Optical Field Ionization Research Articles

Demonstration of a kHz-repetition-rate extreme ultraviolet laser at 41.8 nm.

We demonstrate the operation of a plasma-based extreme ultraviolet (XUV) laser at a 1 kHz repetition rate driven by infrared pump pulses of less than 20 mJ. The 41.8 nm laser pulses were generated in a Xe plasma created by optical-field ionization by the L1 Allegra laser at ELI Beamlines. The output power of the XUV laser lies in the few microwatt range, and the energy efficiency of this pumping scheme opens the way for further scaling in repetition rate and average power.

Benchmarking of hydrodynamic plasma waveguides for multi-GeV laser-driven electron acceleration

Hydrodynamic plasma waveguides initiated by optical field ionization have recently become a key component of multi-GeV laser wakefield accelerators. Here, we present the most complete and accurate experimental and simulation-based characterization to date, applicable to current multi-GeV experiments and future 100 GeV-scale laser plasma accelerators. Crucial to the simulations is the correct modeling of intense Bessel beam interaction with meter-scale gas targets, the results of which are used as initial conditions for hydrodynamic simulations. The simulations are in good agreement with our experiments measuring evolving plasma and neutral hydrogen density profiles using two-color short pulse interferometry, enabling realistic determination of the guided mode structure for application to laser-driven plasma accelerator design. Published by the American Physical Society 2024

Stream instabilities in optical-field ionization of a monatomic dilute neutral gas in fully relativistic regime

Stream instabilities arising from anisotropic electron velocity distribution function (EVDF) are discussed in the optical-field ionization mechanism of a monatomic dilute gas by a circularly polarized laser beam in a fully relativistic regime. It is shown that a relativistically rotating electron beam is derived by a circularly polarized laser field with (p_z>p_perp). We show that the following ionization and before collisions thermalize the electrons, the plasma undergoes Buneman and Weibel instabilities. The Weibel and Buneman modes are co-propagating with k normal to the streaming direction. The theoretical results reveal that for the threshold of the relativistic regime (a_0approx 1), instabilities are aperiodic and grow independently. However, by increasing the laser intensity for a_0>1, two instabilities are coupled. The coupling process increased the growth rate of Weibel instability, while the Buneman instability experienced a decrement in its growth rate. For more intense laser radiation, both instabilities are broken into different oscillatory and aperiodic modes.

Meter-scale plasma waveguides for multi-GeV laser wakefield acceleration

We present results from two new techniques for the generation of meter-scale, low density (∼1017 cm−3 on axis) plasma waveguides, the “two-Bessel” technique, and the “self-waveguiding” technique. Plasma waveguides of this density and length range are needed for demonstration of a ∼10 GeV laser wakefield accelerator module, key for future staging for a ∼TeV lepton collider. Both techniques require the use of high quality ultrashort pulse Bessel beams to efficiently and uniformly ionize hydrogen gas in meter-scale supersonic gas jets via optical field ionization. We review these two techniques, describe our meter-scale gas jets, and present a new method for correction of optical aberrations in Bessel beams. Finally, we briefly present results from recent experiments employing one of our techniques, demonstrating quasi-monoenergetic acceleration of ∼5 GeV electron bunches in 20 cm long, low density plasma waveguides.

Electron Weibel instability induced magnetic fields in optical-field ionized plasmas

Generation and amplification of magnetic fields in plasmas is a long-standing topic that is of great interest to both plasma and space physics. The electron Weibel instability is a well-known mechanism responsible for self-generating magnetic fields in plasmas with temperature anisotropy and has been extensively investigated in both theory and simulations, yet experimental verification of this instability has been challenging. Recently, we demonstrated a new experimental platform that enables controlled initialization of highly nonthermal and/or anisotropic plasma electron velocity distributions via optical-field ionization. Using an external electron probe bunch from a linear accelerator, the onset, saturation, and decay of the self-generated magnetic fields due to electron Weibel instability were measured for the first time to our knowledge. In this paper, we will first present experimental results on time-resolved measurements of the Weibel magnetic fields in non-relativistic plasmas produced by Ti:Sapphire laser pulses (0.8 μm) and then discuss the feasibility of extending the study to a quasi-relativistic regime by using intense CO2 (e.g., 9.2 μm) lasers to produce much hotter plasmas.

Intensity dependence of multiply charged atomic ions from argon clusters in moderate nanosecond laser fields.

We report the laser intensity dependence of multiply charged atomic ions (MCAIs) Arn+ with 2 ≤ n ≤ 8 from argon clusters in focused nanosecond laser fields at 532nm. The laser field, in the range of 1011-1012 W/cm2, is insufficient for optical field ionization but is adequate for multiphoton ionization. The MCAI sections of the mass spectra for clusters containing 3700 and 26 000 atoms are dominated by Arn+ with 7 ≤ n ≤ 9, extending to Ar14+. While the distributions of the MCAIs remain largely constant throughout the intensity range of the laser, the abundance of Ar+ relative to the abundances of the MCAIs increases dramatically with increasing laser intensity. Consequently, exponential fittings of the yields result in a larger exponent for Ar+ than for MCAIs, and the exponents of MCAIs with 2 ≤ n ≤ 8 are similar, with only slight variations for different charge states. The width of the arrival time and, hence, the corresponding kinetic energy of Ar+ also increases with increasing laser intensities, while the width of the arrival time of MCAIs remains constant throughout the range of measurements. These results call for more detailed theoretical investigations in this regime of laser-matter interactions.

Femtosecond X-ray Lasers at λ=32.8 and 44.4 nm in a Plasma Formed by Optical Field Ionization in a Krypton Cluster Jet

Due to high efficiency, X-ray lasers based on transitions of Ni-like krypton (Kr⁸⁺) are being actively studied. The main focus is on an X-ray laser based on the conventional 3d_5/24d_5/2 [J=0] – 3d_3/24p_1/2 [J=1] transition at λ=32.8 nm. Gaseous krypton targets or krypton cluster jets are used in various experiments. X-ray lasers at 32.8 nm in a plasma formed by optical field ionization in a krypton cluster jet are widely used for research of nanoobjects. In this article, the possibility of creating an efficient X-ray laser in Ni-like krypton based on a transition with optical self-pumping 3d_3/24f_5/2 [J=1] – 3d_3/24d_5/2 [J=1] at λ=44.4 nm is predicted for the first time. The plasma filament is excited upon interaction of a jet of krypton clusters with an intense pump laser pulse. Optimal conditions to achieve the duration t_las ≤300 fs of the X-ray laser radiation are determined. The optimal electron density is in a rather narrow interval in the range n_e ~ 10²¹ - 2×10²¹ cm^-3. The optimal electron temperature is several keV. It is likely that this explains the fact that no X-ray laser has been observed on this transition in Kr⁸⁺ so far. The conversion factor per pulse is found to be ~5×10^-5. For an X-ray laser operating on the conventional transition 3d_5/24d_5/2 [J=0] – 3d_3/24p_1/2 [J=1] at λ=32.8 nm, t_las ≤ 300 fs can also be achieved; however, the conversion factor for this transition is times ~5 smaller than that for the former transition.

Optical Guiding in Meter-Scale Plasma Waveguides.

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable laser-driven electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J_{0} beam, generates the core of the waveguide, while the delayed second pulse, here a J_{8} or J_{16} beam, generates the waveguide cladding, enabling wide control of the guide's density, depth, and mode confinement. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on-axis plasma densities as low as N_{e0}∼5×10^{16} cm^{-3}.

Effect of optical field ionisation on the generation of wake fields by femtosecond laser pulses in an inhomogeneous plasma

We consider the interaction of a high-intensity short laser pulse with an argon gas during optical field ionisation. Modelling in a three-dimensional cylindrical symmetric geometry is performed at various moderately relativistic intensities and positions of the focal plane of the laser beam in the case of both optical gas ionisation and a pre-ionised plasma. The influence of the processes occurring during optical field ionisation of the gas on the generation of wake waves is investigated, and the conditions are found under which intense wake fields are produced in a plasma formed from an inhomogeneous argon gas jet. The possibility of using a gas with a large number of electrons on the outer shell (argon) to excite intense wake waves and accelerate electrons is demonstrated. Despite significant ionisation refraction of the laser pulse on a radially inhomogeneous density profile of plasma electrons formed during optical field ionisation, the region of the parameters of the laser pulse and the gas target is determined at which ionisation refraction leads to the excitation of an intense wake wave. It is found that in a pre-ionised plasma a wake wave is not generated even at the same laser radiation intensity.

Ionization yield measurement using metal electrodes with a static electric field in ambient air

The amount of ionization produced in a laser-matter interaction provides critical information in studying strong field physics and attosecond science. The amount of ionization can be accurately measured in vacuum using a photoelectron/ion spectrometer by counting the number of charged particles. However, an ionization yield measurement in ambient air requires the considerations of post processes such as drift and recombination of charged particles, which may add an unwanted nonlinearity to the measurement. Here we investigate the kinetics of the charged particles after optical field ionization in ambient air under a static bias field. A simple kinetic model is used in which coupled rate equations are numerically solved. The kinetic model explains the recombination loss of the charged particles observed in ionization yield measurements. A condition for the static bias field is derived for an accurate ionization yield measurement.

Numerical modelling of chromatic effects on axicon-focused beams used to generate HOFI plasma channels

Hydrodynamic optical-field-ionised (HOFI) plasma channels promise a route towards high repetition-rate, metre-scale stages for future laser plasma accelerators. These channels are formed by hydrodynamic expansion of a plasma column produced by optical field ionisation at the focus of a laser, typically from an axicon lens. Since the laser pulses used to generate the initial plasma column are of sub-picosecond duration, chromatic effects in the axicon lens could be important. In this paper we assess these effects using a numerical propagation code. The code is validated using analytical formulae and experimental data. For the parameter range investigated, dispersive effects are found to be of minor importance, reducing the peak on-axis intensity in the focal region by approximately 10%.

Characterization of ionization injection in gas mixtures irradiated by subpetawatt class laser pulses

Effects of ionization injection in low and high Z gas mixtures for the laser wake field acceleration of electrons are analyzed with the use of balance equations and particle-in-cell simulations via test probe particle trajectories in realistic plasma fields and direct simulations of charge loading during the ionization process. It is shown that electrons appearing at the maximum of laser pulse field after optical ionization are trapped in the first bucket of the laser pulse wake. Electrons, which are produced by optical field ionization at the front of laser pulse, propagate backwards; some of them are trapped in the second bucket, third bucket and so on. The efficiency of ionization injection is not high, several pC/mm/bucket. This injection becomes competitive with wave breaking injection at lower plasma density and over a rather narrow range of laser pulse intensity.

Initializing anisotropic electron velocity distribution functions in optical-field ionized plasmas

Optical-field ionization (OFI) is often used to produce plasmas for myriad laboratory applications. OFI of gases is known to generate electrons that have anisotropic distributions. In this paper we show that at the time of plasma formation the electron velocity distribution functions can be controlled by changing the wavelength and polarization of the pump laser and ionization state of the plasma. Thomson scattering is used to measure the distinct electron velocity distributions of helium plasmas produced by linearly and circularly polarized laser pulses within a few inverse plasma periods after the plasma formation. In both cases, non-thermal and highly anisotropic initial electron distributions are observed that are consistent with expectations from OFI of both He electrons.

Relativistic Interaction of Long-Wavelength Ultrashort Laser Pulses with Nanowires

We report on experimental results in a new regime of a relativistic light-matter interaction employing mid-infrared (3.9-micrometer wavelength) high-intensity femtosecond laser pulses. In the laser generated plasma, the electrons reach relativistic energies already at rather low intensities due to the fortunate lambda^2-scaling of the kinetic energy with the laser wavelength. The lower intensity suppresses optical field ionization and creation of the pre-plasma at the rising edge of the laser pulse efficiently, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. In our experiments about 80% of the incident laser energy has been absorbed resulting in a long living, keV-temperature, high-charge state plasma with a density of more than three orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate.

Hydrodynamic optical-field-ionized plasma channels.

We present experiments and numerical simulations which demonstrate that fully ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical field ionization. Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of 200mm long plasma channels with axial densities of order n_{e}(0)=1×10^{17}cm^{-3} and lowest-order modes of spot size W_{M}≈40μm. These simulations show that the laser energy required to generate the channels is modest: of order 1mJ per centimeter of channel. The simulations are confirmed by experiments with a spherical lens which show the formation of short plasma channels with 1.5×10^{17}cm^{-3}≲n_{e}(0)≲1×10^{18}cm^{-3} and 61μm≳W_{M}≳33μm. Low-density plasma channels of this type would appear to be well suited as multi-GeV laser-plasma accelerator stages capable of long-term operation at high pulse repetition rates.

Ionization assisted self-guiding of femtosecond laser pulses

We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.

Theoretical investigation of tunable polarized broadband terahertz radiation from magnetized gas plasma**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574105, and 61475054) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2017KFYXJJ029).

The mechanism of terahertz (THz) pulse generation with a static magnetic field imposed on a gas plasma is theoretically investigated. The investigation demonstrates that the static magnetic field alters the electron motion during the optical field ionization of gas, leading to a two-dimensional asymmetric acceleration process of the ionized electrons. Simulation results reveal that elliptically or circularly polarized broadband THz radiation can be generated with an external static magnetic field imposed along the propagation direction of the two-color laser. The polarization of the THz radiation can be tuned by the strength of the external static magnetic field.

Toward compact and ultra-intense laser-based soft x-ray lasers

We report here recent work on an optical field ionized (OFI), high-order harmonic-seeded EUV laser. The amplifying medium is a plasma of nickel-like krypton obtained by OFI when focusing a 1 J, 30 fs, circularly-polarized, infrared pulse into a krypton-filled gas cell or krypton gas jet. The lasing transition is the 3d94d (J = 0) → 3d94p (J = 1) transition of Ni-like krypton ions at 32.8 nm and is pumped by collisions with hot electrons. The gain dynamics was probed by seeding the amplifier with a high-order harmonic pulse at different delays. The gain duration monotonically decreased from 7 ps to an unprecedented shortness of 450 fs full width at half-maximum as the amplification peak rose from 150 to 1200 with an increase of the plasma density from 3 × 1018 to 1.2 × 1020 cm−3. The integrated energy of the EUV laser pulse was also measured, and found to be around 2 μJ. It is to be noted that in the ASE mode, longer amplifiers were achieved (up to 2 cm), yielding EUV outputs up to 14 μJ.

Threshold Fluence for Femtosecond Laser Nanoablation for Metals

SUMMARYFemtosecond laser nanoablation of Ti by short‐pulse laser irradiation (800 nm/40 fs) is studied in the laser fluence range of 0.07 J/cm2 to 0.5 J/cm2. To determine the ablation threshold, the ablation rate dependence on laser fluence is precisely measured. Multishot ablation threshold of titanium is found to be 0.074 J/cm2 in which value is good agreement with that reported previously by other group. To discuss the ablation mechanism, all the ablation thresholds for metals previously published are plotted as a function of work function and melting temperature. We found that the ablation thresholds correlated with work function of metals. Experimental data suggested that the femtosecond laser ablation is mainly due to multiphoton absorption and optical field ionization.

Postcompression of high-energy terawatt-level femtosecond pulses and application to high-order harmonic generation

We perform a post-compression of high energy pulses by using optical-field ionization of low pressure helium gas in a guided geometry. We apply this approach to a TW chirped-pulse-amplification based Ti:Sapphire laser chain and show that spectral broadening can be controlled both with the input pulse energy and gas pressure. Under optimized conditions, we generate 10 fs pulses at TW level directly under vacuum and demonstrate a high stability of the post compressed pulse duration. These high energy post-compressed pulses are thereafter used to perform high harmonic generation in a loose focusing geometry. The XUV beam is characterized both spatially and spectrally on a single shot basis and structured continuous XUV spectra are observed.