威廉希尔官方下载Mar 20, 2020
The laser is regarded as one of the greatest inventions in the 20th century. With the end of three industrial revolutions, the laser will be the key to lead the fourth industrial revolution. The emergence of laser has greatly promoted the development of the industry. The laser has become the most advanced and widely used means in machining because of its advantages such as high power, easy focus, high brightness, and good directivity. Laser processing has the advantages of high precision, high speed, and low cost. It can be automatically controlled by computer programming. It can process the structure with a complex shape. Because it is non-contact processing, it will not damage the material and is safe and reliable.
Classification and characteristics of laser processing
According to the mechanism of interaction between laser and matter, laser processing can be divided into two categories: laser thermal processing and non-thermal processing. The types of laser used in thermal processing and non-thermal processing are different. The long-pulse laser or continuous laser is usually used in thermal processing, and the ultrashort pulse laser such as picosecond and femtosecond is usually used in non-thermal processing.
Laser thermal processing uses the thermal effect produced in the process of laser irradiation of materials. The molecular system of the irradiated materials needs to constantly obtain energy from the laser irradiation and convert it into its own internal energy. The temperature of the irradiated area rises sharply to achieve the melting point and boiling point of materials, melting and removal, and achieve the purpose of processing. Because it takes a long time for the energy of the laser to be converted into the internal energy of the molecular system, long pulse laser is often used in thermal processing. This processing method is simple and direct and has been widely used in industrial manufacturing, such as laser cutting, laser additive manufacturing, etc. However, due to the inevitable thermal diffusion in the processing, the accuracy and roughness of laser thermal processing are limited.
Non-thermal processing is to use the nonlinear effects (such as nonlinear ionization, surface scattering, etc.) caused by the disturbance of the electronic system of materials, through the transition and ionization of the electron absorption photons, the physical and chemical properties of materials are induced to change, thus leading to the generation of some novel effects (such as two-photon polymerization, laser self-assembly, etc.), using these novel effects to achieve the increase The purpose of machining accuracy and optimization. Because the energy exchange between the electron system and the laser can be completed in an instant, the non-thermal processing usually uses an ultrashort pulse laser. This method has high precision and various processing methods, which is one of the research hotspots in the field of laser processing.
Advantages and disadvantages of traditional femtosecond laser processing
Ultra-high peak power and ultra short pulse duration are two main advantages of femtosecond laser. The ultra-high peak power is enough to induce a variety of nonlinear effects, which enriches the laser processing methods. The ultra-fast time characteristics also make the interaction process between femtosecond laser and materials very short. The light energy absorbed by the laser irradiation area can not even be transferred to other areas, so as to ensure that the laser energy can be accurately deposited in the irradiation range and realize ultra-fine processing.
At present, the femtosecond laser has been widely used in the field of micro and nano processing, mainly including laser direct writing and laser mask. However, due to the diffraction limit of the machining system, it is impossible to reduce the laser irradiation area without limitation, which limits the further improvement of the machining accuracy. At the same time, due to the different nonlinear characteristics of different materials, femtosecond laser processing has a strong dependence on materials. The same processing method often shows different processing effects for different materials.
Advantages of ultraviolet femtosecond laser processing
With the development of modern industry, the requirement of machining accuracy is increasing, and one of the main factors affecting the laser machining accuracy is the diffraction limit of the machining system. The diffraction limit is a physical parameter describing the imaging or processing accuracy of an optical system. The smaller the diffraction limit is, the higher the processing accuracy is. Generally, the diffraction limit is directly proportional to the wavelength of the incident light, so reducing the laser wavelength becomes the most direct and effective means to improve the diffraction limit. For example, the UV lithography technology widely used in the current industry is to improve the processing accuracy by reducing the laser wavelength.
UV laser refers to the laser whose wavelength is less than 380nm. Compared with the wavelength commonly used by femtosecond laser (mainly in the visible light band, 380nm-760nm), the processing accuracy of the UV femtosecond laser is higher. At the same time, due to the short wavelength of the ultraviolet femtosecond laser and the large energy of a single photon, photons can directly cut off the binding bonds of molecules or atoms, which is essentially a photochemical reaction, basically without melting phenomenon, thus limiting the influence of thermal effect. On the other hand, the UV band is the sensitive band of many polymers, such as photoresists. These polymers will produce a two-photon polymerization effect under the irradiation of ultraviolet femtosecond laser, which makes the flowing colloid polymerize into a solid with high mechanical strength. After processing, the photoresist will be washed away, and the desired structure can be obtained. By using this principle, super fine 3D structure processing can be carried out.
Characteristics and advantages of femtosecond machining in vector and vortex fields
The traditional femtosecond laser processing mainly focuses on the energy characteristics of the laser. The nonlinear effect of materials is induced by the ultra-high-energy of the femtosecond laser field, so as to achieve the purpose of processing. In the process of interaction between light and matter, there is not only the absorption of energy but also the exchange of momentum, which means that the new laser mode can give full play to its advantages in the field of femtosecond processing.
The Vector field and vortex field are two of the most typical new laser modes. Their spatial topological characteristics of polarization and phase make the field have some special physical properties. For example, the vector field can converge to the focal spot beyond the diffraction limit, which is smaller in size, so the machining accuracy is higher. On the other hand, the photon angular momentum carried by the field itself can exchange momentum with the matter. For example, the vortex light field with spiral phase structure carries the photon orbital angular momentum, which drives the particles to rotate around the fixed axis; the left-handed or right-handed circularly polarized light carries the photon spin angular momentum, which can induce the particles to rotate; the vector light field whose polarization state changes with the spatial position can show the interaction between the angular momentum. In the same way, the momentum characteristics of vector and vortex fields can also be used in femtosecond laser processing, such as the use of vortex fields to induce chiral structures, the use of vector fields to induce complex patterns and so on.
Compared with the traditional femtosecond laser processing, the vector and vortex field femtosecond laser processing produced by the high-power ultraviolet laser system with controllable space-time characteristics makes the processing structure diversified and complicated. By designing the phase and polarization distribution of the light field, we can get a variety of surface patterns and even complex three-dimensional topology. 威廉希尔官方下载ing femtosecond laser pulse shaping technology combined with space-time light modulation technology and space-time focusing technology to modulate the ultrafast laser pulse in the time domain and frequency domain, and realize the three-dimensional micro-nano processing and practical application in different materials. These technologies are expected to play an important role in new integrated optics and micro nano-optics.
Advantages and potential applications of ultraviolet, vector, and vortex femtosecond light fields
With the continuous development of the industry, the traditional femtosecond laser processing technology can not meet the growing industrial demand, so it must be developed and optimized. UV femtosecond laser processing technology is an effective way to improve the processing accuracy and has great application value in industrial manufacturing. The femtosecond laser processing technology of vector and vortex field has changed the traditional single processing mode, making the laser processing more flexible and diverse. In addition, UV vector and vortex femtosecond laser processing technology is also the practice and verification of the theory of the interaction between light and matter, which is helpful to reveal the deeper physical mechanism and has positive scientific significance.