The optics of multiple ultrasharp sub-wavelength grooves in metal is studied theoretically. Focus is on the transition from a single groove, where the scattering cross section is significant and can exceed the groove width, to infinitely many grooves in a periodic array with very low reflectance. When the multiple-groove array is illuminated by a plane wave the out-of-plane scattering is found to be extraordinarily large compared with the expected maximum from a geometric-optics estimate even for array widths of many wavelengths. The out-of-plane scattering is even higher per groove compared to the single-groove case. This is explained as an effect of surface plasmon polaritons (SPPs) generated at one groove being scattered out of the plane by other grooves. This is supported by studies of the transmittance, reflectance, and out-of-plane scattering, when an SPP is incident on multiple grooves. When illuminating instead with a Gaussian beam, and observing the limit where the beam is confined well within the multiple-groove array, the total reflectance is very low and practically no scattering occurs.
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