Waveguide produced by fiber on glass method using Er-doped tellurite glass

We report the fabrication of waveguides using the fiber on glass (FOG) method. Taking advantage of a Thermal Mechanical Analyzer (Shimadzu TMA-50), we were able to produce a new type of waveguide by coupling an erbium doped fiber core onto a planar glass substrate. Both optical fiber core and substrate were fabricated from tellurite glass. Important thermal characteristics of the substrate and fiber like the transition temperature T_g, the temperature for the crystallization onset T_x and the maximum crystallization temperature T_c were determined by Differential Thermal Analysis (DTA). The thermal expansion coefficient of the tellurite glass was determined by Thermal Mechanical Analysis (TMA).     

The electronic structure and magnetism of GdSi2 by first-principles study

We have investigated electronic and magnetic properties of hexagonal, tetragonal, and orthorhombic GdSi₂, using the full-potential linearized augmented plane-wave method based on general gradient approximation for exchange-correlation potential. Antiferromagnetic (AFM) states of the GdSi₂ are found from total energy calculations to be energetically more stable, compared to ferromagnetic (FM) states in all of the considered present crystal structures. It is in good agreement with an experimental result. The calculated magnetic moments of valence electrons of the Gd atoms are 0.16, 0.14, and 0.14 μ_B for hexagonal, tetragonal, and orthorhombic crystal structures in AFM states, respectively, and the Si atoms are coupled antiferromagnetically to the Gd atoms irrespective of crystal structure even though their magnitudes are negligible.     

Self Mixing Fringes of Vertical-Cavity Surface-Emitting Lasers under Dual Reflector Feedback

The self-mixing fringes which shift due to every one-twentieth wavelength displacement of the target are observed. Taking advantage of the dual reflectors in the external cavity of lasers, the resolution of the sensors has been improved by 10 times. The role of the each reflector has been discussed in detail.     

Temperature dependence of coherent population trapping resonances

We measure the properties of coherent population trapping (CPT) resonances in Cs vapor cells as a function of temperature. We expected the CPT signal to increase with higher vapor density, but instead the signal fades away above a certain density. Two possible density-dependent explanations are discussed: spin-exchange collisions, which are found to give no relevant contribution at the temperatures considered here, and increased absorption due to the optical thickness of the vapor. The dependence of the dark-line resonance amplitude as a function of cell temperature can be well represented by a simple model based on the optical thickness of the vapor as a function of temperature. Received: 30 November 2001 / Published online: 7 February 2002     

Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements

We introduce a novel concept for optical frequency measurement and division which employs a Kerr-lens, mode-locked laser as a transfer oscillator whose noise properties do not enter the measurement process. We experimentally demonstrate that this method opens up the route to phase-link signals with arbitrary frequencies in the optical or microwave range while their frequency stability is preserved. Received: 23 July 2001 / Published online: 29 November 2001     

500-Hz two-photon Ramsey fringes with a SF6 beam: towards a new frequency standard in the 30-THz spectral region

A two-photon Ramsey-fringe experiment with a supersonic beam of SF₆ has been performed with an interzone distance of up to 50 cm. Using a He-seeded beam with 50% of SF₆, the two-photon transition P(4)E⁰ in the 2ν₃ band reveals its magnetic hyperfine structure and the periodicity of the fringes is 500 Hz. The strength of the central fringe of the main hyperfine component corresponds to a flux of 10¹⁰–10¹¹useful molecules/s, which is very promising for a new frequency standard in the 30-THz spectral region. Received: 27 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Atomic force microscopy of photochemistry and the Baeyer–Villiger reaction of 4,4′-dimethylbenzophenone in the solid state

Nanostructures are formed by photodimerization of crystalline 4,4^′-dimethylbenzophenone (1) through intermolecular hydrogen abstraction and Baeyer–Villiger reaction of 1 with m-chloroperoxybenzoic acid (MCPBA) (5) in the solid state. Atomic force microscopy (AFM) reveals that the crystal face (001) of 1 during photodimerization exhibits volcanoes, whereas the same face (001) of 1 yields both craters and volcanoes all over the surface from the contact edge of the crystals during the Baeyer–Villiger reaction. All the experimental results are correlated with the bulk crystal structure. Molecular interpretation of the AFM features of 1 is given. Received: 18 April 2000 / Revised version: 26 July 2000; accepted: 27 July 2000 / Published online: 30 November 2000     

Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples

Opaque samples are imaged by Scanning Nearfield Optical Microscopy (SNOM) in reflection mode: A quartz glass fiber tip is used both to illuminate the sample and to collect light locally reflected from or emitted by the surface. The collected light is coupled out by a 2×2 fiber coupler and fed into a grating spectrometer for spectral analysis at each sampled point. The tip-sample distance is controlled by a shear-force feedback system. The simultaneous measurement of topography and optical signals allows an assessment of imaging artifacts, notably topography-induced intensity changes. It is demonstrated that an optical reflectance contrast not induced by topographic interference can be found on suitable samples. Local spectral analysis is shown in images of a photoluminescent layer.     

Laser multiphoton mass spectroscopy of organometallic compounds: State selective and mass resolved detection of neutral fragment tellurium atoms from C2H5TeC2H5

Typical features in laser multiphoton ionization of organometallic compounds are well evident in the case of the diethyltelluride C₂H₅TeC₂H₅ molecule. The use of a tunable dye laser coupled with time-of-flight (TOF) mass spectroscopy has allowed to establish that a large amount of tellurium is eliminated from the parent molecule as a neutral atom either in its ground or low excited states. Sharp two- and three-photon atomic Te resonances, which give origin to extraproduction of Te⁺ ions, have been identified in the optical spectra measured by varying the laser wavelength.     

Dead time correction of photon correlation functions

The dynamic range of single photon counting measurements in quasi elastic light scattering is restricted by detector and counter dead time effects. While distortions of single interval statistics have been treated at great length, only lowest order corrections or very special cases of dead time effects on temporal correlation functions were computed in the past.Dead times result in a direct distortion of correlograms on time scales comparable to the dead time. This effect exists even at low count-rates. It is independent of the count rate for paralyzable systems. Nonparalyzable systems show a count rate dependence with increasing correlation times at high count rates.Furthermore, counting saturation produces additional distortions extending to all lag times. These distortions are computed for the rather general case of -distributed intensities with arbitrary shape of the photon correlation function. Such signals are commonly found in multiparticle homodyne experiments with a finite size detector, i.e. arbitrary value of the intercept or contrast of the correlogram. Exact results are provided for the paralyzable system including the effect of fluctuating dead times. The latter case is then used to compute a useful approximation for nonparalyzable systems as well.