Sapphire Laser System Research Articles

Wafer-scale fabrication of target arrays for stable generation of proton beams by laser-plasma interaction.

Large-scale fabrication of targets for laser-driven acceleration of ion beams is a prerequisite to establish suitable applications, and to keep up with the challenge of increasing repetition rate of currently available high-power lasers. Here we present manufacturing and test results of large arrays of solid targets for TNSA laser-driven ion acceleration. By applying micro-electro-mechanical-system (MEMS) based methods allowing for parallel processing of thousands of targets on a single Si wafer, sub-micrometric, thin-layer metallic membranes were fabricated by combining photolithography, physical and chemical vapor deposition, selective etching, and Si micromachining. These structures were characterized by using optical and atomic force microscopy. Their performance for the production of laser-driven proton beams was tested on a purpose-made table-top Ti:Sapphire laser system running at 3 TW peak power with a contrast over ASE of 108. We have performed several test series achieving maximum proton energy values around 2 MeV.

Generation of keV hot near-solid density plasma states at high contrast laser-matter interaction

We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. Experiments were carried out with the Ti:Sapphire laser system where a picosecond pre-pulse was strongly reduced by the conversion of the fundamental laser frequency into 2ω. A complex diagnostics setup was used for evaluation of the electron energy distribution in a wide energy range. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electron production in the case of the high laser contrast. Characteristic x-ray radiation has been used for evaluation of the bulk electron temperature and density. The measured Ti line radiation was simulated both in steady-state and transient approaches using the code FLYCHK that accounts for the atomic multi-level population kinetics. The best agreement between the measured and the synthetic spectrum of Ti was achieved at 1.8 keV electron temperature and 2 × 1023 cm−3 electron density. By application of Ti-foils covered with nm-thin Fe-layers, we have demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature Equation of States, wide-range description of all transport and optical properties, ionization, electron, and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy, first deposited into the nm-thin skin-layer, is then transported into 150 nm depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons.

Laser pulses damage safety eyewear

When used with ultrafast pulsed lasers, safety eyewear may fail to protect against eye damage, reported Hood College chemistry professor Christopher J. Stromberg at the meeting. Laser safety eyewear is typically tested and rated using low-power, continuous-wave conditions, assuming narrow-bandwidth lasers. Ultrafast laser pulses, however, have higher peak powers and wider bandwidths. Stromberg, undergraduate students Max Riedel-Topper and Sarah Wirick, and colleagues at NIST tested 22 eyewear filter samples from five manufacturers using an 800-nm Ti:sapphire laser system with 26-to-40-nm bandwidths and 40-to-80-femtosecond pulses (J. Laser Appl. 2017, DOI: 10.2351/1.5004090). Only 10 of the eyewear samples, all of them glass, performed as expected. All the plastic eyewear samples failed. These filters failed because of dye saturation, dye damage or bleaching, plastic substrate burning or melting, or a combination of these mechanisms. Stromberg and colleagues suspect that laser pulses’ higher bandwidths, along with their higher power punch, contributed to the filter

Highly efficient generation of 0.2 mJ terahertz pulses in lithium niobate at room temperature with sub-50 fs chirped Ti:sapphire laser pulses.

We demonstrate generation of 0.2 mJ terahertz (THz) pulses in lithium niobate driven by Ti:sapphire laser pulses at room temperature. Employing tilted pulse front technique, the 800 nm-to-THz energy conversion efficiency has been optimized to 0.3% through chirping the sub-50 fs pump laser pulses to overcome multi-photon absorption and to extend effective interaction length for phase matching. Our approach paves the way for mJ-level THz generation via optical rectification using existing Ti:sapphire laser systems which can deliver Joule-level pulse energy with sub-50 fs pulse duration.

Investigations of gain redshift in high peak power Ti:sapphire laser systems

Gain redshift in high peak power Ti:sapphire laser systems can result in narrowband spectral output and hence lengthen the compressed pulse duration. In order to realize broadband spectral output in 10 PW-class Ti:sapphire lasers, the influence on gain redshift induced by spectral pre-shaping, gain distribution of cascaded amplifiers and Extraction During Pumping (EDP) technique have been investigated. The theoretical and experimental results show that the redshift of output spectrum is sensitive to the spectral pre-shaping and the gain distribution of cascaded amplifiers, while insensitive to the pumping scheme with or without EDP. Moreover, the output spectrum from our future 10 PW Ti:sapphire laser is theoretically analyzed based on the investigations above, which indicates that a Fourier-transform limited (FTL) pulse duration of 21 fs can be achieved just by optimizing the spectral pre-shaping and gain distribution in 10 PW-class Ti:sapphire lasers.

520-µJ mid-infrared femtosecond laser at 2.8 µm by 1-kHz KTA optical parametric amplifier

We report on a 520-µJ, 1-kHz mid-infrared femtosecond optical parametric amplifier system driven by a Ti:sapphire laser system. The seeding signal was generated from white-light continuum in YAG plate and then amplified in four non-collinear amplification stages and the idler was obtained in the last stage with central wavelength at 2.8 µm and bandwidth of 525 nm. To maximize the bandwidth of the idler, a theoretical method was developed to give an optimum non-collinear angle and estimate the conversion efficiency and output spectrum. As an experimental result, laser pulse energy up to 1.8 mJ for signal wave and 520 µJ for idler wave were obtained in the last stage under 10-mJ pump energy, corresponding to a pump-to-idler conversion efficiency of 5.2%, which meets well with the numerical calculation.

Optimization of highly efficient terahertz generation in lithium niobate driven by Ti:sapphire laser pulses with 30 fs pulse duration

We systematically study the optimization of highly efficient terahertz (THz) generation in lithium niobate (LN) crystal pumped by 800 nm laser pulses with 30 fs pulse duration. At room temperature, we obtain a record optical-to-THz energy conversion efficiency of 0.43% by chirping the pump laser pulses. Our method provides a new technique for producing millijoule THz radiation in LN via optical rectification driven by joule-level Ti:sapphire laser systems, which deliver sub-50-fs pulse durations.

High order harmonics generation by relativistically circularly polarized laser-solid interaction

Coherent extreme ultra-violet (XUV) and soft X-ray light with attosecond duration enable the time-resolved study of electron dynamics in a completely new regime. High order harmonic generation (HHG) from the highly nonlinear process of relativistically intense laser interactions with solid-density plasma offers a very new way to generate such a light source. In this paper, we study the HHG by a relativistically circularly polarized femtosecond laser interacting with solid-density plasma. The experiment is carried out by using a 200 TW Ti:sapphire laser system at the Laboratory for Laser Plasmas in Shanghai Jiao Tong University, China. The laser system can deliver laser pulses at 800 nm with a pulse duration (full width at half maximum, FWHM) of 25 fs and repetition rate of 10 Hz. The circularly polarized laser beam with an energy of 460 mJ is used in the experiment and focused by an F/4 off-axis parabolic mirror at an incidence angle of 40 with respect to the glass target. The focal spot diameter is 6 m (FWHM) with 25% energy enclosed, giving a calculated peak intensity of 1.61019 W/cm2. We detect high order harmonics by a flat-field spectrometer. The experimental results show that high order harmonic radiation can also be efficiently generated by a circularly polarized laser at a lager incidence angle, which provides a straightforward way to obtain a circularly polarized XUV light source. Different plasma density scale lengths are obtained by introducing a prepulse with different delays. We study the dependence of HHG efficiency on plasma density scale length by the circularly polarized laser, and find an optimal density scale length to exist. The influence of laser polarization and plasma density scale length on HHG are studied by two-dimensional (2D) PIC simulations. The good agreement is found between the 2D PIC simulations and experimental results. We plan to measure the polarization characteristics of high order harmonic produced by the interaction of circularly polarized lasers with solid target in the future. It is expected to obtain a compact coherent circularly polarized XUV light source, which can be used to study the ultra-fast dynamic process of magnetic materials.

Intensity Noise Transfer Through a Diode-Pumped Titanium Sapphire Laser System

In this paper, we investigate the noise performance and transfer in a titanium sapphire (Ti:S) laser system. This system consists of a distributed Bragg-reflector tapered diode laser, which is frequency doubled in two cascaded nonlinear crystals and used to pump the Ti:S laser oscillator. This investigation includes electrical noise characterizations of the utilized power supplies, the optical noise of the fundamental light, the second harmonic light, and finally the optical noise of the femtosecond pulses emitted by the Ti:S laser. Noise features originating from the electric power supply are evident throughout the whole transfer chain. It is demonstrated that improving the electrical noise provides an easy method for reducing the relative intensity noise (RIN) in all stages. The frequency doubled light is shown to have a higher RIN than the fundamental light. In particular, the cascaded system is seen to exhibit higher RIN than a setup with only a single nonlinear crystal. The Ti:S is shown to have a cutoff frequency around 500 kHz, which means that noise structures of the pump laser above this frequency are strongly suppressed. Finally, the majority of the Ti:S noise seems to originate from the laser itself, which partly can be explained by the effect of relaxation oscillation frequency.

A new resonant Laser-SNMS system for environmental ultra-trace analysis: Installation and optimization

Localization, analysis and mobility of radioactive contaminated particles is of major concern for assessment of contamination threads and nuclear forensics. For this purpose, a new resonant Laser-SNMS system was developed and set up at the Institute for Radioecology and Radiation Protection for spatially resolved ultra-trace analysis of low concentrated radionuclides directly on environmental samples. This paper describes the adaption and combination of a dedicated Ti:sapphire laser system with a commercial TOF-SIMS instrument for resonant Laser-SNMS. The project includes computer simulations for optimization of the TOF analyzer. Results on synthetic uranium and technetium samples verify the expected excellent suppression of interfering isobaric contaminations.

Measurement of nonlinear refractive indices of air, oxygen, and nitrogen in capillary by changing the temporal width of short laser pulses

Nonlinear refractive indices of air and its constituents were measured as a function of the pulse width in a capillary used to confine the gas and to increase the interaction length. This study was carried out with nonlinear ellipse rotation measurements using an amplified Ti:sapphire laser system providing pulses at 800 nm, continuously tunable from ∼60 fs up to ∼3 ps. Due to the presence of noninstantaneous orientational response, nonlinearities in molecular gases strongly depend on the pulse width. The pulse width dependence and discrimination between instantaneous and noninstantaneous nonlinearities of air gases were obtained with a simple exponential growth model.

Arch-Shaped triboelectric nanogenerator as a facile device for water-wave vibrational energy

We have derived modified Frantz-Nodvik equations that simultaneously account for atomic phase shift (APS) and gain depletion as the chirped laser pulse passes through a gain medium, and have analyzed the effect of temporal pulse distortion in a Ti:sapphire multipass amplifier chain. The combination of APS and gain depletion distorted a temporal pulse and decreased the peak power. The pulse width increased from 21.3 fs to 22.8 fs and the peak power reduced to 89% for the PW class Ti:sapphire CPA laser system in the particular conditions.

A New kHz Velocity Map Ion/Electron Imaging Spectrometer for Femtosecond Time-Resolved Molecular Reaction Dynamics Studies

A new velocity map imaging spectrometer is constructed for molecular reaction dynamics studies using time-resolved photoelectron/ion spectroscopy method. By combining a kHz pulsed valve and an ICCD camera, this velocity map imaging spectrometer can be run at a repetition rate of 1 kHz, totally compatible with the fs Ti:Sapphire laser system, facilitating time-resolved studies in gas phase which are usually time-consuming. Time-resolved velocity map imaging study of NH3 photodissociation at 200 nm was performed and the time-resolved total kinetic energy release spectrum of H+NH2 products provides rich information about the dissociation dynamics of NH3. These results show that this new apparatus is a powerful tool for investigating the molecular reaction dynamics using time-resolved methods.

Contrast ratio enhancement by spectral matching of a seed laser pulse and ASE in a Ti:sapphire laser system.

The ASE (amplified spontaneous emission) level in a laser system consisting of an oscillator and a regenerative amplifier is very important, for example, in the interaction of an intense laser pulse and a thin foil, so a lower ASE level is always required. In this paper, we propose a new method to achieve a lower ASE level, which can be obtained by spectral matching of the seed laser beam and the ASE in a CPA (chirped-pulse amplification) Ti:sapphire laser system. In this method, two baffles are used to control the seed pulse spectrum by blocking a portion of the seed beam in a grating stretcher and it was found that the spectral matching method can reduce the temporal contrast ratio (after the regenerative amplifier) by a factor of 10 in a few hundred picosecond scale. This kind of spectral matching method is simple and it can be easily employed for other CPA laser systems to enhance the contrast ratio.

A pulse-to-pulse timing jitter measurement between two synchronized amplified laser beams for TTX.

In China, Tsinghua Thomson Scattering X-ray Source (TTX) is the dedicated hard X-ray source based on the Thomson scattering between a terawatt ultrashort laser and a relativistic electron beam. In the TTX, two synchronized Ti: sapphire laser systems generate the terawatt ultrashort infrared scattering laser and the ultraviolet driving laser for the photocathode RF gun to produce the electron beam; measuring the timing jitter between the electron beam and the laser beam is an essential task for the X-ray source. In the present study, we report on a single shot, non-collinear cross correlator with fs resolution and measured the timing jitter between the two synchronized laser systems with a pulse-to-pulse method, which is beneficial to estimate the jitter of the X-ray yield in the TTX system. Although it is more important to synchronize the scattering laser to the electron beam and not of the driving laser, the laser-laser jitter measurement would be a good first step towards that goal, and the result generated can be considered as the error signal for the potential feedback stabilization.

What is the most powerful laser researchers can build?

“Theorists say an intense enough laser field would rip photons into electron-positron pairs, dousing the beam. But no one knows whether it’s possible to reach that point.” The theory about electron-positron generation from vacuum is reviewed, also the developing of laser intensity. Schwinger predicted that electron-positron pair would be produced from the vacuum when the field near the critical QED field E s= m e2 c 3/ e ħ =1.32×1016 V/cm (the corresponding intensity I s=4.65×1029 W/cm2). The threshold to generate electron-positron pairs is about 5×1027 W/cm2 for single focused laser beam, and about 1026 W/cm2 for a pair of colliding laser pulses. When one electron-positron pair is generated, it would induce cascade effect under such strong laser field which would deplete energy of the field, so the highest intensity would be about 3×1026 W/cm2. The highest power people have reached is 5.13 PW, the corresponding intensity is about 1024 W/cm2, which was obtained by Chirped Pulse Amplification from Ti: Sapphire laser system. There are several projects in the world to generate 10–15 PW laser in the next few years, and several hundred PW to one Exawatt in the far future which would obtain laser intensity of 1026–1027 W/cm2. It is still not clear if the far future aim could be reached.

Synchronizing single-shot high-energy iodine photodissociation laser PALS and high-repetition-rate femtosecond Ti:sapphire laser system.

A system of precise pulse synchronization between a single-shot large-scale laser exploiting an acousto-optical modulator and a femtosecond high repetition rate laser is reported in this article. This opto-electronical system has been developed for synchronization of the sub-nanosecond kJ-class iodine photodissociation laser system (Prague Asterix Laser System-PALS) with the femtosecond 25-TW Ti:sapphire (Ti:Sa) laser operating at a repetition rate 1 kHz or 10 Hz depending on the required energy level of output pulses. At 1 kHz synchronization regime, a single femtosecond pulse of duration about 45 fs and a small energy less than 1 mJ are exploited as a probe beam for irradiation of a three-frame interferometer, while at 10 Hz repetition rate a single femtosecond pulse with higher energy about 7-10 mJ is exploited as a probe beam for irradiation of a two-channel polaro-interferometer. The synchronization accuracy ±100 ps between the PALS and the Ti:Sa laser pulses has been achieved in both regimes of synchronization. The femtosecond interferograms of laser-produced plasmas obtained by the three-frame interferometer and the femtosecond polarimetric images obtained by the two-frame polaro-interferometer confirm the full usefulness and correct functionality of the proposed method of synchronization.

Spectral-Phase-Modulated Cross-Polarized Wave for Chirped Pulse Amplifier with High Contrast Ratio**Supported by the National Key Basic Research Program of China under Grant No 2013CB922402, the National Natural Science Foundation of China under Grant Nos 61575217 and 11434016, the Key Research Program of Frontier Sciences of Chinese Academy of Sciences under Grant Nos KJZD-EW-L11-03 and QYZDJ-SSW-JSC006, and the Strategic Priority Research

We demonstrate a high-quality cross-polarized-wave filter based on spectral phase modulation. Driven by well-compressed spectral-phase fully-compensated fundamental laser pulses, the filter stretches the pulse bandwidth from 35 nm to 70 nm with a conversion efficiency of 20%. After implementing the filter into a femtosecond TW Ti:sapphire laser system, we generate 40 mJ output pulse energy with pulse duration of 18.9 fs. The temporal contrast of the compressed pulse is enhanced to 109.

Characterization of the ELIMED prototype permanent magnet quadrupole system

The system described in this work is meant to be a prototype of a more performing one that will be installed at ELI-Beamlines in Prague for the collection of ions produced after the interaction Laser-target, [1]. It has been realized by the researchers of INFN-LNS (Laboratori Nazionali del Sud of the Instituto Nazionale di Fisica Nucleare) and SIGMAPHI, a French company, using a system of Permanent Magnet Quadrupoles (PMQs), [2]. The final system that will be installed in Prague is designed for protons and carbons up to 60 MeV/u, around 10 times more than the energies involved in the present work. The prototype, shown in this work, has been tested in collaboration with the SAPHIR experimental facility group at LOA (Laboratoire d'Optique Appliqueé) in Paris using a 200 TW Ti:Sapphire laser system. The purpose of this work is to validate the design and the performances of this large and compact bore system and to characterize the beam produced after the interaction laser-target and its features. Moreover, the optics simulations have been compared with a real beam shape on a GAFChromic film. The procedure used during the experimental campaign and the most relevant results are reported here demonstrating a good agreement with the simulations and a good control on the beam optics.

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system.

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively, the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm2.