08 Feb 2007
Scientists in Israel measure the ability of snow covered targets to absorb high intensity Ti:Sapphire laser pulses.
Nano-sized snow crystals could be an interesting candidate for boosting the coupling efficiency of high intensity laser energy into matter, say researchers in Israel. The team from Racah Institute of Physics (RIOP) and Soreq Research Center has discovered that sapphire targets coated with snow absorb more than 95% of the incident light compared with a figure of around 50% for the bare substrate. (Appl. Phys. Lett. 90 041501)
The experiments were performed using a Ti:Sapphire laser emitting 150 fs, 40 mJ pulses at a wavelength of 800 nm. Input energy and reflected light were monitored using photodiodes. Bandpass filters were used to isolate the laser light and photodiodes were fitted with a diffuser to average any slight differences in alignment. The laser target was scanned from shot to shot so that each pulse was exposed to a fresh area of the sample.
The snow clusters were grown on the substrate by injecting water vapour into a vacuum chamber at temperatures of less than -70 deg C. "The main problem was producing a snow flake with dimensions in the range of several angstroms," Arie Zigler, head of RIOP's high intensity laser laboratory, told optics.org. "This was necessary to allow the laser radiation to penetrate through the entire diameter of the snow flake's characteristic branch."
To vary the size of the snow crystals, the team used target substrates with different textures. A molybdenum sample with a rough surface produced snow flakes measuring several microns, whereas a smooth sapphire target gave much smaller clusters in the range 10 - 100 nm.
Zigler and his team discovered that the large flakes behaved as solid targets in their own right and failed to enhance the absorption of incident laser radiation. In contrast, the researchers believe that the smaller clusters were highly ionized, which helped to increase the laser absorption from 58% for a bare sapphire surface to 97% when the target was covered with snow.
"Our interest is purely academic, but one application of the technique is the generation of an efficient neutron source," commented Zigler. "The next step will be to generate fine structured snow flakes from heavy water."