The laser turns the dielectric into a conductor. Free Technical Library News

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The laser turns the dielectric into a conductor

06.04.2018

An international group of scientists for the first time theoretically described how the so-called Mott dielectrics will behave under the action of ultrashort and very powerful laser pulses and how the spectrum of radiation reflected from their surface should look like.

Calculations show that in this case the dielectric will turn into a conductor. In the future, this phenomenon can be used for electronics, studies of fast processes and quantum states in matter.

Dielectrics are substances that conduct electricity poorly, because they have few free electrons that can easily move and carry charge. In ordinary dielectrics, this is due to the peculiarities of the distribution of electrons over energy levels that arise in the field of the crystal lattice. This is described by one of the main sections of quantum physics - band theory.

Mott dielectrics differ from ordinary dielectrics in that current cannot flow in them for a completely different reason, due to the strong interaction between electrons. In them, the movement of electrons capable of creating a current is "interfered" by other electrons located on neighboring atoms. By their repulsion, they "lock" each electron on their atom and make the substance a dielectric. They are named after the English physicist Neville F. Mott, Nobel Prize winner in 1977, who in 1949 explained their occurrence. Mott dielectrics (as a rule, these are oxides of transition metals, for example, NiO) cease to conduct current when cooled, when the interaction between electrons becomes more significant.

Experiments on the effect of light on matter began about 20 years ago. But the need to take into account the interaction of electrons made it difficult to theoretically study the processes in Mott dielectrics. Therefore, up to now, single atoms or molecules have been considered in order to study the behavior of electrons in the orbitals of atoms. But no one has studied the behavior of the Mott dielectrics themselves in a superstrong light field. However, over the past five years, experimenters have begun to switch to solids, to crystals. Here the picture is much more complicated, since this is a multi-electron problem, where the interacting electrons affect the conductivity.

The simulated researchers investigated how such materials would respond to flashes from a high-power femtosecond laser and modeled what the spectrum of radiation reflected from a surface should look like, since its properties are affected by the characteristics of the material. Under the action of a strong alternating field of a laser beam incident on the surface of a Mott dielectric, the state of the electrons in it changes. Their kinetic energy increases and the material loses its dielectric properties. The process can be investigated using the so-called high harmonic spectroscopy.

The method consists in sending very short, tens or hundreds of femtoseconds (10-15 s) laser pulses with specified characteristics to the material. When the beam is reflected from the material, these characteristics change, including some of the photons acquiring dozens of times more energy and oscillation frequency than the photons of the initial pulse (this is called the generation of high optical harmonics). By changing the characteristics of the beam, one can judge the properties of the material.

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