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

ABSTRACT 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 WSe 2 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 µm 2 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.