Energy-Decoupled Modes (DMs) and DMs-Based Far-Field to Near-Field Transformation of Metasurface Apertures (Meta-Apertures/Metapertures)

The energy-transmitting mechanism and energy-transmitting process of metasurface apertures (meta-apertures/metapertures) are analyzed. The energy transport theorem (ETT) used to quantitatively depict the energy-transmitting process is derived. The operator expression of the power source term contained in the ETT is formulated as so-called input power operator (IPO). The method for eliminating the dependent currents involved in the IPO and the diagonalizing IPO method for calculating the energy-decoupled modes (DMs) of the objective metaperture are developed. The orthogonality relationships satisfied by the DMs are summarized, and the physical meanings of the real and imaginary power orthogonality relationships are clarified. Employing the DMs-based modal expansion, a far-field to near-field transformation method for metapertures is proposed. In addition, some other related topics, such as the physical meanings of
$\boldsymbol{\theta}-\mathbf{factor}$
(or alternatively called generalized
$\boldsymbol{Q}-\mathbf{factor}$
) and modal significance (MS) and the DMs-version of Parseval's theorem, are also discussed simply.