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实验采用水热法合成Bi2Te3,所有试剂纯度均为分析纯,采用的主要试剂如表1所示。
表 1 水热合成所用试剂
Table 1. Chemicals of hydro-thermal synthesis
Name of reagent Chemical formula Dosage Effect Sodium Tellurite Na2TeO3 1.5 mmol Reactant Bismuth chloride BiCl3 1 mmol Reactant Polyvinyl pyrrolidone PVP 100 mg Surfactant, reducing agent 将100 mg PVP加入40 ml乙二醇中搅拌溶解,依次将亚碲酸纳和氯化铋加入溶液中,充分搅拌,将溶液转移到反应釜中,然后放入烘箱,180 ℃反应20 h。将获得溶液用乙醇离心洗涤3次,去除杂质和乙二醇。最终将获得溶液60℃烘干得到Bi2Te3粉末。图1(a)是用AFM测试了Bi2Te3的厚度,从图中可以看出:所测试的厚度为11 nm左右,由这个结果可知该样品的层数较少。图1(b)为样品XRD (X-Ray diffractometer)[JCPDS NO.15-0863])的测试结果,在015和0015有两个特征峰,显示制备的Bi2Te3纳米颗粒处于晶体状态。称取2.5 mg的Bi2Te3粉末加入8 mL的NaCMC溶液中,搅拌超声使溶液混合均匀。用滴管将溶液滴在盖玻片上,60 ℃烘干得到Bi2Te3可饱和吸收体的薄膜。利用分光光度计(Spectrop-hotometer)对制备好的TI:Bi2Te3光开关进行光吸收测试,为了避免NaCMC和玻璃基片对吸收特性的影响,实验分别对掺有Bi2Te3的NaCMC薄膜和非掺Bi2Te3的NaCMC薄膜进行透过率测试,进行了测量,结果如图1(c)所示,红线为纯NaCMC薄膜透射曲线,黑线为添加Bi2Te3的NaCMC薄膜投射曲线。实验表明:Bi2Te3对波段在400~2 000 nm范围内的激光均有吸收。利用扫描电子显微镜(Scanning Electron Microscope, SEM)观察制备样品的表面形貌如图1(d)所示,所制备的Bi2Te3纳米材料大部分呈六边形形状,宽度在400~500 nm左右且可以清晰地观察到纳米颗粒边界,颗粒与颗粒之间没有粘连在一起,表明所制备的材料具有很好的结晶性和分散性。图1(e)所示为Bi2Te3光开关薄膜放置到FC光纤端面。
实验配制了不同浓度Bi2Te3作为光开关,探索其浓度对调Q光脉冲的影响,分别称取了2.5 、5 、7.5、10 mg的Bi2Te3粉末并分别加入8 mL的NaCMC溶液中,搅拌超声使溶液混合均匀。用滴管将溶液滴在盖玻片上,烘干后截取面积1.5 mm×1.5 mm薄膜,用法兰固定在激光器的光路中。为了表征Bi2Te3粉末浓度对调制深度的影响关系,实验使用I扫描测试装置对不同浓度的Bi2Te3光开关的非线性透射曲线进行测量,实验装置如图2(a)所示[14]。I扫描的测试光源为由自制的被动锁模光纤激光器(重复频率为31.4 MHz,中心波长为1 558.5 nm,脉冲宽度为500 fs),测试光脉冲经过光衰减器,被3 dB光分束器均匀地分成两个强度几乎相同的激光。笔者让一束激光通过样品,然后由功率计A测量,而另一束激光作为参考光由另一个功率计B直接测量。同步测量输入和输出脉冲强度,可以有效地降低检测误差。通过不断调整功率,就可以得到一系列输出功率与输入功率之比。测得的不同浓度Bi2Te3薄膜的透射率曲线如图2(b)所示,在2.5 mg/8 mL~10 mg/8 mL的浓度范围内,随着光开光浓度的增加,非饱和损耗由10.9%增加至29%,调制深度由4.4%增加至6.2%。这是因为拓扑材料中表面态和体态对光的吸收和弛豫分为四个过程,第一个过程中,材料中的载流子被入射光激发到表面态和体态的上能级中;第二个过程,上能级的载流子同时向体态和表面态弛豫;第三个过程,体态载流子耦合到表面态;第四个过程,表面态中载流子通过带间散射回到基态完成平衡。而拓扑材料浓度的变化,使得上能级表面态的载流会影响体态与表面态之间的载流子耦合效率,当体态越多时,耦合速度越慢,也子无法及时过度到表面态以弛豫回基态,而造成载流子的积累[15]。结果显示,随着拓扑材料浓度的增加,拓扑样品的比表面积下降,样品中体态到表面态的耦合速度变慢,这使得样品更容易饱和,因此饱和强度下降。浓度较大的Bi2Te3光开光其覆盖率也较高,非饱和损耗就比较大。Bi2Te3光开关可以实现不同浓度状态下电子的直接跃迁, 在光激发下可吸收更多的光子而获得更大的调制深度。
Q-switched fiber laser with Ti: Bi2Te3 of different concentrations
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摘要: 基于二维材料的非线性光开关是调Q光纤激光器的核心器件,二维材料光开关的浓度会直接影响其非线性光学吸收特性,从而改变脉冲的时域特性。因此,针对二维材料浓度对调Q光脉冲的影响进行了研究,并通过实验制作了基于不同浓度Ti: Bi2Te3可饱和吸收光开关,分析了Ti: Bi2Te3浓度对非线性光学吸收特性的影响,获得了调Q光脉冲的调制深度、脉冲宽度、重复频率和单脉冲能量随不同浓度Bi2Te3的变化关系。最终,针对谐振腔参数对Ti:Bi2Te3浓度进行优化,在泵浦功率为71 mW时,获得了中心波长为1 560 nm、脉冲宽度为8 μs,重复频率为14.2 kHz、平均输出功率2.15 mW、对应单脉冲能量为151.4 nJ的脉冲输出。
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关键词:
- 光纤激光器 /
- 被动调Q /
- 水热合成 /
- Bi2Te3可饱和吸收体
Abstract: Nonlinear optical switch based on two-dimensional material is the core device of Q-switched fiber laser. The concentration of two-dimensional material optical switch will directly affect its nonlinear optical absorption characteristics, thus change the time-domain characteristics of the pulse. Therefore, the influence of two-dimensional material concentration on Q-switched optical pulse was studied, and a saturable absorption optical switch based on Ti: Bi2Te3 with different concentrations was made through experiments. The influence of Ti: Bi2Te3 concentration on nonlinear optical absorption characteristics was analyzed, and the relationship among the modulation depth, pulse width, repetition frequency and single pulse energy of Q-switched optical pulse with different concentrations Bi2Te3 was obtained. Finally, the Ti:Bi2Te3 concentration was optimized for the resonator parameters. When the pump power is 71 mW, the central wavelength is 1 560 nm, the pulse width is 8 μs, the repetition frequency is 14.2 kHz, the average output power is 2.15 mW, and the corresponding single pulse energy is 151.4 nJ.-
Key words:
- fiber laser /
- passively Q-switched /
- hydrothermal synthesis /
- Bi2Te3 saturable absorber
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表 1 水热合成所用试剂
Table 1. Chemicals of hydro-thermal synthesis
Name of reagent Chemical formula Dosage Effect Sodium Tellurite Na2TeO3 1.5 mmol Reactant Bismuth chloride BiCl3 1 mmol Reactant Polyvinyl pyrrolidone PVP 100 mg Surfactant, reducing agent -
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