Ulstrup Søren, Johannsen Jens Christian, Crepaldi Alberto, Cilento Federico, Zacchigna Michele, Cacho Cephise, Chapman Richard T, Springate Emma, Fromm Felix, Raidel Christian, Seyller Thomas, Parmigiani Fulvio, Grioni Marco, Hofmann Philip
Department of Physics and Astronomy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Nordre Ringgade 1, 8000 Aarhus C, Denmark.
J Phys Condens Matter. 2015 Apr 29;27(16):164206. doi: 10.1088/0953-8984/27/16/164206. Epub 2015 Apr 2.
In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the π band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and we find that the electronic system is much harder to heat up than on the semiconductor due to screening of the laser field by the metal.
为了利用石墨烯引人入胜的光学特性，必须在超短时间尺度上更好地理解材料中的光与物质相互作用。用强超快激光脉冲激发狄拉克费米子会引发一系列涉及电子、声子和杂质之间相互作用的过程。在这里，我们使用基于高次谐波产生的超快时间和角度分辨光电子能谱（TR-ARPES），研究了在碳化硅（半导体）和铱（金属）衬底上的外延石墨烯中的这些相互作用。对于半导体衬底，我们揭示了一种复杂的热载流子动力学，其表现为电子温度升高和π带线宽增加。通过分析这些效应，我们能够解开石墨烯中的电子弛豫通道。在金属衬底上，发现这种热载流子动力学受到金属存在的严重干扰，并且我们发现由于金属对激光场的屏蔽，电子系统比在半导体上更难被加热。
