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

Abstract

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-based surface gratings for efficient fiber-chip and free-space beam coupling

“…We designed grating couplers with trenches filled by cladding material and sub-wavelength grating metamaterials. SWG metamaterials expand the design space to overcome performance limitations of conventional Si photonic devices by engineering the light propagation characteristics [31][32][33][34]. Figures 1b and 1c shows fiber-to-chip coupling loss for a C-band operated surface grating couplers with and without SWG metamaterials.…”

Asymmetric comb waveguide for strong interactions between atoms and light

“…A periodic variation of the refractive index couples forward and backward propagating modes, resulting in the opening of photonic bandgaps and the apparition of band edges where the group velocity vanishes [32]. Periodic waveguides are usually made of a periodic pattern that is symmetric in transverse directions [44][45][46][47][48][49][50]. Around zero-group-velocity points, the dispersion relation that relates the frequency ω to the wavevector k is most often a parabola.…”

Library of single-etch silicon nitride grating couplers for low-loss and fabrication-robust fiber-chip interconnection

“…By controlling SWG geometry, comprising the widths of Si 3 N 4 blocks ( w Si3N4 ) and air holes ( w Air ), a wide range of equivalent homogeneous metamaterials can be tailored with synthetic refractive index 36 – 38 . Since the first demonstration of SWG metamaterials in integrated photonics 39 – 43 , they have been successfully used as a powerful tool for overcoming performance limitations of conventional photonic devices by customizing the light propagation properties 44 – 46 . SWG-based grating couplers can provide an additional design freedom to optimize the device performance (otherwise limited to DC and period variations only), especially by lowering both the back-reflections and grating strength.…”