The optical absorption in a homogeneous and non-dispersive slab is governed by the well-known Fabry-Perot resonances. We have found that below the lowest order Fabry-Perot resonance, there is another absorption maximum due to the zero frequency mode whose peak frequency is given not by the real part of the complex resonance frequency, as it is the case for all other resonances, but by the imaginary part. This result is of interest, among other applications, for ultra thin solar cells, as tuning the zero frequency mode peak with the maximum of solar irradiance results in an increased efficiency.
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The relation between tangent and natural log functions is: let a = tan z, then z = −i/2 ln[(i − a)/(i + a)].
The natural log must be approximated to its first order Taylor expansion: ln(x) ≈ (x − 1), valid only for 0 < x < 2.
