Fabrication of Fresnel zone plate and phase mask by index change in the bulk of polymers with femtosecond laser pulses

Abstract

In recent years, direct writing in the volume of transparent materials using femtosecond pulses has been growing considerably. Applications are wide, namely from holography purposes to the design of optical waveguides or multifocal lenses for optical corrections in micromachining [1]. It is known that permanent modification in the refractive index of the material can be created by tightly focused femtosecond pulses, as a consequence of non linear absorption within the focal volume [2].In our study, we modified permanently the refractive index in the bulk of a transparent polymer in the visible spectrum. We investigate the range of index change in dependence of laser parameters. Preventing crack apparition or too low index change, we varied notably pulse energy (in the order of 0.2 µJ), spot size (from 2 to 4 µm) on target and laser pulse overlap. For some of the created microstructures, namely the phase masks or diffraction gratings, we evaluated diffraction efficiency on diffraction patterns by power measurements. Indeed, the power ratio of the first order over the zero order of diffraction allows for determining this efficiency.The process uses a Satsuma (Amplitude Systemes) amplified fiber laser that delivers 400 femtosecond pulses (5W@2MHz). Thanks to a frequency conversion to a 515 nm wavelength the spot size can be even reduced and mulptiphotonic absorption efficiency can be increased. This setup allows for design and laser production of multifocal lenses (optical diffractive phase zone plate). In addition, to illustrate the possibilities of our phase masks, we machined patterns which project logos after illumination with coherent light. The machining conditions of these two realizations are very robust.In recent years, direct writing in the volume of transparent materials using femtosecond pulses has been growing considerably. Applications are wide, namely from holography purposes to the design of optical waveguides or multifocal lenses for optical corrections in micromachining [1]. It is known that permanent modification in the refractive index of the material can be created by tightly focused femtosecond pulses, as a consequence of non linear absorption within the focal volume [2].In our study, we modified permanently the refractive index in the bulk of a transparent polymer in the visible spectrum. We investigate the range of index change in dependence of laser parameters. Preventing crack apparition or too low index change, we varied notably pulse energy (in the order of 0.2 µJ), spot size (from 2 to 4 µm) on target and laser pulse overlap. For some of the created microstructures, namely the phase masks or diffraction gratings, we evaluated diffraction efficiency on diffraction patterns by power mea...