We report on laser welding between transparent materials without the insertion of intermediate layers such as glue by use of near-infrared femtosecond laser pulses. When femtosecond laser pulses are focused at the interface of transparent materials, the material around the focal point is melted and resolidified because of the temperature increases due to the localized nonlinear absorption of optical pulse energy. We experimentally succeeded in laser welding between two pieces of silica glass without the insertion of an intermediate layer by femtosecond laser pulses. This technique has a possible application to the joining of semiconductors such as silicon crystals.

By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of $4‐\mu \mathrm{m}$ diameter were more than $200 \mu \mathrm{m}$ deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.

We fabricated a terahertz wire-grid polarizer consisting of a micrometer-pitch Al grating on a Si substrate by photolithography and wet etching. The ratio of TM and TE transmittances (extinction ratio) was over $35\mathrm{dB}$ at $0.5\mathrm{THz}$. At the Brewster angle of the Si substrate, the polarization transmittance of a TM wave through the fabricated polarizer exceeded 95% and the extinction ratio was over $45\mathrm{dB}$ at $\sim 1\mathrm{THz}$. The fabricated polarizer has a higher extinction ratio than conventional free-standing terahertz wire-grid polarizers.

Subcellular organelles in living cells were inactivated by tightly focusing femtosecond laser pulses inside the cells. Photodisruption of a mitochondrion in living cells was experimentally confirmed by stacking three-dimensional confocal images and by restaining of organelles. The viability of the cells after femtosecond laser irradiation was ascertained by impermeability of propidium iodide as well as by the presence of cytoplasmic streaming.

We investigated the relationship between the formation of filaments and local refractive-index changes induced by femtosecond laser pulses in silica glass. In situ observation revealed that the location of a filament coincided with that of the refractive-index change. Observation also showed that the region of refractive-index change was elongated toward the upstream direction of the laser pulses with increasing exposure time. The region of refractive-index change was several hundred micrometers long, and its diameter was smaller than $2 \mu \mathrm{m}$. The refractive-index change was confirmed by two of three different methods to be as large as $0.8\times {10}^{-2}$.