Ms. Natalia G. Kokareva Profile

Natalia G. Kokareva

at MV Lomonosov Moscow State Univ

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Publications (4)

Proceedings Article | 18 April 2021 Presentation + Paper

Femtosecond laser reconstruction of graphene field effect transistor

Nikita Nekrasov, Boris Afinogenov, Natalia Kokareva, Vladimir Bessonov, Andrey Fedyanin, Ivan Bobrinetskiy

Proceedings Volume 11770, 117700N (2021) https://doi.org/10.1117/12.2589254

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Possibility of femtosecond laser pulses to affect the materials properties arises the interest in ultrafast processes based research and technology. In the case of graphene surface modification and functionalization using femtosecond laser, there are several effects appear, such as ablation, covalent bonding of different chemical groups, re-crystallization in three-dimensional shapes. CVD grown graphene was transferred on Si/SiO₂. Through several lithography steps, graphene-based field-effect transistors were formed with Cr/Au source-drain electrodes and Si back gate electrode. For graphene modification we used 100 fs 80 MHz laser with 780 nm wavelength with different irradiation doses. Exposure of graphene to a femtosecond laser pulse is determined by the prevalence of physical or chemical effects during exposure to a laser pulse. The range of laser exposure was narrowed down to values causing the formation of atomic defects in the carbon lattice, which makes it possible to form nanopores in graphene and these doses are below the graphene ablation. The main tool for studying the effect of femtosecond laser irradiation was Raman spectroscopy. By evaluating the intensity ratio of certain peaks, namely the G-band (~1600 cm^-1) and D-band (~ 1350 cm^-1), the degree of functionalization, or amorphization of graphene, was estimated. It was found that the ablation threshold starts from 18 mW at the beam speed in the range of 400-500 μm/s. Just below this range, both graphene functionalization and a change in the graphene surface roughness were observed. Despite the change in the morphology of graphene, the graphene resistance fell by only ~4 times, and the transfer current-voltage curves of the graphene transistor did not change much, showing a shift towards higher voltages. With a decrease in the slope of the transfer current-voltage characteristics, the resistance of the structure also decreases with an increase in the dose of laser exposure, since the number of defects and functional groups in graphene increases. In addition, we found the effect of the laser polarization on the modification of graphene. The difference in parameters between the samples modified with different polarization directions along the direction of the beam motion can be explained as the interference interaction of the electron density in graphene. A beam passing over the graphene region excites hot electrons, which partially cause the graphene modification. After passing by the laser, the electron density does not have time to relax, and the next beam of photons affects the already excited electrons, increasing the total dose of laser radiation.

Proceedings Article | 20 August 2020 Presentation + Paper

Directional excitation of Bloch surface waves by laser-printed Mie nanoparticles

Boris Afinogenov, Natalia Kokareva, Dmitry Gulkin, Daniil Shilkin, Denis Zhigunov, Vladimir Bessonov, Andrey Fedyanin

Proceedings Volume 11461, 114612E (2020) https://doi.org/10.1117/12.2570436

KEYWORDS: Metamaterials, Nanoparticles, Silicon, Photonic crystals, Light wave propagation, Microscopy, Semiconductors

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Proceedings Article | 29 May 2018 Presentation

Fabrication of 3D x-ray compound refractive lenses by two-photon polymerization lithography (Conference Presentation)

Natalia Kokareva, Alexander Petrov, Vladimir Bessonov, Ksenia Abrashitova, Kirill Safronov, Aleksandr Barannikov, Petr Ershov, Nataliya Klimova, Ivan Lyatun, Vyacheslav Yunkin, Maxim Polikarpov, Irina Snigireva, Andrey Fedyanin, Anatoly Snigirev

Proceedings Volume 10675, 106750E (2018) https://doi.org/10.1117/12.2307371

KEYWORDS: Lenses, Two photon polymerization, X-rays, Lithography, X-ray lithography, X-ray optics, Absorption, Beryllium, Visible radiation, Scanning electron microscopy

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X-ray microscopy is advantageous over conventional optical microscopy because of its high resolution and capability to study the inner structure of materials opaque to visible light. Furthermore, this method does not require metallization and vacuum and therefore it can be used to visualize fragile biological samples that cannot be studied by scanning electron microscopy. Focusing X-ray optics may be roughly divided into three groups based on the physical principle of focusing: reflection, diffraction and refraction. The reflection optics includes curved mirrors, multilayers and capillaries; the diffractive optics includes Fresnel zone plates. Refractive optics comprises X-ray compound refractive lenses (CRLs) that are widely used nowadays because of their compactness and ease of fabrication. Focusing performance of the CRL is determined by the refractive index, absorption, the inner structure of the CRL material and the geometry of the lens. The optimal shape for the lens is parabolic with a small radius of curvature, because the smaller radius of the parabola leads to shorter focal distance and therefore allows to achieve higher resolution. The common choice of the CRL material is beryllium. However the resolution of Be lenses is far below theoretically predicted limits because of the parasitic scattering introduced by the grains in the material. Moreover the existing manufacturing technologies do not allow to achieve radius of curvature less than 50 μm. Polymer materials are also popular for the CRL microfabrication because of their amorphous nature, ease of structuring and low price. Among the advanced lithographic techniques the two-photon polymerization lithography (2PP) holds a special place. It is based on polymer solidification by means of two-photon absorption. Nonlinear character of two-photon absorption leads to the transparency of the out-of focus material, while presence of polymerization threshold reduces resolution far below diffraction limit. Therefore 2PP can be used for fabrication 3D structures of almost arbitrary shape including overhanging and self-intersecting structures. In this work we introduce the 3D X-ray CRL fabricated by 2PP from the commercially available photoresist ORMOCOMP. Hundred double concave individual lenses formed a CRL with the 60 μm distance between adjacent lenses. Radius of curvature of a single parabolic surface was 3 μm that is comparable to radius of 2D silicon nano-lens made by conventional lithography and much less than achievable radius of 3D Be lens. Physical aperture was 28 μm. The optimal processing parameters (power, incident on the sample, and velocity of the laser beam waist movement) were determined. The fabricated CRL was studied by scanning electron microscopy. It was shown that surface of the lens is smooth and the geometrical parameters do not deviate significantly from that of the model. Focusing performance of lenses was studied by the knife-edge technique. It was obtained that the focal distance is not larger than 2 cm at the energy of 9.25 keV. The radiation resistance of the CRL was tested at the synchrotron DESY: PETRA-III. The CRL was exposed at the non-focused X-ray radiation with the standard power and the energy of 12 keV for more than 10 hours without visible degradation.

Proceedings Article | 20 February 2017 Presentation + Paper

Nonlinear polymer/quantum dots nanocomposite for two-photon nanolithography of photonic devices

Ksenia Abrashitova, Dmitry Gulkin, Natalia Kokareva, Kirill Safronov, Artem Chizhov, Alexander Ezhov, Vladimir Bessonov, Andrey Fedyanin

Proceedings Volume 10115, 1011510 (2017) https://doi.org/10.1117/12.2253179

KEYWORDS: Quantum dots, Multiphoton lithography, Switching, Polymers, Nanocomposites, Kerr effect, Photoresist materials, Nanolithography, Photonic devices, Photonic crystal devices, Prisms, Transmission electron microscopy, Waveguides

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