  We investigate symmetry-selective surface-mode excitation in a general
periodically time-modulated double-layer system, where the modulation of the
two layers has a constant phase difference. By deriving a semi-analytic
trans-fer matrix formalism of a Drude-dispersive double-layer structure with
periodic time-modulation, we calculate the scattering amplitudes and the
corresponding transmission coefficient. Our results show that the
phase-difference between the modulation of the two sheets plays an essential
role in significantly enhancing and selectively exciting either the even or odd
surface mode with high efficiency. We verify our calculations with full-wave
time-domain simulations, showing that efficient switching between the
surface-wave excitation of the two distinct modal chan-nels can be achieved,
even under illumination from a single off-resonant Gaussian pulse, by
controlling the phase difference between the two modulations. Our results pave
the way towards ultrafast, symmetry-selective mode excitation and switching via
temporal modulation.
