基于电荷掺杂量子点的低阈值激光器的研究开发

While being under intense investigation for nearly two decades, colloidal semiconductor quantum dot (QD) lasing is still not a commercial technology. The fundamental problem is a non-unity degeneracy of QD band-edge states, due to which stimulated emission by an exciton is not sufficient to overwhelm excited state absorption, and hence, at least biexcitons are required for realizing the population inversion in lasing regime. In QDs, however, biexcitons decay on the picosecond time scale via the nonradiative Auger recombination. If using continuous wave excitation, the pumping rate needs to outpace the Auger process so as to effectively maintain population inversion. This requires pump power densities that are not sustainable by colloidal nanomaterials, for which reason QDs lasers reported so far usually requires ultrafast pulsed excitations, diminishing the practical value of this technology. Unlike the reported methodologies of lowering lasing threshold by tuning the size and shape of QDs, here we propose to solve the fundamental problem by effectively lifting the band-edge degeneracy using extra charges doped into QDs. By completely blocking the band-edge state absorption using the doped charges, we expect to observe a very peculiar regime called “zero-threshold” optical gain. In addition, we propose to extend the excited state lifetime of charged QDs by specially engineering the interface for core/shell QDs. Combining these two strategies, we may greatly simplify the realization of QD lasing with ultra-low-intensity continuous wave excitation.

尽管经历了近二十年的研究，基于胶体量子点的激光器还没有成为一项商品化技术。这一难题的根源在于量子点带边跃迁具有至少两重的简并度，它决定了激光媒介当中至少有一些量子点被激发两个以上激子才能实现粒子数反转。然而，量子点中的双激子会通过俄歇复合快速衰退(皮秒尺度)。 用连续光源泵浦的情况下，泵浦速率必须要大于双激子衰退速率才能有效地保持粒子数反转。 满足这一要求的激发功率密度超过了一般胶体纳米材料所能承受的热极限。因此，目前报道的量子点激光器几乎都依赖于超快脉冲泵浦光源，使这一技术失去了实用价值。 不同于文献中通过调控尺寸和形貌来降低量子点激光阈值的思路，本项目中申请人从根本原理出发，拟采用电荷掺杂方法来有效解除带边简并。 在掺杂电荷完全阻挡带边跃迁的情况下，预计观察到“零阈值”光增益的奇异现象。同时，通过界面调控延长掺杂量子点激发态的寿命，以期实现极低阈值的连续光源泵浦量子点激光器。

本项目针对胶体量子点激光器开发的难题，提出采用电荷掺杂方法来有效解除带边简并，同时通过界面调控延长掺杂量子点带电激发态的寿命.两者相结合,以期实现低阈值泵浦的量子点激光器。在此目标下，申请人合成了具有组分梯度式合金结构的核/壳量子点，将量子点的双激子寿命延长至1 ns以上，并且采用电荷掺杂结合光谱动力学测试手段，实现了带负电激子（negative trion）俄歇复合过程的有效抑制，这一实验结果对实现基于trion增益的低阈值量子点激光器具有重要意义。同时，基于该合金层量子点的长寿命双激子特性开展了放大自发辐射和激光性能相关研究，成功实现了纳秒脉冲光激发下的量子点溶液激光输出，为量子点激光器的实际应用开辟了新的途径。此外，考虑到电荷掺杂量子点可作为模型体系来研究多电子光催化和光电转换应用中的一系列重要动力学过程，申请人开展了相关研究工作并取得了一系列重要进展：采用电荷预掺杂的纳米晶量子点构建模型体系，分别研究了捕光材料和催化剂上的积累电荷对后续电荷转移过程的不利影响；考虑到电荷掺杂对研究纳米晶光谱和动力学性质的重要性，发展了一种三脉冲Pump–Pump–Probe “无损”动态掺杂方法，基于该方法对真实光催化体系开展了多电荷转移动力学研究，展示了纳米尺度多余电荷导致的库仑势垒和效率瓶颈；在电荷掺杂的量子点中巧妙实现了“自旋阻塞”和“声子瓶颈”效应，将亚皮秒的热电子寿命延长至近300皮秒；在构建的无机/有机杂化体系中，通过构建单电荷转移态，首次观测到低维材料电荷转移的Marcus反转区间等。

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