Scattering resonances for highly oscillatory potentials

Abstract: We study resonances of compactly supported potentials $ V_\varepsilon = W ( x, x/\varepsilon ) $ where $ W : \mathbb{R}^d \times \mathbb{R}^d / ( 2\pi \mathbb{Z}) ^d \to \mathbb{C} $, $ d $ odd. That means that $ V_\varepsilon $ is a sum of a slowly varying potential, $ W_0 ( x) $, and one oscillating at frequency $1/\varepsilon$. For $ W_0 \equiv 0 $ we prove that there are no resonances above the line $\text{Im} \lambda = -A \ln(\varepsilon^{-1})$, except possibly a simple resonance of modulus $\sim \varepsilon^2$, when $ d=1$. We show that this result is optimal by constructing a one-dimensional example. In the case when $ W_0 \neq 0 $ we prove that resonances in fixed strips admit an expansion in powers of $\varepsilon$. The argument provides a method for computing the coefficients of the expansion. In particular we produce an effective potential converging uniformly to $W_0$ as $\varepsilon \rightarrow 0$ and whose resonances approach resonances of $V_\varepsilon$ modulo $O(\varepsilon^4)$.