After giving reports on femtosecond laser processing at domestic and foreign academic conferences, I am often asked: What is the difference between femtosecond laser and other laser processing?
This is a question that attracts widespread attention but cannot be answered in just a few words, just like the seemingly simple question of why glass is transparent. We need to analyze and elaborate on principles based on theory. Here, the femtosecond laser we refer to refers to the laser with a pulse width of 1-1000 fs (1 fs=10-15s), and other lasers refer to pulsed lasers or continuous lasers with a pulse width greater than 1000 fs (1 ps).
During the laser processing process, factors such as the wavelength, energy (or power), pulse width, spectrum, pulse frequency, polarization, phase, etc. of the laser need to be considered. At the same time, the focusing system, scanning speed and direction, and the composition and structure of the material to be processed must also be considered. and form, and even the environmental conditions such as temperature and atmosphere in which the substance is exposed. Pulse width is one of the most important laser parameters with universal influence. For the sake of simplicity, the following discussion assumes that other conditions are basically similar (in fact, this condition is difficult to establish), but the pulse width is different. Femtosecond laser cutting also mainly refers to the near-infrared femtosecond laser with a wavelength near 1 μm, which is mainly titanium sapphire, Yb3+ doped crystal and fiber laser.
Femtosecond laser systems are expensive
Femtosecond laser has now begun to be used in cutting, drilling, welding, marking, stripping, repair and other processing fields, but its application is not very common yet. On the one hand, other lasers are also used well in many situations, and on the other hand, femtosecond lasers are very expensive. The price of femtosecond laser is much more expensive than long pulse laser and continuous laser. Other femtosecond and picosecond lasers with similar parameters sometimes differ by hundreds of thousands of yuan.
The main reasons why femtosecond laser is expensive are:
- 1) According to the Fourier change relationship, a gain medium with a broad spectrum, such as titanium sapphire, is required to generate ultrashort femtosecond pulses. The bandwidth of the gain medium determines the ultimately achievable pulse width. Therefore, the requirements for use as femtosecond laser gain media will be higher. Currently, they are mainly titanium sapphire, some Yb3+ doped crystals and glass fibers;
- 2) Femtosecond laser pulses generally need to be realized by locking film technology. The femtosecond pulses with large pulse energy used for processing need to be further realized by further processing the femtosecond pulses with low pulse energy through pulse broadening-amplification-compression regenerative amplification technology. , the femtosecond laser system requires finely processed optical components such as chirped mirrors and high-power pump sources, and the laser system is relatively complex.
Femtosecond laser has very good characteristics
Femtosecond processing has many advantages, first of all, it is reflected in its high precision, its characteristics and threshold effects based on multi-photon absorption, and the negligible thermal effects during the processing (that is, the commonly emphasized cold processing). It should be noted here that this refers to a single pulse or a relatively low pulse frequency. In addition, it is only a relative term. The laser wavelength and the characteristics of the target material are ignored here.
In principle, due to its short pulse width, femtosecond laser can obtain extremely high peak power (pulse energy/pulse width) with low pulse energy. When the material is further focused with an objective lens, etc., the energy density near the focus is very high. High, it can cause various strong nonlinear effects.
Laser processing can be considered as a laser-induced reaction, which in principle is divided into induced molecular vibration and electronic excitation. The former is a thermal reaction, and the latter interacts with chemical bonds associated with electrons in the outer shells of atoms that make up matter. Considering the energy band structure of matter, generally long-wavelength lasers such as CO2 lasers utilize thermal reactions caused by molecular vibrations, while short-wavelength lasers such as excimer lasers utilize the cutting of chemical bonds generated by electronic excitation.
Near-infrared femtosecond laser processing uses a multi-photon process, which means that although the material does not linearly absorb at the laser wavelength (λ), the light intensity near the focus is very high. By absorbing multiple (n) photons at the same time, it can reduce short wavelengths. (λ/n) light has the same effect when it is taken inside the material to illuminate the material, achieving spatially selective micro cutting structure manipulation without affecting the surface structure. This is another advantage of femtosecond laser processing.
Because multiple nonlinear processes compete to participate, the phenomena produced often exceed our predictions and imagination. When femtosecond laser interacts with materials, here we consider a medium that does not have linear absorption at the laser wavelength. First, through processes such as multi-photon absorption or ionization, the laser energy is deposited in the electronic system, and then through a series of energy transfer and transport process, resulting in a series of changes in the material. Generally, under laser irradiation, the time for electrons to absorb photons to be excited is in the fs range (during the pulse action), and then electron-phonon coupling occurs. The energy is transferred to the crystal lattice and the time for the crystal lattice to reach thermal equilibrium is within a few seconds. to the order of tens of ps. The time scale of thermal diffusion and material melting varies with different materials, and is basically on the order of tens to hundreds of ps. The time for ablation to form on the material surface ranges from a few hundred ps to ns.
Under the action of nanosecond and picosecond lasers, the laser energy deposited in the electron gas is transmitted to the crystal lattice within the time when the laser pulse irradiates the material, causing heating, melting and even ablation of the material. The thermal effect plays an obvious role in the process. The pulse width of femtosecond laser is smaller than the time scale of electron-phonon interaction, and the laser energy deposited in the electron gas ends before it can be transferred to the ion laser pulse irradiation. At this time, the temperature of the electron gas is very high, but the temperature of the ions is very low. The material undergoes a “cold” ablation process, which suppresses fluid mechanics effects, thermal effects, etc., and the processing accuracy is very high. There have been many works comparing the effect of pulse width on laser processing. It can be seen from Figure 1 that the femtosecond pulse processing structure is relatively steep and clean, and there are ridges and residues caused by thermal effects of picosecond and nanosecond lasers. Precisely due to the high precision and “cold processing” characteristics of femtosecond laser, it can be widely used in industrial fields such as microelectronics and aerospace, as well as in medical cutting treatment, such as myopia correction, brain surgery, etc.
When the pulse width is greater than 20 ps, research shows that the damage threshold of quartz glass is proportional to the square root of the pulse width, which means that thermal damage occurs to the quartz glass at this time. When the pulse width is less than 10 ps, the dependence of the damage threshold on the pulse width obviously deviates from the square root law, and material damage is mainly caused by avalanche breakdown. Under the action of a strong electric field, when an electron is accelerated to an energy higher than the band gap width, it will collide with a valence electron and produce two conduction band electrons with smaller kinetic energy. This chain process is repeated continuously, and the number of electrons increases exponentially with time. If the electron number density generated by the avalanche reaches a critical value within the pulse irradiation time, the material is broken down. Jia Tianqing et al. combined quantum mechanics with classical approximation to study the light absorption of conduction band electrons. They calculated the avalanche rate and photoionization rate respectively based on the flux-double model and Keldysh theory, and studied the evolution of the number density of conduction band electrons in the material. Through analysis, an avalanche breakdown model was developed, which explained the dependence of the material’s damage threshold and pulse width as well as the ablation depth, ablation volume and laser intensity.
When the femtosecond laser acts on metal, since the pulse width of the femtosecond laser is smaller than the time scale of electron-phonon interaction, the laser energy absorbed by the electrons ends before it can be transferred to the ions. Therefore, the temperature of electrons is very high while the temperature of ions is still very low. Femtosecond laser ablation of metal is a non-equilibrium ablation. The two-temperature model and the improved two-temperature model show that the temperature change of the lattice is related to the lattice heat conduction and electron-lattice coupling. Under the action of high-intensity (≥1014W/cm2) femtosecond laser, the ionization of the material is completed before the end of the pulse action time (~100 fs). At this time, the ablation mechanism of metal and dielectric is the same.
Femtosecond laser is cold processing, which is actually a misunderstanding
Some people often think that femtosecond laser must be cold processing. In fact, this is also a misunderstanding. Femtosecond laser processing is also related to laser pulse frequency. When numerous femtosecond laser pulses are strung together into a quasi-continuous pulse array, that is, when the pulse frequency is very high, the residual heat of femtosecond laser processing will produce a heat accumulation effect. Controlling the repetition frequency is expected to achieve the preparation of three-dimensional structures that coexist with multi-photon absorption and thermal effects that are characteristic of femtosecond lasers and long-pulse or continuous lasers, further expanding the types of micro-nano structures and the functions of materials.
The induced structure shows obvious thermal effect, and the structure increases significantly with irradiation time. During the interaction between femtosecond laser and material, the time scale of photon heating of electrons (<1ps) and electron-phonon coupling (ps) is much smaller than the time scale of thermal diffusion (>0.1ns), so the residual heat of laser processing will generate A hot spot. For low repetition frequency femtosecond lasers, due to the long interval between pulses, the temperature of the laser focus area has dropped to the ambient temperature by the time the next pulse reaches the material. For high repetition frequency femtosecond lasers, due to the short interval between pulses, when this time is shorter than the time required for the diffusion of the thermal field generated by laser irradiation, when the next pulse reaches the sample, the previous pulse is generated The thermal field has not completely dissipated, which will lead to the accumulation of heat. As the irradiation time prolongs and the number of laser pulses increases, the temperature of the laser focus area will gradually increase until it reaches dynamic equilibrium. Although the thermal field generated during high repetition rate femtosecond laser irradiation will increase the size of laser-induced micro-nano structures, it is also crucial to the formation of certain micro-nano structures.
There are many descriptions on the Internet that deify femtosecond lasers. We need to restore the true colors of femtosecond lasers, use femtosecond lasers to the extreme, and use them in areas where they can truly exert their characteristics, instead of “using atomic bombs to bomb mosquitoes.” .