Plastic laser on the horizon

09 Oct 2007

Encapsulating polymer chains in nanopores forces luminescent semiconducting plastics to emit polarized light.

The plastic laser has come one step closer, with researchers in the US and Japan developing a new way to make luminescent semiconducting polymers emit and confine polarized light. The results could lead to brighter polarized sources for products with LED-type displays, such as portable computers and mobile phones.

Benjamin Schwartz and Sarah Tolbert of the University of California at Los Angeles and colleagues used conjugated polymers, remarkable materials that combine the electrical and optical properties of semiconductors with the mechanical properties of plastics. The optical properties of these 1D organic semiconductors are not isotropic, but are highly polarized along the axis of the polymer chain. By aligning the polymer chains in one direction, the chains could be made to absorb and emit polarized light.

The scientists achieved this alignment by encapsulating the polymer chains on the nanoscale in an ordered nanoporous silica film. The holes are so small that the polymer chains have no space to coil up, explained Tolbert. They must lie straight so that they all end up pointing in the same direction.

Lining up the chains in this way also means that all of the chains can take part in lasing. This is because the way the polymer fills the pores helps confine light in the material, enhancing lasing by producing a so-called graded-index waveguide.

"The filling is highest at the top of the film and decreases throughout it," Tolbert told nanotechweb.org. "This generates a gradation in the refractive index, which in turn confines the light." In contrast, confining light in most conventional lasers is done by external mirrors. The pores can also make the light polarized without the need for any external optical elements, she added.

According to the team, aligning the polymers and confining the light makes it 20 times easier to get optical gain than if the polymer chains were randomly oriented. Applications include polarized re-emitters and low-cost graded-index waveguides.

The materials were made using self-assembly techniques – the researchers began by fabricating oriented nanoporous silica films with long, straight pores using surfactant templating methods in combination with rubber polyimide substrates. Next, they diffused the semiconducting polymers into the pores and confined them there. Optical gain was generated using a pulsed laser to excite the semiconducting polymer and the emitted light was then wave-guided in the film and collected from the edge. Emission was collected as a function of incident laser power, spatial direction and polarization.

"Under some conditions, we saw normal diffuse luminescence," said Tolbert. "However, when the direction and polarization were matched to the orientation of the polymer chains, we saw directional, bright emission with a narrow spectral width, characteristic of amplified spontaneous emission, or ASE."

The team, which includes researchers at Canon in Japan, is now working on both vertical cavity lasers and electrically driven devices using these aligned polymer-based materials.

The results were published in Nature Nanotechnology.