Laser diode integrated with a low radiation loss thickness-tapered beam expander for highly efficient fiber coupling

The optimal structure of a laser diode monolithically integrated with a thickness-tapered beam expander waveguide is demonstrated by analyzing the relationship between fiber coupling efficiencies and radiation losses. It is also found that mode conversion loss is lowered in a ridge waveguide structure than in a buried hetero structure under equivalent fiber coupling. A fabricated ridge waveguide device based on this design shows threshold current as low as 16 mA and narrow beam divergences of 13° and 12°.     

Polarization memory of luminescence from photo-generated carriers in aSixC1−x:H (x∼0.2)

Photoluminescence of aSi_xC_1?x:H (x～0.2) is studied with a particular interest on the localization of carriers. Taking the polarization memory of luminescence as a probe, we clarify the localized character of photo-generated carriers as functions of excitation photon energy, emitted photon energy, temperature and time. The picosecond time-resolved measurement of the polarization memory is shown useful to study dynamical relaxation processes of carriers in disordered systems.     

Development of variable‐magnification X‐ray Bragg optics

A novel X‐ray Bragg optics is proposed for variable‐magnification of an X‐ray beam. This X‐ray Bragg optics is composed of two magnifiers in a crossed arrangement, and the magnification factor, M, is controlled through the azimuth angle of each magnifier. The basic properties of the X‐ray optics such as the magnification factor, image transformation matrix and intrinsic acceptance angle are described based on the dynamical theory of X‐ray diffraction. The feasibility of the variable‐magnification X‐ray Bragg optics was verified at the vertical‐wiggler beamline BL‐14B of the Photon Factory. For X‐ray Bragg magnifiers, Si(220) crystals with an asymmetric angle of 14° were used. The magnification factor was calculated to be tunable between 0.1 and 10.0 at a wavelength of 0.112 nm. At various magnification factors (M≥ 1.0), X‐ray images of a nylon mesh were observed with an air‐cooled X‐ray CCD camera. Image deformation caused by the optics could be corrected by using a 2 × 2 transformation matrix and bilinear interpolation method. Not only absorption‐contrast but also edge‐contrast due to Fresnel diffraction was observed in the magnified images.     

Quantitative characterization of spatiotemporal patterns II

Disorderness of spatiotemporal patterns which are obtained by nonlinear partial differential equations is characterized quantitatively. The mean Lyapunov exponent for a nonlinear partial differential equation is given. The local Lyapunov exponent which is a finite time average of the mean Lyapunov exponent is shown to have close relation to the spatiotemporal patterns. It is suggested that the systems which are described by nonlinear partial differential equations are characterized statistically through the probability distribution function of the local Lyapunov exponent.     

Natural Convection of Temperature-Sensitive Magnetic Fluids in Porous Media

In this article, natural convection of a temperature-sensitive magnetic fluid in a porous media is studied numerically by using lattice Boltzmann method. Results show that the heat transfer decreases when the ball numbers increase. When the magnetic field is increased, the heat transfer is enhanced; however, the average wall Nusselt number increases at small ball numbers but decreases at large ball numbers due to the induced flow being more likely confined near the bottom walls with a high number of obstacles.     

Structure determination of the Cu(0 0 1)–c(4 × 4)-Sn surface by low-energy electron diffraction

Low-energy electron diffraction (LEED) have been used to determine the Cu(0 0 1)–c(4 × 4)-Sn structure formed at 300 K. It is demonstrated that a structural model suggested by scanning tunneling microscopy observations is correct: The model consists of one substitutional Sn atom and four Sn adatoms in the unit cell. Optimum parameters of the determined c(4 × 4) structure reveal that Sn adatoms laterally are displaced by 0.30 Å away from ideal fourfold-hollow sites along the 〈100〉 directions. It is proposed that such displacements of the Sn adatoms cause the formation of a network of octagonal rings on Cu(0 0 1). The substitutional Sn atom is located at each center of the octagonal rings. The formation conditions of the network are discussed.     

Chaos in Fiber-Optic Stimulated Brillouin Scattering Dependent on Pump Power, Nonlinear Refractive Index, Feedback Power, and Fiber Length

Dependence of Stokes dynamics in stimulated Brillouin scattering generated in optical fibers on the pump power, the nonlinear refractive index, the fiber length, and the feedback powers of the pump and the Stokes is numerically analyzed. The Stokes power fluctuation becomes complicated and chaotic as the nonlinear refractive index and the input pump power increase in a fiber of sufficient length. The Stokes dynamics is less complicated and the chaotic region decreases in a pump power-reflectivity domain in a short fiber. The chaotic behavior appears without regular tendency in relation to the feedback power. Taking into account the pump feedback, the chaotic region expands and some Stokes behaviors are different in the P_P-R domain graph, compared with the case with the Stokes feedback alone, although this feedback does play an essential role in Stokes dynamics.