黑体辐射法测量电介质内部被超短激光脉冲加工后的温度

黑体辐射法可用于测量电介质内部被超短脉冲激光加工后,电子和晶格的瞬时温度.当一个超短激光脉冲通过物镜聚焦到石英玻璃内部时,在焦点附近诱导出微结构.微结构中热影响区的最大宽度为16μm,热影响区发出的黑体辐射谱通过物镜、带耦合透镜的光纤、光谱仪以及ICCD组装成的系统记录.测试系统收集了电介质内部被单个激光脉冲辐照后,热影响区发射的黑体辐射谱,然后用Planck公式拟合黑体辐射谱,得到电介质温度.电介质被超短激光脉冲辐照后,首先电介质中的价带电子通过强场电离和雪崩电离跃迁到导带,高温高压的等离子体以冲击波的形式向外运动,通过对流方式传递能量,该过程发生在激光辐照石英后21 ns内.21 ns后冲击波转化为声波,中心的气态石英通过热扩散方式影响周围的固态区域,石英温度缓慢下降.在时刻t(单位ns),石英玻璃的温度为5333 exp(-t/1289)K.石英经过3.72μs将冷却到室温,因此重复频率在269 k Hz以上的激光,加工石英玻璃时具有热累积效应.

Measuring the internal temperature of dielectrics machined by the ultrashort laser pulse through the black-body irradiation method

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 μJ 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 μm. The microstructure induced by laser near the focal point is 16 μm wide and 104 μm 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 9–81 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 3.72μs after the fused silica has been irradiated by one ultrashort laser pulse. If another laser pulse arrives at fused silica before 3.72μs, 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.