Duty cycle tolerant binary gratings for fabricable short period phase masks
Résumé
Wavelength scale 1D binary gratings of rectangular corrugation profile are often used as diffractive elements acting on incident free space waves under different incidence angle, wavelength and polarization. Their optical function is best understood by considering the interplay of the grating modes propagating up and down the periodic walls and slits of the segmented structure. The interference conditions between modes depend on the difference between the effective index of the interfering modes and on their relative amplitude. This difference and relative amplitude depend critically on the ratio between the wall and slit widths which is difficult to control technologically. The condition for a wide tolerance of the effective index difference and for a balanced mode excitation on the wall/slit ratio is found analytically and once for all for a wide class of 1D gratings. It is also found that TE interference elements may exhibit a very wide wall/slit ratio tolerance domain.