We report on the highly nonlinear behavior observed in the central nervous
system tissue of zebrafish (Danio rerio) when exposed to femtosecond pulses at
1030 nm. At this irradiation wavelength, photo damage becomes detectable only
after exceeding a specific peak intensity threshold, which is independent of
the photon flux and irradiation time, distinguishing it from irradiation at
shorter wavelengths. Furthermore, we investigate and quantify the role of
excessive heat in reducing the damage threshold, particularly during
high-repetition-rate operations, which are desirable for label-free and
multi-dimensional microscopy techniques. To verify our findings, we examined
cellular responses to tissue damage, including apoptosis and the recruitment of
macrophages and fibroblasts at different time points post-irradiation. These
findings substantially contribute to advancing the emerging nonlinear optical
microscopy techniques and provide a strategy for inducing deep-tissue, precise
and localized injuries using near-infrared femtosecond laser pulses