Attosecond current control and timing in a scanning tunnelling microscope

Quantum tunnelling of electrons can be confined to sub-cycle time scales under strong light fields, and this work demonstrates robust attosecond directional control of ultrafast tunnelling currents in an STM junction using two-colour laser pulses. This capability enables triggering and imaging ultrafast charge dynamics at the spatio-temporal microscopy frontier, achieving sub-angstrom topographic sensitivity while operating under ambient conditions without thermal artifacts.

Core Methodology

The researchers induce STM tunnelling currents using two-colour laser pulses—an infrared fundamental pulse and its second harmonic (SH)—by superimposing them and controlling their relative time delay. This technique is employed to achieve a pronounced symmetry breaking of the resulting waveform, which enables sub-cycle control. Specifically, they employ an infrared laser pulse with its second harmonic in the same polarization plane and change their relative time delay. The two-colour pulses lead to field waveforms with controlled symmetry breaking and enable sub-cycle control, allowing them to seamlessly switch between tip-to-sample tunnelling and vice versa, as shown unambiguously and directly by measurements with a temporarily frozen tip.

Measurement Techniques

The experiment is conducted at ambient conditions (room temperature) using an electropolished Pt:Ir nanotip on a gold substrate. The microscope operates in constant-current measurement mode, where a static bias field drives the tunnelling current. To isolate the laser-induced currents from the static tunnelling current, they utilize a sinusoidal modulation of the two-colour delay at an angular frequency of 3.7 kHz, which avoids artifacts associated with conventional lock-in approaches. This modulation allows them to measure the laser-induced current independently of the microscope feedback loop operating at a locking bandwidth below 500 Hz, thereby avoiding thermal artifacts.

Theoretical Framework and Analysis

The measurement data are projected onto several theoretical models: (i) an analytical strong-field (SF) model based on the van Vleck propagator; (ii) a single-active-electron model approach based on a numerical integration of the one-dimensional time-dependent Schrödinger equation (nTDSE); and (iii) many-body time-dependent density functional theory (TDDFT). The nTDSE successfully reproduces the three-step transport process in the non-adiabatic tunnelling regime, revealing a theory-derived current burst duration of 860 as. Furthermore, they analyze power scaling, observing that the current scales approximately like a power law with order 3.8 at low powers and exhibits a soft kink signature indicating the transition to the strong-field tunnelling regime.

Key Findings on Dynamics

The study reveals that the direction of the current is governed by the two-colour delay, as demonstrated by direct measurements with a frozen tip at zero bias. The reconstructed laser-induced current shows that Positive currents indicate that the asymmetric waveform of the two-colour field drives electrons in sub-cycle bursts from the nanotip to the sample. They also demonstrate that peaks in the signal are correlated with small topographic protrusions, suggesting an effective lateral spatial resolution of around 2 nm. The analysis using TDDFT and nTDSE confirms a three-step model of ultrafast STM: (1) Tunnelling through a barrier thinned by the strong light field and concomitant energy gain, (2) further acceleration inside the junction, and (3) Transmission into the sample.

Robustness and Future Outlook

The results show that the sub-femtosecond confinement of waveform-controlled bursts remains robust against reasonable variations of experimental parameters, including peak field strength, intensity ratio of SH to fundamental, junction width, and tip workfunction. The research suggests that replacing two-colour pulses with single-cycle drivers will lead to a single isolated attosecond electron burst with waveform-controlled timing and direction. Future work envisions combining these bursts with all-optical readout for real-time observation of coherent electron-hole dynamics. Furthermore, they discuss the potential for bias-dependent measurements and combining quasi-static THz fields with near-infrared driving fields to further modulate the barrier shape.

The gist

In this work, two-colour laser pulses are used to induce and dynamically control the direction of ultrafast tunnelling currents in an STM junction, achieving sub-angstrom topographic sensitivity without thermal artifacts. The resulting measurements reveal a three-step transport process with a burst duration of 860 as, confirming that the sub-cycle waveform of the light pulse governs ultrafast electric current dynamics.

How it works

1. Two-colour pulses (fundamental and second harmonic) are superimposed, and their relative time delay is controlled using an interferometric setup with a linear delay stage and a piezo-mounted mirror modulated at 3.7 kHz to generate a lock-in signal without thermal artifacts.

As a fastidious and diligent researcher, I have analyzed this cutting-edge work on attosecond control of STM currents. The core breakthrough lies in achieving sub-angstrom spatial resolution (2 nm lateral) while dynamically controlling the direction of attosecond electron bursts using two-color laser pulses in an STM junction, all under ambient conditions without thermal artifacts.

Here are the specific improvements to AI systems that can be derived from this research:

) Improvements to AI Systems and Capabilities

1. A new class of Attosecond Quantum Dynamics Simulators capable of predicting electron transport in complex nanoscale junctions.

2. Enhanced materials informatics for designing novel quantum devices based on predicted current asymmetry.

3. A real-time, closed-loop feedback system for attosecond charge manipulation in next-generation microscopy platforms.

) Specific Improvements and System Capabilities

1. The paper provides a robust theoretical framework (nTDSE, SF model, TDDFT) that accurately models the three-step non-adiabatic tunnelling process (tunnelling, acceleration, transmission).

2. The AI system can be trained on this framework to perform high-fidelity simulations of electron dynamics in arbitrary nanoscale junctions defined by varying geometry and material properties (e.g., tip/sample work functions).

3. The experimental results demonstrate that the directionality of the current is governed by two key controllable parameters:

a) The relative time delay between two laser colors (fundamental vs. second harmonic) to break symmetry.

b) The polarization angle of the combined field, which dictates the alignment with the nanotip geometry for maximum near-field enhancement.

4. The AI system can be used to map current directionality landscapes across a parameter space defined by:

a) Junction width and material work functions (e.g., comparing Pt:Ir/Gold vs. other combinations).

b) Laser intensity ratios (SH/Fundamental ratio).

c) Field polarization angle.

5. The AI system can predict the resulting current burst duration and its spatial profile (the attosecond signature) based on the input parameters, with quantified uncertainty estimates derived from NTDSE/TDDFT error analysis.

) What the Improved AI System Can Do

1. Predict optimal laser pulse configurations (two-color delay, polarization angle) required to achieve a specific electron transport direction (e.g., tip-to-sample vs. sample-to-tip current).

2. Design novel STM junction geometries that are predicted to exhibit desired sub-cycle current asymmetry or reversal based on the AI's predictive modeling of work function and geometric effects (as hinted in Fig 4a and S15).

3. Develop digital twins of experimental setups, allowing researchers to simulate the effect of ambient conditions, mechanical vibrations, and feedback loop bandwidth limitations on attosecond measurements before physical experimentation.

4. Perform automated data analysis on STM current maps to rapidly identify sub-angstrom topographic features correlated with specific laser-induced current signatures (as seen in Fig 1d), enabling faster characterization of surface morphology at the atomic scale.

Quantum tunnelling of electrons can be confined to the sub-cycle time scale of strong light fields, contributing decisively to the extreme time resolution of attosecond science. Because tunnelling also enables atomic-scale spatial resolution in scanning tunnelling microscopy (STM), integrating STM with light pulses has long been a key objective in ultrafast microscopy, spanning the picosecond and femtosecond domains, with first signatures of attosecond dynamics. However, while sub-cycle dynamics on the attosecond time scale are routinely controlled and determined with high precision, controlling the direction of attosecond currents and determining their duration have remained elusive in STM. Here, we induce STM tunnelling currents using two-colour laser pulses and dynamically control their direction, relying solely on the sub-cycle waveform of the pulses. Projecting our measurement data onto one-electron and many-body theory descriptions reveals a three-step transport process in the non-adiabatic tunnelling regime as the physical mechanism, with a theory-derived current burst duration of 860 as. Despite working under ambient conditions but free of thermal artifacts, we achieve sub-angström topographic sensitivity and a lateral spatial resolution of 2 nm. This unprecedented capability to directionally control attosecond bursts will enable triggering and imaging ultrafast charge dynamics at the spatio-temporal microscopy frontier of lightwave electronics.