Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11864045, 11404282, 61465014, 61465015), the Young and Middle-aged Academic and Technical Leaders (Reserve Talent) in Yunnan Province, China (Grant No. 2018HB029), and the China Scholarship Council (CSC) Foundation.
Received Date:11 September 2018
Accepted Date:11 November 2018
Available Online:01 March 2019
Published Online:05 March 2019
Abstract:A bare quartz fiber with single refractive index is implanted into a polydimethylsiloxane (PDMS) microfluidic channel. The lasing gain medium consists of fluorescence resonance energy transfer (FRET) donor-acceptor dye pair Rhodamine B (RhB)-LDS821 mixture solution, which has a lower refractive index than that of the optical fiber and flows in the PDMS microfluidic channel. The circular cross section of the optical fiber forms a ring resonator and hosts high-quality (Q) whispering gallery modes (WGMs). Pumping along the optical fiber axis, the FRET characteristic parameters, i.e., the FRET efficiency $\eta $ and the F?rster distance R0 of donor-acceptor dye pair, are firstly studied by using a continuous wave laser as a pump light source with a wavelength of 532 nm. The excited states are thencreated in the donor (RhB) by using a pulse laser with a wavelength of 532 nm and whose energy is transferred into the adjacent acceptor (LDS821) through the non-radiative FRET mechanism. Finaly, the emission of LDS821 iscoupled into the WGM of the ring resonator to lase. Due to the high energy transfer efficiency and high Q-factor, the acceptor shows a lasing threshold as low as 1.26 ${\text{μ}}{\rm J}$/mm2. Keywords:optofluidic laser/ fluorescence resonance energy transfer/ evanescent wave
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2.1.PDMS芯片的设计与制作
如图1所示, PDMS基片采用铸模法制作而成, 其尺寸为34 mm × 15 mm (长 × 宽). PDMS基片中光纤通道的尺寸为34 mm × 0.2 mm × 0.2 mm (长 × 宽 × 高), 尺寸为20 mm × 0.3 mm × 0.3 mm的微流道包含于光纤通道中. 基片由苏州汶颢微流控有限公司生产制作. 图 1 PDMS芯片结构示意图 Figure1. Structure diagram of the PDMS chip.
实验装置如图2所示, 以波长为532 nm的YAG激光器(北京镭宝公司生产, 脉宽为7 ns)作为抽运光. 抽运光依次经过分束镜BS, 以便由激光能量计PM (MELLES GRIOT, 13PEM001)测出即时抽运能量. 然后经过由透镜L1和L2构成的光学缩束系统后再经一块焦距为75 mm的透镜L3汇聚于光纤前端面, 焦点距石英光纤前端面约为2 mm, 抽运光在光纤内沿光纤轴向以TIR方式传播, 其消逝场(Ep)渗透到包层溶液中, 并在此消逝场区域内激励染料分子产生激光辐射, WGM激光沿光纤表面切向辐射出来, 由导光光纤送至光谱采集系统(ICCD: PI-MAX; spectrometer: Spectrapro 500i)的进光狭缝口. 增益介质溶液经微型蠕动泵BT100-2J上的导管与PDMS上的流体入口和出口相连, 其流速为1.0 mL/min. 图 2 实验装置图 BS, 分束镜; PM, 激光能量计; L1, L2, L3表示透镜; 插图为激光产生的原理图, Ep表示 抽运光的消逝场, Ew表示WGM的消逝场 Figure2. Illustration of the experimental setup. BS, beam splite; PM, power meter; L1, L2, L3, lens. Inset: the schematic diagram of laser: Ep, evanescent field of pump light; Ew, evanescent field of WGM.
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3.1.FRET能量供体浓度的选取
实验中选用RhB和LDS821分别作为FRET的能量供体和能量受体. 首先测量了不同浓度的RhB (0.05, 0.10, 0.30, 0.50, 0.70, 1.00 mM, 1 M = 1 mol/L)的激光阈值. 如图3所示, 随着RhB浓度增加, 激光阈值呈现出先减小后增加的趋势, 当c = 0.5 mM时, 激光阈值达到最低. 因此, 为了实现低供体阈值、低供体浓度条件下高效率的基于FRET机制的激光辐射, 本文中我们选用c = 0.5 mM的RhB作为能量供体. 图 3 不同RhB浓度对应的激光阈值, 误差线是三次测量的平均值 Figure3. Lasing threshold of RhB as a function of the dye concentration. Error bars are obtained with three measurements.
为了系统地表征能量转移效率及测定供受体对之间的F?rster距离(R0)等FRET特性参数, 我们用微泵BT100-2J将RhB和LDS821的乙醇混合溶液通入PDMS芯片的微流道中, 以波长为532 nm的低功率(5 mW) CW 激光器作为激励光, 获得如图5(a)所示的供-受体 (A/D) 对荧光辐射光谱. 如图中蓝色实线所示, 在没有供体RhB时, 由于激励光远离受体LDS821(浓度c = 1.0 mM)的吸收峰且自身的荧光量子效率较低[19], 导致LDS821对激励光的吸收较弱, 从而使得LDS821荧光辐射强度也较弱. 相反, 在没有受体LDS821的情况下, 由于没有发生FRET, 供体RhB (c = 0.5 mM)对激励光有较强的吸收且自身的荧光量子效率较高[19], 所以在$\lambda $ = 580 nm附近的荧光辐射强度达到最强. 保持供体RhB浓度不变(0.5 mM), 随着受体浓度的逐渐增加, 由于FRET机制, 使得受体在$\lambda $ = 800 nm附近的荧光辐射峰强度不断增强, 而供体RhB的荧光辐射峰强度逐渐减弱. 图5(b)是FRET效率$\eta $随受体与供体对比值(A/D)的变化关系, 由图可知, 随着A/D值的增加, $\eta $不断增大. 且由于受供体浓度(0.5 mM)和受体溶解度的限制, 实验中, 当A/D = 8.0/0.5 mM时, 我们认为$\eta $到达最大(约为69.8%). 图 5 (a) 以RhB和LDS821分别作为能量供体和能量受体的归一化荧光辐射光谱, A/D为受体与供体浓度比值, 图中“A/D = 1.0/0 mM”表示没有供体时受体的辐射光谱, 其他值表示固定供体浓度为0.5 mM, 不同受体浓度所对应的FRET光谱, 插图为微流道中荧光辐射的实物图; (b)红色三角形是根据图5(a)计算得到的能量转移效率$\eta $随A/D变化关系的实验值, 实线是根据(1)式得到的理论值 Figure5. (a) Normalized fluorescence spectra of RhB (donor) and LDS821 (acceptor); A/D, acceptor to donor ratio, A/D = 1.0/0 mM was collected for 1.0 mM acceptor in the absence of donor and the other spectra were collected for a constant donor concentration of 0.5 mM and the acceptor concentration changing from 0 to 8 mM; inset, the picture of fluorescent radiation generated in the PDMS microfluidic channel; (b) the red triangle is the experimental value of the energy transfer efficiency $\eta $ as a function of A/D calculated from Fig. 5(a), and the solid line is the theoretical value calculted by formula (1).
通过表征FRET特性参数以后, 我们利用RhB和LDS821分别作为能量供体和能量受体的FRET激光实验. 实验中, 以波长为532 nm的YAG脉冲激光器作为抽运光, PDMS基片微流道中流体的流速仍为1.0 mL/min. 保持抽运光的能量密度不变(约为1.45 ${\text{μ}}{\rm J}$/mm2), 得到图6(a)所示的不同A/D值所对应的激光光谱. 如图所示, 在没有受体LDS821时, RhB (c = 0.5 mM)在$\lambda $ = 585 nm附近具有较强的激光辐射峰. 随着受体LDS821的加入(A/D = 0.5/0.5 mM), 供体激光辐射强度降低, 同时, 在波长较长的一侧出现了较为突出的受体辐射峰(797, 822, 828 nm), 表明在这些波长处首先达到了受体的激光阈值. 然而, 由于实验中采用的是密度g = 150 g/mm的低分辨率光栅, 导致激光辐射峰不能完全被分辨. 图7是采用中等分辨率(g = 1200 g/mm)的光栅所采集的不同A/D值对应的RhB在578—592 nm波长范围以及LDS821在810—838 nm波长范围的激光光谱. 由图可知, 随着A/D值的进一步增加, 由于FRET效应, 供体激光从逐渐减弱到完全淬灭(A/D = 8/0.5 mM), 而受体激光辐射强度从逐渐增强到最强. 图 6 (a)不同A/D值对应的低等分辨率(光栅密度g = 150 g/mm)的FRET激光光谱, 供体浓度保持0.5 mM不变; (b)激光辐射峰强度随抽运光能量密度的变化关系; 供体峰值为585 nm, 阈值约为0.48 ${\text{μ}}{\rm J}$/mm2; A/D = 8/0.5 mM和A/D = 8/0 mM的LDS821的峰值均为822 nm, 其阈值分别为1.26 ${\text{μ}}{\rm J}$/mm2和1.69 ${\text{μ}}{\rm J}$/mm2 Figure6. (a) Low resolution (grating density = 150 g/mm) FRET lasing spectra for various A/D values, the donor concentration is fixed at 0.5 mM; (b) lasing peak intensity vs. pump energy density. The donor peak is at 585 nm and its lasing threshold is approximately 0.48 ${\text{μ}}{\rm J}$/mm2. The peaks of LDS821 for A/D = 8/0.5 mM and A/D = 8/0 mM are at 822 nm and their lasing threshold is approximately 1.26 ${\text{μ}}{\rm J}$/mm2 and 1.69 ${\text{μ}}{\rm J}$/mm2, respetively.
图 7 不同A/D值对应的中等分辨率(光栅密度g = 1200 g/mm)的激光光谱 光谱图从上到下分别对应A/D = 0/0.5, 0.5/0.5, 1/0.5, 4/0.5, 8/0.5 mM; (a) RhB(供体)的激光光谱; (b) LDS821(受体)的激光光谱 Figure7. Medium resolution (grating density = 1200 g/mm) lasing spectra for various A/D values. The spectra correspond to A/D = 0/0.5, 0.5/0.5, 1/0.5, 4/0.5, 8/0.5 mM from top to bottom: (a) Lasing spectra of RhB (donor); (b) lasing spectra of LDS821 (acceptor).