Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Avenue de Rangueil, 31077 Toulouse, France.
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
Phys Rev Lett. 2014 Jan 31;112(4):047401. doi: 10.1103/PhysRevLett.112.047401. Epub 2014 Jan 27.
In monolayer MoS2, optical transitions across the direct band gap are governed by chiral selection rules, allowing optical valley initialization. In time-resolved photoluminescence (PL) experiments, we find that both the polarization and emission dynamics do not change from 4 to 300 K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4 ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady-state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature-induced change in band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300 K, close to the maximum emission polarization for this sample at 4 K.
在单层 MoS2 中，直接带隙的光学跃迁受手性选择定则控制，允许光学谷初始化。在时间分辨光致发光（PL）实验中，我们发现，在我们的时间分辨率内，从 4 K 到 300 K，偏振和发射动力学都没有变化。我们测量了高偏振度，并表明在脉冲激发下，随着激光功率的增加，发射偏振度显著降低。我们发现激子发射衰减时间约为 4 ps。在我们的时间分辨率内没有明显的 PL 偏振衰减表明，初始注入的偏振度主导了稳态 PL 偏振度。随着泵浦光子能量的增加，初始偏振度的减小表明可能存在超快的谷间弛豫，超出了实验的 ps 时间分辨率。通过用激发激光能量补偿带隙能量随温度的变化，可以在 300 K 时恢复 40%的发射偏振度，接近该样品在 4 K 时的最大发射偏振度。
