Clocking and controlling attosecond currents in a scanning tunnelling microscope

This paper demonstrates the first directional control and characterization of attosecond-scale tunnelling currents in a scanning tunnelling microscope using two-colour laser pulses, achieving sub-angstrom spatial resolution and revealing an 860-attosecond current burst duration via a three-step non-adiabatic transport mechanism.

The Gist

Imagine you have a tiny, invisible needle (the tip of a microscope) hovering just above a flat surface (a gold sample). Normally, electrons jump across the tiny gap between them like a frog hopping across a pond. This is called "quantum tunneling."

For a long time, scientists could see where these electrons were (atomic resolution) and they could see when they moved, but only in slow motion (picoseconds or femtoseconds). They wanted to see the electrons move in "real-time" at the fastest speed possible: the attosecond (one-quintillionth of a second). It's so fast that if an attosecond were a second, a second would be the age of the universe.

The problem was that while scientists could control the timing of these jumps, they couldn't control the direction or measure exactly how long the jump lasted without causing the needle to overheat and melt (thermal artifacts).

Here is what this team did, explained simply:

1. The "Two-Color" Flashlight

Instead of using a single beam of light to push the electrons, they used a special "two-color" laser pulse. Think of this like a conductor leading an orchestra with two instruments playing at once: a deep bass note (infrared light) and a higher-pitched note (its "second harmonic").

By mixing these two colors, they created a light wave that wasn't symmetrical. Imagine a wave that has a huge, powerful crest on one side and a tiny, weak trough on the other. This asymmetry is the key.

2. Steering the Electrons

Because the light wave is lopsided, it pushes the electrons in one specific direction.

• The Analogy: Imagine a surfer on a wave. If the wave is perfectly symmetrical, the surfer might just bob up and down. But if the wave has a massive, steep front and a gentle back, the surfer is forced to ride forward.
• The Result: By slightly adjusting the timing (delay) between the two colors of light, the scientists could flip the shape of the wave. This allowed them to instantly switch the direction of the electron flow, making them jump from the needle to the gold, or from the gold back to the needle, with incredible precision.

3. The "Freeze-Frame" Trick

Usually, when you turn a laser on and off to measure a signal, the heat from the laser makes the metal needle expand and contract, creating a messy signal that looks like the electrons are moving when they aren't.

To solve this, the team used a clever trick:

• They didn't turn the laser intensity on and off (which causes heat).
• Instead, they wiggled the timing of the two light colors back and forth very quickly (thousands of times a second).
• This is like wiggling a steering wheel left and right without actually pressing the gas pedal. The needle stays cool, but the electron current wiggles in response to the timing changes. This allowed them to measure the current without any "thermal noise."

4. What They Found

By using this method, they achieved three major things:

• Directional Control: They proved they could steer the electrons to go left or right just by tweaking the light's timing.
• The Speed Limit: They calculated that the burst of electrons jumping across the gap lasts only 860 attoseconds. That is less than one-thousandth of a femtosecond. It's a blink of an eye so fast it barely exists.
• Sharp Vision: Even though they were working in normal air (not a vacuum) and at room temperature, they could still see tiny bumps on the gold surface that were smaller than a single atom (sub-angstrom sensitivity) and distinguish features 2 nanometers wide.

The "Three-Step" Dance

The paper explains that the electron doesn't just teleport. It performs a three-step dance:

1. The Escape: The strong light field thins the wall (barrier) the electron is trapped behind, allowing it to tunnel out.
2. The Sprint: Once out, the electron gets a massive kick from the light field and accelerates across the gap.
3. The Landing: It crashes into the other side (the sample).

Why This Matters (According to the Paper)

This work is a breakthrough because it combines the ability to see where atoms are (like a standard microscope) with the ability to see how fast electrons move (like a high-speed camera). They have created a tool that can trigger and image the movement of electric charge at the absolute limit of speed and space, all without melting the equipment.

In short, they built a microscope that can take "stop-motion" photos of electrons moving at the speed of light, controlling exactly which way they go, using a two-color laser trick to keep the machine cool.

Technical Summary

Technical Summary: Clocking and Controlling Attosecond Currents in a Scanning Tunnelling Microscope

Problem Statement
While attosecond science has successfully controlled electron motion on sub-cycle time scales using strong light fields, achieving simultaneous atomic-scale spatial resolution remains a significant challenge. Scanning Tunnelling Microscopy (STM) offers atomic spatial resolution but has historically struggled to integrate attosecond temporal resolution without introducing thermal artifacts. Previous attempts to induce and control attosecond tunnelling currents in STM using infrared laser pulses and Carrier-Envelope Phase (CEP) modulation were hindered by thermal expansion of the nanotip caused by intensity modulation, which masked the actual laser-induced current signals. Furthermore, while sub-cycle dynamics could be inferred, the direct control of the direction of attosecond currents and the precise determination of their duration within an STM junction had remained elusive.

Methodology
The authors developed an ultrafast STM setup operating under ambient conditions at room temperature, utilizing a two-colour laser pulse scheme to overcome thermal limitations.

• Optical Setup: The system employs an Er:fiber laser oscillator generating 1850 nm fundamental pulses (35 fs duration). These pulses are passed through a highly nonlinear fiber to generate a supercontinuum, from which the fundamental is isolated. A second harmonic (SH) at 925 nm is generated via Second-Harmonic Generation (SHG) in a Bismuth Triborate (BiBO) crystal.
• Two-Colour Control: The fundamental and SH pulses are recombined in a polarization-maintaining interferometer. The relative time delay ($\tau_0$) between the two colours is controlled by a linear delay stage, while a piezo-mounted mirror modulates this delay sinusoidally at 3.7 kHz ($\delta \sin(\Omega t)$).
• Artifact-Free Detection: Instead of chopping the laser intensity (which causes thermal artifacts), the authors modulate the delay between the two colours. This allows for lock-in detection of the laser-induced current at the modulation frequency without modulating the total laser power, thereby avoiding tip expansion/contraction artifacts.
• Experimental Conditions: Experiments were conducted on a Pt:Ir nanotip (5–20 nm radius) and a gold substrate. Measurements were taken in constant-current mode, with specific "tip-freezing" experiments performed at zero bias to isolate the net laser-induced current directly.
• Theoretical Framework: The experimental data was projected onto three theoretical models: (i) an analytical strong-field (SF) model based on the van Vleck propagator; (ii) a numerical integration of the one-dimensional time-dependent Schrödinger equation (nTDSE); and (iii) many-body time-dependent density functional theory (TDDFT) for a 2D model geometry.

Key Contributions and Results

• Directional Control of Attosecond Currents: The study demonstrates robust control over the direction of tunnelling currents by varying the two-colour delay ($\tau_0$). By adjusting the relative phase, the asymmetric waveform of the combined field can be switched to drive electrons from the tip to the sample or vice versa. Direct measurements with a frozen tip at zero bias confirmed that the current direction is governed solely by the sub-cycle waveform, not by static bias fields.
• Sub-Ångström Sensitivity and Spatial Resolution: Despite operating in ambient conditions, the system achieved lateral spatial resolution of approximately 2 nm and sensitivity to topographic features on the sub-Ångström scale. The laser-induced current signal correlated strongly with atomic steps and protrusions on the gold sample.
• Characterization of Transport Regime: Power-scaling measurements of the laser-induced current revealed a transition from a multiphoton regime (scaling $\sim P^{3.8}$) to a strong-field tunnelling regime as laser power increased. Theoretical analysis placed the experiments in the transition region near a Keldysh parameter $\gamma \sim 1$.
• Attosecond Burst Duration: By comparing experimental data with nTDSE and TDDFT calculations, the authors identified the physical mechanism as non-adiabatic tunnelling. The theory-derived duration of the electron current bursts was determined to be 860 ± 90 attoseconds. This duration is shorter than the ~970 as observed in single-colour cases, indicating that the two-colour approach not only breaks symmetry but also compresses the burst duration.
• Three-Step Transport Mechanism: Theoretical models support a three-step process for electron transport in the non-adiabatic regime: (1) tunnelling through a light-field-thinned barrier with concomitant energy gain; (2) further acceleration within the junction; and (3) transmission into the sample.

Significance
The paper claims to demonstrate the first direct, artifact-free control of the direction of attosecond currents in an STM junction. By utilizing two-colour pulses and delay modulation, the authors successfully bypassed the thermal artifacts that have plagued previous ultrafast STM experiments. The ability to trigger and image ultrafast charge dynamics with sub-femtosecond temporal resolution and nanometer spatial resolution under ambient conditions represents a significant step toward the "spatio-temporal microscopy frontier of lightwave electronics." The work establishes a foundation for future applications, such as the real-time observation of coherent electron-hole dynamics and many-body effects in molecules and nanostructures, by combining the spatial resolution of conventional STM with the temporal precision of attosecond science.