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

Abstract

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 media 1-3 , 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 monolayer 4 . 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 plasmonics 5-7 has progressed rapidly, propelled by the electrical tunability, high field confinement 8,9 , potentially long lifetimes 10,11 of plasmons and the strong light-matter interactions 12-15 in graphene. An earlier spectroscopic study has reported photoinduced alteration of the plasmonic response of graphene on optical pumping 16 . 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/hBN 4 .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 resolution 16-18 , 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 plasmons 19 . 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 hBN 10 . The tip of the nanoscope acts as a launcher for surface plasmons of wavelength (λ p ) that propagate radially outwards from the tip. On reflection from the sample edge, these plasmons form sta...

Towards properties on demand in quantum materials

“…Importantly, mesoscopic phenomena that are prevalent in quantum materials lead to new time and energy scales, as documented in various ultrafast studies 110,160 . Modern scanning optical probe tools are set to dramatically advance our understanding of mesoscopic dynamics, visualizing local transient phenomena at nano-and mesoscales [161][162][163] . A future challenge is to create groundstate or continuous-wave-driven steady-state versions of enigmatic nano-and mesoscale phenomena currently attainable only in transient regimes.…”

Tipping solutions: emerging 3D nano-fabrication/ -imaging technologies

“…The absorption technique is better suited for the characterization of organic absorbing surfaces while scattering delivers high-fidelity imaging from metallic reflective surfaces and nanoparticles. Interpretation of data from the near-field requires further knowledge of probe-surface interaction, phase information of the reflected/transmitted light from sub-wavelength structures to reveal complex peculiarities of light-matter interactions at the nanoscale [118][119][120][121][122][123]. Recently, electron tunneling control by a single-cycle terahertz pulse illuminated onto a tip of scanning tunneling electron microscope (STEM) needle was demonstrated at 10 V/nm fields [124].…”

Nonlocal plasmonic response of doped and optically pumped graphene, MoS2 , and black phosphorus

“…The presence of the so-called thermoplasmons when the electronic temperature is sufficiently high has been recently corroborated in highquality graphene through ultrafast near-field spatial imaging [9]. Plasmon switching based upon optical pumping has also been demonstrated [26], while it has been proposed that gain resulting from population inversion could compensate for losses under these circumstances [27][28][29].…”