Near field-based spectroscopic techniques are gaining extensive attention in recent years due to their outstanding potential to access light-induced physico-chemical processes in the nanoscale. In particular, coupling an electromagnetic field into a scanning tunneling microscope (STM) tip results into a highly enhanced and confined near field around the tip apex that can be used as a nanometric excitation source to carry out spectroscopic studies. The precise spatial control of the STM tip enables in situ mapping of different light induced processes in surfaces with atomic spatial resolution. In this talk I will introduce the working principle and applications of different STM-based near field techniques. Firstly, I will discuss the implementation and recent advances on THz-driven STM. By coupling THz light into an STM junction, an ultrafast modulation of the bias voltage between tip and sample can be achieved, enabling the control of the tunneling process in the picosecond to femtosecond time scales. I will describe a method to sample ultrabroadband THz pulses in the STM junction, based on the THz-induced bias modulation of ultrafast photocurrents excited by femtosecond laser pulses.(1) In the second part of the talk, near field vibrational spectroscopy will be discussed. In particular, I will describe the technical extension of tip-enhanced Raman spectroscopy to liquid and electrochemical environments and its application to the investigation of molecular reorientation of DNA bases in the electrochemical interface.(2)
(1) Müller M., Martín Sabanés N., Kamprath T., Wolf. M, ACS photonics 7(8), 2046-2055 (2020.
(2) Martín Sabanés N., Ohto T., Andrienko D., Nagata Y., Domke K.F., Angewandte Chemie, 129(33), 9928-9933 (2017).