Time-resolved femtosecond optical characterization of multi-photon absorption in high-pressure-grown Al_(0.86)Ga_(0.14)N single crystals

摘要

We report our experimental studies on time-resolved pump-probe spectroscopy in high-quality Al_(0.86)Ga_(0.14)N 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 (△T/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 △T/T correlation signal amplitude dependence on the pump-power intensity and wavelength, the multi-photon absorption was determined to be predominantly caused by absorption of a pump photon and a second harmonic photon from the probe.