【作者】 李威；

【导师】 童利民；

【作者基本信息】 浙江大学 ， 光学工程， 2014， 博士

【摘要】 原子层厚度的二维石墨烯晶体具有线性色散的能带结构以及极强的载流子带间跃迁,能实现超快的宽带光与物质相互作用,可以自由贴附于光学表面实现光子结构的功能化,已经在激光锁模、超快光调制、偏振控制、光电探测等方面显示出良好的应用前景,其中石墨烯超快光调制是最具挑战和器件应用前景的前沿研究方向之一。最近,通过外加电场改变石墨烯中的费米能级,Liu等人首次在实验上演示了光学波段的石墨烯光调制器,但是,受限于器件电子回路的寄生电容等效应,最高调制速率为1GHz量级。如果能使用全光调制方式,将可以绕开电子回路的寄生响应,显著提高调制速率。双锥形的氧化硅微纳光纤是从标准光通信单模光纤中直接拉伸而得,通过严格控制光纤拉锥形状和拉伸参数,可以制备出具有具有亚波长直径、超低单模传输损耗的微纳光纤。利用这种双锥形微纳光纤,可以将光从标准单模光纤端以极低的额外损耗经拉锥过渡区耦合进入并以基模(HE11模)沿着微纳光纤传导,再经拉锥过渡区传导回另一端标准单模光纤。这种形式的光传输,不仅能增强微纳光纤表面光与物质的相互作用,同时也能实现微纳结构中的光信号与标准光纤系统的高效衔接。基于上述考虑,本论文提出基于微纳光纤的石墨烯超快全光调制器研究设想。利用微纳光纤的强倏逝场和导波效应来增强光与石墨烯相互作用,首次在光通信波段获得2.2ps(计算得到的等效带宽约为450GHz；考虑到飞秒脉冲为高斯光束,其时间-带宽常数为0.44,则通过计算得到的对应带宽大约是200GHz)的石墨烯超快光调制实验结果,调制深度达到38%。同时,该调制器结构紧凑、与光纤系统兼容,在光通信、光计算、光逻辑等方面均具有潜在的应用前景。全文共分为以下五个章节：本论文第一章为绪论,简要介绍本课题的目的与意义,以及微纳光纤和石墨烯的研究背景,并概述本论文的主要工作。本论文的第二章主要研究微纳光纤的光传输特性及实验制备。首先,研究微纳光纤的模场分布、单模传输条件以及色散等特性。其次,研究了本文中所使用的双锥形氧化硅微纳光纤的制备,并基于微光纤中多模干涉效应用于应变传感、激光加热熔接微纳光纤用于激光发射等工作研究了微纳光纤的功能化方法。本论文的第三章主要介绍光学质量的石墨烯包层微纳光纤的研制及光传输特性表征。在此,我们首次提出石墨烯与低损耗微纳光纤复合结构,用于全光纤饱和吸收及全光调制。我们通过撕胶带法制备出寡层石墨烯薄膜,将其转移并包裹在微纳光纤表面,获得光学质量的石墨烯包层微纳光纤样品。同时,我们对石墨烯包层的光场功率密度和模场分布进行了理论建模和计算,对石墨烯包层微纳光纤的线性及非线性(饱和吸收)传输特性进行了实验测量,结果显示,石墨烯包层微纳光纤具有很好的饱和吸收效应。在本论文的第四章中,我们首次从实验上演示了石墨烯的超快全光调制器。利用纳秒脉冲对石墨烯包层微纳光纤的饱和吸收效应,实现了对1.5微米波段连续光的全光调制。为测试该调制器的超快时间响应,我们搭建了一套基于光纤传输系统的泵浦-探测实验系统,实验测得石墨烯包层微纳光纤的响应时间为2.2ps(对高斯脉冲而言,其对应的调制速率为200GHz),调制深度达到38%,较此前的自由空间实验的光透过率变化率提高了近2个数量级。本文第五章为本工作的总结与展望。简要概括了本文主要研究成果,同时,提出了在本文基础上可以进一步开展的工作。总的来说,本文提出并实现的基于微纳光纤的石墨烯超快全光调制器,是石墨烯光子学和光纤光学结合的一个成功范例,不仅拓展了光纤光学和石墨烯光子学在超快光子技术领域的应用潜力,也为未来超快光信息处理开辟了一条新的途径。更多还原

【Abstract】 Owing to its linearly dispersive conduction and valence bands and the strong interband transitions, the atomic-thickness two-dimensional graphene film allows broadband light-matter interactions with ultrafast responses, and can be readily pasted to surfaces of functional structures for a variety of photonic and optoelectronic applications including mode-locked lasers, ultrafast optical modulators, broadband polarizers and ultrafast photodetectors. Among the above-mentioned possibilities, graphene ultrafast optical modulation is one of the most promising and challenging techniques for device applications. Recently, by electrically tuning the Fermi level of a graphene film to modify the interband transitions of graphene, Liu et al. successfully demonstrated a high-speed graphene-based optical modulator. The modulation bandwidth was however limited to～1GHz by the response time of the bias circuit. Obviously, the "electrical bottleneck" on the modulation rate can be circumvented by an all-optical scheme.Biconical silica optical micro-/nanofibers (MNFs) tapered down from standard telecom single-mode fibers, have been used for launching light into and collecting signal out from micro/-nano scale components or devices. With proper taper geometries, light from the standard single-mode fiber can be guided through the taper region and propagation along the subwavelength-diameter microfiber in single-mode (fundamental HE11mode) with low transmission loss, which does not only enhance the light-matter interaction on the MNF surface, but also realize highly efficient optical connection between micro/nanostructures and standard optical fibers.In this work, we propose, for the first time to our knowledge, an ultrafast all-optical modulator based on a graphene-cladded MNF. Relying on significantly enhanced evanescent light-graphene interaction in a tightly confined MNF waveguiding structure, we experimentally demonstrated a graphene optical modulation with modulation time down to2.2-ps (corresponding to a calculated bandwidth of450GHz; for Gaussian pulses with a time-bandwidth product of0.44, the calculated bandwidth is～200GHz.) in a single-mode optical fiber around1.5-μm wavelength (the C-band of optical communication). The maximum modulation depth is about38%, which is two orders of magnitude larger than that in free-space measurement. This compact modulator is compatible with fiber-optic communication networks, and may find applications in optical communications, optical computing and optical logic devices. The thesis consists of five chapters, as introduced below:The first part is an introductory chapter, which briefly introduces the motivation and aims of this study, followed by the research background of optical MNFs and graphene.The second chapter investigates optical properties and fabrication of MNFs. Firstly, we introduce optical properties of MNFs, including mode distribution, surface power density, single-mode condition and waveguide dispersion. Secondly, we study the flame-heated taper drawing fabrication and optical characterization of biconical MNFs with low optical loss. Then, based on the multimode-interference effect in single microfiber and fusion spliced MNFs via CO2laser heating, we demonstrate the approaches to structural functionalization of MNFs for strain sensing and closed-loop ring lasing.The third chapter focuses on the fabrication and optical characterization of graphene-clad-microfiber (GCM). Here, we propose, for the first time, an ultrafast all-optical graphene modulator based on the hybrid GCM structure. We first prepare a few layer graphene flake by micromechanical exfoliation of highly oriented pyrolytic graphite, and then transfer and wrap a graphene flake around the microfibers via micromanipulation. Besides, we model the surface power density of GCM, and experimentally measure the linear and nonlinear transmission of the GCM, which shows excellent saturable absorption properties of GCMs.In the fourth chapter, we experimentally demonstrate graphene ultrafast all-optical modulation. Firstly, by co-propagating a train of1064-nm5-ns nanosecond laser pulses and a1550-nm CW light in a GCM, we show graphene all-optical modulation around1550-nm wavelength. Secondly, by employing an in-fiber optical pump-probe technique, we measure the ultrafast dynamic response of the GCM modulator. We obtain a measured response time down to2.2ps, corresponding to a calculated modulation rate of～200GHz for Gaussian pulses. The maximum modulation depth is～38%, which is about2orders of magnitude higher than previous free-space normal incident optical differential transmission measurements.Finally, in the last chapter, we give a brief summary and outlook of our work.Overall, the graphene-clad-microfiber ultrafast all-optical modulator demonstrated in this work, represents a successful example of merging graphene photonics and fiber optics, which does not only extend the reach of fiber optics and graphene photonics for ultrafast optical technology, but also opens a new opportunity for future ultrafast optical signal processing.更多还原