Contact-free high-resolution conductivity mapping of large-area graphene covered with an ion gel top-gate

We present contact-free THz near-field measurements at large-area graphene covered by an ion gel top-gate. In dependence of the gate-voltage, we map the THz near-field transmission amplitude and the resulting sheet resistance distribution across the graphene layer. High-resolution access to the local charge carrier properties of the buried graphene enables the visualization of inhomogeneity and defects on a microscopic scale and the time-dependent evolution of the ion gel based doping process.