The high index contrast of the silicon-on-insulator (SOI) platform allows the realization of ultracompact photonic circuits. However, this high contrast hinders the implementation of narrow-band Bragg filters. These typically require corrugations widths of a few nanometers or double-etch geometries, hampering device fabrication. Here we report, for the first time, on the realization of SOI Bragg filters based on sub-wavelength index engineering in a differential corrugation width configuration. The proposed double periodicity structure allows narrow-band rejection with a single etch step and relaxed width constraints. Based on this concept, we experimentally demonstrate a singleetch, 220 nm thick, Si Bragg filter featuring a corrugation width of 150 nm, a rejection bandwidth of 1.1 nm and an extinction ratio exceeding 40 dB. This represents a ten-fold width increase compared to conventional single-periodicity, single-etch counterparts with similar bandwidths. The silicon-on-insulator (SOI) platform with submicrometric thick Si layer has shown outstanding results in the miniaturization of photonic circuits [1]. Highquality materials and mature fabrication processes, together with the potential to leverage already existing CMOS facilities, make it a promising candidate for the large volume production of performant photonic devices. In addition to datacom [2] or sensing applications [3,4], SOI shows a great potential for the generation and manipulation of photonic entanglement [5][6][7][8][9][10]. Such a technology would enable monolithic integration of quantumprocessing circuits, opening new routes for envisioned quantum-based applications, including quantum key distribution [11] and optical quantum computing [12].
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