This Small Business Innovation Research Phase I project describes how one can protect the grooves of the microscopic surface relief pattern in optical diffractive elements by filling them with aerogel, a very fine quartz foam, which protects the grooves against dust and grease and thereby improves the usefulness of various types of these optical diffractive elements by saving them from severely degraded optical performance. In applications of coherent optics, the use of optical diffractive elements is spreading rapidly and they are even important in certain non laser devices. This project combines two recent material technologies and applies them to two important national needs, one of which is energy savings in buildings through innovative window designs, the other is to develop novel optical devices to be used in laser devices. The diffractive structures for windows (when eventually manufactured industrially in large sheets) would be placed in contact with the upper part of a window. Sunlight incident from the southeast, south, or southwest on the structure is diffracted with high efficiency towards the ceiling and from there shines downwards into dark corners, so called `daylighting.` The role of the aerogel is to fill the delicate diffractive grooves and thus protect them against damage from handling and against dust accumulation. The flat aerogel film would further improve thermal insulation and could also serve as a substrate for deposition of heat (infrared) reflecting layers. In that case, the structure would combine the function of daylighting (redirecting of incident light) with keeping out heat rays (low-E glazing) and be thermally insulating. Aerogel is the optimal material to combine with the diffractive plastic for various reasons: 1) Its refractive index is very close to that of air, which is essential in maintaining large efficiency over a wide range of incident angles as will be elaborated on later. 2) Aerogel is optically quite transparent, particularly in a thin layer. 3) Aerogel is lightweight and has recently been deposited in micron thick films. 4) The trapped air gives aerogel very excellent thermal insulating properties
