Wataru Watanabe

Terahertz wire-grid polarizers with micrometer-pitch Al gratings

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 dB at 0.5 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 dB at approximately 1 THz. The fabricated polarizer has a higher extinction ratio than conventional free-standing terahertz wire-grid polarizers.

Space-selective laser joining of dissimilar transparent materials using femtosecond laser pulses

We report on the joining of dissimilar transparent materials based on localized melting and resolidification of the materials only around the focal volume due to nonlinear absorption of focused femtosecond laser pulses. We demonstrate the joining of borosilicate glass and fused silica, whose coefficients of thermal expansion are different. The joint strength and the transmittance through joint volume were investigated by varying the translation velocity of the sample and the pulse energy of the irradiated laser pulses.

Laser micro-welding of transparent materials by a localized heat accumulation effect using a femtosecond fiber laser at 1558 nm

We report on laser micro-welding of materials based on a localized heat accumulation effect using an amplified femtosecond Er-fiber laser with a wavelength of 1558 nm and a repetition rate of 500 kHz. We demonstrated the welding of non-alkali alumino silicate glass substrates, resulting in a joint strength of 9.87 MPa. We also welded a non-alkali glass substrate and a silicon substrate using the 1558-nm laser pulses, resulting in a joint strength of 3.74 MPa. Our laser micro-welding technique can be extended to welding of semiconductor materials and has potential for various applications, such as three-dimensional stacks and assembly of electronic devices and microelectromechanical system devices.

Single-organelle tracking by two-photon conversion

Spatial and temporal information about intracellular objects and their dynamics within a living cell are essential for dynamic analysis of such objects in cell biology. A specific intracellular object can be discriminated by photoactivatable fluorescent proteins that exhibit pronounced light-induced spectral changes. Here, we report on selective labeling and tracking of a single organelle by using two-photon conversion of a photoconvertible fluorescent protein with near-infrared femtosecond laser pulses. We performed selective labeling of a single mitochondrion in a living tobacco BY-2 cell using two-photon photoconversion of Kaede. Using this technique, we demonstrated that, in plants, the directed movement of individual mitochondria along the cytoskeletons was mediated by actin filaments, whereas microtubules were not required for the movement of mitochondria. This single-organelle labeling technique enabled us to track the dynamics of a single organelle, revealing the mechanisms involved in organelle dynamics. The technique has potential application in direct tracking of selective cellular and intracellular structures.

Structural modification in fused silica by a femtosecond fiber laser at 1558 nm

We report on structural modification in fused silica by a novel commercial femtosecond fiber laser with a fundamental wavelength of 1558 nm. The refractive-index change was induced by laser pulses at a repetition rate of 173 kHz and pulse duration of 870 fs. The refractive index change with a magnitude of 1.2 x 10(-3) was estimated from the diffraction efficiencies of an internal grating.

Chromophore-assisted laser inactivation – towards a spatiotemporal–functional analysis of proteins, and the ablation of chromatin, organelle and cell function

ABSTRACT Chromophore-assisted laser or light inactivation (CALI) has been employed as a promising technique to achieve spatiotemporal knockdown or loss-of-function of target molecules in situ. CALI is performed using photosensitizers as generators of reactive oxygen species (ROS). There are two CALI approaches that use either transgenic tags with chemical photosensitizers, or genetically encoded fluorescent protein fusions. Using spatially restricted microscopy illumination, CALI can address questions regarding, for example, protein isoforms, subcellular localization or phase-specific analyses of multifunctional proteins that other knockdown approaches, such as RNA interference or treatment with chemicals, cannot. Furthermore, rescue experiments can clarify the phenotypic capabilities of CALI after the depletion of endogenous targets. CALI can also provide information about individual events that are involved in the function of a target protein and highlight them in multifactorial events. Beyond functional analysis of proteins, CALI of nuclear proteins can be performed to induce cell cycle arrest, chromatin- or locus-specific DNA damage. Even at organelle level – such as in mitochondria, the plasma membrane or lysosomes – CALI can trigger cell death. Moreover, CALI has emerged as an optogenetic tool to switch off signaling pathways, including the optical depletion of individual neurons. In this Commentary, we review recent applications of CALI and discuss the utility and effective use of CALI to address open questions in cell biology.

Optical knock out of stem cells with extremely ultrashort femtosecond laser pulses

Novel ultracompact multiphoton sub-20 femtosecond near infrared 85 MHz laser scanning microscopes and conventional 250 fs laser microscopes have been used to perform high spatial resolution two-photon imaging of stem cell clusters as well as selective intracellular nanoprocessing and knock out of living single stem cells within an 3D microenvironment without any collateral damage. Also lethal cell exposure of large parts of cell clusters was successfully probed while maintaining single cells of interest alive. The mean power could be kept in the milliwatt range for 3D nanoprocessing and even in the microwatt range for two-photon imaging. Ultracompact low power sub-20 fs laser systems may become interesting tools for optical nanobiotechnology such as optical cleaning of stem cell clusters as well as optical transfection.

Density characterization of femtosecond laser modification in polymers

Bragg-type gratings were prepared by femtosecond laser irradiation in a series of optical polymers. The diffraction efficiency of polymethylpentene (PMP) was an order of magnitude higher than those of other polymers. Repeated scanning irradiation with femtosecond laser light formed gratings by refractive index changes inside the polymers. In PMP, whose density was the lowest among the polymers examined, large volume contraction by femtosecond laser irradiation was observed by transmission electron microscopy. The larger refractive index change of PMP was attributed to its large volume contraction based on its low density.

Increasing diffraction efficiency by heating phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate)

The diffraction efficiency of phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate) was increased by more than an order of magnitude by subsequent heating below the glass transition temperature (Tg). The first-order Bragg diffraction efficiency of a 10u2002μm period grating was 1.9% without heating, whereas it increased to 72% when heated at 70u2009°C for 500 h. This is because the induced refractive index changes (Δn) were increased by heating. From the Lorentz–Lorenz equation, one of the reasons why Δn increases by heating could be a slight volume contraction in the irradiated area.

Fabrication of controlled volume scattering medium in poly(methyl methacrylate) by focused femtosecond laser pulses

A volume scattering medium was fabricated in a transparent material by focused femtosecond laser pulses. A random distribution of voids fabricated by femotosecond pulsed laser light acted as a volume scattering medium that may be applied to photonic secure data storage. We fabricated a partially random distribution of voids in poly(methyl methacrylate) and then measured the scattering properties. Comparing beam widths of the output intensity distribution through the scattering medium, obtained from both complex amplitude propagation and Monte Carlo simulation, showed that the scattering coefficient can be controlled by the void density.