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Single photon source emits at higher rate

18 Jan 2008

optics.org speaks to a research team that is "amazed" by the performance of its new single photon source.

Researchers from the US have demonstrated a semiconductor quantum dot (QD)-based single photon source (SPS) with an emission rate five times higher than previous SPSs. Thanks to an innovative microcavity design, the SPS emits photons at a rate of 4.0 MHz. Until now, the highest measured SP rates were around 200 kHz. (Nature Photonics doi:10.1038/nphoton.2007.227)

"We wanted to make a brighter SPS for applications such as quantum cryptography and we have been amazed by how many single photons we can get out of these structures," Stefan Strauf, a researcher at the University of California, Santa Barbara (UCSB), told optics.org. "We also want to study the fundamental light-matter interaction, which is important for developing more efficient optoeletronic devices such as LEDs, lasers and detectors."

The key to the increased performance is a modification of a traditional semiconductor cavity design. In conventional cavity structures, micropillars with a diameter of 1 µm or less are etched onto the semiconductor. This results in rough sidewalls which scatter photons and limit the Q-factor.

Strauf's team opted to confine the photons in a different way. "Instead of etching tiny micropillars, we etched large trenches with diameters of around 20 microns onto the semiconductor," explained Strauf. "We then added embedded oxide-tapers within the GaAs/AlGaAs microcavities."

This approach confines photons into a tiny space but avoids the scattering losses experienced in the conventional design and results in a Q-factor up to 50,000. What's more, the trench design is not as brittle as micropillars, which allowed the team to attach electrical contacts.

"It is impossible to attach electrical contacts to SPSs that are based on etching micopillars with less than 1 micron diameter as they would break," commented Strauf. "Our structure allows embedded electrical contacts and gates to be added. This allows on-chip tuning of the mode-emitter resonance as well as control over the polarization of the emitted photons. The wavelength of our device can be tuned over 900-1000 nm with around 1-2 nm accuracy."

Future work will focus on devices emitting at telecommunication wavelengths and the team hopes that the new design will be adopted by other research groups.

The research team at UCSB is headed by Dirk Bouwmeester, Pierre Petroff and Larry Coldren. Stefan Strauf is now an assistant professor at the Stevens Institute of Technology, US.

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