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Benefiting from the abrupt phase changes within subwavelength thicknesses, metasurfaces have been widely applied for lightweight and compact optical systems. Simultaneous broadband and high-efficiency characteristics are highly attractive for the practical implementation of metasurfaces. However, current metasurface devices mostly adopt discrete micro/nano structures, which rarely realize both merits simultaneously. In this paper, dielectric metasurfaces composed of quasi-continuous nanostrips are proposed to overcome this limitation. Via quasi-continuous nanostrips metasurface, a normal focusing metalens and a superoscillatory lens overcoming the diffraction limit are designed and experimentally demonstrated. The quasi-continuous metadevices can operate in a broadband wavelength ranging from 450 nm to 1000 nm and keep a high power efficiency. The average efficiency of the fabricated metalens reaches 54.24%, showing a significant improvement compared to the previously reported metalenses with the same thickness. The proposed methodology can be easily extended to design other metadevices with the advantages of broadband and high-efficiency in practical optical systems.

For the purpose of detecting waterborne bacteria, a high-phase-sensitivity SPR sensor with an Ag–TiO2–Franckeite–WS2 hybrid structure is designed using an improved seeker optimization algorithm (ISOA). By optimizing each layer of sensor construction simultaneously, the ISOA guarantees a minimum reflectance of less than 0.01 by Ag (20.36 nm)–TiO2 (6.08 nm)–Franckeite (monolayer)–WS2 (bilayer) after 30 iterations for E. coli. And the optimal phase sensitivity is 2.378 × 106 deg/RIU. Sensor performance and computing efficiency have been greatly enhanced using the ISOA in comparison to the traditional layer-by-layer technique and the SOA method. This will enable sensors to detect a wider range of bacteria with more efficacy. As a result, the ISOA-based design idea could provide SPR biosensors with new applications in environmental monitoring.