We consider a system governed by the wave equation with index of refraction n(x), taken to be variable within a bounded region Ω ⊂ R d , and constant in R d \ Ω. The solution of the time-dependent wave equation with initial data, which is localized in Ω, spreads and decays with advancing time. This rate of decay can be measured (for d = 1, 3, and more generally, d odd) in terms of the eigenvalues of the scattering resonance problem, a non-selfadjoint eigenvalue problem governing the time-harmonic solutions of the wave (Helmholtz) equation which are outgoing at ∞. Specifically, the rate of energy escape from Ω is governed by the complex scattering eigenfrequency, which is closest to the real axis. We study the structural design problem: Find a refractive index profile n (x) within an admissible class which has a scattering frequency with minimal imaginary part. The admissible class is defined in terms of the compact support of n(x) − 1 and pointwise upper and lower (material) bounds on n(x) for x ∈ Ω, i.e., 0 < n − ≤ n(x) ≤ n + < ∞. We formulate this problem as a constrained optimization problem and prove that an optimal structure, n (x) exists. Furthermore, n (x) is piecewise constant and achieves the material bounds, i.e., n (x) ∈ {n − , n + }. In one dimension, we establish a connection between n (x) and the well-known class of Bragg structures, where n(x) is constant on intervals whose length is one-quarter of the effective wavelength.