基于石墨烯/硅混合波导的片上超连续谱光源研究

Implementation of integrated supercontinuum source can meet the urgent requirements for airspace integration, next-generation optical communication, and integrated optoelectronics. However, femtosecond source with bulky volume and high power consumption can’t be integrated with waveguide which generating the supercontinuum. This project creatively introduces two-dimensional graphene with ultrafast optical response to the silicon waveguides. The bottleneck of the slow carrier recovery in the silicon waveguide is broken. The strong optical nonlinear effect within the silicon waveguide is fully used. The capability of manipulation of pulse is fully discovered with hybrid graphene/silicon waveguide. We will set up the models for the femtosecond pulse transmitting in the silicon waveguides. The relation between the spectrum broaden of supercontinuum source and waveguide dispersion, Kerr effect, self-phase modulation effect and two-photon absorption effect. The simulated results would be used for design the silicon waveguide which generating the supercontinuum. A reliable process for graphene transfer and graphene pattern will be studied. The parameters of the modulation depth, waveguide configuration and the doping level of graphene will be examined to evaluate of the width and energy of ultrafast pulse. A monolithic silicon chip with a less-than-300 fs pulse width of an ultrafast pulse and a 1400 nm- 1800 nm spectrum range of a supercontinuum source having more than 500 MHz repetition rate will be realized. Within four years, 6-8 journal papers will be published and 2-3 chinese patents will be achieved.4-6 graduated students will be supported by this project.

可集成的超连续谱光源在空天一体化建设、下一代光通信网络和集成光电子学等领域需求迫切。然而，体积大、功耗高的飞秒脉冲源难以与产生超连续谱的波导集成。本项目创造性将具备超快光学响应的二维石墨烯材料集成在硅波导上，突破硅波导的载流子恢复时间慢的瓶颈，发挥硅波导的强光学非线性效能，挖掘石墨烯/硅混合波导调控脉冲的能力。本项目将建立飞秒脉冲在硅波导内传播的理论模型，研究超连续谱的光谱展宽与波导色散、克尔效应、自相位调制和双光子吸收效应的关系，优化设计硅波导。摸索一套可靠的石墨烯转移及其图形化工艺流程，研究石墨烯/硅混合波导的调制深度、波导形貌和石墨烯掺杂浓度等参数对超快脉冲宽度和能量的相对影响程度。在单个硅芯片上实现脉冲宽度优于300 fs的超快脉冲和光谱范围覆盖1400 nm-1800 nm、重复率大于500 MHz的超连续谱。发表6-8篇高水平论文，申请2-3项国家发明专利，培养4-6名研究生。

超连续谱光源具有传统宽带光源的宽光谱特性和激光光源的高空间相干性，在基础研究领域、光通信、光计算和生物成像等领域有着广泛的应用潜力。超连续谱产生依赖于脉冲激光与非线性介质相互作用。相比较于固态超连续谱和全光纤超连续谱光源，利用硅波导的非线性效应实现超连续谱光源不仅能大幅度降低设备大小和功耗，而且具备大规模制备的能力，有效降低超连续谱光源的成本。在硅波导中产生超连续谱光源的核心是使用高功率密度的飞秒脉冲泵浦高非线性波导。然而，受限于硅波导自身载流子复合时间慢的固有瓶颈，利用硅波导自身的饱和吸收效应很难产生飞秒脉冲。相较于三五族量子阱材料，石墨烯带隙为零，饱和吸收的带宽大。并且，石墨烯作为饱和吸收体还具备载流子复合寿命快散热能力强和易于与硅波导集成的优势。依托研究中心的微纳工艺平台，我们开发了一整套石墨烯/硅混合集成的工艺流程。采用标准的CMOS工艺，在硅波导上保留特定长度的石墨烯带，精度可达到微米量级。为了表征石墨烯/硅混合波导的载流子动态过程，我们采用了异步采样技术，即选用两个重频略有差异的光频梳。实验结果显示载流子的复合寿命仅为1.65ps，较硅波导自身提升了三个量级。在突破了载流子寿命长的瓶颈限制后，我们将石墨烯/硅混合集成波导与掺铒光纤连接起来，构成环腔型脉冲激光器。在泵浦激光达到200mW的条件下，硅波导中产生了脉冲宽度仅为543fs的激光。该结果是迄今为止在波导结构中使用石墨烯所产生的的最短的。剔除硅波导自身的插入损耗，该脉冲的工作阈值较光纤激光器小了近两个量级，意味着在进一步优化硅波导插入损耗的基础上，所需掺铒光纤的长度缩减，脉冲的重频可从现有的54.37MHz得到显著提升。不同于光纤，我们还考虑了硅波导中特有的双光子吸收效应来描述超连续谱的产生过程。在输入脉宽为100fs，输入功率仅为15W的条件下，在长度仅为6mm的硅波导内足以产生光谱范围覆盖1200nm-1700nm的超连续谱。