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

Abstract:
{'i': ['E', 'μ', 'R', 'μ', 'η', 'E', 'E', 'μ', 'η'], 'sub': ['p', 'max', 'B', 'p'], '#text': 'We demonstrate the formation of low- and high-density plasmas in aqueous media using sub-nanosecond laser pulses delivered at low numerical aperture (NA\u2009=\u20090.25). 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 \u2009≈\u20094-5\u2009J, depending on media formulation. The onset of this regime occurs over a very narrow interval of pulse energies and produces small bubbles (\u2009=\u20092-20\u2009m) due to a tiny conversion ( < 0.01%) of laser energy to bubble energy . 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 (\u2009=\u200911-20\u2009J), 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 [Formula: see text] due to a roughly two-order-of-magnitude increase in bubble energy conversion ( ≳ 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.'}