Shoichi Kubo

Nano-Dispersed Organic Liquid and Liquid Crystals for All-Time-Scales Optical Switching and Tunable Negative-and Zero- Index Materials

We present an overview of recent results obtained in nano-dispersed nonlinear neat organic liquids and liquid crystals. Experimental observations confirm the enhancement of nonlinear absorption in gold nano-spheres doped neat nonlinear liquids and extension of the bandwidth of efficient all-optical switching operation for visible-near infrared lasers in time scales ranging from sub-picoseconds through nanoseconds and microseconds and longer. Similar nano-particulate doped liquid crystal films also exhibit efficient all-optical switching capabilities for microseconds – cw lasers. Meta-materials fabricated with core-shell nano-spheres dispersed nematic liquid crystals show effective refractive indices that are tunable from negative through zero to positive values.

Synthesis and characterization of the multi-photon absorption and excited-state properties of a neat liquid 4-propyl 4'-butyl diphenyl acetylene

The synthesis, characterization, and quantitative electronic structure modeling of multi-photon absorption properties of a neat liquid L34 (4-propyl 4′-butyl diphenyl acetylene) are reported. The liquid is (linearly) transparent in the visible spectrum, but possesses large two-photon absorption (2PA) and 2PA-induced singlet and triplet excited-state absorption as measured by the Z-scan technique and non-linear transmission measurements using both picosecond and nanosecond pulses. The most dominant contributions to the intensity-dependent non-linear absorption come from the 2PA-induced triplet excited states in the nanosecond time regime. We also present transient absorption spectra of the liquid obtained by nanosecond laser-flash photolysis and compare these results with electronic structure calculations. The energy of the absorption bands, both singlet and triplet are in reasonable agreement with calculations performed with Gaussian 03. The experimentally measured spectra and theoretical electronic structure modeling provide information about the energy levels of the excited states of this liquid, including 2PA and 2PA-induced process that is responsible for its non-linear optical properties.

Photochemical switching behavior of azofunctionalized polymer liquid crystal/SiO2 composite photonic crystal

A photochemically tunable photonic crystal was prepared by infiltrating azopolymer liquid crystal in a SiO2 inverse opal structure. The SiO2 inverse opal film obtained reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation shifted the reflection band to longer wavelength more than 15nm. This is caused by the formation of anisotropic molecular orientation of the azopolymer. The switched state was stable in the dark, and the reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiations.

Fluorescent Microscopy Proving Resin Adhesion to a Fluorinated Mold Surface Suppressed by Pentafluoropropane in Step-and-Repeat Ultraviolet Nanoimprinting

Resin adsorption on fluorinated silica mold surfaces during step-and-repeat ultraviolet nanoimprinting was studied by fluorescent microscopy using a fluorescent UV-curable resist. The fluorescence intensity indicating resin adsorption to the mold surfaces in step-and-repeat UV nanoimprinting under air atmosphere was significantly higher than that under condensable gas pentafluoropropane (PFP) atmosphere. The larger resin adsorption in air was attributable to sticking uncured resin around trapped air bubbles preventing the UV-curable resist from causing acrylate radical photopolymerization and to the small amount of resin components adhering to the mold surface. The entire adsorption of resin components, not only a fluorescent dye doped in a UV-curable resin, was confirmed by high-sensitivity UV–visible absorption spectroscopy and atomic force microscopy in a frictional mode. PFP suppressed obviously stuck uncured resin and entirely adhered resin components to the fluorinated mold surface. The entire adsorption of resin components was compared among three kinds of fluorinated mold surface treated with commercially available antisticking reagents, FAS13 (tridecafluoro-1,1,2,2-tetrahydro-octyltrimethoxysilane), OPTOOL DSX, and OPTOOL AES4-E. It was proved by the fluorescent microscopy that the fluorinated mold surface prepared by chemical vapor surface modification with FAS13 showed the best antisticking property among the fluorinated mold surfaces, because the entire adsorption of resin components was hardly affected by the number of cycles of step-and-repeat UV nanoimprinting and by the positions in the mold surface.

Selection of di(meth)acrylate monomers for low pollution of fluorinated mold surfaces in ultraviolet nanoimprint lithography.

We used fluorescence microscopy to show that low adsorption of resin components by a mold surface was necessary for continuous ultraviolet (UV) nanoimprinting, as well as generation of a low release energy on detachment of a cured resin from a template mold. This is because with low mold pollution, fracture on demolding occurred at the interface between the mold and cured resin surfaces rather than at the outermost part of the cured resin. To achieve low mold pollution, we investigated the radical photopolymerization behaviors of fluorescent UV-curable resins and the mechanical properties (fracture toughness, surface hardness, and release energy) of the cured resin films for six types of di(meth)acrylate-based monomers with similar chemical structures, in which polar hydroxy and aromatic bulky bisphenol moieties and methacryloyl or acryloyl reactive groups were present or absent. As a result, we selected bisphenol A glycerolate dimethacrylate (BPAGDM), which contains hydroxy, bisphenol, and methacryloyl moieties, which give good mechanical properties, monomer bulkiness, and mild reactivity, respectively, as a suitable base monomer for UV nanoimprinting under an easily condensable alternative chlorofluorocarbon (HFC-245fa) atmosphere. The fluorescent UV-curable BPAGDM resin was used for UV nanoimprinting and lithographic reactive ion etching of a silicon surface with 32 nm line-and-space patterns without a hard metal layer.

Split-ring resonators interacting with a magnetic field at visible frequencies

Split-ring resonators (SRRs) are attractive owing to the interaction with a magnetic field of incident light. Here, we report the fabrication of uniform arrays of about 360 million Au SRRs with a line width of approximately 50 nm by reactive-monolayer-assisted thermal nanoimprint lithography over a 5-mm square area. Furthermore, we present an experimental demonstration of the oscillation of free electrons excited by a magnetic field at 690 nm in the visible frequency region. The fabrication and optical investigation of SRR arrays over such large areas will facilitate opportunities to realize advanced optical devices.

Resist Properties of Thin Poly(methyl methacrylate) and Polystyrene Films Patterned by Thermal Nanoimprint Lithography for Au Electrodeposition

Gold (Au) line patterns of 0.5 and 1 µm widths were fabricated on a silicon substrate covered with a Au thin film by thermal nanoimprint lithography with poly(methyl methacrylate) (PMMA) and polystyrene (PS), followed by Au electrodeposition with their thin films as resist masks. We described the differences in the fidelity of the deposited Au patterns for linewidth and linewidth roughness and differences in undesired Au deposition in the masked regions between thin PMMA and PS films. The linewidths of the convex deposited Au pattern were larger than those of the concave nanoimprinted polymer pattern, and the deviation from the polymer concave pattern in the case of PS was significantly smaller than that in the case of PMMA. The linewidth roughness of the Au lines deposited with a PMMA mask was markedly high owing to particle-like Au deposition in comparison with that deposited with a PS mask. Undesired Au deposition occurred even on electrode surfaces masked with the thin PMMA and PS films. It was considered from these results that the difference in the size accuracy of Au electrodeposition between PMMA and PS was responsible for the resistance in the oxygen dry-etching step of a residual layer during thermal nanoimprint lithography in addition to polymer water absorbability in Au electrodeposition.

Photochemically Grafted Polystyrene Layer Assisting Selective Au Electrodeposition

We describe the selective electrodeposition of submicrometer gold (Au) patterns achieved by a thin film resist layer of polystyrene (PS) that was exposed to ultraviolet (UV) light on a photoreactive monolayer of a benzophenone-containing alkylthiol formed on a Au-plated substrate and patterned by thermal nanoimprint lithography. The presence of a PS graft layer caused by the benzophenone monolayer photochemistry at an interface between the PS resist layer and photoreactive monolayer played the important role of suppressing the unfavorable growth of tiny Au grains in regions masked with the PS resist layer, resulting in the selective Au electrodeposition in aperture regions of PS resist patterns. The suppressive effect on selective Au electrodeposition depended on the molecular weight of PS used as a resist material. Among unimodal PSs having weight-average molecular weights (M(w)s) of 2100, 10,900, and 106,000 g mol(-1), the PS of M(w) = 10,900 g mol(-1) functioned most effectively as the resist layer. Au electrodeposition at a low current density allowed the preparation of Au lines having widths of submicrometers and a uniform height independent of line widths in resist aperture regions. Submicrometer bump structures of Au lines could be fabricated on transparent silica substrates by the subsequent wet etching of a Au electrode layer and then a chromium adhesive layer.

Surface-assisted unidirectional orientation of ZnO nanorods hybridized with nematic liquid crystals.

Inorganic semiconductor nanorods are regarded as the primary components of optical and electrical nanoscale devices. In this paper, we demonstrate the unidirectional alignment of monolayered and dispersed ZnO nanorods on a rubbed polyimide alignment layer, which was achieved by a conventional liquid crystal alignment technique. The outermost surfaces of the ZnO nanorods (average diameter 7 nm; length 50 nm) were modified by polymerization initiator moieties, and nematic liquid crystalline (LC) methacrylate polymers were grown by atom transfer radical polymerization. By regulating the densities of the polymerization initiator moieties, we successfully hybridized LC-polymer-grafted ZnO nanorods and small nematic LC molecules. The LC-polymer-modified ZnO nanorods were hierarchically aligned on the substrate via cooperative molecular interactions among the liquid crystal mesogens, which induced molecular orientation on the rubbed polyimide alignment layer.

Facile wide-scale defect detection of UV-nanoimprinted resist patterns by fluorescent microscopy

The occurrence of resist pattern defects in UV nanoimprinting with a spincoated resin thin film on a silicon wafer was studied by fluorescent microscopy using a fluorescent UV-curable liquid resin causing radical photopolymerization. The generation of nonfill defects with a surface-modified silica mold with submicrometer line cavities was compared between UV nanoimprinting atmospheres of air and pentafluoropropane. It was visualized in a rapid and nondestructive manner that nonfill defects were hardly induced by UV nanoimprinting under easily condensable pentafluoropropane atmosphere, while nonfill defects owing to bubble trap and resin adhesion to a mold surface were observed in the case of UV nanoimprinting under air atmosphere. The fluorescent microscopy using the fluorescent UV-curable resin was useful for mold inspection whether or not the submicrometer-scale mold cavities were partially filled with the resin. To investigate a resolution limit to nonfill defect, the authors examined a pattern pitch a...