Polarization imaging camera with a waveplate array fabricated with a femtosecond laser inside silica glass.

In this study, we demonstrate a polarization imaging camera with a waveplate array of a silica glass fabricated by femtosecond (fs) laser direct writing. To use a waveplate array of silica glass for polarization imaging, non-uniformity of the transmittance and retardance in the waveplates must be considered. Therefore, we used a general method of polarization analysis with system matrices determined experimentally for all the units in the waveplate array. We found that a figure of merit based on the determinant of the system matrix could be applied to improve the accuracy of analysis and the robustness to the retardance dispersion for both the simulated and the fabricated waveplate array.

Condensation of Si-rich region inside soda-lime glass by parallel femtosecond laser irradiation.

Local melting and modulation of elemental distributions can be induced inside a glass by focusing femtosecond (fs) laser pulses at high repetition rate (>100 kHz). Using only a single beam of fs laser pulses, the shape of the molten region is ellipsoidal, so the induced elemental distributions are often circular and elongate in the laser propagation direction. In this study, we show that the elongation of the fs laser-induced elemental distributions inside a soda-lime glass could be suppressed by parallel fsing of 250 kHz and 1 kHz fs laser pulses. The thickness of a Si-rich region became about twice thinner than that of a single 250 kHz laser irradiation. Interestingly, the position of the Si-rich region depended on the relative positions between 1 kHz and 250 kHz photoexcited regions. The observation of glass melt during laser exposure showed that the vortex flow of glass melt occurred and it induced the formation of a Si-rich region. Based on the simulation of the transient temperature and viscosity distributions during laser exposure, we temporally interpreted the origin of the vortex flow of glass melt and the mechanism of the formation of the Si-rich region.

Shape control of elemental distributions inside a glass by simultaneous femtosecond laser irradiation at multiple spots.

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

Modulation of laser induced-cracks inside a LiF single crystal by fs laser irradiation at multiple points.

Crack formations inside a LiF single crystal after femtosecond laser irradiation at multiple points were investigated. In the case of sequential laser irradiation at three points, the propagations of some cracks were prevented by the dislocation bands generated by the previous laser irradiation. On the other hand, in the case of simultaneous laser irradiation at three points with a spatial light modulator, cracks in all the <100> directions from the photoexcited regions were generated clearly, but the length of one crack depended on the distribution of laser irradiation positions. The simulation of elastic dynamics after fs laser irradiation at three points elucidated that the interference of laser induced stress waves depended on the distributions of the irradiation positions. We found that the constructive interference of stress waves at a crack tip should have prevented the crack from propagating further and the tensile stress by destructive interference of stress waves along a crack should have facilitated the propagation of the crack.