Synthetic diamond optical elements

An optical element includes a synthetic diamond material and an optical surface pattern formed directly in at least one surface of the synthetic diamond material. The optical surface pattern includes a plurality of projections separated by trenches, the projections spaced apart with a periodicity d. The periodicity d is between 65 and 99% of a zeroeth order diffraction limit above which non zeroeth diffraction orders are observed at an operating wavelength λ. The optical surface pattern has a fill fraction in a range 0.1 to 0.6, the fill fraction defined as [area of projection in one periodic unit]/[area of the periodic unit]. The optical element has an absorption coefficient measured at room temperature of ≤0.2 cm-1 at a wavelength of 10.6 μm.