## 3 Fabrication and Characterization  
UV Soft NIL, which is considered as cost-effective and allowing high throughput, was applied to fabricate the microring resonator. For conventional NIL, silicon and metal are normally utilized to fabricate the hard mold by etching. However, the etching process will result in surface roughness problems which increase the scattering loss of the waveguide. The hard mold in this article was prepared on a negative photoresist SU8 (MicroChemicals) by UV lithography in order to avoid the etching process. The preparation process can be described as below. First, the silicon wafer which was used as the substrate was carefully cleaned using pirahna and acetone. Before preparing the SU8 film, the Ti-prime (MicroChemicals) was spin-coated on the substrate in order to improve the adhesion and the uniformization of SU8 film. Then, the SU8 was spin-coated on the substrate with the spin speed 5600 rpm (rounds per minutes) for 40 s. The thickness was controlled to be about 980 nm by adjusting the spin-coating parameters. After two steps post-baking (65 \({}^{\circ}\)C for 2 min, 95 \({}^{\circ}\)C for 2 min), the UV exposure was carried out using the contact mask aligner, Suss MA6, and the high-resolution mask. Then the sample was baked again using the same parameters as mentioned above. After 40 s developing, the patterns could be prepared on the substrate. At last, the thermal curing at 180 \({}^{\circ}\)C for 10 min was applied to make SU8 cross-linked and solidified completely. The fabrication process of the soft mold is illustrated in Figure 6a-c. Before fabrication, the FFPE acrylate should be prepared by mixing Iragcare 2022 photoinitiator (BASF) with Fluorolink MD 700 (Solvay Solexis) at the proportion 1:20. As the viscosity of the PFPE acrylate was not very high, the bubbles could be rejected out by about 20 min standing. Then the PFPE acrylate was cast on the top of the SU8 hard mask. A Polystyrene foil must be placed on the top of the PFPE acrylate in order to improve the mechanical stability. Afterwards, a roller was applied to press the foil in order to make sure the hard mask was fully filled by the PFPE acrylate and the patterns could be perfectly replicated. Different from the thermal curing of a PDMS soft mold, UV curing with 30 mW/cm\({}^{2}\) for 90 s was used for the PFPE soft mold, which simplified the curing process. After peeling off, the PFPE soft mold was prepared. The imprinting processes are illustrated in Figure 6d-f. The silicon wafer with SiO\({}_{2}\) layer on the top was used as the substrate. The SiO\({}_{2}\) layer with the thickness of 3 \(\upmu\)m acted as the under cladding, which was thick enough to prevent the light leaking out from the waveguide. After carefully  
Figure 5: The propagation of the optical field in the MMI coupler with the designed parameters. cleaning, the Ormoprime was spin-coated on the substrate in order to improve the adhesion and the uniformization of the OrmoCore layer. The OrmoCore needed to be diluted by the thinner maT with the proportion 1:2.3 in order to reach the desired thickness. Then the mixture was spin-coated on the substrate with 3000 rpm for 30 s, followed by thermal curing at 120 \({}^{\circ}\)C for 10 min. Afterwards, the PFPE soft mold was carefully placed on the OrmoCore layer. Extra pressure was not necessary as the capillary force was enough for the imprinting. UV curing was applied with a power of 30 mW/cm\({}^{2}\) for 2 min. After peeling off the soft mold, the patterns could be replicated on the OrmoCore layer. Then, the 150 \({}^{\circ}\)C thermal curing was applied for 3 h and the microring resonator was finally fabricated. The fabricated microring resonator was characterized using scanning electron microscopy. The top views of the entire microring and MMI coupler are shown in Figure 7a,b, respectively. It can be seen that the microring resonator was prepared with UV Soft NIL with good quality. The cross-sections of single-mode waveguide and multimode waveguide are shown in Figure 7b,c.