Exciton characteristics of GaTe single crystals grown by vapor-phase transport were studied by optical measurements. A hydrogenlike exciton series up to nϭ4 was clearly observed in the absorption spectra at 2 K. In the nϭ1 exciton energy region three types of exciton lines were found. By analyzing microphotoluminescence and micro-Raman-scattering spectra on the basis of group theory, it was clarified that these exciton lines are not due to different polytypes but to intrinsic exciton states. Furthermore, optical-absorption spectra in a magnetic field at 4.2 K were measured. In the Voigt configuration, one and two components for Eʈb and EЌb polarizations, respectively, were observed in the nϭ1 and 2 exciton lines. These magnetic-field dependencies cannot be interpreted on the basis of the previously proposed L-S coupling regime. The electronic band structure of GaTe was studied by the ab initio tight-binding linear muffin-tin orbitals method. It was found that GaTe is a direct-gap semiconductor and that the band edge is located at an M point of the Brillouin zone. From a comparison of exciton absorption spectra and the calculated band structure, the existence of the three types of excitons was interpreted from the viewpoint of j-j coupling. Our model calculation was also able to explain the Zeeman splitting and the diamagnetic shift of the exciton peak energies.

The energy states of a particle confined in a narrow space are discrete and lined up in the order of n=1,2,3,.... However, if the particle interacts with a radiation field, modification of the energy, referred to radiative correction, will occur and quantum states are expected to interchange. We investigated the center-of-mass confinement of excitons in CuCl films by a new method based on "nondegenerate two-photon excitation scattering." The energies of confined excitons in a 19.3 nm thick film are found to be lined up in the order of n=1,3,5, because the radiative correction is very weak. On the other hand, in a 35.3 nm thick film, in which the radiative correction becomes large, the energies of quantum states are ordered n=2,3,4,1,5,7. This interchange is confirmed by comparing the calculated scattering spectra, in which radiative correction is taken into account, with the measured ones.

Analytic solutions to the superfocusing modes of surface plasmon polaritons in a conical geometry are theoretically studied using an ingenious method called the quasi-separation of variables. This method can be used to look for fundamental solutions to the wave equation for a field that must satisfy boundary conditions at all points on the continuous surface of tapered geometries. The set of differential equations exclusively separated from the wave equation can be consistently solved in combination with perturbation methods. This paper presents the zeroth-order perturbation solution of conical superfocusing modes with azimuthal symmetry and graphically represents them in electric field-line patterns.

We proposed and demonstrated a simple but deterministic scheme for generating polarization-entangled photon pairs at telecommunication wavelengths with type-II quasi-phase-matched spontaneous parametric down-conversion (QPM-SPDC) having two poling periods. We fabricated a LiNbO3 crystal having two poling periods so as to generate entangled photons at two wavelengths, i.e., 1506 nm and 1594 nm. We characterized the two-photon polarization state with state tomography and confirmed that the state was highly entangled.