Suppression of the photoinduced light scattering in LiNbO3:Fe by redox treatment and incoherent homogeneous illumination

In this paper, the redox treatment and incoherent homogeneous illumination were tried for the suppression of the photoinduced light scattering in LiNbO₃:Fe. Meanwhile, the effects of both the redox treatment and incoherent homogeneous illumination on the holographic storage properties of LiNbO₃:Fe were also studied. Our experimental data show that the reduction treatment sacrifices the diffraction efficiency for the suppression of photoinduced light scattering in LiNbO₃:Fe. By contrast, incoherent homogeneous illumination with the wavelength of 460 nm is efficient to suppress the photoinduced light scattering without any tradeoff on the holographic storage performance of LiNbO₃:Fe. The superiority of homogeneous illumination over the reduction treatment was attributed to the untouched acceptor concentration under the homogeneous illumination.     

High resolution study of permanent photoinduced reflectivity changes and charge-order domain switching in Bi(0.3)Ca(0.7)MnO(3)

We report near-field and far-field optical microscopic studies of photoinduced effects in charge-ordered Bi(0.3)Ca(0.7)MnO(3). Unlike previously reported transient photoinduced effects in manganites, we have observed permanent reflectivity changes following local sample illumination with 488 nm light. High-resolution images of exposed regions reveal optical contrast on a submicrometer scale. This observation indicates that photonic band-gap structures may be created using holographic recording in manganites. We also present images of photoinduced charge-order domain switching in Bi(0.3)Ca(0.7)MnO(3).     

Interfacially controlled transient photoinduced superconductivity

We report on a large transient photoinduced enhancement of the superconducting critical temperature (DeltaTc = 23 K) in epitaxial YBa2Cu3O(6.7)/La(0.7)Ca(0.3)MnO3 bilayers upon visible light illumination. The effect relaxes with a characteristic time of 100 s at low temperatures, which is 4 orders of magnitude faster than the persistent photoconductivity or persistent photoinduced superconductivity previously found in single high-Tc superconducting films. This result is discussed in terms of light induced charge transfer through the interface similar to what happens in semiconductor junctions.     

Investigation into the effects of deposition parameters on TiO2 photocatalyst thin films by rf magnetron sputtering

Titanium dioxide (TiO₂) photocatalyst thin films were deposited on non-alkali glass substrates by radio frequency (rf) magnetron sputtering. The Taguchi method with orthogonal array, signal-to-noise ratio and analysis of variance were employed to study the performance characteristics. The experimental studies were conducted under different rf powers, sputtering pressures, O₂/(Ar+O₂) flow-rate ratios, and substrate temperatures. The deposited TiO₂ films were of the anatase phase with a (101) preferred orientation. We performed both photoinduced decomposition of methylene blue (MB) and photoinduced hydrophilicity under UV light illumination. With the optimized TiO₂ photocatalyst thin film deposition conditions, the water contact angle after 9 min UV illumination was approximately 5, the absorbance of MB was reduced to 0.2 for 240 min UV irradiation, and the deposition rate was 34.18 Å/min.     

Strong Bragg gratings photoinduced by 633-nm illumination in evaporated As2Se3 thin films

Bragg gratings are used in several photonic devices to reflect, and thus to isolate, specific wavelengths of light. Gratings can be photoinduced in chalcogenide glasses by illumination of bandgap light in an interference pattern. We used holographic interferometry to create Bragg gratings in amorphous As2Se3 thin films with a period of 0.56 microm by illumination with 633-nm light. The quality of the gratings was tested in real time, and refractive-index modulations as high as 0.037 were measured. These gratings were found to be stable over a period of several months if they were kept in the dark.     

Investigation of photoinduced scattering in LiNbO3 crystals

A photoinduced scattering effect in LiNbO₃:Fe crystals was investigated in detail. The scattering depends substantially on illumination geometry, light beam profile, doping degree, and appears under coherent illumination only. It was shown that besides the photoinduced scattering a bleaching effect is possible. It consists in the fact that after the stage of low-scale inhomogeneity formation there occurs their effective erasure. The conditions were found out under which the bleaching effect can be observed.     

Relaxation of the optical density of glass modulated with bichromatic radiation

Exposure of oxide glass to two coherent light beams with different frequencies ω and 2 ω leads to the buildup of spatial index gratings in the glass. The kinetics of the writing and relaxation of the gratings is investigated. Illumination with one beam or a background illumination of the sample accelerates the relaxation process; this acceleration is due to the appreciable photoconductivity of the glass. The diffraction is found to reach a residual self-maintaining level when a grating is illuminated continuously with a beam at the fundamental frequency. The results are analyzed on the basis of a model that includes the coherent photogalvanic current, the accumulation of a grating of nonuniform charges under the action of this current, and the back-effect of the static field, via electrooptic effects, on the propagation of the beams. It is shown that an optical instability of the photoinduced index gratings exists in the glass. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 166–170 (10 February 1996)     

Irreversible Photoinduced Insulator-Metal Transition in Charge Ordered Pr 0.75 Na 0.25 MnO 3

Irreversible photoinduced insulator-metal transition has been observed in a perovskite manganite doped with monovalent Na + , Pr 0.75 Na 0.25 MnO 3 . The increase of the conductance is estimated to be of the order of 10 4 -10 5 , considering the shallow penetration depth ( ¨ 0.1 µm) of illumination light. Analysis of the dc- and ac-susceptibility data shows that the system is in a cluster glass state below 45 K, which means that ferromagnetic metallic clusters exist in insulating matrix. The photo-excitation helps the clusters to dislodge from a local energy minimum and to seek more stable configuration. The observed large increase of the conductivity can be well explained by the percolative conduction. We argue that metastability is due to the large charge mismatch between the monovalent Na + ion and trivalent Pr 3+ ion or the deviation of the average Mn valency from 3.5+.     

GeSe2非晶半导体薄膜中光致结构及性能变化

运用X射线衍射分析、红外光谱分析、扫描电镜分析和透射光谱分析，研究了GeSe2非晶半导体薄膜经514.5nm波长的氩离子激光辐照后的结构及性能变化。实验结果表明，经热处理和激光辐照后，薄膜的光学吸收边均移向短波长处，这种移劝随着辐照激光强度和辐照时间的增加而增大，并且在退火薄膜中是可逆的，扫描电镜分析结果表明，薄膜在激光辐照后有微晶析出，这种微晶的析出量随着辐照激光强度的增强而增加。     

Ultrafast semiconductor-metal phase transition in vanadium dioxide induced by a femtosecond laser pulse

A model is proposed for a photoinduced Peierls-type semiconductor-metal phase transition that makes it possible to determine the time dependence of the bandgap width in the electronic spectrum of vanadium dioxide subjected to a light field and the dependence of the time at which a photoinduced semiconductor-metal phase transition occurs on the laser pulse duration. The theoretical results obtained are consistent with experimental data on the illumination of a VO₂ film with an intense laser pulse.     

Photoinduced Barkhausen effect in the ferromagnetic semiconductor (Ga,Mn)As

Magnetization of ferromagnetic materials commonly occurs via random jumps of domain walls between pinning sites, a phenomenon known as the Barkhausen effect. Using strongly focused light pulses of appropriate power and duration we demonstrate the ability to selectively activate single jumps in the domain wall propagation in (Ga,Mn)As, manifesting itself as a discrete photoinduced domain wall creep as a function of illumination time. The propagation velocity can be increased over 7 orders of magnitude varying the illumination power density and the magnetic field.     

Hollow-core grating fiber

We propose a new type of hollow-core fiber where the propagation is ensured by a photoinduced self-pattern acting as a surface relief grating (SRG). The SRG is written by launching a suitable laser beam with proper polarization in a capillary glass fiber with the inner surface previously coated with an azopolymer thin film. Such a grating acts as a wavelength/angle dependant reflective mirror and enhances the confinement and the propagation of the light.     

Optically stimulated optical effects in the ZnO:Tm nanorods

We have performed the studies of photoinduced second harmonic generation in the ZnO nanorods doped by Thulium with different content (0.1; 0.2;0.5% of Tm). The illumination was performed with the Er:glass laser emitting at 1540 nm wavelength of the 10 ns laser and the output second harmonic generation (SHG) was studied. The AFM data analysis was performed. The role of the rare earth Tm ions on the morphology and the optically stimulated SHG is discussed. The SHG yield was found to be strongly dependent on Thulium content and Root Mean Square roughness of the studied layers.     

Gold-bead scanning near-field optical microscope with laser-force position control

We have developed a scanning near-field optical microscope with an optically trapped metallic particle that has a small diameter compared to the wavelength of visible light. In this microscope we employed spot illumination to enhance the intensity of light scattered from a probe particle so we could reduce the diameter of the probe particle to 40nm. We detected slight irregularities of the surface of the cover glass near 10-nm depth. Also, we observed gold colloidal particles on the surface of the cover glass.     

A study of the photoinduced conformational mobility of spin-labeled regenerated rhodopsin by ESR spectroscopy

Saturation transfer EPR spectroscopy is used to study the photoinduced conformational changes of spin-labeled (Cys140 and Cys316) rhodopsin in a photoreceptor membrane. Illumination of rhodopsin with “yellow” light (λ > 450 nm), converting it from the dark form into metarhodopsin II, a physiologically active form, is accompanied by an increase in the mobility of the cytoplasmic loop (Cys140) and eighth alpha-helix (Cys316), whereas the subsequent illumination with “blue” light (λ < 445 nm), which converts metarhodopsin II into a mixture of inactive photoregenerated products, results in a decrease in the mobility. The restoration of the specific features of the photoinduced changes in the conformational mobility of the cytoplasmic loops of rhodopsin regenerated in the dark is demonstrated.     

液体封装LED汽车远光灯光学设计

使用一种导热性能好、粘度小、电绝缘性强、无腐蚀性的透明液体封装LED,研究其封装后的光学特性,以获得一种光效高、散热好、体积小的新型汽车光源.按照汽车用LED前照灯GB 25991-2010配光要求,选用1 300 Lm的大功率白光LED光源,采用椭球面玻璃作为灯具基本外壳.用光学设计软件TracePro进行设计,通过调节光源在灯具内的位置,修改玻璃外壳形状,设计出了一款自投射式LED汽车远光灯.设计结果表明:以目前大功率白光LED光源技术,椭球面玻璃外壳直径在40~50 mm范围内的远光灯完全能满足国标要求,光能收集效率在86.5%以上.最后对制作灯具进行测试,照度值分布与仿真结果基本吻合.     

Photoinduced stable second-harmonic generation in chalcogenide glasses

We report on photoinduced second-harmonic generation (SHG) in chalcogenide glasses. Fundamental and second-harmonic waves from a nanosecond pulsed Nd:YAG laser were used to induce second-order nonlinearity in chalcogenide glasses. The magnitude of SHG in 20G?20A?60S glass was 10(4) larger than that of tellurite glass with a composition of 15Nb(2)O (5) 85TeO(2) (mol.%). Moreover, no apparent decay of photoinduced SHG in 20G?20A?60S glass was observed after optical poling at room temperature. We suggest that the large and stable value of chi((2)) is due to the induced defect structures and large chi((3)) of the chalcogenide glasses.     

液体封装LED汽车远光灯光学设计

使用一种导热性能好、粘度小、电绝缘性强、无腐蚀性的透明液体封装LED,研究其封装后的光学特性,以获得一种光效高、散热好、体积小的新型汽车光源.按照汽车用LED前照灯GB25991-2010配光要求,选用1 300 Lm的大功率白光LED光源,采用椭球面玻璃作为灯具基本外壳.用光学设计软件TracePro进行设计,通过调节光源在灯具内的位置,修改玻璃外壳形状,设计出了一款自投射式LED汽车远光灯.设计结果表明:以目前大功率白光LED光源技术,椭球面玻璃外壳直径在40～50 mm范围内的远光灯完全能满足国标要求,光能收集效率在86.5％以上.最后对制作灯具进行测试,照度值分布与仿真结果基本吻合.     

偶氮苯侧链聚合物液晶薄膜的光致双折射和永久光学性存储研究

研究了一种含偶氮基侧链聚合物液晶薄膜,该液晶薄膜表现出显着的光致双折射(Δn～1.1×10^-2).当关闭写光作用后,光致双折射的大小没有任何衰减,表现出永久光存储特性.光致双折射的擦除可通过将样品加热到接近其清亮点温度,或用一束圆偏振光照射来实现.分析了偶氮侧链聚合物永久性光学存储的物理机制.     

Composition-dependent photosensitivity in As-S glasses induced by bandgap light: structural origin by Raman scattering

Massive photoinduced short- and medium-range structural changes (photopolymerization) in As-S glasses are induced by near-bandgap light and studied by Raman scattering. Structural changes involve bond restructuring in sulfur-rich nanodomains of these nanoscale-phase-separated glasses. The spectral dependence of the photopolymerization effect demonstrates that various wavelengths can be used to optically change the structure of As-S glasses. The immense structural changes are relevant to recent findings about the role of bandgap light illumination for fabricating channel waveguides in noncrystalline arsenic sulfides.