Silicon-on-sapphire mid-IR wavefront engineering by using subwavelength grating metasurfaces

Abstract

Subwavelength high-contrast gratings (sub-λ HCGs) were pursued to design mid-IR optical devices based on wavefront engineering. Unlike metallic counterparts, dielectric and semiconductors offer low-loss operation over a wide spectral range. Silicon gratings on sapphire substrate are proposed here as the device platform due to their transparency at near-IR to mid-IR wavelengths. The proposed generic methodology manipulates the wavefront of the reflected wave based on the phase and magnitude of reflectivity for unit cells with different dimensions. To demonstrate the capability of generating an arbitrary wavefront profile, several design examples are shown: (1) a reflective mirror with ∼100% power efficiency and broad bandwidth of >1 μm as reflectivity >90%; (2) a blazed grating with a reflection peak automatically aligned with its 1st diffraction order at 19.5°; (3) a focusing mirror with a focal length of 35 μm and diffraction-limited focus; (4) a sinusoidal phase profile with a periodicity of 8860 nm.

Low-loss silicon platform for broadband mid-infrared photonics

“…To increase robustness, the under-cladding can instead be replaced by a more transparent material, such as sapphire (transparent out to ∼4.5 µm). Siliconon-sapphire is commercially available and has been extensively studied [39][40][41][42][43][44][45][46][47]; however, it is a hard material that is difficult to etch, leading to scattering losses. Additionally, Chen et al have recently demonstrated a platform based on silicon on calcium fluoride (CaF 2 ), which extends the transparency window even further [48].…”

Absorption enhanced thin‐film solar cells using fractal nano‐structures

“…Therefore, using the proposed method results in a short circuit current which is 2.97 times higher than a simple solar cell using the same amount of crystalline silicon. The short circuit current enhancement achieved in this work is much higher than the values reported in previous works [8][9][10][11][12][13][14][15][16][17][18][19][20][21]. To study the effect of the incident angle, Figure 12 illustrates the short circuit current enhancement for different angles of incidence.…”

“…Furthermore, since the structure is completely symmetric, it provides exactly the same response for TE and TM polarisations of the incident light. The consistency of the absorption for TM and TE polarisations is one of the important advantages of the proposed solar cell when compared with previous works [12, 13, 20].…”

“…When using nanoparticles, the light excites local surface plasmons (LSP) around nanoparticles leading to local field enhancement and absorption improvement [8][9][10][11]. Metasurfaces, 2D version of metamaterials [22][23][24][25], have been also used as absorbers [16,17], lenses [18,19] and beam formers [20,21] to trap sunlight inside the active layer of solar cells.…”

Metacoatings for wavelength-scale, high-numerical-aperture plano–concave focusing lenses

“…Furthermore, suitable designs for terahertz and lower frequencies can also benefit from engineered metamaterials with ultrahigh index of refraction [42]- [44]. Mid-IR wavefront shaping can be carried out also using all-dielectric subwavelength highindex-contrast gratings offering a low-loss performance [45]. Finally, compact multifocal nanolenses might be developed following for instance the procedure of Refs.…”