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Spectral analysis of the Neumann--Poincaré operator on the crescent-shaped domain and touching disks and analysis of plasmon resonance

We consider the Neumann--Poincaré operator on a planar domain enclosed by two touching circular boundaries. This domain, which is a crescent-shaped domain or touching disks, has a cusp at the touching point of two circles. We analyze the operator via the Fourier transform on the boundary circles of the domain. In particular, we define a Hilbert space on which the operator is bounded, self-adjoint. We then obtain the complete spectral resolution of the Neumann--Poincaré operator. On both the crescent-shaped domain and touching disks, the Neumann--Poincaré operator has only absolutely continuous spectrum on the closed interval $[-1/2,1/2]$. As an application, we analyze the plasmon resonance on the crescent-shaped domain and touching disks.