Yoshimasa Kawata

Use of two-photon absorption in a photorefractive crystal for three-dimensional optical memory

We describe the use of two-photon absorption in a photorefractive crystal for recording bit data in multilayered optical memory. A short-pulse near-infrared laser is used for generating the photorefractive effect by two-photon absorption. We succeeded in recording and reading seven layers of data in a LiNbO(3) crystal with a lateral resolution (distance between bits) of 5microm and an axial resolution (distance between layers) of 20 microm .

Feasibility of molecular-resolution fluorescence near-field microscopy using multi-photon absorption and field enhancement near a sharp tip

Aperture-based near-field microscopy suffers from fundamental limitations imposed by the electromagnetic skin depth of the aperture material and a rapidly decreasing throughput as the aperture is made smaller. Apertureless approaches without these limitations have been demonstrated for coherent imaging but are not easily applicable to incoherent processes such as fluorescence or Raman scattering and to photochemical surface modification. Using multi-photon processes in conjunction with the field enhancement that occurs at a sharp tip in close apposition to a substrate should permit substantial localization of absorption and excitation to a nm sized volume. The expected enhancement of the optical field at the tip edge is calculated here for various combinations of metallic and nonmetallic tip and substrate materials. It is estimated that when using 100 fs pulses repeating at 100 MHz average laser powers of about 10 mW should be sufficient to reach saturating field strengths for three-photon absorption. Ste...

Electron beam excitation assisted optical microscope with ultra-high resolution

We propose electron beam excitation assisted optical microscope, and demonstrated its resolution higher than 50 nm. In the microscope, a light source in a few nanometers size is excited by focused electron beam in a luminescent film. The microscope makes it possible to observe dynamic behavior of living biological specimens in various surroundings, such as air or liquids. Scan speed of the nanometric light source is faster than that in conventional near-field scanning optical microscopes. The microscope enables to observe optical constants such as absorption, refractive index, polarization, and their dynamic behavior on a nanometric scale. The microscope opens new microscopy applications in nano-technology and nano-science.

Reflection-type confocal readout for multilayered optical memory

We present a multilayered optical memory for use in reading data with a confocal reflection microscope system. We use a recording medium in which photosensitive thin films and nonphotosensitive transparent films are stacked alternately. Since the photosensitive films are thinner than the depth of focus of the recording beam, the spatial frequency distribution of the recorded bit data is extended in the axial direction. The extended distribution overlaps the coherent optical transfer function of the reflection-type confocal microscope. Urethane-urea copolymer film is used as a photosensitive material. The recording and reading of two layers are demonstrated.

Non-optically probing near-field microscopy

We present a near-field optical microscope without the use of a probe for illumination or detection or scattering of the optical fields. In our system the optical fields near specimens are converted to the topographical change of a photosensitive film, and then the topography is detected with an atomic force microscope. Urethane-urea copolymer films are used for the conversion material from the optical fields to the topography. We succeeded in imaging with a resolution higher than 50 nm.

Three-dimensional patterned media for ultrahigh-density optical memory

We report a recording medium in which a three-dimensional nanoscale structure can be photofabricated for multilayered optical memory using a two-photon process. By fabricating the structures in the medium, we can control the shape of recorded bits and, in effect, their spatial frequency distribution. We succeeded in recording bits with a 0.5μm interval in any particular plane and 2.0μm interval between successive layers. Thus, storage density of 2.0Tbits∕cm3 is achieved.

Fluorescence enhancement with deep-ultraviolet surface plasmon excitation

We report the experimental demonstration of fluorescence enhancement in fluorescent thin film using surface plasmon excitation in deep-ultraviolet (deep-UV) region. Surface plasmon resonance in deep-UV is excited on aluminum thin film in the Kretschmann-Raether geometry. Considering the oxidation thickness of aluminum, the experimentally measured incident angle dependence of reflectance show good agreement with Fresnel theory. Surface plasmon resonance was excited at the incident angle of 49 degrees for 266 nm p-polarized excitation light on the film of 18 nm-thick aluminum with 6.5 nm-thick alumina. Fluorescence of CdS quantum dots coated on this aluminum film was enhanced to 18-fold in intensity by the surface plasmon excitation.

Nanofabrication induced by near-field exposure from a nanosecond laser pulse

We demonstrate a photoinduced nanofabrication method, applicable to dimensions beyond the diffraction limit, by employing a nanosecond laser pulse with a high peak power. The second-harmonic wave of a Nd: yttrium–aluminum–garnet (YAG) laser with a 5 ns pulse width was irradiated onto the sample, which consisted of a hexagonal arrayed monolayer of nanoparticles on an azopolymer film. Topographical changes in the surface after irradiation were observed by atomic force microscopy. A transcription of the arrayed structure in the form of a dent structure was attained. In the case of 100 nm nanoparticles, the resolution of the transcription was beyond the diffraction limit. The dent depth and diameter were changed depending on the diameter of the nanoparticles and the irradiation power used. A depth analysis indicates a threshold for the ablation process.

Dynamic and high-resolution live cell imaging by direct electron beam excitation.

We propose a direct electron-beam excitation assisted optical microscope with a resolution of a few tens of nanometers and it can be applied for observation of dynamic movements of nanoparticles in liquid. The technique is also useful for live cell imaging under physiological conditions as well as observation of colloidal solution, microcrystal growth in solutions, etc. In the microscope, fluorescent materials are directly excited with a focused electron beam. The direct excitation with an electron beam yields high spatial resolution since the electron beam can be focused to a few tens of nanometers in the specimens. In order to demonstrate the potential of our proposed microscope, we observed the movements of fluorescent nanoparticles, which can be used for labelling specimens, in a water-based solution. We also demonstrated an observation result of living CHO cells.

Fabrication of multilayered photochromic memory media using pressure-sensitive adhesives

We fabricated a multilayered medium by a laminating process with pressure-sensitive adhesives. It was possible to reduce the thickness variations of both photosensitive layers and transparent layers by applying laminating films. This method is easy to use to pile up many layers for a multilayered medium. We fabricated twenty recording layers and demonstrated the capability to record with significant reading results. The recorded data in each layer were read out without crosstalk. We evaluated the signal-to-noise ratio and crosstalk between neighboring layers. It was found that the signal-to-noise ratio of a multilayered medium was higher than 50 dB.