Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ∼ 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At sub-nanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single-and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene: yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers. KeywordsInfrared nano-imaging, bilayer graphene, plasmons, tunneling, plasmon-off region Main TextQuantum plasmonics is a rapidly growing field of research that involves the study of light-matter interactions in the quantum regime 1,2 . In particular, tunneling plasmonics explores the role of electron tunneling on the optical responses of coupled plasmonic nanostructures. Previous studies about tunneling plasmonics were focused on coupled metal nanoparticles within sub-nanometer separation [3][4][5][6] . The plasmonic resonance of the coupled system deviates from classical predictions where only Coulomb coupling is considered. In order to fully describe the plasmonic response, quantum tunneling of electrons between the nanoparticles has to be taken into account. Here we report experimental observation of novel plasmonic phenomena due to electron quantum tunneling between adjacent layers of graphene -a novel plasmonic material that carries infrared plasmons with high confinement, low loss and gate tunability [7][8][9][10][11][12][13] . Specifically, we observed a high plasmon confinement and effective plasmon-off state when two graphene layers are placed close to each other in a Bernal-stacking order. These effects are attributed to interlayer electron tunneling that plays a prominent role in graphene due to the reduced dimensionality and relatively low carrier density. In order to study the plasmonic properties of coupled graphene layers, we performed infrared (IR) nano-imaging experiments using an antenna-based nanoscope (Supporting Information). As shown in Figure 1a, the metalized tip of the atomic force microscope (AFM) is illuminated by IR light thus generating strong near fields underneath the tip apex. These fields have a wide range of in-plane momenta q therefore facilitating energy transfer and momentum bridging from photons to plasmons [7][8][9][10][11][12][13] . Our samples were fabricated by mechanical exfoliation of bulk graphite and then transferred to ...

Unraveling exciting new physics in complex novel materials requires access to both material excitations and their dynamics, thus continuously pushing ultrafast pump-probe spectroscopy to its limits. However, most of the materials whose dynamics are at the center of current attention are also known to be inhomogeneous at the nanoscale. Hence, diffraction-limited optical techniques with their inherent areaaveraging character inhibit access to characteristic time scales of nanoscopic, heterogeneous systems. Circumventing the diffraction limit, scattering scanning near-field optical microscopy (s-SNOM) is a well-established technique that enables broad-band infrared spectroscopy with the nanoscale spatial resolution. In s-SNOM backscattered light from an atomic force microscope (AFM) tip reveals the local dielectric function of a sample [1]. Previous infrared s-SNOM studies were static, utilizing primarily continuous wave (CW) laser sources. Here, we extend s-SNOM by merging nano-spectroscopy with ultrafast pump-probe techniques and exemplify new capabilities with the time-resolved control of the plasmonic response of graphene and the semiconductor InAs.The goal of plasmonics is to utilize electromagnetic energy on a sub-wavelength scale in form of collective surface charge oscillations. Amongst various candidates for plasmonic media graphene stands out due to its most favorable properties: ultimate energy confinement in a monoatomic layer along with easy control over its charge density via electrostatic fields. Recently, the strong coupling between Dirac plasmons and the AFM tip of a near-field microscope has proven ideal to investigate their characteristics, so far utilized for their infrared spectroscopy [2] and real space visualization [3,4]. Another interesting material class for plasmonics are semiconductors due to mature processing technologies and carrier density control via doping.In our experiment we combine mid-infrared s-SNOM and ultrafast 100 fs, near-infrared laser excitation (Fig. 1a)) to study the time-dependent behavior of graphene [5] and InAs plasmons [6] at the nanoscale in infrared spectroscopy. For graphene we find strong pump-induced spectral changes in the infrared plasmonic response (Fig. 1b), top) around the SiO 2 substrate phonon resonances at 800 and 1125 cm -1 (Fig. 1b), bottom panel). Modeling reveals that pump-induced heating of carriers up to a temperature of 2100 K is the dominant effect. It results in an increase in Drude weight that s-SNOM is sensitive to. In

No abstract