What happens when gigawatt peak-power femtosecond laser pulses interact with millimetre-scale water droplets? The answer, according to scientists in Russia, is the intense emission of visible light from a laser-induced plasma that forms within the droplets, with a strong spectral broadening as the laser power rises.

The self-focusing properties of transparent spherical water particles can create very intense optical fields, making them a potentially interesting tool for studying a variety of nonlinear optical effects. Yuri Geints and co-workers from the Zuev Institute of Atmospheric Optics in Tomsk and the Institute of Automation and Control Processes in Vladivostok have now investigated the dynamics of the interaction between laser pulses and water droplets as a function of laser power (Opt. Lett. 35, 2717–2719; 2010). The researchers prepared large millimetre-size droplets, which have sufficient curvature and volume for realizing the required focusing effect.

They then focused femtosecond pulses from a Ti:Sapphire chirped-pulse amplification laser (with a central wavelength of 800 nm, repetition rate of 1 kHz, pulse energy of 1 mJ and beam diameter of 7 mm) onto a droplet of distilled water. The peak power of the laser pulses was varied over the range of 1–25 GW. The droplet emission was measured by a spectrometer in the wavelength range of 195–1,150 nm.

Credit: © 2010 OSA

The emission was sparkling white to the naked eye but orange–red when viewed through a neutral density filter. The areas of light emission within the droplet have a distributed and granulated structure, which is indicative of boiling. The researchers measured emission lines of N2 in the droplet (near 430 and 575 nm), and estimated that the laser-induced optical breakdown caused a rise in temperature of approximately 1,000 °C.

For higher peak-power laser pulses with durations of less than 285 fs, the emission spectra widened dramatically and an extended pedestal formed around the incident wavelength of 800 nm. The authors believe that this is attributable to self-phase modulation effects of the laser pulses as they propagate within the droplet.