Quantum Dot Lasers Remain Stable

Daniel C. McCarthy

Quantum dot lasers are, by nature, more thermally stable than quantum well lasers. Their confinement of a limited number of electrons and holes also limits thermal carrier distribution. Used as an active region for semiconductor lasers, quantum dots can help reduce threshold current and temperature sensitivity.

As a rule, however, quantum dot lasers have difficulty operating at the ground state without the aid of highly refractive coatings. And even then, characteristic temperature -- a measure of temperature sensitivity -- is frequently lower than desired. A low characteristic temperature indicates instability in a laser's threshold current density as temperature rises.

Collaborators at the Jet Propulsion Laboratory in Pasadena, Calif., and at the University of New Mexico in Albuquerque have demonstrated ground-state lasing in 1.5-mm-long-cavity quantum dot lasers that maintain a characteristic temperature of 78 K. Heat-sink temperatures were up to 100 °C, and the lasers emitted at 1.3 µm. At room temperature, the lasers demonstrated a differential quantum efficiency of about 55 percent and a single-facet output power of 50 mW.

The device design vertically stacked four InAs quantum dots, embedding each stack in a strained InGaAs quantum well. The structures delivered a high injection efficiency and optical gain.

Researchers fabricated the device structures using molecular-beam epitaxy on GaAs substrates.

"I think the most important thing is the materials," said Yueming Qiu, who fabricated the lasers at the Jet Propulsion Laboratory from materials provided by his collaborators at the university. "I mean, [the] high density of quantum dot and the stacked dot-in-a-well configuration provided enough gain for groundstate lasing."

Single-mode, short-cavity quantum dot lasers are expected to benefit high-speed optical communications applications, but the length of laser cavities needs to be reduced. Uniformity between dots is another issue, because single-mode lasing from the dot ensemble is preferable. The next step of Qiu's work is to develop such single-mode lasers.