Low-density plasma formation in aqueous biological media using sub-nanosecond laser pulses

Abstract

We demonstrate the formation of low-and high-density plasmas in aqueous media using sub-nanosecond laser pulses delivered at low numerical aperture (NA ¼ 0.25). We observe two distinct regimes of plasma formation in deionized water, phosphate buffered saline, Minimum Essential Medium (MEM), and MEM supplemented with phenol red. Optical breakdown is first initiated in a low-energy regime and characterized by bubble formation without plasma luminescence with threshold pulse energies in the range of E p % 4-5 lJ, depending on media formulation. The onset of this regime occurs over a very narrow interval of pulse energies and produces small bubbles (R max ¼ 2-20 lm) due to a tiny conversion (g < 0.01%) of laser energy to bubble energy E B . The lack of visible plasma luminescence, sharp energy onset, and low bubble energy conversion are all hallmarks of low-density plasma (LDP) formation. At higher pulse energies (E p ¼ 11-20 lJ), the process transitions to a second regime characterized by plasma luminescence and large bubble formation. Bubbles formed in this regime are 1-2 orders of magnitude larger in size ðR max տ 100 lmÞ due to a roughly two-order-of-magnitude increase in bubble energy conversion (g տ 3%). These characteristics are consistent with high-density plasma formation produced by avalanche ionization and thermal runaway. Additionally, we show that supplementation of MEM with fetal bovine serum (FBS) limits optical breakdown to this high-energy regime. The ability to produce LDPs using sub-nanosecond pulses focused at low NA in a variety of cell culture media formulations without FBS can provide for cellular manipulation at high throughput with precision approaching that of femtosecond pulses delivered at high NA. V C 2014 AIP Publishing LLC.

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