Free-Form Micro-Optics Out of Crystals: Femtosecond Laser 3D Sculpturing

Optical crystals are ideal materials for complex functional and durable optical components, but their good stability and hardness bring about difficulties in high-precision machining and require surface roughness below lambda/10. Femtosecond laser ablation is a widely applicable processing method indifferent to material types, but its 3D fabrication capability is limited by the accumulation of ablated debris and rough ablated surface. This work demonstrates a universal and flexible technology for the fabrication of crystalline micro-optics with required shape and surface roughness for the most demanding optical phase control. The cavitation-assisted ablation by a direct laser writing mode is followed by a high-temperature treatment to remove the rough non-crystalline layer caused by ablation. The annealing at temperatures below the melting point of the crystal reduces the roughness down to approximate to 2 nm without changing the structure shape. This virtue of maintaining the designed shape without change, which is impossible during thermal morphing of 3D surfaces of glasses, allows for a previously unavailable flexibility of surface finish for the most demanding optical micro-optical elements made in this study. This universal technology with nanoscale resolution and free-form 3D manufacturing capability is applicable for various crystals and provides a new way to fabricate micro-/integrated-optics and nonlinear optical elements.