How Do Silicon Carbide Wafers Suppress The Rainbow Effect

Thanks to its exceptionally high refractive index (≥2.6), silicon carbide offers a fundamental physical solution for eliminating the "rainbow effect" in optical waveguide structures. This phenomenon arises when ambient light passes through an AR waveguide; diffraction angles vary according to wavelength, causing dispersion that splits white light into a rainbow spectrum. Silicon carbide’s high refractive index addresses this challenge in two key ways:

Its high refractive index grants greater design flexibility, enabling the use of finer grating periods. When the grating period is reduced to the nanoscale, the diffraction angles of ambient light increase significantly—often shifting the light outside the human eye's range of perception—thereby visually diminishing the visibility of distracting diffracted light.

From the perspective of wave optics, the phase vector imparted by a grating to incident light is directly proportional to the wavelength and inversely proportional to the grating period; consequently, a smaller period results in a larger deflection angle for the diffracted light. For light incident at large angles, an excessively small grating period causes the diffracted light to shift outside the annular region of the waveguide's K-space that supports guided modes, thereby limiting the effective field of view. Achieving a wide field of view with full-color transmission in a monolithic system therefore requires avoiding grating periods that are too small.

The advantage of silicon carbide (SiC) in mitigating the "rainbow effect" stems fundamentally from the design flexibility afforded by its high refractive index. High-refractive-index structures compress the effective wavelength of light, allowing for significantly smaller grating periods—for instance, a 300 nm period is achievable on a SiC substrate, whereas 500 nm is typically required on conventional glass substrates. At a fixed angle of incidence, this period-compression effect substantially reduces the dispersion angle variance across different wavelengths. Experimental data indicate that within the 400–700 nm visible light range, SiC-based gratings exhibit approximately 40% less dispersion angle variance than those made of conventional materials. Through optimized design, stray light responsible for the rainbow effect can be directed away from the human eye's visible range, effectively suppressing the appearance of rainbow artifacts.

Furthermore, research indicates that employing a design featuring a SiC substrate combined with a multilayer SiO2/TiO2/ITO thin-film structure can effectively reduce visible light reflection at the lens surface, thereby enhancing light transmission. On one hand, the multilayer structure minimizes color deviation caused by light reflection, indirectly mitigating the rainbow effect; on the other hand, the multilayer thin-film structure blocks blue light, which also helps reduce the rainbow effect—as blue light has a shorter wavelength and is more prone to dispersion, a primary cause of the rainbow effect.

The exceptional hardness and chemical stability of silicon carbide make it ideally suited for precision processes such as nanoimprint lithography and electron-beam lithography, enabling the high-precision fabrication of sub-micron grating structures to resolve the rainbow effect issue.

Source: xiamenzhongxinjingyan