Algorithm for subcycle terahertz scanning tunneling spectroscopy

Abstract

Terahertz scanning tunneling microscopy (THz-STM) enables ultrafast measurements of surfaces, single molecules, and nanostructures with simultaneous sub-picosecond temporal resolution and atomic spatial resolution. In pump-probe THz-STM experiments employing femtosecond optical pump pulses, lightwave-driven tunneling by a time-delayed THz probe pulse accesses the evolving differential conductance of the tunnel junction following photoexcitation. However, a general theoretical approach to extract the time-and voltage-dependent differential conductance from THz-STM measurements is lacking. Here, we introduce an algorithm for pump-probe THz scanning tunneling spectroscopy (THz-STS) analysis. Our approach allows us to reliably reconstruct the tunnel junctions differential conductance in steady-state or time-dependent scenarios from simulated THz-STS data. The algorithm achieves subcycle time resolution, which we demonstrate by retrieving dynamics faster than the bandwidth of the input THz voltage transient. Subcycle THz-STS will make lightwave-driven microscopy yet more powerful as a tool for characterizing ångström-scale ultrafast dynamics in novel materials.

Efficient and Continuous Carrier-Envelope Phase Control for Terahertz Lightwave-Driven Scanning Probe Microscopy

“…Continuous CEP tuning by frustrated total internal reflection is applicable to any THz-based far- and near-field technique requiring precise CEP control, including THz-STM, THz scanning tunneling spectroscopy, THz luminescence, THz action spectroscopy, THz scanning near-field optical microscopy (THz-SNOM), THz electro-optic modulation, and THz-based electron bunch compression. − In particular, our method enables precise state-selective tunneling in THz-STM, providing a strong basis for the investigation of localized quantum states in low-dimensional materials systems …”

“…This concept proves as an efficient and cost-effective method for CEP control of THz pulses, enabling continuous modulation between unipolar and bipolar waveforms. It is an enabler for ultrafast scanning tunneling spectroscopy and for perspective experiments where bipolar field transients drive luminescent excitations by generating local excitons. We demonstrate how our implementation transfers to the optical near field , by characterizing the waveform at the STM tip using photoemission sampling (PES) …”

Single-Molecule Continuous-Wave Terahertz Rectification Spectroscopy and Microscopy

“…Coupling ultrafast THz pulses − into an STM junction generates transient voltage pulses , that can induce nanoscale ultrafast dynamics. − However, the voltage pulses in the tunneling junction often have a high peak intensity of up to several volts and are too strong for low-energy spectroscopy of single molecules. Moreover, obtaining high energy resolution in THz time domain spectroscopy (TDS) requires an extended delay scan range, which can be very time-consuming in application. , Therefore, it is desirable to incorporate THz frequency domain spectroscopy (FDS) as a new approach for single-molecule spectroscopy.…”

“…Coupling ultrafast THz pulses 7−10 into an STM junction generates transient voltage pulses 11,12 that can induce nanoscale ultrafast dynamics. 13−20 However, the voltage pulses in the tunneling junction often have a high peak intensity of up to several volts and are too strong for low-energy spectroscopy of single molecules.…”

On the Possibility of Exploring Tip-Molecule Interactions with STM Experiments

“…For studies of diffusion of molecules on surfaces, a time resolution in the microsecond regime should be sufficient in general, while it is possible to monitor STM current fluctuations even in the picosecond time regime. 7,8 Fluctuations caused by molecule diffusion are also analyzed in other fields as, for example, fluorescence correlation spectroscopy. [9][10][11][12][13] In previous studies, different techniques have been developed and investigated to evaluate the times series of the tunneling current.…”