Researchers from the Gas Lasers Laboratory at the Lebedev Physical Institute of the Russian Academy of Sciences (LPI RAS), Russia, have developed two methods for precision microscale labeling of artificial and natural diamonds. The markings, which are invisible to the naked eye, are created using a femtosecond laser.

“Most of the globally produced diamonds (about 80%) are used for a range of technical needs. The remaining 20%, which satisfy the gem quality requirements, are much more expensive, and their price varies greatly depending on the weight and quality of the gemstone. Therefore, the development of non-destructive methods for creation of diamond markings, which are invisible to the naked eye (according to the Kimberley Process certification standard), is a very important issue for the modern jewelry industry”, said Dr. Andrey Ionin, the head of the Gas Lasers Laboratory at the Lebedev Physical Institute.

A promising way to create these “quality markings” has been developed by the laboratory scientists Sergey Kudryashov, Alexey Levchenko, Leonid Seleznev, Dmitry Sinitsyn and others. The team processed a diamond using a light beam from a femtosecond laser, which generates an increased concentration of vacancies – defects in the diamond lattice, where there are no carbon atoms at the particular location of the gem.

“Normally such vacancies are created using an electron beam or beams of any high-energy particles. This typical method produced a uniform concentration of vacancies throughout the sample. On the other side, you can achieve a similar but localized effect with a femtosecond laser. For example, you may draw some image using small clusters of vacancies”, said Alexey Levchenko, a member of the laboratory team.

Later the generated vacancies interact with impurity atoms of nitrogen, which are typically present in the volume of diamond in high concentrations, and form so-called nitrogen-vacancy centers, or NV-centers. These particular defects are very “useful” for the purposes of diamond marking. The fact is, that under irradiation using visible light the NV-centers start emitting a characteristic fluorescence light, and the external application of the high-frequency electromagnetic field can also alter the intensity of this fluorescence.

“A diamond may contain other impurities that illuminate under the presence of external radiation, therefore, when we create a high-frequency electromagnetic field, we are able to observe our signal changing in the background of all these noises. If we create an invisible microcluster of NV-centers in the volume of the gemstone using a sharply focused laser beam with [positioning] precision down to a micron, we can consequently read the label of our diamond according to fluorescence induced by the high-frequency field. We have successfully completed such experiments with the support of the company “New Energy Technologies”, also registered a patent and now are looking for prospective buyers”, said Mr. Levchenko.

A femtosecond laser is also used for the second method of gemstone marking. This time, the amorphous carbon phase inclusions are formed in the diamond volume instead of creating the lattice vacancies. These small dark micro-inclusions contained in the crystal can be observed using an optical microscope, but still remain indistinguishable to the naked eye.

The label is not visible under normal conditions, even with microscope, and therefore does not reduce the value of the stone. But under the influence of external laser radiation, the label starts emitting bright fluorescent light. This label created inside the diamond may not be polished or abraded, and its transcerse dimension is only about 1 micron.

An intense multi-photon excitation of the material takes place at the focal region under the impact of femtosecond laser pulses, which can cause the “softening” of its crystal lattice. Later, after thermalisation of the absorbed radiation energy, the temperature and pressure inside the diamond increases greatly, which can also lead to structural disorders.

“Femtosecond laser beam can be focused at different depths inside transparent materials. Due to this effect, the technology allows you to create unique three-dimensional markings. We have successfully formed microscale dimensional labels during our experiments, both in synthetic and in natural diamonds”, commented Sergey Kudryashov, a senior researcher at the Lebedev Physical Institute.

Source: FIAN-INFORM (in Russian)