LED Lasers Cost Less

Plastic lasers that convert light from LEDs and can be tuned to every color of the rainbow can be created at a fraction of the cost of existing technologies, said the physicists who developed them at the University of St. Andrews.

The lasers could be suitable in applications such as medical diagnostics or explosives sensing.

Polymer distributed feedback laser pumped by an Indium gallium nitride light-emitting diode. (Photo: Georgios Tsiminis and Ying Yang, University of St. Andrews)

"For more than 40 years, visible organic lasers have required another laser to make them shine. We have now developed a low-cost, easy-to-make plastic laser that converts the light from an LED (of the kind used in flashlights and traffic lights) into laser light," said Graham Turnbull, a member of the research team along with Ifor Samuel and graduate student Ying Yang.

"Conventional visible lasers can cost anything between a few hundred pounds to tens of thousands, but our new laser could be created for less than five pounds (about $10). These LED-lasers offer a better, smaller and brighter alternative to conventional light sources. They are the next generation of low-cost lasers."

The technology provides simple, compact lasers that use plastic-like semiconducting materials. The materials,which combine the virtues of semiconductors with the simple manufacture of plastics. have even been used to make a light-emitting sticking plaster for the treatment of skin cancer.

"One of the major outstanding problems in the field of organic semiconductors is to produce an electrically pumped organic semiconductor laser," Turnbull said. While the first organic LEDs were demonstrated nearly 20 years ago, electrically driven lasing has not yet been demonstrated.

"The reasons for this are the high current densities required, losses due to the presence of contacts and strong absorption by the injected charge carriers," Turnbull said. "All these issues relate to the low charge mobility of the very disordered organic semiconductors."

Optically pumped lasing in organic semiconductors, on the other hand, is readily achievable, "and we have taken a strategy of reducing lasing thresholds through improved optical design to work toward the most simple and compact optically pumped systems."

Historically, he said, this started with a move away from tabletop lasers to pulsed microchip lasers, then to direct excitation with blue diode lasers and most recently with InGaN LEDs

"In our most recent work, we have made the first demonstration of a polymer laser pumped by a light-emitting diode. his makes a very compact and inexpensive integrated package for tunable visible lasers that can be (indirectly) electrically driven."

In a paper published Applied Physics Letters, they demonstrate the feasibility of pulsed-LED pumping of organic lasers and observe the onset of distributed feedback lasing at 568 nm (a wavelength difficult to produce with conventional semiconductor lasers) for drive currents above ~140A (~6 kA/cm2).

A prime advantage of organic semiconductors is that they can be very simply processed -- like plastics -- allowing optoelectronics devices to be printed (for example, organic LED displays and organic field-effect transistor [OFET) drive circuits], Turnbull said. "This means that the polymer laser itself could be manufactured at extremely low cost as a single molded unit of gain medium and resonator." He said this could be used as a disposable unit by itself, or when integrated with the LED pump, which currently costs a few dollars.

One potential application of the technology is for point-of-care medical diagnostics, integrated into a lab-on-chip microanalysis system. A fluorescence-labeled antibody would be mixed with the sample being tested, and the polymer laser would be tuned to one or more wavelengths suitable for excitation of the bound analyte-antibody fluorescence, Turnbull said.

Another potential use is sensing vapors from explosive material, as pioneered by Timothy M. Swager and his colleagues at MIT. "In such an application, the polymer gain medium acts not just as a light source, but as the sensing element itself," Turnbull said. "The interaction of a very low concentration of nitroaromatic explosive molecules (such as TNT) with the polymer chains can strongly affect the light emission from the polymer. This leads to a reversible change in the laser output depending on the presence/absence of the explosive vapors. In each of these applications, a cheap disposable laser would be very attractive."

The laser is now in the laboratory proof-of-concept stage, and the team is working to enhance the laser performance through improved optical and electrical design. "We also aim to expand the range of potential lasing wavelengths through, for example, suitable blends of organic gain media. We hope these improvements will ultimately lead to commercial organic semiconductor lasers," Turnbull said.

For more information, visit: www.st-andrews.ac.uk/