Bloch wave – MoM method - AbsoluteAstronomy.com

Bloch wave – MoM is a technique for determining the band structure of triply-periodic electromagnetic media such as photonic crystal

Photonic crystal

Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons...

s. This technique uses the method of moments

Boundary element method

The boundary element method is a numerical computational method of solving linear partial differential equations which have been formulated as integral equations . It can be applied in many areas of engineering and science including fluid mechanics, acoustics, electromagnetics, and fracture...

(MoM) in combination with a Bloch wave expansion of the electromagnetic field in the structure. This approach is very efficient in terms of the number of plane waves needed for good convergence and is analogous to the spectral domain MoM method commonly used for analyzing 2D periodic structures such as frequency selective surfaces (FSS). In the latter case, the electromagnetic field is expanded in terms of a plane wave spectrum

Fourier optics

Fourier optics is the study of classical optics using Fourier transforms and can be seen as the dual of the Huygens-Fresnel principle. In the latter case, the wave is regarded as a superposition of expanding spherical waves which radiate outward from actual current sources via a Green's function...

(Scott [1989]). In both cases, the field is expanded in terms or a set of eigenfunction modes (either a Bloch wave in 3D or a plane wave spectrum in 2D), and an integral equation is enforced on the surface of the scatterers in each unit cell. In the FSS case, the unit cell is 2-dimensional and in the photonic crystal case, the unit cell is 3-dimensional.

Field equations for 3D PEC photonic crystal structures

For perfectly electrically conducting (PEC) structures admitting only electric current sources J, the electric field E is related to the vector magnetic potential A via the well-known relation:

and the vector magnetic potential is in turn related to the source currents via:

where

Bloch wave expansion of the fields

To solve equations (1.1) and (1.2), we may assume a Bloch wave expansion for all currents, fields and potentials:

where for simplicity, we assume a cubic lattice in which α only depends on m, β only depends on n and γ only depends on p. In the equations above,

and,

where l_x, l_y, l_z are the dimensions of the unit cell in the x,y,z directions respectively, λ is the effective wavelength in the crystal and θ₀, φ₀ are the directions of propagation in spherical coordinates

Spherical coordinate system

In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the radial distance of that point from a fixed origin, its inclination angle measured from a fixed zenith direction, and the azimuth angle of...

. Note that k in equations (1.1) and (1.2) comes from the time derivative in Maxwell's equations and is the free space propagation constant, proportional to frequency as we see in equation (1.3). On the other hand, k₀ in the equations above comes from our assumed Bloch wave solution given by equations (2.1) & (2.2). As a result, it represents the propagation constant in the periodic medium. These two k's, i.e. the free space propagation constant and the propagation constant of the Bloch wave, are in general different thereby allowing for dispersion in our solution.

Integral equation for PEC media

Substituting equations (2.1) into (1.1) and (1.2) yields the spectral domain Greens function relating the radiated electric field to its source currents:

where,

With this, the electric field boundary condition on the surface of PEC material within a unit cell becomes:

Since we are seeking characteristic modes (eigenmodes) of the structure, there is no impressed E-field on the RHS of this electric field integral equation (EFIE). Equation (3.3) is not strictly correct, since only the tangential electric field is zero on the surface of the PEC scatterer. This inexactness will be resolved presently when we test with the current basis functions, defined as residing on the surface of the scatterer.

Method of Moments solution

As is usual in the method of moments, we assume an expansion for the source currents over some known set of basis functions with unknown weighting coefficients J_j:

Substituting (3.4) into (3.3) and then testing the resulting equation with the i-th current basis function (i.e., dotting from the left and integrating over the domain of the i-th current basis function, thereby completing the quadratic form) produces the i-th row of the matrix eigenvalue problem as:

This equation is very simple to implement and requires only that the 3D FT of the basis functions be computed, preferably in closed form. With this method, computing bands of a 3D photonic crystal is as easy as computing reflection and transmission from a 2D periodic surface. In fact, equation (3.5) is identical to the basic EFIE for PEC FSS (Scott [1989]), the only difference being the stronger singularity in 3D which accelerates convergence of the triple sums.

Computing bands

To compute bands of the crystal (i.e. k-k₀ diagrams), we may assume values for (k₀, θ₀, φ₀) and then search for those values of k which drive the determinant of the impedance matrix to zero. Equation (3.5) has been used to compute bands in various types of doped and undoped photonic crystals