The interaction mechanism between laser and solid

Let us first look at the interaction mechanism between the laser and the solid: 　　 (1) The laser first excites the electrons in the solid, and the electrons absorb the energy of the photon within 100 fs and jump to a higher energy level; 　　 (2) Because the electrons are relative to the crystal lattice The temperature is higher, so it is in a non-equilibrium state. In order to achieve equilibrium, electrons will transfer energy to the lattice within 1ps; 　　(3) Within 10ps, these energy will be gradually transferred to the inside of the material.　　 Therefore, for picosecond laser processing of about 10 ps, u200bu200bthe material has enough time to transfer heat to its interior, and then the etching takes place, so the thermal effect is actually unavoidable. For femtosecond lasers, the pulse action time is already less than 1 ps, and the electrons do not have enough time to transfer energy to the crystal lattice. As a result, a large number of plasmas are generated on the surface of the material, and the energy is dissipated along with the removal of the material, so a strong etching effect occurs. In other words, when the laser pulse width is much smaller than the heating time of the lattice, the ablation time does not depend on the laser pulse width. Figure 1 shows the comparison between long pulse laser and femtosecond laser. From the above three stages, we can see that the development of laser processing is basically synchronized with the development of lasers. A new technological breakthrough may usher in a leap in the application of laser industry. Of course, it will also bring new questions. : 　　 First, since the picosecond laser cannot completely avoid the thermal effect during processing, is it meaningless for fine processing, or has it lost its value and necessity? of course not. Although the picosecond laser cannot achieve fine cold processing, compared to the femtosecond laser, the price is lower, the structure is relatively simple, and the power is higher. Therefore, for such a transitional product of ultrashort pulse technology, how to make full use of it is worthy of our thinking. The current method worthy of reference is to triple the frequency of the picosecond laser and use the 'photoetching effect of ultraviolet light and the material. High-energy photons directly destroy the chemical bond of the material through the 'cold treatmentAt present, this system has great advantages for processing transparent materials. There are many such lasers, such as the 15 ps 355 nm laser of Photonics industry. 　　 Second, the femtosecond laser can achieve non-thermal etching, is it pulsed? The shorter the width, the finer the processing? In fact, the etching effect is related to six main factors: average power, pulse width, wavelength, spectral width, single pulse energy, and frequency. 　　 Generally speaking, the shorter the pulse width, the more It is difficult to obtain high average power. Therefore, in this respect, short pulses are at the expense of average power. 　　 In addition, the narrower the pulse width, the larger the spectral width. Therefore, the color difference will also affect the processing effect. 　　 is followed by The influence of wavelength. For general materials, wavelength has little effect on the etching effect. It is only considered when processing transparent materials (such as SiO2), and the issue of wavelength should be considered. As mentioned earlier, transparent materials have little effect on the etching effect. Ultraviolet light has strong absorption, but the transmittance of visible and infrared light is very high. Therefore, the triple frequency light of ultrafast lasers is usually used for processing. For lasers of tens of femtoseconds, the spectral width reaches 20 nm, even 50 nm, so the triple frequency efficiency is very low. But picosecond laser can get a higher triple frequency output. 　　 Frequency will affect the processing speed. Generally speaking, the higher the better, but usually only oscillation The oscillator is only about 80 MHz, and the single pulse energy of the oscillator is too low, so it needs to be amplified. The frequency of the amplifier generally ranges from 1 kHz to several hundred kilohertz. 　　 Therefore, for ultra-fast laser processing, you cannot blindly pursue ultra-short pulses. , We must look at the actual demand. And the extremely short pulse and high-power femtosecond lasers are expensive. As far as the current fine processing requirements are concerned, lasers with a frequency of hundreds of femtoseconds and hundreds of kilohertz should be enough to cope with the usual For fine processing requirements, such as the D2.fs industrial laser (4 W, 400 fs, 300 kHz, 1025 nm, 20 μJ @ 200 kHz) from JENOPTIK. The laser uses a fiber laser oscillator, which can achieve long life and avoid Maintenance is a bright spot. From the point of view of indicators, price, stability, and overall cost performance, the laser is very good. 　　 In the past two years, femtosecond laser technology has been rapidly improved, China’s economy has also developed steadily, and more overseas Advanced products are pouring in. For example, KMlabs' 12 fs oscillators and peak power terawatt amplifiers, menlosystems' femtosecond fiber lasers, and Lumera and photonics industry's picosecond lasers are some of the more successful commercial ultrafast lasers. 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