Metric deformation and boundary value problems in 3D

A novel perturbative method, proposed by Panda {\it et al.} [1] to solve the Helmholtz equation in two dimensions, is extended to three dimensions for general boundary surfaces. Although a few numerical works are available in the literature for specific domains in three dimensions such a general analytical prescription is presented for the first time. An appropriate transformation is used to get rid of the asymmetries in the domain boundary by mapping the boundary into an equivalent sphere with a deformed interior metric. The deformed metric produces new source terms in the original homogeneous equation. A deformation parameter measuring the deviation of the boundary from a spherical one is introduced as a perturbative parameter. With the help of standard Rayleigh-Schr{\"o}dinger perturbative technique the transformed equation is solved and the general solution is written down in a closed form at each order of perturbation. The solutions are boundary condition free and which make them widely applicable for various situations. Once the boundary conditions are applied to these general solutions the eigenvalues and the wavefunctions are obtained order by order. The efficacy of the method has been tested by comparing the analytic values against the numerical ones for three dimensional enclosures of various shapes. The method seems to work quite well for these shapes for both, Dirichlet as well as Neumann boundary conditions. The usage of spherical harmonics to express the asymmetries in the boundary surfaces helps us to consider a wide class of domains in three dimensions and also their fast convergence guarantees the convergence of the perturbative series for the energy. Direct applications of this method can be found in the field of quantum dots, nuclear physics, acoustical and electromagnetic cavities.