  Tuning the properties of materials using external stimuli is crucial for
developing versatile smart materials. A strong coupling among order parameters
within a single-phase material constitutes a potent foundation for achieving
precise property control. However, cross-coupling is pretty weak in most single
materials. Leveraging first principles calculations, we demonstrate the layered
mixed anion compound MoBr$_2$O$_2$ exhibits electric-field switchable
spontaneous polarization and ultrastrong coupling between polar distortion and
electronic structures as well as optical properties. It offers feasible avenues
of achieving tunable Rashba spin-splitting, electrochromism, thermochromism,
photochromism, and nonlinear optics by applying an external electric field to a
single domain sample, heating, as well as intense light illumination.
Additionally, it exhibits an exceptionally large photostrictive effect. These
findings not only showcase the feasibility of achieving multiple order
parameter coupling within a single material, but also pave the way for
comprehensive applications based on property control, such as energy
harvesting, information processing, and ultrafast control.
