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Nanodiscs boost polymer lasers

05 Dec 2003

A diffraction grating made from tiny gold discs could lead the way towards electrically-pumped organic polymer lasers.

Researchers at Ludwig-Maximillians University in Germany believe they have taken a crucial step towards making an electrically-pumped organic polymer laser. By using an array of miniature gold discs as a diffraction grating, the team says its approach could eliminate some of the problems that have held back the development of current-driven devices. (Advanced Materials 15 1726)

“Electrically-pumped lasing has proven difficult because metal contacts tend to quench molecular excitation energy,” researcher Thomas Klar told optics.org. “Our research shows that gold nanodiscs on indium-tin-oxide (ITO), both of which are conducting materials, can be used as distributed feedback (DFB) mirrors.”

Klar’s laser is built on a glass substrate. It consists of a 110 nm thick ITO layer which is covered by a 2D grating of 110 nm diameter gold nanodiscs. This is topped with a 460 nm layer of a polymer called LPPP. Klar says gold was chosen as it is an inert material that does not oxidize.

The researchers pump the device using 130 femtosecond pulses from a regeneratively amplified Ti:Sapphire laser emitting at 400 nm. The device itself relies on second-order diffraction from the nanodisc grating to provide feedback.

According to Klar, a pump pulse energy of 2.7 nJ results in a strong emission at 492 nm. “The surprising result is that this lasing threshold is only a factor of two larger than the lasing threshold for a metal-free LPPP-DFB laser,” report Klar and colleagues. “This is only a small increase as the common opinion has been that introducing metals into DFB structures should increase the threshold much more.”

The team also believes it will be able to tune the laser’s emission by varying the grating constant of the thickness of the LPPP layer. “We expect that we can tune the laser emission from 480 to 505 nm,” says Klar. “Other polymers will give different spectral regions of gain.”

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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