Comparative analysis of acceleration gradients for chip structures with different refractive indices

The results of numerical studies of accelerating gradients in accelerators based on dielectric chip structures with different refractive indices, excited by a titanium-sapphire laser pulse, are presented. A comparative analysis of the influence of the refractive index on the rate of acceleration of...

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Published in:Вопросы атомной науки и техники
Date:2021
Main Authors: Vasyliev, A.V., Bolshov, O.O., Svistunov, O.O., Povrozin, A.I., Zaitcev, V.P., Leshchenko, V.P., Sotnikov, G.V.
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Language:English
Published: Національний науковий центр «Харківський фізико-технічний інститут» НАН України 2021
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Online Access:https://nasplib.isofts.kiev.ua/handle/123456789/195642
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Cite this:Comparative analysis of acceleration gradients for chip structures with different refractive indices / A.V. Vasyliev, O.O. Bolshov, O.O. Svistunov, A.I. Povrozin, V.P. Zaitcev, V.P. Leshchenko, G.V. Sotnikov // Problems of Atomic Science and Technology. — 2021. — № 6. — С. 75-79. — Бібліогр.: 25 назв. — англ.

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Digital Library of Periodicals of National Academy of Sciences of Ukraine
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spelling Vasyliev, A.V.
Bolshov, O.O.
Svistunov, O.O.
Povrozin, A.I.
Zaitcev, V.P.
Leshchenko, V.P.
Sotnikov, G.V.
2023-12-05T17:39:08Z
2023-12-05T17:39:08Z
2021
Comparative analysis of acceleration gradients for chip structures with different refractive indices / A.V. Vasyliev, O.O. Bolshov, O.O. Svistunov, A.I. Povrozin, V.P. Zaitcev, V.P. Leshchenko, G.V. Sotnikov // Problems of Atomic Science and Technology. — 2021. — № 6. — С. 75-79. — Бібліогр.: 25 назв. — англ.
1562-6016
PACS: 41.75.Jv, 41.75.Ht, 42.25.Bs
DOI: https://doi.org/10.46813/2021-136-075
https://nasplib.isofts.kiev.ua/handle/123456789/195642
The results of numerical studies of accelerating gradients in accelerators based on dielectric chip structures with different refractive indices, excited by a titanium-sapphire laser pulse, are presented. A comparative analysis of the influence of the refractive index on the rate of acceleration of electron bunches is carried out. Promising materials for the manufacture of dielectric laser accelerators are proposed.
Представлені результати чисельних досліджень прискорюючих градієнтів у прискорювачах на основі діелектричних ЧІП-структур з різними показниками заломлення, збуджуваних титан-сапфіровим лазерним імпульсом. Проведено порівняльний аналіз впливу показника заломлення на темп прискорення електронних згустків. Запропоновано перспективні матеріали для виготовлення діелектричних лазерних прискорювачів.
Представлены результаты численных исследований ускоряющих градиентов в ускорителях на основе диэлектрических ЧИП-структур с различными показателями преломления, возбуждаемых титан-сапфировым лазерным импульсом. Проведен сравнительный анализ влияния показателя преломления на темп ускорения электронных сгустков. Предложены перспективные материалы для изготовления диэлектрических лазерных ускорителей.
Work supported by The National Research Foundation of Ukraine, program "Leading and Young Scientists Research Support" (project # 2020.02/0299).
en
Національний науковий центр «Харківський фізико-технічний інститут» НАН України
Вопросы атомной науки и техники
Novel and non-standard acceleration technologies
Comparative analysis of acceleration gradients for chip structures with different refractive indices
Порівняльний аналіз градієнтів прискорення для ЧІП-структур з різними показниками заломлення
Сравнительный анализ градиентов ускорения для ЧИП-структур с различными показателями преломления
Article
published earlier
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
title Comparative analysis of acceleration gradients for chip structures with different refractive indices
spellingShingle Comparative analysis of acceleration gradients for chip structures with different refractive indices
Vasyliev, A.V.
Bolshov, O.O.
Svistunov, O.O.
Povrozin, A.I.
Zaitcev, V.P.
Leshchenko, V.P.
Sotnikov, G.V.
Novel and non-standard acceleration technologies
title_short Comparative analysis of acceleration gradients for chip structures with different refractive indices
title_full Comparative analysis of acceleration gradients for chip structures with different refractive indices
title_fullStr Comparative analysis of acceleration gradients for chip structures with different refractive indices
title_full_unstemmed Comparative analysis of acceleration gradients for chip structures with different refractive indices
title_sort comparative analysis of acceleration gradients for chip structures with different refractive indices
author Vasyliev, A.V.
Bolshov, O.O.
Svistunov, O.O.
Povrozin, A.I.
Zaitcev, V.P.
Leshchenko, V.P.
Sotnikov, G.V.
author_facet Vasyliev, A.V.
Bolshov, O.O.
Svistunov, O.O.
Povrozin, A.I.
Zaitcev, V.P.
Leshchenko, V.P.
Sotnikov, G.V.
topic Novel and non-standard acceleration technologies
topic_facet Novel and non-standard acceleration technologies
publishDate 2021
language English
container_title Вопросы атомной науки и техники
publisher Національний науковий центр «Харківський фізико-технічний інститут» НАН України
format Article
title_alt Порівняльний аналіз градієнтів прискорення для ЧІП-структур з різними показниками заломлення
Сравнительный анализ градиентов ускорения для ЧИП-структур с различными показателями преломления
description The results of numerical studies of accelerating gradients in accelerators based on dielectric chip structures with different refractive indices, excited by a titanium-sapphire laser pulse, are presented. A comparative analysis of the influence of the refractive index on the rate of acceleration of electron bunches is carried out. Promising materials for the manufacture of dielectric laser accelerators are proposed. Представлені результати чисельних досліджень прискорюючих градієнтів у прискорювачах на основі діелектричних ЧІП-структур з різними показниками заломлення, збуджуваних титан-сапфіровим лазерним імпульсом. Проведено порівняльний аналіз впливу показника заломлення на темп прискорення електронних згустків. Запропоновано перспективні матеріали для виготовлення діелектричних лазерних прискорювачів. Представлены результаты численных исследований ускоряющих градиентов в ускорителях на основе диэлектрических ЧИП-структур с различными показателями преломления, возбуждаемых титан-сапфировым лазерным импульсом. Проведен сравнительный анализ влияния показателя преломления на темп ускорения электронных сгустков. Предложены перспективные материалы для изготовления диэлектрических лазерных ускорителей.
issn 1562-6016
url https://nasplib.isofts.kiev.ua/handle/123456789/195642
citation_txt Comparative analysis of acceleration gradients for chip structures with different refractive indices / A.V. Vasyliev, O.O. Bolshov, O.O. Svistunov, A.I. Povrozin, V.P. Zaitcev, V.P. Leshchenko, G.V. Sotnikov // Problems of Atomic Science and Technology. — 2021. — № 6. — С. 75-79. — Бібліогр.: 25 назв. — англ.
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fulltext ISSN 1562-6016. ВАНТ. 2021. № 6(136) 75 https://doi.org/10.46813/2021-136-075 COMPARATIVE ANALYSIS OF ACCELERATION GRADIENTS FOR CHIP STRUCTURES WITH DIFFERENT REFRACTIVE INDICES A.V. Vasyliev, O.O. Bolshov, O.O. Svistunov, A.I. Povrozin, V.P. Zaitsev, V.P. Leshchenko G.V. Sotnikov National Science Center “Kharkov Institute of Physics and Technology”, Kharkiv, Ukraine E-mail: o.bolshov@student.csn.khai.edu The results of numerical studies of accelerating gradients in accelerators based on dielectric chip structures with different refractive indices, excited by a titanium-sapphire laser pulse, are presented. A comparative analysis of the influence of the refractive index on the rate of acceleration of electron bunches is carried out. Promising materials for the manufacture of dielectric laser accelerators are proposed. PACS: 41.75.Jv, 41.75.Ht, 42.25.Bs INTRODUCTION Particle accelerators are an important tool in basic scientific research, industry and medicine. Traditional RF accelerators are often expensive and too large, which hinders their to be widespread, and their accelera- tion gradients are limited by the low breakdown thresh- old of the materials used and are usually equal to 20…30 MeV/m. In this regard, it became necessary to develop more compact and cheaper accelerators, which, in this case, would provide greater efficiency. One of such options is the concept of dielectric wakefield ac- celerators driven by a long sequence of electron bunches [1]. Another proposed by us turned out to be dielectric laser accelerators (DLA) based on chip structures, which will be considered in this work. The dielectric structures used in such accelerators have a high damage threshold when operating in the optical range. Dielectrics have higher gradients due to the fact that they withstand fields exceeding 1 GV/m [2]. Due to advanced nanomanufacturing techniques, it has become possible to create precise and low-cost nanostructures from a variety of dielectric materials. In addition, they are transparent to the operating wave- lengths of high-power and commercially available femtosecond laser systems. Thus, DLAs exhibit larger acceleration gradients than RF accelerators, are smaller and less expensive. 1. ACCELERATION GRADIENT Following [2] give the expression for the accelera- tion gradient depending on the refractive index. To quantify DLA efficiency, an indicator such as the accel- eration gradient Gacc is used, which is usually written as: 0 1 ( ( ), ) g acc z g G E z t t dz     , (1) where g is the grating period of the structure, z is the direction of propagation of electrons, Ez(z(t), t) is the longitudinal electric field. The acceleration gradient shows the electron energy gain per unit length and is usually measured in MeV/m. Two other important indicators are: 1. The acceleration factor fA: 0 acc A G f E  , (2) where E0 is the input electric field inside the structure after Fresnel reflection at the interface between the two media. The acceleration coefficient is a dimensionless quantity that shows the efficiency of converting an inci- dent electric field into an acceleration gradient. 2. The field enhancement factor : max 0 E E   , (3) where Emax is the maximum electric field in the struc- ture. This coefficient determines the ability of the struc- ture to enhance the input field. From equation: 2 2 0 0 0 0 1 1 2 2 inc inc r E E E E n      , (4) where Einc is the input electric field outside the struc- ture, n = r is the refractive index of the material; it follows that the acceleration gradient can be written as: 0 2 incA A acc inc p Ff f G E n n c       , (5) where Finc is the power density of laser radiation, p is the pulse duration. In Section 5, we will compare Eqs. (4) and (5) with the results of numerical simulation. 2. MATERIAL SELECTION CRITERIA The material of the structure is an important compo- nent of the DLA and therefore some criteria should be followed when choosing it: 1. Transparency. The dielectric material must be transparent in the selected region of the laser spectrum. This is necessary to transfer maximum power to the accelerator and reduce material heating. Most dielectric materials are transparent to infrared radiation, so this criterion is not essential for the creation of DLA. 2. Laser-induced damage threshold (LIDT). From Eqn. (5) it follows that the acceleration gradient is pro- portional to the square root of the laser pulse power and, therefore, the maximum acceleration gradient is limited by the breakdown of the dielectric material. Thus, to obtain large acceleration gradients, materials should be used that will withstand high electric field strengths. It should also be taken into account that grating structures will be less durable than the bulk material [3]. mailto:o.bolshov@student.csn.khai.edu ISSN 1562-6016. ВАНТ. 2021. № 6(136) 76 3. Refractive index. DLA is an optical phase mask that modulates the amplitude and phase shift of electro- magnetic waves when laser radiation is incident on the chip structure. Higher refractive indices give greater limiting of the electric field and create greater phase contrast. It should be noted that since DLA operates in the re- verse mode of Cherenkov radiation, for greater energy modulation, it is necessary to adhere to the rule that the electron velocity c should be greater than the phase velocity of light in the material c/n, which means 1/n  . 4. Simplicity of production. One of the conditions for synchronization between the first spatial harmonic and the electron bunch is that the period of the electrodynamic structure of the accelerator must satisfy the relation g  [4], where  is the wavelength of laser radiation. That is, the grating period of the chip structure must be equal to the operating wavelength for relativistic electrons. Moreover, the production of such structures should be relatively quick and cheap. 5. Resistance to radiation of relativistic electrons. Materials such as Fused Silica, YAG, Lithium niobate and Sapphire are not damaged by relativistic electrons, while borosilicate glass (BK7) is. 3. MATERIALS OVERVIEW Fused Silica was used in the first DLA demonstra- tions [2, 5]. The material has a relatively high LIDT – 2.1 J/cm 2 (at a wavelength 800 nm, with a pulse dura- tion of 30 fs [6]), and the methods of producing nanostructures from it are well studied [7 - 10]. In addi- tion, as mentioned above, Fused Silica is resistant to electrons. One of the disadvantages of Fused Silica is its low refractive index 1.45. Recently, the production of DLA from Sapphire and Gallium Oxide has been demonstrated [11]. Both mate- rials outperform Fused Silica in terms of LIDT and re- fractive index. Sapphire has a LIDT of 11 J/cm 2 (800 nm, 100 fs) [12] and a refractive index of 1.76. Gallium Oxide has, respectively, 2.6 J/cm 2 (760 nm, 9 fs) and a refractive index of 1.9 [13]. The main disad- vantage of these materials is the complexity of manufac- turing chip structures from them. One of the materials mentioned that can withstand the high energies of relativistic electrons is Lithium nio- bate. This material is also interesting because it has the highest refractive index among those considered in this work, it is equal to 2.26. However, its LIDT is lower than that of the previous two materials, it is equal to 2.0 J/cm 2 [14]. The last material reviewed is the commercially available BK7 optical glass. In terms of refractive index, it is close to Fused Silica, it is equal to 1.51. But it has the lowest LIDT, only 2.55 J/cm 2 (760 nm, 200 fs) [15]. At the same time, as already noted, BK7 is destroyed by the radiation of relativistic electrons. Nevertheless, such material can be used in research in a number of cases, due to its availability. Based on the works [16 - 18], for clarity, we intro- duce an equation for approximating the available data of LIDT of materials and reduce all values to the parame- ters of laser radiation equal to 800 nm wavelength and 120 fs pulse duration: 2 2 2 2 1 1 1 1 ( , ) ( , )LIDT LIDT            , (6) where 1 and 1 are the wavelength and duration of the pulse for the known LIDT, 2 and 2 are the values of the determined LIDT (in our case, 800 nm and 120 fs). All of these materials are transparent for a wave- length of 800 nm and have a low absorption index (~10 -8 …10 -7 ) [19 - 24]. For this reason, dielectric losses were not taken into account in the simulation. The materials considered and their parameters are shown in Table 1. Table 1 DLA materials and their characteristics: Material LIDT (J/cm 2 ) Approx. LIDT (J/cm 2 ) Refractive index, n Trans- mittance Fused Silica 2.10 4.20 1.45 0.90 BK7 2.55 2.07 1.51 0.90 Sapphire 11.00 12.05 1.76 0.85 Gallium oxide 2.60 9.99 1.90 0.80 Lithium niobate 2.00 3.43 2.26 0.75 4. SIMULATION AND RESULTS We used the particle-in-cell method to simulate DLA. A structure with a single grating was irradiated perpendicularly for transmission with a Gaussian pulse. The electron source was located at a height /2 above the surface of the structure and emitted one electron bunch. The parameters of the chip structure, Gaussian pulse, and electron bunch are given in Table 2. Table 2 Parameters of the chip structure, Gaussian pulse, and electron bunch used in the numerical simulation Chip structure Period, g 800 nm Pillars height, h 400 nm Grooves width, w 400 nm Gaussian pulse Center wavelength,  800 nm Pulse duration, p 120 fs Beam waist, w0 14 um Electric field intensity, Einc 1 GV/m Electron bunch Bunch width 100 nm Electron energy 50 MeV Bunch length 0.35 fs From the previously investigated profiles of chip structures [25], the profile of the “grooves” type was chosen as the main one for this work. The geometric image of the profile is shown in Fig. 1. Numerical mod- eling was carried out for all materials listed in Table 1. ISSN 1562-6016. ВАНТ. 2021. № 6(136) 77 Fig. 1. Geometric image of profile of the used structure Fig. 2 shows energy gain of the electrons depending on the longitudinal coordinate of electron propagation z for different materials of chip structures. The figure shows that energy gain is proportional to the refractive index of the material. Fig. 2. Energy gain of the electrons for different materials of the accelerator Fig. 3,a-c show the dependence of the electric field strength along the z coordinate (accelerating channel). The blue curve indicates the distribution of the electric field at the time of the maximum intensity of the accel- erating field when the electron is above the pillars of the chip structure, the red curve indicates the distribution of the field strength after half the optical period, during which the electron passes half the grating period and will be above the grooves. With an increase in the re- fractive index, the difference in intensity decreases. Thus, in the case of Fused Silica, although electrons experience a stronger accelerating field, they also expe- rience a strong decelerating field after a time equal to the phase change in the /2 interval. Whereas for Lithi- um niobate, due to the higher refractive index, the am- plitude of the fields in the channel is less [see eqs. (4) and (5)], but the electron is in the accelerating phase throughout the entire path. As a result, Lithium niobate gives greater acceleration and a smoother curve in Fig. 2 (compared to Fused Silica curve). It follows from this that a more uniform electric field acts on the electron. Fig. 3,b shows an intermediate case that corresponds to the average refractive index of the selected materials (for Sapphire). In this case, the maximum intensity of the accelerating field is less than for the case with Fused Silica. However, when the phase changes by /2, the electron is still affected by the decelerating field, which is why the total acceleration gradient is lower than for the case with Lithium niobate. Fig. 3,d-f show the distribution of the longitudinal electric field at the moment of maximum intensity at the height of the flight of electrons. Red color corresponds to the accelerating field, blue one to the decelerating. The presented figures clearly show how the refractive index of the structure affects the formation of the longi- tudinal accelerating component of the electric field in space. In Fig. 3,d field has a more uniform distribution throughout the entire flight of electrons. It can be seen that for materials with a higher refractive index (Fig. 3,e,f), the field modulation becomes more pro- nounced. When the wave phase changes in the /2 in- terval, the field will change to a decelerating one and the electrons located above the grooves will be affected by the decelerating field, the stronger the lower the re- fractive index of the material. Consequently, the most advantageous in this case will be a material that, due to its refractive index, makes it possible to change the field to an accelerating one. Fig. 3. Electric field intensity along the propagation of electrons for various materials (a-c); distribution of electric fields formed by the chip structure in accordance with a-c (d-f). The dotted line indicates the height of the flight of electrons /2 Table 3 shows the acceleration gradients obtained at modeling for various materials, as well as the ratio of refractive indices and acceleration gradients with those of Fused Silica, the main material used in DLA. ISSN 1562-6016. ВАНТ. 2021. № 6(136) 78 Table 3 Refractive indices and acceleration gradients of various materials, and the ratio of these parameters to those of Fused Silica Material Refrac- tive index Acceleration gradient, (MeV/m) Fused Silica refractive index ratio Fused Silica gradient ratio Fused Silica 1.45 73.7 1.00 1.00 BK7 1.51 92.5 1.04 1.26 Sapphire 1.76 102.9 1.21 1.40 Gallium oxide 1.90 113.3 1.31 1.54 Lithium niobate 2.26 140.4 1.56 1.91 Simulation results and material characterization in- dicate that Sapphire, Gallium oxide and Lithium niobate are promising candidates as primary materials for DLA research. CONCLUSIONS In this paper, various criteria that are desirable to consider when choosing a material for DLA were de- scribed, and materials common in research were consid- ered, as well as those that have more preferred charac- teristics. Simulation of the acceleration of electrons was car- ried out using the considered materials as the material of the dielectric structure. The results obtained showed the increase of the en- ergy gain of the accelerated electron bunch with an in- crease in the refractive index. At the same time, as fol- lows from expressions (4) and (5), the amplitude of the longitudinal electric field decreases with an increase in the refractive index. But the integral effect during the motion of the accelerated bunch along the structure con- sists in an increase in its energy gain, since for half the period of the structure, the bunch is in a weaker deceler- ating field with an increase in the refractive index. Moreover, for some materials, the bunch can be in the accelerating phase throughout the entire period of the structure, which can lead to an even greater energy gain. Sapphire, Gallium oxide and Lithium niobate have been identified as promising materials for future DLA research. ACKNOWLEDGEMENTS Work supported by The National Research Founda- tion of Ukraine, program "Leading and Young Scien- tists Research Support" (project # 2020.02/0299). REFERENCES 1. I.N. Onishchenko et al. Concept of dielectric wake- field accelerator driven by a long sequence of elec- tron bunches // Proc. IPAC. 2013, p. 1259. 2. E.A. Peralta et al. Demonstration of electron accel- eration in a laser-driven dielectric microstructure // Nature. 2013, 503, 7474, p. 91-94. 3. K. Soong et al. Experimental determination of dam- age threshold characteristics of IR compatible opti- cal materials // Particle Accelerator Conference Proceedings. 2011, v. 277. 4. T. Plettner, R.L. Byer, and B. Montazeri. Electro- magnetic forces in the vacuum region of laser-driven layered grating structures // Journal of Modern Op- tics. 2011, v. 58.17, p. 1518-1528. 5. J. Breuer, P. Hommelhoff. 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Поврозин, В.П. Зайцев, В.П. Лещенко, Г.В. Сотников Представлены результаты численных исследований ускоряющих градиентов в ускорителях на основе ди- электрических ЧИП-структур с различными показателями преломления, возбуждаемых титан-сапфировым лазерным импульсом. Проведен сравнительный анализ влияния показателя преломления на темп ускорения электронных сгустков. Предложены перспективные материалы для изготовления диэлектрических лазерных ускорителей. ПОРІВНЯЛЬНИЙ АНАЛІЗ ГРАДІЄНТІВ ПРИСКОРЕННЯ ДЛЯ ЧІП-СТРУКТУР З РІЗНИМИ ПОКАЗНИКАМИ ЗАЛОМЛЕННЯ А.В. Васильєв, О.О. Большов, О.О. Свістунов, А.І. Поврозін, В.П. Зайцев, В.П. Лещенко, Г.В. Сотніков Представлені результати чисельних досліджень прискорюючих градієнтів у прискорювачах на основі ді- електричних ЧІП-структур з різними показниками заломлення, збуджуваних титан-сапфіровим лазерним імпульсом. Проведено порівняльний аналіз впливу показника заломлення на темп прискорення електронних згустків. Запропоновано перспективні матеріали для виготовлення діелектричних лазерних прискорювачів.