Ultrafast on-demand exciton formation in a single-molecule junction by tailored terahertz pulses

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The ultrafast manipulation of molecular states by charge transfer is essential for characterizing and controlling molecular dynamics. In this study, we demonstrated exciton formation in a single molecule through ultrafast electron tunneling processes between a molecule and a metal tip of a scanning tunneling microscope (STM) using a phase-controlled terahertz (THz) pulse. The pronounced luminescence of the well-defined molecular system under the distinct carrier-envelope phase of the THz pulse revealed that sequential state-selective electron-tunneling processes to the frontier molecular orbitals promoted ultrafast exciton formation in the molecule at the STM junction. Furthermore, ultrafast control of exciton formation was achieved using phase- and delay-controlled THz pulse pairs, providing a route for the regulation of molecular dynamics and the emergence of new molecular functions.