Thermal stability of oxygen-modulated chemical-composition gratings in standard telecommunication fiber

Experimental results and a discussion on the formation and decay of oxygen-modulated chemical-composition gratings in a standard telecommunication fiber are presented. Comparison between the decay experiment and the model provides a diffusion coefficient with an activation energy of 490 kJ/mol, which is in close agreement with reported values of oxygen self-diffusion in silica. The gratings have a diffusion-controlled decay behavior, with more than 50% of the reflectivity remaining after 7.5 h at a temperature of 1230 degrees C. The gratings show higher thermal stability when heated in air than in an inert argon atmosphere.     

Dynamics of pulsed recording of holographic diffraction gratings in photopolymer materials

The dynamics of recording holographic diffraction gratings in photopolymer material using 10-ns pulses of Nd:YAG laser radiation (532 nm) was investigated. The formation of permanent and intermediate gratings was observed. Optimization of the photopolymer composition for pulsed recording was performed. The kinetic photopolymerization models taking into account the mechanisms of radical destruction due to recombination and by impurities are verified. Measurement of the recording kinetics at different recording angles showed that diffusion does not affect the hologram formation for times of the same order of magnitude as the recording duration (0.1 s) at a length of the grating half-period. Samples of the photopolymer composition in triethanolamine were investigated and diffusion of oligomeric acrylamide molecules was revealed. The diffusivity was found to be 1.1 × 10^?9 cm²/s.     

Annealing properties of gratings written into UV-presensitized hydrogen-outdiffused optical fiber

Accelerated aging of gratings written in UV presensitized hydrogen outdiffused optical fibers show that these gratings are more stable than standard gratings written in hydrogen-loaded fibers. They are observed to grow initially by as much as 2%. The predicted decay after 25 years at 80 degrees C is ~0.1% . The results suggest that the index modulation decay is so slow that postfabrication annealing can be avoided.     

Novel non-linear optical techniques for diagnostics: laser-induced gratings and two-colour four-wave mixing

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool.The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C₂, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame.Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields.     

Evolution of the density profiles and flows of charged particles during the diffusive decay of an electronegative gas plasma

Different scenarios of the spatiotemporal evolution of the parameters of the diffusive decay of a pulsed electronegative gas plasma in the absence of plasma chemical processes are studied. It is shown that nonlinear diffusion in a plasma with negative ions occurs in several stages. The rate of electron density decay increases with time and, in the beginning of the second stage, almost all the electrons escape from the discharge volume. On the other hand, the ion density profile is smoothed out due to ion-ion ambipolar diffusion and the flow of negative ions toward the wall is absent in the first stage of decay. In the second stage, the main diffusion mode is first established and then the ion-ion (electronless) plasma decays exponentially with a characteristic time determined by ion-ion ambipolar diffusion.     

Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses

We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500 degrees C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique will be useful for fabrication of fiber gratings with a superior aging characteristic.     

Hyperfine interaction-dominated dynamics of nuclear spins in self-assembled InGaAs quantum dots

We measure the dynamics of nuclear spins in a single-electron charged self-assembled InGaAs quantum dot with negligible nuclear spin diffusion due to dipole-dipole interaction and identify two distinct mechanisms responsible for the decay of the Overhauser field. We attribute a temperature-independent decay lasting ～100 sec at 5 T to intradot diffusion induced by hyperfine-mediated indirect nuclear spin interaction. By repeated polarization of the nuclear spins, this diffusion induced partial decay can be suppressed. We also observe a gate voltage and temperature-dependent decay stemming from cotunneling mediated nuclear spin flips that can be prolonged to ～30 h by adjusting the gate voltage and lowering the temperature to ～200 mK. Our measurements indicate possibilities for exploring quantum dynamics of the central spin model.     

Effect of diffusion nonlinearity and an external electric field on the formation of ion-exchange diffraction gratings in glass

The formation of gratings in glass by the ion exchange technique is analyzed. The ion exchange is modeled, and the effect of diffusion nonlinearity and the external electric field on the diffusant concentration profile and diffraction characteristics of gratings are studied. It is shown that the character of diffusion nonlinearity has a substantial effect on the diffraction efficiency of the gratings produced by thermal ion exchange. It is also shown that the diffraction efficiency can be considerably increased by using an external electric field.     

Spectral dependence of cross-talk between photorefractive gratings in Bi12SiO20 in diffusion regime

12 SiO₂₀ crystals in the diffusion regime. We find unusually high cross-talk for two gratings with close spatial frequencies at wavelengths 488 and 476 nm. The results indicate that the density of free charge carriers does not directly follow light modulation at low spatial frequencies. Received: 2 December 1996/Revised version: 20 February 1997     

Postexposure evolution of a photoinduced grating in a polymer material with phenanthrenequinone

The specific features of the postexposure evolution of photoinduced (holographic) gratings in a model medium that consists of poly(methyl methacrylate) with distributed phenanthrenequinone and is characterized by a diffusion enhancement are investigated using the holographic relaxation technique. It is established that the evolution process occurs in four stages controlled by different mechanisms: (i) initial nonmonotonic changes governed by the diffusion of an intermediate radical photoreaction product and its transformation into a stable product due to attachment to the macromolecules, (ii) hologram enhancement through the diffusion of phenanthrenequinone molecules, (iii) rapid partial degradation associated with the spatially confined motion of polymer chain segments, and (iv) slow degradation as a result of macromolecular diffusion. In the course of polymer relaxation after a temperature jump, the processes associated with the mobility of molecules are retarded and the depth of rapid degradation decreases. The effective diffusion coefficient characterizing the destruction of reflection gratings is smaller than that for transmission gratings. This can be explained by the spatial inhomogeneity of the polymer.     

用非相干光时间延迟四波混频测量TiO2超微粒子的扩散系数及载流子复合时间

本文报导了用非相干光时间延迟四波混频方法测得Ti2O超微粒子的扩散系数和载流子复合时间分别为0.016cm2/s和1.08ns.两个光栅衰减时间分别是1.07ns和0.56ns.讨论了引起两个光橱衰减时间不同的原因.     

Comparative study of sub-micrometer polymeric structures: Dot-arrays, linear and crossed gratings generated by UV laser based two-beam interference, as surfaces for SPR and AFM based bio-sensing

Two-dimensional gratings are generated on poly-carbonate films spin-coated onto thin gold-silver bimetallic layers by two-beam interference method. Sub-micrometer periodic polymer dots and stripes are produced illuminating the poly-carbonate surface by p- and s-polarized beams of a frequency quadrupled Nd:YAG laser, and crossed gratings are generated by rotating the substrates between two sequential treatments. It is shown by pulsed force mode atomic force microscopy that the mean value of the adhesion is enhanced on the dot-arrays and on the crossed gratings. The grating-coupling on the two-dimensional structures results in double peaks on the angle dependent resonance curves of the surface plasmons excited by frequency doubled Nd:YAG laser. The comparison of the resonance curves proves that a surface profile ensuring minimal undirected scattering is required to optimize the grating-coupling, in addition to the minimal modulation amplitude, and to the optimal azimuthal orientation. The secondary minima are the narrowest in presence of linear gratings on multi-layers having optimized composition, and on crossed structures consisting of appropriately oriented polymer stripes. The large coupling efficiency and adhesion result in high detection sensitivity on the crossed gratings. Bio-sensing is realized by monitoring the rotated-crossed grating-coupled surface plasmon resonance curves, and detecting the chemical heterogeneity by tapping-mode atomic force microscopy. The interaction of Amyloid-β peptide, a pathogenetic factor in Alzheimer disease, with therapeutical molecules is demonstrated.     

Dynamic phase holograms in anthracene-containing polymer layers under conditions of rapid diffusion

A model of the dynamics of phase holographic gratings in a polymer layer containing monosubstituted anthracene and a solvent-plasticizer is developed for the case when diffusion of all low-molecular components occurs. Good agreement between this model and the experimental data is demonstrated. It is shown that the photoinduced trapping and diffusion of solvent molecules also occur in highly plasticized polymer matrices, affecting significantly the dynamics of holographic gratings. Differences in the dynamics of holograms under conditions of rapid diffusion and thermolysis of the photoproduct (the photodimer) are noted. The trends in variations of the recording parameters as a function of the solvent content are revealed.     

Characterization of thermal (in)stability and temperature-dependence of type-I and type-IIA Bragg gratings written in B-Ge co-doped fiber

The temperature-dependence and the thermal stability of UV-exposed type-I and type-IIA fiber Bragg gratings written in boron-germanium co-doped silica fiber were examined, in terms of their refractive-index modulations and the change in the Bragg wavelengths. The decay in the reflectivity of the gratings with time at different temperatures was studied and modelled according to a power-law and a master aging curve approach. The distribution of the activation energy, for these types of gratings, was evaluated to describe their thermal stability and then the comparison of their stabilities has been made. Temperature-induced reversible and irreversible changes in the grating characteristics were observed. The temperature-induced reversible spectral characteristics of type-I and type-IIA gratings were found to be different in nature. All these results were discussed within the framework of the current theoretical approaches for predicting the stability and operational lifetime of these types of gratings.     

Anomalous behaviour of long-time increase in diffraction efficiency of photorefractive grating in Ce：BaTiO3 in dark

Anomalous long-time increase of the diffraction efficiency is observed in dark-decay experiments of photorefractive gratings in Ce：BaTiO3. It is deduced that a phase-conjugate beam is induced by the writing beam at acute angle to the ＋c axis of the crystal and it interferes with the other writing beam to form a second grating which is perpendicular to the first grating formed by the interference between two writing beams. The rising behaviour of the diffraction efficiency results from the different decay rates of these two photorefractive gratings. Furthermore, a simplified model of two gratings, both induced by two deep traps, is proposed to account for this phenomenon and the fitting results agree well with the experimental results.     

Spectral diffusion in glasses at low temperatures

We give the results of a substantially exact treatment of the effect of spectral diffusion on an ultrasonic two-pulse echo in glasses below 100 mK. The results indicate that more extended observations of the echo decay would enable one to verify the predictions of the tunnelling model.     

不同增感对AgCl乳剂中光电子时间行为的影响

借助微波吸收介电谱检测技术,对AgCl立方体乳剂中光电子的时间行为进行了检测,同时获得了自由光电子与浅束缚光电子在不同增感条件下的时间分辨谱。实验结果表明：化学增感时由于硫加金增感中心的浅电子陷阱作用有效地抑制了空穴与光电子之间的复合,化学增感使得光电子的衰减相对未增感的减缓、衰减时间得到延长;光谱增感时由于染料J聚集体增加了卤化银晶体中添隙银离子的浓度、促进了光电子与添隙银离子之间的结合,光谱增感使得光电子的衰减相对未增感的加剧、衰减时间变短;化学与光谱共同增感使光电子的衰减时间在光谱增感的基础上得到了延长,且硫加金化学增感中心的浅电子陷阱效应在光谱增感的基础上更明显。     

Transport properties and polarization phenomena in intercalated Ag x HfSe2 compounds

The electrical properties of intercalated Ag _x HfSe₂ compounds (x = 0.1, 0.2) have been investigated for the first time. Investigations have been performed using various current electrodes, which make it possible to pass either the electron current or the ion current across the sample. Polarization effects, which indicate the self-consistent migration of charge carriers in the samples, have been found for the samples at room temperature. Based on the characteristic features of polarization decay, coefficients of conjugated chemical diffusion have been evaluated.