For the purpose of laser-material interaction, the definition of "ultrafast" typically refers to a non-thermal regime of energy absorption. This "cold" processing is a primary distinguishing factor between lasers with picosecond and femtosecond pulse durations and lasers with pulse durations measured in nanoseconds. The non-thermal absorption regime is critically important for some applications because it virtually eliminates thermal damage to materials, enables the creation of smaller features, and allows for more precise control over the process. Two major mechanisms are responsible for the shift from thermal to non-thermal regimes.

High Peak Power: Relative to average output power, ultrafast lasers generate higher peak powers when compared to continuous wave, quasi-continuous wave, and nanosecond lasers. This exceptionally high peak power results in the simultaneous aborption of several photons by the target material. The energy of these photons is additive due to reaching the target at the same time and in the same space.

Cold Ablation: The short pulse duration of ultrafast lasers greatly limits the time available for heat to diffuse into the bulk material. Individual pulses instantly remove or ablate an extremely small volume of material while creating virtually no heat affected zone. Increasing the pulse energy leads to higher material removal rates rather than additional heating.