Researchers at Harvard University have developed a new diamond-based device, which has great potential for applications in electronics. It is a microchip made of silver-coated diamond nanoposts, which show a higher and more controllable photon emission.
Electromagnetic Radiation and Photons
The term “electromagnetic radiation” normally refers to energy that has the form of a wave; the energy emitted by the sun, for instance, is an electromagnetic radiation.
The sun emits many different types of waves; visible light and microwaves are just two examples. The radiation’s main characteristics are the intensity I and the wavelength λ, as shown here. Considering these two radiations, a has a shorter wavelength than b; a also has a higher intensity.
Electromagnetic radiation can also be considered as being made of very small particles, all moving together, with the same frequency. These particles are called photons. Therefore, the photon is the basic unit of which each radiation is made.
The energy of the photon – and hence of the electromagnetic radiation – is inversely proportional to its wavelength. This means that the photons belonging to radiation a have smaller energy than those of radiation b.
Some materials can emit energy in form of photons. If a material comes into contact with an electromagnetic radiation, it can absorb some of its energy. Subsequently, this energy can be re-emitted as photons.
An example is the color of a crystal: the crystal is hit by the light, absorbs some of it and then re-emits it; the photons re-emitted will have the wavelength corresponding to the crystal’s color.
This phenomenon happens in a different way for different materials, depending on the structure. For instance, different materials may have different colors; other materials have no color, this may mean that the photons emitted have frequencies which do not fall into visible light.
Many electronic devices – like the ones used for a computer screen – are based on the principle of converting energy and re-emitting it as photons. The materials employed for this must have the appropriate characteristics to perform this conversion.
Photon emission in diamond: the nitrogen-vacancy center
Diamond is best known for its use a gem stone; it has, however, many other technological applications, due to the coexistence of several interesting properties. Considering photon emission, this can take place in diamond if its structure is not completely regular, but it has some imperfections, called defects.
In diamond’s regular structure, each carbon atom is bonded to another 4 carbons in a tetrahedral geometry. Defects can occur if, for instance, a carbon atom is missing; this is called a vacancy, V. A carbon atom could also be replaced by another element, such as nitrogen, boron, aluminum, etc (a substitution).
The nitrogen-vacancy center is a defect where a vacancy and a nitrogen atom (N) replace two adjacent carbon atoms. This defect is indicated as N-V; it can be either neutral, positively charged, or negatively charged. With this defect, the diamond can emit photons in the visible region; it has, in fact, a characteristic yellow color.
As this photon emission can be used in electronic devices, these defects are purposely placed into the structure of synthetic diamonds.
Dr Lončar and his coworkers, in the Laboratory for Nanoscale Optics of Harvard University, studied in detail the photon emissions from diamonds containing these N-V defects. In October, 2011, the team published new and interesting results in the journal Nature Photonics.
Starting from a homogenous diamond sample, they performed a controlled etching of the diamond surface; in this way, they obtained small diamond nanoposts which contain individual N-V defects, with a radius between 50 and 70 nm and a height of about 180 nm. Afterwards, they applied a silver coating on the sample surface, with a thickness of about 500 nm. The picture here shows the different phases of the process.
A diamond chip prepared like this showed much better properties as a photon emitter. In fact, the presence of the silver layer caused an increase in the emission of almost 7 times.
Results with great potentialities
Jennifer Choy, co-worker and co-author of the paper, told Decoded Science: “Our work showed that it is possible to improve and modify the photon emission of NV centers using metallic resonators that can be tuned by changing the diamond nanopost diameter. Besides, this procedure is easily scalable, as thousands of posts are made each time on each diamond chip. This process can therefore facilitate the development of quantum photonic networks based on diamond chips.”
Potential applications for these kinds of devices could be in the fields of communications and high-speed computer networks.
J.T. Choy et al.: “Enhanced single-photon emission from a diamond–silver aperture.” Nature Photonics, DOI: 10.1038/NPHOTON.2011.249.
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