Deformation potential driven photostriction in layered ferroelectrics

TL;DR

The paper resolves the long-standing debate on photostriction in layered ferroelectrics by showing that deformation-potential effects dominate inverse piezoelectric screening in multilayer SnS, causing polar-axis expansion even within ferroelectric stacking domains. By combining polarization-resolved SHG to map stacking motifs with ultrafast pump–probe reflectivity and DFT calculations, the authors reveal an anisotropic lattice response and disentangle intrinsic photostrictive strain from thin-film interference artifacts. The findings are corroborated by a three-layer optical model and highlight DP as the primary mechanism for ultrafast actuation in SnS, positioning stacking-engineered SnS as a versatile platform for ultrafast optomechanical transduction in van der Waals ferroics. These insights offer a general design principle for light-driven, direction-selective actuation in layered ferroelectrics and ferroelastics through stacking and domain engineering.

Abstract

The coupling between electronic excitations and lattice deformation in van der Waals ferroelectrics is governed by a competition between the electron deformation potential and the inverse piezoelectric effect. While theory predicts that piezoelectric screening should drive a polar-axis contraction in monolayer group-IV monochalcogenides, we demonstrate that in multilayer SnS, the deformation potential provides the dominant contribution, driving a polar-axis expansion even within ferroelectric domains. By correlating polarization-resolved second-harmonic generation microscopy with ultrafast reflectance spectroscopy and first-principles calculations, we resolve the anisotropic lattice response and disentangle intrinsic photostrictive strain from extrinsic thin-film interference artifacts. These results establish a microscopic hierarchy of photostrictive mechanisms and position stacking-engineered SnS as a platform for ultrafast optomechanical transduction.

Figures (12)

• Figure 1: The two key mechanisms for photostriction. (a) Deformation-potential-driven photostriction, where optical excitation redistributes electronic population into excited states whose strain-dependent energies exert an internal electronic stress on the lattice. In polar crystals, occupation of antibonding electronic states leads to a repulsive force along the polar axis, driving lattice expansion or contraction depending on the sign of the deformation potential. (b) Inverse piezoelectric contribution to photostriction, where illumination of a ferroelectric with spontaneous polarization (P$_s$) generates photocharges that partially screen the polarization, modifying the internal electric field and inducing crystal expansion/compression.
• Figure 2: Anisotropy in SHG generated by stacking. (a) Schematics showing the presence of SHG in AA(FE) stacking and its absence in AB(AFE) stacking. (b) Optical image of monodomain SnS with thickness of 49.5 nm on MgO substrate. Red arrows show its AC and ZZ directions. (c-g) Polarization-resolved SHG intensity plot for (c,d) 49.5 nm thick monodomain SnS on MgO substrate under (c) parallel, and (d) perpendicular configuration. (e,f) 7.7 nm thick monodomain SnS on mica substrate under (e) parallel, and (f) perpendicular configuration. (g) 65.5 nm thick monodomain SnS on mica substrate under parallel configuration. Solid line represents the simulated fit using the two-tensor model.
• Figure 3: Coexistence of FE and AFE stacking in a SnS island. (a,b) Cross polarized microscopy image of multidomained SnS on MgO substrate at (a) 0$^{\circ}$, and (b) 90$^{\circ}$ orientation. Relative bright and dark stripes represent the different crystallographic domains. The magnified image shows the AC and ZZ directions for the bright and dark striped region for 0 $^{\circ}$ orientation of the sample. (c-e) SHG mapping of multidomained SnS island under parallel configuration at (c) 0$^{\circ}$, (d) 45$^{\circ}$, and (e) 90$^{\circ}$ orientation. Bright regions in the SHG map represent the SHG active region. The white solid lines represent the outline of the pattern extracted from cross polarized optical image of multidomained SnS. Scale bar represents 10 $\mu$$m$.
• Figure 4: Sign reversal in transient reflection signal. (a) Schematics showing photostriction in SnS (left). Dielectric ellipsoid showing the change in dielectric constant due to photostriction (right). (b) Color map of polarization dependent transient reflection signal for monodomain SnS. (c) Representative positive and negative reflective signal from the color map of (b) for 180$^{\circ}$ and 90$^{\circ}$, respectively. Blue and red circles represents the experimental data points whereas the solid yellow line represents the biexponential fit to the decay dynamics. (d-f) Color map of peak intensity of transient reflection signal from multi-domain SnS for sample orientation at (d) 45$^{\circ}$, (e) 135$^{\circ}$, and (f) 0$^{\circ}$, while horizontal probe polarization direction was kept. Dark solid lined overlaid in (d) and (e) is a guide to identify the domains. Scale bar represents 10 $\mu$$m$.
• Figure 5: Optical interference effect in transient reflectivity signal. (a) Pictorial representation of multireflection in SnS film on MgO substrate. (b) Thickness dependent reflectance taken with probe polarization parallel to AC. Red dots shows the experimental data, and the blue solid curve is the simulated fit generated considering interference effect and complex refractive index of SnS. (c) Transient reflectivity change as a function of SnS film thickness, with probe polarization parallel to AC. Red dots represent the experimental data, and blue solid curve shows the simulated fit incorporating refractive index changes from photostriction and interference effects.