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Waveguides fabricated in crystals, created by utilizing techniques such as ion implantation, femtosecond laser inscription, and proton exchange, have low refractive index contrast with the crystal substrate, which limits their potential development to create compact passive devices, such as waveguide interferometers. In this work, a new waveguide multi-mode interferometer fabrication strategy based on low-effective-index mode interference is presented. Numerical tools have been used for the analysis of this new guided-wave approach used for the device design. The research has demonstrated that a series of high-order modes with an effective refractive index much lower than the substrate can exist in trench-cladded ridge crystal waveguides that have a tiny index contrast in the vertical direction. Simple trench-cladded tapered waveguide configurations to excite such modes and to realize compact waveguide multi-mode interferometers with lengths of several tens and up to hundreds of micrometers are presented. The waveguide multi-mode interferometer design is compact in size, easy to modulate, and with low insertion loss. Furthermore, refractive index sensing is realized, with a sensitivity of ∼490 nm/RIU for aqueous solution samples. The novel multi-mode interference phenomenon present here offers new possibilities and significant opportunities for waveguide modulation and, thus, the development of compact waveguide refractometers.

Nanogap-rich silver nanoislands with fascinating optical properties are desirable substrates for surface-enhanced Raman scattering (SERS). Here, we propose a simple and high-throughput approach through the laser molecular beam epitaxy (LMBE) technique for preparing silver nanoislands containing large numbers of intra-nanogaps on a silicon wafer (6×6 cm2). By optimizing the deposition time, the enlarged silver nanoislands with ∼5 nm interstitial gaps of abundance and homogeneity were formed. Remarkably, the optimized SERS substrate with high-density hotspots demonstrated a high analytical enhancement factor (AEF) as large as 1.17×105, excellent reproducibility with relative standard deviation (RSD) of 7.76% over the entire substrate, and good stability after storage for 21 days. The electromagnetic field distribution of the optimized SERS substrate was simulated using the software COMSOL Multiphysics based on the actual SEM image of the fabricated sample, and the calculated enhancement factor (EF) is as high as 109. Furthermore, it can enable sensitive and quantitative detection of malachite green at concentrations as low as 10−8 M. This simple fabrication of silver nanoislands with homogeneous ∼5 nm interstitial gaps provides a practical solution for wafer-scale, sensitive, and reproducible SERS substrates.