Graphene Q-switched 0.9-um solid-state laser
The 4F3/2 4I9/2 quasi-three-level transition of laser crystal produces 915-nm continuous-wave laser. Through the graphene saturable absorber, the laser changes to giant pulses, which manifests that the graphene has great potential for the modulation of short wavelength laser.
The research group, led by Prof. Zhengping Wang, from State Key Laboratory of Crystal Materials, Shandong University, realized Q-switched output of 0.9-um laser with graphene as the saturable absorber. The graphene was prepared on K9 glass substrate with liquid phase exfoliation method. To elevate pulse energy and shorten pulse width, a mixed crystal, Nd:La0.11Y0.89VO4, was utilized as the laser medium whose emission cross section is smaller than that of Nd:YVO4 crystal. At an absorbed pump power of 7.62 W, the maximum average output power was 0.62 W, and the highest pulse energy, minimum pulse width were 2.58 uJ, 84 ns, respectively. The wavelength was 915 nm, corresponding to the 4F3/2 4I9/2 quasi-three-level transition of Nd:La0.11Y0.89VO4 crystal. Such laser has important applications in water vapor detection, ozone detection, and differential absorption radar. Its second harmonic generation can supply blue coherent light, which can be employed for biomedicine, underwater optical communication, high density optical storage, and laser color display. It is reported in Chinese Optics Letters Vol. 12, No.1, 2014 .
As a novel developed photoelectric material, the zero band-gap structure of graphene leads its saturable absorption (SA) characteristic that is wavelength insensitive, so full wave band saturable absorber can be fabricated. While the current popular SA materials such as Cr4+:YAG crystal or GaAs semiconductor just respond to certain wave bands, which have no universality. What’s more, compared with Cr4+:YAG or GaAs, graphene owns shorter fabrication period and lower production cost, which are favorable for large scale promotion. Here it is the first time to the best of our knowledge that graphene is used to the pulse modulation for the laser with wavelength shorter than 1 um, which fully demonstrates the advantages of preparation convenience, wavelength non-selectivity, short recovery time, and small loss. The present research manifests graphene possesses great potential and broad prospects for short wavelength laser modulation.
Next, the overall laser performance is planned to be improved by further optimizing the graphene, the laser medium, and the resonator parameters. Combing with nonlinear frequency doubling technique, new type solid-state blue laser source will be developed. At the same time, the graphene saturable absorber will be applied to more wave bands.
石墨烯调Q的0.9 um固体激光器
图片说明：激光晶体内4F3/2 4I9/2的准三能级跃迁产生915 nm的连续波激光，该激光通过石墨烯可饱和吸收体后变为巨脉冲激光输出，展示了石墨烯对于短波长激光的调制潜力。
山东大学晶体材料国家重点实验室王正平教授课题组利用液相剥离方法制备的石墨烯，实现了0.9 m激光的调Q输出。激光工作介质选用Nd:La0.11Y0.89VO4晶体, 其受激发射截面比Nd:YVO4更小，能够提高脉冲能量、缩短脉冲宽度。在7.62 W的吸收抽运功率下，获得了平均功率为0.62W，单脉冲能量为2.58 uJ，脉冲宽度为84 ns的准三能级激光输出，波长为915 nm。这种光源在水蒸汽探测、臭氧探测以及差分吸收雷达等方面都有重要应用。此外，经过非线性倍频可以产生蓝色激光，应用于生物医学、水下光通信、高密度光信息存储、激光彩色显示等领域。该研究成果将发表在Chinese Optics Letters 2014年第1期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-1-011401）。
作为一种新兴光电材料，石墨烯具有零带隙能带结构，这决定了它具有对波长不敏感的可饱和吸收特性，可作为全波段可饱和吸收器件，而目前常见的可饱和吸收材料，无论是Cr4+:YAG晶体还是GaAs半导体都只针对某一特定波段响应，不具有普适性。此外，石墨烯的制备周期、生产成本也远远优于上述材料，便于大规模推广。但是，到目前为止关于石墨烯可饱和吸收的报道仍局限于波长1 um以上激光。本研究实现了石墨烯对于1um以下激光的脉冲调制，充分展示了该类材料制备便利、适用波段宽、恢复时间短、损耗低的优点，在短波长激光领域显示出巨大潜力和广阔前景。
下一步将通过石墨烯、激光介质、谐振腔等参数的调整来进一步提高激光器的整体性能，并结合非线性倍频技术进行新型固体蓝光光源的研制。同时，也将探索石墨烯可饱和吸收体在更多新波段的应用。