激光诱导石墨烯全光调谐超高Q值回音壁微腔【增强内容出版】
1 引言
回音壁微腔基于光的全反射原理,将光波束缚在μm量级的谐振腔内,对局域在微腔中的光波起着频率选择的作用。回音壁微腔具有超高的品质因子Q值和极小的模式体积,在低阈值激光器[1]、高精度传感[2]、非线性光学[3]、腔量子动力学[4]等领域有巨大的应用潜能。
固态回音壁微腔制备完成后,结构参数已固定,但在实际应用中,通常希望能对谐振波长进行调谐。目前,调谐回音壁微腔谐振波长的方法主要包括机械拉伸与压缩[5-8]、气压调谐[9-11]等改变微腔尺寸形貌,或电热调谐[12-14]、全光调谐[15-17]改变微腔折射率和微腔体积。2009年,Pöllinger等[8]通过驱动器拉伸二氧化硅微瓶腔两端的光纤实现了400 GHz的大范围调谐,Q值保持在
柔性聚合物材料激光诱导石墨烯(LIG)具有光吸收率高、吸收波长范围广、光热转换效率高等优点[26]。此外,其还具有鲁棒性好、工作温度范围宽、响应时间快等特点,且可通过计算机设计制造成任意形状[27]。基于前期工作实现的LIG电热调谐回音壁微腔,本文提出了一种基于LIG的回音壁微腔全光调谐方法。将超高Q值回音壁微球腔固定于自制的LIG上,采用980 nm激光对LIG表面进行光激励,光能被LIG高效吸收后,LIG产生的热量被传导到回音壁微腔模场区域。在热光效应以及热膨胀效应的作用下,实现对微球腔谐振波长的调谐。回音壁微腔的模场区域没有直接与LIG接触,因此微腔Q值仅受微弱影响。这种基于LIG的调谐方法可以实现约1.09 nm的调谐范围,灵敏度S约为8.8 pm/mW,微腔Q值保持在
2 结构装置与原理
2.1 制备方法
LIG的制备方法[27]和表征,如
图 1. LIG的制备方法和表征。(a)LIG的加工示意图;(b)LIG的实物图;(c)LIG的拉曼光谱表征;(d)LIG的透射光谱图;(e)扫描电镜图(刻度为20 μm);(f)固定于LIG上微球腔的显微图(刻度为100 μm)
Fig. 1. Manufacturing method and characteristics of LIG. (a) Schematic of manufacturing LIG; (b) image of LIG; (c) Raman spectrum of LIG; (d) transmission spectrum of LIG; (e) SEM image of LIG (scale is 20 μm); (f) micrograph of a microsphere fixed on LIG (scale is 100 μm)
2.2 实验装置
全光调谐实验装置如
2.3 基本原理
光波沿着微腔表面传输一周的光程为该光波波长的整数倍时,可以在微腔内相干相长并形成稳定传播的回音壁谐振模式。该波长为微腔的谐振波长
式中:R为微腔的半径;
LIG吸收激励光能量后产生热量,热量被传输到回音壁微腔的模场区导致微腔温度升高,造成微腔体积和折射率改变,使谐振波长发生漂移。波长漂移量
式中:二氧化硅的热光系数
3 分析与讨论
基于LIG的回音壁微球腔的透射光谱特性与调谐特性,如
图 3. 基于LIG的回音壁微球的光谱特性及调谐特性。(a)1558~1563 nm范围内的透射光谱;(b)单一模式的透射光谱;(c)模式红移随激励光功率的变化;(d)波长漂移量随激励光功率的变化;(e)Q值随激励光功率的变化
Fig. 3. Characteristics of microsphere with LIG and its tunability. (a) Transmission spectrum in the range of 1558 from 1563 nm; (b) transmission spectrum of a single resonance mode; (c) mode redshift with increasing pump power; (d) wavelength shift with pump power; (e) Q factor with pump power
对反射光谱进行全光调谐,如
表 1. 不同全光调谐方法性能
Table 1. Performance of different all-optical tuning schemes
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图 4. 反射光谱的全光调谐。(a)1559.7~1560.1 nm范围内的透射光谱与反射光谱;(b)透射光谱与反射光谱的放大图;(c)随着激励光功率增加反射光谱的红移;(d)反射光谱波长漂移随激励光功率的变化
Fig. 4. All-optical modulation of the reflection spectra. (a) Transmission and reflection spectra in the range of 1559.7 to 1560.1 nm; (b) enlarged transmission and reflection spectra; (c) redshift of the reflection spectra with increasing pump power; (d) wavelength shift of the reflection spectra with pump power
使用980 nm激光器周期性输出激励光,并测量经过回音壁微腔的透射率。当980 nm激光器输出激励光时,激光被LIG高效吸收后产生热量,导致谐振波长红移,透射率增加。当980 nm激光器无激励光信号输出时,回音壁微腔散热,导致谐振波长蓝移,透射率减小。
图 5. 光学响应的测量。(a)980 nm激励光的同步信号;(b)全光调谐的响应信号
Fig. 5. Measure optical response. (a) Synchronization signal of the 980 nm pump light; (b) all-optical tuning of the response signal
已发表的回音壁微腔全光调谐文献的性能,如
4 结论
提出了一种基于LIG超高Q值回音壁微球腔的全光调谐方法,其具有成本低廉、无机械干扰、保持超高Q值、调谐范围宽等优点。将制备的二氧化硅微球腔球杆固定于LIG上,在光激励下,LIG产生的热量将传导到微球腔的模场区域。微球腔的模场区域与LIG未直接接触,回音壁微腔Q值仅受微弱影响。对微球腔的透射光谱和反射光谱分别进行研究,在整个调谐过程中微腔Q值保持在
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