Fresnel-type solid immersion lens for efficient light collection from quantum defects in diamond.

Quantum defects in diamonds have been studied as a promising resource for quantum science. The subtractive fabrication process for improving photon collection efficiency often require excessive milling time that can adversely affect the fabrication accuracy. We designed and fabricated a Fresnel-type solid immersion lens using the focused ion beam. For a 5.8 µm-deep Nitrogen-vacancy (NV-) center, the milling time was highly reduced (1/3 compared to a hemispherical structure), while retaining high photon collection efficiency (> 2.24 compared to a flat surface). In numerical simulation, this benefit of the proposed structure is expected for a wide range of milling depths.

Robust, ultralow-loss, and broadband light-recycling for a nanophotonic delay line.

The optical delay line is a key building block for applications in photonics. It requires low loss, wide bandwidth, and small footprint. In this work, we adopt the light recycling approach based on spatial-mode multiplexing, and achieve high performance with a Mach-Zehnder interferometer (MZI)-based design, which significantly increases the fabrication error tolerance. The proposed nanophotonic delay lines allow fabrication errors of ±10 nm over the broad bandwidth of 100 nm while maintaining the excess insertion loss below 0.5 dB. It will enable power-efficient, ultralow-loss, small-footprint, and broadband optical information processing capabilities in diverse applications.