Lightwave STM: Ultrafast Atomic Probing

• Lightwave-driven STM is a technique using ultrafast electromagnetic pulses to modulate tunnelling currents, achieving attosecond temporal and atomic spatial resolution.
• It employs nonlinear tunnelling and phase-stabilized waveforms to capture real-time electron dynamics and photonic emissions at the nanoscale.
• The method enables applications in ultrafast electron videography, quantum nano-optoelectronics, and multimodal spectroscopic mapping with effective artifact suppression.

Lightwave-driven scanning tunnelling microscopy (STM) encompasses a suite of experimental methodologies that harness ultrafast optical fields to control, probe, and map electronic processes at atomic-scale spatial resolution and sub-picosecond to attosecond temporal resolution. By coupling radio-frequency, terahertz, and optical (near-infrared) light sources to the tunnel junction of a conventional or customized STM, these approaches establish the capability for real-time observation and manipulation of electron dynamics, photonic emission, and optical coupling at the nanometre and atomic scales.

1. Fundamental Concepts and Evolution of Lightwave-Driven STM

Lightwave-driven STM refers to the use of externally applied, coherent electromagnetic fields—ranging from radio-frequency (RF), through terahertz (THz), to near-infrared (NIR) and optical frequencies—to induce, modulate, or detect tunnelling currents between a sharp metallic (or functionalized) tip and a conducting or semiconducting surface. Classical STM achieves atomic resolution through bias-controlled electron tunnelling, with typical temporal resolution limited by preamplifier electronics (milliseconds to microseconds). Lightwave-driven paradigms exploit ultrafast pulses, nonlinear tunnelling processes, and phase-coherent optical control to overcome this limitation, enabling real-time, sub-picosecond, and even attosecond-resolved measurements without loss of lateral (atomic) precision (Saunus et al., 2013, Jelic et al., 2023, Maier et al., 14 Jul 2025, Davidovich et al., 14 Jul 2025, Rossetti et al., 22 Jul 2025).

This field evolved from RF-optimized STM techniques implementing ~120 ps pulse-pair pump–probe sequences to THz and optical field-driven methodologies leveraging non-adiabatic tunnelling, phase-stable single-cycle fields, and the advanced control of waveform-dependent tunnelling current.

2. Technical Architectures and Implementation Strategies

The technical realization of lightwave-driven STM methods varies depending on the spectral regime and target resolution:

• RF-Pulsed STM: Achieves 120 ps time resolution and concomitant atomic spatial resolution using RF-optimized coaxial wiring (18–40 GHz bandwidth), direct voltage pulse delivery to the tip, and lock-in detection of nonlinear excess current originating from I(V) nonlinearity (Saunus et al., 2013).
• THz-STM and Ultrafast Pump–Probe STM: Utilizes single-cycle THz pulses generated by photoconductive antennas driven by femtosecond fiber lasers (e.g., 1560 nm, <90 fs) and focused to the tip-sample junction via double off-axis parabolic mirrors. A variable time delay between pump and probe arms (motorized delay up to 670 ps) allows for cross-correlation and time-domain sampling at repetition rates up to 100 MHz. The current detected is f_Rep-integrated, i.e.,

$$I_{\mathrm{THz}} = f_{\mathrm{Rep}} \int_0^{1/f_{\mathrm{Rep}}} G(V_{\mathrm{THz}}(t)) \, dt$$

where $G$ is the nonlinear conductance and $V_{\mathrm{THz}}(t)$ is the THz transient (Azazoglu et al., 2023, Jelic et al., 2023).

• Optical/Attosecond STM: Employs CEP-controlled, phase-stabilized single-cycle NIR waveforms (typically 1.5 μm) synthesized by combining two spectrally separated pulses from an octave-spanning continuum with fine acousto-optic phase adjustment. These pulses can reach peak fields of several MV/cm in the junction. Ultrafast, coherent tunnelling is induced by transient barrier deformation, with detection focused on CEP-dependent (coherent) current components, $I_{\mathrm{CEO}}(t)$, isolated via innovative electronic phase modulation of the laser (Maier et al., 14 Jul 2025, Rossetti et al., 22 Jul 2025). Advanced lock-in detection at frequencies exceeding the STM feedback bandwidth isolates the derivative $dI/d\phi$ (with respect to CEP), minimizing background and thermal artifacts:

$$I(\phi_0 + \delta\phi \sin(2\pi \nu_{\mathrm{CEP}} t)) \approx I(\phi_0) + \left.\frac{dI}{d\phi}\right|_{\phi_0} \delta\phi \sin(2\pi \nu_{\mathrm{CEP}} t)$$

3. Mechanisms for Spatial and Temporal Resolution Enhancement

• Spatial Confinement: All lightwave-driven STM methods exploit the exponential spatial localization of the electron wavefunction at the tip apex. This ensures sub-nanometre (frequently sub-angstrom) precision in current maps and topographic imaging, even as the tunnelling process is modulated on fs and as timescales.
• Temporal Modulation and Detection: The temporal response is set by the duration and shape of the applied field: traditional pump–probe approaches (120 ps pulses) (Saunus et al., 2013), sub-cycle rectification from THz pulses (few hundred fs) (Jelic et al., 2023, Azazoglu et al., 2023), and attosecond-resolved, waveform-dependent tunnelling using optical single-cycle pulses (Maier et al., 14 Jul 2025, Rossetti et al., 22 Jul 2025).
• Nonlinear Detection: The crucial requirement is a pronounced nonlinearity in the junction I(V) characteristic. For pulsed or field-driven STM, this enables detection of nonlinear excess current—e.g.,

$I(2V_{\text{pulse}}) > 2 I(V_{\text{pulse}})$

arising due to the positive curvature of I(V), which strongly enhances time-dependent signal response (Saunus et al., 2013).

4. Experimental Modalities and Supported Spectroscopic Techniques

The architectural approach determines the suite of experiments possible:

Technique Category	Physical Principle	Supported Modalities
RF-Driven STM	Nonlinear I(V), lock-in at RF	Pump–probe, time-resolved imaging
THz-STM	Strong-field rectification, near-field	THz pump–probe, THz-STS, TD spectroscopy
Optical/Attosecond STM	Single-cycle phase-coherent driving	Attosecond-resolved currents, waveform control, directional current injection
Optical Emission and Collection	Etendue-limited collection	STM-induced light emission (STM-LE), in situ PL, Raman, CL, angular imaging (Román et al., 2022)

Examples include:

• STM-LE and Cathodoluminescence (STM-CL): Light collection devices (off-axis parabolic mirrors, θ_max ≈ 80°, 72% hemispherical collection efficiency) enable detection of plasmonic and excitonic emission from the junction, facilitating multimodal optical/electronic correlation (Román et al., 2022).
• In Situ Spectroscopies: Integration with free-space optical beams (via external lasers) supports PL and Raman on nanoscale samples (graphite, h-BN, TMDs), with feedback-concurrent atomic resolution imaging preserved (Román et al., 2022, Zhang et al., 2018).
• Quantum Emitter Manipulation: STM-induced emission reveals photon antibunching, visualization of superradiant/subradiant dimer modes, and Fano resonance via tip-positioned molecule–plasmon hybridization (Zhang et al., 2018).

5. Results: Performance Metrics, Resolution and Operational Considerations

Key reported performance metrics include:

Metric	Value/Details	Regime
Temporal Resolution	120 ps; sub-ps; down to 900 as	RF-pulse, THz, attosecond optical
Spatial Resolution	Atomic (Å); 2 nm (ambient attosecond STM)	All
Sensitivity	50 pA dynamic change at 120 ps; 0.6 electrons/pulse	RF; THz
Collection Efficiency (Light Emission)	72% hemispherical (off-axis parabolic mirror)	STM-LE, PL, CL
Spectral Resolution (collected light)	0.5 nm with 600 gr/mm grating	STM-LE, PL, Raman
Temperature Range	10–300 K (THz-STM)	UHV STM-THz combined system

Experiments confirm atomically resolved imaging in all modalities, robust lock-in demodulation and effective artifact suppression by CEP-only or two-color pulse delay modulation. In STM-LE, photon antibunching $g^{(2)}(0) = 0.12(2)$ is reached, confirming single-photon emission in optically coupled quantum emitters (Zhang et al., 2018).

6. Underlying Physical Mechanisms and Theoretical Formulation

• Sub-cycle Tunnelling Dynamics: Lightwave control (particularly two-color field synthesis) induces non-adiabatic tunnelling, as modeled by the time-dependent Schrödinger equation (TDSE):

$$i\hbar\frac{\partial\Psi(x,t)}{\partial t} = \left[ -\frac{\hbar^2}{2m}\frac{\partial^2}{\partial x^2} + V(x) - exE(t) \right]\Psi(x,t)$$

where $E(t)$ is the asymmetric, sub-cycle electric field waveform, supporting attosecond-scale control and directional current generation (Davidovich et al., 14 Jul 2025).

• Charge Transfer and Coherence: The net transferred charge is the time-integral of the coherent current,

$Q(t) = \int_{-\infty}^t I(t') dt'$

and exhibits explicit CEP dependence and quantum retardation, as confirmed by TD-DFT simulations and nTDSE models (Maier et al., 14 Jul 2025, Davidovich et al., 14 Jul 2025).

• Noise and Artifact Suppression: Fast electronic modulation, with frequencies ($\nu_{\mathrm{CEP}} >$ feedback bandwidth), enables lock-in amplification at the derivative of the current with respect to CEP, suppressing thermal and mechanical noise sources intrinsically (Rossetti et al., 22 Jul 2025).

7. Applications and Future Directions

Lightwave-driven STM provides a platform for:

• Ultrafast, Attosecond-Scale Electron Videography: Real-space imaging of wavepacket motion in atoms, molecules, and condensed matter with combined attosecond and atomic-scale resolution (Maier et al., 14 Jul 2025).
• Quantum Nano-Optoelectronics and Emitter Control: Visualization and coherent control of excitonic coupling, charge transfer, photon emission, and light–matter hybridization at the single-entity level (Zhang et al., 2018).
• Atomic-Resolution THz and Petahertz Electronics: Time-domain THz-STS exploits extreme near-field localization for material-specific dynamics and device-relevant ultrafast processes, e.g., at step edges, adatoms, and local defects (Jelic et al., 2023, Azazoglu et al., 2023).
• Multimodal Nano-optical–Electronic Correlation: Integration of STM-LE, PL, CL, and Raman spectroscopy on 2D heterostructures, quantum dots, and atomic defects with direct spatial and spectral correlation (Román et al., 2022).

A plausible implication is that ongoing advances in lightwave-driven STM—especially those extending robust attosecond methods to variable temperature and broader environmental compatibility—will define new regimes of transport, quantum optics, and coherent control at essentially the fastest and smallest accessible scales. This positions lightwave-driven STM as a crucial tool for petahertz logic, ultrafast quantum sensing, and real-space probing of non-adiabatic, strongly correlated electron phenomena.