Article

Coherent Control of Ballistic Photocurrents in Multilayer Epitaxial Graphene Using Quantum Interference

Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109-2099, USA.
Nano Letters (Impact Factor: 13.59). 03/2010; 10(4):1293-6. DOI: 10.1021/nl9040737
Source: PubMed

ABSTRACT

We report generation of ballistic electric currents in unbiased epitaxial graphene at 300 K via quantum interference between phase-controlled cross-polarized fundamental and second harmonic 220 fs pulses. The transient currents are detected via the emitted terahertz radiation. Because of graphene's special structure symmetry, the injected current direction can be well controlled by the polarization of the pump beam in epitaxial graphene. This all optical injection of current provides not only a noncontact way of injecting directional current in graphene but also new insight into optical and transport process in epitaxial graphene.

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Available from: Dong Sun, Aug 22, 2014
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    • "In addition, graphene exhibits a large nonlinear optical response arising from the linear carrier energy dispersion, together with the high electron velocity near the Dirac point [2]. Harmonic generation at THz frequencies relying on a third-order nonlinearity has been recently demonstrated in graphene using a femtosecond optical excitation at normal incidence [3]. Although second-order nonlinear effects are generally considerably stronger and therefore of importance in applications, these are forbidden by symmetry as graphene is a centrosymmetric material. "
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    ABSTRACT: Graphene has been proposed as a particularly attractive material for the achievement of strong optical nonlinearities, in particular generation of terahertz radiation. However, owing to the particular symmetries of the C-lattice, second-order nonlinear effects such as difference-frequency or rectification processes are predicted to vanish in a graphene layer for optical excitations (ħω>2Ef ) involving the two relativistic dispersion bands. Here we experimentally demonstrate that graphene excited by femtosecond optical pulses generate a coherent THz radiation ranging from 0.1 to 4 THz via second-order nonlinear effect. We fully interpret its characteristics with a model describing the electron and hole states beyond the usual massless relativistic scheme. This second-order nonlinear effect is dynamical photon drag, which relies on the transfer of light momentum to the carriers by the ponderomotive electric and magnetic forces. The model highlights the key roles of next-C-neighbor couplings and of unequal electron and hole lifetimes in the observed second-order response. Finally, our results indicate that dynamical photon drag effect in graphene can provide emission up to 60 THz, opening new routes for the generation of ultra-broadband terahertz pulses.
    Full-text · Article · Sep 2014 · Nano Letters
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    • "Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field [1] and by quantum interference [2], we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample [3]. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. "
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    ABSTRACT: Strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena making graphene a unique playground for studying many-body effects. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain.
    Full-text · Conference Paper · May 2013
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    • "An additional manifestation of the carrier–carrier scattering on the THz generation is observed in the dependence of the THz amplitude on the 2ω beam power, as shown in figure 4(b). Since the coherently controlled photocurrent generation is a third-order nonlinear process, the generated current is determined by I ∼ |E ω | 2 |E 2ω | ∼ |P ω ||P 2ω | 1/2 [22], so the emitted THz signal from the injected photocurrent should be proportional to|P 2ω | 1/2 for fixed |P ω |. We verified that this power dependence is strictly followed when probing the third-order nonlinear tensor of ZnTe crystal with 1.6 µm/3.2 µm as shown in figure 4(b) (for details see the supplementary information). "
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    ABSTRACT: In this paper, we present direct time-domain investigations of the relaxation of electric currents in graphene due to hot carrier scattering. We use coherent control with ultrashort optical pulses to photoinject a current and detect the terahertz (THz) radiation emitted by the resulting current surge. We pre-inject a background of hot carriers using a separate pump pulse, with a variable delay between the pump and current-injection pulses. We find the effect of the hot carrier background is to reduce the current and hence the emitted THz radiation. The current damping is determined simply by the density (or temperature) of the thermal carriers. The experimental behavior is accurately reproduced in a microscopic theory, which correctly incorporates the nonconservation of velocity in scattering between Dirac fermions. The results indicate that hot carriers are effective in damping the current, and are expected to be important for understanding the operation of high-speed graphene electronic devices.
    Full-text · Article · Oct 2012 · New Journal of Physics
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