Charge transport processes in LiNbO3:Fe at high intensity laser pulses

Light-induced refractive index changes in LiNbO₃:Fe crystals are investigated at high light intensities (>10⁹ Wm^–2). Holographic gratings are recorded and erased with frequency-doubled pulses of a Q-switched Nd:YAG laser. We find new intensity dependent contributions to the holographic sensitivity, to the photoconductivity, and to the saturation value of refractive index change. Light-induced absorption changes are also detected. These results indicate that the Fe²⁺/Fe³⁺ charge transport model, well established for low intensities, has to be modified for high intensities by assuming additional centers which trap and supply electrons.     

LiNbO3 with the damage-resistant impurity indium

We report on a detailed investigation (EPR, SHG, optical absorption, luminescence and Raman scattering) on the new damage-resistant impurity indium in LiNbO₃, where the increased photoconductivity strongly reduces the photorefractive effect. EPR and optical absorption measurements point to a complete disappearance of the Nb antisite in LiNbO₃: In for all In concentrations. We believe that the very effective driving out of Nb antisites by In is due to the trivalent charge state of In and the possibility of charge self-compensation. Similarities in the properties of Mg-, Zn- or Indoped samples are discussed. Simultaneous doping with In and Zn leads to an addition of both contributions, in particular for optical frequency doubling and luminescence. Raman studies prove that In does not improve the stoichiometry of the crystals. Indium doping provides the possibility to control simultaneously phase-matching conditions and to reduce drastically photorefraction. Therefore, In co-doped LiNbO₃ compositions are promising materials for applications after solving contemporary growth problems.     

Performance of two different types of passive samplers for the GC/ECD-FID determination of environmental VOC levels in air

Two types of passive samplers differing in their geometry (OVM 3500 by 3M, ORSA 5 by Dr?ger) were compared with respect to their suitability for typical environmental indoor and outdoor VOC concentrations. Benzene, toluene, o-, m-, p-xylene, ethylbenzene, tetrachloroethene, trichloroethene, nonane and ethyl acetate were representatively analyzed by dual-column capillary gas chromatography with tandem ECD-FID detection. There was a good correlation between the results obtained with OVM 3500 and ORSA 5 monitors indicating that both monitors can be used for this kind of application. The ratio between the results for indoor air sampling with OVM 3500 and ORSA 5 monitors was between 0.89 and 1.14 showing no systematic variation. For outdoor air sampling the ratio was between 1.06 and 1.26 indicating that the results obtained with OVM 3500 monitors were slightly higher. Reproducibility was slightly better when using ORSA 5 monitors. But, due to the higher sampling rates which are a result of the larger cross-sectional area, signal-to-noise ratios obtained with OVM 3500 monitors were between six to nine times higher than those of ORSA 5 samplers. Blank values of the unexposed samplers were comparable for both sampler types. As a consequence, detection limits were by a factor of 1.5 to 4 better for OVM 3500 monitors. Received: 18 August 1998 / Revised: 6 October 1998 / Accepted: 15 October 1998     

Characterization of stoichiometric LiNbO3 grown from melts containing K2O

The growth of LiNbO₃ single crystals from a melt with the Li/Nb ratio of 0.946, to which 6 wt.% K₂O has been added, leads to stoichiometric specimens, essentially free of potassium, with (50±0.15) mol% Li₂O in the crystal. This is established by studying the composition dependence of the following properties: linewidths of the electron paramagnetic resonance (EPR) of Fe³⁺, energy of the fundamental absorption edge, Raman linewidths of phonon modes, and dispersion of the optical birefringence. Comparison of the results with relevant calibration scales leads to the above composition. In all cases the Li₂O content was found to be closer to 50% than that of a LiNbO₃ crystal vapor-phase equilibrated to 49.9mol% Li₂O. The photorefractive effect at light intensities I10⁷ W/m² is suppressed in this stoichiometric material. The features of the ternary system K₂O-Li₂O-Nb₂O₅, which are possibly responsible for the unexpected growth of stoichiometric LiNbO₃ from the indicated melts, are discussed.     

Estimates on the zeros of E_2

We improve the results from El Basraoui (Proc Amer Math Soc 138(7):2289–2299, 2010) about the Eisenstein series $E_2(z)=1-24\sum _{n=1}^\infty \frac{nq^n}{1-q^n}$ . In particular we show that there exists exactly one (simple) zero in each Ford circle and give an approximation to its location.     

Intrinsic photorefractive centers in LiNbO3:Fe

In LiNbO₃:Fe,Me crystals (Me = K, Mg, Zn) elementary holograms are recorded and erased with frequency-doubled, ordinarily polarized pulses of a Q-switched Nd:YAG laser. At high light intensities (I > 10⁹ Wm^–2) holographic sensitivities and saturation values of refractive index changes grow with increasing intensity and a contribution to photo-conductivity appears increasing quadratically withI. The intensity-dependent parts of the holographic quantities are diminished by increasing the lithium content of the samples or by doping the crystals with magnesium or zinc. Experimental results can be described by a two-center charge transport model, which is the Fe^2+/3+ model supplemented by shallow traps, namely niobium ions occupying lithium sites.     

Infrared holographic recording in LiNbO3:Cu

Holograms may be recorded in photorefractive LiNbO₃:Cu with pulsed infrared light (wavelength =1064 nm, Q-switched Nd:YAG laser), if the crystals are previously or simultaneously illuminated with a green (=532 nm) light pulse. We study refractive index changes and time constants of as-grown and thermally treated crystals with different copper concentrations. A model explaining this effect is discussed.     

Light-induced thermal gratings in LiNbO3: Fe

Holographic gratings are recorded in LiNbO³: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelength = 532 nm, pulse durationt _p = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe²⁺ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO₃: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.     

Passive sampling for volatile organic compounds (VOCs) in air at environmentally relevant concentration levels

A sensitive and reliable method is described for the determination of aromatic and chlorinated hydrocarbons (benzene, toluene, o-, m-, p-xylene, trichloromethane, trichloroethane, trichloroethene and tetrachloroethene) in indoor and outdoor air at environmental concentration levels. The procedure can be easily extended to other VOCs. Using passive samplers the VOCs have been adsorbed onto charcoal during a four-week sampling period and subsequently desorbed with carbon disulfide. After injection with a cold split-splitless multi-injector the VOCs have been separated by capillary gas chromatography. Quantification has been achieved using an electron capture detector (ECD) and a flame ionization detector (FID) switched in series. A limit of about 1 g/m³ for aromatic hydrocarbons and of about 0.01 g/m³ for chlorinated hydrocarbons has been obtained. The procedure has been successfully applied in the framework of a field study to measure indoor and outdoor air concentrations in Essen and Borken, two differently polluted areas of Northrhine-Westphalia.     

Performance of two different types of passive samplers for the GC/ECD-FID determination of environmental VOC levels in air

Two types of passive samplers differing in their geometry (OVM 3500 by 3M, ORSA 5 by Dräger) were compared with respect to their suitability for typical environmental indoor and outdoor VOC concentrations. Benzene, toluene, o-, m-, p-xylene, ethylbenzene, tetrachloroethene, trichloroethene, nonane and ethyl acetate were representatively analyzed by dual-column capillary gas chromatography with tandem ECD-FID detection. There was a good correlation between the results obtained with OVM 3500 and ORSA 5 monitors indicating that both monitors can be used for this kind of application. The ratio between the results for indoor air sampling with OVM 3500 and ORSA 5 monitors was between 0.89 and 1.14 showing no systematic variation. For outdoor air sampling the ratio was between 1.06 and 1.26 indicating that the results obtained with OVM 3500 monitors were slightly higher. Reproducibility was slightly better when using ORSA 5 monitors. But, due to the higher sampling rates which are a result of the larger cross-sectional area, signal-to-noise ratios obtained with OVM 3500 monitors were between six to nine times higher than those of ORSA 5 samplers. Blank values of the unexposed samplers were comparable for both sampler types. As a consequence, detection limits were by a factor of 1.5 to 4 better for OVM 3500 monitors.