Subwavelength engineering in silicon photonics

Subwavelength engineering in silicon photonic integrated circuits is a powerful design tool that allows one to synthesize an effective photonic medium with adjustable refractive index. This creates a new degree of freedom in photonic circuit design. We present an overview of the fundamental concept and its application to address several important practical challenges in the implementation of silicon photonics as a next generation photonics platform for telecom, datacom and sensing. In particular, we report our results in developing highly efficient and broadband fiber-chip couplers for silicon photonic wire waveguides using subwavelength engineered edge coupling structures. We experimentally demonstrate a coupling efficiency of −0.4 dB and polarization independent operation for a broad spectral range exceeding 100 nm for optical fiber with a core diameter of 3.2 µm. For coupling to standard SMF-28 fiber with 10.4 µm mode field diameter we numerically demonstrate a subwavelength engineered overlayer structure composed of SiO
2
and Si
3
N
4
which exhibits an overall coupling efficiency exceeding 90%. We have also used our subwavelength structure for coupling experiments with a conventional In- GaAsP/InP buried heterostructure laser at λ = 1.3 µm with a measured near field mode size of 2.1 µm×2.8 µm. Peak coupling efficiency is 1.5 dB with 1-dB alignment tolerance of approximately ±1.2 µm horizontally and ±0.8 µm vertically. We further present subwavelength engineered grating couplers fabricated in a single-etch step for the telecom (1.55 µm) and datacom (1.3 µm) wavelengths with efficiencies exceeding −0.5 dB. Further applications that will be discussed include waveguide crossings, microspectrometers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators and colorless interference couplers.