The combination of electronic and plasmonic circuitries may in the near future become of technological relevance for the development of hybrid chips on which improved signal transmission is realized by surface plasmon polariton (SPP) waves. The element required at the joint of electronic and plasmonic circuits is a coupler which converts voltage pulses to light pulses at clock rates in the GHz range. We introduce such an element in a compact form on the nanometer scale.
In an earlier study we demonstrated that the electronic states of a quantum system inside a tunnel junction can act as a passive filter for the excitation of plasmons . By adding an active tunability to the element a fast dynamic control can be implemented . Figure 1(a) shows a sketch of the element which we realized inside a low temperature (4 K) scanning tunneling microscope (STM). As quantum system we choose an individual single fac-tris(2-phenylpyridine)iridium(III) molecule (briefly: Ir(ppy)3) deposited on a close-packed semiconducting bilayer of C60 fullerene molecules on top of a flat metal substrate. The molecule is contacted from the opposite side through a vacuum tunnel barrier by the second electrode realized by the STM tip. The current from source to drain electrodes cannot pass efficiently through the extended double barrier structure. However, the tunnel efficiency substantially increases when an electronic state is present inside the double barrier and when this state is energetically accessible to the tunneling electrons. Controlling the energy alignment of the electronic levels of the molecule thus allows to either block or permit the passage of an electric current. The additional benefit of the tunnel junction is that the entire applied source–drain voltage falls off within the junction so that inelastically tunneling electrons may convert their potential energy completely to create a plasmon. The created plasmons are quanta of the electromagnetic field in the cavity between source and drain electrode. The generation of plasmons in an STM tunnel junction can readily be monitored by the luminescence emitted from the junction, which is detectable by external photo detectors. Here, we use the term "plasmonic emission" to denote both, the generation of plasmons and the detectable emission of light from the junction because both processes are inextricably related to each other.
The response of the molecule to an electric field in the junction can be compared to the gating of a conventional field effect transistor (FET). In both devices, the application of an electric field can open a current channel between the source and the drain. While the conduction channel in a FET is due to the field-induced generation of mobile charge carriers, in our case, the channel is realized by a single well defined electronic level. With respect to its mechanism, the concept rather resembles a resonant tunneling transistor. We realize the function in an extremely compact element with low space consumption and the tunnel current passing vertically to the surface plane. For the experimental demonstration, we employ the fact that single Ir(ppy)3 molecules (colored violet in Fig. 1(a)) adsorbed on top of a C60 bilayer possess a separate unoccupied electronic state only 0.5 eV above the Fermi energy of the source electrode. The state is indicated by the violet bar in the energy diagram of the tunnel junction in Fig. 1(b). By an external electric field, the molecular state can be shifted downwards to become aligned with the substrate Fermi energy as illustrated in Fig. 1(c). The alignment closes the electric circuit and switches on the plasmonic emission. Subtle changes of the molecular electrostatic potential thus substantially modify the conductivity and − even more efficiently – the plasmonic emission. The response can be extremely fast because possible delays due to line capacitances may be minimized. The element thus provides a suitable candidate for a field-controlled transistor or a local sensor for a large variety of physical or chemical processes involving a change of the electric field.
Figure 2 shows the performance of the element. The yellow color represents the intensity of the detected light as a function of source–drain voltage (horizontal scale) and electric field (diagonal lines, with field values given in V/nm). The source-drain voltage and the gate field can be adjusted independently of each other by varying the distance between molecule and drain electrode (vertical axis). This is possible since in the experiment the drain is realized by the movable tip of the tunneling microscope. The onset of plasmonic emission follows accurately a line of constant electric field of 2.5 V/nm. This proves that the observed onset of emission is controlled by the gate field and not by the applied source–drain voltage. Figure 2(b) zooms in on the data recorded at the steep rise of plasmonic emission. The increase by 3 orders of magnitude in intensity occurs over a small range of electric field at 2.51 V/nm. The steepest ascend of light emission reaches a slope of one order of magnitude per 11 mV/nm. An interesting detail is, that the control mechanism does not only work for the current passing through the molecule itself, but also for the current passing through the semiconducting buffer layer in proximity to the molecule. Whenever the molecule becomes charged, it induces a significant band bending in the semiconducting C60 layer and in consequence also increases the current flow through that layer followed by a boost of plasmonic emission. The existence of this proximity effect which separates charge transfer (i.e. gating) and conduction (i.e. source-drain current) is illustrated by the large extension of the disk of light excitation in Fig. 3(a) whose diameter exceeds the geometric size of the Ir(ppy)3 molecular states, Fig. 3(b).
The time response of the transistor can be probed by charging and discharging the molecule repeatedly through a train of voltage pulses. We employed 100 ns rectangular pulses with 150 mV amplitude added to a fixed 2.85 V bias. The time-resolved plasmonic emission from the tunnel junction is displayed by the yellow symbols in Fig. 3(c). The curve follows the time response function of the experiment (black dotted curve) without any measurable delay. This suggests that a band width well in the GHz range can be achieved by the device.
In conclusion, we showed that an individual quantum system, exemplified by a single molecule, is able to dynamically control the electrical plasmon generation in a tunnel junction. Other quantum systems, such as quantum dots or single atoms, may be operated similarly and an ultra-thin insulator may, for instance, replace the vacuum gap. In fact, the plasmon gating only relies on the alignment of an individual confined electronic state within the tunnel barrier between the contacting electrodes. A proper choice of the electronic structure of the junction allows the realization of electronically driven and ultrafast gateable single surface plasmon polariton (SSP) sources. Future studies may enhance and apply the demonstrated effects in planar tunnel junction and metal–insulator–metal waveguides. Such quantum interfaces, bridging nanoelectronics and nanophotonics, might provide new routes to realize single-plasmon-on-demand sources and to convert electronic qbits into photonic qbits.