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由Nd:YAG激光器输出三倍频脉冲355 nm的泵浦光,通过光学参量振荡器OPO可输出波长覆盖410~2 500 nm的宽带单波长脉冲。而闪光灯泵浦的Nd:YAG激光器的脉宽通常情况下为5~7 ns,适用于纳秒−微秒尺度范围内的瞬态光谱研究。
光学参量振荡器以准相位匹配技术[23]为基础,在非线性介质中的差频过程中,每湮灭一个高频的光子就可以产生两个低频的光子。泵浦光与信号光同时入射非线性晶体后,每一个泵浦光光子都可以分为一个信号光光子与一个闲频光光子。泵浦光与信号光多次通过非线性晶体,可以得到经过多次放大的信号光。与其他几种诸如量子级联激光器[24]、自由电子激光器[25]以及稀土掺杂固体激光器[26-27]等相比具有体积紧凑、调谐范围宽、不受抽运光波长的限制等优点。
激光闪光光解技术是应用OPO调谐过的短激光脉冲激发样品,研究激发产生的瞬态物种的一种光谱表征手段[28]。在文中,此项技术被应用于研究三重态−三重态湮灭上转换的动力学过程。
以三重态光敏剂BDP-PXZ-1与苝的上转换体系为例,首先研究了氮气气氛下甲苯溶液中只含有光敏剂的溶液,其纳秒瞬态吸收光谱以及三重态寿命衰减曲线如图8所示。
Figure 8. (a) Nanosecond transient absorption spectra of BDP-PXZ-1 excited by a 495 nm OPO tunable pulse laser and (b) the decay curve of triple state at 504 nm
由纳秒瞬态吸收光谱表明,光敏剂BDP-PXZ-1的基态漂白峰位于504 nm处,与紫外可见稳态吸收光谱的数据相吻合,其激发态吸收峰位于350~700 nm处,这是Bodipy三重态的特征信号,其拟合后的三重态真实寿命为539.0 μs,在氮气甲苯溶液中的三重态量子产率为54%[19]。同时监测了三重态光敏剂BDP-PXZ-1/苝上转换体系的TTA上转换延迟荧光。
在上转换体系的延迟荧光光谱中(见图9),处于420~600 nm范围内的发光峰可归属为上转换延迟发光。通过监测445 nm处的荧光衰减曲线,可以明显地看到其衰减曲线由一个长达16.0 μs的上升段与71.9 μs的下降段构成。此上升段为三重态受体的三重激发态的激发形成过程(通过分子间三重态能量转移),而下降段则属于三重态−三重态湮灭产生的上转换延迟荧光的衰减过程,其寿命长达71.9 μs。利用OPO可调谐纳秒脉冲激光器激发得到的荧光光谱与荧光衰减曲线提供了上转换发光过程中的动力学数据。
Figure 9. (a) Delayed fluorescence spectra of TTA upconversion excited by 510 nm OPO tunable nanosecond pulse laser and (b) decay curves of fluorescence detected at 445 nm
通过对445 nm处监测的荧光衰减曲线的分析计算可以得到光敏剂的上转换相关参数(见表1)。其中KSV为Stern−Volmer淬灭常数,Kq为双分子淬灭常数,Rq为淬灭剂的分子半径,Dq为能量受体的扩散系数,k0为扩散控制的双分子淬火速率常数,fQ为淬灭效率,Φuc为上转换量子效率。结合对此三重态光敏剂的稳态光谱的研究以及密度泛函理论计算结果,可以得到此三重态光敏剂/能量受体体系在受光激发时的光物理过程的近似雅布隆斯基示意图(见图10)。
Parameter BDP-PXZ-1 KSV/105 M−1 7.3 Kq /109 M−1s−1 6.8 Rq /10−10 m 3.74 Dq /10−6 cm2s−1 9.74 k0 /1010 M−1s−1 1.24 fQ 56.5% Φuc 12.3% Table 1. Upconversion-related parameters of the photosensitizer
Figure 10. Modified Jablonski diagram illustrating the process of the triplet photosensitizer BDP-PXZ-1/perylene upconversion
三重态光敏剂BDP-PXZ-1吸收激光器发出的510 nm激光到达单重激发态(1*Bodipy),通过电荷分离过程(charge separation, CS)产生电荷分离态(charge transfer state, CT state),经过系间窜越过程产生光敏剂的三重态(3*Bodipy);通过三重态−三重态能量转移过程将能量传递给苝的三重态(3*Perylene),然后两分子苝的三重态发生三重态−三重态湮灭过程形成高能级的苝的单重态(1*Perylene (hot)),经内转换过程形成苝的单重态(1*Perylene),弛豫到基态并发出蓝色荧光。
Application of continuous wave and pulsed lasers in triplet−triplet annihilation upconversion (Invited)
doi: 10.3788/IRLA20201068
- Received Date: 2020-09-10
- Rev Recd Date: 2020-10-28
- Available Online: 2021-01-14
- Publish Date: 2020-12-24
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Key words:
- continuous wave diode pumped solid state laser /
- OPO tunable pulsed laser /
- upconversion /
- photosensitizer
Abstract: The triplet−triplet annihilation upconversion is a new technology for photonic upconversion, which has the advantages such as continuous wave excitation, tunable upconversion wavelength and high upconversion quantum yields. In this upconversion process, as the energy donor, photosensitizer absorbs the light excitation, and intersystem crossing occurs, then sensitizes the energy acceptor through the triplet−triplet energy transfer process. Finally, the triplet−triplet annihilation of the energy acceptor in the triplet state generates a single excited state which can produce high−efficiency fluorescence (i.e. upconversion luminescence), thus the low−energy light is converted into higher−energy upconversion luminescence, which provides a feasible method for improving the photoelectric conversion efficiency of solar cells or the efficiency of photocatalysis, etc. It is desired to select appropriate lasers to excite the photosensitizer/energy donor system to study the steady upconversion luminescence and the upconversion kinetics. For instance, continuous wave diode pumped solid state laser (DPSSL) was selected as the light source to excite photosensitizer/acceptor system, upconversion luminescence was observed, and the effect of laser power density on upconversion luminescence can be studied conveniently. Additionally, in order to analyze the kinetic process of upconversion luminescence, with optical parametric oscillator (OPO) tunable pulsed laser as the light source, the lifetime of the triplet state, the kinetic characteristics of intermolecular energy transfer and triplet annihilation of photosensitizers can be studied. The application of continuous wave and pulse lasers in triplet–triplet annihilation upconversion experiments was introduced.