  Let $\Gamma$ be a Schottky subgroup of $\mathrm{SL} (2,\mathbb{Z})$. We
establish a uniform and explicit lower bound of the second eigenvalue of the
Laplace-Beltrami operator of congruence coverings of the hyperbolic surface
$\Gamma \backslash \mathbb{H}^2$ provided the limit set of $\Gamma$ is thick
enough.
