3D nano and micro structures in transparent materials by in-volume femtosecond laser processing
Nano as well as micro structuring in the volume of transparent materials is enabled by ultrafast laser radiation. By laser radiation with pulse durations in the fs and ps regime multi photon processes are efficiently induced resulting in a high resolution of less than 1 µm3, a very low heat input and a high writing flexibility in all three dimensions. High transparent materials such as sapphire and glasses are locally modified in the volume to change the refractive index for optical applications or to increase the corrodibility selectively for the manufacturing of micro channels and micro structured parts for the use in micro systems and medical technology. The miniaturization of products for micro optics, the medical technology and micro systems engineering requires transparent components with structure sizes in the micrometer range and accuracies of approx. 100 nm. In-volume Selective Laser-induced Etching (ISLE) is an appropriate manufacturing process for micro machining of transparent materials such as sapphire and glasses, e.g. fused silica. By focusing the laser radiation (wavelength 1040 nm, pulse duration 500 fs, repetition rate 0.1-5 MHz, pulse energy <1 µJ) in the volume the material is locally modified. By scanning the laser focus with pulse overlap inside the material, connected volumes of modified material are created. The modified volumes are subsequently removed by chemical etching in a second processing step using aqueous solution of e.g. HF or KOH. Periodical nano structures (ripples, nanoplanes) are fabricated inside transparent materials exploiting fs-laser induced nano optics with potential applications such as nano sieves for filtering, optical gratings or functionalized internal surfaces. Direct laser writing of waveguides in transparent materials by fs laser radiation is well known. Further applications include crack-free 3D micro markings in transparent materials consisting e.g. of micro-gratings, which result in a colorful experience in the observers eye due to diffraction and interference. To exploit the high manufacturing velocities possible with ultrafast lasers with repetition rates of several MHz a high speed scanning system with large numerical aperture and pre-compensation of spherical aberrations has been developed for in-volume nano and micro structuring. With this system high writing velocities (50-400 mm/s), small focus size (0.6-2.2 mm) and high precision (100-400 nm) are combined for the first time on rather large scanning fields (0.6-1.5 mm) and material depth of up to 2 mm.