Observation of particle ejection behavior following laser-induced breakdown on the rear surface of a sodium chloride optical window

Laser-induced rear surface breakdown process of sodium chloride (NaCl) optical window was investigated based on the time-resolved shadowgraphy and interferometry. Violent particle ejection behavior lasting from tens of nanoseconds to tens of microseconds after the breakdown was observed. Classified by the particle velocity and propagating direction, the ejection process can be divided into three phases: (1) high-speed ejection of liquid particles during the first 100-ns delay; (2) micron-sized material clusters ejection from ∼100-ns to ∼1-μs delay; (3) larger and slower solid-state particles ejection from ∼1 μs to tens of microseconds delay. The moving directions of particles in the first and third phases are both perpendicular to the sample surface while particles ejected in the second phase exhibits angular ejection and present a V-like particle pattern. Mechanisms include explosive boiling, impact ejection, and shockwave ejection are discussed to explain this multiple phase ejection behavior. Our results highlight the significance of impact ejection induced by recoil pressure and backward propagating internal shockwave for laser-induced rear surface breakdown events of optical materials with low melting point.