Femtosecond lasers are lasers with a pulse width of 1-1000 Fs (1 Fs =10-15s).Other lasers are pulsed lasers or continuous lasers with a pulse width of 1000FS (1ps).
Laser processing needs to consider the laser wavelength, energy (or power), pulse width, frequency, pulse frequency, polarization and stage, focusing system and scanning speed and direction, material composition, structure and shape of the processed object, even temperature, atmosphere and other material environmental conditions.
Femtosecond processing has many advantages, first of all reflected in its high precision, based on the characteristics of multi-photon absorption and threshold effect, the thermal effect in the treatment can be ignored (that is, the cold treatment is often emphasized).It should be noted that this refers to the case of monopulse or low pulse frequency, but relatively speaking, the wavelength of the laser and the properties of the object material are ignored here.
Femtosecond laser has a short pulse width and can theoretically obtain a high peak power (pulse energy/pulse width) at a lower pulse energy.When the laser focuses the material further through the objective lens, various strong nonlinear effects will occur due to the high energy density near the focus.
Laser processing can be regarded as a kind of laser induced reaction, whose principle can be divided into induced molecular vibration and electron excitation.The former is a thermal reaction, while the latter interacts with the chemical bonds formed by electrons in the atomic shells that make up the material.Considering the energy band structure of matter, long wavelength lasers such as CO2 lasers generally adopt thermal reaction caused by molecular vibration, while short wavelength lasers such as excimer lasers adopt chemical bond cutting caused by electron excitation.
Near-infrared femtosecond laser processing through the multiphoton process, that is to say, although the material in laser wavelength (lambda) no linear absorption, but near the focal point of light intensity is very high, by absorbing multiple (n) photons at the same time, the shorter wavelengths of light (lambda/n) material in the material has the same effect, through selective control of the micro structure of the space, and shall not affect the surface structure, which is another advantage of the femtosecond laser processing.
When the femtosecond laser interacts with the material, we consider a medium that has no linear absorption within the laser wavelength range.First, the laser energy is absorbed or ionized by multiple photons in the electronic system, and then passes through a series of energy transfer and transmission processes, leading to a series of changes in the material.In general, under laser irradiation, electrons absorb the FS(action) pulse process within the excitation time range of the photon, followed by electron phonon coupling, and lattice energy transfer and thermal equilibrium orders from a few dollars to dozens of Ps.Thermal diffusion time scale and melting of materials using different materials, material surface ablation time from hundreds of PS NS.
Under the action of nanosecond and picosecond lasers, the laser energy deposited in the electron gas is transmitted to the crystal lattice during the laser pulse irradiation of the material, which causes the heating, melting and even combustion of the material.In this process, the thermal effect is obvious.However, the pulse width of the femtosecond laser is smaller than the time scale of the electron-phonon interaction, and the laser energy deposited in the electron gas has no time to transmit to the ion laser pulse.Now, the electron gas is very hot, and the ion is very cold.The material has a "cold" ablation process, which inhibits hydrodynamics and thermal effects.High machining accuracy.Therefore, it is widely used in microelectronics, aerospace and other industrial fields, as well as medical fields, such as myopia correction, brain surgery and so on.