Ultrafast Strong-Field Tunneling Emission in Graphene Nanogaps

Abstract

We demonstrate subcycle electron pulse generation in a nanogap of graphene when irradiated by a femtosecond laser pulse in the near-infrared region (800 nm). A strong photoinduced emission was produced when the gap area was irradiated by the ultrashort pulse laser. The graphene, which has atomically sharp edges with a large damage threshold, enables us to achieve a strong tunneling regime for the subcycle field emission. The photoinduced signals exhibited an anomalous increase in nonlinear order as a function of incident pulse energy in the presence of static electric field. A dynamical analysis of tunneling electrons based on the semiclassical model, which considers the contribution from the recoil electrons, reproduced our observation successfully. The large field enhancement near the graphene edge enabled us to reach the deep tunneling regime with the extraordinary Keldysh parameter of 0.2 in the near-infrared region, which has not been accessible by conventional metal nanostructures.

Ultrashort field emission in metallic nanostructures and low-dimensional carbon materials

“…Most of the previous experiments on ultrafast field emission have been performed in the absence of DC electric fields; the photoelectrons were measured by the anode electrode, which is located far from the cathode metal tip. In a number of recent studies, in which the photoemission occurred through a small gap [71,109], a strong static field induced by a DC bias voltage across the gap significantly influences the emission efficiency and dynamic motion. In this case, while the DC field significantly narrows the vacuum level, the fs light triggers the photo-field emission by additional electronic bending or by thermal electron generation.…”

“…A more simplified model of electron motion in a semi-classical limit has been developed recently, taking the DC field contribution into consideration [71]. Here, the photoelectron generation, and their subsequent motion, has been simulated with the assumption of two-dimensional metal edge structures, a theoretical counterpart for the graphene edge emitter, as will be discussed in Section IV.…”

“…Graphene has been recently demonstrated in an ultrafast electron emitter, specifically in the strong-field tunneling regime [71]. As schematically illustrated in Figure 13(a), graphene field emission devices were fabricated with a nanogap of 100 nm in the middle of the conducting channel, fabricated by focused ion beam (FIB) milling.…”

“…Even though strong-field tunneling emissions have been successfully explored using metal nanoprobes or metal nanostructures [55,57,58,60,[63][64][65][66][67][68][69][70], they have been limited in terms of achieving a large ponderomotive potential, due to the thermal damage that frequently leads to the meltdown of nanostructures [67]. This will also restrict the possible quantity of ultrashort electron emissions from the metal nanotip [69,71]. Their functionality is minimally variable, and therefore, the control of electron motion has been primarily manifested by varying the geometry of the metal structure (such as the tip apex and conical angles) and the physical properties of the laser [60-62, 72, 73].…”

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

“…Using a THz field or a dc applied bias along with the optical excitation offers additional possibilities [14][15][16][17][18][19] for the coherent control of electron dynamics. In this context, among the plasmonic nanoobjects that can be applied for light wave electronics [20][21][22][23][24][25], the dimer antenna with a nanoscale gap is particularly relevant. On the one hand, the coupling between * andrei.borissov@u-psud.fr electrons and photons in narrow gaps of dimer antennas leads to light emission originating from inelastic electron tunneling events [26][27][28][29].…”