Highly Efficient Air‐Mode Silicon Metasurfaces for Visible Light Operation Embedded in a Protective Silica Layer

Abstract

Dielectric metasurfaces have significant potential for delivering miniaturized optical systems with versatile functionalities, leading to applications in various fields such as orbital angular momentum generation, imaging, and holography. Among the different materials, crystalline silicon has the advantage of technological maturity and high refractive index, which increases design flexibility and processing latitude. The second, and often overlooked, advantage of silicon is that it affords embedding the metasurface in a protective material such as silica, which is essential for practical applications. The trade‐off against this high refractive index is silicon's absorption at visible wavelength, which requires new design strategies. Here, such a strategy based on metasurfaces supporting air modes is identified that can lead to a transmission efficiency as high as 87% at a wavelength of 532 nm. This exceptional efficiency is obtained by using the high index to confine the electric field in the periphery of the meta‐atoms, thereby reducing absorption losses. As an example, the design of a fully embedded metasurface is described that can generate vortex beams with various orders of orbital angular momentum. It is envisioned that the proposed strategy paves the way for practical applications of high‐efficiency metasurfaces based on crystalline silicon.

Ultracompact multifunctional metalens visor for augmented reality displays

“…is a high refractive index material, that is compatible with the process of Complementary Metal Oxide Semiconductor (CMOS) mass production and industrialization, and it has already been used for realizing high performance metalenses [24,[42][43][44]. Compared to amorphous silicon, crystalline silicon has a lower absorption coefficient in the visible wavelengths and has been reported to be a material platform for high efficiency metalenses [45][46][47]. Second, to preserve a good see-through property, the size of the nanofin and the filling ratio of the unit cell are critical due to the non-negligible absorption of silicon in the visible region [46,48], and therefore need to be subtly tuned.…”

“…Compared to amorphous silicon, crystalline silicon has a lower absorption coefficient in the visible wavelengths and has been reported to be a material platform for high efficiency metalenses [45][46][47]. Second, to preserve a good see-through property, the size of the nanofin and the filling ratio of the unit cell are critical due to the non-negligible absorption of silicon in the visible region [46,48], and therefore need to be subtly tuned. Third, to achieve high diffraction efficiency for the reflected light, it is desired that the orthogonal polarization components to experience similar amplitude attenuation but a phase retardation close to π upon reflection, according to the PB phase.…”

Ultracompact multifunctional metalens visor for augmented reality displays

“…Compared to amorphous silicon, crystalline silicon has a lower absorption coe cient in the visible wavelengths and has been reported to be a material platform for high e ciency metalenses [42][43][44]. Second, to preserve a good seethrough property, the size of the nanorod and the aperture ratio of the unit cell are critical due to the nonnegligible absorption of silicon in the visible region [43,45], and therefore need to be subtly tuned. Third, to achieve high diffraction e ciency for the re ected light, it is desired that the orthogonal polarization components to experience similar amplitude attenuation but a phase retardation close to π upon re ection, according to the PB phase.…”

“…Silicon is a high refractive index material, that is comparable with the process of Complementary Metal Oxide Semiconductor (CMOS) mass production and industrialization, and it has already been used for realizing high performance metalenses [21,[39][40][41]. Compared to amorphous silicon, crystalline silicon has a lower absorption coe cient in the visible wavelengths and has been reported to be a material platform for high e ciency metalenses [42][43][44]. Second, to preserve a good seethrough property, the size of the nanorod and the aperture ratio of the unit cell are critical due to the nonnegligible absorption of silicon in the visible region [43,45], and therefore need to be subtly tuned.…”

RGB Achromatic Metalens Doublet for Digital Imaging

“…The meta-atoms of the first layer are made of 400 nm thick crystalline silicon (c-Si) nanostructures. The silicon nanorods maintain high accuracy of phase control and high diffraction efficiency in transmission at visible wavelengths even when embedded in a silica layer. , The meta-atoms in the second layer, which are surrounded by air, consist of 600 nm thick silicon nitride (Si 3 N 4 ) nanostructures. To simplify the fabrication, only cylindrical and rectangular nanostructures (Figure b) are employed in the meta-atoms’ design, thus ensuring polarization insensitivity.…”