Ultrafast tunneling microscopy by optical field control of quantum currents

ended

The project aims at imaging electronic dynamics in molecules with atomic precision and sub-femtosecond temporal resolution. This result will be achieved by establishing new experiments at the boundary of ultrafast optics and scanning probe microscopy where the electric field of single-cycle light pulses is harnessed to control currents in nanojunctions. The basic concept relies on the fact that state-of-the-art femtosecond optical wave packets exhibit only one cycle of radiation with a defined electric field maximum. These pulses need to be phase locked to a “cosine-like” electric field profile. If such radiation is focused onto a junction with a nonlinear current-voltage characteristics, a net charge flow results solely due to the bias induced by the optical field. In detail, we want to exploit the time resolution provided by this new technique and induce electron transport at the probe tip of a scanning tunneling microscope (STM). The optical control of the current over a sub-optical-cycle interval will guarantee a temporal resolution better that one femtosecond, thus improving by several orders of magnitude what can be achieved with standard electronic bias. The core of the experimental system will be an ultrabroadband and passively phase-locked Er:fiber laser that is designed to generate single-cycle optical pulses in the near/mid-infrared, i.e. off resonant to the transition energies of III-V and II-VI semiconductors and large molecules. This laser will operate at 80-MHz repetition rate for enhanced sensitivity and stability when coupled to an ultra-high-vacuum STM. The setup will allow for the direct combination of independent pulse trains to resonantly excite few-femtosecond dynamics and then probe the electron density via the optically driven tunneling. In this pump-probe scheme it will be possible to map with atomic resolution the coherent evolution of electronic wavefunctions that in molecules and nanosystems follows an impulsive photoexcitation.