Sample preparation

We prepared our sample via UV lithography. Standard (100) silicon wafer with 50 mm diameter is used as a substrate, where we spin-coat an SU-8 3025 (Microchem, USA) photoresist layer with 21-μm thickness. After the soft baking (1 min at 65°C followed by 10 min at 95°C on hot plates), a periodic pattern of exposed rectangles (300 μm by 80 μm, distributed in a rectangular array with 130- and 160-μm gaps in two directions, respectively) is generated by UV exposure (i-line of a Hg arc lamp, ~200-mJ/cm² power density at 365 nm) using a mask aligner (MJB4, Suss MicroTec, DE) and a soda-lime optical mask. Then, we perform a postexposure baking (5 min at 65°C, followed by 5 min at 95°C). A change in the chemical composition of the exposed polymer occurs during the postexposure baking. The 400-μm-thick CaF₂ cylindrical window is placed on top of the photoresist during the postbake.