Diffraction-less propagation beyond the sub-wavelength regime: a new type of nanophotonic waveguide

Sub-wavelength grating (SWG) metamaterials have garnered a great interest for their singular capability to shape the propagation of light. However, practical SWG implementations are limited by fabrication constraints, such as minimum feature size. Here, we present a new nanophotonic waveguide grating concept that exploits phase-matching engineering to suppress diffraction effects for a period three times larger than those with SWG approaches. This long-period grating not only facilitates fabrication, but also enables a new diffraction-less regime with additional degrees of freedom to control light propagation. More specifically, the proposed phase-matching engineering enables selective diffraction suppression, providing new tools to shape propagation in the grating. We harness this flexible diffraction control to yield single-mode propagation in, otherwise, highly multimode waveguides, and to implement Bragg filters that combine highly-diffractive and diffraction-less regions to dramatically increase light rejection. Capitalizing on this new concept, we experimentally demonstrate a Si membrane Bragg filter with record rejection value exceeding 60 dB. These results demonstrate the potential of the proposed long-period grating for the engineering of diffraction in nanophotonic waveguides and pave the way for the development of a new generation of high-performance Si photonics devices. Silicon photonics is widely recognized as an enabling technology for next generation optical interconnects, holding the promise of providing ultra-compact and low power consumption opto-electronic transceivers, fabricated at large volumes leveraging existing CMOS facilities 1. Driven by the impressive technological development over the recent years, silicon photonics is expanding its frontiers towards new applications beyond datacom 2. These include, among others, chemical and biological sensing 3 , radio-over-fiber 4 , spectroscopy 5 , and quantum cryptography 6. In this very diverse ecosystem, combining demanding optical interconnects with disruptive new applications , sub-wavelength grating (SWG) engineering has gained momentum due to its unmatched flexibility in controlling the propagation of light in nanophotonic devices 7,8. SWG metamaterial waveguides rely on periodic silicon patterning, with a structural period shorter than half the wavelength, to synthesize refractive index and chromatic dispersion that can, in principle, be engineered at will 9,10. Unlike photonic crystals that rely on resonant light confinement 11 , SWG waveguides operate well below the bandgap, guiding light by (synthetic) refractive index difference. This way, they provide flexible control over modal confinement, birefringence and dispersion, non-achievable in conventional waveguide arrangements, alongside with low propagation loss and remarkably wide spectral bandwidth 9,10,12,13. These key advantages allowed the demonstration of several SWG-based devices