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Using focus ion beam (FIB), we milled in an opaque gold film a clockwise (right R ) or anticlockwise (left L ) Archimedian spiral grooves around a central subwavelength hole. The polar equation ( ρ, θ ) of the left handed Archimedian spiral is ρ = P · θ/ (2 π ), and the right handed enantiomeric spiral is obtained by reflection across the y axis (see Fig. 1). The 

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geometrical parameter P is the radial grating period and we take its value equal to the 

