Light trapping by arbitrarily thin cavities

Simple plasmonic multilayers are found to perfectly trap electromagnetic waves around specific incidence directions, regardless of the sample thickness. Such a counterintuitive feature is acquired via increasing unboundedly the effective path length of the ray into the film with proper tilt of the optical axis. The obtained optimal setups support wide-band operation while being substantially robust with respect to fabrication defects. Accordingly, the reported designs may exceptionally serve a range of objectives, from photovoltaic conversion and sensing to light modulation and detection, as parts of photonic devices requiring high efficiency combined with compact packaging.