Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy

Abstract

The graphene photodetector is illustrated in Fig. 1a. A monolayer graphene sheet was encapsulated between two h-BN layers 15 . The h-BN(13 nm)-graphene-h-BN (42 nm) heterostructure is placed on top of a pair of 15-nm-thick AuPd gates, which are laterally separated by a gap of 50 nm. Applying individual voltages to the gates allows for controlling independently the carrier concentrations n 1 and n 2 in the graphene sheet at the left and right sides of the gap.In Fig. 1a we also introduce the concept of THz photocurrent nanoscopy, and its application for GPs mapping. The setup is based on a s-SNOM (Neaspec), where the metal tip is illuminated with the THz beam of a gas laser (SIFIR-50 from Coherent, providing output power in the range of a few 10 mW). Owing to a lightning-rod effect, the incident field is concentrated at the tip apex yielding a THz nanofocus 16 . Once brought into close proximity of the sample, the near fields of the nanofocus induce a current in the graphene sheet, similar to IR photocurrent nanoscopy 14,17 . Recording the current as a function of the tip position yields nanoscale-resolved THz photocurrent images. For the current measurement, the graphene is contacted electrically in a lateral geometry (i.e. metal contacts were fabricated at both sides of the heterostructure, as shown in Fig. 1a). Analogously to s-SNOM 18 and scanning photocurrent nanoscopy 14, 17 , we isolate the near-field contribution to the total photocurrent, I PC , by (i) oscillating the tip vertically at frequency Ω and (ii) demodulating the detector signal at 2Ω. This 3 technical procedure is required because of the background photocurrent generated by the diffraction limited illumination spot. We achieved a spatial resolution of about 50 nm (supplementary information S1), which is an improvement of nearly 4 orders of magnitude compared to diffraction-limited THz imaging. Fig. 1b shows a photocurrent image of the photodetector, recorded at 2.52 THz (λ 0 = 118.8 µm). Choosing graphene charge carrier densities n 1 = 0.77 and n 2 = -0.77x10 12 cm -2 , we generate a sharp pn-junction in the graphene above the gap between the gates.We observe a strong near-field photocurrent, I PC , which is localized to an about 1 µm wide region centred above the gap (central part of Fig. 1b). It can be explained by a photo-thermoelectric effect: due to a variation of the local Seebeck coefficient S in graphene (generated by the carrier density gradient), a local temperature gradient (caused by the THz nanofocus at the tip apex) generates a net charge current 14,17 .Because the variation of the carrier concentration -and thus ΔS -is largest between the two gates, we expect a maximum in the photocurrent at this location. In Fig. 1b, however, we observe a slight decrease of the photocurrent between the gates. We explain it by the reduced near-field intensity when the tip is above the gap, owing to the weaker near-field coupling between the tip and the metal gates. To corroborate the photo-thermoelectric origin of the THz photocurrent, we carrie...

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