以光激發中紅外光探測光譜研究石墨烯之超快動力學

Abstract

本論文利用自行架設之光激發中紅外探測系統研究不同層數石墨烯之載子超快動力學行為。當探測光光子能量大於二倍費米能時，光激發可導致中紅外波段反射率增加；反之，探測光光子能量小於二倍費米能時，光激發將導致中紅外波段吸收率增加。因此，藉由改變光激發中紅外探測系統之探測光子能量，即可觀察到石墨烯中能帶結構內之載子動力學行為。因基板效應以及層間交互作用，不同層數的石墨烯，其內部之載子動力學行為也各有不同。利用上述分析方法，本論文成功地得到化學氣相沈積（Chemical Vapor Desposition, CVD）成長不同層數石墨烯的費米能階。除此之外，透過分析電子聲子散射行為之弛緩時間，可得到費米能階附近之電子聲子耦合強度，且分析結果與其他研究團隊實驗及理論都非常吻合。因此，本實驗室所建立之光激發中紅外光探測光譜技術可望成為檢驗石墨烯費米能階位置之新利器。

Using ultrafast optical pump mid-infrared probe spectroscopy (OPMP), we study the relaxation dynamics of photoexcited carriers in monolayer and multilayer graphene. The substrate-induced charge density profile in graphene is determined through the spectra of transient reflectivity changes which are well described by carrier occupation probability near Fermi energy with tuning the probe photon energy. We succeeded in getting the Fermi energy of graphene films grown by chemical vapor deposition (CVD) on sapphire substrates. For photon energies about twice the value of the Fermi energy, the transient responses of pump-induced absorption will change to pump-induced reflection. Additionally, we analyzed the carrier-phonon coupling strength by the relaxation dynamics of carriers, which are consistent with other experimental and theoretical results. Finally, the optical pump mid-infrared probe spectroscopy would be a new technique to study the Fermi level in graphene.