(b) Damage by laser-induced breakdown:
Laser induced damage (LIDT) may come from randomly distributed intrinsic faults/impurities or laser induced ionized free carriers, which cause materials to absorb input laser energy with value far above that would be experienced at lower intensity levels and thus cause the laser-induced breakdown:
- Intrinsic randomly distributed faults or impurities: Most initial carriers are generated by intrinsic randomly distributed faults or impurities; in such a case, the tendency for damage involving such extrinsic effects substantially depends on the exact spot which is illuminated on a sample.
- Ionized free carriers: high input peak power or multi-photon absorption might also be the starting point to generate some ionized free carrier, which causes subsequent strong absorption for induced damage. As the generated ionized free carriers have a limited lifetime, a sufficiently high optical energy must be provided within a short time to induce the damage. If the initial carriers are generated by multi-photon absorption, the related intrinsic damage process can be rather deterministic as it is largely defined and set by input laser parameters.
The physical mechanisms of laser induced damage (LIDT) for above two scenarios may depend on the pulse duration. For nanosecond and longer pulses, non-deterministic laser-induced damage is often observed, where the probability of damage increases smoothly around the threshold and damage is mainly through intrinsic randomly distributed faults or impurities. In such a case, a larger safety margin for safety operation is required to avoid laser damage through random impurities. For femtosecond pulses, deterministic laser-induced damage is often observed, where the damage is set in when the damage threshold is reached. Figure 6 (b) illustrates the damage by laser-induced breakdown.