We theoretically investigate the formation of dynamic optical microcavities in a multi-level atomic medium induced by colliding quasi-unipolar attosecond pulses. These pulses, characterized by a dominant unipolar burst and a weak trailing edge, create population gratings through coherent interference effects. The microcavity dynamics is shown to be sensitive to the trailing-edge structure. In the weak-field regime, the effect stems from the medium-mediated interference between the electric area of the leading half-cycle burst and the envelope area of the trailing edge. The strong-field regime reveals additional nonlinear phenomena when pulse durations are shorter than the medium’s phase relaxation time T2. This approach enables subcycle spatiotemporal control of medium properties, opening new possibilities in attosecond-scale light manipulation and light-induced quantum state structuring. © 2025 Elsevier B.V.