Kazuhiko Yamasaki

Recording by microexplosion and two-photon reading of three-dimensional optical memory in polymethylmethacrylate films

We report the recording of a three-dimensional (3D) pattern in bulk polymethyl metacrylate (PMMA) films by microexplosion induced by a tightly focused (numerical aperture of oil-immersion objective was 1.3) single laser shot. Recording was done by picosecond (ps) single-pulse irradiation at 532 nm and by femtosecond at a 400 nm wavelength. The light-induced damage threshold of PMMA was found at ∼4 MW/cm2 for ps damaging. Readout of the 3D pattern was carried out by the recording of the transmission and the photoluminescence (PL) of bits. We demonstrate the possibility exciting PL of the optically damaged area inside the PMMA by one- and two-photon absorption.

Glass transition-assisted microstructuring in polystyrene

We report on the use of a shape transition process to resize patterns prerecorded inside polystyrene film. The shape transition, which is shrinkage in two dimensions and expansion in the third (the volume is conserved), was brought about by annealing polystyrene above the glass transition temperature. This caused approximately twofold lateral shrinkage and fourfold axial stretching of the film, inside which micrometer-sized patterns had been recorded by femtosecond pulses. The transformation of these patterns corresponded to the macroscopic shape transformation of the film. The shape transition was also used to transform a diffraction grating. This allowed the transformation to be confirmed by the corresponding change in diffraction efficiency. The applicability of the shape transition process to nanofabrication is discussed.

Microstructuring of silica and polymethylmethacrylate glasses by femtosecond irradiation for MEMS applications

We report the fabrication of complex 3D microstructures in silica and polymethylmethacrylate glass by a combination of femtosecond laser microfabrication and chemical wet etching techniques. It is demonstrated that fabrication of interconnected network of channels having lengths of about 200 μm, and diameters as small as 10 μm is possible due to the enhanced etching selectivity (typically 20 60) in the laser-irradiated regions. Thus, it becomes feasible to form 3D micro-fluidic and photonic crystal structures in transparent glass-like materials using this approach. In addition, preliminary results on microstructuring of rubber are presented.

Fabrication of gallium nitride grating by interferometric irradiation using focused femtosecond laser

We report the optical modification of gallium nitride by femtosecond laser irradiation. A 5-nm-band-gap shift, observed by spectral ellipsometry, results a reduction of 0.01 in the refractive index at a wavelength of 400 nm. An indistinct circular pattern was also observed using nano-beam diffraction. These results suggest that femtosecond-laser irradiation induces stress–strain, rather than slight amorphization. We also observed that the angle of first-order diffraction from the grating with a 1.0 µm period, fabricated by interferometric irradiation using a femtosecond laser without any damage, corresponds to the calculated angle. The first-order diffraction efficiency for a wavelength of 403 nm was estimated to be 4.0×10-3%.

Three-Dimensional Optical Memory in a Photoacid-Induced Recording Medium

We report a photoacid-induced write-once read-many recording medium based on poly(methyl methacrylate) for three-dimensional (3D) optical memory. Three-layered recording was achieved using only an 800 nm-femtosecond oscillator with a 82-MHz-repetition rate without structural damage. Bits of arbitrary length used in the present modulation method of Compact Disc type memory can be recorded by photo-modification in the medium. The results suggest that the medium is an attractive candidate to solve the technical problems of 3D recording, in which micro-structural integrity of medium is altered.

Transient light-induced refractive index change made by laser microfabrication in nitroaniline-doped PMMA film

We report the observation of high light-induced change in refractive index (recognizable by observation in conventional microscope) in PMMA film doped with an optically non-linear dye 2-nitroaniline (NO2(C6H4)NH2 abbreviated as 2NA). The optically altered micrometer-sized regions were fabricated by single-shot radiation of 120 fs laser pulses into doped PMMA film using high numerical aperture 1.3 and high magnification x100 objective lens. The doping of films can be achieved in a wide range of 2NA concentrations (up to 40 wt%) without precipitation. This allows to control a storage time of an optically altered region up to one month by the adjusting the energy of the femtosecond (fs) recording pulse at 800 nm. Typical recording energy was 10-80 nJ/pulse at the point of irradiation. Total recovery of transmission of the PMMA2NA film was confirmed by optical transmission measurements in a microscope. The light induced damage threshold (LIDT) (for permanent damage) was increased more than by four times (up to 40 nJ/pulse) when 2NA doping were ca.1e wt%. While the LIDT for transient damage was decreased by 1.5-2 times. Total optical recovery was observed single exponential with decay time of ca. 0.5-1 minute for moderate irradiation intensities (0.1xLIDT of permanent damage). The damage induced with at the higher intensities lasts up to a month, but the recovery was not total (residual transmission changes were observable). The phenomenon can be applied for the optical memory, photonic crystal, and micro-mechanical applications. The underlying mechanism of the phenomenon is discussed in terms of anelastic (alpha) and (beta) -relaxation (polymer backbone and side chain relaxation, respectively).

Three-dimensional optical recording in tert-butoxycarbonyl-protected quinizarin methacrylate polymers

We report photoacid-induced nonerasable recording in tert-butoxycarbonyl-protected quinizarin methacrylate polymers for three-dimensional optical memory. 1 m/s-speed recording was achieved using an 800 nm femtosecond laser at an 80 MHz-repetition rate, without a postexposure bake after laser irradiation. Three-layer recording indicates that no optical damage during recording enables modulation of the Compact Disc type memory. The linewidth of recorded marks of 0.4 µm indicates a 49 Gbits/cm2 areal density when the numerical aperture is 1.35.