Micro-pyramids offer route to polarization controlled photon emitters
In short: We have developed a concept for emission of strongly polarized light with good control of the linear polarization direction. The light is generated by quantum dots formed on top of elongated hexagonal micro-pyramids. Our results could find use in applications such as energy-efficient backlighting of displays and polarized single-photon sources for quantum communication.
Polarized light is the basis for various technologies, including common LCD displays and futuristic quantum cryptography. The conventional way to generate polarized light is to filter out the desired polarization from an unpolarized light source – a process in which at least 50 percent of the light from the source is lost. A more efficient approach would be to use a source that directly generates light with the desired polarization.
Quantum dots – nanoscopic inclusions of one semiconductor material in another, small enough to invoke quantum mechanical effects – are known to improve the general performance of light emitting devices. Moreover, one quantum dot is an emitter of single light quanta, photons, suitable for e.g. quantum cryptography. However, the polarization of today’s quantum dots is either weak or non- controllable.
Here we demonstrate a new type of nitride-based quantum dots formed on top of elongated hexagonal micro-pyramids. Standard lithography is used to define the elongation prior to growth of the crystalline pyramids with dots. The resulting quantum dots have a high degree of linear polarization (average 84 percent), with a high probability to be well aligned with the axis of elongation (up to about 90 percent for one micron elongation). Best polarization alignment is obtained for elongations parallel with the principal axes of the crystal, arranged with multiples of 60-degree angle with respect to each other, but polarization control in 30-degree steps is also shown possible.
Our experimental results are obtained on quantum dots emitting violet light (wavelength 415 nm), but the photon color can in principle be shifted across the visible spectrum towards the infrared by adding more and more indium into the dots. Thus, it could be possible to reach blue, green and red colors as well as standard infrared wavelengths for telecom. Our theoretical predictions indicate that an increased content of indium in the dots further improves the polarization degree of the emitted photons.
The proposed concept is promising for more energy efficient polarized light-emitting diodes, e.g. for back illumination of LCD displays. The concept is also highly prospective for polarized single photon emitters, e.g. for quantum cryptography, but it requires further optimization of the fabrication process to ensure formation of a single quantum dot on each pyramid. A light source of variable polarization can be made by combining several pyramids, individually addressed and individually designed for different polarization directions.
Details of the research are described in Light: Science & Applications 3, 140 (2014)
- Nano-N consortium funded by the Swedish Foundation for Strategic Research (SSF).
- Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University.
- Swedish Research Council (VR).
- Knut and Alice Wallenberg Foundation (KAW).
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Last updated: 05/08/14