Two timed laser pulses enlarge interlayer sliding in WTe₂ at the same total fluence as a single pulse
In the sliding ferroelectric WTe₂, one strong laser pulse saturates the interlayer phonon. Splitting the same energy into two well-timed pulses gave markedly larger amplitudes.
Researchers from the Universities of Würzburg and Rostock and ETH Zurich reported in Nature Physics that splitting laser energy into two well-timed pulses produces a larger interlayer sliding motion in WTe₂ than a single pulse of the same total fluence doi.org
Nature Physicsdoi.orgOpen source ↗. WTe₂ is a sliding ferroelectric, a material in which neighbouring atomic layers can slide against each other. The first author is Gerrit Horstmann, who heads an Emmy Noether junior research group at the University of Würzburg. This is a laboratory study of the material, not a device.
Why a single pulse saturates
Displacive excitation of coherent phonons, the principal mechanism for launching structural motion, typically assumes that the forces driving the lattice scale linearly with the photo-excited carrier density. At high excitation this linearity can break down, which limits the lattice displacements that can be reached doi.org
Nature Physicsdoi.orgOpen source ↗.
In WTe₂, time-resolved second-harmonic generation showed that the interlayer-sliding phonon, which governs ferroelectricity, saturates and even diminishes under single-pulse excitation doi.org
Nature Physicsdoi.orgOpen source ↗. First-principles calculations attribute this to band-specific electron–phonon coupling that induces competing forces on the lattice
doi.org
Nature Physicsdoi.orgOpen source ↗. According to Horstmann, under very strong excitation electrons are promoted into high-energy states that counteract the directed motion of the atoms, so additional light energy no longer yields a proportionally larger displacement.
Two pulses at fixed fluence
The authors divided the optical energy into two pulses separated in time and chosen to avoid populating the counteracting states. After the first pulse, the electronic system has time to partially relax before the second pulse arrives. Horstmann compares the choice of delay to pushing a child on a swing. At fixed total fluence, this approach gave markedly larger phonon amplitudes than single-pulse excitation doi.org
Nature Physicsdoi.orgOpen source ↗.
The enhanced sliding motion exposes a regime of anharmonic phonon coupling that emerges only far from equilibrium doi.org
Nature Physicsdoi.orgOpen source ↗. The processes unfold within femtoseconds. Horstmann says that timing opens an additional way to make use of the interaction between electrons and atomic motion. He notes that the relative motion of the layers can influence material properties such as ferroelectricity and topology.
