Waveguide mode in the box with an extraordinary flat dispersion curve

Area of Science:

• Photonics and Optical Engineering
• Waveguide Optics
• Resonant Structures

Background:

• Guided-mode resonance filters (GMRFs) are typically limited by narrow angular acceptance.
• Cavity resonator integrated guided-mode resonance filters (CRIGFs) exhibit unusually wide angular acceptance.
• The underlying physical mechanism for this enhanced performance in CRIGFs is not fully understood.

Purpose of the Study:

• To analyze and explain the extraordinary flattening of the dispersion curve in CRIGFs.
• To identify the phenomenon responsible for the wide angular acceptance of CRIGFs.
• To compare CRIGF performance with "doubly periodic" gratings.

Main Methods:

• Numerical calculation of the dispersion curve for modes excited in CRIGFs.
• Development of an approximate coupled four-wave model.
• Analysis of the role of external Bragg reflectors (DBRs) and grating couplers (GMRFs).

Main Results:

• The dispersion curve of CRIGFs shows a flattened region where resonance wavelength is nearly independent of incidence angle.
• Flattening increases with the width of the DBRs.
• A novel intramode coupling mechanism, provided by the Bragg gratings, was identified as the cause of the wide angular acceptance, absent in "doubly periodic" gratings.

Conclusions:

• The extraordinary wide angular acceptance of CRIGFs is attributed to intramode coupling facilitated by the external Bragg resonators.
• The developed coupled four-wave model accurately predicts this phenomenon.
• CRIGFs offer superior angular performance compared to traditional grating structures.