Iterative wavefront optimization of ultrafast laser beams carrying orbital angular momentum.

Structured intense laser beams offer degrees of freedom that are highly attractive for high-field science applications. However, the performance of high-power laser beams in these applications is often hindered by deviations from the desired spatiotemporal profile. This study reports the wavefront optimization of ultrafast Laguerre-Gaussian beams through the synergy of adaptive optics and genetic algorithm-guided feedback. The results indicate that the intensity fluctuations along the perimeter of the target ring-shaped profile can be reduced up to ∼15%. Furthermore, the radius of the ring beam profile can be tailored to a certain extent by establishing threshold fitting criteria. The versatility of this approach is experimentally demonstrated in conjunction with different focusing geometries.

High-energy nanosecond parametric source at 2.7 µm.

We report a high-energy nanosecond KTiOAsO4-based optical parametric oscillator and amplifier system driven by a 1064-nm, injection-seeded, Q-switched Nd:YAG laser to provide mid-infrared pumping for few-cycle optical parametric chirped-pulse amplification operating in the long-wave infrared spectral regime. An output energy of 10.5 mJ at 2726 nm was obtained from the final power amplifier with a spectral width of 2.8 nm, pulse duration of 1.2 ns, and M2 value of 2.4. The parametric source provides for the second mode of operation, where 10-ns pulses are produced by disabling the electro-optic slicing that enables the 1-ns mode. For 10-ns operation, the source produced energy of 25 mJ with a M2 value of 2.8.

Adaptive control of laser-wakefield accelerators driven by mid-IR laser pulses.

There has been growing interest both in studying high intensity ultrafast laser plasma interactions with adaptive control systems as well as using long wavelength driver beams. We demonstrate the coherent control of the dynamics of laser-wakefield acceleration driven by ultrashort (∼ 100 fs) mid-infrared (∼ 3.9 µm) laser pulses. The critical density at this wavelength is 7.3 × 1019 cm-3, which is achievable with an ordinary gas target system. Interactions between mid-infrared laser pulses and such near-critical-density plasma may be beneficial due to much higher absorption of laser energy. In addition, the normalized vector potential of the laser field a0 increases with longer laser wavelength, lowering the required peak laser intensity to drive non-linear laser-wakefield acceleration. Here, MeV level, collimated electron beams with non-thermal, peaked energy spectra are generated. Optimization of electron beam qualities are realized through adaptive control of the laser wavefront. A genetic algorithm controlling a deformable mirror improves the electron total charge, energy spectra, beam pointing and stability at various plasma density profiles. Particle-in-cell simulations reveal that the optimal wavefront causes an earlier injection on the density up-ramp and thus higher energy gain as well as less filamentation during the interaction, which leads to the improvement in electron beam collimation and energy spectra.

Enhancement of THz generation by feedback-optimized wavefront manipulation.

We apply active feedback optimization methods to pyroelectric measurements of a THz signal generated by four wave mixing in air using 1 mJ to 12 mJ, 35 fs laser pulses operating at 12 kHz repetition rate. A genetic algorithm, using the THz signal as a figure of merit, determines the voltage settings to a deformable mirror and results in up to a 6 fold improvement in the THz signal compared with settings optimized for the best focus. It is possible to optimize for different THz generation processes using this technique.

Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator.

Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. Here, we demonstrate the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes.

High-Flux Femtosecond X-Ray Emission from Controlled Generation of Annular Electron Beams in a Laser Wakefield Accelerator.

Annular quasimonoenergetic electron beams with a mean energy in the range 200-400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays.

Coherent control of plasma dynamics.

Coherent control of a system involves steering an interaction to a final coherent state by controlling the phase of an applied field. Plasmas support coherent wave structures that can be generated by intense laser fields. Here, we demonstrate the coherent control of plasma dynamics in a laser wakefield electron acceleration experiment. A genetic algorithm is implemented using a deformable mirror with the electron beam signal as feedback, which allows a heuristic search for the optimal wavefront under laser-plasma conditions that is not known a priori. We are able to improve both the electron beam charge and angular distribution by an order of magnitude. These improvements do not simply correlate with having the 'best' focal spot, as the highest quality vacuum focal spot produces a greatly inferior electron beam, but instead correspond to the particular laser phase front that steers the plasma wave to a final state with optimal accelerating fields.

A novel femtosecond-gated, high-resolution, frequency-shifted shearing interferometry technique for probing pre-plasma expansion in ultra-intense laser experiments.

Ultra-intense laser-matter interaction experiments (>10(18) W/cm(2)) with dense targets are highly sensitive to the effect of laser "noise" (in the form of pre-pulses) preceding the main ultra-intense pulse. These system-dependent pre-pulses in the nanosecond and/or picosecond regimes are often intense enough to modify the target significantly by ionizing and forming a plasma layer in front of the target before the arrival of the main pulse. Time resolved interferometry offers a robust way to characterize the expanding plasma during this period. We have developed a novel pump-probe interferometry system for an ultra-intense laser experiment that uses two short-pulse amplifiers synchronized by one ultra-fast seed oscillator to achieve 40-fs time resolution over hundreds of nanoseconds, using a variable delay line and other techniques. The first of these amplifiers acts as the pump and delivers maximal energy to the interaction region. The second amplifier is frequency shifted and then frequency doubled to generate the femtosecond probe pulse. After passing through the laser-target interaction region, the probe pulse is split and recombined in a laterally sheared Michelson interferometer. Importantly, the frequency shift in the probe allows strong plasma self-emission at the second harmonic of the pump to be filtered out, allowing plasma expansion near the critical surface and elsewhere to be clearly visible in the interferograms. To aid in the reconstruction of phase dependent imagery from fringe shifts, three separate 120° phase-shifted (temporally sheared) interferograms are acquired for each probe delay. Three-phase reconstructions of the electron densities are then inferred by Abel inversion. This interferometric system delivers precise measurements of pre-plasma expansion that can identify the condition of the target at the moment that the ultra-intense pulse arrives. Such measurements are indispensable for correlating laser pre-pulse measurements with instantaneous plasma profiles and for enabling realistic Particle-in-Cell simulations of the ultra-intense laser-matter interaction.

Ionization-induced self-compression of tightly focused femtosecond laser pulses.

As lasers become progressively higher in power, optical damage thresholds will become a limiting factor. Using the nonlinear optics of plasma may be a way to circumvent these limits. Here, we present a new self-compression mechanism for high-power, femtosecond laser pulses based on geometrical focusing and three dimensional spatiotemporal reshaping in an ionizing plasma. By propagating tightly focused, 10-mJ femtosecond laser pulses through a 100-µm gas jet, the interplay between ionization gradients, focusing, and diffraction of the light pulse leads to stable and uniform self-compression of the pulse, while maintaining a high-energy throughput and excellent refocusability. Self-compression down to 16 fs from an original 36-fs pulse is measured using second-harmonic-generation frequency-resolved optical gating. Using this mechanism, we are able to maintain a high transmission (>88%) such that the pulse peak power is doubled. Three-dimensional numerical simulations are performed to support our interpretation of the experimental observations.

Table-top laser-based source of femtosecond, collimated, ultrarelativistic positron beams.

The generation of ultrarelativistic positron beams with short duration (τ(e+) ≃ 30 fs), small divergence (θ(e+) ≃ 3 mrad), and high density (n(e+) ≃ 10(14)-10(15) cm(-3)) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

Compressor optimization with compressor-based multiphoton intrapulse interference phase scan (MIIPS).

The multiphoton intrapulse interference phase scan (MIIPS) technique is modified to optimize the compressor settings of a chirped pulse amplification (CPA) laser system. Here, we use the compressor itself to perform the phase scan inherent in MIIPS measurement . A frequency-resolved optical gating measurement shows that the pulse duration of the compressor optimized using the modified MIIPS technique is 33.8 fs with a 2.24 rad temporal phase variation above 2% intensity. The measured time-bandwidth product is 0.60, which is close to that of transform-limited Gaussian pulse (0.44).

Relativistic generation of isolated attosecond pulses in a lambda 3 focal volume.

Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (lambda(3)) can create relativistic intensity with maximal gradients, using minimal energy. With particle-in-cell simulations we found, that single 200 attosecond pulses could be produced efficiently in a lambda(3) laser pulse reflection, via deflection and compression from the relativistic plasma mirror created by the pulse itself. An analytical model of coherent radiation from a charged layer confirms the pulse compression and is in good agreement with simulations. The novel technique is efficient (approximately 10%) and can produce single attosecond pulses from the millijoule to the joule level.