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arXiv · 2610.32341

Flexo-Photoresponses from First Principles

Zhichao Guo·Hua Wang·Haowei Xu
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Abstract

Many functionally important materials, such as silicon, possess high-symmetry crystal structures that forbid even-order nonlinear optical responses, thereby limiting their application in e.g., photonics and optoelectronics. Strain gradients can effectively break symmetries, activating otherwise forbidden optical responses. Although such flexo-photoresponses have attracted considerable interest recently, a comprehensive microscopic theoretical framework is still lacking. Here, we develop a first-principles approach that can systematically evaluate a wide range of flexo-photoresponses in realistic materials. By mapping strain gradients to long-wavelength phonons within the established theory of electron-phonon coupling, our approach captures the nonlocal quantum-mechanical effects of lattice deformations on nonlinear optical responses. Applying the framework to prototypical centrosymmetric materials, including Si, cubic $\rm SrTiO_3$, and monolayer black phosphorus, we demonstrate that a strain gradients of $10^5\sim10^6~\rm m^{-1}$ can induce flexo-second-harmonic generation and flexo-bulk photovoltaic effects comparable to the intrinsic responses of non-centrosymmetric crystals. Our work establishes a rigorous foundation for strain gradient engineering of light-matter interactions, opening new avenues for nanoscale optomechanics, optoelectronics, and beyond.

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Zhichao Guo, Hua Wang, Haowei Xu. 2026-10-23. Flexo-Photoresponses from First Principles. https://arxiv.org/abs/2610.32341

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