“…This last feature -that the bandwidth can be enhanced and engineered "by design" at the nanoscale -is a key characteristic of the herein reviewed (ultra)slow-light structures. Specific applications range from enhanced and more efficient nonlinear effects (68,73,74,108), to light-harvesting (64,83,89), bio-sensing (87,88), nano-imaging (80,111), optical and acoustic spectral demultiplexing (40-42, 55, 56, 61, 62, 64, 68, 75-81), on-chip spectroscopy (82), non-classical light sources (90,91), cavity-free plasmonic nanolasing (92)(93)(94), enhanced acoustic sensors operating beyond the noise-threshold limit (61,62), and tunable, deepsubwavelength, ultraslow guided Dirac fermions (102,103), plasmons and surface phononpolaritons in atomically-thin crystals and heterostructures (104)(105)(106)(107)(108)(109)(110). Broadband slow-light effects are also attained in other structures above the diffraction-limit, including photonic crystals and CROWs where broadband slow light is usually obtained with group indices of ~30-100 (21)(22)(23)(24)(25)(26)(27)(28), and PT-symmetric structures, which can be broadband and with the light speed reducing to zero at the exceptional point (58,114).…”