ORGANIC LASERS: LED pumps polymer laser

Polymer lasers have long promised cheap, simple fabrication and broad tunability but have so far suffered from the need for an external laser pump source.

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Polymer lasers have long promised cheap, simple fabrication and broad tunability but have so far suffered from the need for an external laser pump source. Not for long, according to new research from the University of St. Andrews (Fife, Scotland).

While laser pump sources have steadily gotten smaller, they remain the limiting step in terms of cost and ease of fabrication. “There is great interest in removing the need for this laser, for example by making an electrically pumped injection laser,” says Ifor Samuel of St. Andrews. “However, that is extremely challenging.”

The reason for that is that polymer lasers, as highly disordered systems, have very low charge mobility. That means that in direct electrical excitation, high current densities are needed, and significant losses are incurred through absorption of charge carriers and at the metal contacts.

The group has over recent years worked to reduce the lasing threshold for polymer lasers and thereby the size of the pump lasers required. Now, Samuel and his colleagues Graham Turnbull and Ying Yang at St. Andrews’ Organic Semiconductor Centre have demonstrated a hybrid approach that is the closest yet to directly electrically pumped polymer lasers. They take advantage of the favorable electronic properties of light-emitting diodes (LEDs), close-coupling light from an indium gallium nitride (InGaN) LED into the polymer gain medium.

“We are dealing with the low mobility of the organic semiconductors by taking a hybrid approach—that is, we generate light in a high-mobility nitride LED but then have the lasing properties of the polymer,” Samuel says. “Such hybrid approaches are very powerful—for example, a liquid-crystal-display TV combines inorganic transistors with an organic display element.”

Thin film of dye

The team started with corrugated silica substrates, which form the Bragg grating of a distributed-feedback laser. Such a structure narrows the linewidth of the lasing output by Bragg scattering as the optical-feedback mechanism, as opposed to mirrors. The researchers then spin-cast thin films of a commercially available fluorene copolymer onto the substrates. The dye’s absorption spectrum has a broad shoulder at 450 nm, providing a well-matched gain medium to be pumped by their LEDs. The polymer is then coated with Cytop (a low-refractive-index, highly transparent fluorinated polymer) as a barrier to oxidation and water absorption. The composite structure serves as a waveguide supporting the lowest-order modes of the polymer emission.

Pump light derived from a commercially available LED is coupled in through the Cytop layer. A dichroic mirror fixed to the rear surface of the substrate allows a double pass of the pump light through the polymer medium, reflecting 98% at the pump wavelength and greater than 90% at the lasing wavelength (see figure). The high current density required to induce lasing is overcome by running the LED in a pulsed mode, with a laser-diode driver delivering pulses of duration 36 ns at 20 Hz.

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A semiconducting-polymer laser is pumped with an InGaN LED (top; Courtesy of Graham Turnbull). Organic semiconductors can have various colors, and can be dissolved and made into various shapes (bottom; Courtesy of Ifor Samuel).1
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Above a threshold of about 140 A (corresponding to an intensity of 217 W/cm2), a narrow peak in the broad emission arises at 568 nm, with its intensity climbing sharply with current—the signature of lasing.

The output was analyzed through angle-resolved photoluminescence measurements, which showed that the surface emission arises from light in the lowest-order TE0 and TM0 modes, which is then Bragg-scattered out of the polymer from the diffraction grating formed by the substrate.

“We think that this result shows a new route to very inexpensive visible lasers, making polymer lasers a more practical option for applications,” says Turnbull. “This approach promises few-dollar lasers tunable across the visible spectrum and so should have a wide range of potential applications. In particular, we envisage applications in point-of-care medical diagnostics, spectroscopy and sensing, and displays.”

“It’s a simple but effective implementation,” says John de Mello of Imperial College London. The importance of the work, he says, is in its inexpensive nature. “Substituting the usual laser diodes with an LED is a logical and cost-effective thing to do. In that context it’s very nice work.” One quick way to push the approach ahead, he says, will come as the coupling of the pump light into the organic layer is optimized.

The authors note that because the commercial LEDs they used have not been electronically designed for pulsed operation, significant improvements are in store for the approach as LED and driver design are matched to the application. For now, the group plans to develop the resonator design to increase output power and demonstrate lasing at other wavelengths.

D. Jason Palmer

REFERENCE

1. Yang et. al., Appl. Phys. Lett., DOI: 10.1063/1.2912433 (2008).

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