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.