solves for the TE-mode cutoff wavenumbers in a dielectric slab
[u,v] = dslab(R,Nit)
[u,v] = dslab(R) (equivalent to Nit=3)
frequency radius = k0*a*NA = (2*pi*a/la0)*NA, where NA = sqrt(n1^2-n2^2)
number of Newton iterations (default Nit=3)
k1*a = cutoff wavenumber inside n1
alpha2 * a = cutoff wavenumber inside n2
measure of approximation error = norm(u.*tan(u-m*pi/2)-v)
dslab(R,Nit) solves for the TE-mode cutoff wavenumbers in a dielectric slab. solves the equations v = u*tan(u) or v = -u*cot(u), and u^2 + v^2 = R^2 the equivalent system is v = u*tan(u-m*pi/2), for u in range (m*pi/2,(m+1)*pi/2). uses J. F. Lotspeich's approximation as the initial values to Newton's iteration of solving u*tan(u-m*pi/2)-v = 0. convergence is extremely fast since the initial values are good. Nit = 0 produces the Lotspeich approximation. if u = [u(1),u(2),u(3),u(4),...], then the E-even modes are [u(1),u(3),...] and the E-odd modes [u(2),u(4),...]
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