Typically, optical devices such as imaging systems require the use of multiple lenses to form a high quality image with low aberrations. Use of a material with a constant index of refraction may require that each lens has a particular shape such as a convex or concave shape to direct light passing through the lens. Such designs often have structural limitations relating to a shape of a lens or a requirement for a number of different lenses and structural supports that add cost to an optical design or add a complexity to the design of such a structure that limits their applicability in different products. For instance, cost and design constraints can limit the application of such lenses.
Gradient refractive index (also referred to as “GRIN”) optics use materials with variable index of refraction and thus often can be made to conform to size and shape constraints in many optical devices, apparatuses and applications. GRIN materials are often made using an ion exchanged cation modification to an optical device made of a base oxide glass matrix to create a modified refractive index profile that is different from the refractive index profile of the starting base glass matrix. Other GRIN optics have been formed by using a nanometer (nm, 1 nm=10−9 meters) thick layer (nanolayer) polymer coextrusion forced assembly process to stack thousands of polymer layers to fabricate a GRIN sheet, or by solidifying through a lamination process thin layers of glass of varying index to form a solid GRIN structure. Additionally, a low refractive index polymer matrix doped with specific concentrations of high refractive index, nanometer sized, particles (nanopowders) dispensed through an inkjet dispenser can yield droplets of varying refractive index that can be layered to form a GRIN structure. Such GRIN optics, however, have drawbacks.
For instance, the ion exchange approach is often limited to oxide matrices that are strongly absorbing in the infrared spectrum of light, which limits their applicability. Organic polymer matrices also have strong absorption bands in the infrared. As another example, ion exchange processing requires an optical device to be submerged in a high temperature molten salt bath for hours of time, which limits their integration onto many types of substrates and their applicability for use with different optical apparatuses. Further, the fabrication method employed for ion exchange or the above referenced polymer coextrusion can require long exchange times or integration of thousands of multi-layer polymer stacks, which make the processes time consuming and difficult to scale for large production volumes. Organic polymer materials have limited thermo-mechanical and environmental stability. Laminated materials have cumbersome manufacturing protocols and limited applicability and stability. Additionally, such GRIN optics may only provide a gradient along one or two dimensions relative to a face of the material.