Three-dimensional nanofabrication of hollow wavelength-thick lenses deep inside solid-state optical crystals

Efficient light control and sensing in harsh environments using small footprint sensors require robust photonic components capable of withstanding extreme temperatures, chemical conditions, and ionizing radiation. In this work, we present a novel approach for manufacturing nanostructured diffractive hollow lens (NDHL) embedded inside highly resistant yttrium aluminum garnet (YAG) laser crystals using 3D femtosecond-pulsed laser subtractive nanolithography. To ensure high precision and fidelity to the design, laser writing parameters such as the pulse repetition rate and pulse energy were optimized. This technique enables high-fidelity nanostructuring, up to 100 nm feature sizes, inside the volume of ultra-hard optical crystal, overcoming longstanding fabrication challenges. The NDHL here reported has a diffraction-limited focus, confirmed by both numerical simulations and experimental validation, obtaining a high fidelity (>95 %) between simulated and experimental data. The proof-of-concept microlens has a numerical aperture NA of 0.49 and is embedded 10 μm inside the crystal. This work therefore demonstrates the potential for seamlessly integrating 3D nanostructured focusing elements inside key optical crystals such as YAG, and opens a path towards monolithic, ultra-compact photonic devices for extreme environment applications.