  In this article, we report, for the first time, broadband multifunctional
metasurfaces with more than four distinct functionalities. The constituent
meta-atoms combine two different phase change materials, $\mathrm{VO_2}$ and
$\mathrm{Sb_2S_3}$ in a multi-stage configuration. FDTD simulations demonstrate
a broadband reflection amplitude switching between the four states in visible
range due to the enhanced cavity length modulation effect from the cascaded
Fabry-Perot cavities, overcoming the inherent small optical contrast between
the phase change material (PCM) states. This, along with the reflection phase
control between the four states, allows us to incorporate both amplitude and
phase-dependent properties in the same metasurface - achromatic deflection,
wavelength beam splitting, achromatic focusing, and broadband absorption,
overcoming the limitations of previous functionality switching mechanisms for
the visible band. We have used a Tandem Neural network-based inverse design
scheme to ensure the stringent requirements of different states are realized.
We have used two forward networks for predicting the reflection amplitude and
phase for a meta-atom within the pre-defined design space. The excellent
prediction capability of these surrogate models is utilized to train the
reverse network. The inverse design network, trained with a labeled data set,
is capable of producing the optimized meta-units given the desired
figure-of-merits in terms of reflection amplitude and phase for the four
states. The optical characteristics of two inverse-designed metasurfaces have
been evaluated as test cases for two different sets of design parameters in the
four states. Both structures demonstrate the four desired broadband
functionalities while closely matching the design requirements, suggesting
their potential in visible-range portable medical imaging devices.
