Debye Representation and Multipole Expansion of the Quantized Free Electromagnetic Field

  • A. J. Devaney
Conference paper


Quite often in classical electrodynamics, a problem is simplified, considerably, by choosing an appropriate representation for the field. Well-known examples are the Cauchy initial-value problem and the Dirichlet or Neumann boundary-value problem. For the Cauchy problem an ideal representation for the field is a plane-wave expansion [1]; this being so because the plane-wave fields form a complete set of solutions to the wave equation into which the Cauchy data can be easily expanded. For the case of Dirichlet or Neumann boundary conditions given on an infinite plane surface, together with Sommerfeld’s radiation condition at infinity, a plane-wave expansion is again appropriate, although in this case it is necessary to include evanescent (inhomogeneous) plane waves in the expansion [2], Plane-wave expansions are not always the most appropriate representation to use, however. For example, with Dirichlet or Neumann conditions prescribed on the surface of a sphere, it is desirable to expand the field into a multipole expansion since the multipole fields form a complete, orthogonal set of functions on such surfaces [3].


Multipole Moment Multipole Expansion Angular Momentum Operator Debye Representation Magnetic Field Operator 
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    Here we assume that the operator a(ω/c s) possesses the necessary continuity properties required in Hodge’s theorem. This assumption seems justifiable since, in a coherent state representation, the (diagonal) matrix elements of this operator are of the form of a classical plane-wave amplitude and, in the classical case, the plane-wave amplitudes are generally entire analytic functions of ω/c s (cf. reference 8).Google Scholar
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Copyright information

© Plenum Press, New York 1973

Authors and Affiliations

  • A. J. Devaney
    • 1
  1. 1.University of RochesterRochesterUSA

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