Ultrabroadband THz Conductivity of Gated Graphene In- and Out-of-equilibrium

TL;DR

This study uses ultrabroadband THz spectroscopy to investigate the high-frequency electrical properties of monolayer graphene under various doping conditions, revealing both equilibrium and transient conductivity behaviors up to 15 THz.

Contribution

It provides the first comprehensive measurement of graphene's THz conductivity across a broad frequency range, including transient responses after femtosecond excitation, and models the underlying carrier dynamics.

Findings

Transient conductivity changes are resolved up to 15 THz.

An additional broad-frequency component (~8 THz) is observed in photo-induced response.

Quantum-critical transport mode is identified in large-area CVD graphene.

Abstract

We employ ultrabroadband terahertz (THz) spectroscopy to expose the high-frequency transport properties of Dirac fermions in monolayer graphene. By controlling the carrier concentration via tunable electrical gating, both equilibrium and transient optical conductivities are obtained for a range of Fermi levels. The frequency-dependent equilibrium response is determined through a combination of time-domain THz and Fourier-transform infrared spectroscopy for energies up to the near-infrared, which also provides a measure of the gate-voltage dependent Fermi level. Transient changes in the real and imaginary parts of the graphene conductivity are electro-optically resolved for frequencies up to 15 THz after near-infrared femtosecond excitation, both at the charge-neutral point and for higher electrostatic-doping levels. Modeling of the THz response provides insight into changes of the carrier spectral weights and scattering rates, and reveals an additional broad-frequency ($\approx$ 8 THz) component to the photo-induced response, which we attribute to the zero-momentum mode of quantum-critical transport observed here in large-area CVD graphene.