Plastic laser one step closer

Materials World magazine
,
1 Feb 2008

Polymer lasers could be one step closer to reality thanks to research conducted at the University of California at Los Angeles (UCLA), USA, using a luminescent semiconducting polymer. Such devices offer potential for brighter and cheaper polarised light sources.

Working with experts at Canon’s Nanocomposite Research division in Tokyo, Japan, the UCLA team has developed a method for making the polymer MEH-PPV (methoxy-ethyl-hexyloxy-phenylene-vinylene) emit and confine polarised light.

The optical properties of the material are highly polarised along the axis of its polymer chains. Researchers have determined that aligning the chains in one direction causes them to conduct light.

This alignment was achieved by forcing the chains into one to two nanometre-thick holes that were drilled in an ordered series on a nanoporous silica film using surfactant templating methods. The tight fit of the holes prevents the chains from coiling, keeping them straight and pointing in the same direction.

Optical gain is generated by a pumped laser that excites the MEH-PPV. The emitted light is then wave-guided into the film and collected from the edge.

Polymer chain concentration is thickest at the top of the silica film, as the chains are incorporated by difffusion from the top surface. ‘If the pores are not filled completely down to the bottom, the pores on top fill to a greater extent,’ explains Professor Ben Schwartz of UCLA. ‘This is what creates the graded-index waveguide that confines the light and makes the laser possible.’

All of the chains are involved in the lasing, and emit polarised light without any external optical elements. According to Schwartz, using this process makes it 20 times easier to achieve optical gain than if the polymer chains were randomly oriented.

The main advantage of a plastic laser over other polarised light sources is price, he adds. Polymers are easy to manipulate and process. However, Schwartz sees no immediate future for a commercial plastic laser product, largely because the UCLA device relies on optical pumping. ‘The hurdle of being able to produce electrically-driven laser light from these materials is a large one that may or may not be solved in the next few years.’

Professor Sir Richard Friend of the Optoelectronics Group at Cambridge University, UK, agrees. He notes that the presence of the silica will act as an insulator, preventing electrical current from flowing. ‘But achieving the alignment and improving the optical structure of MEH-PPV is very impressive, and I’m sure the lessons here will be transferrable [to future organic lasers].’