  The full control of the fundamental photophysics of nanosystems at
frequencies as high as few THz is key for tunable and ultrafast nano-photonic
devices and metamaterials. Here we combine geometrical and ultrafast control of
the optical properties of halide perovskite nanoparticles, which constitute a
prominent platform for nanophotonics. The pulsed photoinjection of free
carriers across the semiconducting gap leads to a sub-picosecond modification
of the far-field electromagnetic properties that is fully controlled by the
geometry of the system. When the nanoparticle size is tuned so as to achieve
the overlap between the narrowband excitons and the geometry-controlled Mie
resonances, the ultrafast modulation of the transmittivity is completely
reversed with respect to what is usually observed in nanoparticles with
different sizes, in bulk systems and in thin films. The interplay between
chemical, geometrical and ultrafast tuning offers an additional control
parameter with impact on nano-antennas and ultrafast optical switches.
