another embodiment the silver containing dopant can be introduced into substrate 156 using a method other than ion implantation for example in one embodiment substrate 156 along with a patterned mask formed thereon can be immersed in a heated bath containing a silver salt such as silver nitrate silver can diffuse into the exposed portions of the substrate from the bath to locally increase the refractive index of the substrate the bath can be under pressure in this embodiment fig8 a and 8b illustrate a diffractive lens 160 constructed in accordance with another embodiment of our invention comprising alternating transparent and opaque regions 162 and 164 opaque regions 164 of lens 160 are formed by depositing an opaque material such as cr on a transparent substrate 166 and lithographically patterning the cr so that the cr covers regions 164 of importance regions 162 and 164 form a diffraction pattern that causes lens 160 to focus light onto a small spot however the spacing of regions 162 and 164 increases and decreases as one proceeds circumferentially around lens 160 thus different regions of lens 160 exhibit different focal lengths lenses comprising alternating opaque and transparent regions are described in more detail in the above incorporated 39 608 application fig9 represents a numerical reconstruction of the focal spot in a plane along the optical axis of a lens a collimated laser beam 71 is diffracted by lens 72 and focused into a spot 73 onto recording media.