  We considered the multiphoton resonance in the periodically driven quantum
oscillator with Kerr nonlinearity in the presence of weak high-order
nonlinearities. Multiphoton resonance leads to the emergence of peaks and dips
in the dependence of the stationary occupations of the stable states on
detuning. We demonstrated that due to high-order nonlinearities, these peaks
and dips acquire additional fine structure and split into several closely
spaced ones. Quasiclassically, multiphoton resonance is treated as tunneling
between the regions of the oscillator phase portrait, and the fine structure of
the multiphoton resonance is a consequence of a special quasienergy dependence
of the tunneling rate between different regions of the classical phase
portrait. For different values of damping and high-order nonlinearity
coefficients, we identified the domain of quasienergies where tunneling
strongly influences the system kinetics. The corresponding tunneling term in
the Fokker-Planck equation in quasienergy space was derived directly from the
quantum master equation.
