Polarization and geometric phase shaping via a space-variant anisotropy has attracted considerable interest for fabrication of flat optical elements and generation of vector beams with applications in various areas of science and technology. Among the methods for anisotropy patterning, imprinting of self-assembled nanograting structures in silica glass by femtosecond laser writing is promising for the fabrication of space-variant birefringent optics with high thermal and chemical durability and high optical damage threshold. However, a drawback is the optical loss due to the light scattering by nanograting structures, which has limited the application. Here, we report a new type of ultrafast laser-induced modification in silica glass, which consists of randomly distributed nanopores elongated in the direction perpendicular to the polarization, providing controllable birefringent structures with transmittance as high as 99% in the visible and near-infrared ranges and >90% in the UV range down to 330 nm. The observed anisotropic nanoporous silica structures are fundamentally different from the femtosecond laser-induced nanogratings and conventional nanoporous silica. A mechanism of nanocavitation via interstitial oxygen generation mediated by multiphoton and avanlanche defect ionization is proposed. We demonstrate ultralow-loss geometrical phase optical elements, including geometrical phase prism and lens, and a vector beam convertor in silica glass.

通过空间变化各向异性实现的偏振和几何相位整形引起了人们对平面光学元件的制造和矢量光束的生成以及在各个科学技术领域的应用的极大兴趣。在各向异性图案化方法中，通过飞秒激光写入在石英玻璃中压印自组装纳米光栅结构有望用于制造具有高热稳定性和化学稳定性以及高光学损伤阈值的空变双折射光学器件。然而，其缺点是由于纳米光栅结构的光散射造成的光学损失，这限制了其应用。在这里，我们报告了一种新型的超快激光诱导石英玻璃改性，该改性由随机分布的纳米孔组成，在垂直于偏振的方向上延伸，提供了可控双折射结构，在可见光和近红外区域的透射率高达99%范围内，>90% 的紫外线范围低至 330 nm。观察到的各向异性纳米多孔二氧化硅结构与飞秒激光诱导纳米光栅和传统纳米多孔二氧化硅有根本不同。提出了一种通过多光子和雪崩缺陷电离介导的间隙氧产生的纳米空化机制。我们展示了超低损耗几何相位光学元件，包括几何相位棱镜和透镜，以及石英玻璃中的矢量光束转换器。