Toshiaki Tamamura

Highly ordered nanochannel-array architecture in anodic alumina

The development of the ordered channel array in the anodic porous alumina was initiated by the textured pattern of the surface made by the molding process, and growth of an almost defect-free channel array can be achieved throughout the textured area. The long-range-ordered channel array with dimensions on the order of millimeters with a channel density of 1010 cm−2 was obtained, and the aspect ratio was over 150. The master for molding could be used many times, which makes it possible to overcome problems in the conventional nanolithographic technique, such as low through-put and high cost.

Self-collimating phenomena in photonic crystals

We found that self-determining collimated light is generated in a photonic crystal fabricated on silicon. The divergence of the collimated beam is insensitive to that of the incident beam and much smaller than the divergence that would be generated in conventional Gaussian optics. The incident-angle dependence of the self-collimated light propagation including lens-like divergent propagation was interpreted in terms of the highly modulated dispersion surfaces with inflection points, where the curvature changes from downward to upward corresponding to respectively a concave/convex-lens case. This demonstration is an important step towards controlling beam profile in photonic crystal integrated light circuits and towards developing “photonic crystalline optics.”

Photonic crystals for micro lightwave circuits using wavelength-dependent angular beam steering

Light-beam steering that is extremely wavelength dependent has been demonstrated by using photonic crystals fabricated on Si. The scanning span reached 50° with only a 1% shift of incident wavelength at around 1 μm. The resulting angular dispersion is two orders of magnitude larger than that achieved with conventional prisms or gratings. The application of such superprism phenomena promises to enable the fabrication of integrated micro lightwave circuits that will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.

Conditions for Fabrication of Ideally Ordered Anodic Porous Alumina Using Pretextured Al

The conditions for the fabrication of ideally ordered anodic porous alumina with a high aspect ratio were examined using pretextured Al in oxalic acid solution. The obtained anodic porous alumina has a defect-free array of straight parallel channels perpendicular to the surface. The channel interval could be controlled by changing the interval of the pretextured pattern and the applied voltage. However, the depth at which perfect ordering could be maintained depended on the anodizing conditions, that is, the hole array with a high aspect ratio could be obtained only under the appropriate anodizing voltage, which corresponded to that of the long-range ordering conditions in the oxalic acid solution. Under the most appropriate condition, ideally ordered channels with an aspect ratio of over 500 could be obtained. From these results, it was concluded that the long-range ordering conditions significantly influenced the growth of channels in anodic porous alumina even in/on the pretextured Al.

Superprism phenomena in photonic crystals: toward microscale lightwave circuits

The superprism phenomenon, the dispersion of light 500 times stronger than the dispersion in conventional prisms, was demonstrated at optical wavelengths in photonic crystals (PCs) fabricated on Si. Drastic light-beam steering in the PCs was achieved by slightly changing the incident wavelength or angle. The scanning span reached 50/spl deg/ with only a 1% shift of incident wavelength, and reached 140/spl deg/ with only a 14/spl deg/ shift of the incident angle at wavelengths around 1 /spl mu/m. The propagation direction was quantitatively interpreted in terms of highly anisotropic dispersion surfaces derived by photonic band calculation. The physics behind this demonstration will open a novel field called photonic crystalline optics. The application of these phenomena promises to enable the fabrication of integrated microscale lightwave circuits (/spl mu/LCs) on Si with large scale integrated (LSI)-compatible lithography techniques. Such /spl mu/LCs will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.

Photonic Crystal Using Anodic Porous Alumina

Two-dimensional (2D) photonic crystals in the visible wavelength region were fabricated using anodic porous alumina with a highly ordered hole array configuration. The transmission properties of an ordered triangular array of an air cylinders with high aspect ratio in alumina matrix showed a stop band in the spectrum which corresponds to the band gap in the 2D photonic crystals.

Directional lasing oscillation of two-dimensional organic photonic crystal lasers at several photonic band gaps

We fabricated a series of two-dimensional (2D) hexagonal organic photonic-crystal lasers whose lattice constant varies from 0.18 to 0.44 μm, and observed clear lasing oscillation at the four lowest band gap frequencies (M1, K2, M2, and Γ1). We used in-plane beam propagation analysis to clarify the 2D feedback mechanism at each gap frequency, which differs for different gaps. The observed K1 lasing oscillation is due to coupling of three nonparallel diffracted waves, which has a purely 2D character. This shows that photonic crystal lasers can operate with various feedback essentially different from that of conventional 1D distributed feedback lasers.

Self-repair of ordered pattern of nanometer dimensions based on self-compensation properties of anodic porous alumina

The self-repair of an ordered pattern of nanometer dimensions based on the self-compensation properties of anodic porous alumina is demonstrated. In a pretextured pattern formed on Al using the nanoindentation process with an array of convexes, the deficiency sites of the pattern were found to be compensated automatically during the anodization. Combining the self-compensation properties of the pore configuration of the anodic porous alumina with the preparation of a replica of the compensated porous structure allowed us to develop a process which has the capability of self-repairing the imperfections in the starting pattern. It was confirmed that deficiencies in the starting pattern could be repaired automatically in the Ni pattern regenerated using the self-compensated anodic porous alumina as a template.

Quantum Wire Fabrication by E-Beam Elithography Using High-Resolution and High-Sensitivity E-Beam Resist ZEP-520

We have evaluated the resolution of the positive electron-beam (E-beam) resist ZEP-520 using finely focused E-beam exposure for the application of quantum wire fabrication in a large area. Compared with the poly-methylmethacrylate (PMMA) resist conventionally used for nanofabrication, ZEP resist shows almost the same resolution under sensitivity improvement of one order of magnitude, and the throughput is increased by a factor of more than 100 by introducing a highly bright Zr/O/W thermal field emitter as an E-beam source. Other excellent performance characteristics, such as high dry-etching durability and process stability, allow us to apply ZEP resist for larger-area, high-density quantum wire fabrication. By both wet chemical etching and dry-etching combined with CBE selective growth, InGaAs nanostructures as small as 15 nm can be obtained with a pitch of 70 nm over several hundred µm squares.

Fabrication of ideally ordered anodic porous alumina with 63 nm hole periodicity using sulfuric acid

Nanochannel arrays with an ideally ordered hole configuration with a 63 nm hole periodicity and 15–40 nm hole diameter were fabricated using anodization of the pretextured Al in sulfuric acid solution. The SiC mold with an array of convexes, which was prepared by electron beam lithography, was used for the nanoindentation of the Al. The periodicity of convexes was adjusted to the self-organized periodicity in sulfuric acid solution. The obtained concaves on Al initiated the hole development during the anodization and generated the ideally ordered hole configuration with a 63 nm period. Under the appropriate anodization conditions, anodic porous alumina with an aspect ratio of over 20 was obtained.