Giant Multiphoton Luminescence and Band Renormalization with Hot Electron–Hole Plasma in Multilayer GaSe

Efficient photoluminescence (PL) of layered semiconductors is crucial for advancing next‐generation photonic devices. However, thermal effect‐induced destruction typically hinders the practical applications, such as biosensing and imaging. Here, the upconversion PL of multilayer GaSe is reported, which circumvents thermal damage. A high‐order multiphoton (up to 8‐photon) PL is first reported in multilayer GaSe. Both experimental and theoretical results reveal a power‐dependent redshift of the PL peak (≈40 meV, equivalent to 2% of the bandgap) and PL spectral broadening (full width at half maximum increased by ≈2 times), attributed to the hot electron–hole plasma. Time‐resolved PL resolves the multistage of carrier relaxation, revealing an ultrafast transition (≈58 ps) from electron–hole plasma to excitonic states, which establishes hot electron–hole plasma engineering as a critical mechanism for manipulating PL processes in Group‐III–VI chalcogenides. Furthermore, wavelength‐dependent two‐ and three‐photon PL spectra are explored. These results establish a microscopic framework connecting hot electron–hole plasma dynamics with macroscopic optoelectronic phenomena, providing critical insights for designing ultrafast photonic modulators and nonlinear optical devices based on 2D layered semiconductors.