Near-infrared femtosecond laser machining initiated by ultraviolet multiphoton ionization

Abstract

We report on the experimental study of microstructures fabricated on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm beam followed by a loosely focused 800 nm beam. By setting the fluence of each pulse below the damage threshold, visible microstructures are fabricated using the combined beams. Our results suggest that the ultraviolet pulse generates seed electrons through multiphoton absorption, and the near-infrared pulse utilizes these electrons to cause damage by avalanche ionization.

Self-consistent modeling of photoionization and the Kerr effect in bulk solids

“…Once the fields are combined, the resulting yield increases by an order of magnitude, which is much more than simply adding the ionization yields of the two individual pulses, as is sometimes done in the literature. 5 This fact is even more pronounced once the 267 nm pulse is switched with an otherwise 400 nm pulse in Fig. 7.…”

“…In calculating laser material interactions, this fact is typically ignored in the interest of simplicity. This is consequential in the case of ionization, [5][6][7] but it is also important for other areas of nonlinear optics. 8,9 In particular, for dielectrics the Kerr effect ceases to be well approximated as an instantaneous contribution to the polarization when the laser frequency approaches a two-photon resonance of the material.…”

“…[1][2][3][4][5] One major advantage of such pulses is that their short duration can avoid the onset of thermal effects while the pulse is in transit through the material. Another advantage is that the femtosecond is on the time scale of processes found within the atom, making ultrashort pulses ideal to study the details of fast interactions that are typically time-averaged.…”

Fabricating nanostructures on fused silica using femtosecond infrared pulses combined with sub-nanojoule ultraviolet pulses

“…Synchronized femtosecond UV (266 nm) and IR (800 nm) pulses are generated from the same Ti:sapphire laser that operates at a repetition rate of 1 kHz and delivers 60 fs (full width at half-maximum, FWHM) IR pulses. The UV beam (estimated pulse duration 70 fs FWHM), generated from third-harmonic generation (THG) [13], first goes through a spatial filter which consists of two thin lenses (L1, f 500 mm and L2, f 1 m), and a pinhole drilled through a borosilicate microscope cover glass (thickness 150 μm) by another IR beam. The estimated UV pulse duration after L2 is 85 fs because of dispersion of the two lenses.…”

“…The energy fluctuations of the UV and IR beams are within 5% and 1%, respectively. For temporal overlapping, multiple-shot UV damage threshold is measured at different delays (temporal step 60 fs) at a fixed IR pulse energy below its damage threshold and a slow sample moving speed of 0.4 mm/s, and the optimal delay (∼60 fs) is determined when the lowest UV damage threshold is observed [13]. In the following single-shot measurements, the sample moves at a speed of 20 mm/s, ensuring that each UV-IR pulse pair irradiates at a fresh site.…”