  Nanothermochromic diffraction gratings based on the metal-insulator
transition of $\mathrm{VO_2}$ are fabricated by site-selective ion beam
implantation in a $\mathrm{SiO_2}$ matrix. Gratings were defined either (i)
directly by spatially selective ion beam synthesis or (ii) by site-selective
deactivation of the phase transition by ion beam induced defects. The strongest
increase of the diffracted light intensities was observed at a wavelength of
1550\,nm exceeding a factor of 20 for the selectively deactivated gratings. The
observed pronounced thermal hysteresis extending down close to room temperature
makes this system ideally suited for optical memory applications.
