  Quark-gluon plasma's (QGP) properties at non-hydrodynamic and
non-perturbative regimes remain largely unexplored. Here, we examine the
response functions describing how a QGP-like plasma responds to initial
energy-momentum disturbance in both static and Bjorken-expanding plasma at
non-hydrodynamic gradient using the Boltzmann equation in the relaxation-time
approximation (RTA). We show that the resulting response functions are
remarkably similar in both static and expanding backgrounds at non-hydrodynamic
gradients. While non-hydrodynamic response can not be described by the
conventional first-order and second-order theories, its behavior is reasonably
captured by the extended version of hydrodynamics proposed by us (arXiv:
2208.01046). The potential sensitivity of the Euclidean correlator to
non-hydrodynamic response is also illustrated.
