Engineered omnidirectional external-reflectivity spectra from one-dimensional layered interference filters

Area of Science:

• Optics and Photonics
• Materials Science

Background:

• Optical refraction limits external rays incident on high-refractive-index media to a small internal cone angle.
• Periodic Bragg structures can overcome this limitation by reflecting light over a wider angular range than the internal cone.

Purpose of the Study:

• To investigate the conditions for omnidirectional external reflection using Bragg structures.
• To explore methods for achieving wide spectral gaps and arbitrary spectral shapes with reduced refractive index contrast.

Main Methods:

• Analysis of refractive index ranges for omnidirectional reflection.
• Theoretical proposal of chirping or grading the periodicity of Bragg structures.

Main Results:

• Omnidirectional external reflection is possible with sufficient internal refractive-index contrast.
• Chirping or grading periodicity enables coverage of wide spectral ranges with modest index contrast.
• The graded-periodicity approach is effective when the average refractive index exceeds 2.

Conclusions:

• Modifying Bragg structure periodicity offers a pathway to broadband and shape-tunable spectral reflection.
• This approach reduces the need for high refractive index contrast, making the technology more accessible.