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| Photonic crystal nanocavity resonances in a patterned gallium phosphide membrane (a indicates the lattice constant, and r the hole radius). From [1] |
We are interested in creating photonic crystal nanocavities operating in the visible part of the spectrum. Such devices would not only enable low-power, on-chip optoelectronic devices operating in the visible part of the spectrum for applications such as light sources, but also allow photonic crystals to be combined with novel emitters such as visible colloidal quantum dots, fluorescent molecules, and nitrogen vacancy centers.
Operating in these wavelengths requires a departure from the traditional set of materials used in photonic crystals--gallium arsenide and silicon--because these materials absorb strongly in the visible. The small body of experimental work in visible photonic crystals to date has relied on materials that suffer from lower refractive index, which ultimately limits device quality factors, or require quaternary materials systems, which are difficult to grow and process.
To circumvent these difficulties, we are developing high refractive index gallium phosphide-based nanocavities. Gallium phosphide is not typically used for devices requiring very high brightness because it has an indirect band gap (at 550 nm); however incorporating InP quantum dots or quantum wells, or colloidal quantum dots should greatly increase the quantum efficiency.
So far, we have fabricated cavities in this system that show resonances between 645 nm and 750 nm with quality factors between 500 and 1700, measured via a cross-polarized reflectivity setup. Even these Qs can still enable a significant Purcell effect, up to 185, as mode volume is only 0.7(lambda/n)^3.