Role of molecular photodissociation in ultrafast laser surgery

Transparent ocular tissues such as cornea and crystalline lens can be precisely ablated or dissected using ultrafast ultraviolet, visible, and infrared lasers. In refractive or cataract surgery, cutting of the cornea, lens, and lens capsule is typically produced by dielectric breakdown in the focus of a short-pulse laser which results in explosive vaporization of the interstitial water and mechanically ruptures the surrounding tissue. Here, we report that tissue can also be disrupted below the threshold of bubble appearance using 400 nm femtosecond pulses with minimal mechanical damage. Using gel electrophoresis and liquid chromatography/mass spectrometry, we assessed photodissociation of proteins and polypeptides by 400 nm femtosecond pulses both below and above the cavitation bubble threshold. Negligible protein dissociation was observed with 800 nm femtosecond lasers even above the threshold of dielectric breakdown. Scanning electron microscopy of the cut edges in porcine lens capsule demonstrated that plasma-mediated cutting results in the formation of grooves. Below the cavitation bubble threshold, precise cutting could still be produced with 400 nm femtosecond pulses, possibly due to molecular photodissociation of the tissue structural proteins.