Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides

Abstract

Subwavelength grating (SWG) structures are an essential tool in silicon photonics, enabling the synthesis of practical metamaterials with controllable refractive index. Here we propose, for the first time, tilting the grating elements to gain control over the anisotropy of the metamaterial. Rigorous FDTD simulations demonstrate that a 45°tilt results in an effective index variation on the fundamental TE mode of 0.23 refractive index units, whereas the change in the TM mode is 20 times smaller. Our simulation predictions are corroborated by experimental results. We furthemore propose an accurate theoretical model for designing tilted SWG structures based on rotated uniaxial crystals, which is functional over a wide wavelength range and for both the fundamental and higher order modes. The proposed structures open up promising venues in polarization management of silicon photonic devices.

Design of a Broadband Polarization Splitter Based on Anisotropy-Engineered Tilted Subwavelength Gratings

“…Indeed, this concept can be exploited to develop MMIs that act as broadband polarization splitters [27]. Recently, tilted SWGs were proposed as a means to control the anisotropy of the metamaterial, enabling independent engineering of the propagation constants of TE and TM polarized modes [28].…”

Subwavelength structured silicon waveguides and photonic devices

“…With the combination of adiabatic coupling techniques and subwavelength structure-enabled dispersion engineering, ultrabroadband devices are expected to be within reach in the next few years. The anisotropic optical properties of subwavelength structures have rarely been explored so far but will potentially provide another degree of freedom to manipulate the propagation of light at the subwavelength scale [109]. In summary, advanced theoretical analysis and fabrication technologies have significantly facilitated research on subwavelength engineering and paved the way toward future commercialization of these kinds of novel functional devices.…”

High performance silicon photonic devices based on practical metamaterials

“…(c) Waveguides with subwavelength cladding allow for suspended waveguide which exhibit low losses in the mid-infrared [14], [15]. (d) Tilting of subwavelength segments provides fine control over waveguide birefringence; the insets show the field propagation of the TE and TM modes [16]. The scanning electron microscope pictures have been colored to highlight different parts of the devices.…”

“…2(d). We have recently shown that tilting affects mainly the TE polarized modes, with virtually no effect on the TM polarized modes [16]. As shown in the inset, the phase front of the TE modes tilts strongly when the segments are tilted, whereas the phase front of the TM modes is much less distorted.…”