Active control of ultrafast electron dynamics in plasmonic gaps using an applied bias

Abstract

In this joint experimental and theoretical study we demonstrate coherent control of the optical eld emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that a small applied dc bias or an external THz eld allows to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultra-fast nanoscale vacuum diode for lightwave electronics. Using Time-Dependent Density Functional Theory calculations we elucidate the main physical mechanisms behind the observed eects and show that an applied dc eld modies the optical eld emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device which allows us to establish a new paradigm for controlling nanocircuits at Petahertz frequencies. The interaction of intense short laser pulses with matter provides access to the dynamics of electronic excita-tions in a highly nonlinear regime characterised by emission of energetic electron bursts of sub-cycle duration and by generation of high harmonics used to track the evolution of the quantum systems at attosecond time scales [1 4]. For metal surfaces and metal nanoparticles, the coupling of light with collective electronic excitations (plas-mons) allows to engineer enhanced optical elds at the hot spots with characteristic sizes well below the dirac-tion limit [5, 6]. Thus, the optical eld emission regime can be reached for incident eld strengths signicantly smaller than those required for molecular and atomic species in the gas phase [4, 7, 8]. In contrast to electron photoemission via multiphoton absorption, optical eld emission can be seen as an electron tunneling at the metal vacuum interface in a situation where the potential barrier is strongly modied by the instantaneous optical eld. The in-depth studies performed for metallic nanotips and plasmonic nanoparticles revealed that optical eld electron emission can be manipulated at femtosecond time scales via the carrier-envelope phase (CEP) of the driving laser pulse [911]. Using THz elds or an applied bias along with the optical excitation oers additional possibilities [1217] of coherent control. In this context, among the plasmonic nanoobjects that can be applied for light wave electronics [1823], the dimer antenna with a nanoscale gap is very relevant. On the one hand, the coupling between electrons and photons in narrow gaps of dimer antennas leads to light emission originating from inelastic electron tunneling events [2427]. On the other hand, the highly nonlinear optical eld electron emission process [19, 22, 23, 28, 29] as well as optically assisted electron tunneling [3032] allow rectication at optical frequencies and CEP control of the electron transport across the junction [21, 33]. In this Letter, we demonstrate coherent control of the net electric current in a nanocircuit comprising a single bowtie nanoantenna with a 6 nm wide gap as presented in Fig. 1. It follows from our results that, along with the CEP of the incident pulse, the petaherz currents of the optically emitted electrons in the gap of the nanoantenna can be directed and controlled by applying a dc eld two orders of magnitude smaller than the optical eld in the junction. Notably, such a dc bias alone does not trigger any electron ow across the gap, i.e. electron tunneling is impossible for this width of the junction. Our study thus extends the possible application of the static or THz elds beyond the control of the electron (photo)emission from metallic tips [1217] and electron tunneling [3436]. Along with dielectric, semiconductor [3739], graphene-based [18, 40], and tunneling [33] structures, the analogue of the ultrafast rectifying vacuum diode demonstrated here (see also [29]) paves the way towards petaherz electronics [41]. The gold bowtie nanoantenna has been fabricated by electron beam lithography on a silica substrate. The two arms of the antenna are interfaced macroscopically (see Fig. 1b) with a transimpedance amplier which allows for a readout of the optically driven currents via a lock-in scheme. An additional DC bias can be applied across the antenna gap by means of a bias tee. The ultrafast currents are driven by the electric eld transients of single-cycle light pulses of a carrier wavelength of 1250 nm and a duration of 4.2 fs. These pulses are generated by a