Bulk Modification of N-BK7 Glass Induced by Ultrashort Pulse Pairs: Influence of Pulse Offset and Energy Contrast

Temporally shaped femtosecond pulses provide a versatile approach for controlling energy deposition in transparent materials. In this work, bulk modification of N-BK7 glass is investigated using double femtosecond pulses with variable delay and energy contrast. Each sub-pulse is set below the single-pulse modification threshold, so any resulting modification arises from their combined interaction within the focal volume. The results reveal an additive effect in which two individually sub-threshold sub-pulses jointly produce permanent bulk modification. The observed effect is most pronounced for comparable sub-pulse energies, whereas the range of energy contrasts supporting modification narrows with increasing delay. The accompanying reduction in the effective transmittance of the second sub-pulse indicates that the first pulse creates a transient material state that promotes subsequent energy deposition. These findings identify pulse delay and energy contrast as effective parameters for controlling bulk modification and suggest a potential route toward more localized energy deposition and smaller feature sizes in three-dimensional glass microfabrication.