These Diamonds Don’t Just Sparkle, They Fluoresce!

By introducing germanium to the diamond crystal lattice, scientists have succeeded in producing single photons in a narrow, high energy wavelength band.

AsianScientist (Oct. 6, 2015) – According a study published in Scientific Reports, germanium defects in a diamond crystal lattice act as a reliable source for single photons, which could find applications in quantum cryptography and as bright luminescent makers in living cells.

Pure diamonds are naturally colorless, but gaps in the crystal structure or impurities of other elements can create colors and even emit fluorescence. Now, Assistant Professor Takayuki Iwasaki and co-workers at Tokyo Institute of Technology (Tokyo Tech), together with scientists across Japan and Germany, have demonstrated a new type of diamond crystal defect that fluoresces to produce single photons in a narrow, high energy wavelength band. The defects, which have been named germanium-vacancy (GeV) centers, are relatively easy to fabricate in a reliable, reproducible way.

Iwasaki and co-workers were inspired by recent work that demonstrated fluorescence from nitrogen-vacancy (NV) and silicon-vacancy (SiV) defects in diamond. They used an ion implantation method to insert germanium atoms into diamond films, before heating the films at 800°C.

The resulting samples showed fluorescence only after heating, which induces diffusion of vacancies in the diamond lattice. By using Raman spectroscopy and confocal microscopy they observed fluorescent light emerging from the samples at a wavelength of around 602 nm. The team used theoretical calculations to deduce that this fluorescence resulted from combined defects, each comprising a germanium atom next to a lattice vacancy.

The biggest step forward in the work was when Iwasaki and co-workers managed to create the same types of defects through a different method, microwave plasma chemical vapor deposition (MPCVD). MPCVD involves reactions of volatile chemicals on a substrate, and is often used to make synthetic diamonds.

The defects in the sample prepared using MPCVD gave off more consistent fluorescence with a narrower and more stable peak. Moreover, MPCVD provides closer control over the fabrication process, and is less likely to produce unwanted damage to the samples than ion implantation.

Further work is needed to refine the fabrication process so that diamond films with germanium defects can be incorporated into devices for reliable, on demand, single photon generation. The ability to generate individual photons of known wavelength is an important step towards securing data transmission by quantum cryptography.

Furthermore, the bright emission from the GeV centers could also find biological applications. Due to the high biological compatibility of diamond, fluorescent defects in diamond nanostructures are stable biomarkers that do not face the problem optical bleaching.

The article can be found at: Iwasaki et al. (2015) Germanium-Vacancy Single Color Centers in Diamond.
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Source: Tokyo Institute of Technology.
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