Nanoscale Terahertz Conductivity and Ultrafast Dynamics of Terahertz Plasmons in Periodic Arrays of Epitaxial Graphene Nanoribbons

TL;DR

This study investigates the ultrafast dynamics of terahertz plasmons in epitaxial graphene nanoribbons, revealing carrier cooling, defect-induced localization, and edge effects using time-resolved spectroscopy and near-field microscopy.

Contribution

It provides new insights into the ultrafast carrier dynamics and plasmon behavior in graphene nanoribbons, highlighting defect effects and edge scattering at sub-picosecond timescales.

Findings

Carrier temperature drops from >5000 K to lattice temperature in ~7 ps.

Graphene nanoribbons contain defects causing charge localization.

Edges slightly increase carrier scattering, affecting conductivity.

Abstract

Dynamics of plasmons in nanoribbons of (hydrogen intercalated) quasi-free-standing single layer graphene is studied by terahertz spectroscopy both in the steady state and upon photoexcitation by an ultrashort near infrared laser pulse. The use of two-dimensional frequency domain analysis of the optical pump - THz probe signals allows us to determine the evolution of carrier temperature and plasmon characteristics with ~100 fs time resolution. Namely, we find that the carrier temperature decreases from more than 5000 K to the lattice temperature within about 7 ps and that during this evolution the carrier mobility remains practically constant. The time-resolved THz conductivity spectra suggest that graphene nanoribbons contain defects which act as low potential barriers causing a weak localization of charges; the potential barriers are overcome upon photoexcitation. Furthermore, the edges of graphene nanoribbons are found to slightly enhance the scattering of carriers. The results are supported by complementary measurements using THz scanning near-field microscopy which confirm a high uniformity of the THz conductivity across the sample and demonstrate high enough sensitivity to resolve even the impact of nanometric terrace steps on SiC substrate under the graphene monolayer.