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.