Optics of a single ultrasharp groove in metal

Optical properties of a single ultrasharp groove of subwavelength width cut in an otherwise flat metal surface are examined theoretically. We calculate optical extinction, scattering, and absorption cross-section spectra for a wide range of groove profiles, establishing several fundamental trends. As grooves are made sharper, amplitudes of oscillations in cross-section spectra and their period decrease, while the absorption level increases, leading eventually to efficient broadband (nonresonant) absorption. Oscillations in scattering spectra are generally more pronounced than those in corresponding absorption spectra. For ultrasharp grooves, oscillations in all spectra can be suppressed by increasing the groove depth. Finally, the level of absorption relative to that of scattering increases as the top groove width decreases, a trend that is analogous to that found when decreasing the size of metal nanoparticles.