Subwavelength Structure (SWS) of Alumina Treated by Aluminum Diphosphate

The subwavelength structure (SWS) surface of γ -AlOOH transformed from sol-gel derived amorphous alumina on a Ge prism and quartz and subsequently followed by the treatment of aqueous aluminum dihydrogen phosphate (AlP) was investigated by optical spectroscopy in the visible region and attenuated total reflection (ATR) infrared spectroscopy, respectively. To facilitate the mechanism of anti-reflection in the SWS based on a graded region in the refractive index, the layer of amorphous alumina was widened by using a two-layer coating and by employing the precursor’s solution with prolonged aging for 6 months. As for the treatment of aqueous phosphate, the enrichment of D 2 O was applied in combination with infrared spectroscopy. The anti-reflection of the SWS coating on quartz became superior by increasing the thickness of amorphous alumina up to 400 nm, which was reproduced in the simulations with an increase in width of the graded region having the principal gradient of the refractive index. By ATR infrared spectroscopy, aqueous aluminum dihydrogen phosphate was inferred to penetrate into the amorphous layer of alumina left at the interface with the substrate and reacted with it, increasing the refractive index of the amorphous layer. By the enrichment of D 2 O, it was revealed that there were few pores accessible by the vapor of D 2 O in the SWS coating treated by AlP. The reaction of the amorphous layer with aluminum dihydrogen phosphate is inferred to prevent corrosion from moisture in the optical glasses beneath in practical camera lenses.