Enhanced harmonic generation efficiency using focussing devices based on Fresnel diffraction

We propose the use of focusing devices based on Fresnel diffraction (such as a Fresnel zoneplate ) as a means to enhance the efficiency of nonlinear-optical processes starting from radiation in the ultraviolet region, where ordinary lenses cannot be used. We present experimental results of a proof-of-concept experiment involving the efficient generation of second- and third-harmonic laser radiation in the visible range. An enhancement of a factor 6 in the third-harmonic yield is obtained by simply limiting the laser beam with a circular aperture. Higher enhancement factors (up to 16) are obtained using a Fresnel zoneplate. The experimental findings are in good agreement with the results of a simple theoretical analysis