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  • 1
    Electronic Resource
    Electronic Resource
    Effects of dephasing and vibronic structure on the first hyperpolarizability of strongly charge-transfer molecules (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1917-1924 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dispersion of the first molecular hyperpolarizability β of strongly charge-transfer chromophores is investigated. The investigation extends the well-known Oudar–Chemla nonresonant two-level model into the resonant regime. An equation for β that includes the effect of dephasing and vibrational structure of electronic states is derived. It is shown that if only the dephasing mechanism is included in the two-level model, the intrinsic hyperpolarizability β0 calculated using the Oudar–Chemla equation from the hyper-Rayleigh scattering (HRS) experimental data will decrease with decreasing the excitation wavelength. The trend is reversed when the vibrational structure is also incorporated. It is concluded that in order to obtain a consistent β0 in the HRS experiment using an excitation wavelength in the resonance regime, it is inadequate to include only the dephasing mechanism. The vibronic structure also plays an important role and needs to be considered in the data analysis in order to obtain a β0 to agree with the off-resonance experimental result. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480754
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  • 2
    Electronic Resource
    Electronic Resource
    Cathodoluminescence analysis of cleaved facets of a ZnSe p–n junction (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 3823-3828 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: ZnSe diodes grown on (100) GaAs substrates by molecular beam epitaxy were investigated using cathodoluminesence (CL) measurements at sample temperatures between 50 and 300 K. The CL line scans at different photon energies were performed on cleaved p–n junctions at 50 and 300 K, respectively. Taking into account surface recombination, carrier generation volume, carrier diffusion and internal built-in electric field and related carrier drift, the CL measurements from cleaved p–n junctions could be qualitatively explained. The charge depletion layer has a strong influence on the CL measurements. The calculated charge depletion width is in good agreement with CL measurements. The experimental data from the spatially resolved CL on the cleaved ZnSe diodes revealed important information of the carrier dynamics and recombination processes in these devices. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.372420
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  • 3
    Electronic Resource
    Electronic Resource
    Resonant and nonresonant hyper–Rayleigh scattering of charge-transfer chromophores (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 4209-4217 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The first molecular hyperpolarizabilities (β) of a series of charge-transfer nonlinear optical (NLO) chromophores are measured with the hyper-Rayleigh scattering (HRS) technique using two excitation wavelengths at 1064 and 1907 nm. The 1907 nm wavelength is the longest excitation wavelength used for the HRS experiment. For some of these chromophores, β values in excess of 1000×10−30 esu at 1907 nm are obtained, and due to two-photon enhancement, even greater β values are found with the 1064 nm excitation. Chromophores with such large hyperpolarizability are expected to have potential applications in practical electro-optical devices. The dispersion of β is analyzed using a two-vibronic-state model developed previously in our laboratory. The study shows that it is necessary to consider the vibronic structure of the chromophore in the excited state in order to account for the behavior of the first molecular hyperpolarizability of the charge-transfer NLO chromophores. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1354636
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  • 4
    Electronic Resource
    Electronic Resource
    Nanostructure of thin amorphous hydrogenated carbon films studied by positron annihilation and photoluminescence (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 404-410 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We deposited polymer-like a-C:H films using the plasma-enhanced chemical vapor deposition technique and characterized film microstructure by variable-energy positron lifetime spectroscopy, photoluminescence (PL), and UV-visible absorption spectroscopy. It was confirmed that PL occurs from a chromophore in a sp2 cluster as a result of fast recombination of a photoexcited electron-hole pair. Positron annihilation lifetime spectroscopy showed that positronium (Ps) formation takes place via electron-positron recombination in the sp3 matrix. The lifetime of ortho-positronium (o-Ps) in our a-C:H films was similar to that in polyethylene, indicating their polymer-like nature. The relative PL efficiency increased by about an order of magnitude with increasing film band gap from 1.3 to 3.4 eV, which can be related to the decreasing concentration of nonradiative centers. On the other hand, Ps formation was much less influenced by the band gap and nonradiative centers. Comparison of this result with that for polyethylene mixed with carbon-black nanoparticles, where a considerable reduction in Ps formation was observed, showed that nonradiative centers were of a different nature from the defects on the carbon nanoparticle surface. This work demonstrated the usefulness of positron lifetime spectroscopy combined with optical measurements to study the nanostructure of a-C:H. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1378063
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  • 5
    Electronic Resource
    Electronic Resource
    Shear force scanning near-field optical microscope based on a piezoelectric bimorph cantilever (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 2344-2349 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In this article we present a simple, nonoptical shear force detection scheme to control probe–sample distance for a scanning near-field optical microscope (SNOM). Shear force detection is realized by attaching a tapered optical fiber probe to a piezoelectric bimorph cantilever in which one piezo layer generates a maximum piezo voltage when the cantilever is excited at resonance by the other piezo layer. The amplitude of the piezo voltage will decrease as the probe approaches a sample's surface due to probe–sample interacting shear force. Keeping the piezo voltage constant provides a very sensitive method by which to control probe–sample distance. Based on the shear force detection scheme, a shear force SNOM system has been built, operating in transmission collection mode. Shear force topographic and optical images have been taken using uncoated optical fiber probes fabricated by a chemical etching technique. The results suggest that the system is very reliable, repeatable, and easy to use. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1367360
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  • 6
    Electronic Resource
    Electronic Resource
    Stokes slip flow through a grid of circular cylinders (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 14 (2002), S. 3358-3360 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Stokes flow through a row of equally spaced circular cylinders with partial slip on the boundaries is studied by domain decomposition and matching. The pressure drop or resistance across the grid is found for various slip and normalized cylinder radii. Asymptotic formulas are found for sparcely and closely spaced cylinders. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1499127
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  • 7
    Electronic Resource
    Electronic Resource
    Quinazolines. IX. A Study of an Unusual Reaction of 5-Diazoacetyl-1,3-dimethyl-2,4-quinazolinedione1 (1950)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 72 (1950), S. 4887-4890 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja01167a010
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  • 8
    Electronic Resource
    Electronic Resource
    Local vibrational modes of carbon in GaSb and GaAsSb (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 80 (2002), S. 1942-1944 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have measured the Raman spectra of heavily carbon doped (p〉1019 cm−3) GaSb and GaAsSb. A local vibrational mode (LVM) due to carbon residing on group-V lattice sites was observed at 540 cm−1 for GaSb and 568 cm−1 for GaAs0.44Sb0.56. A gap mode at 164 cm−1 was observed for GaSb. The frequency of the LVM as well as the gap mode is in quantitative agreement with recent theoretical predictions. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1461871
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  • 9
    Electronic Resource
    Electronic Resource
    Tellurium antisites in CdZnTe (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 2728-2730 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electrical properties of CdTe and Cd1−xZnxTe crystals grown under excess tellurium by a modified Bridgman technique are critically dependent on the zinc content. Below an x value of 0.07, the as-grown CdZnTe crystals are n type while, above this value, CdZnTe crystals are p type. The origin of the shallow donor level at 0.01 eV below the conduction band is most likely singly ionized Te antisites (Te at Cd sites). The origin of the deep donor level at 0.75 eV below the conduction band is therefore doubly ionized tellurium antisites. Based on this model, the conduction type of CdZnTe crystals is determined by the results of compensation between the shallow donors of Te antisites and the shallow acceptors of Cd vacancies. High resistivity Cd0.9Zn0.1Te crystals are produced by compensating the p-type crystals with indium impurity at a low doping level of 1–5×1015 cm−3. At room temperature, CdZnTe radiation detectors can resolve the six low energy peaks in the 241Am spectrum, a performance comparable to that of the best CdZnTe detectors reported. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1412588
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  • 10
    Electronic Resource
    Electronic Resource
    Correlation between surface step structure and phase separation in epitaxial GaInAsSb (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 76 (2000), S. 2077-2079 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A strong correlation between the surface step structure and phase separation in metastable GaInAsSb epitaxial layers grown by organometallic vapor-phase epitaxy is reported. The full width at half maximum (FWHM) of the 4 K photoluminescence (PL) peak energy is used as a semiquantitative measure of the degree of phase separation: FWHM values increase with phase separation. The step structure of GaInAsSb grown at 525 °C is vicinal, while it is step bunched for layers grown at 575 °C. The corresponding 4 K PL FWHM data indicate smaller FWHM values for layers grown at the lower temperature, and suggest a lower degree of phase separation. Extreme PL broadening is associated with an aperiodic surface structure. Furthermore, the PL FWHM values decrease when the growth rate increases from 1.2 to 5 μm/h. It is proposed that longer adatom lifetimes, which are associated with longer terrace lengths of a step-bunched surface compared to a vicinal one, allow more time for the adatoms to reach equilibrium and thus phase separate. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.126260
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