Photo-induced terahertz near-field dynamics of graphene/InAs heterostructures

Abstract

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (/ 6000 λ), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely "screens" the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures

“…To put our concept to a final test, we compare the delayed onset of the pump-probe signal from the heterostructure with the tunnelling dynamics independently measured by laser terahertz emission nanoscopy 38,41,43 (LTEN). Interlayer tunnelling should generate a time-dependent out-of-plane current of density j z (t) that produces coherent terahertz emission 20 , providing direct access to the ultrafast charge-transfer dynamics.…”

“…Here, we use ultrafast scattering-type scanning near-field optical microscopy 29,32,33 (s-SNOM) at terahertz (THz) frequencies 27,[34][35][36][37][38][39][40][41][42][43] as a non-invasive, subcycle, and contact-free probe of charge-transfer dynamics in WSe 2 /WS 2 heterostructures on insulating substrates. Our approach builds on a critical change of the polarizability of e-h pairs during interlayer tunnelling, as explained by ab initio density functional theory and confirmed by terahertz emission directly linked to the ultrafast charge separation.…”

Nanoimaging and Nanospectroscopy of Polaritons with Time Resolved s‐SNOM

“…As the system is pumped away from equilibrium, the dynamics of the photo‐excited electrons and their recovery to thermal equilibrium will be detected by the probe beam arriving at various time delays after pumping. The setup has been carried out successfully in the infrared, multi‐THz, and THz frequency regions. This is also the basic scheme for the detection of ultrafast polaritons using s ‐SNOM.…”

Quantitative terahertz emission nanoscopy with multiresonant near-field probes

“…Still, the approach lacks nanoscale spatial sensitivity. To overcome this challenge, TES has recently been merged with THz near-field microscopy [11][12][13][14][15][16][17][18] and termed terahertz emission nanoscopy (TEN) [11,14] paving the way for investigations of nanostructures. While the near-field probe can efficiently confine the emerging long-wavelength radiation to its sharp apex and subsequently scatter it to the far field, the metallic tip also alters the temporal structure of the detected waveforms.…”