Cumulative Defocusing Of Sub-Mhz-Rate Femtosecond-Laser Pulses In Bulk Diamond Envisioned By Transient A-Band Photoluminescence

High-rate direct femtosecond (fs) laser writing is a well-established technology for fabricating various micro-optical elements in bulk dielectrics. In this technology, the "heat accumulation" effect, occurring during high-repetition rate (similar to 1 MHz) exposure in dielectrics by a fs laser, enables ultralow-energy micro-modification via cumulative heating. Meanwhile, in this work in the transient multi-photon A-band photoluminescence studies, we demonstrate that this effect underlies dynamic thermal lensing even in diamond with its high thermal conductivity, dynamically shifting the laser focus upstream. Our study paves the way for more precise, accurate and robust direct fs-laser writing of advanced three-dimensional structures in diamond and other dielectrics for a variety of photonic applications. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement