The mode fields of optical fibers and high-index contrast photonic waveguides have different dimensions, making difficult the coupling of light from one to the other. To solve this problem, surface grating couplers are one of the most used alternatives. However, in conventional grating couplers the bandwidth is limited. To overcome this drawback, we have recently proposed zero-order grating couplers, which, making use of a high-index prism and subwavelength grating structures, substantially mitigate the wavelength dependency of conventional grating couplers. In the near-infrared (λ = 1.55 μm) it is not possible to take full benefit from the potential bandwidth enhancement of zero-order grating couplers. Thus, in this work we migrate our zero-order grating coupler design from the near-infrared wavelength range to the mid-infrared one, which is attaining increasing attention because of its sensing applications. Subdecibel coupling efficiencies and broad bandwidths up to ~680 nm are achieved at λ = 3.8 μm for silicon-on-insulator and germanium-on-silicon nitride platforms.
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