  Polaritonic modes in low-dimensional materials enable strong light-matter
interactions and provide a platform for light manipulation at nanoscale. Very
recently, a new class of polaritons has attracted considerable interest in
nanophotonics: image polaritons in van der Waals crystals, manifesting when a
polaritonic material is in close proximity to a highly conductive metal, so
that the polaritonic mode couples with its mirror image. Image modes constitute
an appealing nanophotonic platform, providing an unparalleled degree of optical
field compression into nanometric volumes while exhibiting lower normalized
propagation loss compared to conventional polariton modes in van der Waals
crystals on non-metallic substrates. Moreover, the ultra-compressed image modes
provide access to the nonlocal regime of light-matter interaction. In this
Review, we systematically overview the young yet rapidly growing field of image
polaritons. We discuss their dispersion properties, showcase the diversity of
image modes in various van der Waals materials, and highlight the experimental
breakthroughs owing to the unique properties of image polaritons.
