Three-dimensional recording by tightly focused femtosecond pulses in LiNbO₃

The authors report on a three-dimensional single-shot optical recording by 150fs pulses at 800nm wavelength in Fe doped LiNbO₃. The rewritable bits (2ₓ×2y×8zμm³) are demonstrated. The highest refractive index modulation of ∼10⁻³ per single pulse has been formed by preferential photovoltaiceffect at close to the dielectric breakdownirradiance of ∼TW/cm² and was independent of polarization (in respect to the c axis). The achievable refractive index modulation is evaluated and the recording mechanisms are discussed.One of the authors M.S. thanks the Matsumae International Foundation for the research fellowship. Another author E.G.G. acknowledges support of the Australian Research Council through its Center of Excellence

The Homogeneity of Interstellar Oxygen in the Galactic Disk

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of magnitude in mean sight line density, and that extend up to 6.5 kpc in length. Consequently, we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight line density and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with large mean density intervals; it is centered near a density of 1.5 cm^-3 and is similar to trends evident in gas-phase abundances of other elements. Paths less dense than the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or 390+/-10 ppm), which is consistent with averages determined for several long, low-density paths observed by STIS (Andre et al. 2003) and short low-density paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The datapoints for low-density paths are scattered more widely than those for denser sight lines, due to O/H ratios for paths shorter than 800 pc that are generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication in ApJ, tentatively in Oct 200

The effective temperature scale of FGK stars. II. Teff : color : [Fe/H] calibrations

We present up-to-date metallicity-dependent temperature vs. color calibrations for main sequence and giant stars based on temperatures derived with the infrared flux method (IRFM). Seventeen colors in the following photometric systems: UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO, Hipparcos-Tycho, and 2MASS, have been calibrated. The spectral types covered range from F0 to K5 (7000 K<Teff<4000 K) with some relations extending below 4000 K or up to 8000 K. Most of the calibrations are valid in the metallicity range -3.5<[Fe/H]<0.4, although some of them extend to as low as [Fe/H]=-4.0. All fits to the data have been performed with more than 100 stars; standard deviations range from 30 K to 120 K. Fits were carefully performed and corrected to eliminate the small systematic errors introduced by the calibration formulae. Tables of colors as a function of Teff and [Fe/H] are provided. (Abridged)Comment: To appear in ApJ. For online tables and figures, see http://webspace.utexas.edu/ir68/tef

Dynamics of Bloch Oscillations: Influence of Excitation Conditions

The dynamics of the Bloch oscillations in GaAs/Al(0.3)Ga(0.7)As superlattices are studied experimentally applying spectrally-resolved four-wave mixing in a self-diffraction geometry. We have found that the interband dephasing rate and the decay rate of the Bloch oscillations have different dependencies on excitation density. We have explored the dynamical features of the Bloch oscillations depending on the number of Wannier-Stark states excited. The conditions for observation of the Bloch oscillations and their harmonics are determined, the destructive role of excitons in the decay of the Bloch oscillations is demonstrated

Direct measurement of the spatial displacement of Bloch-oscillating electrons in semiconductor superlattices

We present a novel experimental technique which allows to precisely measure the spatial displacement of Blochoscillating electrons in semiconductor superlattices as a function of time: The dipole field caused by the motion of the electrons is traced by small shifts of the Wannier-Stark ladder states. The electron wave packet displacement can then be derived from these shifts with the excitation density as the only free parameter. Using this method, we show that the optically generated electron wave packets perform harmonic oscillations, as predicted by Zener for the semiclassical motion of electrons in 1934. The absolute amplitudes of the wave packets depend inversely on the static field and are close to the values expected from semiclassical theory

New features of spatial-temporal dynamics of Bloch oscillations

A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn lead to a formation of a time-varying coherent wave packet. A real spatial oscillations of the excited wave packet identified quantum beats of the Wannier-Stark states as Bloch oscillations. Using a new technique for measurement of the spatial dynamics of Bloch wave packets in semiconductor superlattices we investigate the features of spatial-temporal dynamics on the initial experimental conditions and the composition of the wave packet

Reversible photomodification of LiNbO 3 and LiTaO 3 by femtosecond laser pulses

We present experimental and theoretical study of refractive index modification induced by femtosecond laser pulses in photorefractive crystals. The single pulses with central wavelength of 800 nm, pulse duration of 150 fs, and energy in the range of 6-130 nJ, tightly focused into the bulk of Fe-doped LiNbO3 and stoichiometric LiTaO3 crystals induce refractive index change of up to about 10-3 within the volume of about (2,0 × 2,0 × 8,0) μm3. The photomodification is independent of the polarization orientation with respect to the crystalline c-axis. The recorded region can be erased optically by a defocused low-intensity single pulse of the same laser. Recording and erasure can be repeated at the same position many times without loss of quality. These findings demonstrate the basic functionality of the ultrafast three-dimensional all-optical rewritable memory. Theoretically they are interpreted by taking into account electron photogeneration and recombination, as well as formation of a space-charge field. The presented analysis indicates dominant role of photovoltaic effect for our experimental conditions, and suggests methods for controlling various parameters of the photomodified regions. Rewritable optical memory,

Reversible photomodification of LiNbO3 and LiTaO3 by femtosecond laser pulses

Abstract not reproduced here by request of the publisher. The text is available from http://dx.doi.org/10.1117/12.75923