Juan Song

Redistribution of elements in glass induced by a high-repetition-rate femtosecond laser.

The redistribution of elements in a multicomponent oxyfluoride glass is induced by a 250 kHz femtosecond laser. Elemental distribution in the cross section of the modified region along the laser propagation axis is analyzed by an electron microprobe analyzer. The results indicate that the relative concentrations of network formers of the glass are higher in the central area of the modified region and lower in the periphery of the modified region compared with the unirradiated areas. However, the relative concentrations of network modifiers are as opposed to that of network formers. Fluorescence spectra confirm that the distribution of fluorescence intensity of Yb(3+) in the modified region is consistent with that of its concentration. The effects of spherical aberration of the incident beam on the elemental redistribution are also discussed.

Femtosecond laser-induced inverted microstructures inside glasses by tuning refractive index of objective’s immersion liquid

We report the formation of inverted microstructures inside glasses after femtosecond laser irradiation by tuning the refractive index contrast between the immersion liquid and the glass sample. By using water as well as 1-bromonaphthalene as immersion liquids, microstructures with similar shape but opposite directions are induced after femtosecond laser irradiation. Interestingly, the elemental distribution in the induced structures is also inverted. The simulation of laser intensity distribution along the laser propagation direction indicates that the interfacial spherical aberration effect is responsible for the inversion of microstructures and elemental distribution.

Void-nanograting transition by ultrashort laser pulse irradiation in silica glass.

The structural evolution from void modification to self-assembled nanogratings in fused silica is observed for moderate (NA > 0.4) focusing conditions. Void formation, appears before the geometrical focus after the initial few pulses and after subsequent irradiation, nanogratings gradually occur at the top of the induced structures. Nonlinear Schrödinger equation based simulations are conducted to simulate the laser fluence, intensity and electron density in the regions of modification. Comparing the experiment with simulations, the voids form due to cavitation in the regions where electron density exceeds 1020 cm-3 but is below critical. In this scenario, the energy absorption is insufficient to reach the critical electron density that was once assumed to occur in the regime of void formation and nanogratings, shedding light on the potential formation mechanism of nanogratings.

Mechanism of femtosecond laser inducing inverted microstructures by employing different types of objective lens

In this paper, an interesting inverted structure, fabricated by focusing femtosecond laser pulses through two different objective lenses (an oil-immersion objective lens and a dry objective lens), is reviewed. According to the interface spherical aberration theory, reasonable models are built and the simulated laser fluence in the focal region is given. The distribution of laser fluence agrees with the structural characteristics of the experimentally obtained void array. We propose that the monotonic tendency that the aberration function changes with the azimuth angle determines the final ?growth? direction of the void array.

Formation and partial recovery of optically induced local dislocations inside CaF2 single crystal

We reported on the formation of a microstructure (skew cross-shaped pattern) in bulk CaF(2) single crystal, which originates from local dislocations and microcracks around the focal point of a single infrared femtosecond laser beam. Relations between morphology of the microstructure and the laser power as well as the number of laser pulses were discussed. Furthermore, it was observed that the optically induced microstructure could be partially erased by additional irradiation of its neighboring area with femtosecond laser pulses. High-resolution transmission electron microscope (HRTEM) observations confirmed the disappearance of some local dislocations after the additional femtosecond laser irradiation.

Polarization dependence of the self-organized microgratings induced in SrTiO3 crystal by a single femtosecond laser beam

In this paper, self-organized microgratings are fabricated in SrTiO(3) crystal just by scanning the focus of a tightly-focused linearly-polarized femtosecond laser beam to form a single line. The polarization direction of the laser beam is rotated by a λ/2 waveplate to check the effect of the polarization azimuth on the micrograting morphology. Fourier analyzing of the microscopic images of the microgratings indicates that the polarization plane azimuth of the laser beam does have influence on the microgratings in the aspects of groove orientation and groove spacing. A possible mechanism of polarization dependence is also proposed.

Simultaneous Formation of Two types of Periodic Nanostructure in ZnO Single Crystal by Femtosecond Laser Direct Irradiation

In this paper, we report the simultaneous fabrication of nanogratings and three-dimensional (3D) concentric circles in the same bulk of ZnO single crystal by femtosecond laser irradiation of a single spot. For nanogratings, polarization dependence and selective growth characteristics are two main characteristics; for 3D concentric circles, a noticeable feature is their close relationship with the development of the deep crater ablated by the femtosecond laser. On the basis of the above key points observed from scanning-electron-microscope (SEM) images, possible formation mechanisms for these nanostructures are analyzed.