1清华大学机械工程系, 北京 100084
2北京理工大学机械车辆学院, 北京 100081
3清华大学化学系, 北京 100084
Significance Transparent dielectrics generally refer to materials with a transmittance of more than 80% in the visible light range, such as glass, gemstones, diamonds, some organic polymers, and various crystals. They have been extensively used in aerospace, electronic elements, flexible photonic devices, and other advanced fields because of their high transmittance and corrosion resistance. However, processing transparent dielectrics with traditional methods like mechanical physical means in the micro-nano scale is rather difficult because they can easily cause breakage and cracks.
With ultrahigh intensity and ultrashort pulse duration, an ultrafast laser (pulse duration <10 ps) can break the diffraction limit due to its wide material adaptability and extreme precision and can minimize the heat-affected zone, providing an advanced approach for micro-nano fabrication. Hence, an ultrafast laser has become the most appropriate tool for the micro-nano fabrication of transparent dielectrics. The corresponding phenomena and physical mechanisms in the ultrafast laser fabrication of transparent materials must be investigated to optimize the micro-nano scale processing of transparent dielectrics and utilize it in more application fields.
Progress The structural changes induced by the ultrafast laser in the irradiation area can be divided into three types according to the different pulse energy levels focused inside the transparent dielectrics: 1) low pulse energy can induce the formation of areas where the refractive index change occurs inside the dielectrics, 2) under moderate pulse energy, a nanograting structure can be induced, and 3) high pulse energy induces a nanovoid structure at the irradiation point.
If the energy of the laser incident into transparent dielectrics slightly exceeds the modification threshold, the structural change in the irradiation area will be induced, which causes the refractive index to change from the initial n0 to n1(
If the energy of the pulse incident into transparent dielectrics exceeds the material's modification threshold, but is less than the optical breakdown threshold, the formation of a nanograting structure will be induced, which consists of multiple layers of materials with different refractive indexes and thus it results in the birefringent change of the irradiation area. The mechanism behind this phenomenon can be explained in three ways: some studies believe that the interference between the incident laser and induced plasma accounts for the nanograting formation, some studies attribute the structural change to the asymmetric growth of the nanoplasma, and the other studies think excitons and defect assistance play a more pivotal part in the nanograting's evolution.
When the peak intensity of the incident laser is higher than 10 14 W·cm -2, the high energy density and the electromagnetic field will cause an optical breakdown of transparent dielectrics. The optical properties of the irradiated area will also be greatly changed. Meanwhile, extremely high temperature and pressure will be engendered in the center of the irradiation area. A strong expansion shock wave propagating outward and a sparse wave propagating inside the focal point will be generated under such extreme conditions. The shock wave compresses the material around the absorption volume, and the sparse wave creates a void structure in the center of the focal volume.
Numerous applications of transparent dielectrics have been implemented based on these three structural changes induced by an ultrafast laser. For instance, optical waveguides can be directly written in materials with a refractive index increase or decrease. Diffractive optical elements can be fabricated by laser regulating dielectric birefringence or chemical etching after irradiation. Micro-holes and micro-channels can be produced by direct ablation, laser-induced backside wet etching, and laser-assisted chemical etching. Information and data storage can be achieved by the unique feature difference between processed and unprocessed areas like refractive index and structural color. A micro-nano-connection can be realized by local melting and resolidification at the interface of two dielectrics under a suitable energy ultrafast pulse irradiation.
Conclusion and Prospect Interm of the incident laser energy, three types of structural changes are induced by ultrafast lasers in transparent materials: refractive index changes, nanogratings, and nanovoids. The micro-nano device preparation method developed based on the ultrafast laser-induced structural changes has shown advantages and potentials in the application of the micro-nano fabrication of transparent dielectrics. Research on the ultrafast laser micro-nano processing of transparent dielectrics can not only help understand the interaction between dielectric materials and lasers, but also provide various applications in the micro-nano field based on the phenomena such as ultrafast laser-induced refractive index changes and nanograting structures. With the continuous development of the ultrafast laser technology and an in-depth understanding of the ultrafast laser micro-nano fabrication process, research on the ultrafast laser micro-nano processing of transparent materials will make a new progress in aerospace, biomedicine, energy engineering, and other fields.