Antireflection coating with fluoropolymer for a novel organic nonlinear optical crystal: 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (ADPA).
Prior to the frequency doubling of a laser diode through a ring resonator, reflection loss on the surface of a novel organic second-order nonlinear optical crystal, 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (APDA), was decreased. An optically flat crystal surface with a flatness of λ/10 (λ = 633 nm) and surface roughness of 1.5 nm rms was obtained when an ultraprecision diamond turning lathe was used to machine the crystal surface. After this surface was coated antireflectively with an amorphous fluorinated polymer, Teflon AF-2400, the transmittance at 810 nm was increased to 98%. A laser-diode-pumped ring resonator was built with this crystal, and a blue second-harmonic emission at 405.5 nm wassuccessfully generated.