Binary synthetic Bragg holograms: fabrication issues and first results

The high transverse resolution available with modern microfabrication also allows the manufacture of binary surface relief gratings with subwavelength features. Previous authors have proposed that by employing pulse width modulation these grating could attain the high diffraction efficiencies of multi-level diffractive elements. The major disadvantage of this modulation scheme, however, is the high ratio of groove depth, h, to the minimum transverse feature Δ _min (i.e. h/Δ_min≫10). To overcome this problem, the present authors propose a modulation scheme based on Bragg incidence and pulse position (or frequency) modulation of the (minus) first diffraction order. The ratio h/Δ_min is then typically reduced by an order of magnitude