Roles of vaporization and thermal decomposition in the dynamic evolution of laser-induced bubble on the surface of a submerged metal plate.

This paper aims to explain when the vaporization or thermal decomposition prevails during laser-induced bubble growth and how they influence bubble morphology. Bubbles were generated by irradiating a 304 stainless steel plate submerged in degassed water using millisecond lasers with a pulse width of 0.4 ms and powers of 1.6 kW and 3.2 kW, respectively. The dynamic evolution of bubbles was recorded by a high-speed camera. Moreover, the numerical models were developed to obtain a vaporization model and a decomposition model by incorporating the source terms due to the vaporization and decomposition mass fluxes into the governing equations, respectively. The simulated dynamic bubble evolution is consistent with the experimental results. When the laser power is 1.6 kW, a thin-layer bubble is formed, which gradually shrinks and eventually disappears after the laser stops irradiating. When the laser power is 3.2 kW, a spherical bubble is formed, and its volume decreases significantly after the laser stops irradiating. Subsequently, it remains relatively stable during the observation period. The fundamental reason for the difference between the bubble morphologies obtained from the vaporization model and the decomposition model lies in the presence of a condensation zone in the gas phase. When water vaporization or thermal decomposition dominates, the temperatures obtained from the models align with the decomposition ratios at varying temperatures reported in the literature. Our findings are significant for understanding the dynamic behavior of bubbles, with implications for various laser processing underwater.

Index Modulation Embedded in Type I Waveguide Written by Femtosecond Laser in Fused Silica.

Slit-shaped laser beams focused in bulk optical materials can realize embedded waveguides with circular cross sections consisting of positive index change type I traces. In these kinds of waveguide traces, a peculiar periodical refractive index modulation was observed in type I waveguides with two different femtosecond lasers. The direction of refractive index modulation can be controlled with the slit configuration, and its period can be controlled by mechanical perturbation of the stages and the scanning speed. We argue that platform perturbation and dynamical thermal transport processes during the scan are generating factors in the appearance of this modulation. The embedded microstructures in waveguides can provide spectrum modulation, which may have potential applications in optical sensing, filtering, and phase control.

3D waveguide element fabrication in Gorilla glass by an ultrafast laser.

Waveguide fabrication with an ultrafast laser system and the mechanism of index modification have been investigated in Corning Gorilla glass. Type I waveguides were obtained when the pulse duration was in the range of 250 fs to 15 ps. With the increase of pulse energy, single-mode waveguides converted to ring-mode waveguides. The variation tendency of Raman peak at 580cm-1 band is nonmonotonic with the increase of pulse energy, and the negative index change appears finally in the waveguide core. The alkali ions migrated towards the outside with different diffusivities after the laser irradiation. Finally, bend waveguides and hexagon-link waveguide connectors were produced.

Transmission performance of 90°-bend optical waveguides fabricated in fused silica by femtosecond laser inscription.

The L-shape waveguide was written in fused silica using a femtosecond laser with beam shaping. The guiding structure supports good light turning; 0.88 dB/turn was achieved at the silica-air interface. By using the finite-different time-domain method, the turn loss due to the turning structure and refractive index of the L-shape waveguide has been simulated. The results show that the proposed method has unprecedented flexibility in fabricating a 90°-bend waveguide.

Rutile TiO2 Flocculent Ripples with High Antireflectivity and Superhydrophobicity on the Surface of Titanium under 10 ns Laser Irradiation without Focusing.

We report on the formation of rutile TiO2 flocculent laser-induced periodic surface structures (LIPSSs) with high antireflectivity and superhydrophobicity on the surface of titanium under 10 ns 1064 nm laser irradiation without focusing. The center part of the Gaussian laser beam is used to deposit flocculent structure and the edge part used to produce LIPSSs. The melt and modification thresholds of titanium were determined first, and then, the melt and modification spot-overlap numbers, several responsible for the formation of flocculent structure and LIPSSs, were introduced. It is found that both the melt and modification spot-overlap numbers increase with an increase in laser fluence and spot-overlap number, contributing to the production of flocculent LIPSSs. LIPSSs are obtained with the modification spot-overlap number above 300, and the amount of flocculent structures increases with an increase in the peak laser fluence and spot-overlap number. Then, considering that the fine adjustment of the melt and modification spot-overlop numbers in one-time line scanning is quite difficult, the composite structure, of which both LIPSSs and flocculent structures are distinct, was optimized using laser line scanning twice. On this basis, a characterization test shows the sample full of the flocculent LIPSSs represents best antireflectivity with the value around 10% in the waveband between 260 and 2600 nm (advance 5 times in infrared wavelengths compared to the initial titanium surface), and shows the no-stick hydrophobicity with the contact angle of 160° and roll-off angle of 25° because of the pure rutile phase of TiO2.