Arrays of planar, Fresnel-like microlenses are prepared by a spontaneous chemical process of periodic precipitation (PP) occurring in a thin layer of a dry gel, and initiated by wet stamping. The PP lenses focus white light more efficiently than the conventional Fresnel zone plates of similar dimensions. Nanoscale topographies of the micropatterned gels can be replicated into transparent elastomers, and used for focusing based on optical path differences. Experimental observations for both types of structures are in agreement with the Fresnel diffraction calculations.
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The nonlinear correction to the purely linear Jablczynski’s dependence was due in part to the finite amount of the inner electrolyte inside the circle and in part due to the curvature of the source. These effects will be discussed in detail in an upcoming communication.
Formation of periodic precipitation bands requires purely diffusive transport of participating chemicals, and is suppressed by hydrodynamic effects in the vicinity of the source of the outer electrolyte (here, ). The higher the concentration of this electrolyte, the larger the difference in osmotic pressures between the stamp and the gelatin film, and the larger the transient precipitation zone over which the hydrodynamic effects settle down (cf. Ref. 14).
Fresnel lenses have many focal distances (see Ref. 1), but only the farthest focal distance has the greatest intensity. This focal distance was chosen for all of our calculations.
As the PDMS is transparent, it was difficult to provide an exact measurement of the experimental focal point.
