title: 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 = 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 ofEp≈ 4-5μJ, depending on media formulation. The onset of this regime occurs over a very narrow interval of pulse energies and produces small bubbles (Rmax= 2-20μm) due to a tiny conversion (η< 0.01%) of laser energy to bubble energyEB. 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 (Ep= 11-20μJ), 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 (η≳ 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.
authors.forename: Suzanne L
authors.initials: SL
authors.lastname: Genc
authors.forename: Huan
authors.initials: H
authors.lastname: Ma
authors.forename: Vasan
authors.initials: V
authors.lastname: Venugopalan