Ultrafast Lateral Photo-Dember Effect in Graphene Induced by Nonequilibrium Hot Carrier Dynamics

The photo-Dember effect arises from the asymmetric diffusivity of photoexcited electrons and holes, which creates a transient spatial charge distribution and hence the buildup of a voltage. Conventionally, a strong photo-Dember effect is only observed in semiconductors with a large asymmetry between the electron and hole mobilities, such as in GaAs or InAs, and is considered negligible in graphene due to its electron hole symmetry. Here, we report the observation of a strong lateral photo-Dember effect induced by nonequilibrium hot carrier dynamics when exciting a graphene metal interface with a femtosecond laser. Scanning photocurrent measurement reveal the extraction of photoexcited hot carriers is driven by the transient photo-Dember field, and the polarity of the photo current is determined by the device's mobility asymmetry. Furthermore, ultrafast pump probe measurements indicate the magnitude of photocurrent is related to the hot carrier cooling rate. Our simulations also suggest that the lateral photo-Dember effect originates from graphene's 2D nature combined with its unique electrical and optical properties. Taken together, these results not only reveal a new ultrafast photocurrent generation mechanism in graphene but also suggest new types of terahertz sources based Oil 2D nanomaterials.