Time-resolved femtosecond optical characterization of multi-photon absorption in high-pressure-grown Al0.86Ga0.14N single crystals

We report our experimental studies on time-resolved pump–probe spectroscopy in high-quality Al0.86Ga0.14N single crystals, grown using a solution technique in a high-nitrogen-gas-pressure system. Our optical measurements were performed using a non-traditional, two-beam [one ultraviolet (UV) and one infrared (IR)], femtosecond pump–probe approach, in which the photon energies of both beams were below the bandgap of the sample and each electron–hole pair was generated by a multi-photon process of absorption of a pump photon together with another photon produced by second harmonic generation from two probe photons. Temporal scanning of the probe while monitoring the normalized transient differential transmissivity (ΔT/T) signal, produced a 310-fs-wide, Gaussian-shaped correlation signal caused by the multi-photon absorption process, followed by a >100-ps-long relaxation of photo-excited carriers. By studying the ΔT/T correlation signal amplitude dependence on the pump-power intensity and wavelength, the mult...