Mid-Infrared Graphene Photoresponse*

Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since its discovery in 2004. Due to its zero band gap band structure, it has a broad-band optical absorption ranging from the far-infrared all the way to the visible making it potentially useful for infrared photodetectors. Electrostatically gated p-n junctions have demonstrated photocurrents in the near-IR ($\lambda =$ 850nm), primarily due to hot carrier mechanisms. In order to study these mechanisms at longer wavelengths ($\lambda =$ 10 $\mu $m), high quality chemically vapor grown (CVD) graphene is necessary to fabricate electrostatically controlled p-n junctions due to the longer optical length scales. Moreover, at these low energies ($\sim$ 125 meV), optical phonon scattering is suppressed and is predicted to lead to increased carrier lifetimes and enhanced photo-response. Using electrostatic gating, we are able to study the absorption mechanisms in graphene by selecting between conventional photovoltaic effects and photo-thermoelectric effects. Experiments suggest that the photocurrent signal is enhanced by electrostatic gating near the Dirac peak and reduced disorder in the graphene sample.