Princeton Promises Much on Organic Lasers

R. Winn Hardin

PRINCETON, N.J. -- It may not carry the fury of the big bang or death gene debates, but to those in the diode laser business organic lasers have sparked more than polite dinner conversation. Princeton University researchers say they have the evidence to prove that organic lasers are more than superluminescent light bulbs.
Touted as a tunable laser medium, organic lasers offer low costs (compared with semiconductors), small packages and flexible designs for various applications.
The Princeton group began by doping a small molecular weight organic material of aluminum and hydroxyquinoline (Alq₃) with DCM laser dye and depositing the film on an SiO₂-coated InP substrate. A nitrogen laser operating at 337 nm and 50 Hz pumped the organic laser's active region with 1.2 µJ cm²², eliciting 50 W of emission at 645 nm. "There are four or five things that are clear indicators of stimulated emission," said Princeton's Stephen Forrest. "Others have shown a couple, but we've shown all five."
Clear thresholds, polarization, dispersion, cavity modes and line narrowing are some clear indicators of lasing, he said. Princeton's experiments show evidence of longitudinal modes centered around 645 nm, line narrowing of less than 1 Å and polarization of >15 dB along the plane of the active material.

Double-heterostructure performs
Researchers tried both slab and double-heterostructure layers of thin films when testing the organic laser. Forrest's results indicate thresholds of 3.5 nJ at 30 percent optical efficiency for the monolith and a 1-nJ threshold and 70 percent optical efficiency for the heterostructure. A monolith is less efficient because it does not sandwich the active region between two semiconductor layers to form a laser cavity.
Despite these results, "the debates won't be resolved until after a lot of data has been accumulated," said Larry Dalton of the University of Southern California at Los Angeles.
An area where Princeton has shown leadership, Dalton said, is the manufacturing process of organic lasers. In a recent paper published in the Lasers and Electro-Optics Society newsletter, Princeton demonstrates organic lasers from the IR to the blue on plastic chips.
Whether or not Princeton answers the eternal question, "to lase or not to lase," it still wins in the end. Universal Display Corp. of Bala Cynwyd, Pa., recently licensed the technology for commercial use.