Tuning laser-induced optical breakdown and cavitation through the ionic environment in aqueous media

Laser-induced cavitation in liquids originates from optical breakdown processes that depend sensitively on both laser–plasma dynamics and the chemical microenvironment of the solvent. Herein, we experimentally decouple the effects of ionic strength and ion specificity on cavitation inception in aqueous electrolytes spanning neutral, acidic, and alkaline regimes. Using focused nanosecond laser pulses, we show that increasing ionic strength universally lowers the cavitation threshold by enhancing charge screening and seed-electron availability. However, under constant ionic strength, strongly asymmetric behavior emerges: acidic (hydrogen chloride) solutions inhibit cavitation, whereas alkaline (sodium hydroxide) solutions enhance it. This asymmetry arises from hydrated-electron kinetics that depends on the ion specificity. In acidic solutions, hydronium ions act as diffusion-limited scavengers of hydrated electrons, quenching their lifetime and inhibiting avalanche ionization. In contrast, hydroxide ions reduce hydronium availability and extend electron survival, promoting more efficient plasma formation. Notably, the electrical conductivity remains nearly constant despite large variations in breakdown thresholds, underscoring that microscopic electron chemistry, rather than macroscopic charge transport, governs cavitation onset. These results establish a mechanistic connection between electrolyte chemistry and optical breakdown, showing that ion-specific reaction dynamics fundamentally controls laser-induced cavitation in aqueous environments.