Coherent laser control of the current through molecular junctions

The electron tunneling through a molecular junction modeled by a single site weakly coupled to two leads is studied in the presence of a time-dependent external field using a master equation approach. In the case of small bias voltages and high carrier frequencies of the external field, we observe the phenomenon of coherent destruction of tunneling, i.e. the current through the molecular junction vanishes completely for certain parameters of the external field. In previous studies the tunneling within isolated and open multi-site systems was suppressed; it is shown here that the tunneling between a single site and electronic reservoirs, i.e. the leads, can be suppressed as well. For larger bias voltages the current does not vanish any more since further tunneling channels participate in the electron conduction and we also observe photon-assisted tunneling which leads to steps in the current-voltage characteristics. The described phenomena are demonstrated not only for monochromatic fields but also for laser pulses and therefore could be used for ultrafast optical switching of the current through molecular junctions. Introduction. – Electronic transport through molecular wires and junctions has recently attracted much attention experimentally as well as theoretically [1–3]. Under the influence of a bias voltage and because of the coupling to the leads which act as electron source and drain, a current through the molecular junction is established. When an external time-dependent field, such as a laser field or an additional ac voltage is applied to the molecular junction, several interesting effects arise. One phenomenon is the well-known photon-assisted tunneling (PAT) [4]. It was studied already in the early 1960’s experimentally by Dayem and Martin [5] and theoretically by Tien and Gordon using a simple theory which captures already the main physics of PAT [6]. The main idea is that an external field periodic in time with frequency ω can induce inelastic tunneling events when the electrons exchange energy quanta h̄ω with the external field. Another important effect is the famous phenomenon of coherent destruction of tunneling (CDT). Grossmann et al. [7–9] first studied this effect and found that tunneling can be quenched in a periodically driven quantum system. In the context of molecular wires, this phenomenon can be explained using Floquet theory in the case of a periodic (a)Formerly International University Bremen laser field [10], and CDT occurs for certain amplitudes of the laser field at fixed frequencies [11–15]. Different scenarios of controlling the tunneling in molecular wires and quantum dots have been proposed based on different mechanisms [4, 14–18]. Also current-induced light emission in molecular junctions has been studied [19]. In the current paper we focus on the tunneling through a single-site molecular junction. This might be a quantum dot (though the temperatures in the current examples are rather high for quantum dots) or a single molecular level acting as a molecular wire. The theoretical foundation is a density matrix formalism using a perturbative treatment within the molecule-lead coupling to second order. Applying this technique it is possible to calculate the time-dependent population in and the current through the molecular junction under the influence of a time-varying external field [15,20]. Since the effect of the external field on the coupling between molecule and leads is treated exactly and not neglected as for example in Redfield theory, effects based on the influence of the laser on this coupling can be investigated. The tunneling between the molecule and the leads can, for small bias voltages, be suppressed by a monochromatic laser. This result is different from previous studies [13–15,21] in which the current vanished because of the CDT between the sites of the wire.