The spectral properties
of a quantum system are essential when
probing theoretical predictions against experimental data. For an
open quantum system strongly interacting with its environment, spectral
features are challenging to calculate. Here we demonstrate that the
stochastic Hierarchy of Pure States (HOPS) approach is well suited
to calculate the response of an open quantum system to a, possibly strong, coherent probe driving. For weak driving,
where Kubo’s linear response theory is applicable, it turns
out that the HOPS method is highly efficient since fluctuations inherent
to the stochastic dynamics cancel for the response function and, thus,
allow us to obtain the susceptibility easily. Our results are in agreement
with experimental data for a strongly damped spin system showing that
the transition from oscillatory to overdamped motion is also reflected
by the transmission spectrum. As a further application we demonstrate
that the susceptibility, quantifying the amplitude of the response,
as a function of temperature exhibits a maximum which is the hallmark
of stochastic resonance. Beyond the linear regime, the exact open
system dynamics shows the asymptotic Floquet state. We use the topic
of probe driving and response to present the HOPS approach in a novel
and self-contained way. This includes the importance sampling scheme
which yields the nonlinear HOPS as well as the stochastic treatment
of a thermal initial environmental state within the zero temperature
formalism. Special attention is given to the exponential representation
of the algebraic Ohmic bath correlation function and the truncation
condition for the hierarchy.