PHYSICAL REVIEW B 91, 035422 (2015) Nonlinear terahertz field-induced carrier dynamics in photoexcited epitaxial monolayer graphene Hassan A. Hafez, 1 Ibraheem Al-Naib, 2 Marc M. Dignam, 2 Yoshiaki Sekine, 3 Katsuya Oguri, 3 Franc¸ois Blanchard, 4 David G. Cooke, 4 Satoru Tanaka, 5 Fumio Komori, 6 Hiroki Hibino, 3 and Tsuneyuki Ozaki 1 , * 1 INRS-EMT, Advanced Laser Light Source, INRS, Varennes, Qu´ ebec, Canada J3X1S2 2 Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, Ontario, Canada K7L 3N6 3 NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan 4 Department of Physics, McGill University, Montr´ eal, Qu´ ebec, Canada H3A 2T8 5 Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0395, Japan 6 Institute for Solid State Physics, University of Tokyo, Chiba, 277-8581, Japan (Received 17 October 2014; published 16 January 2015) We report nonlinear terahertz (THz) field transmission through photoexcited monolayer epitaxial graphene via differential transmission measurements enabled by optical-pump/intense-terahertz-probe (OPITP) spectroscopy. After photoexcitation of graphene, a transmission enhancement, defined by a positive differential transmission of the intense terahertz probe pulse, is observed. This is due to suppression of the graphene photoconductivity arising from an increased carrier scattering rate due to the increase in the carrier density and the extra energy from the photoexcited hot carriers. Thus, the transient enhancement in transmission increases as the optical pump fluence increased. Most interestingly, we observe that the transmission enhancement after photoexcitation decreases as the THz field strength is increased, which we attribute to the combined effects of the intense THz electric field and the optical pump fluence on the carrier scattering rate. We model the carrier dynamics in the graphene using the length gauge interaction Hamiltonian with the inclusion of short-range scattering by neutral impurities and the interaction of the carriers with optical phonons. Comparing the experimental and simulated transmission results, we extract the nonequilibrium effective lattice temperature of graphene as a function of the optical pump fluence and THz field strength. DOI: 10.1103/PhysRevB.91.035422 PACS number(s): 78.67.Wj, 78.47.D−, 78.67.−n, 78.30.−j The exceptional quantum properties of graphene, exempli- fied by its relativistic massless Dirac-fermion physics [1–5], have great potential for innovative applications in high-speed optoelectronics [6,7] and photodetection [8–12]. This has sparked great interest in exploring the ultrafast carrier transport properties of graphene. Noninvasive spectroscopic techniques have been employed to study such properties over a broad range of the electromagnetic spectrum, ranging from the ultraviolet to the far-infrared frequencies [13–36], includ- ing numerous studies based on one- [20,21] and two-color [16–19,22–29] pump/probe spectroscopy. In such experi- ments, selecting the correct wavelength is crucial for exploring inter- and intraband carrier dynamics in graphene. Due to the Pauli-blocking effect [30–32], interband transitions occur linearly only when the exciting photon energy ex- ceeds twice the Fermi-level energy of the graphene sam- ple. Therefore, the optical and infrared frequencies induce interband transitions in graphene [13–20], while the tera- hertz (THz) frequencies usually induce intraband dynamics [13,18–23], especially in highly doped graphene. Therefore, THz spectroscopy is a powerful tool for probing intraband dynamics. Optical-pump/THz-probe (OPTP) spectroscopy is fre- quently used for studying the intraband nonequilibrium carrier dynamics in photoexcited graphene. In previous OPTP exper- iments [19,22–29], the THz probe beam is sufficiently weak so that the THz-graphene interaction is in the linear regime. These OPTP studies have revealed different responses of * Author to whom correspondence should be addressed: ozaki@emt.inrs.ca graphene under various experimental conditions. For example, photoexcitation-induced THz absorption has been observed in multilayer epitaxial graphene [19,22,23], while several studies have recently reported enhancement in the THz transmission induced by photoexcitation of monolayer CVD graphene [24–29]. To explain the observations of THz transmission enhancement after photoexcitation, various interpretations have been presented. Some works [24–26] have suggested THz stimulated emission due to population inversion [33,34] through energy relaxation of the photoexcited carriers, while others report suppression in the photoconductivity [27–29]. More recently [35,36], the THz response of photoexcited monolayer graphene has been found to be strongly dependent on the Fermi level of the graphene sample, giving rise to a transition behavior between the metal-like and the semiconductorlike properties of graphene. This is exemplified by switching between transmission enhancement and absorp- tion after photoexcitation when the Fermi level is tuned. In most cases of OPTP spectroscopy with graphene, the transient change in transmission takes place within a few hundreds of femtoseconds after photoexcitation. Then, a subsequent cooling (energy relaxation) takes place in a few picoseconds via phonon emission [22,27–29,37]. With the recent progress in developing intense THz sources during the last decade, the nonlinear properties of various materials in the THz frequency range are under investigation [38–41]. For graphene, it has been theoretically predicted that the relativistic massless Dirac-fermion behavior of its charge carriers and the linear dispersion of its energy spectrum will lead to strong nonlinear responses to relatively high THz field excitations [42–46]. Nonlinear processes such as fre- quency multiplication [42,44,47], intense THz-field-induced 1098-0121/2015/91(3)/035422(9) 035422-1 ©2015 American Physical Society