Etchless InSe Cavities Based on Bound States in the Continuum for Enhanced Exciton‐Mediated Emission

Abstract Recently, fervent research interest is sparked to indium selenide (γ‐InSe) due to its dazzling optical and electronic properties. The direct bandgap in the near‐infrared (NIR) range ensures efficient carrier recombination in InSe, promoting impressive competency for lavish NIR applications. Nevertheless, the photoluminescence (PL) efficiency of InSe is significantly limited by out‐of‐plane (OP) excitons, adverse to practical devices. Herein, a facile and effective solution is proposed by introducing photonic bound‐states‐in‐the‐continuum (BIC) modes to enhance excitons in InSe through strengthened exciton‐photon coupling. This cavity is constructed simply by patterning a polymer grating onto the InSe flake without an etching process, achieving an impressive PL enhancement of over 200 times. By adjusting the cavity resonance wavelength, it can selectively amplify the exciton emission or the exciton‐exciton scattering process, which is not observable off‐cavity at room temperature. Additionally, the second harmonic generation (SHG) process in InSe can also be largely enhanced by over 400 times on the cavity. Notably, the etchless cavity design can be further extended to other nanostructures beyond grating. This research presents a feasible and efficient approach to enhancing the optical performance of OP excitons, paving a prospective avenue for advanced linear and nonlinear photonic devices.