  We propose a novel scheme for the population and depletion of nuclear
isomers. The scheme combines the $\gamma$-photons with energies $\gtrsim 10$
keV emitted during the interaction of a contemporary high-intensity laser pulse
with a plasma and one or multiple photon beams supplied by intense lasers. Due
to nonlinear effects, two- or multi-photon absorption dominates over the
conventional multi-step one-photon process for an optimized gamma flash.
Moreover, this nonlinear effect can be greatly enhanced with the help of
externally supplied photons. These photons act such that the effective
cross-section experienced by the $\gamma$-photons becomes tunable, growing with
the intensity $I_0$ of the beam. Assuming $I_{0}\sim 10^{18}$ Wcm$^{-2}$ for
the photon beam, an effective cross-section as large as $10^{-21}$ cm$^2$ to
$10^{-28}$ cm$^2$ for the $\gamma-$photon can be achieved. Thus, within
state-of-the-art 10 PW laser facilities, the yields from two-photon absorption
can reach $10^6$ to $10^9$ isomers per shot for selected states that are
separated from their ground state by E2 transitions. Similar yields for
transitions with higher multipolarities can be accommodated by multi-photon
absorption with additional photons provided.
