Ultrafast Laser Breakdown

“Photograph of the high-pressure chamber and dense microplasma formed during ultrafast laser breakdown.  The chamber (center) is filled with high-pressure gas through a pressure inlet (bottom) and is sealed on all sides by pressure windows. 100 femtosecond laser pulses are focused through a lens (top) and into the chamber.  At the location of strongest laser intensity, blackbody plasma is generated and analyzed by a streak system (not shown).” – Description adapted from Bataller, Alexander William. Exploring the Universality of Sonoluminescence. University of California, Los Angeles, 2014.  Photograph taken by A. Bataller in the Putterman research group at UCLA.

In 2010, Dr. Brian Kappus and coauthors in the Putterman research group at UCLA submitted their remarkable findings on Sonoluminescence to Physical Review Letters.  In this work they reported a transparent-to-opaque (blackbody) transition in the visible region of the Sonoluminescence spectrum, a seeming theoretical contradiction given the plasma’s temperature and density.  Through scientific rigor and creativity, they resolved the discrepancy by presenting a new model that re-imagines the very nature of Sonoluminescence.  In this model, Sonoluminescence is described as a completely new state of matter defined by its extraordinarily high level ionization, similar to the Mott transitions observed in solid-state systems.  The compelling evidence demanded follow-up experiments in other Sonoluminescence systems, all of which supported such a model (Khalid et al., Kappus et al., and Bataller et al.).

Inspired by these efforts, I formulated my thesis question: if Sonoluminescence is a new state of matter, can it be recreated outside its liquid confines?  To answer this question, I designed two different systems for reproducing the conditions of Sonoluminescence and measuring its optical response.  Both methods generated blackbody plasma of similar temperature and density as Sonoluminescence by injecting energy into an already dense system, specifically, high-pressure gases.  The first method of excitation was the focusing of ultrafast (femtosecond) laser pulses.  Like focusing the sun through a magnifying glass, large amounts of energy can be squeezed into a tiny volume and cause intense heating.  Temperatures of over 20,000K are easily achieved with modern laboratory lasers using this method!  At these temperatures and densities, the dense plasma observed in Sonoluminescence was recreated outside a liquid (see photo above).

Although these experiments were successful in supporting the universal nature of Sonoluminescence, the expensive hardware required for ultrafast laser breakdown limits its practical uses.  The second method of recreating Sonoluminescence utilizes a primordial form of excitation that was available to scientists dating back to the days of Benjamin Franklin…the spark discharge.

Would you like to know more?

• http://acoustics-research.physics.ucla.edu/sonoluminescence/ – Research website of Professor Seth Putterman at UCLA where ultrafast laser breakdown research is ongoing.
• https://lasers.llnl.gov/ – Link to the biggest laser on earth (4 MJ).  Laser light is focused to such extremes that fusion occurs!
• Hercules – Link to the most intense laser on earth (2×10²² W cm^-2).  At these mind-boggling intensities, the fabric of space and time is predicted to rip open, out of which will emerge the Borg.
• Bataller, A., Latshaw, A., Koulakis, J., and Putterman, S. “Gas to Liquid Transition in Femtosecond Laser Breakdown.” Manuscript in preparation.
• Bataller, A., Plateau, G. R., Kappus, B., and Putterman, S. “Blackbody Emission from Laser Breakdown in High-Pressure Gases.”  Physical Review Letters, 113(7), 075001 (2014).