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Summary form only given. During SHG experiments of GaN using a femtosecond Cr:forsterite laser, we also observed strong third harmonic generation (THG) in the bulk GaN. Since THG exists in centrosymmetric structures it can be an excellent label for the mapping of the crystal property itself, in contrast to SHG as the label for the strength of piezoelectric field. With 40 mW 100 fs Cr:forsterite laser input, we can easily obtain THG at purple wavelength on the order of 10 nW with an 80/spl times/ objective. The input and output spectra of THG in a 2 /spl mu/m-thick bulk GaN are shown. With input wavelength centered at 1230 nm, we observed the THG spectrum centered at 410 nm. The room temperature bandgap of GaN is located around 365 nm. The THG microscopy can then by accomplished by scanning the GaN sample using a computer controlled XYZ stage. By comparing the THG microscopy with SHG microscopy, detailed information on the crystal properties can then be obtained. In this presentation we compare the THG microscopy with the SHG microscopy as well as two-photon and three-photon confocal microscopy. By comparing these images obtained through different nonlinear optical microscopy, we can obtain complete and interesting information not observed before.