  The quasar mode of Active Galactic Nuclei (AGN) in the high-redshift Universe
is routinely observed in gas-rich galaxies together with large-scale AGN-driven
winds. It is crucial to understand how photons emitted by the central AGN
source couple to the ambient interstellar-medium to trigger large-scale
outflows. By means of radiation-hydrodynamical simulations of idealised
galactic discs, we study the coupling of photons with the multiphase galactic
gas, and how it varies with gas cloud sizes, and the radiation bands included
in the simulations, which are ultraviolet (UV), optical, and infrared (IR). We
show how a quasar with a luminosity of $10^{46}$ erg/s can drive large-scale
winds with velocities of $10^2-10^3$ km/s and mass outflow rates around $10^3$
M$_\odot$/yr for times of order a few million years. Infrared radiation is
necessary to efficiently transfer momentum to the gas via multi-scattering on
dust in dense clouds. However, IR multi-scattering, despite being extremely
important at early times, quickly declines as the central gas cloud expands and
breaks up, allowing the radiation to escape through low gas density channels.
The typical number of multi-scattering events for an IR photon is only about a
quarter of the mean optical depth from the center of the cloud. Our models
account for the observed outflow rates of $\sim$500-1000 M$_\odot$/yr and high
velocities of $\sim 10^3$ km/s, favouring winds that are energy-driven via
extremely fast nuclear outflows, interpreted here as being
IR-radiatively-driven winds.
