highrefractiveindex

In optics, the refractive index n of a material describes how light slows and bends as it passes through the material. Materials with a high refractive index, typically those with n around 2 or greater in the visible range, are described as high refractive index materials. The high index arises from the material's electronic structure and its interaction with electromagnetic waves.

Examples and typical values include diamond (n ≈ 2.42 in the visible), titanium dioxide in the rutile phase (n ≈ 2.4), zinc sulfide (n ≈ 2.2), gallium phosphide (n ≈ 3.3), and silicon (n ≈ 3.5 but with substantial absorption in the visible). Transparent oxides such as hafnium oxide (n ≈ 2.0–2.1) and tantalum pentoxide (Ta2O5, n ≈ 2.0–2.2) are common in coatings.

High refractive index increases optical contrast at interfaces, enabling compact lenses and waveguides, but it also raises Fresnel reflections at boundaries with lower-index media. Therefore, anti-reflective coatings and index-matching layers are important in high-index systems. High index materials often exhibit dispersion, meaning n varies with wavelength; designers use dispersion models to predict performance.

Applications include dielectric mirrors and Bragg reflectors, optical coatings for cameras and displays, photonic integrated circuits, LEDs, laser optics, and solar cells where index contrast helps light management. In microfabrication, high-index materials enable high numerical aperture lenses and efficient light confinement in waveguides and resonators.

Limitations and considerations include absorption outside transparent windows, thermal and mechanical properties, chemical stability, and manufacturing cost. The choice of high-index material depends on wavelength range, required transparency, and integration with other optical components.