Full Repetition Rate Research Articles

Balanced air-biased detection of terahertz waveforms.

A novel, to the best of our knowledge, balanced air-biased coherent detection scheme for capturing ultrabroadband terahertz (THz) waveforms is implemented. The balanced detection scheme allows for coherent detection at the full repetition rate of the laser system without requiring bias modulation, signal generators, or lock-in amplifiers while doubling the dynamic range and quadrupling the signal-to-noise ratio compared to conventional air-biased coherent detection. These advantages are achieved by rotating the bias electrodes by 90° relative to the conventional scheme. With a 1 kHz driving laser, the scheme enables sub-second, high-fidelity waveform acquisition with a continuously moving delay stage, demonstrated by collecting 200 waveforms in 100 s. The balanced detection scheme paves the way for much faster and higher quality 2D ultrabroadband terahertz spectroscopy.

Spectrally shaped and pulse-by-pulse multiplexed multimode squeezed states of light

Spectral- and time-multiplexing are currently explored to generate large multipartite quantum states of light for quantum technologies. In the continuous variable approach, the deterministic generation of scalable entangled states requires the generation of a scalable number of squeezed modes. Here, we demonstrate the simultaneous generation of 21 squeezed spectral modes at the repetition rate of our laser, i.e., 156 MHz. We exploit the full repetition rate and the pulse shaping of a femtosecond light source to combine, for the first time, frequency- and time-multiplexing in multimode squeezing. This paves the way for the implementation of multipartite entangled states that are both scalable and fully reconfigurable.

FLASH and the FLASH2020+ project—current status and upgrades for the free-electron laser in Hamburg at DESY

The free-electron laser in Hamburg, FLASH, is the first extreme ultra-violet and soft X-ray free-electron laser (FEL) user facility and has been continuously upgraded since its start in 2005. Further major works are currently underway within the FLASH2020+ project that pioneeringly implements full repetition rate external seeding at a superconducting accelerator facility. With fully tunable undulators providing variable polarization FEL pulses, we expect FLASH to turn into the ideal spectroscopy machine for ultrafast processes within the coming years.

Rapid-Scan Nonlinear Time-Resolved Spectroscopy over Arbitrary Delay Intervals

Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise, and accurate measurement mode with easy registration of the combs beat note in the radio frequency domain. Extensions of this technique already found application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump–probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of 2 free-running oscillators. We confirm the versatility of this delay generation method by measuring χ (2) cross-correlation and χ (3) multicomponent population recovery kinetics.

High-speed two-dimensional terahertz spectroscopy with echelon-based shot-to-shot balanced detection.

By using a reflective-echelon-based electro-optic sampling technique and a fast detector, we develop a two-dimensional terahertz (THz) spectrometer capable of shot-to-shot balanced readout of THz waveforms at a full 1-kHz repetition rate. To demonstrate the capabilities of this new detection scheme for high-throughput applications, we use gas-phase acetonitrile as a model system to acquire two-dimensional THz rotational spectra. The results show a two-order-of-magnitude speedup in the acquisition of multidimensional THz spectra when compared to conventional delay-scan methods while maintaining accurate retrieval of the nonlinear THz signal. Our report presents a feasible solution for bringing the technique of multidimensional THz spectroscopy into widespread practice.

Bandwidth extension and conversion efficiency improvements beyond phase matching limitations using cavity-enhanced OPCPA.

The conversion efficiency and phase matching bandwidth of ultrafast optical parametric amplification (OPA) are constrained by the dispersion and nonlinear coefficient of the employed crystal as well as pulse shaping effects. In our work we show that an enhancement cavity resonant with the pump seeded at the full repetition rate of the pump laser can automatically reshape the small-signal gain in optical parametric chirped-pulse amplification (OPCPA) to achieve close-to-optimal operation. This new method termed cavity-enhanced OPCPA or C-OPCPA significantly increases both the gain bandwidth and the conversion efficiency, in addition to boosting gain for high-repetition-rate amplification. The goal in C-OPCPA is to arrive at a condition of impedance matching at all temporal coordinates, such that, in the absence of linear losses, all the incident pump power is dissipated in the nonlinear loss element, i.e., converted to signal and idler. The use of a low finesse enhancement cavity resonant with a low average power (<1W) and a high repetition rate (78MHz) pump source is shown to achieve more than 50% conversion efficiency into signal and idler from the coupled pump in an optical parametric process, whereas an equivalent amount of pump power in a single-pass configuration leads to negligible conversion. Additionally, the gain bandwidth is extended by a factor of 3-4 beyond the phase-matching limit. Our empirical observations are corroborated by a numerical analysis of depletion optimizing the single-pass case, which assesses the underlying impedance matching that is responsible for the observed performance improvements.

High-speed spatial encoding of modulated pump–probe signals with slow area detectors

Most of today’s pump–probe experiments, which rely on two-dimensional detectors, suffer from low read-out rates that prevent the implementation of fast lock-in techniques to increase the signal-to-noise ratio. The first cameras running at true kHz-frame rates are available at large-scale facilities, but require sophisticated data management strategies. Here we present a scheme for high-speed spatial encoding of modulated pump–probe signals using slow area detectors at full repetition rate of a laser system without an increase in data rate or change of the sample environment. Towards that end we block the probe light in front of the detector alternatingly with two inverted masks at the same frequency as the signal is modulated, e.g. the chopping frequency of the pump light. Modulation frequencies up to 500 Hz are demonstrated using a commercial mechanical chopper and have been applied to a time-resolved Faraday microscopy experiment probing all-optical magnetic switching of a GdFe-alloy with femtosecond temporal resolution. We believe that our concept bridges the gap between today’s slow area detectors and the upcoming generation of true kHz-frame-rate cameras.

Megahertz single-particle imaging at the European XFEL

The emergence of high repetition-rate X-ray free-electron lasers (XFELs) powered by superconducting accelerator technology enables the measurement of significantly more experimental data per day than was previously possible. The European XFEL is expected to provide 27,000 pulses per second, over two orders of magnitude more than any other XFEL. The increased pulse rate is a key enabling factor for single-particle X-ray diffractive imaging, which relies on averaging the weak diffraction signal from single biological particles. Taking full advantage of this new capability requires that all experimental steps, from sample preparation and delivery to the acquisition of diffraction patterns, are compatible with the increased pulse repetition rate. Here, we show that single-particle imaging can be performed using X-ray pulses at megahertz repetition rates. The results obtained pave the way towards exploiting high repetition-rate X-ray free-electron lasers for single-particle imaging at their full repetition rate.

Design, realization, and high power test of high gradient, high repetition rate brazing-free S -band photogun

rf photoguns find several types of applications as high brightness electron sources for free-electron lasers, energy recovery linacs, Compton and Thomson sources, and high-energy linear colliders. The high peak current and low transverse emittance of the generated beam are obtained with the combination of a high peak electric field ($g100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$) at the cathode surface, a proper choice of the solenoid field around, or immediately after, the gun, and special fabrication and treatments of the cathode itself. On the other hand, to increase the average electron current, a high repetition rate ($g100\text{ }\text{ }\mathrm{Hz}$) and/or a multibunch rf gun have to be developed. These types of devices are, in general, fabricated by brazing processes of copper machined parts. The brazing processes require a large vacuum furnace, are very expensive, and pose a not negligible risk of failure. A new fabrication technique for this type of structure has been recently developed and implemented at the Laboratories of Frascati of the National Institute of Nuclear Physics (INFN-LNF, Italy) and already applied to an rf gun now operating at a relatively low cathode peak field and low repetition rate [D. Alesini et al., Phys. Rev. Accel. Beams 18, 092001 (2015)]. It is based on the use of special rf-vacuum gaskets that allow a brazing-free realization process. The $S$-band gun of the Extreme Light Infrastructure-Nuclear Physics Gamma Beam System, under construction in Magurele (Bucharest, Romania), has been realized with this new technique and represents a further and fundamental step toward the consolidation of this technology for high gradient particle accelerator fabrication. It operates at 100 Hz with a $120\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ cathode peak field and $1.5\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{s}$-long rf pulses to house the 32 bunches necessary to reach the target gamma flux. High gradient tests, performed at full power and a full repetition rate, have shown the extremely good performances of the structure in terms of the breakdown rate and conditioning time and definitively demonstrated the reliability and suitability of such fabrication process for high gradient structure realization. In this paper, we report and discuss the electromagnetic and thermomechanical design, the realization process, and all the experimental results at low and high power at a full repetition rate.

Experimental station for ultrafast extreme ultraviolet spectroscopy for non-equilibrium dynamics in warm dense matter

Recent interest in highly excited matter generated by intense femtosecond laser pulses has led to experimental methods that directly investigate ultrafast non-equilibrium electronic and structural dynamics. We present a tabletop experimental station for the extreme ultraviolet (EUV) spectroscopy used to trace L-edge dynamics in warm dense aluminum with a temporal resolution of a hundred femtoseconds. The system consists of the EUV probe generation part via a high-order harmonic generation process of femtosecond laser pulses with atomic clusters, a beamline with high-throughput optics and a sample-refreshment system of nano-foils utilizing the full repetition rate of the probe, and a flat-field EUV spectrograph. With the accumulation of an order of a hundred shots, a clear observation of the change in the aluminum L-shell absorption was achieved with a temporal resolution of 90 fs in a 600-fs window. The signature of a non-equilibrium electron distribution over a 10-eV range and its evolution to a 1-eV Fermi distribution are observed. This demonstrates the capability of this apparatus to capture the non-equilibrium electron–hole dynamics in highly excited warm dense matter conditions.

Demonstration of a time-resolved x-ray scattering instrument utilizing the full-repetition rate of x-ray pulses at the Pohang Light Source.

We report on the development of a new experimental instrument for time-resolved x-ray scattering (TRXS) at the Pohang Light Source (PLS-II). It operates with a photon energy ranging from 5 to 18 keV. It is equipped with an amplified Ti:sappahire femtosecond laser, optical diagnostics, and laser beam delivery for pump-probe experiments. A high-speed single-element detector and high trigger-rate oscilloscope are used for rapid data acquisition. While this instrument is capable of measuring sub-nanosecond dynamics using standard laser pump/x-ray probe techniques, it also takes advantage of the dense 500 MHz standard fill pattern in the PLS-II storage ring to efficiently record nano-to-micro-second dynamics simultaneously. We demonstrate this capability by measuring both the (fast) impulsive strain and (slower) thermal recovery dynamics of a crystalline InSb sample following intense ultrafast laser excitation. Exploiting the full repetition rate of the storage ring results in a significant improvement in data collection rates compared to conventional bunch-tagging methods.

Characterization of pseudosingle bunch kick-and-cancel operational mode

Author(s): Sun, C; Robin, DS; Steier, C; Portmann, G | Abstract: Pseudosingle-bunch kick-and-cancel (PSB-KAC) is a new operational mode at the Advanced Light Source of Lawrence Berkeley National Laboratory that provides full timing and repetition rate control for single x-ray pulse users while being fully transparent to other users of synchrotron radiation light. In this operational mode, a single electron bunch is periodically displaced from a main bunch train by a fast kicker magnet with a user-on-demand repetition rate, creating a single x-ray pulse to be matched to a typical laser excitation pulse rate. This operational mode can significantly improve the signal to noise ratio of single x-ray pulse experiments and drastically reduce dose-induced sample damage rate. It greatly expands the capabilities of synchrotron light sources to carry out dynamics and time-of-flight experiments. In this paper, we carry out extensive characterizations of this PSB-KAC mode both numerically and experimentally. This includes the working principle of this mode, resonance conditions and beam stability, experimental setups, and diagnostic tools and measurements.

Fast spatial beam shaping by acousto-optic diffraction for 3D non-linear microscopy.

Acousto-optic deflection (AOD) devices offer unprecedented fast control of the entire spatial structure of light beams, most notably their phase. AOD light modulation of ultra-short laser pulses, however, is not straightforward to implement because of intrinsic chromatic dispersion and non-stationarity of acousto-optic diffraction. While schemes exist to compensate chromatic dispersion, non-stationarity remains an obstacle. In this work we demonstrate an efficient AOD light modulator for stable phase modulation using time-locked generation of frequency-modulated acoustic waves at the full repetition rate of a high power laser pulse amplifier of 80 kHz. We establish the non-local relationship between the optical phase and the generating acoustic frequency function and verify the system for temporal stability, phase accuracy and generation of non-linear two-dimensional phase functions.

High-power multi-megahertz source of waveform-stabilized few-cycle light.

Waveform-stabilized laser pulses have revolutionized the exploration of the electronic structure and dynamics of matter by serving as the technological basis for frequency-comb and attosecond spectroscopy. Their primary sources, mode-locked titanium-doped sapphire lasers and erbium/ytterbium-doped fibre lasers, deliver pulses with several nanojoules energy, which is insufficient for many important applications. Here we present the waveform-stabilized light source that is scalable to microjoule energy levels at the full (megahertz) repetition rate of the laser oscillator. A diode-pumped Kerr-lens-mode-locked Yb:YAG thin-disk laser combined with extracavity pulse compression yields waveform-stabilized few-cycle pulses (7.7 fs, 2.2 cycles) with a pulse energy of 0.15 μJ and an average power of 6 W. The demonstrated concept is scalable to pulse energies of several microjoules and near-gigawatt peak powers. The generation of attosecond pulses at the full repetition rate of the oscillator comes into reach. The presented system could serve as a primary source for frequency combs in the mid infrared and vacuum UV with unprecedented high power levels.

Ramsey-comb spectroscopy with intense ultrashort laser pulses

Frequency combs provide a broad series of well-calibrated spectral lines for highly precise metrology and spectroscopy, but this usually involves a trade-off between power and accuracy. A comb created by adjusting the time delay between two optical pulses now enables both. This so-called Ramsey comb could probe fundamental problems such as determining the size of the proton. Optical frequency combs based on mode-locked lasers have revolutionized the field of metrology and precision spectroscopy by providing precisely calibrated optical frequencies and coherent pulse trains1,2. Amplification of the pulsed output from these lasers is very desirable, as nonlinear processes can then be used to cover a much wider range of transitions and wavelengths for ultra-high precision, direct frequency comb spectroscopy3,4. Therefore full repetition rate laser amplifiers5,6 and enhancement resonators7,8 have been employed to produce up to microjoule-level pulse energies9. Here we present a spectroscopic method to obtain frequency comb accuracy and resolution by using only two frequency comb pulses amplified to the millijoule pulse energy level, orders of magnitude more energetic than what has previously been possible. The new properties of this approach, such as cancellation of optical light-shift effects, are demonstrated on weak two-photon transitions in atomic rubidium and caesium, thereby improving the frequency accuracy by up to thirty times.

Optical design of the short pulse x-ray imaging and microscopy time-angle correlated diffraction beamline at the Advanced Photon Source

The short pulse x-ray imaging and microscopy beamline is one of the two x-ray beamlines that will take full advantage of the short pulse x-ray source in the Advanced Photon Source (APS) upgrade. A horizontally diffracting double crystal monochromator which includes a sagittally focusing second crystal will collect most of the photons generated when the chirped electron beam traverses the undulator. A Kirkpatrick-Baez mirror system after the monochromator will deliver to the sample a beam which has an approximately linear correlation between time and vertical beam angle. The correlation at the sample position has a slope of 0.052 ps/μrad extending over an angular range of 800 μrad for a cavity deflection voltage of 2 MV. The expected time resolution of the whole system is 2.6 ps. The total flux expected at the sample position at 10 keV with a 0.9 eV energy resolution is 5.7 × 10(12) photons/s at a spot having horizontal and vertical full width at half maximum of 33 μm horizontal by 14 μm vertical. This new beamline will enable novel time-dispersed diffraction experiments on small samples using the full repetition rate of the APS.

Advanced photoinjector experiment photogun commissioning results

The Advanced Photoinjector Experiment (APEX) at the Lawrence Berkeley National Laboratory is dedicated to the development of a high-brightness high-repetition rate (MHz-class) electron injector for x-ray free-electron laser (FEL) and other applications where high repetition rates and high brightness are simultaneously required. The injector is based on a new concept rf gun utilizing a normal-conducting (NC) cavity resonating in the VHF band at 186 MHz, and operating in continuous wave (cw) mode in conjunction with high quantum efficiency photocathodes capable of delivering the required charge at MHz repetition rates with available laser technology. The APEX activities are staged in three phases. In phase 0, the NC cw gun is built and tested to demonstrate the major milestones to validate the gun design and performance. Also, starting in phase 0 and continuing in phase I, different photocathodes are tested at the gun energy and at full repetition rate for validating candidate materials to operate in a high-repetition rate FEL. In phase II, a room-temperature pulsed linac is added for accelerating the beam at several tens of MeV to reduce space charge effects and allow the measurement of the brightness of the beam from the gun when integrated in an injector scheme. The installation of the phase 0 beam line and the commissioning of the VHF gun are completed, phase I components are under fabrication, and initial design and specification of components and layout for phase II are under way. This paper presents the phase 0 commissioning results with emphasis on the experimental milestones that have successfully demonstrated the APEX gun capability of operating at the required performance.

Carrier-envelope phase stabilization of a terawatt level chirped pulse amplifier for generation of intense isolated attosecond pulses

We demonstrate the first carrier-envelope phase (CEP)-stabilized chirped pulse amplification system with pulse peak-powers in the terawatt regime. The system, which eventually is intended to be used in the generation of isolated attosecond pulses, consists of two consecutive multipass amplification stages. The first amplification stage is a commercial CEP-stable kHz system including a single 13-pass amplifier reaching a pulse energy of 2.3 mJ. Pulses are picked after the first stage at a repetition rate of 50 Hz and are further amplified in a 5-pass power-amplifier to pulse energies that reach up to 80 mJ before compression. After compression the pulse energy is 35mJ at a pulse duration of 32 fs, signifying a peak power of 1.1 terawatt. Peak-powers exceeding 1.5 TW should easily be achievable by improving the efficiency of the grating compressor. The CEP-stability of the terawatt system is demonstrated by single shot measurements of the residual CEP jitter at the full repetition rate and show an excellent root-mean-square value of 315 mrad.

Performance scaling via passive pulse shaping in cavity-enhanced optical parametric chirped-pulse amplification

We show that an enhancement cavity seeded at the full repetition rate of the pump laser can automatically reshape small-signal gain across the interacting pulses in an optical parametric chirped-pulse amplifier for close-to-optimal operation, significantly increasing both the gain bandwidth and the conversion efficiency, in addition to boosting gain for high-repetition-rate amplification. Applied to a degenerate amplifier, the technique can provide an octave-spanning gain bandwidth.

Phase-stable single-pass cryogenic amplifier for high repetition rate few-cycle laser pulses

We demonstrate cryogenic Ti:sapphire single-pass amplification of sub-7 fs laser pulses with 80 MHz repetition rate. We amplify the output of a broadband Ti:sapphire oscillator by more than a factor of two, re-compress the pulses down to sub-7 fs, and show that the rms carrier-envelope phase jitter stays below 70 as after amplification. The amplified output exceeds 2 MW of peak power and 1 W of average power. In addition, we demonstrate amplification of ∼200 fs, 75 MHz oscillator pulses up to 1.6 W with a gain of four. This work opens a new way to explore phase sensitive and highly nonlinear phenomena at the full oscillator repetition rate. As a first example, we demonstrate white light generation in a bulk crystal at the full oscillator repetition rate.