vrijdag 15 april 2016

Why can diamond be used to sense a magnetic field?

Diamond is known as a material with excellent mechanical and thermal properties. Synthetic diamond coatings are often used for drills or cutting blades. Synthetic diamond can also be used to work as an electronic sensor to measure ultraviolet light or even magnetic fields! And the latter is what we want to do on a stratospheric balloon. But where do these sensing capabilities originate from?   

Atoms, molecules, diamond lattices … are all characterized by the states their electrons are in and what energy levels these states have. States with the same energy levels are referred to as degenerate states.

When no external forces are present, these energy levels are well defined and fixed. However, when an external force is applied to a system, the energy levels can shift or even split in the case of degenerate states. If a magnetic field is used as an external force, this splitting is called Zeeman splitting (Not to confuse with the clothing store http://www.zeeman.com)

By using a certain production method, it is possible to introduce nitrogen vacancy defects within the diamond. A nitrogen vacancy defect in diamond has three ground states, with two degenerate states at a slightly higher energy level than the third state. If an electron from the latter state gets excited, it will relax back to the ground state while sending out red light. This phenomenon is known as photoluminescence. If in the other case the electron gets excited from the slightly higher states (by first exciting electrons from the lowest to the two degenerate states by microwaves), it will also relax while sending out red light, but this time the light will be less intense. This will show up as a dip when graphing the intensity against the frequency of the microwaves.

When a magnetic field is applied, the two degenerate states will split at the same rate as the strength of the magnetic field. Now, two dips will show up in the graph, one for each state. By exploiting this effect, the magnetic field can be measured! 

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