Damping in Duffing equation

Greeting every one.

In Duffing equation, the damping coeffecient usually assume possitive values (<<1). However, at certain cases, the damping coeffecient may asume negative values.

Can any one explain the meaning of positive and negative damping in Duffing equation as well as in real physical systems.

Further, in modelling the nonlinear response of a nonlinear material driven by optical laser source using Duffing equation, what type of damping we have in this case.

any contribution is appreciated.