Fig. 1. (a) Linear absorption spectrum of GaN: Ge crystal. The inset shows the two-photon excited photoluminescence photograph of sample; (b) open-aperture Z-scan data of GaN: Ge at several input pulse energies, the solid lines are theoretical fitting curves.

Fig. 2. (a) Ultrafast TAS in GaN: Ge using 2PE under the excitation fluence of 0.8 mJ/cm²; (b) ultrafast TAS in GaN: Ge using 1PE under the excitation fluence of 0.5 mJ/cm². The insets show the TAS probed at visible wavelengths.

Fig. 3. (a) The transient absorption kinetics in GaN: Ge under various excitation fluence probed at 1050 nm, the solid lines denote the theoretical curves using bi-exponential decay, and the inset illustrates the transient absorption kinetics in a 7 ps time window; (b) the fast and slow relaxation time (τ₁ and τ₂, respectively) extracted from transient absorption kinetics under various excitation fluence.

Fig. 4. Energy band diagram used to model the carrier dynamics of GaN under 2PE. Straight broken arrows denote non-radiative transitions and curvy downwards arrows denote emissions via radiative recombination.

Fig. 5. Fitting and simulation of ultrafast carrier relaxation dynamics in GaN: Ge using carrier recombination model: (a) The fitting of experimental results; (b) under higher excitation fluence and 1PE.

Fig. 6. Ultrafast transient absorption kinetics in GaN:Ge probed at communication band 1310 nm under both 1PE (0.8 mJ/cm²) and 2PE (1.6 mJ/cm²).