High Repetition Rate Research Articles

Time-resolved momentum microscopy with fs-XUV photons at high repetition rates with flexible energy and time resolution

Time-resolved momentum microscopy is an emerging technique based on photoelectron spectroscopy for characterizing ultrafast electron dynamics and the out-of-equilibrium electronic structure of materials in the entire Brillouin zone with high efficiency. In this article, we introduce a setup for time-resolved momentum microscopy based on an energy-filtered momentum microscope coupled to a custom-made high-harmonic generation photon source driven by a multi-100 kHz commercial Yb-ultrafast laser that delivers fs pulses in the extreme ultraviolet range. The laser setup includes a nonlinear pulse compression stage employing spectral broadening in a Herriott-type bulk-based multi-pass cell. This element allows flexible tuning of the driving pulse duration, providing a versatile time-resolved momentum microscopy setup featuring two operational modes designed to enhance either the energy or time resolution. We show the capabilities of the system by tracing ultrafast electron dynamics in the conduction band valleys of a bulk crystal of the 2D semiconductor WS2. Using uncompressed driving laser pulses, we demonstrate an energy resolution better than (107 ± 2) meV, while compressed pulses lead to a time resolution better than (48.8 ± 17) fs.

Methane-filled hollow fiber: A cost-effective pathway to compress high repetition rate ytterbium femtosecond lasers using self-phase modulation

Abstract Hollow-core fiber (HCF) based temporal pulse compression is widely used to generate few-cycle pulses. Here, we demonstrate the strength of methane (CH4) as a nonlinear medium for HCF-based pulse compressors to achieve high average power few-cycle pulses of sub-16 fs duration with > 70% transmission and up to 250 kHz repetition rate in a compact setup and present it as a cost-effective alternative to highly expensive and scarcely available noble gases.

Stable laser-acceleration of high-flux proton beams with plasma collimation

Laser-plasma acceleration of protons offers a compact, ultra-fast alternative to conventional acceleration techniques, and is being widely pursued for potential applications in medicine, industry and fundamental science. Creating a stable, collimated beam of protons at high repetition rates presents a key challenge. Here, we demonstrate the generation of multi-MeV proton beams from a fast-replenishing ambient-temperature liquid sheet. The beam has an unprecedentedly low divergence of 1° (≤20 mrad), resulting from magnetic self-guiding of the proton beam during propagation through a low density vapour. The proton beams, generated at a repetition rate of 5 Hz using only 190 mJ of laser energy, exhibit a hundred-fold increase in flux compared to beams from a solid target. Coupled with the high shot-to-shot stability of this source, this represents a crucial step towards applications.

Next-Generation Ultrafast Photoluminescence Spectroscopy: Integration of Transient Grating Optical Gate and Advanced Femtosecond Laser Technology.

We demonstrate a high-performance ultrafast broadband time-resolved photoluminescence (TRPL) system based on the transient grating photoluminescence spectroscopy (TGPLS) technique. The core of the system is a Kerr effect-induced transient grating (TG) optical gate driven by high repetition rate ultrashort laser pulses at 1030 nm with micro-Joule pulse energy. Satisfying the demands of spectroscopy applications, the setup achieves high sensitivity, rapid data acquisition, ultrafast time resolution, and a wide spectral window from ultraviolet to near-infrared. The time resolution can be further improved to achieve <80 fs instrument response function by employing the multiple plate compression (MPC) technique to temporally compress the driving pulses. This work presents a new benchmark for ultrafast TRPL.

Fast‐Tuning and Narrow‐Linewidth Hybrid Laser for FMCW Ranging

AbstractPossessing a long coherent length, high repetition rate, and fast frequency‐sweeping laser sources with narrow linewidth is crucial components in coherent frequency‐modulated continuous‐wave (FMCW) light detection and ranging (LiDAR) systems. While these attributes are realized individually in standalone devices, the integration of these features into a single laser represents a significant advancement in the field. In this study, a hybrid integrated laser that achieves a linewidth of 9 kHz, a wide frequency‐modulation response extending up to 68 MHz, and a low chirp nonlinearity of 4.3 × 10−6 at a repetition rate of 100 kHz is presented. The achievement of this performance is made possible through self‐injection locking of a DFB laser diode to a low‐loss Si3N4 micro‐ring resonator on the dual‐layer Si3N4‐Si platform. Through the application of a fast‐converging pre‐distortion algorithm and a driving signal with 150 mV amplitude, a linear FMCW signal with 1.05 GHz frequency excursion is generated. Exploiting the wideband FM response of the PIN phase shifter, a frequency‐agile FMCW light source engine capable of generating linear FMCW signals at repetition rates of up to 2 MHz is successfully developed. Leveraging these cutting‐edge capabilities, an FMCW LiDAR ranging system for target detection across varying distances, achieving a high ranging precision of 0.4 cm for targets at 6.2 m, is implemented. This innovative work not only demonstrates the feasibility of integrating multiple advanced functionalities into a single laser but also demonstrates the potential for enhancing the resolution and precision of FMCW LiDAR systems for a wide range of applications.

A System Control and Waveform Acquisition Framework for the Kicker System at SHINE

A lumped kicker magnet system is being developed as part of the electron beam switchyard of the Shanghai High Repetition Rate X-Ray Free-Electron Laser (XFEL) and Extreme Light (SHINE) facility at Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS). SHINE is a linac-based facility with an energy of 8 GeV and a repetition rate of 1 MHz for multiple users. The kicker system is a critical element of the deflection system and determines the stability performance of the beamlines. To ensure autonomous and reliable operation in a harsh environment, a dedicated control system—consisting of a custom-designed programmable logic controller (PLC) and waveform acquisition system—has been developed based on the Experimental Physics and Industrial Control System (EPICS). The PLCsystem enables status acquisition, control implantations, and alarm handling, allowing for automated operation. The waveform acquisition system provides an intuitive demonstration and enables the real-time diagnosis of kicker waveforms. The results demonstrate a half quasi-sinuous waveform with a peak current of 11.5 A and a full width at half maximum of 218 ns at a repetition rate of 10 kHz. The detailed design, overall concept, achieved performance, and initial test results of two modular controls are provided for further demonstration.

High SNR, high repetition rate, compact, picosecond, mid-infrared optical parametric oscillator pumped synchronously by a harmonic mode-locked fiber laser

High SNR, high repetition rate, compact, picosecond, mid-infrared optical parametric oscillator pumped synchronously by a harmonic mode-locked fiber laser

Realization of a 4 kHz, large energy single-frequency ns optical parametric oscillator by suppressing optical parametric generation.

Large energy single-frequency nanosecond (ns) near-infrared light source is an essential device in the field of the remote chemical analysis based on the laser-induced breakdown spectroscopy (LIBS). In this paper, a large energy single-frequency ns 824 nm light source with high repetition rate is presented, which is generated from a seed-injection locked optical parametric oscillator (OPO). By optimizing the spot radius of the pump laser and the mode-matching between the pump laser and signal light, the optical parametric generation (OPG) process is effectively eliminated. On this basis, with the assistance of the seed-injection locking, a single-frequency 4 kHz ns 824 nm light source with an output pulse energy and a spectral width of 3.39 mJ and 32.42 MHz is obtained. For the best of our knowledge, it is the largest energy for the single-frequency ns near-infrared light source with the repetition rate of kHz level.

Thermal stress-induced depolarization compensation in wide bandwidth, high energy, high repetition rate multi-slab laser amplifiers

Thermal stress-induced depolarization compensation in wide bandwidth, high energy, high repetition rate multi-slab laser amplifiers

Pulse duration dependent effects of ultrafast laser induced damage on a 1030 nm multi-layer dielectric mirror for high repetition rate, high average power laser systems

High repetition rate, high peak, and average power laser systems are crucial for next-generation particle accelerators, inertial confinement fusion, and secondary particle sources. These applications demand durable laser optics, particularly interference coatings on optics lasting millions of shots at high fluence. This study focuses on designing, testing, and simulating multi-layer dielectric (MLD) mirrors for pulse durations of 260 fs, 77 fs, and 25 fs at 1030 nm wavelength and 45-degree incidence angle with p-polarization. S-on-1 laser-induced damage thresholds (LIDT) for varying pulse numbers were determined, with single-shot LIDT values of 0.98 Jcm-2, 1.63 Jcm-2, and 2.3 Jcm-2 for 25 fs, 77 fs, and 260 fs respectively. A strong correlation between blister shape and local fluence was observed, implying that the layer expansion in a blister depends on local fluence. We have also examined mechanisms responsible for laser-induced stress generation and energy release rates in blister formation. Damage mechanisms are further explored by finite-difference time-domain (FDTD) simulations, incorporating Keldysh strong field ionization, whose predictions were in excellent agreement with the onset of damage determined experimentally. These findings offer insights for enhancing MLD coating technology, promising more efficient and resilient laser systems for diverse scientific and industrial applications.

Femtosecond laser fabricated semi-tapered waveguide for Q-switched vortex laser generation.

Dielectric waveguides are widely recognized as excellent and versatile components for integrated multifunctional photonic chips, thanks to their strong optical confinement capabilities. In this study, we present a novel semi-tapered depressed-cladding waveguide structure, designed and fabricated using femtosecond laser direct writing technology. The optical guiding performance of this semi-tapered waveguide is systematically analyzed by characterizing its loss characteristics. Leveraging the high gain and superior laser performance of the semi-tapered waveguide platform, we successfully demonstrate an efficient vortex waveguide laser. Moreover, we achieve a stable Q-switched vortex laser with short pulse duration and high repetition rate by utilizing an Sb2Te3 thin film as a saturable absorber. This work exemplifies the potential for developing three-dimensional waveguides for on-chip integrated pulsed lasers.

Multi-segment cooling design of a reflection mirror based on the finite-element method.

High-repetition-rate free-electron lasers impose stringent requirements on thermal deformations of optics in the beamline. The Shanghai HIgh-repetition-rate XFEL aNd Extreme light facility (SHINE) experiences high average thermal power and demands wavefront preservation. To effectively manage thermal deformation in the first reflection mirrors M1, we optimized the cooling length and position of the cooling groove with numerical calculations. For example, the root mean square of the height error of the thermal deformation of the mirror at a photon energy of 900 eV was optimized, resulting in a 12.7× reduction, from 13.76 nm to 1.08 nm. This optimized design also eliminated stray light in the focus spot at the sample and resulted in a 177% increase in the peak intensity of the beam's focus spot at the sample, from 3.08 × 105 to 8.53 × 105. The multi-segment cooling design of the mirror advanced the quality of the beam's focus spot at the sample and ensured the stable operation of SHINE under high repetition rates.

Selective growth of ZnO micro- and nano-structures on fs-laser processed metallic Zn substrates for large area applications

The synthesis of large area micro- and nano-structured ZnO surfaces has been successfully achieved through a two-step process. It involves the irradiation of Zn metal sheets with femtosecond laser pulses (350 fs at 1030nm) at high repetition rates (100 - 500kHz), and fast scanning speeds (cm/s). Subsequently, the irradiated sheets are thermally treated in an Argon flux at 380 oC, a temperature significantly lower than that typically required for growing micro- and nanostructures in ZnO. Fs-laser irradiation promotes the initial development of topography and the localized oxidation of the metal. This enables the further growth of micro- and nanostructures at preferential sites with good crystalline quality and luminescent properties. Analysis of the material at different processing steps shows that the initial laser-induced oxidation is crucial in defining ZnO growth mechanisms upon thermal treatment, and determining the final properties. We have tested the potential use of these structures as reusable photocalyst. The ease of catalyst recovery in photocatalysis experiments and the degree of degradation achieved may be considered as key performance indicators. Photocatalytic activity tests performed with a Rhodamine B solution showed degradation values up to 43% over 90min. The morphology of the samples remains unaltered after photocatalysis experiments.

A newly designed laser-based time- and angle-resolved photoelectron spectroscopy with a time-of-flight electron analyzer

Angle-resolved photoemission spectroscopy can directly detect the energy and momentum resolved electronic structure of solids, serving as a central role in the discovery and understanding of quantum materials. Here, we report the development of a novel time-resolved ARPES setup equipped with a table-top vacuum ultraviolet laser source with a photon energy of 10.8 eV and a time-of-flight analyzer. The light source is obtained through the generation of ninth harmonics of a 1030 nm Yb fiber-based amplified laser (290 fs, 100 μJ). The photon flux can reach 5 × 1012 photons/s at 333 kHz. We demonstrate its performance in ARPES measurements of the polycrystalline gold film and the electronic structure of the topological insulator Bi2Te3. By introducing a pump beam, we make a pump–probe experiment to detect unoccupied electronic states of Bi2Te3. This setup can achieve an energy resolution of 21.6 meV and a temporal resolution of 296 fs with the tunability of the polarization and repetition rates. This system can provide an important platform to study the non-equilibrium band structure of complex quantum materials with exceptional energy resolution at high repetition rates.

Toward a storage ring coherent light source based on an angular dispersion-induced microbunching scheme.

The combination of reversible angular dispersion-induced microbunching (ADM) and the rapid damping storage ring provides a storage-ring-based light source with the capability to produce longitudinal coherent radiation with a high repetition rate. This paper presents a prototype design for a test facility based on the study by Jiang et al. [Sci. Rep. (2022), 12, 3325]. The modulation-demodulation section is inserted into a long straight section of the storage ring instead of a bypass line, which poses great challenges for the optimization of the nonlinear dynamics of the storage ring. However, this design avoids the challenging injection and extraction system connecting to the bypass line. To utilize mature laser technology and reduce the difficulty of the reversible ADM lattice design, we use a long-wavelength 1030 nm seed laser. In the simulation, we achieved 20th harmonic radiation with a bunching factor of about 7.2%. The growth rate of vertical emittance and energy spread of the electron beam for a single pass are about 11% and 0.02%, respectively. When the energy of the electron beam is 800 MeV and two sets of damping wigglers are employed, the damping time in the vertical plane is reduced to 8.31 ms. This results in a 438 kHz repetition rate of the coherent radiation at the new equilibrium state.

A high repetition rate millimeter wavelength accelerator for an X-ray free-electron laser

A compact collinear wakefield accelerator has been designed for an X-ray free-electron laser capable of operating at a pulse repetition rate in the tens of kilohertz. The maximum achievable accelerating gradient has been determined, with its limitation linked to beam breakup instability. The fabrication techniques for the principal components of the accelerator including wakefield generation, couplers for excess power extraction and diagnostics, focusing quadrupoles, and a novel undulator have been discussed. Results from various laboratory and beam-based tests on these components have been compared to their original design specifications and demonstrated very good agreement. A preliminary design for the XFEL has been presented, featuring a novel small-period, force-neutral, adjustable-phase undulator.

Generation of highly stable electron beam via the control of hydrodynamic instability

By employing the stabilizer in the supersonic gas nozzle to produce the plasma density profile with a sharp downramp, we have experimentally demonstrated highly stable electron beam acceleration based on the shock injection mechanism in laser wakefield acceleration with the use of a compact Ti:sapphire laser. A quasi-monoenergetic electron beam with a peak energy of 315 MeV ± 12.5 MeV per shot is generated. The electron pointing fluctuations are less than 1 mrad, which is a significant improvement over previous results. This is due to the precise control of the target density distribution and the relative distance between the shock and the laser focal position. The Particle-in-cell simulations demonstrate the sensitivity of electron acceleration to the target profile, while the computational fluid dynamics prove the stabilizer’s effect on gas formation. Further developments of this scheme have the potential to deliver a high repetition rate gas target. The corresponding reproducibility of the accelerated electron beam paves the way for the realisation of compact laser plasma accelerators and the potential application of free electron lasers.

Compression stability enhancement with low temperature-dependent SBS compressors at high repetition rates

The stimulated Brillouin scattering (SBS) pulse compression with stable compression efficiency in low temperature-dependent SBS compressors is demonstrated by suppressing the rising edge broadening under high repetition rates. Theoretical analysis reveals that the difficulty in achieving stable SBS compression is caused by the broadening of the phonon lifetime owing to thermal effect, and this undesirable fluctuation can be effectively suppressed by a low temperature-dependent medium. Reductions of 66.33% on Stokes pulse width broadening and 50% on rising time with repetition rate were achieved under 36.87% enhancement of kinematic viscosity stability. Benefiting from a stable gain coefficient, the maximum fluctuation of 3.51% in transient Stokes pulse energy was demonstrated at maximum pump power with stability enhancement of 16.28% on steady-state energy reflectivity.

Mitigation of thermal artifacts in 100kHz ultrafast 2D IR spectroscopy.

Ytterbium lasers make possible shot-to-shot data collection of two-dimensional infrared (2D IR) spectra at 100kHz and higher repetition rates. At those rates, the power absorbed by the sample is appreciable and creates a steady state temperature rise and an accumulated thermal grating artifact in the spectra that can obscure weak or low concentration IR chromophores. We report the magnitude of the temperature rise, the pulse ordering by which it is created, and ways to mitigate it. Using a calibrant molecule, we measured a steady-state temperature up to 32.5 and 45 °C for laser light at 4 µm in H2O and 6 µm in D2O, respectively, for a typical optical density used in 2D IR experiments. The temperature reached a steady state in ∼60s. The temperature rise scales with the integrated optical density of the sample across the laser spectrum. By cooling the sample cell, we returned the steady state temperature to room temperature within the laser focus. For samples that undergo rotation, the accumulated thermal grating artifact is removed using a perpendicular ⟨XXYY⟩ polarization because the permuted time-orderings of the thermal grating artifact has the orientational response ⟨XYXY⟩, which decays to zero during the delay between consecutive laser pulses. The procedure described in this study can be used to characterize and minimize the thermal effects in experiments where repetition rate and/or pulse energy cause an appreciable temperature rise.

Experimental Investigation of the Optical Nonlinearity of Laser-Ablated Titanium Dioxide Nanoparticles Using Femtosecond Laser Light Pulses.

In this report, the nonlinear optical (NLO) properties of titanium dioxide nanoparticles (TiO2 NPs) have been explored experimentally using femtosecond laser light along with the Z-scan approach. The synthesis of TiO2 NPs was carried out in distilled water through nanosecond second harmonic Nd:YAG laser ablation. Characterization of the TiO2 NPs colloids was conducted using UV-visible absorption spectroscopy, transmission electron microscopy (TEM), inductively coupled plasma (ICP), and energy-dispersive X-ray spectroscopy (EDX). The TEM analysis indicated that the size distribution and average particle size of the TiO2 NPs varied from 8.3 nm to 19.1 nm, depending on the laser ablation duration. The third-order NLO properties of the synthesized TiO2 NPs were examined at different excitation laser wavelengths and incident powers through both open- and closed-aperture Z-scan techniques, utilizing a laser pulse duration of 100 fs and a high repetition rate of 80 MHz. The nonlinear absorption (NLA) coefficient and nonlinear refractive (NLR) index of the TiO2 NPs colloidal solutions were found to be influenced by the incident power, excitation wavelength, average size, and concentration of TiO2 NPs. Maximum values of 4.93 × 10⁻⁹ cm/W for the NLA coefficient and 15.39 × 10⁻15 cm2/W for the NLR index were observed at an excitation wavelength of 800 nm, an incident power of 0.6 W, and an ablation time of 15 min. The optical limiting (OL) effects of the TiO2 NPs solution at different ablation times were investigated and revealed to be concentration and average size dependent. An increase in concentration results in a more limiting effect.