Waveguide electro-optic modulator in fused silica fabricated by femtosecond laser direct writing and thermal poling

An integrated electro-optic waveguide modulator is demonstrated in bulk fused silica. A Mach-Zehnder interferometer waveguide structure is fabricated by direct writing with a femtosecond laser followed by thermal poling. A 20 degrees electro-optic phase shift is achieved at an operating wavelength of 1.55 microm with an applied voltage of 400 V and an interaction length of 25.6 mm, which correspond to an estimated effective electro-optic coefficient of 0.17 pm/V for the TE-polarized mode.     

Structural and compositional modification of a barium boroaluminosilicate glass surface by thermal poling

In addition to inducing second-order nonlinear properties, significant structural and compositional alteration can be imparted to glass surfaces during the process of thermal poling. In this work, we focus on how thermal poling affects a structurally complex, nominally alkali-free boroaluminosilicate display glass composition. We provide evidence for electrolysis of the glass network, characterized by the migration of both cations (Ba²⁺, Na⁺) and anions (O^?, F^?) towards opposing electrode interfaces. This process results in oxidation of the positively biased electrode and forms a network-former rich, modifier-depleted glass surface layer adjacent to the anodic interface. The modified glass layer thickness is qualitatively correlated to the oxidation resistance of the electrode material, while extrinsic ions such as H⁺/H₃O⁺ at not found in the depletion layer to compensate for the migration of modifier cations out of the region. Rather, FTIR spectroscopy suggests a local restructuring of the B₂O₃–Al₂O₃–SiO₂ network species to accommodate the charge imbalance created by the exodus of network-modifying cations, specifically the conversion of tetrahedral B(4) to trigonal B(3) as Ba or Na ions are removed from B-related sites in the parent network. The resultant poling-induced depletion layer exhibits enhanced hydrolytic resistance under acidic conditions, and the IR spectra are substantially unlike those produced by acid leaching the same glass.     

Relaxation time and viscosity of fused silica glass at room temperature

Cases of long-term deformation of fused silica glass at room temperature attributed to the action of gravity have been reported. Further experimental investigations now provide evidence of time-dependent viscous behavior, with a time constant of the order of 10 years. Data relating to a pair of fused silica reference plates are presented, showing the overall deformation occurred over the years; considerations on the pertaining viscosity with aging are also given. An account of the observed relaxation process in terms of the Kelvin-Voigt model for linear viscoelasticity is provided.     

熔石英表面划痕附近电磁场分布模拟分析

 亚表面缺陷是造成固体激光器光学器件损伤阈值过低的重要原因，而表面划痕是缺陷中重要的一种。使用时域有限差分方法（FDTD）模拟了熔石英表面圆柱形、三角形划痕对激光电磁场的调制作用，绘出了2维电磁场强度分布图，计算出划痕尺寸不同时电磁场的最大强度。数值计算结果表明，尺寸为二倍波长的划痕可以获得最大的电磁场强度，此时容易导致自聚焦；亚波长级和足够大尺寸的划痕作用基本可以忽略。一定尺寸的划痕，深度越大，最大场强也越大，但当划痕过深时最大场强反而会降低。     

熔石英表面划痕附近电磁场分布模拟分析

亚表面缺陷是造成固体激光器光学器件损伤阈值过低的重要原因,而表面划痕是缺陷中重要的一种。使用时域有限差分方法（FDTD）模拟了熔石英表面圆柱形、三角形划痕对激光电磁场的调制作用,绘出了2维电磁场强度分布图,计算出划痕尺寸不同时电磁场的最大强度。数值计算结果表明,尺寸为二倍波长的划痕可以获得最大的电磁场强度,此时容易导致自聚焦;亚波长级和足够大尺寸的划痕作用基本可以忽略。一定尺寸的划痕,深度越大,最大场强也越大,但当划痕过深时最大场强反而会降低。     

Femtosecond laser erasing and rewriting of self-organized planar nanocracks in fused silica glass

Tightly focused, linearly polarized, femtosecond laser radiation can produce highly birefringent nanograting structures inside fused silica glass. Here we report that when the polarization direction of the femtosecond light is changed, old nanogratings are erased and simultaneously replaced with new ones whose orientation is solely determined by the polarization of the rewrite beam. We also show that these volume nanogratings can be rewritten 1000 times with little degradation in their quality.     

熔石英表面热致应力对激光损伤行为影响的研究

为了研究热致应力对光学元件损伤特性的影响,通过实验测试退火处理消除热应力和未消热应力石英基片的激光损伤特性,研究了热致应力对石英元件初始损伤阈值、损伤增长阈值以及损伤增长规律的影响.结果表明,热致应力对熔石英光学元件的初始损伤阈值有影响,初始损伤阈值随着热致应力增大而降低;热致应力会加剧激光引发的损伤增长,相同的激光通量下,表面应力越大的区域拥有越高的损伤增长因子,但损伤增长仍遵从指数增长规律.热致应力对损伤增长阈值没有明显的影响.本文的研究将为CO2激光预处理工艺能否被应用于大口径光学元件提供一个必要的技术参考.     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

Design and development of low-cost room temperature electric field poling system for the fabrication of quasi-phase matching devices

Based on density functional theory, we perform first-principles investigations to study the optical properties of the O-, F- and H-terminated SiC nanoribbons with armchair edges (ASiCNRs). By irradiating with an external electromagnetic field, we calculate the dielectric function, reflection spectra, energy loss coefficient and the real part of the conductance. It is demonstrated that the optical constants are sensitive to the low-energy range, different terminal atoms do not make much difference in the shape of the curves of the optical constants for the same-width ASiCNR, and the optical constants of wider nanoribbons usually have higher peaks than that of the narrower ones in low energy range. We hope that our study helps in experimental technology of fabricating high-quality SiC-based nanoscale photoelectric device.     

Fabrication and stitching of embedded multi-layer micro-gratings in fused silica glass by fs laser pulses

Fabrication and stitching of internal 2D, 1D and multi-layer micro-gratings in fused silica glass using amplified Ti:sapphire femtosecond laser were reported. These gratings have the pitch of 4 μm and the size of 400 μm×400 μm. For a two-layer 1D micro-grating where a second-layer grating was overwritten on a first-layer grating at the exact X,Y position and the different Z depth, the diffraction efficiency can reach more than 25% due to the grating thickness increase. If a second-layer grating was stitched with a first-layer by the shift of 2 μm in the X direction and at the different Z depth, the diffraction angle was doubled but the diffraction efficiency was about 9%. The last result has the potential application for fabricating high-density micro-/nano-structures beyond the diffraction limit through 3D stitching. PACS 42.79.Dj; 42.40.Lx; 42.62.Cf     

Ultrafast time-resolved photography of femtosecond laser induced modifications in BK7 glass and fused silica

The dynamics of absorption after excitation of fused silica and BK7 glass with femtosecond laser radiation are visualized by transient absorption spectroscopy. Focusing laser radiation with pulse durations in the picosecond time regime in BK7 glass generates free electrons with relaxation by emission of radiation or by formation of defects. The temporal and spatial emission characteristics are observed by high-speed photography in the streak mode. The beam waist moves within the pulse duration towards the incoming laser radiation by self-focusing and with the laser radiation absorbed by multi-photon processes. The dynamics of the long lasting stress formation is visualized by time-resolved Nomarski-Photography. The modification of the glass is investigated during and after irradiation with ultra-short pulsed laser radiation (100 fs<t_p<3 ps) at the wavelength =810 nm. The formation of a sound wave in fused silica and BK7 glass is observed and the mechanical stress, depending on the excitation pulse duration, is measured. PACS 06.60.Jn; 42.50.Md; 78.47.-p; 81.16.-c; 82.53.-k     

Nondestructive method for characterization of the second-order nonlinear profile and charge distribution in thermally poled fused silica

Noncollinear frequency conversion of two femtosecond pulses is used to characterize the nonlinear profile induced in thermally poled fused silica. This technique permits one to deduce the nonlinear profile and the charge distribution induced by the poling treatment. These data and the current that passes through the sample during the poling are consistent with the occurrence of a built-in electric field associated with the diffusion of two ionic species.     

Synthesis and thermal stability of gold nanowires within monolithic mesoporous silica

This paper describes the preparation, by a novel and simple method, and the thermal stability of gold nanowires within monolithic mesoporous silica, involving soaking monolithic mesoporous silica in HAuCl₄ aqueous solution, followed by drying and subsequent step-annealing. It has been shown that reduction of Au³⁺ within silica pores can occur during the drying process at 80 °C without any special reduction treatment. After initial annealing at 300 °C, Au nanowires are formed within the pores and are stable at temperatures up to 500 °C. Increasing the annealing temperature leads to a wire-to-rod-to-sphere morphological transformation of the Au nanowires. The surface-mediated reducing groups (-OH) on the silica pore are responsible for the low-temperature reduction of Au³⁺ ions, and the formation of Au nanowires is attributed to the uni-directional diffusion of Au atoms and the confinement of the pore channels. Spheroidization and breaking at some defects in the Au nanowires during annealing at elevated temperature result in the wire-to-rod-to-sphere transformation, accompanied by a blue-shift of the surface plasmon resonance over a very wide region in the optical spectrum. PACS 81.07.-b; 81.40.-z; 81.05.Rm     

熔石英和磷酸盐钕玻璃表面三角形微裂纹对入射光的散射分析

采用时域有限差分方法(FDTD)模拟了石英玻璃和掺钕磷酸盐激光玻璃表面三角形微裂纹对入射光调制后的分布。TE模式光波入射,最大调制电场幅值出现在玻璃体内部;TM模式光波入射,最大调制电场幅值则发生在裂纹的缝隙中。两种入射光模式下,后表面的调制电场幅值增大倍数在相同裂纹条件下均比前表面大。TE模式入射时,当入射光在裂纹和玻璃表面相继发生全反射时,得到的调制电场幅值最大;后表面最大调制电场幅值随着裂纹在界面投影长度的增加、开口宽度的变大及裂纹深度的增加而分别增大。     

熔石英和磷酸盐钕玻璃表面三角形微裂纹对入射光的散射分析

 采用时域有限差分方法(FDTD)模拟了石英玻璃和掺钕磷酸盐激光玻璃表面三角形微裂纹对入射光调制后的分布。TE模式光波入射,最大调制电场幅值出现在玻璃体内部;TM模式光波入射,最大调制电场幅值则发生在裂纹的缝隙中。两种入射光模式下,后表面的调制电场幅值增大倍数在相同裂纹条件下均比前表面大。TE模式入射时,当入射光在裂纹和玻璃表面相继发生全反射时,得到的调制电场幅值最大;后表面最大调制电场幅值随着裂纹在界面投影长度的增加、开口宽度的变大及裂纹深度的增加而分别增大。     

Xe离子束辐照硼硅酸盐玻璃和石英玻璃效应对比研究

玻璃固化体作为放射性废物地质处置的第一道安全屏障,它的耐辐照性能研究至关重要.玻璃固化体主要网络结构硅氧四面体与石英玻璃的硅氧四面体是一致的,所以这里用石英玻璃代替玻璃固化体作为研究对象.本文采用Xe离子在相同条件下辐照石英玻璃和硼硅酸盐玻璃.利用纳米压痕技术和椭圆偏振仪表征了辐照前后样品的硬度、模量以及折射率的变化情况.结果表明:硼硅酸盐玻璃和石英玻璃的硬度均随着辐照剂量的增大而减小,硼硅酸盐玻璃的模量随着辐照剂量的增大而减小;石英玻璃的模量随着辐照剂量的增大而增大.模量的变化可能和密度的变化有关,这点与折射率的结果相符.     

Influence of electrode composition on the second-order nonlinearity profile in thermally poled silica glass

Silica glass plates (Infrasil from Heraeus) have been thermally poled with different types of electrodes (evaporated or pressed-on) having different chemical compositions (Au, Al, n-doped Si or p-doped Si). We show for the first time to our best knowledge that the profile of the induced second-order nonlinearity strongly depends on the composition of electrodes used for the thermal poling process.