Z-scan is one of the common measurement method to characterize the third-order optical nonlinearity of materials and has ability to determine both real and imaginary components of the nonlinearity with a single scan. With the optical intensity and its distribution in time and space properly measured, each component of the nonlinearity can be determined without measuring the reference material whose nonlinearity was already known (i.e. absolute method). However, precise characterizations of the incident optical intensity and its distributions are laborious tasks, especially for femtosecond pulses for a wide range of the wavelengths. Thus, to save measurement time for reproducible results, the relative method by comparing unknown sample with a known reference material is still effective for the Z-scan method. For two-photon absorption (TPA), i.e. the imaginary component, a number of dyes have been reported as reference materials.[1,2] However, their TPA spectra have been reported from 550 nm or longer. Virtually no reliable reference material was reported for the shorter wavelength region than that although the TPA property of the blue-green region is interesting because of the potential for high resolution in applications such as 3D photolithography.
As standard material, strong and broad spectrum of TPA is desireble around the wavelength of interest, in addition to stability and availability of the materials. Thus, here we propose to use GaN as standard for the TPA measurement for the blue-green excitation. Semiconductor is known to have structureless, broad TPA spectrum well descried by the band theory at the excitation photon energies higher than the half of the band gap energy (Eg). As Eg=3.4 eV for GaN, its TPA spectrum covers all wavelengths of the blue-green region. Reported TPA coefficients (β) of GaN were found to be scattered and different from a report to another. We measured TPA spectrum of single crystal of GaN substrate (thickness = 270 μm) by the femtosecond Z-scan method from 400 nm to 700 nm. The obtained magnitude of the TPA coefficient was found to be close to that reported by van Stryland et al. although it is smaller than other reports. The TPA spectral measurement of a short wavelength TPA dye, bis(o-methylstyrylbenzene), based on the GaN as standard gave consistent result of the previous report. In conclusion, GaN can be use as convenient standard material for the Z-scan measurement in the blue-green region.
 C. Xu and W. W. Webb, J. Opt. Soc. Am. B 1996, 13, 481.
 N. S. Makarov, et al., Opt. Expr. 2008, 16, 4029.
 M. Sheik-Bahae, et al., IEEE J. Quantum Electron. 1991, 27, 1296.
 E. W. van Stryland et al., Opt. Lett. 1985, 10, 490.