Sidelobe suppression of spectral response in holographic optical filter

In order to suppress sidelobes of holographic filter’s spectral response we propose to use weighting of grating intensity distribution along light propagation. This can be realized in photorefractive material (for example in LiNbO₃:Fe) by using spatial selective decaying of conventional uniform grating by illumination of light through some transparent mask. Experimental results show that it is able to obtain 8 dB improvement of sidelobe level compared to uniform grating. But it is accompanied by 1.7 times expansion of main lobe and two times decrease of diffraction efficiency.     

损耗对屏蔽光伏空间孤子演化特性的影响

研究了晶体损耗对屏蔽光伏空间孤子在加偏压的光伏光折变晶体中演化特性的影响.结果表明,损耗除了造成屏蔽光伏孤子光波振幅的减小外,还造成了孤子横截面的改变.对于明孤子,当入射波光强较低时,横截面随传播距离的增长而不断增大;而当入射波光强较高时,起初横截面不断压缩,而后再膨胀,并在某一传播距离上孤子横截面恢复到入射时的尺寸.对于暗孤子,不论入射光孤子光强的高低,横截面都随传播距离的增长而不断增大.虽然在传播距离足够短时,可以不考虑损耗的影响,但距离较大时,损耗将最终导致孤子崩溃. 关键词： 非线性光学 空间光孤子 光折变效应     

光折变光栅在超短脉冲光照射下衍射的研究

利用Kogelnik单色光在体光栅中衍射效率公式推导出超短脉冲光读出稳态光折变体光栅时的衍射光频谱表达式,并在此基础上导出了脉冲光读出时的总衍射光强及总光强衍射效率.研究发现衍射光的频谱分布、总光强及衍射效率与光栅周期Λ、光栅厚度d和读出光脉冲宽度Δτ有密切的关系.通过调整这些参量,光折变光栅可以对入射的超短脉冲光实现不同程度的滤波作用.该结论可用于超短脉冲的整形研究.     

Ultraviolet photorefractive effect in Mg-doped near-stoichiometric LiNbO3

The ultraviolet photorefractive effect of Mg-doped near-stoichiometric LiNbO₃ crystals prepared by vapor transport equilibration (VTE) technique was studied at 351 nm. It was found in the near-stoichiometric LiNbO₃ crystals that the ultraviolet photorefractive effect could be enhanced greatly with the increase of Mg concentration. Based on the activation energy of dark decay of the photorefractive grating, possible centers responsible for the ultraviolet photorefractive effect were also discussed.     

Double optical waveguide in Cu-doped KNSBN crystal formed by MeV boron ion implantation

We reported for what is to be believed the first time a double optical waveguide in a Cu-doped potassium sodium strontium barium niobate (KNSBN) crystal formed by double boron ion implantation. The energy and dose of B⁺ and B³⁺ ions were (3+6) MeV and (2+2)×10¹⁴ ions/cm², respectively. The refractive index profile of the waveguide showed a double-barrier confined shape, which suggested the formation of a two-layer waveguide structure. The two waveguide layers were with thickness of 2.6 and 2 μm, respectively, which was in a good agreement with the parameters obtained from transport and range of ions in matter 98 (TRIM) code simulation. The nuclear energy loss distribution of the MeV B ions implanted into this crystal had a similar shape to that of the waveguide index profile, which means an inherent relationship between the waveguide formation and the energetic energy deposition.     

Manakov solitons in biased photorefractive polymer

The coupled equations for the incoherently coupled soliton pairs in biased photorefractive polymer are provided. It is shown that the coupled soliton equations reduce to Manakov equations when the total intensity of two coupled solitons is much lower than the background illumination. The bright-bright, dark-dark, and grey-grey soliton pair solutions of these Manakov equations are obtained, and the characteristics of these Manakov solitons are also discussed.     

一维标量光折变大光强、饱和光强空间灰孤子

推导出了光折变空间灰孤子在大光强和饱和光强条件下所满足的非线性波方程,并进行了数值求解,给出了大光强和饱和光强一维标量光折变灰孤子的复振幅空间分布。     

Temporal response of photorefractive multiple quantum wells in Franz-Keldysh geometry

Transient behavior of the photorefractive grating generated in multiple quantum well structure working in the transverse Franz-Kieldysh geometry is considered. Analysis was performed in the framework of the band transport model with material parameters of GaAs/AlGaAs system. The influence of carrier transport nonlinearity caused by the intervalley scattering of electrons was taken into account. An analytical expression for the grating formation time is derived and compared with numerical solutions as well as with the available experimental results. The results show that in the case of stationary interference pattern the nonlinear transport of electrons does not considerably improve the response time of the structure. This conclusion, checked for parameters used in majority of experiments, does not confirm the statements of earlier papers.     

Photoinduced effects in hybrid sol-gel materials

Hybrid silica-based sol-gel films containing Disperse Red 1 (DR1), carbazole units (CBZ) and 2,4,7-trinitro-9-fluorenone (TNF) at different concentrations show reversible photoinduced birefringence. Polarization sensitive holographic gratings have been produced and characterized. Dichroism measurements give information on the orientation mechanisms and on the aggregation of the dye molecules. The alignment of the DR1 molecules by corona poling is verified by second harmonic generation (SHG) and a moving grating technique is used to investigate an asymmetric energy exchange in a two-beam coupling experiment, typical of photorefractive gratings.     

Hydrothermal synthesis of organic hybrid BaTiO3 nanoparticles using a supercritical continuous flow reaction system

Barium titanate (BaTiO₃: BT) nanoparticles were synthesized by the hydrothermal method in the presence of dispersants using a continuous supercritical flow reaction system. The reactants of TiO₂ sol/Ba(NO₃)₂ mixed solution and KOH solution were used as starting materials and that was heated quickly up to 400 °C under the pressure of 30 MPa for 8 ms as reaction time. The dispersant solution such as polyacrylic acid (PAA) and polyoxyethylene(20) sorbitan monooleate (Tween 80) was injected in the cooling process after the reaction. The crystal phase of the obtained particles was identified as perovskite cubic BaTiO₃ by X-ray diffractometry (XRD) and Raman spectroscopy. Raman spectra and thermogravimetric data revealed that PAA and Tween 80 fabricated hybrid BT nanoparicles. Primarily particle size of the BaTiO₃ nanoparticle was determined by means of BET surface area, as small as less than 10 nm irrespective of dispersants. In contrast, dispersed particle size in solution measured by dynamic light scattering (DLS) technique decreased from 282 nm to less than 100 nm depending on the dispersant. Aggregation of BaTiO₃ nanoparticles might be depressed in the presence of dispersants, especially PAA is the most effective among the dispersants examined.