Non-Local Transfer of Ultrafast Currents Generated by Few-Cycle Laser Pulses

Ultrafast optoelectronics promises a several-order-of-magnitude speed-up of computational devices. In recent years several experiments emerged that demonstrated the possibility of steering electrons in matter with optical fields on femtosecond timescales and this way they established building blocks for PHz devices [1], [2]. Such a typical experiment usually exploits the carrier-envelope phase (CEP) of laser pulses which causes symmetry breaking driving an oriented current in a medium. These ultrafast, CEP-dependent currents appear in a wide range of media spanning graphene, semiconductors or dielectrics. A common feature of these experiments is that currents are detected using metal electrodes and slow electrical circuit is contacted to the illuminated volume. This raises the question on what basis the current transfer from the medium to the electrodes is and whether the optical-to-electrical conversion is possible without the loss of bandwidth. A recent study [3] hints that the photocurrent transfer might be non-local, i.e. there is no need for physical transport of carriers from the volume to the electrodes. Having that in mind, we experimentally studied the relationship between the ultrafast photo current current, material, laser beam and geometry. We observed signatures that can be modelled with the Ramo-Shockley theorem that support the manifestation of non-local, i.e. quasi-instantaneous dynamics.