The ultrafast modulation of light transmitted by a metamaterial making up an extraordinary optical transmission geometry is investigated by means of optical pump-probe spectroscopy. Using a sample consisting of a lattice of square nano-holes in a gold film on a glass substrate, we monitor the high-frequency oscillations in the intensity of transmitted infrared light. A variety of gigahertz acoustic modes, involving the opening and shutting motion of the holes as well as the straining of the glass substrate below the holes, are revealed to be active in the optical modulation. Numerical simulations of the transient deformations and strain fields elucidate the nature of the vibrational modes contributing most strongly to the variations in optical transmission, and point to the hole-area modulation as the dominant effect. Potential applications include ultrafast acousto-optic modulators.

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