The periodic structure of a photonic crystal causes the propagating waves to be governed by Bloch's theorem: they are composed of multiple wave vectors or harmonics. We found, by measuring the field with phase-sensitive near-field microscopy, that the evanescent field of the composite Bloch wave decays nonexponentially as a function of height. Even the individual Bloch harmonics, having only a single wave vector along the propagation direction, do not necessarily decay single exponentially,

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... h has its origin in the spread of wave vectors required to confine the light to the waveguide. The complex decay leads to an evolution of the mode pattern as a function of the height above the structure.