Gallium oxide (Ga2O3) is a promising candidate in future photonic integrated circuits devices due to its ultra-wide band gap and high stability. However, the nonlinear optical properties of Ga2O3 still remain elusive. In this work, we investigate the free-carrier absorption and refraction in pristine β-Ga2O3 single crystal via the degenerate pump-probe measurement with phase object technique at 355 nm wavelength. The free-carrier absorption cross-section and free-carrier refraction index of β-Ga2O3 on the sub-nanosecond time scale are determined to be ∼0.6 × 10−22 m2, and ∼−0.62 × 10−28 m3, respectively. And the two-photon absorption coefficient of β-Ga2O3 is characterized as ∼0.53 × 10−11 m W−1. It is found that the free-carrier absorption and free-carrier refraction effect in β-Ga2O3 are significantly smaller than that in other wide bandgap semiconductors such as GaN and ZnO. Our results can serve as guidelines for designing β-Ga2O3 based ultra-low loss waveguides and integrated photonic applications in UV spectral range.

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We report the different nonlinear optical mechanisms and defect-related carrier dynamics in Sn-doped β-Ga2O3 crystal by utilizing time-resolved pump-probe technique based on phase object under UV excitation. The obtained nonlinear optical parameters arise from bound electron can be well explained by the theoretical calculation of two-band model and Kramers-Kronig transformation. By tuning the probe wavelength, the carrier nonlinearity can be modulated greatly due to additional absorption of defects within the bandgap. The results reveal that by choosing a proper probe wavelength that matches the defect state to the valence band, the nonlinear absorption and refraction of the carriers can be greatly enhanced, which provides an important reference for the design of gallium oxide-based waveguide materials and all-optical switching materials in the future.

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In the red region (650–800 nm), the nonlinear absorption of trithiophene-based compound INT3 is greater than that of triphenylamine-based compound INB3, while in the NIR region (800–1100 nm), the strength of nonlinear absorption is the opposite.