Gas-filled Hollow Fiber Research Articles

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

Ultrafast laser sources in the far ultraviolet (100–300 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 fiber 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.

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

Gas-filled hollow capillary fibers can be used for continuum generation and pulse compression. Here, the authors change the gas pressure inside hollow capillary fibers and investigate how ultrashort laser pulses propagating through the fibers respond to these changes. The use of short pulses and a long capillary fiber enables the authors to survey a much wider range of dynamics than previously observed.

Measurements of microjoule-level, few-femtosecond ultraviolet dispersive-wave pulses generated in gas-filled hollow capillary fibers.

To the best of our knowledge, we demonstrate the first time-domain measurement of µJ-level, few-fs ultraviolet dispersive-wave (DW) pulses generated in gas-filled hollow capillary fibers (HCFs) in an atmosphere environment using several chirped mirrors. The pulse temporal profiles, measured using a self-diffraction frequency-resolved optical gating setup, exhibit full width at half maximum pulse widths of 9.6 fs at 384 nm and 9.4 fs at 430 nm, close to the Fourier-transform limits. Moreover, theoretical and experimental studies reveal the strong influences of driving pulse energy and HCF length on temporal width and shape of the measured DW pulses. The ultraviolet pulses obtained in an atmosphere environment with µJ-level pulse energy, few-fs pulse width, and broadband wavelength tunability are ready to be used in many applications.

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 (HCF). 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 LP11 mode of an argon-filled HCF, 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.

Single-Cycle, Multigigawatt Carrier–Envelope-Phase-Tailored Near-to-Mid-Infrared Driver for Strong-Field Nonlinear Optics

A short-pulse, frequency-tunable idler-wave output of a two-stage optical parametric amplifier is compressed in a gas-filled antiresonance-guiding hollow-core photonic-crystal fiber to yield ultrashort field waveforms of near-to-mid-infrared (near-IR/mid-IR) radiation with a pulse energy of up to 28 μJ, a peak power of up to 4 GW, and a pulse width smoothly tunable, via gas-pressure adjustment inside the gas-filled hollow fiber, all the way down to a single field cycle at ≤6.6 fs. With the active carrier–envelope phase (CEP) stabilization extended to its entire multioctave bandwidth, the single-cycle near-to-mid-IR fiber output is CEP-stable within 140 mrad and can be finely tailored toward a desired CEP profile. High-order harmonics generated by such pulses in solids exhibit clear signatures of strong-field single-cycle nonlinear optics. Well-resolved harmonic peaks, which dominate the visible–ultraviolet spectrum of the nonlinear-optical response of a solid driven by multicycle near-to-mid-IR pulses, tend to broaden and merge together as the driver pulse width approaches the field cycle, eventually giving rise to visible–ultraviolet supercontinua as a signature of single-cycle light–matter interactions.

Spectroscopic application of few-femtosecond deep-ultraviolet laser pulses from resonant dispersive wave emission in a hollow capillary fibre.

We exploit the phenomenon of resonant dispersive wave (RDW) emission in gas-filled hollow capillary fibres (HCFs) to realize time-resolved photoelectron imaging (TRPEI) measurements with an extremely short temporal resolution. By integrating the output end of an HCF directly into a vacuum chamber assembly we demonstrate two-colour deep ultraviolet (DUV)-infrared instrument response functions of just 10 and 11 fs at central pump wavelengths of 250 and 280 nm, respectively. This result represents an advance in the current state of the art for ultrafast photoelectron spectroscopy. We also present an initial TRPEI measurement investigating the excited-state photochemical dynamics operating in the N-methylpyrrolidine molecule. Given the substantial interest in generating extremely short and highly tuneable DUV pulses for many advanced spectroscopic applications, we anticipate our first demonstration will stimulate wider uptake of the novel RDW-based approach for studying ultrafast photochemistry – particularly given the relatively compact and straightforward nature of the HCF setup.

Pulse self-compression down to the sub-cycle regime in hollow capillary fibers with decreasing pressure gradients

We theoretically demonstrate an enhancement in the generation of clean, near-infrared sub-cycle laser pulses by soliton self-compression in gas-filled hollow capillary fibers using decreasing pressure gradients. Furthermore, we identify the optimal input parameters for high quality compression and the main advantages of this promising technique which paves the way towards ultrafast vacuum experiments.

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 µJ 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.

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 27 µJ 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.

Extreme UV Light Generation Through Dispersive Wave Trapping in a Tapered Gas-Filled Hollow Fiber

In this letter, we demonstrate how the soliton–plasma interaction initiates trapping of the generated dispersive waves (DWs) in an experimentally feasible tapered He-filled hollow-core anti-resonant fiber (HC-ARF). We show that the taper gradient strongly influences the pulse trapping dynamics and thus determines the intensity and blueshift of the trapped DW. This process leads to an efficient DW generation down to 100 nm with a 3.4-octave supercontinuum spanning 100–1150 nm (2.73 PHz) by tapering a 36- $\mu \text{m}$ core HC-ARF to 18 $\mu \text{m}$ under 19-bar He, pumped at 800 nm with 6- $\mu \text{J}$ pulse energy. The proposed fiber taper structure could be an alternative route to generate light in the extreme ultra-violet (EUV) spectral range using moderate gas pressure and relatively low pulse energy.

Passive coherent combining of CEP-stable few-cycle pulses from a temporally divided hollow fiber compressor.

We demonstrate a simple and robust passive coherent combining technique for temporal compression of millijoule energy laser pulses down to few-cycle duration in a gas-filled hollow fiber. High combining efficiency is achieved by using carefully oriented calcite plates for temporal pulse division and recombination. Carrier-envelope phase (CEP)-stable, 6-fs, 800-nm pulses with more than 0.6 mJ energy are routinely generated. This method could aid in the energy scaling of CEP-stable hollow-fiber compressor systems.

Optical parametric amplification of a supercontinuum in a gas

Amplification of supercontinuum light via four-wave optical parametric amplification in a gas-filled hollow fiber is investigated. By pumping with a femtosecond pulse in the near-infrared, amplification was observed in the wavelength range 500–700 nm, with an amplification gain up to 100. The amplification bandwidth obtained here supports sub-10-fs pulse durations. The wavelength and gas-pressure dependence of the amplification gain in this regime significantly differ from those predicted from only the linear phase mismatch. The nonlinear phase mismatch, arising from nonlinear phase modulations induced by a pump pulse, dominates the features observed for the amplification, i.e., a low phase-matching pressure and an amplification gain depending on wavelength. To investigate the effect of phase modulations on the parametric amplification, numerical analysis and simulations are used to reproduce the measured wavelength dependence of the amplification gain.

Acetylene frequency references in gas-filled hollow optical fiber and photonic microcells

Gas-filled hollow optical fiber references based on the P(13) transition of the ν1+ν3 band of 12C2H2 promise portability with moderate accuracy and stability. Previous realizations are corrected (<1σ) by using proper modeling of a shift due to line-shape. To improve portability, a sealed photonic microcell is characterized on the 12C2H2 ν1+ν3 P(23) transition with somewhat reduced accuracy and stability. Effects of the photonic crystal fiber, including surface modes, are explored. Both polarization-maintaining (PM) and non-PM 7-cell photonic bandgap fiber are shown to be unsuitable for kilohertz-level frequency references.

Modelling beam-transport diffraction in near-infrared few-cycle strong-field experiments

Few-cycle high peak power laser pulses produced in gas-filled hollow fibres have a EH11 Bessel mode. Control of the intensity and phase-matching conditions requiring no additional dispersion is introduced, and is often achieved by closing a hard aperture around the beam. We use the Huygens–Fresnel diffraction integral to propagate such a beam through a numerical model of a typical attosecond beamline, quantifying the distribution of spatial intensity and the shape of the wavefront through the focal volume as the beam is truncated by finite-diameter optics. State-of-the-art attosecond and single-harmonic experiments combining the fundamental NIR with high-harmonic XUV radiation also employ annular optics, again as material dispersion must be avoided. We simulate the additional changes to wavefront shape and spatial intensity distribution on reflection from a spherical annular mirror for a range of annulus and aperture sizes. Finally we predict the change in intensity and focal position as the aperture is reduced. The model's usefulness as a tool for optimisation of an experimental beamline is therefore highlighted.

Multi-mJ pulse compression in hollow fibers using circular polarization

We develop a numerical model to explore the polarization-dependent compression of multi-mJ laser pulses in a gas-filled hollow fiber. We show how losses and instabilities due to cycling of pulse energy between fiber modes can be efficiently minimized using circularly polarized light and adjusting simple experimental parameters such as pulse energy, chirp and gas pressure. This should help scale the peak power of few-cycle pulses available for high-field experiments using standard hollow fiber compressors. We also discuss the limits of this approach.

Generation of intense frequency-tunable few-cycle femtosecond pulses in hollow fiber

We theoretically show an approach to generate intense frequency-tunable few-cycle femtosecond pulses by molecular phase modulation combined with self-phase modulation. An intense pump pulse propagates through a gas-filled hollow fiber to excite alignment of the gas molecule, which induces temporal modulation of the refractive index. Then the spectrum of a time-delayed intense probe pulse is continuously tuned by the alignment and broadened simultaneously by self-phase modulation. The modification to the pump-induced alignment by the intense probe pulse is considered. Frequency-tunable intense few-cycle pulses in the visible and near-infrared spectrum range are obtained using initial 20 fs, 800 nm pulses.

Optimizing anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers

Anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers is discussed. It is shown that the efficient anti-Stokes generation observed under conditions of significant wave mismatch is caused by phase locking of the interacting fields. This leads to the establishment of a phase difference that is independent of the optical path. An optimization technique, based on the adjustment of the wave mismatch along a gas-filled hollow fiber using pressure control, is proposed. Anti-Stokes conversion efficiencies close to the theoretical maximum of 50% are predicted.

Femtosecond pulse propagation in temperature controlled gas-filled hollow fiber

We propose in this paper a novel technique of controlling spectrum expansion of intense femtosecond pulses in argon gas by temperature. The simulation demonstrated that the temperature gradient gas-filled hollow fiber allows more incident pulse energy and therefore more spectra broadening. This scheme has the similar effect as differential pumping, while avoiding the disadvantages of gas flowing through the fiber.

Broadband dynamic phase matching of high-order harmonic generation by a high-peak-power soliton pump field in a gas-filled hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to enable dynamically phase-matched high-order harmonic generation by a gigawatt soliton pump field. With a careful design of the waveguide structure and an appropriate choice of input-pulse and gas parameters, a remarkably broadband phase matching can be achieved for a soliton pump field and a large group of optical harmonics in the soft-x-ray-extreme-ultraviolet spectral range.

Colloquium: Optimal control of high-harmonic generation

High-harmonic generation provides an attractive light source of coherent radiation in the extreme-ultraviolet (XUV) and soft-x-ray regions of the spectrum and allows for the production of single attosecond pulses or pulse trains. This Colloquium covers the control of high-harmonic spectra by temporal and spatial pulse shaping of the driving laser pulses and its implications on time-resolved XUV spectroscopy and attosecond pulse shaping. It summarizes important steps for extending existing pulse shaping techniques and control schemes from the near-infrared or visible part to shorter wavelengths. Using adaptive pulse shaping of the driving laser pulses, several groups have demonstrated control of the high-harmonic spectrum, including the author's work on the complete control over the XUV spectrum of high-order harmonics, generated in a gas-filled hollow fiber. It is possible to achieve both the enhancement and the suppression of single or several selected harmonic orders. These arbitrarily shaped soft-x-ray spectra will allow for important modifications of the resulting harmonic pulses in the temporal domain. This constitutes first steps towards direct attosecond pulse shaping in the soft-x-ray domain. Moreover, high-harmonic generation in a hollow-core fiber can be enhanced by coupling into a single fiber mode using a feedback-controlled adaptive two-dimensional spatial light modulator.