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Photonic crystals: Dotting the P’s

 
Photonic crystals offer the intriguing possibility to manipulate the properties of light. In particular, if quantum dots are embedded within photonic crystals, a strong coupling between electrons and photons can be achieved. However, so far such coupling has only been realized in two-dimensional photonic crystals. Researchers from the University of Tokyo¹ now demonstrate a three-dimensional photonic crystal with a strong coupling of quantum dots to photonic modes, enabling a number of applications.

The advantage of three-dimensional photonic crystals is their ability to confine light in all directions. This makes the coupling of photons to electronic states particularly strong in case the energetic states of a quantum dot are matched to the resonance frequency of a defect embedded within the photonic crystal. “Such systems will be particularly useful as single quantum dot lasers or as single photon emitters, which could be used in quantum computing applications,” says team leader Yasuhiko Arakawa.

So far, however, progress has been hampered because the assembly of three-dimensional photonic crystals requires a precision much greater than the wavelength used, particularly for the telecommunications wavelength around 1.5 µm. Therefore, in this research, a careful alignment procedure in combination with micromanipulation was required to fabricate multiple layers of GaAs in the typical photonic crystals woodpile structure (Fig. 1).

The InAsSb quantum dots were embedded into the middle of a layer with a point defect structure in the GaAs photonic crystal.  Importantly, their emission wavelength was matched to the resonance frequency of a defect deliberately created within the crystal structure. Evidence for strong coupling between quantum dots and the photonic modes was seen by the sharp light emission lines from the photonic crystals whose intensity increased as more were placed on top of the quantum dots.

Although the necessary precision in fabrication cannot be sustained for too many layers, the so-called quality factor for these three-dimensional photonic crystals—a measure of the sharpness of a resonance— reached a new record value of 2,300.

This achievement represents a significant step towards the use of such photonic crystal devices in applications. “It will be no problem to combine the photonic crystals with other photonic devices such as waveguides, modulators and other optical components,” says Arakawa.
 

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