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

Abstract

In their comment on Optica 8, 1559 (2021)OPTIC82334-253610.1364/OPTICA.433562, Liang and Vogel performed a theoretical calculation to show that with a 1.1 NA objective and 140 femtosecond (fs) laser pulses with a 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 [Optica 9, 868 (2022)OPTIC82334-253610.1364/OPTICA.454469]. 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 femtosecond (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 cortex of an adult mouse. Our results show that laser irradiation at 930 nm wavelengths with power ranging from 19 to 300 mW can all lead to a brain tissue temperature rise of more than 0.3 K at 1 s at a distance of 25 µm from the laser focus, and the 300 mW laser can cause a temperature increase of more than 28 K at 6 s. 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.