Probing non-equilibrium states at atomic scale by time-resolved scanning probe microscopy
Non-equilibrium states of molecules or solids play crucial roles across a broad range of fields such as the photovoltaics, chemical reactions and phase transition, etc. The detection and control of local non-equilibrium states are extremely crucial for understanding and tuning many basic processes involving electron and energy transfer, but they remain a great challenge to date. Optical methods with the ultrafast laser have proven to be very powerful in detecting various non-equilibrium states, but they suffer from a poor spatial resolution which is about half the wavelength due to the optical diffraction limit. Scanning probe microscopies such as the scanning tunneling microscope (STM) and atomic force microscope (AFM) have the advantage of ultrahigh spatial resolution down to atomic level [1, 2]. However, they are usually only accessible to the equilibrium ground states due to their poor temporal resolution (~0.1 ms). Here I will show how we broke this constraint by developing a novel electronic pump-probe AFM technique, which allows us to probe the lifetimes of molecular excited states (triplet states) at atomic scale for the very first time . The advantage in ultrahigh spatial resolution of our technique was demonstrated by atomically observing the triplet quenching induced by single oxygen molecules. In the end, I will briefly show how the temporal resolution was further improved by combining the STM with ultrashort optical or terahertz pulses, realizing the atomic spatial resolution and femtosecond temporal resolution simultaneously [4, 5]. Such a technique will make it possible to track various ultrafast dynamics at atomic scale such as molecular vibrations and rotations, phonons, carrier dynamics, spin dynamics, phase transitions, etc.
 Peng et al., Nature communications 9, 122(2018).
 Peng et al., Nature 557, 701 (2018).
 Peng et al., Science 373, 452 (2021).
 Terada et al., Nature Photonics 4, 869 (2010).
 Cocker et al., Nature Photonics 7, 620 (2013).