Optical and ultrasonic signatures of femtosecond pulse filamentation in fused silica

Abstract

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica by single, moderately focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy Eopt=5 μJ corresponding to a few megawatt power, the pulses collapse due to self-focusing and the nonlinear focus moves upstream with increasing pulse energy. Behind the focus, elongated, gradually narrowing awl-shaped channels of electron-hole plasma and luminescent defects are produced. In the channels, whose dimensions generally depend on the pulse energy, supercontinuum emission propagating downstream the channels occurs, although its observation requires elevated pulse energies above 25 μJ in order to compensate energy dissipation in the channels. Ultrasonic side-view imaging of the channels, conducted from a few millimeters distance, reveals predominantly compressive pressure transients. The compressive signals are observed above the same threshold pulse energy Eopt, and their amplitude increases linearly with the laser pulse energy, simultaneously exhibiting significant temporal broadening of the corresponding pulsewidths, reflecting square root dependence of the channel length and sublinear (∝E3/4) dependence of the source pressure on the pulse energy. Altogether, these optical and ultrasonic studies demonstrate filamentary pulse propagation with considerable dissipation (∼10 cm−1) in the awl-shaped subcritical plasma channels rich with generated point defects and optical damage sites.

Femtosecond laser induced phenomena in transparent solid materials: Fundamentals and applications

“…Ashcom et al suggest that competing nonlinear optical effects are involved in the interaction of femtosecond laser pulses with transparent materials: self-focusing, supercontinuum generation and multiphoton-induced bulk damage [76]. At low NA (<0.65), self-focusing and supercontinuum generation play an important role in the energy deposition before the damage takes place, and this is confirmed by optical and ultrasonic signatures [76][77][78]. At high NA, multiphoton-induced bulk damage predominates the femtosecond microfabrication process.…”

Towards Safer Primers: A Review

“…Pressure sensors and optical fibres in pressure tubes or chambers can also measure gas generation and shockwave velocity by spacing the measurement points along the length of the tube in increments away from the igniter [75]. Piezoelectric sensors are small and can be placed in close proximity to optically induced initiators as demonstrated by detecting the acoustic signature of an electron plasma formed in glass [146].…”

In vitro femtosecond laser subsurface micro-disruption inside human cornea and pre-cleared sclera

“…Once strongly focused, high power fs pulses provide in such materials at pre-determined focal depths sub-or micro-scale sub-or critical laser plasmas [7,8], appearing via multi-photon or tunnel ionization of the condensed matter [9] at absence of macroscopic (micrometer-sized) thermal or shock-affected zones [4], and any laser damage along the laser beam path with except for the focal volume. Depending on input peak powers P -sub-or critical ones, strongly focused fs laser pulses fabricate singlefocus or multi-foci features -nanovoids (sub-critical P ) [4] or quasi-continuous sequences of nano-or micro-voids (supercritical P ) [5,6], which can be arranged in threedimensional arrays significantly modifying local optical, mechanical and other characteristics of the host materials.…”