Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene

The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at subpicosecond timescales. This handicap has prompted the search for alternative plasmonic media1–3, with graphene emerging as one of the most capable candidates for infrared wavelengths. Here we visualize and elucidate the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer4. We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks infrared plasmonic response at equilibrium. In combination with static nano-imaging results on plasmon propagation, our infrared pump–probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene. Graphene plasmonics5–7 has progressed rapidly, propelled by the electrical tunability, high field confinement8,9, potentially long lifetimes10,11 of plasmons and the strong light–matter interactions12–15 in graphene. An earlier spectroscopic study has reported photoinduced alteration of the plasmonic response of graphene on optical pumping16. In this work, we harnessed ultrafast optical pulses to generate mid-infrared (mid-IR) plasmons in a sample that lacks a plasmonic response at equilibrium. We examined the real-space aspects of non-equilibrium plasmon–polariton generation and propagation under femtosecond (fs) photo-excitation using a new ultrafast nano-infrared (IR) technique that fuses realspace plasmon imaging with spectroscopy. We applied this method to investigate high-quality graphene specimens encapsulated in hexagonal boron nitride: hBN/G/hBN4. We performed time-resolved broadband nano-IR experiments using antenna-based near-field nanoscopy (see Methods). This set-up (Fig. 1a,b) combines exceptional spatial, spectral and temporal resolution16–18, allowing an experimental probe of the dispersion of graphene plasmons under photo-excitation—a feat previously considered technologically infeasible. In our measurements, the metalized tip of an atomic force microscope (AFM) was illuminated by a focused IR probe beam, generating strong evanescent electric fields beneath the tip. These fields possess a wide range of in-plane momenta q and therefore facilitate efficient coupling to graphene plasmons19. Such evanescent fields extend ∼20 nm beneath the top surface of our structures, which is sufficient to launch and detect surface plasmons in a graphene microcrystal protected by a thin (10 nm) encapsulating layer of hBN10. The tip of the nanoscope acts as a launcher for surface plasmons of wavelength (λp) that propagate radially outwards from the tip. On reflection from the sample edge, these plasmons form standing waves between the tip and the reflector. While scanning the tip towards a graphene edge, one can collect the near-field signal, which displays oscillations with a period of λp/2. Recent nano-IR imaging experiments with encapsulated high-mobility graphene have registered oscillations with both λp/2 and λp periodicity; the latter were assigned to plasmons launched directly from the sample edge by incident light10. We begin with the principal result of this work: the ultrafast dynamics of plasmons in encapsulated graphene revealed by means of nano-IR pump–probe spectroscopy (Fig. 1c,d). The broadband mid-IR probe allows for visualization of the frequency–momentum dispersion of graphene plasmons in the course of a single line-scan across the sample surface20. We investigated the photo-induced changes in near-field scattering amplitude s(ω,x) collected from sequential 20 × 20 nm spatial pixels that together constitute a hyperspectral line-scan. Acquired at varying pump–probe delay times, hyperspectral scans reveal a rich spatiotemporal plasmonic response, which arises and then decays according to the dynamics of photo-excited carriers. It is instructive to present hyperspectral line-scans in the form of two-dimensional frequency-position maps s(ω,x) plotted in Fig. 1c,d (and see Supplementary Information (SI)). This representation highlights the novelty of our experimental approach, which combines non-equilibrium spectroscopy with imaging of plasmonic standing waves. The strongest photo-induced signal is recorded at zero time delay between pump and probe pulses (Fig. 1c): we observe a set of three dispersing peaks in the s(ω,x) hyperspectral map. The dashed lines shown in Fig. 1c trace peaks of the signal determined from the spatial derivative of the raw data, ds(ω,x)/dx = 0 (Fig. 1e). The spatial period of oscillations, as well as the relative separation between peaks and troughs, systematically increases with decreasing ω. These findings are consistent with expectations for plasmonic modes dispersing with positive group velocity. The photo-induced plasmonic signal decays as the probe pulse is temporally delayed with respect to the pump pulse (Fig. 1d,f ). After t = 2 ps pump–probe delay, we observe only a single peak in close proximity to the sample edge. Plasmonic features completely vanish after 5 ps (Supplementary Section 6): a timescale consistent with earlier diffraction-limited measurements21–23. Figure 1c,d