Few-cycle Laser System Research Articles

Numerical Investigation of Polarization-Dependent Ultrashort Pulse Amplification in Erbium-Doped Fluoride Fibers

We have theoretically investigated the polarization-dependent ultrashort pulse amplification in erbium-doped fluoride fibers. The numerical model is based on the coupled generalized nonlinear Schrödinger equations and the rate equations. It is found that the Raman effect can be weakened by converting the seed polarization from linear to circular. Compared with the linearly polarized seed pulses, amplification of the circularly polarized seed pulses can generate Raman solitons with a larger pulse energy of the same central wavelength, although more pump power is needed. Furthermore, the amplification dynamics when no pulse splitting occurs have also been investigated. We demonstrate that amplification of the circularly polarized seed pulses allows the main pulse to accumulate more energy while maintaining a single-pulse profile. It is predicted that sub-two-cycle laser pulses at 2.8 μm with megawatt peak power and hundreds of nanojoules pulse energy can be directly generated from the erbium-doped fluoride fiber amplifiers. The investigations conducted in this paper can provide guidelines for the design of Raman-soliton-based tunable fiber laser systems and high-power few-cycle fiber laser systems.

Water jet space charge spectroscopy: route to direct measurement of electron dynamics for organic systems in their natural environment

The toolbox for time-resolved direct measurements of electron dynamics covers a variety of methods. Since the experimental effort is increasing rapidly with achievable time resolution, there is an urge for simple and robust measurement techniques. Within this paper prove-of-concept experiments and numerical simulations are utilized to investigate the applicability of a new setup for the generation of ultrashort electron pulses in the energy range of 300 eV up to 1.6 keV. The experimental approach combines an in-vacuum liquid microjet and a few-cycle femtosecond laser system, while the threshold for electron impact ionization serves as a gate for the effective electron pulse duration. The experiments prove that electrons in the keV regime are accessible and that the electron spectrum can be easily tuned by laser intensity and focal position alignment with respect to the water jet. Numerical simulations show that a sub-picosecond temporal resolution is achievable.

Carrier-envelope phase stable few-cycle laser system delivering more than 100 W, 1 mJ, sub-2-cycle pulses

Two-stage multipass-cell compression of a fiber-chirped-pulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107 W average power, 1.07 mJ pulses at 100 kHz centered at 1030 nm with excellent spatial beam quality (M2 = 1.1, Strehl ratio S = 0.98), pointing stability (2.3 µrad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360 mrad in the frequency range from 10 Hz to 50 kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation.

Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate

The development of attosecond pump–probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system.

Broadband phase-shifting mirrors for ultrafast lasers.

Metal-dielectric phase-shifting multilayer optical elements have been developed, providing broadband, virtually dispersion-free polarization manipulation down to the few-cycle level. These optical elements are Ag/Al2O3 mirrors that operate in the spectral range from 500 to 100 nm, exhibiting reflectance higher than 95%, and a differential phase shift between the s- and p-polarization of about 90° distributed over four bounces. The mirrors have been designed, produced, and reliably characterized based on spectral photometric and ellipsometric data using a non-parametric approach as well as a multi-oscillator model. The optical elements were implemented into a few-cycle laser system, where they transformed linearly polarized few-cycle light pulses to circular polarization.

Self-induced ionization injection LWFA and generation of sub-fs electron bunches with few-cycle sub-TW laser pulses

AbstractWe investigate an ionization injection scheme in a “weakly” non-linear regime of a wakefield, driven by sub-TW, few-cycle laser pulses in a single-stage, high-Z gas. This medium simultaneously provides the background wake fluid electrons from its lower ionization states and the necessary dephased electrons from its higher ionization states. Two dimensional-particle-in-cell simulations show the generation of relativistic electron beamlets having up to 15 MeV peak energy, with a narrow energy-spread and sub-fs duration. Since the currently-available sub-TW, few-cycle laser systems operate at kHz repetition rates, the presented scheme is capable of producing kHz attosecond electron bunches and their associated radiations which can find unique applications, for instance, in attosecond diffraction and microscopy.

Extreme-ultraviolet high-order harmonic generation from few-cycle annular beams

An annular infrared (IR) laser beam has been used for high-order harmonic generation reaching a cut-off energy of 90eV for extreme-ultraviolet-infrared (XUV-IR) pump-probe experiments in an intrinsically stable attosecond beamline. The generation of harmonics along the laser axis in the missing portion of the laser beam decreases the IR power load on thin metallic foils that are used for removing the residual IR and shaping the XUV pulses from high-harmonic generation. This finds applications in high-average-power few-cycle laser systems, where high-average IR power destroys the foils. The spatial separation of IR and XUV will, moreover, simplify the realization of attosecond time-resolved measurements.

Continuous every-single-shot carrier-envelope phase measurement and control at 100 kHz.

With the emergence of high-repetition-rate few-cycle laser pulse amplifiers aimed at investigating ultrafast dynamics in atomic, molecular, and solid-state science, the need for ever faster carrier-envelope phase (CEP) detection and control has arisen. Here we demonstrate a high-speed, continuous, every-single-shot measurement and fast feedback scheme based on a stereo above-threshold ionization time-of-flight spectrometer capable of detecting the CEP and pulse duration at a repetition rate of up to 400kHz. This scheme is applied to a 100kHz optical parametric chirped pulse amplification few-cycle laser system, demonstrating improved CEP stabilization and allowing for CEP tagging.

Carrier frequency tuning of few-cycle light pulses by a broadband attenuating mirror.

We demonstrate the performance of a novel multilayer dielectric reflective thin-film attenuator capable of reshaping the super-octave spectrum of near-single-cycle visible laser pulses without deteriorating the phase properties of the reflected light. These novel broadband attenuating mirrors reshape in a virtually dispersion-free manner the incident spectrum such that the carrier wavelength of the reflected pulses shifts from ∼700 nm (Eγ=1.77 eV) to ∼540 nm (Eγ=2.25 eV) or beyond while maintaining their initial near-single-cycle pulse duration. This constitutes a viable approach to convert a number of established few-cycle ultrafast laser systems into sources with a selectable excitation wavelength to meet the requirements of single-color/multicolor high temporal resolution spectroscopic experiments.

Few-cycle Laser System for Visualization of Electron Density Modulation in Laser Wakefield

Few-cycle Laser System for Visualization of Electron Density Modulation in Laser Wakefield

Energetic sub-2-cycle laser with 216 W average power.

Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.

53 W average power few-cycle fiber laser system generating soft x rays up to the water window.

We report on a few-cycle laser system delivering sub-8-fs pulses with 353 μJ pulse energy and 25 GW of peak power at up to 150 kHz repetition rate. The corresponding average output power is as high as 53 W, which represents the highest average power obtained from any few-cycle laser architecture so far. The combination of both high average and high peak power provides unique opportunities for applications. We demonstrate high harmonic generation up to the water window and record-high photon flux in the soft x-ray spectral region. This tabletop source of high-photon flux soft x rays will, for example, enable coherent diffractive imaging with sub-10-nm resolution in the near future.

High repetition rate plasma mirror device for attosecond science

This report describes an active solid target positioning device for driving plasma mirrors with high repetition rate ultra-high intensity lasers. The position of the solid target surface with respect to the laser focus is optically monitored and mechanically controlled on the nm scale to ensure reproducible interaction conditions for each shot at arbitrary repetition rate. We demonstrate the target capabilities by driving high-order harmonic generation from plasma mirrors produced on glass targets with a near-relativistic intensity few-cycle pulse laser system operating at 1kHz. During experiments, residual target surface motion can be actively stabilized down to 47 nm (root mean square), which ensures sub-300-as relative temporal stability of the plasma mirror as a secondary source of coherent attosecond extreme ultraviolet radiation in pump-probe experiments.

Front-End Light Source for aWaveform-Controlled High-Contrast Few-Cycle Laser System for High-Repetition Rate Relativistic Optics

We present the current development of an injector for a high-contrast, ultrashort laser system devoted to relativistic laser-plasma interaction in the few-cycle regime. The front-end is based on CEP-stabilized Ti:Sa CPA followed by XPW filter designed at the mJ level for temporal cleaning and shortening. Accurate characterization highlights the fidelity of the proposed injector. Measured CEP drift is 170 mrad rms.

The circular-polarization phase-meter

A new method to measure the carrier-envelope phase (CEP) of a strong few-cycle laser pulse is introduced. The new concept relies on above threshold ionization (ATI) in circularly polarized laser fields and allows for CEP tagging every single laser pulse generated in few-cycle laser systems at multi-kHz repetition rates. The implementation of the new method is described and discussed quantitatively. It is shown that the introduced circular-polarization phase-meter may offer a series of advantages over existing CEP-tagging schemes based on ATI in linearly polarized laser fields.

Two-photon polymerization technique with sub-50 nm resolution by sub-10 fs laser pulses

Nanofabrication of structures with a feature size of sub-50 nm with ultrashort-laser based two-photon polymerization (2PP) technique is presented. The spatial resolution of the 2PP structures depends on the characteristics of the polymer material and the laser system used for fabrication. Here we compare the successful creation of sub-100 nm structures with two different few-cycle laser systems and chemically modified zirconium-based sol-gel composite material using cross-linker for resolution enhancement.

Phase-independent generation of relativistic electron sheets

A uniform and stable relativistic electron sheet can be generated by the two-layer target scheme, where a linearly polarized drive laser is originally employed. The energy and density of the electron layer are found to be sensitive to carrier-envelope phases of few-cycle laser pulses. To circumvent this problem, the present letter proposes to use a circularly polarized laser. The produced electron layer becomes completely independent of the phase of the laser, avoiding the rigorous requirement for phase stabilization in an ultra-intense few-cycle laser system. The improved scheme makes coherent x-ray sources based on relativistic electron sheets more attainable.

State-of-the-art attosecond metrology

Tracking and controlling electron dynamics in the interior of atoms, molecules as well as in solids is at the forefront of modern ultrafast science [1–5]. Time-resolved studies of these dynamics require attosecond temporal resolution that is provided by an ensemble of techniques consolidated under the term “attosecond metrology” [6,7]. This work reports the development and commissioning of what we refer to as next-generation attosecond beamline technology: the AS-1 attosecond beamline at the Max-Planck Institute of Quantum Optics. It consists of a phase-stabilized few-cycle laser system, for the generation of XUV radiation, and modules tailored for the spectral filtering and isolation of attosecond pulses as well as for their temporal characterization. The setup produces the shortest attosecond pulses demonstrated to date and combines them with advanced spectroscopic instrumentation (electron-, ion- and XUV-spectrometers). These pulses serve as temporally confined trigger events (attosecond streaking and tunneling spectroscopy) or probe pulses (attosecond absorption and photoelectron spectroscopy) enabling attosecond chronoscopy to be applied to a broad range of systems belonging to the microcosm.

Half-cycle cutoffs in harmonic spectra and robust carrier-envelope phase retrieval

In recent years the use of high-order harmonic radiation to create and control events on attosecond timescales has grown at a phenomenal rate. With the use of carrier-envelope phase (CEP) stabilization and few-cycle laser systems it is possible to probe physical processes on unprecedented timescales. Here, we report the first experimental observation of high-harmonic emission at individual half-cycles of a laser pulse. We show that these half-cycle emissions are extremely sensitive to the CEP, providing a route to a new single-shot measurement technique of the CEP. We use this technique to measure the CEP of an 8.5 fs pulse at a centre wavelength of 800 nm with an accuracy of 20 as (1 as=1×10−18 s). With appropriate spatio-spectral filtering of the harmonic spectra, our calculations show that we can isolate emission from an individual half-cycle cutoff, which corresponds to a single isolated attosecond pulse of duration <300 as.