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The coherent interaction of light with matter imprinted the phase information of the light field on the wave function of the photon-dressed electronic state. Driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a sub-cycle timescale in a GaAs quantum well in the presence of a multi-cycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical 1 formula that accounts for the optical susceptibilities caused by the dressed state of excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions.