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Single quantum dot lowers lasing threshold

03 May 2007

Applications such as telecommunications and optical computing could benefit from a quantum dot laser with a lasing threshold of 500 nW.

A quantum dot (QD) microcavity laser with a sub-microwatt lasing threshold could narrow the gap to producing a single state laser. Such a low power laser could have applications in on-chip and off-chip communications as well as optical computing, claim scientists in the US. (Physical Review Letters 98 117401)

"The goal of a single state laser is an important one," Glenn Solomon from NIST told optics.org. "We try to make lasers with lower and lower threshold. Our 'few emitter' laser requires very little input power and begins to lase almost immediately."

Developed by NIST as well as Stanford and Northwestern Universities, the QD microdisk laser has a threshold of just 500 nW and emits at 905 nm. The team says it reduced the lasing threshold by aligning a single QD state with a high quality cavity mode.

"The key feature is that for the first time we can see the effect of a single QD state on the operating characteristics of a laser," said Solomon.

The team fabricated its microdisks using molecular beam epitaxy. Each device consists of a layer of indium arsenide QDs sandwiched between various layers of gallium arsenide. The end result is a 1.8 micron diameter disk with a QD density of 50 QDs/µm2 and a quality factor exceeding 15000 before the system lases. Optically pumped by a Ti:Sapphire laser emitting at 780nm, the microdisk lased at temperatures between 6 K and 55 K corresponding to a tuning range of 1.5 nm.

The team chose to couple the QDs to a microdisk as opposed to a micropillar or 2D photonic crystal cavity due to the high quality of its microdisks.

"It is a system in which we have extensive experience. We find the microdisk easy to work with and the cavity mode quality is more forgiving to the nuances of processing. The key difficulty we had to overcome was to make a very high quality microdisk cavity which remains small enough to limit the number of interacting QDs," commented Solomon.

The team intends to continue to work towards creating a single emitter laser. "We will look more closely at the theory of these structures so that we can compare them to the other systems in solid-state," commented Solomon. "Those using different types of emitters and those using different cavities as well as the single atom system."

One of the key difficulties left to be overcome is the cavity quality and the quantum dot coupling. "We cannot predetermine where the QD will be on the surface without sacrificing the cavity quality. Thus we must look for both good cavities and good alignment between the QD and the cavity," concludes Solomon.

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