The interaction of high-intensity laser pulses with solid targets results in the generation of secondary radiation including beams of energetic ions suitable for many applications. Depending on the particular mechanism, the laser-driven ion acceleration is generally very sensitive to the target conditions at the moment of the main pulse arrival. The scale length, density, spatial distribution of preplasma and the time of its generation (with respect to the main pulse arrival) on both target's surfaces influence the laser absorption, electric and magnetic field generation and subsequent ion acceleration. The observation of the target condition close to the interaction is therefore beneficial for understanding the plasma parameters and reaching desired acceleration mechanisms. Conventional optical probing based on the pump-probe method usually records a single image of the interaction in a particular shot at a specific time. Repeated measurements with the different time delays between the probe and the pump then provide the preplasma dynamics relying on accurate reproducibility and potentially a high occurrence rate of the laser-target interaction. Although there is a variety of single-shot multi-frame probing techniques, they usually work with probes at fundamental or harmonic frequencies. However, pump pulse scattering and plasma self-emission, which is dominantly generated at these frequencies, often saturate the detector and cover the information carried by the probe beams. The utilization of off-harmonic probes generated by stand-alone laser systems is limited due to the need for precise synchronization with the pump at sub-ps level. Ideally, several time-delayed probe pulses with off-harmonic frequencies that are inherently synchronized with the pump pulse should propagate through the laser-target interaction just before the main pulse arrival to reveal any preplasma effects.

The research group from Extreme Light Infrastructure ERIC experimentally demonstrated that optical parametric amplifiers can generate such probe pulses employing a beam picked off the main pump pulse as an amplifier input. The research results are published in High Power Laser Science and Engineering, Volume 11, Issue 4 (Filip Grepl, Maksym Tryus, Timofej Chagovets, Daniele Margarone. Parametric amplification as a single-shot time-resolved off-harmonic probe for laser–matter interactions[J]. High Power Laser Science and Engineering, 2023, 11(4): 04000e45). In particular, the optical setup based on standard broadband non-collinear optical parametric amplification (NOPA) in beta barium borate (BBO) crystal was used to amplify different spectral bandwidths of a temporally chirped white-light continuum (WLC) by short pulses of second harmonic (SH) radiation. The NOPA was initially optimized for amplification of radiation over several hundreds of nanometers. The WLC was generated by focusing a small fraction of the input beam into the sapphire crystal and stretched to units of ps by propagation through a certain thickness of glass. The residual part of the input beam was split into three pulses with equal energies. Each of these pulses, propagated through a dedicated delay line before it was frequency doubled while conserving its temporal duration. This resulted in a sequence of three time-delayed and collinearly propagating pulses of SH acting as pumps in NOPA. Each pump temporally overalled with a different part of the long WLC pulse (acting as a seed) in BBO crystal. Thus, three different bandwidths of WLC were separately amplified and later used as probes. Since both the WLC and all the SHs were initially split from the original beam driving the laser-target interaction, the overall temporal synchronization was achieved only by optical delay lines without any additional electronics. During the experimental verification of the method, the laser plasma created in the air was probed by 3 generated pulses with different central wavelengths within a single shot. The shortest temporal separation of probe pulses was 170 fs.

Broadband NOPA provides the wavelength tunability of the probe pulses over its amplification range without any change of geometry. The temporal chirp of WLC and the duration of SHs determine the bandwidth of probe pulses while their central wavelengths, which are directly proportional to their delays, are set by delay lines. Generally, the tested optical setup can generate several time-delayed collinearly propagating pulses with different off-harmonic frequencies suitable for probing the interaction of high-intensity laser with matter. The primary application of the developed method is the differentiation of the preplasma on the target's front surface from its expansion induced by the main laser pulse while avoiding detector saturation by harmonic laser frequencies.

"The probing technique recently developed and tested by members of our team can help investigate advanced ion acceleration mechanisms for user-oriented research in ELI Beamlines and can be generally applied in characterization of laser plasma physics dynamics" says Dr. Giuffrida, Head of Department of Ion Acceleration and Applications of High Energy Particles.

Further development of the method will focus on the implementation of additional pump pulses to extend the total number of recorded frames. Additional optimization of the pump pulses durations as well as the temporal chirp of WLC can increase the temporal resolution and decrease the interframe interval. Implementation of the method in ion acceleration experiments is expected in the near future at ELI Beamlines facility.

Figure 1. The implementation of the optical setup for time-resolved off-harmonic probing in the interaction chamber for the acceleration of ions by short intense laser pulses at ELI-Beamlines facility.

Figure 2. The plasma channel captured at three different times. The top subpicture corresponds to the first probe, the middle subpicture shows the plasma channel recorded by the second probe and the bottom subpicture corresponds to the channel recorded by the third probe. The solid cyan line marks the middle of the frame and the dashed blue line indicates the end of the developed channel recorded at a specific time point.