Direct laser writing of volume fresnel zone plates in silicon

Functional optical elements fabricated on silicon (Si) constitute fundamental building blocks of Si photonics [1]. For the fabrication of these elements, conventional lithography and etching techniques are used. In spite of the success of these techniques, a functional optical element embedded inside silicon simply does not exist. Here, we present a maskless, one-step laser writing technique for creating phase-type Fresnel zone plates in the bulk of Si. Due to their effectiveness over a broad spectra, Fresnel zone plates (FZPs) are widely used in various micro-imaging applications [2]. Similar lenses have been fabricated inside silica [3,4], but are limited to the transparency window of silica. The silicon counterpart of these elements have been impossible to fabricate so far. By exploiting nonlinear absorption within the focal volume of a tightly focused laser, we generated permanent refractive index changes in Si. The imprinted high-index contrast was then used to fabricate a FZP inside Si. This three dimensional (3D) method can allow for alignment-free multilens systems. Moreover, using silicon as the lens material is fully CMOS compatible and applicable to silicon integrated optics, including single and array detectors.