Ultrafast molecular state control enabled by terahertz-field-driven scanning tunneling microscopy
ORAL · Invited
Abstract
Excitons localized within individual molecules play a central role in molecular optoelectronics, energy conversion, and photochemical reactions. Directly accessing their dynamics, however, remains an experimental challenge, since it requires the simultaneous achievement of atomic-scale spatial resolution and ultrafast time resolution. While electroluminescence spectroscopy based on scanning tunneling microscopy (STM) enables atomic-scale optical spectroscopy by using tunneling electrons as a nanoscale excitation source, its temporal resolution is fundamentally restricted because tunneling electrons flow stationary by the electrical circuit.
In this work, we overcome this limitation by introducing single-cycle terahertz (THz) electric fields to drive tunneling processes on ultrafast timescales. By coupling phase-controlled THz pulses to an STM, we realize terahertz-field-driven scanning tunneling luminescence (THz-STL) spectroscopy, which enables sub-picosecond control of charge injection into individual molecules. In this regime, the tunneling probability is governed by the instantaneous electric field of the THz waveform and the tip-sample separation, providing access to time-domain control of electron transfer at the atomic scale.
We applied THz-STL to a single Pd phthalocyanine (PdPc) molecule adsorbed on a NaCl ultrathin film grown on Ag(111) surface and observed pronounced luminescence from the PdPc, whose yield exhibits a strong dependence on the carrier-envelope phase of the THz pulse. We also demonstrated pico-second control over exciton formation by superimposing two THz pulses
Our findings establish THz-STL as a powerful technique for investigating ultrafast exciton dynamics at the single-molecule level. More broadly, this approach opens a pathway toward atomic-scale non-equilibrium molecular dynamics, in which electron injection, relaxation, and light emission can be coherently controlled with atomic-scale spatial resolution.
In this work, we overcome this limitation by introducing single-cycle terahertz (THz) electric fields to drive tunneling processes on ultrafast timescales. By coupling phase-controlled THz pulses to an STM, we realize terahertz-field-driven scanning tunneling luminescence (THz-STL) spectroscopy, which enables sub-picosecond control of charge injection into individual molecules. In this regime, the tunneling probability is governed by the instantaneous electric field of the THz waveform and the tip-sample separation, providing access to time-domain control of electron transfer at the atomic scale.
We applied THz-STL to a single Pd phthalocyanine (PdPc) molecule adsorbed on a NaCl ultrathin film grown on Ag(111) surface and observed pronounced luminescence from the PdPc, whose yield exhibits a strong dependence on the carrier-envelope phase of the THz pulse. We also demonstrated pico-second control over exciton formation by superimposing two THz pulses
Our findings establish THz-STL as a powerful technique for investigating ultrafast exciton dynamics at the single-molecule level. More broadly, this approach opens a pathway toward atomic-scale non-equilibrium molecular dynamics, in which electron injection, relaxation, and light emission can be coherently controlled with atomic-scale spatial resolution.
*This work was supported by KAKENHI (JP20H05662, JP23H03979, JP22H04967, JP25K18005) and National Research Foundation of Korea.
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Presenters
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Kensuke Kimura
- RIKEN