  The ultimate miniaturization of any optical system relies on the reduction or
removal of free-space gaps between optical elements. Recently, nonlocal flat
optic components named spaceplates were introduced to effectively compress
space for light propagation. However, space compression over the visible
spectrum remains beyond the reach of current spaceplate designs due to their
inherently limited operating bandwidth and functional inefficiencies in the
visible range. Here, we introduce multi-color spaceplates performing achromatic
space compression at three distinct color channels across the visible spectrum
to markedly miniaturize color imaging systems. In this approach, we first
design monochromatic spaceplates with high compression factors and high
transmission amplitudes at visible wavelengths based on a scalable structure
and dielectric materials widely used in the fabrication of meta-optical
components. We then show that the dispersion-engineered combination of
monochromatic spaceplates with suitably designed transmission responses forms
multi-color spaceplates that function achromatically. The proposed multi-color
spaceplates, composed of amorphous titanium dioxide and silicon dioxide layers,
efficiently replace free-space volumes with compression ratios as high as 4.6,
beyond what would be achievable by a continuously broadband spaceplate made of
the same materials. Our strategy for designing monochromatic and multi-color
spaceplates along with the presented results show that strong space-compression
effects can be achieved in the visible range, and may ultimately enable a new
generation of ultra-thin optical devices for various applications.
