Yusuke Taki

Optical properties of fluoride thin films deposited by RF magnetron sputtering

Fluoride thin films for 193-nm lithography were deposited by three different types of RF magnetron sputtering. Systematic analysis of the relation between optical properties and deposition conditions of these thin films is discussed.

Superresolution fluorescence imaging by pump-probe setup using repetitive stimulated transition process

We proposed superresolution nonlinear fluorescence microscopy with pump-probe setup that utilizes repetitive stimulated absorption and stimulated emission caused by two-color laser beams. The resulting nonlinear fluorescence that undergoes such a repetitive stimulated transition is detectable as a signal via the lock-in technique. As the nonlinear fluorescence signal is produced by the multi-ply combination of incident beams, the optical resolution can be improved. A theoretical model of the nonlinear optical process is provided using rate equations, which offers phenomenological interpretation of nonlinear fluorescence and estimation of the signal properties. The proposed method is demonstrated as having the scalability of optical resolution. Theoretical resolution and bead image are also estimated to validate the experimental result.

Visible light-responsive cell scaffolds with bilayer structures for single cell separation

Collagen is a major component of the extracellular matrix, and collagen gels have been used as cell scaffolds. We previously prepared gold nanoparticle (AuNP)-embedded collagen gels (AuCol) to serve as cell scaffolds that were sensitive to visible light. We performed single cell detachment from this cell scaffold using a microscope equipped with a laser irradiation system. In the present study, we adjusted hydrogel thickness and AuNP concentration in AuCol, with a goal of improving cell detachment efficiency. Thin hydrogels became blackened after the laser irradiation, and thick hydrogels with high AuNP concentrations were not permeable to the laser light. We, therefore, prepared bilayer gels, composed of AuCol as the upper layer and intact collagen gel (Col) as the bottom layer. These bilayer gels allowed more effective cell detachment, because they were thick and optically transparent. Our results indicated that an AuCol/Col ratio of 2 enabled the highest cell detachment efficiency. Essentially, no cell damage was observed in our system, suggesting that this is a cell-friendly single cell separation system.

Visible Laser-Induced In Situ Cell Detachment from Gold Nanoparticle-Embedded Collagen Gel

Cell sorting is important for cell biology and regenerative medicine. A visible light-responsive cell scaffold is produced using gold nanoparticles and collagen gel. Various kinds of cells are cultured on the visible light-responsive cell scaffold, and the target cells are selectively detached by photoirradiation without any cytotoxicity. This is a new image-guided cell sorting system.

Optical resolution enhancement and background reduction by stimulated emission depletion structured illumination microscopy with structured excitation

We propose stimulated emission depletion (STED) structured illumination microscopy (SIM), which has structured excitation and structured STED light with the same grating vector. Numerical simulation shows the possibility of improving resolution and reducing background fluorescence.

Selective Growth of Carbon Nanotubes and Their Application to Transparent Conductive Plastic Sheets and Optical Filters

Carbon nanotubes (CNTs) have incomparable physical properties and their applications in various fields have been examined. In particular, CNTs composed of a few cylindrical walls are useful for optoelectronic applications. The electronic properties of CNTs, however, significantly change depending on their chirality and the number of graphene walls. Therefore, first of all, the selective growth of graphene walls is required, and ultimately, chiral selection technology should be established. In single-walled CNTs (SWCNTs) and double-walled CNTs (DWCNTs), both semiconducting and metallic characteristics exist according to the chirality of the CNTs. On the other hand, it was predicted that all triplewalled CNTs (TWCNTs) have semimetal characteristics. Therefore, TWCNTs may be used in electronic applications without the need for chiral selection. Moreover, because TWCNTs is the finest multi-walled CNTs (MWCNTs), it is academically interesting to clarify the formation mechanism and various properties of TWCNTs. Nowadays, as-grown SWCNT films are well synthesized on substrates by several types of chemical vapor deposition (CVD) (Hata et al., 2004; Murakami et al., 2004; Zhong et al., 2005). In addition, a synthetic process of producing high-purity SWCNT powder was established at the end of the last century (Nikolaev et al., 1999). As-grown DWCNT films on substrates have also been reported (Hiramatsu et al., 2005; Yamada at al., 2006). From the viewpoint of obtaining DWCNT powder, the CVD of DWCNT powder with supporting material was reported (Muramatsu et al., 2005). The combustion removal of SWCNTs from a mixture of SWCNTs and DWCNTs was also reported (Ramesh et al., 2006). The combustion removal method required a post-treatment to obtain high-yield DWCNT powder. On the other hand, the synthesis of TWCNTs has not reported at all, regardless of the form, for example, films and powder. For the purpose of obtaining CNT films with high DWCNT and TWCNT contents, post-treatments after CNT synthesis are not suitable, because CNTs have almost same chemical properties not related to the number of graphene walls. Posttreatments are only effective for removing amorphous carbon and metallic catalysts from CNTs. Therefore, it is necessary to develop an as-grown synthetic process for DWCNT and TWCNT films.

Postfluorination of fluoride films for vacuum-ultraviolet lithography to improve their optical properties.

Transmittance of fluoride films prepared by various kinds of deposition methods is not sufficient for a vacuum-ultraviolet (VUV) stepper application. Moreover, moisture penetrates into fluoride films because of porous structures. Then the moisture reacts with the fluoride chemically, and the photoabsorption of the films increases with elapsed time. Postfluorination treatment, in which F-poor areas of fluoride films are sufficiently fluorinated and the film structures are modified to be denser, has been developed. The postfluorination treatment was performed at 0.1 MPa of diluted F2 gas in a special apparatus made of nickel. This treatment reduces photoabsorption of fluoride films and prevents photoabsorption from increasing again with elapsed time. In a long-time F2 laser irradiation test, the transmittance of as-deposited fluoride films remarkably degraded with irradiation. On the other hand, there is no tendency of transmittance degradation in the case of postfluorinated fluoride films. The postfluorination is able to improve the optical performance of fluoride films, promising the highest coating level for VUV lithography.