金属介电反射光栅的设计
文献类型：学位论文
| 作者 | 胡安铎 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 周常河 |
| 关键词 | 反射光栅 简化模式方法 傅里叶模态法 1/4波片 分束器 |
| 其他题名 | Design of metal-dielectric reflective gratings |
| 中文摘要 | 光栅是一类重要的衍射光学元件，广泛应用于测量、显示、激光等技术领域。在拍瓦级飞秒激光系统等应用中需要宽带高效率、高激光破坏阈值的反射光栅。传统的金属反射光栅可以实现接近平顶的宽带高效率，但是其激光破坏阈值较低。多层介电膜反射光栅具有高效率、高激光破坏阈值的优点，可以在20纳米的带宽内实现效率接近100%的衍射。然而，如果要将飞秒脉冲的脉宽压缩至20飞秒以下，所需要高效率光栅的带宽将超过200纳米，对于多层介电膜反射光栅而言，这是一个巨大的挑战。金属介电反射光栅利用金属的宽带反射和介电光栅的衍射，在波长800纳米附近可以实现超过200纳米带宽的高效率衍射。因此，金属介电反射光栅有着重要的应用前景。本文围绕金属介电反射光栅的设计与分析，介绍以下几个方面的工作： 1）高效率、宽带低电场强度增强的金属介电反射光栅的设计。实现高激光破坏阈值需要降低光栅内部的电场强度增强。因此，设计了具有高效率、宽带低电场强度增强特性的金属介电反射光栅。该光栅的结构为在基底上依次镀上铬膜、金膜和二氧化硅膜，在二氧化硅膜上刻蚀矩形槽。通过分析光栅的−1级衍射效率和固体材料内的最大电场强度增强随光栅深度、占空比的变化，发现存在高效率、低电场强度增强的参数区域。利用傅里叶模态法和模拟退火算法，得到了优化的光栅结构参数。优化的光栅在波长1053纳米附近26纳米的带宽内衍射效率高于95%, 超过70纳米带宽内最大电场强度增强低于1.2。为了指导光栅的制作，分析了衍射效率和最大电场强度增强随光栅深度、连接层厚度等参数的变化。考虑到顶部光栅形貌对衍射效率及电场增强的影响，优化设计了正弦脊、三角脊金属介电反射光栅，这对金属介电反射光栅的设计具有重要的参考价值。 2）宽带金属介电反射光栅的简化模式分析。从功能上看，金属介电反射光栅可以划分为顶部透射光栅与一个由连接层和金属层构成的高反射率的反射镜。当光栅的线密度比较高时，入射光在顶部透射光栅内仅激发两个传播模式。如果两个传播模在光栅区传播一次积累的位相差为π/2的奇数倍，则反射光栅0级方向的衍射波干涉相消，−1级方向的衍射波干涉加强，因此, −1级衍射效率达到最大。利用简化模式方法，给出了估算−1级和0级衍射效率以及实现−1级高效率所需的光栅深度的表达式。根据干涉原理，给出了实现高效率所需连接层厚度的表达式。分析了两个传播模式的有效折射率之差随入射波长和光栅占空比的变化，发现优化的宽带金属介电反射光栅的占空比趋于模式有效折射率之差的最大值对应的占空比。在简化模式分析相关结论的指导下，设计了偏振无关宽带金属介电反射光栅。 3）基于亚波长金属介电光栅的宽带反射式1/4波片。传统1/4波片，利用晶体的双折射性质，不易实现宽带功能。利用亚波长光栅的形式双折射性质，设计了基于亚波长金属介电光栅的宽带反射式1/4波片。在垂直入射条件下，该波片对TE（电场矢量平行于光栅槽）和TM（磁场矢量平行于光栅槽）偏振入射波的衍射效率均高于97%，在182纳米（1455~1627纳米）带宽内，TE和TM衍射波的位相差为90±1°。利用简化模式方法，对优化的宽带1/4波片进行了分析。 4）基于金属介电光栅的反射式1×2和1×3分束器。所设计的反射式分束器针对波长为1064纳米的TE偏振平面波。当入射角为30o时，优化的1×2分束器可以实现−1级和0级垂直输出，−1级和0级衍射效率分别为49.2%和49.1%。优化的1×3分束器，可以实现−1、0、+1三个衍射级次输出，当入射角度为5°时，三个级次的衍射效率分别为32.66%、32.71%和32.72%。优化的反射式1×2和1×3分束器具有高效率和较好的均匀性。 本论文设计了−1级高效率光栅、宽带1/4波片、1×2和1×3分束器等反射式微光学元件，拓展了金属介电反射光栅的应用。简化模式分析揭示了−1级高效率金属介电反射光栅衍射的物理图像，对于包括多层介电膜反射光栅在内的顶部有一层透射光栅的反射光栅的设计具有指导意义。 |
| 英文摘要 | As important diffractive optical elements, gratings are widely used in the fields of optical measurement, optical display, laser, etc. In applications such as petawatt-class femtosecond laser system, reflective gratings with wideband high efficiency and high laser-induced damage threshold (LIDT) are needed. Conventional metallic gratings can realize quasi-top-hat wideband high efficiency, but their LIDTs are low. Multilayer dielectric (MLD) gratings have the merits of high efficiency and high LIDT, and they can achieve diffraction efficiency close to 100% over a bandwidth of 20 nm. However, in order to compress pulses to be shorter than 20 femtoseconds, the reflective grating should have high efficiency over a bandwidth of 200 nm, which is a great challenge for a MLD grating. Metal-dielectric gratings, which utilize the wideband reflection of the metal layer and the diffraction of a dielectric grating, can realize high-efficiency diffraction over bandwidth of 200 nm around wavelength of 800 nm. Therefore, metal-dielectric reflective grating should be useful in such applications. Some works on the design and applications of metal-dielectric reflective gratings are presented in this doctoral dissertation. 1) Metal-dielectric reflective gratings with high efficiency and wideband low enhancement of electric field intensity are designed. The high LIDT is relative to low electric field intensity in solid material. Therefore, a metal-dielectric reflective grating, which has rectangular grooves in a silica film coated on a gold film, a chromium film and a substrate in sequence, is designed to achieve high efficiency, wideband, and low field enhancement. By calculating the variations of the diffraction efficiency at the −1st order and maximum enhancement of electric field intensity in solid material with different grating depths and duty cycles, it is found that there are parameter regions for high efficiency and low enhancement of electric field intensity. With Fourier modal method (FMM) and simulated annealing (SA) algorithm, optimized parameters are obtained. Around wavelength of 1053 nm, the optimized grating can achieve diffraction efficiency higher than 95% over 26 nm wavelength bandwidth and the maximum enhancement of electric field as low as 1.2 over more than 70 nm wavelength bandwidth. Moreover, with the influence of the shape of grating ridge considered, gratings with sinusoidal and triangular ridges are optimized, which should be useful in design of meta-dielectric reflective gratings. 2) A simplified modal analysis of a wideband metal-dielectric reflective grating is presented. Functionally, a metal-dielectric grating consists of a transmission grating in the top layer and a highly reflective mirror composed of a connecting layer and a metal layer. When the line density of grating is high, only two propagating modes can be excited in the transmission grating in the top layer. If the two propagating modes accumulate a phase difference that is an odd multiple of π/2 after propagating through the grating region, possible diffraction waves in the direction of the 0th order will interfere destructively and those in the direction of the −1st order will interfere constructively, resulting in the maximum efficiency at the −1st order. By means of simplified modal method, simple expressions of diffraction efficiencies and grating depth to realize high efficiency are derived. A simple expression to estimate the thickness of connecting layer is presented according to interference principle. The variation of the difference of effective indices with duty cycles and wavelengths is investigated and it is found that the duty cycle of an optimized grating tends to that corresponding to the largest difference of effective indices. Under the guidance of simplified modal analysis, a polarization-independent wideband reflective grating is designed. 3) A wideband reflective quarter-wave plate is designed based on subwavelength metal-dielectric reflective gratings. Conventional quarter-wave plates which utilize the birefringence of crystals cannot work in a wide wavelength range. Using the form birefringence of a subwavelength grating, a wideband reflective quarter-wave plate based on a metal-dielectric grating is optimized. The quarter-wave plate can achieve diffraction efficiency higher than 97% for both TE (electric field parallel to grating grooves) and TM (magnetic field parallel to grating grooves) polarizations and the phase difference between TE and TM polarizations is in the range of 90±1° for wavelength from 1455 nm to 1637 nm. The simplified modal method is used to analyze the optimized quarter-wave plate. 4) Reflective 1×2 and 1×3 beam splitters in the form of metal-dielectric gratings are designed. These beam splitters work at wavelength of 1064 nm for a TE-polarized plane wave. When the incident angle is 30°, the −1st order and the 0th order of optimized 1×2 beam splitter are perpendicular, and their diffraction efficiencies are 49.2% and 49.1%, respectively. The optimized 1×3 beam splitter has three propagating diffraction orders, i.e., the −1st, 0th, +1st orders. At incident angle of 5°, the diffraction efficiencies of the three orders are 32.66%, 32.71% and 32.72%, respectively. The reflective 1×2 and 1×3 beam splitters based on metal-dielectric gratings exhibit high efficiency and good uniformity. In this dissertation, a series of reflective micro optical elements such as gratings with high efficiency at the −1st order, wideband quarter-wave plate, 1×2 and 1×3 beam splitters are designed. These optical elements extend the applications of metal-dielectric gratings. The modal analysis reveals the physical picture of the diffraction of high-efficiency metal-dielectric reflective gratings and can guide the design of the other reflective gratings that have a transmission grating in the top layer. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15725]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 胡安铎. 金属介电反射光栅的设计[D]. 中国科学院上海光学精密机械研究所. 2013. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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