Measuring the internal temperature of dielectrics machined by the ultrashort laser pulse through the black-body irradiation method
Authors: Wang Cheng-Wei  , Zhao Quan-Zhong  , Qian Jing  , Huang Yuan-Yuan  , Wang Guan-De  , Li Yang-Bo  , Bai Feng  , Fan Wen-Zhong  , Li Hong-Jin  ,   School of Physics Science and Engineering, Tongji University, Shanghai 200092, China;,   State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Black-body irradiation method can be utilized for measuring the instantaneous temperatures of electrons and lattice in dielectric machined by the ultrashort laser. One ultrashort laser pulse, of which the pulse energy and pulse duration are 240 J and 599 fs respectively, is focused into the fused silica by objective lenses with a magnification of 10 times. The focal point is at the position of 874 m. The microstructure induced by laser near the focal point is 16 m wide and 104 m long. The central region of the microstructure is heavily damaged, and the marginal region is slightly modified. The black-body irradiation spectra are recorded by the system that is composed of objective lenses, a fiber with two lenses, a spectrometer and an intensified charge coupled device (ICCD). Furthermore, other imaging elements can also be used as alternative to objective lenses, for measuring black-body spectra. The image point, which is conjunctive with the machined region due to the imaging effect of the objective lenses, is coupled into the fiber by one lens. Another lens collimates the diverging light beam from the fiber. The collimated light is incident into the spectrometer and dispersed on the ICCD. Because the minimum gate width of ICCD is much larger than the coupled time of electron and lattice, the temperature of electron equals that of lattice when they are characterized by the black-body irradiation method. The temperatures of the electrons and the lattice are regarded as the temperature of dielectric. When the system acquires the reflection peak of incident ultrashort laser, the delay is set to be 0 ns, and the central wavelength of the peak is 784 nm. Therefore, to eliminate the reflection peak, the second harmonic and supercontinuum spectra, the delay for black-body irradiation acquirement is set to be above 6 ns and the machined region should be confined inside the dielectric. The system collects the black-body spectra emitted by the heat-affected zone in fused silica 981 ns after the fused silica has been irradiated by single ultrashort laser pulse. And then the spectra are fitted by the Planck formula to obtain the temperature of dielectric. After the dielectric is processed by the ultrashort laser pulse, the valence electrons of the dielectric transit to the conduction band via strong filed ionization and avalanche ionization. The plasma with high temperature and pressure moves outward in the form of shockwave. The shockwave transfers energy by convection after fused silica has been machined by laser pulse. Due to inverse Bremsstrahlung effect during the avalanche ionization, nearly all the incident laser energy is absorbed by the fused silica. The irradiated energy is only 1.3% of the absorbed energy, so the ways of heat transfer are mainly convection and heat diffusion. 21 ns later the shock wave turns into acoustic wave, so central gaseous fused silica affects the surrounding region through heat diffusion and the temperature of fused silica decreases slowly. The temperature of fused silica is 5333 exp(-t/1289) K at time t (unit: ns). The temperature drops down to room temperature 3.72s after the fused silica has been irradiated by one ultrashort laser pulse. If another laser pulse arrives at fused silica before 3.72s, the temperature rises on the basis of the previous laser pulse. In other words, the heat accumulation effect cannot be ignored if the repetition rate of ultrashort laser is more than 269 kHz.