Graphene Terahertz Lasers: Injection versus Optical Pumping
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Graphene Terahertz Lasers: Injection versus Optical Pumping Taiichi Otsuji1,4, Akira Satou1,4, Maxim Ryzhii2,4, Vladimir Mitin3, and Victor Ryzhii1,4 1
RIEC, Tohoku University, Sendai, 980-8577, Japan CNEL, University of Aizu, Aizu Wakamatsu, 965-8580, Japan 3 Dept. EE, University at Buffalo, SUNY, Buffalo, New York 14260-1920, USA 4 JST-CREST, Tokyo, 102-0075, Japan 2
ABSTRACT In this paper we demonstrate that graphene is one of the best materials for new types of terahertz lasers as optical and/or injection pumping of graphene can exhibit negative-dynamic conductivity in the terahertz spectral range. We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forward-biased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The latter provides a significant advantage of the injection pumping in realization of graphene terahertz lasers. We benchmark graphene as a prospective material for injection-type terahertz lasers. INTRODUCTION In the research of modern terahertz electronics, development of compact, tunable and coherent sources operating in the terahertz regime is one of the hottest issues [1]. Graphene, a monolayer carbon-atomic honeycomb lattice crystal, has attracted attention due to its peculiar carrier transport properties owing to the massless and gapless energy spectrum [2-6]. When we consider the nonequilibrium carrier relaxation/recombination dynamics of optically pumped graphene, a very fast energy relaxation of photoexcited electrons/holes via the optical phonon emission and a relatively slow recombination will lead to the population inversion in the wide terahertz range under sufficiently high pumping intensity. This will make it possible to obtain the negative dynamic conductivity or gain in the terahertz spectral range [7-9]. The authors recently succeeded in observation of amplified stimulated terahertz emission in exfoliated monolayer graphene and/or heteroepitaxial multilayer graphene pumped by an infrared femtosecond pulse laser at room temperature, testifying to the occurrence of the terahertz negative conductivity [1013]. Such an active mechanism can be utilized for creating the graphene-based coherent laser sources of terahertz radiation [9, 14-16]. The authors have first analytically found the possibility of terahertz gain in such systems under a cryogenic condition [7] and have recently numerically verified the occurrence of the terahertz gain even at 300K [17-19]. In this paper we demonstrate that graphene is one of the best materials for new types of terahertz lasers since optical and/or injection pumping of graphene can exhibit negative-dynamic conductivity in the terahertz spectral range. We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forward-biased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The model used accounts for intraband and interband relaxation processes as well as
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