Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany.
Nat Mater. 2013 Dec;12(12):1119-24. doi: 10.1038/nmat3757. Epub 2013 Oct 6.
The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.
石墨烯的线性能带结构和狄拉克点处的态密度消失使它具有独特的光学性质。有人提出，即使没有能隙，狄拉克点处的弛豫瓶颈也可能允许在任意长波长处产生粒子数反转和激光。此外，在光收集应用中，已经讨论了通过碰撞离化实现有效的载流子倍增。然而，所有这些效应都很难通过仅测量瞬态光学性质来进行定量测试，因为这些性质仅间接地反映了能量和动量依赖的载流子分布。在这里，我们使用飞秒极紫外脉冲的时间和角度分辨光发射光谱法，直接探测布里渊区 K 点附近狄拉克电子的非平衡响应。在轻掺杂空穴的外延石墨烯样品中，我们在中红外和近红外范围内进行了探索，这两个范围都低于和高于直接带间跃迁的最小光子能量。中红外激发仅导致平衡载流子分布的加热，而带间激发则导致粒子数反转，这表明太赫兹激光可能是可行的。然而，在这两种激发情况下，我们都没有发现任何载流子倍增的迹象，这对石墨烯在光收集方面的适用性提出了质疑。
