飞秒激光诱导自组织纳米周期结构及其光学特性的研究进展

飞秒激光具有超快、超强的特点.飞秒激光微纳加工发展非常迅速. 本文综述了近十年来利用飞秒激光在金属、半导体、介质等各类材料中制备的纳米周期结构, 阐述了若干关于飞秒激光诱导纳米周期结构的物理机理的观点.讨论了基于偏振调制的多光束 干涉在半导体表面制备纳米周期结构,简要叙述了周期结构对材料光学特性的影响. 关键词： 飞秒激光 纳米周期结构 多光束干涉 光学特性     

不同烧蚀条件下飞秒激光脉冲诱导ZnO纳米结构研究

烧蚀条件对飞秒激光脉冲诱导氧化锌纳米结构有重要影响.研究了800 nm,150 fs,250 kHz的飞秒激光脉冲分别在空气中,去离子水中以及无水乙醇中垂直聚焦于氧化锌晶体表面,诱导形成不同形态的纳米结构.实验结果表明,在空气中利用飞秒激光脉冲辐照样品表面,形成了周期为180 nm的纳米线;在去离子水中辐照诱导形成了由氧化锌纳米线聚集而成的"纳米球";在无水乙醇中形成出现分叉结构的纳米线.拉曼光谱分析辐照前后晶体晶相结果表明,形成的纳米结构相对于辐照前特征峰437 cm-1强度有所下降,在570 cm-1处的峰值则显著增强.分析了在各种烧蚀条件下诱导形成纳米结构的演化过程以及物理机理.     

单光束飞秒激光诱导的电子态密度分布对双周期纳米光栅的影响EI北大核心CSCD

研究了不同脉冲能量下1kHz飞秒激光脉冲在石英玻璃内部诱导的损伤痕迹、纳米光栅结构及其双折射特性,发现在激光辐照区域顶端形成的微纳结构具有两种周期性:沿光传输方向的周期为ΛK;沿激光偏振方向的周期为ΛE.通过数值模拟飞秒脉冲在石英玻璃内部的传输过程,研究了入射能流密度分布及自由电子密度分布对双周期纳米光栅结构的影响.结果表明,较大的入射能流密度有利于纳米光栅的形成,且产生的电子密度会影响周期ΛK,电子密度越大,周期ΛK越大.从理论上分析了双周期纳米光栅结构的形成过程,认为等离子体非对称生长及其引起的局域场强分布影响了双周期纳米光栅结构的形成.     

单光束飞秒激光诱导的电子态密度分布对双周期纳米光栅的影响

研究了不同脉冲能量下1kHz飞秒激光脉冲在石英玻璃内部诱导的损伤痕迹、纳米光栅结构及其双折射特性,发现在激光辐照区域顶端形成的微纳结构具有两种周期性:沿光传输方向的周期为ΛK;沿激光偏振方向的周期为ΛE.通过数值模拟飞秒脉冲在石英玻璃内部的传输过程,研究了入射能流密度分布及自由电子密度分布对双周期纳米光栅结构的影响.结果表明,较大的入射能流密度有利于纳米光栅的形成,且产生的电子密度会影响周期ΛK,电子密度越大,周期ΛK越大.从理论上分析了双周期纳米光栅结构的形成过程,认为等离子体非对称生长及其引起的局域场强分布影响了双周期纳米光栅结构的形成.     

飞秒激光直写偏振转换器件和几何相位器件(特邀)

光学器件的微型化和集成化是当前研究的热点,其中利用微纳结构实现局域电磁改性的超构光学更是引人关注.本文从微纳结构的制备出发,总结了利用超衍射精密加工的飞秒激光直写技术,制备偏振转换器件和几何相位器件的工作.介绍了微纳结构改性的物理机制,飞秒激光直写光刻胶、飞秒激光烧蚀金属薄膜、飞秒激光诱导纳米光栅等手段在偏振转换器件和...     

Al/ZnO/Al薄膜的结构与磁性分析

采用直流磁控溅射的方法制备了Al/ZnO/Al纳米薄膜,并对薄膜分别在真空及空气中进行退火处理.利用X射线衍射仪(XRD)和物理性能测量仪(PPMS)分别对薄膜样品的结构和磁性进行了表征.XRD分析表明,不同的退火氛围对薄膜的微结构有着很大的影响.采用了一种新的修正方法对磁测量结果进行修正,计算了基底拟合误差的最大值,并对修正后样品的磁性进行了分析.结果显示,室温铁磁性可能与Al和ZnO基体之间发生的电荷转移以及在不同退火氛围下Al在ZnO晶格中的地位变化有关. 关键词： Al/ZnO/Al薄膜 铁磁性 磁性表征     

WS2纳米薄膜厚度依赖的三阶非线性吸收特性研究

利用液相剥离法制备了WS2纳米片,结合真空抽滤技术,控制上清液体积制备了不同厚度的WS2薄膜.在此基础上,使用800 nm飞秒激光的Z扫描技术表征了WS2纳米薄膜的三阶非线性吸收特性.研究发现,制备的不同厚度的WS2都表现出可饱和吸收特性,可饱和吸收主要是由于单光子吸收所引起的泡利阻塞效应引起;随着厚度的增加,饱和强度...     

飞秒激光辐射诱导金属表面微纳结构研究

通过1 kHz的飞秒脉冲(脉宽130 fs,中心波长800 nm)对厚度为60 μm的不锈钢65Mn表面进行飞秒微加工,通过拟合得到65Mn的消融阈值为0.5 J·cm-2。研究了飞秒激光作用下表面形成的多种微结构,其中包括纳米孔及纳米柱状物。同时讨论了激光能量和作用脉冲个数对微结构形成的影响。随着周期波纹结构的形成,发现在各种能量和脉冲个数条件下,周期结构的周期约为入射脉冲的波长。相同的激光功率下,在不同加工速度和加工次数下对不锈钢进行表面微加工,得到了规则的圆孔阵列结构。     

飞秒激光双光束干涉高效率地制备硅表面超亲水结构

利用飞秒激光双光束干涉技术,结合柱透镜线扫描技术,在30 s内制备了面积为10 mm×10 mm的微米-纳米复合结构,极大地提升了激光加工效率.飞秒激光刻蚀后的硅表面包含双光束干涉引起的长周期微米结构,以及飞秒激光诱导的纳米周期结构.微米-纳米复合结构极大地提升了表面粗糙度,在毛细效应的作用下,硅在空气中显示出超亲水性...     

Al/ZnO:Al薄膜结构的荧光增强效应

利用物理气相沉积设备制备了Al/ZnO:Al薄膜样品,研究了该薄膜结构的发光特性。结果表明,在ZnO:Al薄膜表面镀一层Al岛薄膜可以增强其带边荧光,同时在475 nm附近产生蓝光峰。通过在Al岛薄膜和ZnO:Al薄膜之间引入一层5 nm的Ta₂O₅绝缘层可以使ZnO:Al薄膜的带边荧光和蓝光显著增强,并随着Ta₂O₅绝缘层厚度的增大而减弱。通过对Al/ZnO:Al样品进行退火处理可以使带边荧光和蓝光峰分别增强9倍和83倍。基于局域表面等离子体共振理论,计算了Al/ZnO:Al纳米结构的光学散射和吸收截面曲线。实验结果与理论计算相一致。     

Interaction of ultra-short laser pulses with CIGS and CZTSe thin films

The thin-film solar cell technologies based on complex quaternary chalcopyrite and kesterite materials are becoming more attractive due to their potential for low production costs and optimal spectral performance. As in all thin-film technologies, high efficiency of small cells might be maintained with the transition to larger areas when small segments are interconnected in series to reduce photocurrent and related ohmic losses in thin films. Interconnect formation is based on the three scribing steps, and the use of a laser is here crucial for performance of the device. We present our simulation and experimental results on the ablation process investigations in complex CuIn_1?x Ga _x Se₂ (CIGS) and Cu₂ZnSn(S,Se)₄ (CZTSe) cell’s films using ultra-short pulsed infrared (~1 μm) lasers which can be applied to the damage-free front-side scribing processes. Two types of processes were investigated—direct laser ablation of ZnO:Al/CIGS films with a variable pulse duration of a femtosecond laser and the laser-induced material removal with a picosecond laser in the ZnO:Al/CZTSe structure. It has been found that the pulse energy and the number of laser pulses have a significantly stronger effect on the ablation quality in ZnO:Al/CIGS thin films rather than the laser pulse duration. For the thin-film scribing applications, it is very important to carefully select the processing parameters and use of ultra-short femtosecond pulses does not have a significant advantage compared to picosecond laser pulses. Investigations with the ZnO:Al/CZTSe thin films showed that process of the absorber layer removal was triggered by a micro-explosive effect induced by high pressure of sublimated material due to a rapid temperature increase at the molybdenum-CZTSe interface.     

氧压对飞秒激光沉积ZnO/Si(100)薄膜光学性能的影响

利用飞秒脉冲激光沉积法在n-Si（100）单晶衬底上制备了ZnO薄膜, 分析了衬底温度、激光能量、氧压及退火处理对薄膜结构和光学性能的影响. X射线衍射结果表明, 当激光能量为15?mJ、氧压为10?mPa时, 80?℃生长的薄膜取向性最好. 场扫描电子显微镜结果显示薄膜的晶粒尺寸随激光能量的增加而减小、随衬底温度的升高而增大且退火后明显变大. 紫外-可见透射光谱显示薄膜具有90%以上的可见光透过率.光致发光谱表明当氧压为10 mPa时,除了ZnO的紫外本征峰外, 还有一波长为410 nm的强紫光峰, 当氧压增至20 mPa以上, 所有缺陷峰均消失, 只有376 nm处的紫外本征峰. 与纳秒激光法所制备的薄膜特性进行了比较, 结果表明, 虽然纳秒激光沉积所制备的薄膜具有更高的c轴取向度, 但飞秒激光沉积制备的薄膜具有更好的发光性能. 关键词： 氧化锌 飞秒脉冲激光沉积 透过率 光致发光     

Fabrication of ZnO thin films by femtosecond pulsed laser deposition

The growth of ZnO thin films on sapphire substrate using the femtosecond PLD technique is reported. The effect of substrate temperature and oxygen pressure on the structural properties of the films was studied. Highly c-axis oriented ZnO films can be grown on sapphire substrates under vacuum conditions using the femtosecond PLD process. There is an optimum substrate temperature for the pulsed laser deposition of ZnO film that enhances the thermodynamic stability and allows the formation of well-crystallized thin films. The crystal quality of the films can be further improved by increasing the deposition time and introducing oxygen during the pulsed laser deposition process.     

Light trapping for a‐Si:H/µc‐Si:H tandem solar cells using direct pulsed laser interference texturing

We present a‐Si:H/µc‐Si:H tandem solar cells on laser textured ZnO:Al front contact layers. Direct pulsed laser interference patterning (DLIP) was used for writing arrays of one‐dimensional micro gratings of submicron period into ZnO:Al films. The laser texture provides good light trapping which is indicated by an increase in short‐circuit current density of 20% of the bottom cell limited device compared to cells on planar ZnO:Al. The open‐circuit voltage of the cells on laser textured ZnO:Al is almost the same as for cells on planar substrates, indicating excellent growth conditions for amorphous and microcrystalline silicon on the U‐shaped grating grooves. DLIP is a simple, single step and industrially applicable method for large area periodic texturing of ZnO:Al thin films. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400-800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.     

Dielectric function dynamics during femtosecond laser excitation of bulk ZnO

Using a broad band dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers causes damping of the exciton resonance and renormalization of the band gap causing broadband (2.3–3.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.     

Excimer laser deposited CuO and Cu<Subscript>2</Subscript>O films with third-order optical nonlinearities by femtosecond <Emphasis Type="Italic">z</Emphasis>-scan measurement

By controlling the oxygen pressure, single-phase CuO and Cu₂O thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV–VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ ⁽³⁾ and Im χ ⁽³⁾, of the films were determined. Both CuO and Cu₂O films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with Cu₂O films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and Cu₂O films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and Cu₂O materials.     

飞秒激光诱导铝等离子体时间分辨光谱研究

在大气环境下利用中心波长800nm、脉宽为30fs的激光聚焦在铝靶上,测定了激光诱导铝等离子体中铝原子的时间分辨发射谱。在局部热平衡条件近似下,根据实验测定的谱线相对强度得到了等离子体的电子温度;研究了激光脉冲能量对等离子体电子温度的影响和等离子体电子温度的时间演化特性。同时,实验发现了394.4nm和396.1nm两条铝原子谱线存在较强的自吸收效应,实验结果表明随着激光脉冲能量的减少和延时的增加,自吸收现象逐渐消失。     

XRD study on the effect of the deposition condition on pulsed laser deposition of ZnO films

Using a pulsed laser deposition (PLD) process on a ZnO target in an oxygen atmosphere, thin films of this material have been deposited on Si(111) substrates. An Nd: YAG pulsed laser with a wavelength of 1064 nm was used as the laser source. The influences of the deposition temperature, laser energy, annealing temperature and focus lens position on the crystallinity of ZnO films were analyzed by X-ray diffraction. The results show that the ZnO thin films obtained at the deposition temperature of 400°C and the laser energy of 250 mJ have the best crystalline quality in our experimental conditions. The ZnO thin films fabricated at substrate temperature 400°C were annealed at the temperatures from 400°C to 800°C in an atmosphere of N₂. The results show that crystalline quality has been improved by annealing, the optimum temperature being 600°C. The position of the focusing lens has a strong influence on pulsed laser deposition of the ZnO thin films and the optimum position is 59.5 cm from the target surface for optics with a focal length of 70 cm.