We demonstrate controlled squeezing of visible light waves into nanometer-sized optical cavities. The light is perpendicularly confined in a few-nanometer-thick SiO2 film sandwiched between Au claddings in the form of surface plasmon polaritons and exhibits Fabry-Perot resonances in a longitudinal direction. As the thickness of the dielectric core is reduced, the plasmon wavelength becomes shorter; then a smaller cavity is realized. A dispersion relation down to a surface plasmon wavelength of 51 nm for a red light, which is less than 8% of the free-space wavelength, was experimentally observed. Any obvious breakdowns of the macroscopic electromagnetics based on continuous dielectric media were not disclosed for 3-nm-thick cores.

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The existence of the critical curve for p-q systems for nonlinear wave equations was already established by D. Del Santo, V. Georgiev, and E. Mitidieri [1997, Global existence of the solutions and formation of singularities for a class of hyperbolic systems, in``Geometric Optics and Related Topics'' (F. Colombini and N. Lerner, Eds.), Progress in Nonlinear Differential Equations and Their Applications, Vol. 32, pp. 117 139, Birkha user, Basel] except for the critical case. Our main purpose is to prove a blow-up theorem for which the nonlinearity ( p, q) is just on the critical curve in three space dimensions. Moreover, the lower and upper bounds of the lifespan of solutions are precisely estimated, including the sub-critical case. 2000Academic Press

The controlled plasmon resonance in nanometer-sized optical cavities with a closed end has been demonstrated. A nanosheet plasmon cavity is a metal/insulator/metal waveguide with a finite length. Its lowest-order transverse-magnetic guided mode is reflected at the ends and exhibits the Fabry-Pérot resonance. In this study, one of the ends was closed by an obliquely evaporated Au film, and the so-called organ pipe resonances were observed as reflection dips. Since such closed configurations offer a higher field enhancement and higher detection efficiency of the scattered light, they are promising as fundamental structures for Raman enhancement.

We have successfully prepared silicon quantum dots/amorphous silicon carbide multilayers by the thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon-rich hydrogenated amorphous silicon carbide (a-Si 1Àx C x ) multilayers. Raman scattering spectroscopy and transmission electron microscopy (TEM) revealed that silicon quantum dots were formed in only a-Si 1Àx C x layers. We also found that the size of silicon quantum dots can be controlled by the thickness of a-Si 1Àx C x layers.

We prepared size-controlled silicon quantum dots superlattices (Si-QDSLs) by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon-rich hydrogenated amorphous silicon carbide (a-Si 1þx C:H) multilayers. Transmission electron microscope (TEM) observation revealed that silicon quantum dots were formed in only a-Si 1þx C:H layers. The size of silicon quantum dots can be controlled by the thickness of the a-Si 1þx C:H layers. It was found that hydrogen plasma treatment (HPT) significantly enhanced the photoluminescence of the Si-QDSLs. The luminescence peaks shifted to shorter wavelength with decreasing the diameter of the silicon quantum dots in the Si-QDSL.

The effects of conduction band grading in a Cu(In,Ga)Se2 (CIGS) thin film with an average bandgap of 1.4 eV on solar cell performance were investigated by changing the minimum bandgap (E gmin) and its position, employing the software wxAMPS. The calculation was carried out, taking CdS/CIGS heterointerface recombination into account, by incorporating a thin defective layer into the interface. For CIGS with a flat conduction band profile, i.e., without conduction band grading, the effects of the valence band offset (ΔE V) between a CdS layer and a CIGS layer with bandgaps from 1.05 to 1.6 eV were investigated. It was found that efficiency was increased by up to 3% by changing the conduction band profile from flat to double-graded, with a deep notch located in the vicinity of the CdS/CIGS interface. On the other hand, efficiency was increased by over 6% and reached 22% by increasing ΔE V up to 0.3 eV in the case of CIGS with a bandgap of 1.35 eV. Finally, an efficiency of 23.4% was achieved by combining a single-graded conduction band profile with a ΔE V of 0.3 eV. This result shows that a single-graded conduction band profile is required for high-efficiency wide-bandgap CIGS solar cells if the recombination at the CdS/CIGS heterointerface can be suppressed.

Theoretical studies have revealed that coupling with a dielectric substrate significantly affects the transmission spectrum of a 2D photonic crystal of monolayer dielectric spheres. The dielectric constant of a semiinfinite substrate has been found to have a threshold, above which dips in the transmission spectrum broaden drastically. A substrate of finite thickness yields additional dips in the spectrum corresponding to localized eigenstates within the substrate. The transmission spectrum is well explained by the anticrossing of the eigenstates of a monolayer and a substrate.Research on photonic crystals 1 ͑PC's͒ has been focused for many years on the realization of a complete-gap PC, i.e., a PC having a common photonic gap in all directions. A complete-gap PC could be used, for example, in fundamental QED experiments to control the atomic lifetime by the suppression of spontaneous emission. 2 A variety of possible complete-gap PC's have been found in theoretical studies. 3 Since gaps appear as a result of interference, long-range order in all directions is indispensable for three-dimensional ͑3D͒ PC's. To realize complete-gap PC's, therefore, highly sophisticated methods such as self-assembly or autocloning 4 have been developed in addition to the advanced technology of lithography. 5 At present, however, it is still difficult to obtain 3D PC's of high quality.In contrast, 2D slablike PC's with long-range order, 6 a typical example being a monolayer of periodically arrayed dielectric spheres on a substrate, can easily be fabricated. Many theoretical 7 and experimental 8-10 studies on the transmission spectra and photonic band structures of such PC's are currently being carried out. A slablike PC is different from ordinary 2D PC's composed of infinitely long parallel cylinders, in that it has a finite thickness in one direction. Consequently, there is an energy dissipation in that direction, which gives rise to a finite lifetime of its eigenstates. There is also a significant enhancement of the electric field near the surface ͑near field͒ by diffracted evanescent waves due to the 2D periodicity. 7 This enhancement can be observed by the use of a scanning near-field optical microscope. 11 There is, in fact, a strong demand for the development of new photon technology by utilizing enhanced near fields such as laser manipulation of atoms. 12 The relevant wavelength of a slab-type PC at present is comparable with the 2D periodicity. For example, it lies in the lower Mie resonance region (lХ3,4) for monolayer spheres. 13 An electric field of such lower resonances is not strongly localized within the spheres and leaks out of the monolayer. This extended nature of the electric field is expected to induce strong coupling with the substrate. However, this coupling was not taken into account in our previous study. 7 A few theoretical attempts 14 have been made to ana-lyze the optical properties of a 2D periodic system on a semiinfinite substrate, but all of them lack the viewpoint of PC's such as band structure. There has...