Second harmonic generation efficiency of Bessel beams

Summary form only given. Second-harmonic generation is widely used in commercial systems where one major aim is to achieve high conversion of the fundamental into second-harmonic light. For a plane wave the second harmonic power grows with the square of the interaction length and to the square of the input intensity. For a Gaussian beam the intensity can be increased by focusing it into the crystal. This leads to an enhanced conversion close to the beam waist. But away from the waist the intensity, and thus the conversion, drops quickly due to diffractive spreading. Detailed studies have shown that there is an optimum degree of focusing where the size of the focus depends on the crystal length. If the diffractive spreading of the fundamental beam is overcome and thus the high intensity retained over a greater distance, the conversion efficiency can be increased. This can be achieved using optical waveguides, but recently the use of ´non-diffracting´ Bessel beams was proposed to obtain higher conversion.