Organic Diode Lasers One Step Closer

Principle of an optically pumped organic laser diode made of organic/inorganic perovskites. (Source: Y. Jia, PSU / NPG)

Diode lasers use inorganic semi­conductors grown in elaborate high vacuum systems. Now, a team of researchers from Penn State and Princeton Univer­sity have taken a big step toward creating a diode laser from a hybrid organic-inor­ganic material that can be deposited from solution on a labora­tory benchtop. “It’s usually not a big leap to turn a light emitting diode into a laser,” said Chris Giebink, assistant professor of electrical engi­neering, Penn State. “You essen­tially just add mirrors and drive it harder. Once organic light-emitting diodes were invented 30 years ago, everybody thought that as soon as we had rela­tively effi­cient OLEDs, that an organic laser diode would soon follow.” As it turned out, organic diode lasers proved to be really hard to make.

An organic laser diode could have advan­tages. First, because organic semi­conductors are relatively soft and flexible, organic lasers could be incor­porated into new form factors not possible for their inor­ganic counter­parts. While inorganic semi­conductor lasers are relatively limited in the wave­lengths, or colors, of light they emit, an organic laser can produce any wave­length a chemist cares to synthesize in the lab by tailoring the structure of the organic molecules. This tuna­bility could be very useful in applications ranging from medical diag­nostics to environ­mental sensing.

Nobody has yet succeeded in making an organic laser diode, but the key may well involve related materials — organic/inorganic perov­skites — that have gotten a lot of attention in the research community over the last few years. This hybrid material has already been responsible for a meteoric rise in the efficiency of photo­voltaics, Giebink said. Perov­skites are fairly common minerals that share a similar cubic crystal structure. Somewhat para­doxically, one of the reasons these hybrid perovskite materials work so well in solar cells is that they are good light emitters. For that reason, they are also of interest for use in LEDs and lasers. The material Giebink and his colleagues are studying is composed of an inorganic perov­skite sub­lattice with relatively big organic molecules confined in the middle.

“The ultimate goal is to make an elec­trically driven perovskite laser diode,” said Giebink. “That would be a game changer. It is fairly easy to make the perovskite material lase by optical pumping, that is, by shining another laser on it. However, this has only worked for very short pulses due to a poorly understood phenomenon we call lasing death. Getting it to go conti­nuously is a key step toward an eventual electrically driven device. What we found in this recent study is a curious quirk. We can avoid lasing death entirely just by lowering the tempera­ture of the material a little bit to induce a partial phase tran­sition.”

“When we lowered the tempera­ture below the phase transition, we were surprised to find that the material initially emitted light from the low tempera­ture phase, but then changed over within 100 nano­seconds and began lasing from the high-tempera­ture phase — for over an hour,” said Yufei Jia, a graduate student in Giebink’s lab. “It turned out that as the material heated up, although most of the material remained in the low-tempera­ture phase, small pockets of the high-tempera­ture phase formed, and that was where the lasing was coming from.”

In some inorganic lasers there are narrow regions called quantum wells where charge carriers can be trapped as the electrons and holes fall into the wells. The intensity of the lasing depends on how many charge carriers can be packed into the quantum wells. In the perovskite material, the arrange­ment of the high-tempera­ture-phase inclusions inside the low tempera­ture bulk seems to mimic these quantum wells and may play a role in enabling the continuous lasing. “The jury is still out on this expla­nation,” Giebink said. “It may be something more subtle.”

Never­theless, these results do point toward an oppor­tunity to engineer a material that has the built-in qualities of this mixed phase arrange­ment, but without having to actually cool the material to low tempera­ture. The current paper points to a couple of ideas for how those materials could be designed. The next big step then is to switch from optical pumping with an external laser to a perov­skite laser diode that can be powered directly with electrical current. “If we can solve the elec­trical pumping problem, perovskite lasers could turn into a tech­nology with real commercial value,” Giebink said. (Source: PSU)

Reference: Y. Jia et al.: Continuous-wave lasing in an organic–inorganic lead halide perovskite semiconductor, Nat. Photonics, online 20. November 2017; DOI: 10.1038/s41566-017-0047-6

Link: Applied Optoelectronics & Photonics Lab, Pennsylvania State University, University Park, USA Thin film optics, materials and devices (B. Rand), Princeton University, USA

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