Casting technique puts new spin on AR coatings

21 Feb 2007

Researchers in China believe that spin-casting polymer latex onto glass is an effective way to mass-produce antireflective coatings.

Spin-casting polymer latex onto glass is a convenient and reproducible way to fabricate antireflection (AR) coatings, say researchers at Sichuan University in China. As it is widely used in the electronic industry, the team believes that spin-casting could be used in mass-production environments (Optics Letters 32 575).

Polymer latex coating

"Polymer latex is on the nanoscale and can be prepared easily, so we tried to assemble a nanoporous surface by spin-casting," Yuechuan Wang from Sichuan's College of Polymer Science and Engineering told optics.org. "The present work is a promising start and is applicable to flat optics."

Forming nanoporous structures on the surface of highly transmissive optical components is an attractive way to produce AR coatings. Although methods such as particle deposition have yielded excellent AR properties, the authors say that these approaches are time-consuming when compared with spin-casting.

Wang says that it only takes a few minutes to coat an area of 100 cm². He adds that optical transmittances at 550 nm of 95.7% for a single-sided coating, and 99.5% for a double-sided coating, have been achieved.

The refractive index and the thickness of the coating control the antireflective properties and, as Wang explains, both of these factors can be adjusted during spin-casting.

"The porosity of the coating (which influences the refractive index) can be adjusted by changing the spin rate and the coating-solution concentration," he said. "The thickness of the monolayer particle film can be adjusted by using latex particles of the desired diameter and/or by forming multilayers."

Wang and colleagues Hao Jiang and Ke Yu prepared their polymer latex by emulsion polymerization of methyl methacrylate. Filtered before use, the polymer latex was typically 110 nm in diameter.

The team studied the resulting coatings using both atomic-force microscopy and a scanning-electron microscope. These methods revealed that the antireflective coatings were highly porous and highlighted the dependence on factors such as spin speed. For example, the team says that some overlaying of particles was visible at low spin speeds of approximately 1000 rpm, while large gaps existed between particles at speeds approaching 7000 rpm.

"We are now extending our studies, by spin-casting and other ways, to fabricate nanoporous structures with nanoparticles for applications requiring large-area optical surfaces," concluded Wang.