An optical element comprising: synthetic diamond material; and an anti-reflective surface pattern formed directly in at least one surface of the synthetic diamond material, wherein the optical surface pattern comprises a plurality of projections separated by trenches, the projections spaced apart with a periodicity d, wherein 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, wherein 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], and wherein the synthetic diamond material of the optical element has an absorption coefficient measured at room temperature of ≤ 0.2 cm -1 at a wavelength of 10.6 μm, characterized in that the optical element has a transmittance of at least 98% at the operating wavelength of the optical element; and characterized in that the optical element is obtained by a method of: forming a patterned resist layer on at least one surface of the synthetic diamond material ; etching the at least one surface of the synthetic diamond material through the patterned resist layer ; and removing the patterned resist layer to leave the anti-reflective surface pattern formed directly in the at least one surface of the synthetic diamond material , wherein the etching comprises an inductively coupled plasma reactive ion etching process comprising an oxygen flow rate of between 20 to 50 sccm O 2 , a chamber pressure of between 5 and 20 mTorr, and an ICP power of between 600 and 1100 W. synthetic diamond material; and an anti-reflective surface pattern formed directly in at least one surface of the synthetic diamond material, wherein the optical surface pattern comprises a plurality of projections separated by trenches, the projections spaced apart with a periodicity d, wherein 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, wherein 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], and wherein the synthetic diamond material of the optical element has an absorption coefficient measured at room temperature of ≤ 0.2 cm -1 at a wavelength of 10.6 μm, characterized in that the optical element has a transmittance of at least 98% at the operating wavelength of the optical element; and characterized in that the optical element is obtained by a method of: forming a patterned resist layer on at least one surface of the synthetic diamond material ; etching the at least one surface of the synthetic diamond material through the patterned resist layer ; and removing the patterned resist layer to leave the anti-reflective surface pattern formed directly in the at least one surface of the synthetic diamond material , wherein the etching comprises an inductively coupled plasma reactive ion etching process comprising an oxygen flow rate of between 20 to 50 sccm O 2 , a chamber pressure of between 5 and 20 mTorr, and an ICP power of between 600 and 1100 W. forming a patterned resist layer on at least one surface of the synthetic diamond material ; etching the at least one surface of the synthetic diamond material through the patterned resist layer ; and removing the patterned resist layer to leave the anti-reflective surface pattern formed directly in the at least one surface of the synthetic diamond material , wherein the etching comprises an inductively coupled plasma reactive ion etching process comprising an oxygen flow rate of between 20 to 50 sccm O 2 , a chamber pressure of between 5 and 20 mTorr, and an ICP power of between 600 and 1100 W. <EOS>