Create femtosecond laser optics coatings - Sputtering coating
Introduction
Optical coatings alter lens surface reflectivity across applications from spectacles to high-power lasers. Femtosecond lasers emit ultra-short pulses with durations measured in femtoseconds (1 femtosecond = 10^-15 seconds) and have very high peak power. Key femtosecond laser coating attributes are low dispersion and high damage thresholds. This article explores three common sputtering techniques for dielectric and metal optical coatings, explaining the physics behind them. It also discusses combining metal and dielectric layers in the coatings. In short, sputtered films enable tailored femtosecond laser optics by layering metals and dielectrics.
Ion Bombardment Coating
Sputtering involves ion bombardment that excites particles from solid targets. The accelerated ions strike and collide with target atoms, initiating cascades as ions and recoils interact with other atoms. Most scattered particles remain in the target, but some recoils reach the surface and eject. These ejected atoms can migrate to the substrate, condensing as a film. In summary, ion collisions in the target trigger atomic collisions that scatter some atoms to the surface which sputter away. The ejected atoms then deposit, forming the sputtered coating. Careful ion beam targeting controls the sputtering process for uniform, adherent coatings.
Magnetic-Enhanced Sputtering
Ions originate from gas discharges, either DC or RF, adjacent to the target. DC sputtering utilizes high purity metal targets, while RF enables dielectric target sputtering. Introducing reactive gases like oxygen or nitrogen creates compound coatings. In short, sputtering relies on gas ionization near the target and can leverage reactive gases to deposit coatings. Both DC and RF power sources enable sputtering from metallic or insulating targets.
Initially a lab technology, magnetron sputtering is now an efficient industrial coating process. It produces high performance optical coatings, especially for visible and near-infrared spectra.
Ion Bombardment Sputter Deposition
Ion beam sputtering utilizes a separate ion source, often supplying reactive gases like oxygen or nitrogen. This enables precise reactive ion production for superior layers. In short, dedicated ion sources allow tight control over sputtering ions and reactions.
Magnetron sputtering integrates ion generation, target, and substrate while ion beam sputtering separates them. This complete separation in IBS enables independent optimization of each component for superior control. In contrast, the tight integration in magnetron sputtering couples the components.
Sputter coater
Characteristics of Sputtered Films
The high kinetic energy of sputtered particles creates femtosecond optical films with advantageous properties like: amorphous microstructure, high density, low stray light loss, thermally and environmentally stable optics, high laser-induced damage threshold, strong mechanical stability, and intrinsic oxidation without external heating for minimal absorption. In summary, sputtering's energetic deposition drives exceptional thermal, optical, and mechanical femtosecond film performance.
Heat Vaporization and Electron Stimulated Vapor Deposition
Thermal and electron beam evaporation frequently produce optical coatings. Also called evaporation, these techniques mainly deposit UV coatings. In summary, heating via hot air or electron beams enables common optical coating production, especially effective for UV applications.
Evaporation sources at the chamber bottom contain coating materials. Electron guns or resistive heating methods bring the material to evaporation, depending on properties like melting point and optical specifications. Vaporized material then deposits on substrates above. In summary, heating techniques tailored to the coating material located in lower sources vaporize and deposit optical coatings on the chamber substrates.
Substrates mount to rotating holders on the chamber top, ensuring uniform coatings. Substrate heating from 150-400°C also occurs, with temperatures dependent on the substrate and coating material. In summary, substrate rotation and strategic heating enable optimized uniform depositions during the top-down thermal evaporation process.
heat thermal evaporation coating
Merits of Electron Beam Coatings
Unlike sputtering, evaporation's low energy deposition (1eV) produces coatings with lower density and microcrystals. Consequently, oblique light losses can reach 10%, although this depends on wavelength. In short, the gently deposited evaporated films, while microcrystalline and porous, still demonstrate substantial scattering losses.
Environmental moisture diffusion alters the porous evaporated films, shifting reflection bands by about 1.5% of wavelength. However, evaporated coatings offer a high laser damage threshold. Consequently, despite atmospheric sensitivity, thermal evaporation remains widely used for lasers and optics needing microcrystallinity. In summary, while susceptible to environmental effects, evaporated optical coatings provide useful microstructured, high damage threshold depositions.
Conclusion
Femtosecond laser optical coating technology is a highly advanced field that is crucial for enhancing the performance of femtosecond laser systems. With the continuous advancement of femtosecond laser technology and its expanding applications, the demands for optical coating technology are also increasing, driving the development of related materials science and optical engineering.