Effect of intermediate states on optical nonlinearities in Fe-doped GaN single crystals

Semiconductor doping strongly influences its electrical and optical properties. The transition metal Fe doping can induce the deep levels to compensate residual donors to generate semi-insulating GaN, and make carrier lifetime short due to carrier trapping, which thus has potential applications in ultrafast optical and optoelectronic devices. For such applications, it is critical to understand the effect of Fe doping on optical nonlinearities and ultrafast carrier dynamics in GaN crystals. In this paper, we studied the dependence of GaN ultrafast nonlinearities on Fe-related intermediate states using femtosecond two-photon Z-scan and pump-probe with phase object (PO) techniques. In particular, we investigated the nonlinearities around 458 nm (2.71 eV) and 540 nm (2.29 eV) where it shows dips in transmission spectra. It is found that the two-photon absorption coefficient has a minimum value at the peak of 458 nm, in contrast, the coefficient has a maximum value at the peak of 540 nm, and enhances around 1.5 times more. It thus indicates that these two absorption resonances are attributed to different physical reasons. We further investigated the ultrafast carrier dynamics by the time-resolved PO pump-probe techniques with 190 fs laser pulses. We observed that the transient refraction curves are recovered once pump and probe pulses are separated in time delay. It is considered that carrier lifetime is dramatically reduced due to Fe trapping centers. Our finding that Fe-doped semi-insulating GaN has ultrafast carrier lifetime expect GaN potential applications in ultrafast detectors and all-optical switches.