Quartz glass has the advantages of high mechanical strength, good heat resistance, small thermal expansion coefficient, and stable chemical properties, so it is widely used in photolithography, space optics and laser fusion systems. These fields mostly involve the precision machining of quartz glass microstructures, such as the fabrication of surface microchannels and the thinning of surface grooves.
However, the hardness of quartz glass is high and it is easy to be broken. Traditional processing methods such as photolithography corrosion, ultrasonic wave and ion etching ablation are difficult to perform precise processing of microstructures such as dots, lines, and surfaces on its surface. Femtosecond laser has unique advantages such as ultra-short pulse width and ultra-high power density, which makes it have the characteristics of small action area, high processing precision, and breakthrough of diffraction limit when interacting with substances. Therefore, femtosecond laser is carried out on the surface of quartz glass. Precision machining possible
Femtosecond Laser Ablation Processing Technology
Laser micromachining traditionally refers to the processing of materials in the micron to submicron range. Femtosecond laser is a new type of processing method gradually developed. Its time resolution is ultra-fast, so it shows a different mechanism from the interaction between traditional long-pulse-width laser and matter, and the thermal effect of the long-pulse laser can be ignored .
As a non-contact processing method, femtosecond laser technology has the characteristics of obvious threshold effect, extremely small heat-affected zone, breaking through the diffraction limit precision, and three-dimensional processing of transparent materials. It injects all its energy into a small area of action at a high speed, and instantly produces high-density energy deposition, which changes the absorption and movement of electrons, and realizes high-quality, high-precision, and high-efficiency micro-nano processing. A potential surface micro-nano structure processing technology .
In addition, according to the spot characteristics of the femtosecond laser beam, by adjusting the peak energy and then controlling the laser intensity near the focal point, the processing area of the femtosecond laser can be precisely controlled, making it possible to make the processing size break through the diffraction limit, which is a promising tool for micro- and nano-scale devices. Ultrafine machining offers feasibility .
Mechanism Of Thinning Quartz Glass By Femtosecond Laser Ablation
The interaction process between femtosecond laser pulses and quartz glass is very complex, involving many factors, such as laser pulse energy, scanning speed, scanning times, etc. .
Compared with the visible light band, quartz glass is a transparent dielectric. When the femtosecond laser is used to burn candles, due to the extremely high peak power, a nonlinear ionization process will be excited inside the quartz glass material, causing the bound valence band in the material. The electrons transition to the conduction band by absorbing multiple laser photons, become conduction band free electrons, and eventually lead to the deposition of incident laser energy into the material.
The research believes that the nonlinear ionization mechanism of transparent dielectric materials is mainly avalanche ionization, collision ionization and multiphoton ionization. The free electrons formed by these two nonlinear ionization effects will strongly absorb laser energy. When the conduction band electrons in the dielectric material When the density reaches or exceeds the critical density value, laser damage to the dielectric material will occur.
the femtosecond laser processing system consists of a high-power femtosecond laser system, a microscopic optical path system, a high-precision six-dimensional precision electronic control platform, a control system host and a pulse shaper. Laser precision thinning of quartz glass is based on the precise layer-by-layer removal of point, line, and surface laser ablation processes.
Common Femtosecond Laser Ablation Thinning Quartz Glass Methods
Relevant scholars have carried out extensive research on the femtosecond laser processing of quartz glass, mainly including three methods: corrosion after femtosecond laser modification, water-assisted femtosecond laser ablation, and femtosecond laser direct ablation removal.
1. Corrosion after femtosecond laser modification
The modified etching method can process any complex hollow structure. Bellouard et al. used the femtosecond laser modified etching method to prepare channels and microgrooves with large aspect ratios inside and on the surface of quartz glass, respectively.
2. Water-assisted femtosecond laser ablation
The water-assisted femtosecond laser ablation method is to focus the femtosecond laser directly on the sample-liquid interface for processing, so that the debris generated during the processing can be taken away in time to ensure the continuous processing. For example, Xing Songling et al. used a 400 nm femtosecond laser obtained by 800 nm and frequency doubling to etch the surface of quartz glass by a water-assisted method to obtain micropores with high quality and high aspect ratio (hole diameter of 72 μm and hole depth of 1824 μm). by using the water-assisted femtosecond laser ablation method to drill holes on the rear surface of quartz glass researcher obtained deep holes with a diameter of 4 μm and a depth of more than 200 μm.
3. Femtosecond laser direct ablation
The femtosecond laser direct ablation method is to focus the laser directly into the transparent material, and directly process it without any auxiliary liquid, and the processing process is simpler. Chinese researchers studied the effect of polarization state on the surface quality of femtosecond laser processing of quartz glass, but the ablation surface has the characteristics of low sides on both sides, which makes the bottom surface flatness low, and did not study the geometric characteristics of grooves.
Application Of Micro-functional Structural Components
1. Micro lens
The common basic application of microlenses is imaging, and other applications include beam shaping and collimation. A single microlens can achieve functional combination and integration by coupling optical signals and optical fibers. Chen et al. first processed square and regular hexagonal array features on the surface of quartz glass by femtosecond laser, and then used a certain concentration of hydrofluoric acid solution for etching, and finally prepared and processed regular polygonal micro-concave lens arrays with regular shapes, as shown in the figure below. Show
2. Fiber Bragg Grating
Fibers with gratings inside or on their surfaces are called fiber gratings. The femtosecond laser can realize processing and removal in the inner space of the fiber or on the surface of the fiber. Its spatial resolution is high, and the grating period can be adjusted as needed. Inspection measurement.
3. Microcavity structure
The microcavity structure is a disc microstructure with a size in the micron or submicron order, and is mainly used in the transformation of optical signals, nonlinear optical systems, and measurement and control sensing systems. Lin et al. used a combination of ultrashort pulse laser direct writing and corrosion to fabricate a three-dimensional space annular microcavity structure with high Q value on quartz glass, as shown in the figure below.
This microcavity structure is a passive device and can be coated with a gain medium on the surface of the structure, which can be used to improve the optical output characteristics of the system.
4. Other
In addition to being widely used in the above fields, femtosecond laser-induced preparation of quartz glass micro-functional structures is also widely used in micro-channels, optical storage, optical waveguides, ordinary gratings and micro-disk lasers. In addition, according to different applications, micro-functional structural parts are also used in various fields such as semiconductor materials, metal materials and polymer materials.
