Fig. 2. Hot carrier cooling modulated by alkali cations.

(A and B) Normalized TA spectra of pump at 3.10 eV with excitation density of 1.3 × 10¹⁸ cm⁻³ at different pump-probe delays for MAFA (A) and RbCsKMAFA (B), respectively. Spectral broadening at early delay time indicates hot carrier distributions. The black dashed lines are fits to the high-energy bleach tail with the Maxwell-Boltzmann distribution to extract effective carrier temperature T_c, as described in the main text. (C and D) Extracted T_c of MAFA and RbCsKMAFA as a function of delay time at different carrier densities. (E) Carrier cooling curves for different samples at N₀ of 1.3 × 10¹⁸ cm⁻³, displaying strong dependence on alkali cations. (F) Similar initial T_c and cooling behavior can be achieved in RbCsKMAFA with an order of magnitude lower N₀ than the reference MAFA (3 × 10¹⁷ versus 3 × 10¹⁸ cm⁻³). Solid lines are the fitted with biexponential or triexponential functions, and the fitting parameters are given in table S1.