Regime 4 — Photomechanical

Photomechanical cleaning removes particles and thin films through laser-induced shock waves and rapid thermomechanical expansion rather than direct ablation or thermal decomposition. Ultra-short pulses (picosecond to femtosecond) or specific nanosecond pulse trains create rapid pressure transients that eject loosely bonded particles from the surface. The mechanism is substrate-gentle because peak temperatures remain low — energy converts to kinetic stress rather than heat.

This regime is used for delicate substrates where even sublimation-level fluences would cause damage: semiconductor wafer surfaces, optical coatings, cultural heritage objects, and contaminated precision optics. The primary limitation is throughput — lower fluences and pulse energies mean slower areal cleaning rates compared to sublimation or interfacial detachment regimes.