Large‐Area Photonic Membranes Achieving Uniform and Strong Enhancement of Photoluminescence and Second‐Harmonic Generation in Monolayer WSe ₂

Two-dimensional (2D) transition metal dichalcogenides exhibit strong excitonic responses, direct bandgaps, and remarkable nonlinear optical properties, making them highly attractive for integrated photonic, optoelectronic, and quantum applications. Here, we present a large-area freestanding membrane photonic platform that achieves exceptional enhancement of light-matter interactions in monolayer WSe₂ via quasi-bound states in the continuum (quasi-BICs). The freestanding architecture effectively suppresses radiative losses and supports high-Q optical resonances, leading to enhanced light-matter interactions. This results in significant photoluminescence emission and second-harmonic generation (SHG) enhancement factors of 1158 and 378, respectively, with spatial uniformity sustained across a 450 × 450 µm² area. This uniform SHG enhancement further enables polarization-resolved mapping of crystal orientation and grain boundaries, offering a practical method for large-area structural characterization of 2D materials. Moreover, femtosecond-pumped SHG spectra reveal multiple narrowband peaks originating from distinct quasi-BIC modes-providing direct spectral evidence of resonantly enhanced nonlinear coupling. The combined attributes of strong optical enhancement, spectral selectivity, and wafer-scale compatibility establish this platform as a scalable interface for 2D semiconductor integration in next-generation optoelectronic, nonlinear, and quantum photonic technologies.