Question

Describe the difference between photoablation and plasma-induced ablation. Discuss the advantages and disadvantages of these interaction mechanisms for refractive corneal surgery.

Answer 1

Laser ablation or photoablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approximately 200 nm. Laser ablation is a useful technique for surface machining. Pulsed Laser Ablation provides an advantage of minimizing the heat-affected zone .

Plasma-mediated ablation makes use of high energy laser pulses to ionize molecules within the first few femtoseconds of the pulse. This process leads to a submicrometer-sized bubble of plasma that can ablate tissue with negligible heat transfer and collateral damage to neighboring tissue.

The growth of the plasma occurs as a two-step process. In the first step, bound electrons are freed from their molecular orbitals by interaction with the electric field of the laser pulse by the process of multiphoton absorption . In the second step, the free electrons seed an impact ionization cascade that involves acceleration of the electrons by inverse-Bremsstrahlung absorption, in which an electron absorbs photons while colliding with molecules. After several absorption events, the free electrons achieve sufficiently high kinetic energy to ionize another molecule by impact ionization. This cascade, along with the continued generation of photoelectrons, leads to the exponential growth of a micrometer-sized plasma bubble. As the electron density grows, the plasma eventually becomes sufficiently conductive to limit the penetration of the incident light to a skin depth of only tens of nanometers. This restricted penetration depth provides better axial localization.