Lightwave-Driven THz-STM: Ultrafast Atomic Spectroscopy

• Lightwave-driven THz-STM is a technique that employs focused ultrafast THz fields at a metallic tip to create a transient bias across an atomic-scale tunnel junction.
• It combines extreme near-field enhancement and calibrated time-domain biasing to perform nonlinear tunneling spectroscopy and image ultrafast lattice, charge, and topological dynamics.
• The method enables transient spectroscopic measurements, coherent control of phase transitions, and defect-specific charge dynamics in materials such as WTe₂, 1T-TaS₂, and WSe₂.

Lightwave-driven terahertz scanning tunneling microscopy (THz-STM) is a scanning tunneling modality in which a focused single-cycle or few-cycle terahertz field is concentrated at a metallic tip apex and converted into a transient bias across an atomic-scale tunnel junction. In this geometry, the THz near field acts as an ultrafast voltage transient, typically written as $V(t)=V_{dc}+V_{THz}(t)$, that modulates the tunneling barrier, the energy alignment between tip and sample, and, in suitable materials, lattice, charge, spin, and topological degrees of freedom while retaining atomic or even subatomic spatial selectivity. Recent work has established THz-STM as both a probe and a control platform: it can sample the local THz waveform in situ, perform differential THz tunneling spectroscopy, drive metastable structural or electronic states, and image ultrafast dynamics in real space and time (Müller et al., 2020, Jelic et al., 2023, Jelic et al., 2024, López et al., 26 May 2025, Jelic et al., 28 Apr 2026).

1. Operating principle and junction electrodynamics

In lightwave-driven THz-STM, the metallic tip functions as a nanoscale antenna. A free-space THz pulse focused onto the tip is concentrated at the apex; the resulting near field produces a transient junction bias $V_{THz}(t)$ across the nanometer-scale vacuum gap. A common description is

$$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$

with $z_0$ the absolute tip height and $\alpha$ the field-to-voltage coupling coefficient (Jelic et al., 2024, Jelic et al., 28 Apr 2026).

In the quasi-static regime, the THz field modulates the instantaneous barrier and the tunneling current follows the exponential barrier sensitivity of STM,

$$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$

or, in simplified form, $I(V,z)\propto V e^{-2\kappa z}$. In stronger-field operation, the same transient bias can generate a unipolar ultrafast current burst each cycle, so the measured observable is no longer a perturbative sideband correction but the rectified consequence of strongly nonlinear barrier modulation (Jelic et al., 2024, López et al., 26 May 2025, Jelic et al., 2023).

A defining feature of the technique is extreme near-field enhancement. In WTe$_2$, the tip apex provides field enhancement $>10^5$, converting free-space peaks of tens of V/cm into junction fields on the order of $1$ V/nm; in other implementations, prior THz-STM literature is cited for $V_{THz}(t)$0 at the apex (Jelic et al., 2024, Azazoglu et al., 2023). This places THz-STM in a regime where modest free-space THz fields become spectroscopically and dynamically consequential at the junction.

The electrodynamics of the junction are often approximately quasi-static. Direct phase-resolved measurements of the THz voltage transient inside a metallic STM junction showed that the detected near-field spectrum can extend to $V_{THz}(t)$1 THz and that the integrated junction voltage is nearly invariant over tip–sample distances from a few nm to $V_{THz}(t)$2m, with changes of $V_{THz}(t)$3 at $V_{THz}(t)$4 THz and $V_{THz}(t)$5 at $V_{THz}(t)$6 THz from $V_{THz}(t)$7 nm to $V_{THz}(t)$8 nm (Müller et al., 2020). This supports the standard picture that the THz pulse acts primarily as an ultrafast bias across a sub-wavelength junction, even though the detailed tip transfer function can low-pass filter the incident waveform.

2. Waveform generation, delivery, and calibration

Implementations of THz-STM span several source architectures, but all rely on coupling a phase-stable THz transient into the STM picocavity and calibrating the resulting junction voltage rather than only the free-space field.

Implementation	THz generation and repetition rate	Representative junction-level result
(Müller et al., 2020)	Spintronic THz emitter, $V_{THz}(t)$9–$$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$0 THz, $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$1 MHz	Up to $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$2 THz detected in the junction; peak THz bias up to $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$3 V
(Azazoglu et al., 2023)	Dual fiber-coupled PCAs, $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$4 MHz	Picosecond pump–probe cross-correlation; junction voltage on the order of a few hundred millivolts
(Jelic et al., 2024)	Tilted-pulse-front optical rectification of $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$5 fs, $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$6 nm Yb:KGW pulses, $$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$7 MHz	$$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$8–$$V(t)=V_{dc}+V_{THz}(t),\qquad V_{THz}(t)=\alpha E_{inc}(t),\qquad E_{enh}(t)=\frac{V_{THz}(t)}{z_0},$$9 V/cm gives $z_0$0–$z_0$1 V
(Allerbeck et al., 2024)	Tilted-pulse-front optical rectification in lithium niobate, $z_0$2–$z_0$3 MHz	In situ calibration yields $z_0$4, $z_0$5, and $z_0$6 V

Several calibration schemes are now established. In one class of experiments, the strong-field THz voltage is related to the incident field by $z_0$7, and $z_0$8 then defines the near-field enhancement (Jelic et al., 2024). In another, the peak junction voltage is obtained by shifting multiple $z_0$9 curves taken at different $\alpha$0 onto a universal curve, giving a calibrated relation $\alpha$1 (Jelic et al., 2023). For defect spectroscopy in WSe$\alpha$2, the THz peak voltage at the junction was calibrated in situ by the strongly nonlinear onset of conduction-band tunneling (Allerbeck et al., 2024).

Waveform acquisition has likewise progressed from external electro-optic characterization to direct in-junction sampling. Phase-resolved detection of ultrabroadband THz pulses inside an STM junction was demonstrated via THz-field-induced modulation of ultrafast photocurrents, enabling direct measurement of the junction voltage transient rather than a scattered optical surrogate (Müller et al., 2020). A complementary THz-only method used a strong “Gate” pulse and a weak “NF” pulse in a two-pulse superposition geometry, with lock-in detection on the weak pulse to measure the local waveform inside the junction in real time; the gate-driven current burst had a full width at half maximum of $\alpha$3 fs at $\alpha$4 V/cm and remained $\alpha$5 fs up to $\alpha$6 V/cm (Li et al., 2023). Another approach combined waveform sampling and tailoring with THz scanning tunnelling spectroscopy, verifying that only an isolated unipolar THz-induced current pulse yields a physically correct waveform for differential THz-STS (Jelic et al., 2023).

These methods are not interchangeable in detail. The tip transfer function can introduce strong low-pass filtering, and several studies therefore separate the sample response from antenna coupling by referencing to Au(111) or by comparing in-junction waveforms at distinct tip positions (Müller et al., 2020, Jelic et al., 2024, Jelic et al., 28 Apr 2026). This suggests that waveform calibration is not ancillary but constitutive: the same local voltage transient that drives tunneling defines the energy and time axes of the experiment.

3. Detection channels, spectroscopy, and imaging modes

The primary measured quantity in many THz-STM experiments is the rectified charge or average THz-induced current. A standard definition is

$\alpha$7

with $\alpha$8 the THz repetition rate (Jelic et al., 2023). In practice, this charge can be mapped versus THz amplitude, static bias, delay, or position to obtain a family of measurement modes that have no direct analogue in conventional STM.

A central distinction is between instantaneous lightwave-driven tunneling and delayed responses associated with long-lived sample states. In WTe$\alpha$9, chopping the THz pulse train at $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$0 Hz and using lock-in detection yielded an in-phase component $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$1, identified as the lightwave-driven tunneling response, and an out-of-phase component $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$2, which measures the differential signal due to a long-lived metastable structural or electronic state (Jelic et al., 2024). In 1T-TaS$$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$3, the corresponding channels were denoted $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$4 and $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$5; a finite $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$6 with $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$7 up to $$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$8 indicated delayed, millisecond-scale modulation of the conductance due to a THz-induced metastable state (López et al., 26 May 2025).

Differential THz-STS exploits a strong-field pulse as an ultrafast gate and a weak-field replica as a linear probe. In the unipolar current-pulse regime,

$$I(z,V)\propto \exp[-2\kappa(z,V)z],\qquad \kappa(z,V)=\sqrt{2m[\Phi-eV]}/\hbar,$$9

so the weak-field modulation becomes directly analogous to AC-modulated $I(V,z)\propto V e^{-2\kappa z}$0 spectroscopy, but with the probe defined by a calibrated THz waveform rather than a low-frequency sinusoid (Jelic et al., 2023). On Au(111), this normalized differential signal revealed Gundlach oscillations that were obscured in the integrated $I(V,z)\propto V e^{-2\kappa z}$1.

Pump–probe operation introduces a second time axis. In 1T-TaS$I(V,z)\propto V e^{-2\kappa z}$2, a NIR pump and THz probe were combined so that $I(V,z)\propto V e^{-2\kappa z}$3 measured coherent dynamics over $I(V,z)\propto V e^{-2\kappa z}$4–$I(V,z)\propto V e^{-2\kappa z}$5 ps, with FFTs revealing a $I(V,z)\propto V e^{-2\kappa z}$6 THz charge-density-wave amplitude mode and a localized $I(V,z)\propto V e^{-2\kappa z}$7 THz interlayer shear mode (López et al., 26 May 2025). In GaAs(110), a three-pulse arrangement with an optical pump, a strong THz probe, and a weak THz replica was used to recover the local, time-dependent differential conductance and intrinsic band-bending dynamics from multidimensional $I(V,z)\propto V e^{-2\kappa z}$8 and $I(V,z)\propto V e^{-2\kappa z}$9 datasets (Jelic et al., 28 Apr 2026).

A recurrent point of interpretation is the distinction between photon-assisted tunneling and instantaneous-field gating. Several studies write the Tien–Gordon expression

$_2$0

as a useful context, but then conclude that the operative regime is lightwave-driven barrier modulation and rectification rather than resolvable sidebands in STS, because the drive is broadband and single-cycle and because the experimental observables include strong quadrature components associated with metastability or delayed conductance changes (López et al., 26 May 2025, Jelic et al., 28 Apr 2026). A common misconception is therefore that THz-STM is merely high-frequency photon-assisted STM; the modern experiments are more accurately described as time-domain biasing of a nonlinear tunnel junction, often with simultaneous access to intrinsic sample dynamics.

4. Structural, electronic, and topological phenomena accessed by THz-STM

The most direct demonstrations of THz-STM as a control technique come from lattice-driven phase transitions. In bulk WTe$_2$1, ultrafast THz fields enhanced at an atomically sharp tip resonantly couple to the interlayer shear mode at $_2$2 THz via an out-of-plane ferroelectric dipole at the surface, inducing a structural phase transition of the top layer into a metastable state (Jelic et al., 2024). Constant-height differential imaging with sampling as fine as $_2$3 pm/pixel and drift of $_2$4 pm/min resolved features of dimension $_2$5 one-tenth of the interatomic spacing, and comparison to hybrid-PBE0 charge-density maps extracted a top-layer shift of $_2$6 pm. The same work linked the THz-driven structural change to a reversible annihilation of the topologically protected Fermi arc surface states at the vacuum interface, with the arcs relocating to the buried interface where $_2$7 stacking remains (Jelic et al., 2024).

In the correlated material 1T-TaS$_2$8, THz-STM was used to investigate a THz-light-induced metastable state near a defect and to follow its dynamics in real space and time. THz excitation induced quasi-stationary changes in the insulating gap on angstrom scales, associated with interlayer stacking changes, while lightwave-driven tunneling simultaneously provided access to ultrafast dynamics within the same metastable state (López et al., 26 May 2025). The measured coherent modes comprised a $_2$9 THz charge-density-wave amplitude mode and a $>10^5$0 THz mode attributed to an interlayer shear vibration emerging near the defect.

Single-defect charge dynamics constitute another major application. In monolayer and bilayer WSe$>10^5$1, individual selenium vacancies act as atomic-scale quantum dots with discrete in-gap levels. THz pump–THz probe sampling captured transient Coulomb blockade, identified back tunneling of localized charges to the tip as a key challenge when charge-state lifetimes exceed the pulse duration, and showed that back tunneling can be mitigated by the Franck–Condon blockade (Allerbeck et al., 2024). The instrument response gave a temporal resolution of $>10^5$2 fs; representative charge-state lifetimes were $>10^5$3 ps for $>10^5$4 ML and $>10^5$5–$>10^5$6 ps for $>10^5$7 ML WSe$>10^5$8.

THz-STM has also been extended to coherent phonon excitation in semiconducting 2H-MoTe$>10^5$9. In a THz pump–probe scheme, long-lived oscillatory signals persisting to $1$0 ps were assigned to an in-plane shear mode at $1$1 THz and an out-of-plane breathing mode at $1$2 THz; both are forbidden in the centrosymmetric bulk and become active at the surface (Rai et al., 9 Jun 2025). The relative excitation strength of these modes varied near atomic-scale defects, and the inversion of their relative intensities under certain bias conditions was not reproduced by a static convolution of the THz pulses with the nonlinear $1$3–$1$4 curve, indicating genuine defect-tunable phonon coupling.

On semiconducting GaAs(110), lightwave-driven THz-STM was used for femtosecond tunneling spectroscopy of transient band bending. The method tracked the transient photocurrents produced by ultrafast shifts in the energy alignment of surface and bulk states near individual defects and resolved position-dependent photocurrent lifetimes of $1$5 fs on a gallium vacancy, $1$6 fs on a bright ring $1$7 nm away, and $1$8 fs on a pristine region (Jelic et al., 28 Apr 2026). Atomic-scale THz time-domain spectroscopy at the tip showed that the local THz waveform remained effectively invariant under optical pumping, which enabled separation of the coherent sub-cycle drive from the intrinsic band-bending response.

THz-STM has further entered the domain of driven topology. A nonequilibrium theory for graphene predicted that ultrafast THz-STM enables direct local detection of bulk Floquet gaps, distinct Floquet edge state signatures in graphene nanoribbons, and Floquet quasiparticle interference that reconstructs driven dispersions (Jacobsen et al., 16 Feb 2026). This remains a proposal rather than an experimental demonstration, but it places THz-STM in the same conceptual space as ultrafast transport and time- and angle-resolved photoemission, with the added capability of atomic-scale spatial selectivity.

5. Theoretical descriptions and dynamical regimes

Theoretical treatments of THz-STM fall into several connected but distinct classes. At the most phenomenological level, lattice control in WTe$1$9 was described by a driven oscillator equation for the shear coordinate,

$V_{THz}(t)$00

with the crucial point that an out-of-plane tip field can resonantly drive an in-plane shear mode through an interfacial ferroelectric dipole (Jelic et al., 2024). This model explains why the threshold for the field-driven phase transition occurs when $V_{THz}(t)$01 V/nm and why the observed displacement remains picometre-scale.

For strongly correlated and metastable electronic states, models must include both ultrafast rectification and slower state evolution. In 1T-TaS$V_{THz}(t)$02, the measured THz response was decomposed into a quasi-stationary LDOS component $V_{THz}(t)$03 and an ultrafast lightwave-driven tunneling component $V_{THz}(t)$04, combined as

$V_{THz}(t)$05

with a 1D Bardeen current and a time-dependent LDOS shift used to reproduce the polarity inversion of $V_{THz}(t)$06 near $V_{THz}(t)$07 mV and the phase-dependent shift of the $V_{THz}(t)$08 curve by $V_{THz}(t)$09 V/cm (López et al., 26 May 2025). This formalism makes explicit that THz-STM signals can be mixed observables: they need not isolate a single tunneling mechanism.

For single-defect transport, a master-equation approach becomes natural. In WSe$V_{THz}(t)$10, the occupation vector of a vibronic defect manifold obeys

$V_{THz}(t)$11

with $V_{THz}(t)$12 describing sample-, tip-, and phonon-mediated transitions (Allerbeck et al., 2024). The model incorporates Franck–Condon factors $V_{THz}(t)$13, voltage division across the vacuum and dielectric, and the measured THz waveform including window reflections. It reproduces the nonlinear recovery of $V_{THz}(t)$14 as $V_{THz}(t)$15, the $V_{THz}(t)$16-dependent maximum in $V_{THz}(t)$17, and the pump–probe traces of transient Coulomb blockade.

A more microscopic route uses nonequilibrium Green’s functions. In a DFT-NEGF-informed model of hydrogenated graphene, three methods were compared: steady-state adiabatic results, lowest-order dynamical expansion, and numerically exact auxiliary-mode propagation without approximations in the time variation (Svaneborg et al., 2023). For the pulse

$V_{THz}(t)$18

the near-adiabatic first-order dynamical expansion provided a good description for STM voltage pulses up to the $V_{THz}(t)$19 V range and for $V_{THz}(t)$20 THz, while more strongly nonadiabatic behavior required full auxiliary-mode propagation. This result is theoretically useful because it delineates when a THz-STM experiment can be interpreted as a sequence of instantaneous steady states and when memory effects become essential.

The most general nonequilibrium tunneling framework in the present corpus is the Floquet-Keldysh theory developed for graphene under circular pumping. There the measurable current is

$V_{THz}(t)$21

with the tip self-energy carrying the phase

$V_{THz}(t)$22

so that the current depends on the history of the THz bias rather than only on its instantaneous value (Jacobsen et al., 16 Feb 2026). In the continuous-wave Floquet steady state, the cycle-averaged current reduces to a Meir–Wingreen-type expression involving the time-averaged Floquet LDOS under the tip. This places THz-STM on a formally similar footing to standard STM, but with the static LDOS replaced by a driven, nonequilibrium spectral function.

Taken together, these models indicate that “lightwave-driven THz-STM” is not a single theoretical limit. Depending on field amplitude, carrier lifetime, dissipation, and the relevant material coordinate, it can behave as a quasi-static barrier-modulation experiment, a rectified nonequilibrium spectroscopy, a driven-vibronic master-equation problem, or a fully time-nonlocal Keldysh transport problem.

6. Instrumentation, artifacts, and broader significance

The instrumental landscape of THz-STM ranges from amplifier-based systems optimized for strong-field control to compact, high-repetition-rate platforms optimized for deployability. A home-built variable-temperature UHV instrument integrated with a compact commercial fiber-laser THz unit operated from $V_{THz}(t)$23 K to $V_{THz}(t)$24 K, used dual fiber-coupled photoconductive emitters at $V_{THz}(t)$25 MHz, and demonstrated picosecond cross-correlation together with atomic-scale THz contrast on Ag(111), step edges, and CO-like impurities (Azazoglu et al., 2023). This implementation showed that high pulse energy is not the only path to useful THz-STM performance; strong apex enhancement and rectification-based detection can compensate for much lower free-space THz power.

A persistent experimental concern is whether THz-induced signals are actually photothermal artifacts. A direct comparison on the same Ag(111)/W junction found clear thermal expansion under red-light illumination but negligible thermal effects under free-space THz pulses (Azazoglu et al., 2023). With red light at $V_{THz}(t)$26 mW, the current showed a fast increase of $V_{THz}(t)$27 pA within $V_{THz}(t)$28 ms followed by a slow rise saturating after $V_{THz}(t)$29 ms, whereas THz excitation at a maximum free-space average power of $V_{THz}(t)$30 pW produced a step-like $V_{THz}(t)$31 pA modulation limited by the $V_{THz}(t)$32s current-amplifier bandwidth and no slow drift component. The paper therefore provides direct evidence that THz gating can yield photo-induced currents of similar magnitude to visible illumination while suppressing thermal expansion by orders of magnitude (Azazoglu et al., 2023).

Other artifacts are subtler. The exact near-field distribution and in-junction $V_{THz}(t)$33 remain uncertain in some experiments, especially on insulating or semiconducting surfaces where screening is incomplete; several studies therefore calibrate with in situ photoemission sampling, Au(111) references, or waveform inversion and forward modeling (López et al., 26 May 2025, Jelic et al., 2023). The junction and tip also act as a low-pass filter: direct measurements with a spintronic THz emitter showed strong high-frequency attenuation even though components up to $V_{THz}(t)$34 THz remain detectable (Müller et al., 2020). In differential imaging of metastable states, too strong a field can suppress the very contrast one wishes to measure, as in Regime III of the WTe$V_{THz}(t)$35 experiments where the metastable phase dominates continuously and $V_{THz}(t)$36 (Jelic et al., 2024).

Several misconceptions can therefore be addressed directly. First, THz-STM does not always read out a purely instantaneous tunneling current; in multiple materials the measured signal contains both femtosecond rectification and millisecond metastability (Jelic et al., 2024, López et al., 26 May 2025). Second, THz-STM is not reducible to heating, and the visible-versus-THz comparison shows why (Azazoglu et al., 2023). Third, local THz waveforms cannot be assumed to equal the far-field waveform; the tip transfer function and sample response must be measured or validated in the junction itself (Müller et al., 2020, Li et al., 2023, Jelic et al., 2023).

The broader significance of THz-STM lies in the conjunction of three capabilities: nanoscale field concentration, calibrated time-domain biasing, and atomically local readout of a nonlinear tunnel junction. In current experiments, this combination has enabled coherent control of a specific lattice mode and topological switching in WTe$V_{THz}(t)$37, defect-specific ultrafast charge-order dynamics in 1T-TaS$V_{THz}(t)$38, transient Coulomb blockade in WSe$V_{THz}(t)$39, defect-tunable coherent phonon excitation in 2H-MoTe$V_{THz}(t)$40, and femtosecond band-bending spectroscopy in GaAs (Jelic et al., 2024, López et al., 26 May 2025, Allerbeck et al., 2024, Rai et al., 9 Jun 2025, Jelic et al., 28 Apr 2026). A plausible implication is that THz-STM is becoming a general platform for atomically resolved nonequilibrium spectroscopy, in which the same tip-enhanced field can drive a local state, probe its response, and calibrate the waveform that links the two.