  We show that a laser pulse can always be found that induces a desired optical
response from an arbitrary dynamical system. As illustrations, driving fields
are computed to induce the same optical response from a variety of distinct
systems (open and closed, quantum and classical). As a result, the observed
induced dipolar spectra without detailed information on the driving field is
not sufficient to characterize atomic and molecular systems. The formulation
may also be applied to design materials with specified optical characteristics.
These findings reveal unexplored flexibilities of nonlinear optics.
