Delocalization of femtosecond laser radiation in crystalline Si in the mid-IR range

Abstract

The strong delocalization of the energy of femtosecond pulses in silicon appears to be an essential factor for preventing laser damage inside a crystal and seemingly excludes the possibility of direct laser writing in the bulk, at least in the one-and two-photon absorption (1 PA and 2 PA) wavelength regions. Previously, the prefocal depletion of the pulse energy and laser-induced free-carrier plasma defocusing of the light were considered to be the main causes of the unlocalized dissipation of light energy. Here, we consider whether the delocalization could be significantly reduced by using longer wavelengths, at which the role of 1 PA and 2 PA decreases and higher orders of nonlinearity come into play. We numerically simulate propagation of focused femtosecond pulses at a wavelength of 1.2-5.25 μm. Plasma defocusing was found to be the crucial delocalization mechanism that prevents the enhancement of material excitation, even in the five-photon absorption region.

Competing Nonlinear Delocalization of Light for Laser Inscription Inside Silicon with a 2- µ m Picosecond Laser

“…This is caused by a strong clamping of the intensity due to nonlinear effects developing in the prefocal region. These contributing effects include Kerr-induced phase distortions, beam depletion by the highly efficient multiphoton absorption inherent to narrow gap materials, and increased plasma effects (screening and defocusing) due to the long wavelengths that are inevitably used [8][9][10].…”

Positive- and negative-tone structuring of crystalline silicon by laser-assisted chemical etching

“…Nonlinear processes (especially the plasma defocusing phenomenon) impose severe limits on the laser energy that can be deposited inside crystalline silicon (c-Si) with femtosecond pulses [1], and make material modification with individual pulses extremely challenging [2]. The advent of high-power nanosecond infrared (IR) laser sources has helped to circumvent these limitations [3,4].…”

Characterization and control of laser induced modification inside silicon

“…We make a simple estimate and find that the energy density in the modified volume is between 0.5 to 4.6 kJ/cm 3 , orders-of-magnitude below the damage threshold (MJ/cm 3 in glass) for cracks and voids. While using high-order photo-ionization from longer wavelengths can achieve tighter focusing, other mechanisms, such as plasma defocusing [20], might impair the effectiveness in realizing high energy density. It is critical to understand the fundamental mechanisms in the laser energy deposition process in order to further improve the stealth dicing technique.…”