We demonstrate a graphene nanoribbon (GNR) metagrating on a rectangular dielectric ridge that exhibits plasmonically induced transparency (PIT) through the excitation of both odd- and even-order dark modes. The metagrating integrates a top bright mode GNR coupled directly to light and a sidewall dark mode GNR isolated from direct excitation. Increasing the dark mode width generates sequentially ordered dark mode based PIT effects. Taking the PIT effects formed by the fundamental bright mode coupled to the first six-order dark modes as examples, we employ Fabry-Pérot theory, Lorentzian oscillator modeling, and finite-difference time-domain (FDTD) simulations to reveal the underlying physics: The resonant phase remains fixed near $\pi /2$ at the bright mode distal edge but transitions continuously from $-\pi /2$ to $+\pi /2$ at its bright mode facing edge. Consequently, the corresponding phase shift is nearly invariant at the distal edge but varies substantially at the facing edge. This transitional resonant phase—together with the resulting asymmetric phase shift—permits noninteger mode orders which correspond to the conversion from the high-frequency mode of the $j\mathrm{th}$-order PIT to the low-frequency mode of ($j+1$)th-order PIT under variation of the sidewall GNR width while high absorption is maintained. Finally, using refractive index sensing as an example, we demonstrate that the high-order modes of this PIT device outperform conventional gratings and other lower-order PIT devices in key metrics, including the quality factor and figure of merit.

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• Electromagnetically induced transparency
• Plasmonics
• Surface plasmon polariton
• Graphene
• Fabry-Pérot interferometry
• Finite-difference time-domain method