![light diffraction light diffraction](https://i.ytimg.com/vi/cTT5Q0sqWD0/maxresdefault.jpg)
By utilizing the new principles of diffraction, we introduce and analyze novel FCE metasurfaces that operate in the vicinities of the higher, i.e., third, fourth, and sixth, stopbands. In this study, we present new principles of diffraction that enable the identification of the dominant Fourier components causing out-of-plane diffraction orders at the higher stopbands in the nonsubwavelength regions. However, FCE metasurfaces are enabled because only some of Fourier harmonic components are dominant at band edges with Bragg conditions. In principle, resonant diffraction phenomena are governed by the superposition of scattering processes, owing to higher Fourier harmonic components of periodic modulations in lattice parameters. Realizing BICs and highly efficient Fano resonances at higher stopbands is important as it can provide new opportunities for manipulating electromagnetic waves in artificial periodic structures. However, higher (beyond the second) stopbands open in the nonsubwavelength regions λ < nΛ, where the Bloch modes are diffracted in multiple directions, have so far attracted little attention for fundamental studies as well as practical applications because the unwanted higher diffraction orders render it difficult to achieve BICs and highly efficient Fano resonances. However, the discovery of Wood’s anomalies in 1902 prompted the study and development of subwavelength (Λ nΛ, where the Bloch modes are diffracted only in the zero-order direction this is important for practical applications because the second stop bands admit BICs and diverse zero-order spectral responses with 100% diffraction efficiency. The directions of the diffracted light can be predicted exactly from the grating equation. Since the pioneering studies by Young and Fraunhofer, frequency-selective functionalities have been realized by utilizing higher diffraction orders when the period of the grating (Λ) is larger than the wavelength of incident light ( λ).
![light diffraction light diffraction](https://cdn.slidesharecdn.com/ss_thumbnails/diffractionoflight-171106075541-thumbnail-4.jpg)
The analysis of light diffracted by periodic structures has a history of more than 200 years. It is demonstrated that these FCE metasurfaces with appropriately engineered spatial dielectric functions can exhibit BICs and highly efficient Fano resonances even beyond the subwavelength limit. Based on the new diffraction principles, novel Fourier-component-engineered (FCE) metasurfaces are introduced and analyzed. Here, we present new principles of light diffraction, that enable identification of the dominant Fourier components causing multiple diffraction orders at the higher stopbands, and show that unwanted diffraction orders can be suppressed by engineering the dominant Fourier components. But only some of Fourier components are dominant at band edges with Bragg conditions. Higher (beyond the second) stop bands open beyond the subwavelength limit have attracted little attention thus far. Indeed, most of the previous studies, that treat anomalous resonance effects, utilize quasiguided Bloch modes at the second stop bands open in the subwavelength region. In conventional diffraction theory, a subwavelength period is considered a prerequisite to achieving the highly efficient resonant physical phenomena. Resonant physical phenomena in planar photonic lattices, such as bound states in the continuum (BICs) and Fano resonances with 100% diffraction efficiency, have garnered significant scientific interest in recent years owing to their great ability to manipulate electromagnetic waves.