The effect of pulse shape on 3D modelling of laser cleaning fluences

Prior research in the field of laser cleaning has suggested that shorter pulses are preferable to achieve low laser cleaning threshold fluences. These predictions were based mainly on exponential pulse shapes. In the present work the three-dimensional model of laser cleaning developed by Boris Luk’yanchuk (B. S. Luk’yanchuk et al., Appl Phys A, 77,2, 209) which accounts for near-field focussing, has been used to calculate the laser cleaning threshold fluence for three different pulse shapes. These were rectangular, sinusoidal and exponential. For each pulse shape, the threshold fluence was determined as a function of pulse width (1-200 ns) and height (1-15 GW/cm²). It was found that the threshold fluence is strongly dependent on the laser pulse shape, particularly for pulses greater than 100 ns in width. The threshold fluence of the rectangular pulse oscillated with a period equal to that of the period of oscillation of the particle on the substrate. In contrast, for both the exponential and sinusoidal pulses, the threshold fluence increases monotonically with pulse length.