Precise optical systems, especially those sensitive to information loss, are typically designed to minimize the use of mirrors. This is due to the Airy disk effect inherent to all conventional mirrors, which is a result of diffraction of the incident light. This effect represents information loss between the incident light and the reflected image. In applications such as space-based observation or lithography information, loss is a serious concern.
NASA Goddard developed a diffraction suppressed mirror to eliminate the problem of undesirable diffraction in optical system mirrors. The impedance mismatch between the edge of the mirror and the vacuum beyond it is responsible for the occurrence of diffraction. To eliminate diffraction, the impedance mismatch must be eliminated.
To achieve this, a gradient of wavelength scale via nanostructures is fabricated on the surface of the mirror substrate. The via nanostructure gradient density is highest at the edge of the mirror and decreases linearly or in a step-wise fashion toward the center of the mirror where the density is the lowest. This technique effectively matches the impedance of the mirror to that of free space and allows for 100 percent transmission of incident light to the reflective layer of the mirror. This is effective for a wavelength of choice. Broadband capability can be achieved by multiple layers of via nanostructures decreasing in feature size from top to bottom layer.