Effect of pulse duration on the energy delivery under nonlinear propagation of tightly focused Cr:forsterite laser radiation in bulk silicon

Abstract

We have investigated nonlinear laser ̶ matter interaction inside silicon under tight focusing conditions by continuously tuning driving pulse duration from femtosecond to picosecond timescales. Such tailoring of laser pulse width provides a new route for energy delivery into a microvolume avoiding two-photon absorption and plasma defocusing in the pre-focal region. As a result, we have achieved values of saturated deposited energy density and plasma electron concentration of as high as 1 kJ cm −3 and 10 19 cm −3 respectively, which is lower than the threshold of irreversible structural transformation. For further increase of energy delivery inside silicon, a two-color technique supported by extremely tight focusing can be realized, forming a roadmap to the 3D industrial micromachining of planar bulk silicon.

Taming Ultrafast Laser Filaments for Optimized Semiconductor–Metal Welding

“…In the second regime, for 10 nJ nJ, the position of the maximum intensity z max is unambiguously shifted upstream the incoming laser with respect to the position . This focal shift toward the prefocal region in accordance with observations in the femtosecond regime [ 17–21 ] highlights the existence of nonlinear propagation effects. Nevertheless, the resulting focal zone is still comparable in its extension to the linear case, while the shape of the fluence distribution deviates from the linear one with increasing pulse energies.…”

“…However, this optimization is limited by the intensity clamping phenomenon saturating the maximum fluence in silicon. [ 17–19 ] In contrast, the energy reaching the silicon‐copper interface increases with E in without any saturation. One must emphasize that, again, the precompensation of is crucial for enhancing , especially for high E in values.…”

“…First, the significant losses induced by two‐photon absorption and the filament formation limit the maximum intensity reachable inside silicon at a level below its modification threshold. [ 16–18 ] This intensity clamping phenomenon thus implies that increasing the input pulse energy is not a well‐adapted strategy for modifying permanently silicon through a plane entrance surface. [ 19 ] The second consequence is the nonlinear focal shift at increasing input power, away from the desired location.…”

“…[ 19 ] The second consequence is the nonlinear focal shift at increasing input power, away from the desired location. [ 15,17–21 ] Taken together, these processes lead to a drastic depletion of the light at the exit surface of silicon to levels insufficient for laser welding applications. To date, the most efficient and reliable method for circumventing these nonlinear effects is to employ nanosecond laser pulses.…”

In‐Volume Laser Direct Writing of Silicon—Challenges and Opportunities

“…recently investigated the pulse duration dependence of the energy deposition in silicon. [ 60 ] The nonlinear propagation imaging relies on a silicon‐based camera. [ 61 ] While this type of camera is usually less costly than infrared cameras, the dynamic range is reduced for laser pulses of wavelength m due to the fact that the two‐photon response of the device has to be taken into account.…”