Single Molecule Coherence with Femtosecond THz-STM
The main goal of the studies presented in this dissertation is to demonstrate the measurement and manipulation of ultrafast quantum coherence in single molecules. Low temperature scanning tunneling microscopy (STM) with inelastic electron tunneling spectroscopy (IETS) allows determination of molecular fingerprints with atomic-scale spatial resolution. Femtosecond (fs) lasers in the terahertz (THz) range can be used to monitor the quantum coherence of molecules and atoms. Here we combine STM-IETS with fs THz pulses into a fs THz-STM and perform the time-resolved THz rectification spectroscopy (t-TRS) in the STM junction.When THz pulses are introduced into the STM junction, an effective voltage will be developed by the THz electric field which behaves as a modulation of the static sample bias. A non-zero rectification current can then be monitored due to the nonlinearity originated from the molecular excitation in the STM junction. THz photon absorption can also occur when the energy separation of the ground and excited states in the molecule, such as the hydrogen molecule (H2), matches the frequency range of the THz pulses. It thus becomes possible to prepare the superposition state of the H2 two-level system and track its temporal evolution from time domain pump-probe spectroscopy. The coherence measurement is successfully demonstrated on a single H2 molecule by our fs THz-STM. The extreme sensitivity of the H2 TLS coherent oscillation to the underlying surface chemical environment is discovered by scanning across the Cu2N surface while performing pump-probe spectroscopy, which shows promising applications in quantum sensing. Both the oscillation frequency and the dephasing time of the initial wave packet fo the H2 TLS are highly dependent on the applied external electric field under the giant Stark effect. By sweeping the sample bias and the corresponding electric field across the STM junction, we realized the electric manipulation of the quantum coherence of the H2 TLS. An avoided crossing in the energy levels and a quantum state transition are revealed from fitting the oscillation frequency spectrum to a model Hamiltonian. The surface electrostatic field is then quantified and scanned with sub-Ångstrӧm spatial resolution. We also demonstrate the optical tuning of the Kondo state in a single cobalt (Co) atom in the STM junction by fs near-infrared (NIR) laser illumination. Laser pulses are found to quench the Kondo state completely, and the continuous tuning of the Kondo amplitude is realized by adjusting the laser power. Our study is preliminary to future investigations of the temporal dynamics of Kondo collapse and revival in a single atom.