Abstract
Subwavelength metamaterials exhibit a strong anisotropy that can be leveraged to implement high-performance polarization handling devices in silicon-on-insulator. Whereas these devices benefit from single-etch step fabrication, many of them require small feature sizes or specialized cladding materials. The anisotropic response of subwavelength metamaterials can be further engineered by tilting its constituent elements away from the optical axis, providing an additional degree of freedom in the design. In this work, we demonstrate this feature through the design, fabrication and experimental characterization of a robust multimode interference polarization beam splitter based on tilted subwavelength gratings. A 110-nm minimum feature size and a standard silicon dioxide cladding are maintained. The resulting device exhibits insertion loss as low as 1 dB, an extinction ratio better than 13 dB in a 120-nm bandwidth, and robust tolerances to fabrication deviations.
Highlights
Subwavelength gratings (SWGs) are periodic arrangements of alternating materials with a pitch smaller than the wavelength of light propagating through them [1]
The availability of high-resolution lithography techniques has enabled the use of SWGs in a wide array of applications, ranging from high-performance fiber-to-chip couplers [3,4], to enhanced photonic biosensors [5,6,7], advanced filters [8,9] and mid-infrared waveguide platforms [10]
When the fundamental modes with TE and TM polarization are launched into input 1, the self-image of the TE polarized mode is formed at output 2, while the TM polarized mode is imaged at output 3
Summary
Subwavelength gratings (SWGs) are periodic arrangements of alternating materials with a pitch smaller than the wavelength of light propagating through them [1]. The metamaterial synthesized by a SWG is strongly anisotropic, because the electric field experiences different boundary condition depending on whether the subwavelength segments are oriented parallel or perpendicular to the direction of propagation [2] This property has been exploited to demonstrate broadband beam-splitters [11] and to minimize coupling between adjacent waveguides [12], and finds direct application in on-chip polarization handling. Tilting each individual segment of a SWG waveguide away from the transverse direction significantly modifies TE effective refractive index, whereas TM mode remains almost unaffected This principle was leveraged in the theoretical design of an MMI polarization beam splitter [26], resulting in a compact device under 100 μm long. Our device exhibits IL below 1 dB and an ER better than 13 dB in a 120-nm bandwidth, exceeding the requirements for optical communications in the full C and L bands [28]
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