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

Abstract 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 λ/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 ≈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.