We theoretically demonstrate the possibility to produce unipolar pulses of varying temporal profile in an extended layer of a resonant medium with the nonlinear coupling to the driving electric field. The proposed approach relies on the coherent control of the low-frequency medium oscillations by a sequence of ultrashort excitation pulses as well as the collective emission of multiple resonant centers along an optically thick medium layer. In particular, we show that creating an inhomogeneous spatial profile of the density of resonant centers allows obtaining unipolar pulses of adjustable waveform, such as rectangular or triangular ones. Such optical response could be realized in Raman-active media, e.g., in molecular crystals with well-resolved vibrational terahertz resonances.