The researchers performed their experiments using a Ti:Sapphire laser emitting 150 fs, 40 mJ pulses at a wavelength of 800 nm and monitored the input energy and reflected light using photodiodes. They used bandpass filters to isolate the laser light and fitted photodiodes 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 team grew snow clusters on the substrate by injecting water vapour into a vacuum chamber at temperatures of less than -70°C. "The main problem was producing a snow flake with dimensions in the range of several angstroms," said Arie Zigler, head of RIOP's high intensity laser laboratory. "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, Zigler and colleagues 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 of 10 to 100 nm.
The researchers 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 team 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," explained Zigler. "The next step will be to generate fine structured snow flakes from heavy water."
The researchers reported their work in Appl. Phys. Lett..