Probing neuronal functions with precise and targeted laser ablation in the living cortex: reply

In the comment on the article [Optica 8, 12 (2021)], the authors performed theoretical calculation to show that with a 1.1 NA objective and 140 fs laser pulses with single pulse energy of 2.2 nJ, and 80 MHz repetition rate, the focal point temperature rises 0.3 K and reaches equilibrium after 100 μs in water. They suggest that the damage to brain tissue by laser could not be caused by thermal effects but rather by plasma-mediated chemical effects. To quantify the thermal accumulation due to the fs laser illumination in living animals, we used a thermocouple sensor to measure the temperature change in the vicinity of the laser focus at a depth of 300 μm in the adult mouse cortex. Our results show that at 930 nm wavelength, 25 μm from the laser focus, 19-300 mW laser can all lead to a brain tissue temperature rise of more than 0.3 K at 1 second and the maximum equilibrium temperature rise of more than 28 K. These experimental measurements are significantly higher than theoretically calculated values in the comment. These results suggest that the thermal accumulation effect of focused low-energy pulses from fs laser oscillators could contribute significantly to the collateral damage in the living brain.