Figure 2: Schematic of the optical streaking technique for laser-driven proton bursts.

Protons accelerated from a thin contamination layer on the rear surface of 10-μm-thick Au foils via the TNSA mechanism undergo electronic stopping in a high-purity SiO₂ sample at ∼300 K placed 5 mm behind the interaction. The corresponding transient opacity (grey) is recorded using a synchronized 1,053-nm probe pulse with a variable linear frequency sweep, or chirp²⁶. This permits observation of the interaction over time windows ranging from 0.4±0.02 (fully compressed pulse for direct imaging of interaction and represents the fundamental temporal resolution of this system) to ∼1,400 ps (maximum chirp for optical streaking). In a and b the proton bunch is incident from below and collimated to a width of 100 μm using a 1-mm-thick Al slit collimating slit (CS). Co-propagating keV electrons are stopped with the use of a 50-μm Al foil at the interaction facing surface of the SiO₂ (not shown). A chirped pulse (>50 ps) is incident from the left. Different frequency components traverse the irradiated region at different times, thus encoding the temporal evolution in the observed spectrum. It is important to note that the bandwidth of the optical probe (4 nm) is narrow compared with the width of the absorption spectrum for conduction band electrons in SiO₂. The optical streak is obtained by spectrally resolving the chirped pulse (c) using a 1-m imaging spectrometer with a 1,200 l mm⁻¹ grating (dimensions 10 × 10 cm). The region of interest (ROI, (d)) for the ion burst interaction is a 10-μm scale slice along the central axis of the driving laser pulse. This is imaged onto the entrance slit of the spectrometer with a magnification of ∼10. The fundamental temporal resolution of the system described here is limited only by the resolution of the spectrometer²⁶. For a 200-ps probe this system provides a resolution of 0.45±0.05 ps.