In the last decade, photonic crystals have attracted an increasing interest, mainly because the optical field, i.e., radiation field and light propagation characteristics can be thereby controlled in principle. This is important not only in the basic science such as physics, but also in developing new devices in optoelectronics. For the latter, it is expected, for example, that using a variety of novel unique features of photonic crystals, an extremely-miniaturized planar light circuit will be developed in future. In this review article, the present status of researches in this field is highlighted. First, several pronounced features of two- and three-dimensional photonic crystals are summarized. Second, some aspects of recent development of the crystals are presented with special concern to two-dimensional ones. Third, examples of controlling optical field are presented, which include laser action, enhancement of second-harmonic generation signal, photonic crystal waveguides, and light delay component.

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Semiconductor photonic crystals are expected to realize the ultimate control of spontaneous emission, so-called Purcell effect. However, there are two important issues. One is a fine dry etching process for a high aspect ratio over 10 in submicron period structures. The other is a surface treatment for the reduction in surface recombination. With regard to these issues we have studied InP-based materials, since they have a low surface recombination velocity ν_s. We developed the etching process using the inductively coupled plasma. The surface recombination was evaluated through the lifetime measurement of photoluminescence at a wavelength over 1 µm using the phase resolved spectroscopy. We investigated various surface treatments and found that the CH₄ plasma irradiation is effective for realizing crucial ν_s of 1×10³ cm/s order. The SIMS analysis suggests that it is due to the electronic insulation near the surface by the shallow implantation of C.

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A semiconductor photonic crystal and its application to functional devices are described. The main feature of the crystal is that it has a photonic band gap and we can realize various functions, such as waveguides, cavities and so on, only by introducing artificial defects in the crystal. As one of the examples, we describe here a trapping and emission of photons by a single defect in two-dimensional (2 D) photonic crystal slab. We show the tunability of trapping wavelength by defect engineering and the possibility to realize ultra-small channel drop filters for WDM optical communication system. As another example, we show the band edge engineering, where we demonstrate that 2 D coherent lasing oscillation can be achieved by using the 2 D photonic crystal.

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We have proposed a method for fabricating photonic crystals, which is named autocloning. It is based on sputtering deposition and sputter etching. When we stack multilayer with the autocloning method on a substrate patterned periodically, we can preserve corrugated surface shape whose symmetry and periodicity reflect those of the substrate pattern. Consequently, we can realize the multi-dimensional structures automatically. Almost all the processes are carried out in a vacuum chamber without alignment process, accordingly the autocloning is simple, reliable, and suitable for mass-production. In this paper, we report the mechanisms of the autocloning and its applications to optical devices. Particularly, we describe the polarization splitters and the channel waveguides utilizing lattice modulation of the photonic crystals.

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Photomodification of transparent dielectric materials, induced by tightly focused laser beams, can be utilized for the fabrication of 3 D photonic crystal (PhC) structures. The photomodification occurs in the focal region of the beam due to multiphoton absorption. Nonlinearity of this process allows to reduce the size of the photomodified region even below the diffractive limit of the focusing optics. We utilize two types of photomodification for the PhC fabrication: (i) damage in silica glass that leaves empty voxels, and (ii) polymerization in liquid resins that leaves solid voxels. By damaging/solidifying the initial materials at precisely controlled periodic locations, we have recorded 3 D PhC structures. Using the (i) process we have recorded PhCs in silica with 3 D fcc lattice types, which exhibit clear signatures of photonic bandgaps in the near IR spectral region. With (ii) process we have recorded log-pile PhC structures, which exhibit photonic bandgaps and microcavity effects in the same spectral region.

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The molecular orientation and structure of vacuum deposited (VD) films of titanyl phthalocyanine (TiOPc) were revealed by X-ray diffraction (XRD) and electron paramagnetic resonance (EPR). The XRD pattern of the VD film of TiOPc shows a diffraction peak yielding the plane separation of 14 Å. The EPR spectra indicate that the molecular orientation strongly depends upon the deposition time. Thermal annealing at 45^oC for two hours induced an orientation change in the TiOPc films where molecule aligns perpendicular to the substrate surface. However, shorter or longer thermal process for one or three hours resulted in the formation of amorphous structure. The XRD patterns of the powder and thin film of TiOPc suggest that TiOPc is oriented perpendicular to the glass substrate. Angular variation in the linewidth of EPR spectra shows a two-dimensional spin-chain in the VD films, indicating a planer network for the spin-chain interaction. Annealing of the VD films promotes molecular reorientation in the film.

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Microscopic processes, such as adsorption and migration of an As adatom on a Ga-terminated GaAs(100) surface are investigated by first-principle calculations using density functional theory and a slab model of the surface, because the importance of the kinetics for both Ga and As adatoms has been reported. The obtained values of barrier energy are compared with those to hopping in the kinetic Monte Carlo simulation. The clarified anisotropic migration of the As adatom agrees with the result of the kinetic Monte Carlo simulation as well as the properties obtained by the scanning tunneling microscopy observation. It is shown that an As adatom is more mobile than a Ga adatom and the kinetics of both As and Ga atoms on the growing surface are important during molecular beam epitaxial growth.

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Epitaxial growth processes on a GaAs(111)A surface were investigated by using first-principles pseudopotential calculations. Stable adsorption sites of adatoms, stable microstructures formed by adatoms, and interactions among the adatoms were investigated. The formation energies of various microstructures were estimated in order to investigate relative stability of the microstructures. We found that formation of a certain stable microstructure is essential to initiate the epitaxial growth. Based on the calculation results, we propose an epitaxial growth mechanism on the (111)A surface, which can qualitatively explain the characteristic properties of the surface.

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We developed a novel processing method, the reactive templated grain growth (RTGG), for the fabrication of highly textured polycrystalline ceramics with a regular perovskite-type structure. Plate-like particles with a layered perovskite-type structure were used as a reactive template, of which the preferred orientation was preserved in the in-situ formed regular perovskite-type polycrystals after heat-treatment. The textured ceramics exhibited a brick-wall-like microstructure and enhanced piezoelectric properties. The mechanism of conversion from a layered perovskite-type template into a regular perovskite-type product was rather topotactic. Furthermore, the RTGG processing was also applied to the fabrication of more complex bismuth layer-structured (BLS) ceramics with a preferred {001} orientation. The textured BLS ceramics exhibited 3–4 times as high piezoelectric coefficients and electromechanical coupling factors as the conventionally sintered BLS ceramics of the same compositions. The textured BLS ceramics showed a layered-slate-like microstructure. The unique microstructures of the two types of textured ceramics ({100}-oriented regular perovskite and {001}-oriented BLS) reflected anisotropy in terms of the surface energy for the crystal structures of regular perovskite and BLS.

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