning from 440 nm to 1200 nm, with an additional deep ultraviolet continuum from 250 nm to 360 nm. Numerical results suggest that this approach can produce even broader supercontinuum spectra extending from the ultraviolet to mid-infrared.

physics.optics↗

arXiv2021

Soliton self-compression and resonant dispersive wave emission in higher-order modes of a hollow capillary fibre

We investigate soliton self-compression and ultraviolet resonant dispersive wave emission in the higher-order modes of a gas-filled hollow capillary fibre. Our simple analytical scaling rules predict shorter required waveguides and different energy scales when moving from the fundamental to higher-order modes. Experimentally, we demonstrate soliton self-compression and ultraviolet dispersive wave emission in the double-lobe LP$_{11}$ mode of an argon-filled hollow capillary fibre, which we excite by coupling into the fibre at oblique incidence. We observe the generation of ultraviolet dispersive waves which are frequency-shifted and more narrowband as compared to fundamental-mode generation due to the stronger modal dispersion, and a suppression of the supercontinuum between the dispersive wave and the pump pulse. With numerical simulations, we confirm the predictions of our scaling rules and find that the use of higher-order modes can suppress photoionisation and plasma effects even while allowing for much higher pulse energy to be used in the self-compression process. Our results add another degree of freedom for the design of hollow-waveguide systems to generate sub-cycle field transients and tuneable ultrashort laser pulses.

physics.optics↗

arXiv2021

Ultrafast circularly polarized pulses tunable from the vacuum to deep ultraviolet

We experimentally demonstrate the efficient generation of circularly polarized pulses tunable from the vacuum to deep ultraviolet (160-380 nm) through resonant dispersive wave emission from optical solitons in a gas-filled hollow capillary fiber. In the deep ultraviolet we measure up to 13 microjoule of pulse energy, and from numerical simulations, we estimate the shortest output pulse duration to be 8.5 fs. We also experimentally verify that simply scaling the pulse energy by 3/2 between linearly and circularly polarized pumping closely reproduces the soliton and dispersive wave dynamics. Based on previous results with linearly polarized self-compression and resonant dispersive wave emission, we expect our technique to be extended to produce circularly polarized few-fs pulses further into the vacuum ultraviolet, and few to sub-fs circularly polarized pulses in the near-infrared.

physics.optics↗

arXiv2021

Timing and energy stability of resonant dispersive wave emission in gas-filled hollow-core waveguides

We numerically investigate the energy and arrival-time noise of ultrashort laser pulses produced via resonant dispersive wave emission in gas-filled hollow-core waveguides under the influence of pump-laser instability. We find that for low pump energy, fluctuations in the pump energy are strongly amplified. However, when the generation process is saturated, the energy of the resonant dispersive wave can be significantly less noisy than that of the pump pulse. This holds for a variety of generation conditions and while still producing few-femtosecond pulses. We further find that the arrival-time jitter of the generated pulse remains well below one femtosecond even for a conservative estimate of the pump pulse energy noise, and that photoionisation and plasma dynamics can lead to exceptional stability for some generation conditions. By applying our analysis to a scaled-down system, we demonstrate that our results hold for frequency conversion schemes based on both small-core microstructured fibre and large-core hollow capillary fibre.

physics.optics↗

arXiv2020

Generation of broadband circularly polarized deep-ultraviolet pulses in hollow capillary fibers

We demonstrate an efficient scheme for the generation of broadband, high-energy, circularly polarized femtosecond laser pulses in the deep ultraviolet through seeded degenerate four-wave mixing in stretched gas-filled hollow capillary fibers. Pumping and seeding with circularly polarized 35 fs pulses centered at 400 nm and 800 nm, respectively, we generate idler pulses centered at 266 nm with more than 25 microjoule of energy and over 95% spectrally averaged ellipticity. Even higher idler energies and broad spectra (27 nm bandwidth) can be obtained at the cost of reduced ellipticity. Our system can be scaled in average power and used in different spectral regions, including the vacuum ultraviolet.

physics.optics↗

arXiv2020

Resonant dispersive wave emission in hollow capillary fibres filled with pressure gradients

Resonant dispersive wave (RDW) emission in gas-filled hollow waveguides is a powerful technique for the generation of bright few-femtosecond laser pulses from the vacuum ultraviolet to the near infrared. Here we investigate deep-ultraviolet RDW emission in a hollow capillary fibre filled with a longitudinal gas pressure gradient. We obtain broadly similar emission to the constant-pressure case by applying a surprisingly simple scaling rule for the gas pressure and study the energy-dependent dispersive-wave spectrum in detail using simulations. We further find that in addition to enabling dispersion-free delivery to experimental targets, a decreasing gradient also reduces the pulse stretching within the waveguide itself, and that transform-limited pulses with 3 fs duration can be generated by using short waveguides. Our results illuminate the fundamental dynamics underlying this frequency conversion technique and will aid in fully exploiting it for applications in ultrafast science and beyond.

physics.optics↗