GoKawiilExperimental details
The experimental results presented in this paper were obtained using the Gemini laser system. A DPM system is used to improve the laser contrast to I max /I(t) > 108 at times more than 1 ps before the peak of the pulse, whereas the sub-ps contrast is discussed in the text referring to Fig. 1. A total throughput of 50% was measured, which leads to on-target pulse energies of 5 J in the 50 ± 5 fs duration pulses with λ L = 800 nm, which are focused by an f/2 parabola onto a polished fused silica target.
Pulses with energies of up to 12 J (before DPM system) in 50 ± 5 fs at a central wavelength of 800 nm were used, which, when focused to a FWHM spot size of 2 μm, reach peak intensities I > 1021 W cm−2. As shown in Fig. 1a, these were focused onto optical-grade fused silica targets in p-polarization at an incidence angle of 45°, and the spectrum of extreme ultraviolet radiation emitted in the direction of specular reflection was recorded.
The on-target intensity was varied by apodizing the beam, which both reduces the laser pulse energy and increases the focal spot size but maintains the same near-field intensity so that the DPM response and contrast are unchanged. The reflected harmonic beam was detected using a cylindrically curved XUV flat-field spectrometer consisting of a 300 lines per mm grating imaging the source in the spectral dimension. Aluminium filters with thicknesses ranging from 0.2 μm to 3 μm were used to attenuate optical emission. No focusing optic was used so that the XUV signal is incident directly onto the charge-coupled device (CCD). The harmonic spectra were detected using a back-thinned ANDOR CCD (Andor DV436) with a resolution-limited pixel size of 13.5 μm placed 1.2 m from the interaction point.
The plasma density gradient was controlled by a 25 mm diameter, 3 mm thick fused silica substrate with an anti-reflection coating on the front side and high reflectivity on the rear. This pick-off mirror was inserted into the main beam line in front of the last mirror before the parabola. This introduced a prepulse, which is focused by the same parabola as the main beam onto the target but with a lower intensity because of the larger focal spot size. Precise adjustment of the distance between the substrate and the full-beam mirror allowed for prepulse timing to be controlled to within 25 fs of the main pulse. This fine timing control enabled accurate tailoring of plasma expansion before the arrival of the driving pulse.
Laser contrast enhancement was achieved through measurements that determined that the anti-reflection coating on the first plasma mirror (PM) of the DPM was breaking down too early in the rise time of the native (unaltered) Gemini pulse contrast. This resulted in the ‘slow rise time’ DPM configuration (t HDR = 711 ± 25 fs; Fig. 1b, red trace). By replacing the first PM with an uncoated substrate, we improved the DPM performance to have a t HDR = 351 ± 25 fs (Fig. 1b, blue trace). The motivation for making this change comes from a separate branch of study on ultrafast materials science that focuses on the part played by materials that are highly structured on the nanoscale in the lifetime of excited electrons before material breakdown45,46. Note that for different peak intensities, t HDR will describe different absolute intensities, whereas the ratio remains the same. This should, therefore, be considered carefully in the context of a given peak intensity interaction.
Harmonic energy deconvolution
To obtain the overall efficiency, the spectral response as a function of wavelength, λ, of all components of the spectrometer were accounted for separately as indicated by
$$S(\lambda )=\text{Al}\times \text{QE}\times \text{G}\,\times {\text{Al}}_{2}{\text{O}}_{3}\times \text{CH},$$ (1)
where Al is the aluminium filter transmission47 (0.2–3 μm), QE is the quantum efficiency of the back-thinned Andor DV436 (ref. 48), G is the calculated efficiency of the SHIMADZU-L0300-20-80 flat-field grating49, Al 2 O 3 is the contaminant aluminium oxide layer present on the filters (see aluminium oxide layer calibration below) and CH is the hydrocarbon contaminant layers47.
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