# Two timed laser pulses amplify layer sliding in ferroelectric WTe2 at the same total fluence

- Published: 2026-11-08 20:56 UTC
- Section: Science › Physics
- Tags: tungsten ditelluride, sliding ferroelectric, coherent phonons, double pulse excitation, interlayer sliding mode, time-resolved second-harmonic generation, electron–phonon coupling, anharmonic phonon coupling, Nature Physics, ultrafast ferroelectric control
- URL: https://realtech.media/news/rt-a505614e81ba49ea8409f03a_two_timed_laser/

**Splitting one pulse into two delayed ones gave larger interlayer sliding amplitudes in WTe2 than a single pulse of equal fluence, researchers from Würzburg, Rostock and ETH Zurich report.**

![Two timed laser pulses amplify layer sliding in ferroelectric WTe2 at the same total fluence](https://realtech.media/media/2026/11/90b22df717ec4e94.png)
*nature.com*

Researchers at the Universities of Würzburg and Rostock and at ETH Zurich have shown that splitting a femtosecond laser pulse into two time-separated pulses drives a larger interlayer sliding motion in tungsten ditelluride (WTe2) than a single pulse with the same total fluence. The study, published in Nature Physics and available on nature.com, was led by Gerrit Horstmann as first author. [1] WTe2 is a van der Waals sliding ferroelectric: its polarization arises from the relative displacement of adjacent layers. [1]

## Why a single strong pulse saturates

Displacive excitation of coherent phonons, the main way to launch structural motion with light, usually assumes that the force on the lattice scales linearly with the density of photo-excited carriers. [1] At high excitation this linearity breaks down and limits the attainable displacement. [1] Time-resolved second-harmonic generation showed that the interlayer-sliding phonon, which governs ferroelectricity, saturates and even diminishes under single-pulse excitation. [1]

First-principles calculations attribute this to band-specific electron–phonon coupling. Occupation of high-lying electronic states produces forces that counteract the sliding. [1] In Horstmann's words, the extra light energy then no longer produces a correspondingly larger displacement of the layers.

## Two pulses instead of one

In the Td polymorph, stable below 565 K, WTe2 has a trilayer structure that breaks inversion symmetry, with polarization along the c-axis. [1] Optical excitation launches a sliding mode at 0.24 THz, which pushes the lattice toward a high-symmetry configuration and eventually restores inversion symmetry. [1]

The team divided the optical energy between two well-timed pulses chosen to avoid populating the counteracting states. At fixed total fluence this gave markedly larger phonon amplitudes. [1] According to the authors, staggered stimulation restores linearity and amplifies sliding beyond the single-pulse limit. [1] The researchers describe the delay as a control parameter: after the first pulse the excited electrons partly relax before the second arrives.

The stronger lattice response also gave greater control over the ferroelectric polarization. The authors report a strong correlation between sliding-mode amplitude, damping and frequency, which they interpret as anharmonic coupling mediated by coherent nuclear motion. This regime emerges only far from equilibrium. [1]

## Sources

1. [nature.com](https://www.nature.com/articles/s41567-026-03491-1)
2. [SciTechDaily](https://scitechdaily.com/quantum-discovery-reveals-why-more-powerful-lasers-can-actually-slow-atoms-down)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-a505614e81ba49ea8409f03a_two_timed_laser/