Abstract
A comparative experimental study of the X-ray emission in the water-window spectral region has been performed using carbon nanofibers (CNFs) of different sizes and graphite plate targets, irradiated with ultra-short (Ti:sapphire) laser pulses. More than an order of magnitude enhancement in the X-ray yield is observed from CNFs of 60-nm diameter with respect to graphite targets. The X-ray emission from CNFs of 160-nm diameter was also high. The integrated X-ray yield of these carbon-based targets scales with the laser intensity (IL) as \(I_{\mathrm{L}}^{\sim 1.3-1.4}\) in the intensity range of 4×1016–4×1017 W/cm2. The effect of the laser pulse duration on the X-ray emission from the CNFs was also studied by varying the pulse duration from 45 fs up to 3 ps at a constant fluence of 2×104 J/cm2. The optimum laser pulse duration for maximum X-ray emission increases with the diameter of the CNFs used. The results are explained from physical considerations of heating and hydrodynamic expansion of the CNF plasma in which resonance field enhancement takes place while passing through two times the critical density. The results add to the efforts towards achieving an efficient low-cost water-window X-ray source for microscopy.
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