Investigation of optical properties of injection moulded subwavelength gratings

Surface reflections from optical transmission components are in many cases unwanted and cumbersome. Thin film coating is the conventional technique used for anti-reflection treatment of optical components. In recent years subwavelength gratings have been studied as a replacement for thin films. Subwavelength gratings are microstructures that can be formed on one or both sides of a substrate. Typically an optical component needs to be AR-coated on both sides. We have fabricated injection moulded subwavelength gratings superimposed upon a blazed grating structure in polycarbonate. The gratings are initially formed by electron-beam lithography and subsequently replicated using the same process which is used to manufacture standard plastic compact discs (CDs). There are several problems when trying to characterize a component such as a blazed transmittance grating. First of all there is the spread of internal reflections. Light that is reflected inside the substrate is shifted in lateral position due to the angle of the grating. We have thoroughly investigated the effects of decrease in grating efficiency due to internal reflections and also tried to minimize these effects by appropriately treating both sides of the plastic CD.