TEMPORAL CHARACTERISTICS OF DECAY OF LIGHT-INDUCED ABSORPTION IN PHOTOREFRACTIVE BaTiO3 CRYSTALS

We report the observations of the decay processes of light-induced absorption in one undoped and two Ce-doped BaTiO₃ crystals. Two decay times were found, they had not a strong dependence on intensity in our experiment. We present a model with two different shallow trap centers to explain the experimental results. The theoretical and experimental resales agree well with each other. The light-induced absorption coefficients and decay times corresponding to the two shallow levels are also given.     

Absorption changes under UV illumination in doped PMMA

The absorption spectra of poly(methyl methacrylate) (PMMA) doped with the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) change under illumination with UV light. Time-resolved measurements of the kinetics of the absorption changes at 352 nm prove the existence of a second light-induced reaction besides the expected light-induced decay of the photoinitiator. We propose a model for this second process which is in good agreement with the observed experimental facts. Received: 17 July 2000 / Published online: 5 October 2000     

Polarons in magnesium-doped lithium niobate crystals induced by femtosecond light pulses

Strong light-induced absorption has been observed in lithium niobate crystals doped with magnesium after application of femtosecond illumination. In this material there are no Nb-on-Li-site defects and hence no antisite polarons occur, but small free polarons close to the conduction band can be generated. The light-induced absorption observed is attributed to these polarons. For LiNbO₃:Mg, their decay times are about two orders of magnitude smaller than those of the Nb-on-Li-site polarons in undoped material. The results are relevant for a better understanding of the suppression of the so-called optical damage in these crystals and for their use in femtosecond applications.     

锰掺杂近化学计量比铌酸锂晶体的电荷输运过程

用泵浦(350 nm)-探测(780 nm)法研究了轻度锰掺杂近化学计量比铌酸锂晶体的光致吸收的特征,结果表明光致吸收的暗衰减过程以扩展指数的形式衰减,并测量了饱和光致吸收、衰减时间常量和扩展因子对泵浦光光强的依赖关系.在z轴方向上加直流电场测量了紫外光照射下光伏打电流强度和光电导,发现它们与光强之间成超线性关系.实验结果可以用考虑本征缺陷能级和掺杂能级之间激发和复合的修正的双中心电荷输运模型进行定量的解释.     

Evidence for light-induced hole polarons in LiNbO3

Transient light-induced absorption in LiNbO3 is observed in the blue-green spectral range after pulsed illumination with 532 nm. Its buildup and decay in Fe-doped LiNbO3 is satisfactorily described by a sum of two stretched exponential functions. For undoped LiNbO3, however, only one stretched exponential decay is observed. These experimental results are explained by the formation of both small Nb(Li)4+ electron polarons and O- hole polarons. The mechanism is discussed on the basis of a proposed band scheme.     

掺铁近化学配比铌酸锂晶体的紫外光致吸收和弛豫过程的研究

生长了系列掺杂低铁(质量分数分别为0×10-6,3×10-6,5×10-6,10×10-6,25×10-6,50×10-6、100×10-6)的近化学配比铌酸锂(SLN)晶体,测量了它们的紫外-可见光谱,并通过477nm处的线性吸收系数估算晶体中Fe2+和Fe3+的浓度。利用抽运(365nm)-探测(632.8nm)法测量不同掺杂浓度晶体光致吸收的动态过程和稳态特性,结果表明光致吸收是以扩展指数的形式衰减的,其衰减时间常数(即小极化子的寿命)随掺杂浓度和抽运光强增加而减小,扩展指数因子随抽运光强的增加而减少。根据电子的输运方程,利用四阶龙格-库塔方法对电子输运过程进行数值求解,模拟了掺铁铌酸锂(Fe:SLN)晶体的光致吸收的全过程,与实验所得结果符合得很好。     

Conductivity and light-induced absorption in BaTiO3

A charge transport model including deep and shallow traps explains both the nonlinear relation between photoconductivity and light intensity and the light-induced absorption in BaTiO₃. A correlation between measurements of photoconductivity and light-induced absorption as a function of temperature yields parameters for the shallow center, among them thermal activation energy and generation rate.     

Light-induced absorption of cerium-doped lead barium niobate crystals

Tetragonal lead barium niobate (Pb(1-x)Ba(x)Nb(2)O(6);1-x<0.63) crystals are promising photorefractive materials for applications such as holographic data storage. Measurements of light-induced absorption in Ce-doped lead barium niobate (Pb(0.5)Ba(0.5) Nb(2)O(6)) were performed. The induced absorption that was due to the presence of shallow traps was as large as 0.3cm(-1) at 633 nm and exhibited saturation behavior at green-pump intensity greater than 2W/cm(2) . The temporal profile of the dark decay was a double exponential, and the activation energies of the slow- and the fast-decaying components were determined to be 0.16 and 0.3 eV, respectively.     

Studies of impurity levels in Rh-doped and Ce-doped photorefractive BaTiO3

Impurity levels, light-induced and thermo-induced charge transfer processes in Rh-doped and Ce-doped photorefractive BaTiO₃ were studied. The thermal depths and optical transitions of these impurity levels in the crystals were determined by using such methods as light-induced absorption, thermo-induced absorption, and grating dark decay. The origins of these impurity levels were discussed. We demonstrated, for the first time to our knowledge, that thermo-induced absorption spectroscopy is a useful and complementary technique for investigating the impurity levels in photorefractive crystals. With this technique, the deep impurity level generated by Ce in BaTiO₃:Ce was revealed and identified. Received: 7 July 1999 / Revised version: 24 September 19999 / Published online: 2 February 2000     

Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO(3)

The recombination of photoinduced free Nb(4+)(Nb) and bound Nb(4+)(Li) small polarons toNb(4+)(Li): Nb(4+)(Nb) bipolarons is investigated in nominally pure, reduced LiNbO(3) single crystals by means of excited-state-absorption spectroscopy. We discovered a two-component decay of the light-induced absorption alpha(li)(t) for probe light at lambda=785 nm and moderate pump beam intensities (I(p) < 670 GW/m(2)). These experimental results give strong evidence for the existence of a two-path recombination of the photoinduced polarons. A corresponding model taking into account hopping charge transport and trapping is presented.     

Dynamics of the Light-Induced Absorption in a Bi12TiO20 : Ca Crystal

The results of investigations into the dynamics of the light-induced absorption and of its dark relaxation in a Bi₁₂TiO₂₀ : Ca single crystal are presented. To illuminate the crystal and to measure the induced changes in the absorption, narrow-band radiation of an incoherent semiconductor light-emitting diode with the wavelength = 660 nm close to that of a helium-neon laser was used. The material parameters of the crystal are estimated based on a comparison of the experimental data with the results of calculations for a theoretical model that takes into consideration doubly ionized donor centers and shallow traps.     

Role of the hydrogen in the light-induced defects in undoped hydrogenated amorphous silicon

We have investigated the effect of the deposition temperature (i.e. the hydrogen content) on the light-induced effects in undoped hydrogenated amorphous silicon (a-Si:H). Combined junction capacitance-temperature (C-T), ESR and IR absorption measurements are carried out in both the dark annealed state (A) and the saturated light-soaked state (B), as well as after partial annealing of the samples, starting from state B. The experimental results indicate that the films deposited at the highest substrate temperature (i.e. the lowest H content) exhibit a completely different behaviour from those deposited at lower substrate temperature (i.e. with higher H concentration), when the samples are left for long times at room temperature in the dark after partial annealing. These results are discussed in detail in relation to the different models proposed to explain the light-induced effects in a-Si:H.     

Ultra-Fast Decay Process of Electrons in LiNbO3 Crystal Induced by Femtosecond Pulse

Temporal evolution of absorption induced by single femtosecond pulse （13Ors, 800nm） with high intensity in LiNbO3 is obtained using the probe shadow imaging technique in order to investigate light-induced electron relaxation processes. By saturating the polaron density with a high intensity laser pulse, ultra-fast decay process on picosecond time scale is observed. The decay time constant is about 141 ps and it is attributed to the direct interband electron-hole recombination process.     

Optical properties of excitons bound to neutral SiGa-donors in GaP and the degeneracy of the SiGa-donor ground state

Low temperature near band-edge absorption and luminescence spectra of Si-doped n-type GaP are reported. Electrical and optical evidence is presented that these spectra are due to the creation and decay of excitons bound to neutral Si donors on Ga sites. Several zero-phonon transitions, each with strong replications by momentum conserving phonons, were observed in both absorption and luminescence. From these measurements it is concluded that the ground state of the Si_Ga donor is split into two sublevels 0.6 meV apart. Crystals containing a sufficiently low concentration of S to be suitable for a reliable analysis of temperature-dependent Hall-effect measurements were selected by combining 300°K Hall-effect data with low temperature S-exciton absorption data. The electrical measurements support the identification of Si as the main shallow donor. Moreover, it is concluded from these measurements that the degeneracy of the ground state of a Ga-site donor is 3 times that of a P-site donor, in agreement with theoretical expectations.     

Measurement of upper level lifetime in presence of radiation trapping: A revisitation of the pinhole method

In this work we expose a recently developed theoretical model for the analysis of the measurements obtained with the so-called pinhole method, for the determination of the upper level lifetime in materials where the superimposition between fluorescence and absorption spectra determines significant radiation trapping effect. Under fairly general conditions the fluorescence decay curve after a pulsed excitation of the sample obtained with this experimental set-up can be conveniently described by a double exponential functional form. The faster of the two decay times is close to the intrinsic fluorescence lifetime, with a difference that can be calculated with geometrical considerations. The theoretical results were tested with a suitably designed experiment, where the upper level lifetime of the laser transition of some Yb doped samples were investigated, obtaining results that are consistent with literature data and in good agreement with the theoretical results as for the temporal dependence of the fluorescence decay.     

Photoconductivity and light-induced absorption in KNbO3:Fe

Measurements of photoconductivity and light-induced absorption in KNbO₃: Fe are performed at different light intensities and crystal temperatures. The results are interpreted in terms of a two-center charge transport model. Different model parameters may be evaluated from the experimental data. A complete set of parameters is suggested explaining the dependences of photoconductivity and light-induced absorption on light intensity and temperature for the KNbO₃: Fe crystal investigated.     

Light-induced absorption instability in weakly reduced congruent Er:LiNbO3 crystal

Light-induced absorption (LIA) characteristics in weakly reduced (or strongly annealed) congruent and/or vapor transport equilibration (VTE)-treated Er-doped LiNbO₃ crystals have been investigated in comparison with their corresponding as-grown ones and undoped crystals by using a polarized 632.8-nm beam as probe light and another polarized 632.8- or 488-nm beam as pump light. Under He-Ne pump, the LIA was observed only in strongly reduced pure VTE LiNbO₃ crystal. Under 488-nm pump, LIA is still not observed in the doped or undoped as-grown crystals. The weakly reduced VTE-treated Er(0.2 mol %):LiNbO₃ crystal displays weak and stable LIA. On the other hand, the corresponding weakly reduced congruent crystal displays a rather unpredictable light-induced absorption instability phenomenon. The instability was shown by the random competition of the LIA and light-induced transparency (LIT). When both crystals were further reduced, the VTE sample still shows stable LIA but with increased LIA, while the LIA in the congruent sample also becomes stable enough. The instability was experimentally proved to be associated with the presence of the Er³⁺ ion that performs the role of an extrinsic defect instead of photoluminescence. A three-level model is suggested that consists of a deep level (the bipolaron) and two shallow levels: the small polaron level and the level with respect to the Er³⁺ ion. The model has been employed to qualitatively explain the LIA characteristics of the weakly reduced congruent Er:LiNbO₃ crystal, including the the instability, the effect of the state of reduction, the pump intensity and the pump–probe polarization dependences. The inhomogeneity of the defects caused by the weak reduction and the simultaneous participation of the two shallow levels in the light-induced electron-transport process result in the random competition between LIA and LIT, and consequently result in the LIA instability. PACS 42.70.Hj; 42.70.Ln; 42.70.Mp; 42.65.Hw; 81.05.-t     

质子交换MgO:LiNbO3波导的双层结构

本文报导MgO:LiN_bO₃质子交换波导X射线双晶衍射和红外吸收的实验结果。这些结果表明质子交换波导由浅表面层和深表面层构成。浅表面层是一个连续应变层,有着大量随机排列的O—H—O形式的缔合OH^-基团。深表面层是一个均匀应变层,其内H⁺以自由OH^-基团形式出现。质子交换波导折射串不稳定性与双层结构中的结构驰豫密切相关。通过对质子交换过程的分析,提出两条避免双层结构的途径,其一为降低交换剂中的H⁺浓度,另一条是采用有限源工艺。     

Selective optical generation of coherent acoustic nanocavity modes

Femtosecond pump-probe experiments on a Ga0.85In0.15As nanocavity enclosed by two Ga(0.85)In(0.15)As/AlAs phonon Bragg mirrors reveal selective generation of terahertz confined acoustic modes and regular folded phonons. Selective generation of the confined modes alone is achievable for laser excitation at certain energies below the mirror absorption edges, corresponding to electronic transitions within the cavity layer only. Calculations based on the photoelastic effect explain the experimental results. Decay times of cavity and regular modes evidence longer decay times and anharmonic effects for the cavity mode.