Binary Lenses For Use At 10.6 µm

By combining advances in lithography and electromagnetic grating theory, we have recently demonstrated the ability to produce highly efficient binary gratings and binary lenses for use at 10.6μm. Electromagnetic theory predicts that binary gratings with the proper parameters can achieve a first order diffraction efficiency of nearly 100%. If the periodicity of the grating is on the order of the radiation wavelength, all of the orders become evanescent except for the zero and +1st orders. By choosing the depth to period ratio and duty cycle properly, the zero order can be suppressed, placing virtually all of the incident radiation into the 1st diffracted order. Theoretical calculations have been done only for constant period gratings. However, assuming a lens pattern to be a minor perturbation of a grating, we succeeded in producing an F/5 binary lens with a diffraction efficiency of 96% at 10.6pm. Furthermore, because of the high efficiency of these elements, it becomes practical to consider using more than a single diffractive element in a system. We have constructed a simple afocal telescope from two binary lenses. The telescope has a 2 inch entrance aperture and a magnification of 5. A final point to be considered is the wavefront quality of these elements. Electron beam machines, which are used to write the lens patterns, are designed to draw the pattern in a raster fashion. This quantization sets a limit on the quality of the lens pattern.