Ultrafast time-resolved single-shot birefringence microscopy for laser-induced anisotropy.

The interaction between ultrashort laser pulses and materials in the ultrafast time domain, especially regarding the effect of laser polarization, has attracted much attention. In this study, ultrafast time-resolved single-shot birefringence microscopy is performed to observe laser-induced anisotropy. The birefringences of the optical Kerr effect and laser-induced anisotropic nanostructures by femtosecond laser pulses in silica glass are measured, and their slow axis is confirmed to correspond to the linear polarization angle of the pump light. We discuss the time variations of these birefringences in the picosecond time domain.

Independent contribution of optical attenuation length in ultrafast laser-induced structural change.

Although laser irradiation with femtosecond pulses is known to generate crystallization and morphological changes, the contribution of optical parameters to material changes is still in discussion. Here, we compare two structures irradiated near Si-L2,3 edges by an extreme ultraviolet femtosecond pulse. Our result implies that, despite the femtosecond irradiation regime, these values of the optical attenuation length between the wavelengths of 10.3-nm and 13.5-nm differ by one order of magnitude. From the structural comparison, the original crystalline state was maintained upon irradiation at 13.5-nm, on the other hand, transition to an amorphous state occurred at 10.3-nm. The difference in optical attenuation length directly influence to the decision of material crystallization or morphological changes, even if the irradiation condition is under the femtosecond regime and same pulse duration. Our result reveals the contribution of optical attenuation length in ultrafast laser-induced structural change.

Surface processing of PMMA and metal nano-particle resist by sub-micrometer focusing of coherent extreme ultraviolet high-order harmonics pulses.

We demonstrate sub-micrometer processing of two kinds of thin films, polymethyl methacrylate (PMMA) and metal nano-particle resist, by focusing high-order harmonics of near-IR femtosecond laser pulses in the extreme ultraviolet (XUV) wavelength region (27.2-34.3 nm) on the thin film samples using an ellipsoidal focusing mirror. The ablation threshold fluences for the PMMA sample and the metal nano-particle resist per XUV pulse obtained by the accumulation of 200 XUV pulses were determined to be 0.42mJ/cm2 and 0.17mJ/cm2, respectively. The diameters (FWHM) of a hole created by the ablation on the PMMA film at the focus were 0.67 µm and 0.44 µm along the horizontal direction and the vertical direction, respectively. The fluence dependence of the Raman microscope spectra of the processed holes on the PMMA sample showed that the chemical modification, in which C=C double bonds are formed associated with the scission of the PMMA polymer chains, is achieved by the irradiation of the XUV pulses.