Overcoming the indirect bandgap: efficient silicon emission via momentum-expanded photonic states
Silicon's inherently indirect bandgap severely limits its radiative efficiency, posing a fundamental challenge to the development of practical silicon-based light sources. While strategies such as nanoscale confinement of electrons and holes (quantum dots), Mie resonators, and hybrid plasmonic structures have improved emission, they typically require complex fabrication workflows. Here, we demonstrate a conceptually distinct and scalable approach to enable light emission from a bulk silicon wafer by decorating its surface with gold or copper nanoparticles. Remarkably, the effect is nearly identical for Au and Cu, with particle size emerging as the dominant factor. We show that strong luminescence from the bulk wafer emerges only when the nanoparticle diameter is below 2 nm. We attribute this effect to the formation of spatially confined photonic states with broadened momentum distributions, which must enable diagonal, phonon-independent optical transitions that bypass the limitations imposed by silicon's indirect bandgap. This mechanism yields broadband emission across the visible and near-infrared spectrum, with quantum efficiencies comparable to direct bandgap semiconductors, representing a 10^5-fold increase in integrated spectral intensity. This discovery challenges the conventional understanding of silicon's optical constraints and opens a practical pathway toward high-performance silicon-based optical and optoelectronic components.