Sub-cycle transient scanning tunneling spectroscopy with visualization of enhanced terahertz near-field

The recent development of optical technology has enabled the practical use of a carrier-envelope phase (CEP)-controlled monocycle electric field in the terahertz (THz) regime. By combining this technique with metal nanostructures such as nanotips, which induce near-field enhancement, the development of novel applications is anticipated. In particular, THz scanning tunneling microscopy (THz-STM) is a promising technique for probing ultrafast dynamics with the spatial resolution of STM. However, the modulation of the THz waveform generally accompanies the enhancement of the electric field, which is unknown in actual measurement environments. Here, we present a method capable of directly evaluating the enhanced near field in the tunnel junction of THz-STM in the femtosecond range, which is essential for the use of THz near field. In the tunneling regime, it was also demonstrated that the transient electronic state excited by an optical pulse can be evaluated using the THz-STM, and the ultrafast carrier dynam...