Molybdenum disulfide

2021: The staff of MIPT measured optical anisotropy in layered crystals of a disulfide of molybdenum

On February 11, 2021 it became known that the staff of the Center of photonics and two-dimensional materials of MIPT together with foreign colleagues from Spain, Great Britain, Sweden and Singapore including the pioneer of two-dimensional materials and the Nobel laureate Konstantin Novosyolov for the first time measured optical anisotropy in layered crystals of a disulfide of molybdenum. Scientists assume that similar crystals of dikhalkogenid of transition metals will succeed silicon in photonics. Birefringence with a huge difference in indexes of refraction, inherent to these substances, will allow to create more high-speed and at the same time miniature optical devices. Work is published in the Nature Communications magazine.

Valentin Volkov, the head of a research in laboratory. Photographer: Natalya Arefyeva

As it was explained, some of the first still the Scandinavian Vikings paid attention to polarizing effects in optics. They found out that when viewing through the Icelandic spar the image doubles that received afterwards the name of birefringence. This effect is connected with the fact that the arrangement of atoms in some materials is asymmetrical. As the result, depending on light traveling direction it differently refracts in material, as leads to bifurcation of the image. Index of refraction for one beam remains to constants, and this beam is called ordinary, and for the second — unusual — it depends on light incidence angle.

Icelandic spar. Source: Directory of minerals

It appears, this phenomenon is very useful in practice. For example, Vikings used it for navigation, and modern liquid crystal monitors use effect of birefringence in liquid crystals for creation of the image. This effect is also used for creation of polarizers, wave plates and other optical components. At the same time it is desirable that indices of refraction of ordinary and unusual beams differed as much as possible — then the desirable effect can achieve when passing light through a plate of smaller thickness that will allow to reduce the device sizes, and in a number of applications and to increase its high-speed performance. Recently scientists showed a possibility of creation of compact wave guides on the basis of the anisotropic materials allowing to reach and even to overcome a diffraction limit. For achievement of this effect materials with value of birefringence more than 1 are required. Till 2021 layered crystals of a perovskite of BaTiS3 and hexagonal nitride of a pine forest h-BN had record value of birefringence (0.8). Desire to make modern optics of more and more compact stimulated search of the natural materials having the huge optical anisotropy exceeding 1. Dikhalkogenida of transition metals in this respect are extremely perspective. These connections on the basis of sulfur, selenium, tellurium and 3d - elements of the periodic table of Dmitry Mendeleyev have a sandwich structure. So, the disulfide of molybdenum (MoS2) consists of the alternating layers turned from each other on 1800 which are kept by weak Van der Waals forces (figure 1).

Figure 1. Scheme of the structure of a disulfide of molybdenum. Source: Nature Communications

From a problem of measurement of optical properties of a disulfide of molybdenum we came to perfect other task — actually, to studying of anisotropy and search of prospective applications of anisotropy of such crystals in photonics. Georgy Yermolaev, the graduate student of MIPT and the original author of a research explained motivation of authors.

Such anisotropic building could not affect optical properties of material. It was known in the second half of the twentieth century. Nevertheless quantitative measurements of anisotropy were absent. It including is connected with considerable experimental difficulties. For their overcoming researchers combined methods of near and far electric fields. In other words, in addition to usual radiation of substance under different corners and detectings of a signal, authors of a research studied distribution of the waveguide modes in material that allowed to define unambiguously material birefringence which in near infrared made 1.5, and in visible reaches 3. These values several times exceed values of the previous champions.

We used a combination of methods — a spectral ellipsometry, blizhnepolny optical microscopy and verified our data numerical calculations. Work demanded efforts of a large number of scientists from different scientific groups of the different countries with different competences. For all of us this work became the beginning of large-scale researches on creation of anisotropic nanophotonics on dikhalkogenida of transition metals. Alexey Arsenin, the leading researcher of MIPT told

Data retrieveds were compared to quantum calculations which, to surprise of scientists, showed absolutely the same result. It validated the constructed kvantomekhanichesky model of layered materials and suggests that the theory and outputs published in article are applicable for all class of dikhalkogenid of transition metals.

Scientists absolutely in a different way opened to the world well-known as it seemed earlier, a class of the materials having optical anisotropy. This opening gives additional degree of freedom when developing compact photon devices and, for example, allows to reach a diffraction limit in optics for the volnovedushchy systems with characteristic sizes about 100 nanometers.

Work is performed under the leadership of professor Valentin Volkov who in September, 2019 moved from the University of the Southern Denmark to MIPT where he headed the Center of photonics and two-dimensional materials.

If earlier for creation of optical circuits and devices we were limited to changes of geometry and effective index of refraction, then the huge anisotropy gives additional degree of freedom for manipulation of light. Unexpectedly for us it turned out that natural anisotropic materials allow to create compact wave guides literally on the verge of a diffraction limit. It gives us the chance to compete with silicon photonics, and now we safely can not only speak about post-silicon photonics, but also implement it in practice. Valentin Volkov reported

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