In THz-gated Scanning Tunneling Microscopy (THz-STM), the electric field of a single-cycle THz pulse acts as a transient bias modulating the STM-junction, enabling control of the tunneling current on femtosecond time scales. Optimal operation of a THz-STM requires exact knowledge and precise control of the THz near field waveform. In this regard, we demonstrate THz near field sampling via THz-induced modulation of ultrafast photocurrents in a metal-metal junction, and characterize in detail the coupling of broadband (1-30 THz) single-cycle THz pulses generated from a spintronic emitter to the STM tip. Specifically, we show that employing NIR laser pulses with a curved wavefront for THz generation allows for precise control of the phase, amplitude and bandwidth of the THz near field. Depending on the excitation conditions, THz near fields with frequencies up to 10 THz and peak voltages of several volts can be achieved at 1 MHz repetition rate.
Tip-enhanced Raman spectroscopy (TERS) in air and ultra-high vacuum (UHV) has been refined over the years through the study of various adsorbates in different experimental configurations. Developing the technique toward more realistic working conditions would render possible the investigation of more complex solid/liquid systems like bio-membranes or energy conversion and storage devices, providing a powerful tool to characterize nanoscale electrochemical processes occurring at the interface with high sensitivity and resolution. However, the extension to solid/liquid interfaces and electrochemical conditions still remains a challenge and few reports have been published. We have built an electrochemical TERS setup with side-illumination geometry that adapts easily to different experimental conditions such as opacity, shape and dimensions of the sample. The instrument features a specially designed solid/liquid sample holder that is implemented in a standard commercial STM. The experimental scheme can, in principle, be adapted to upgrade classic air TERS setups for work in liquids. Here, we show potential-dependent EC-TER spectra of a monolayer of adenine adsorbed on Au(111). The intensity of the ring-breathing mode at 735 cm-1 decreases with increasing sample potential and is recovered again upon potential reversal. The intensity variation is attributed to orientational changes of adenine upon (dis)charging of the Au substrate.