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    Area of Science:

    • Photonics
    • Condensed Matter Physics
    • Electromagnetism

    Background:

    • Photonic crystals exhibit unique band structures, including Dirac cones.
    • Exceptional points (EPs) arise in non-Hermitian systems with gain/loss.
    • Understanding EP formation in photonic crystals is crucial for novel device applications.

    Purpose of the Study:

    • To develop a non-Hermitian effective medium theory for interpreting exceptional points in photonic crystals.
    • To predict novel band dispersions in passive photonic crystals based on this theory.
    • To introduce new concepts like complex Dirac-like cones in non-Hermitian photonics.

    Main Methods:

    • Formulation of a non-Hermitian effective medium theory.
    • Analysis of band structures in photonic crystals with loss/gain.
    • Theoretical prediction and demonstration of unique band dispersions.

    Main Results:

    • The theory successfully interprets the spawning of exceptional rings from Dirac cones.
    • Two novel types of band dispersions in fully passive photonic crystals were predicted and demonstrated.
    • An exceptional ring can shrink into a complex Dirac point, forming a complex Dirac-like cone.
    • A point of quadratic degeneracy was realized in the imaginary frequency spectrum.

    Conclusions:

    • The proposed theory offers a unified framework for understanding exceptional points in effective media.
    • The study introduces the novel concept of complex Dirac-like cones in non-Hermitian photonics.
    • This work opens new avenues for designing photonic devices with tailored spectral properties.