BROADBAND LIGHT ABSORBERS BASED ON MULTILAYER METAL-INSULATOR-METAL STRUCTURES Текст научной статьи по специальности «Физика»

i l St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.2 Научно-технические ведомости СПбГПУ. Физико-математические науки. 15 (3.2) 2022

Conference materials UDC 535.3

DOI: https://doi.org/10.18721/JPM.153.234

Broadband light absorbers based on multilayer metal-insulator-metal structures

A. I. Kashapov 12 E. A. Bezus D. A. Bykov N. V. Golovastikov 12, L. L. Doskolovich 12

1 IPSI RAS - Branch of the FSRC "Crystallography and Photonics" RAS, Samara, Russia;

2 Samara National Research University, Samara, Russia H ar.kashapov@outlook.com

Abstract. We study optical properties of multilayer resonant metal-insulator structures. It is shown that a simple three-layer metal-insulator-metal (MIM) structure can provide near-perfect absorption of light (more than 90%) in a 732 nm-wide wavelength range. We demonstrate that by making the MIM structure slightly more complex by adding additional layers and optimizing their thickness, it is possible to broaden the near-perfect absorption band by almost a factor of two, covering the visible, near ultraviolet and near-infrared wavelength ranges with an average absorption level of 98.5%. Both structures provide a considerably low sensitivity to changes in the angle of incidence of the incident light. The obtained results can be applied for solar energy harvesting, photovoltaics and thermophotovoltaics, absorption filtering, and sensing.

Keywords: broadband light absorption, metal-insulator structures, resonant diffractive structures

Funding: This work was funded by the Ministry of Science and Higher Education of the Russian Federation (state assignment for research to Samara University (laboratory "Photonics for Smart Home and Smart City", project FSSS-2021-0016), investigation of the broadband MIM absorbers; and state assignment to the FSRC "Crystallography and Photonics" RAS (agreement 007-GZ/Ch3363/26), implementation of the simulation software) and by the Russian Science Foundation (project 19-19-00514, analytical design the of initial "double" MIM structure).

Citation: Kashapov A. I., Bezus E. A., Bykov D. A., Golovastikov N. V., Doskolovich L. L., Broadband light absorbers based on multilayer metal-insulator-metal structures, St. Petersburg State Polytechnical University Journal. Physics and Mathematics. 15 (3.2) (2022) 184—189. DOI: https://doi.org/10.18721/JPM.153.234

This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)

Материалы конференции УДК 535.3

DOI: https://doi.org/10.18721/JPM.153.234

Широкополосные абсорберы света на основе многослойных структур металл-диэлектрик-металл

А. И. Кашапов 12 е, Е. А. Безус 12, Д. А. Быков 12, Н. В. Головастиков 12, Л. Л. Досколович 12

1 Институт систем обработки изображений РАН - филиал ФНИЦ

«Кристаллография и фотоника» РАН, г. Самара, Россия;

2 Самарский национальный исследовательский университет имени академика С.П. Королёва, г. Самара, Россия н ar.kashapov@outlook.com

Аннотация. Исследованы оптические свойства многослойных резонансных металло-

© Kashapov A. I., Bezus E. A., Bykov D. A., Golovastikov N. V., Doskolovich L. L., 2022. Published by Peter the Great St.Petersburg Polytechnic University.

диэлектрических (МДМ) структур. Показано, что путем усложнения простой трехслойной МДМ структуры, состоящего в добавлении дополнительных слоев и оптимизации их толщины, можно расширить полосу близкого к полному поглощения, охватив при этом диапазон длин волн от ближнего УФ до ближнего ИК включительно. Полученные результаты могут найти применение в фотовольтаике и термо-фотогальванике, сенсорике и для создания поглощающих оптических фильтров.

Ключевые слова: широкополосное поглощение света, металлодиэлектрические структуры, резонансные дифракционные структуры

Финансирование: Работа выполнена при финансовой поддержке Министерства науки и высшего образования Российской Федерации (государственное задание Самарскому университету (лаборатория «Фотоника для умного дома и умного города», проект ФССС-2021-0016), исследование широкополосных поглощающих МДМ-структур и государственное задание ФНИЦ «Кристаллография и фотоника» РАН (соглашение № 007-ГЗ/Ч3363/26 в части разработки программных средств для моделирования) и Российского научного фонда (проект 19-19-00514, аналитический расчет исходных «двойных» МДМ-структур).

Ссылка при цитировании: Кашапов А. И., Безус Е. А., Быков Д. А., Головастиков Н. В., Досколович Л. Л. Широкополосные абсорберы света на основе многослойных структур металл-диэлектрик-металл // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2022. Т. 15. № 3.2. С. 184-189. БОГ: Шрв://^. 0^/10.18721/ JPM.153.234

Статья открытого доступа, распространяемая по лицензии СС БУ-МС 4.0 (ИЦря:// creativec0mm0ns.0rg/licenses/by-nc/4.0/)

Introduction

Light absorbers based on nanophotonic structures are promising for a wide range of applications, including solar energy harvesting, photovoltaics and thermophotovoltaics, photodetection, absorption filtering, and sensing. Metal-insulator layered structures are widely used for broadband absorption of light [1—3]. One of the possible geometries of broadband absorbing structures is a set of pairs of metal and dielectric layers [1—3]. For example, in recent work [3], a non-resonant structure consisting of pairs of MgF2 — Cr layers was studied, which provides absorption exceeding 90% in a wavelength range with a width of about 1 ^m (900—1900 nm).

In our recent works [4, 5], we theoretically proved the possibility of obtaining an exact zero in the reflection spectrum of a three-layer metal-insulator-metal (MIM) structure at a given wavelength and angle of incidence by choosing the layer thicknesses. In the case, in which the lower metal layer of the MIM structure is sufficiently thick, it will "prohibit" the transmission of light through the structure, and, therefore, a zero in the reflection spectrum R will simultaneously mean complete absorption A = 1 - R. By combining two MIM structures, it is possible to obtain a reflection zero of the 2nd order in the spectrum of such a double MIM structure [6]. The authors of the present work believe that this is an important factor for broadening the band of near-perfect light absorption. In this work, absorbers based on single and double MIM structures are studied. It is shown that the optimization of the thicknesses of double MIM structures makes it possible to significantly expand the near-perfect absorption band. The broader near-perfect absorption band is explained by the presence of several resonances supported by the double MIM structure. The structure presented in this work absorbs more than 90% (98.5% on average) of the incident light with wavelengths ranging from 360 nm to 1750 nm, which is wider than the corresponding range in similar works [2, 3].

Materials and Methods

As it was noted earlier, an MIM structure can provide complete absorption due to the proper choice of layer thicknesses [5]. Let us consider an MIM structure with metal and dielectric layers made of chromium (Cr) and silicon dioxide (SiO2), respectively. The refractive indices for

© Кашапов А. И., Безус Е. А., Быков Д. А., Головастиков Н. В., Досколович Л. Л., 2022. Издатель: Санкт-Петербургский политехнический университет Петра Великого.

chromium and silicon dioxide were taken from the source [7]. The absorption spectra of metal-dielectric structures were calculated using an in-house implementation of the enhanced transmission matrix method [8]. As the optimization algorithm, we used the standard implementation of the interior-point algorithm in the MATLAB environment.

Results and Discussion

Fig. 1, a shows the high absorption bandwidth at 90% (near-perfect absorption) at normal incidence as a function of the wavelength X0, at which total absorption is achieved. It is important to note that in order to obtain the graph in (Fig. 1, a), for each wavelength X0, the thicknesses of the MIM structure layers were calculated from the condition of zero reflection (total absorption) at this wavelength [5]. It can be seen from (Fig. 1, a) that a simple three-layer structure provides a wide near-perfect absorption band in the near-IR. Also, in Fig.1, a, bold dots show the wavelengths used to calculate the structures, the spectra of which are shown in (Fig. 1, b). Consider a structure providing total absorption at a wavelength of 1300 nm. The thicknesses of the structure layers are described by the array [5.5, 197.1, 200.0] nm. In (Fig. 1, b), the yellow solid line shows the absorption spectrum of this structure, and the black dotted line shows the absorption level of 90%. Note that the width of the near-perfect absorption band is 732 nm (968—1700 nm).

The thicknesses of the layers of the MIM structure can be optimized to broaden the absorption band. For this, the following objective function can be used:

F (h) = J A ( h, X) dX, (1)

where A is the absorption, h is the array of thicknesses of the structure layers, Xj and X2 are the boundaries of the "target" wavelength range, in which it is required to obtain high absorption.

Fig. 1, c shows the absorption spectra of the "initial" MIM structure (blue line) and the structure with optimized thicknesses (red line) maximizing the function (1). In this case, the thicknesses of the initial MIM structure were used as the starting point for the optimization. The thicknesses of the resulting optimized MIM structure are described by the following array: [6.0, 197.1, 167.0] nm. It can be seen from (Fig. 1, c) that the optimization of the thicknesses of the layers of the MIM structure makes it possible to only slightly increase the width of the absorption band.

For most applications, it is important for the absorption to remain high as the angle of incidence of the incident light changes. Fig. 1, d shows a rather low sensitivity to changes in a wide range of angles of incidence. In addition, let us note that concerning the application of photonic absorbing structures for solar energy harvesting, modern techniques in the field of reinforcement learning enable effectively choosing the position of a solar panel with such a coating, so the "uncovered" ranges of incidence angles can be compensated by such approaches [9].

Next, we make the studied structure more complex by "combining" two MIM structures with a SiO2 dielectric layer separating them. Thus, we will obtain the following configuration (which we will refer to as "double" MIM structure): Cr-SiO2-Cr-SiO2-Cr-SiO2-Cr. The structure calculated in this way has a broader near-perfect absorption band (Fig. 2, a) as compared to the simple three-layer structure. In the vicinity of the wavelength, at which total absorption is provided, the absorption spectrum can be approximated by the function .P4(X) = 1 - aX4 (Fig. 2, a). It should be noted that the thickness of the upper chromium layer of such a structure is close to zero, and, as the results of numerical simulation demonstrate, the exclusion of this layer does not affect its optical properties. Thus, we have obtained a structure in the SiO2-Cr-SiO2 Cr-SiO2-Cr configuration, the layer thicknesses of which are described by the array [217.9, 5.6, 422.8, 8.9, 199.6, 200.0] nm.

Let us now consider the possibility of broadening the near-perfect absorption band by numerically optimizing the six layer thicknesses of the obtained structure. The objective function (1) given above can also be used to optimize the thicknesses of a multilayer structure. The spectra of three-layer structures in Fig. 1, b show a sharp drop in absorption with the shift to the visible wavelength range. Let us choose Xj = 380 nm and X2 = 1200 nm to expand the near-perfect absorption band of the multilayer MIM structure to the visible range. The thicknesses of the "double" MIM structure were used as the starting point for maximizing the objective function (1).

a)

à

c &

c

I

A„ = 1500 i

A0 = . A» = 1100 nn

X0, pm

c)

b)

d)

-A0 = 0.9 firn

-A0 = 1.1 firn

1.3 fim

-A,= 1.5 fim

X, pm

X, pm

30 40

8, grad

00

90

80

70

60

60

50

10

20

50

60

70

Fig. 1. Bandwidth of near-perfect absorption (at the level of 90%) vs. wavelength, at which total absorption is achieved, the black bold dots show the structures used to calculate the spectra in Fig. 1, b (a); absorption spectra of MIM structures calculated from the condition of zero reflection at normal incidence at different wavelengths, the dotted black line shows the absorption level of 90% (b); absorption spectrum of the analytically calculated MIM structure (blue line) and the structure with optimized thicknesses (red line) (c); average absorption over the wavelength range from 968 nm to 1700 nm vs. the angle of incidence for unpolarized light (black solid line), TE-polarized (red dashed line), and TM-polarized (blue dashed line) light (d)

X, pm

X,pm

iJ

c &

c

I

r

■ ■ \

8,grad

X,pm

Fig. 2. Absorption spectrum of the "double" MIM structure without the upper metal layer and its approximation by the function P4(X) = 1 - aX4 (inset) (a); absorption spectra of the three-layer MIM structure (blue line) and the optimized multilayer MIM structure (red line), the black dashed line shows the absorption level of 90% (b); Average absorption in the 360-1750 nm wavelength range vs. angle of incidence for unpolarized light (black solid line), TE-polarized (red dashed line), and TM-polarized (blue dashed line) light (c); numerically calculated spectrum of the optimized structure (black solid line) and its resonant

approximation by expression (2) (red dashed line) (d)

The red line in (Fig. 2, b) shows the absorption spectrum of the optimized structure, the thicknesses of which are described by the following array: [91.7, 5.0, 95.7, 12.3, 87.6, 223.0] nm.

Fig. 2, b shows that the addition of two dielectric layers separated by a metal layer to the three-layer MIM structure and subsequent optimization of their thicknesses makes it possible to extend the near-complete absorption band to the visible wavelength range, and that the width of the near-perfect absorption band of such a structure is almost two times greater than the corresponding value in the case of a simple three-layer structure. Also, the obtained structure provides rather low sensitivity to a change in the angle of incidence of the incident light (Fig. 2, c).

It should be noted that this effect has a resonant nature and is due to the excitation of eigenmodes supported by such a structure. Hence, the absorption spectrum of the six-layer structure can be approximated by the following expression:

A (X) = 1 -

i=1 x-'ki' p

(2)

where p is the non-resonant reflection coefficient, X and X. are the complex wavelengths corresponding to zeros and poles (eigenmodes) of the reflection coefficient r(X), respectively.

In Fig. 2, d, the red dashed line shows the resonant approximation (2) of the numerically calculated spectrum of the multilayer MIM structure with optimized thicknesses (black solid line). As it can be seen from Fig. 2, d, the resonant model well approximates the obtained result, which confirms the assumption about the resonant nature of the studied effect. The numerically obtained approximation parameters (2) are given in Table. 1.

Table 1

Numerical values of approximation parameters (2) for the absorption spectrum of the optimized six-layer structure.

2

Approximation parameter P

Value 0.837 0.485+ 0.083i 0.909+ 0.189i 1.273+ 0.187i 0.452+ 0.270i 0.921+ 1.489i 1.297+ 0.466i

Conclusion

We demonstrated that a three-layer metal-insulator-metal structure can provide a wide near-perfect absorption band in the near-IR wavelength range, and high absorption is also maintained for a wide range of incidence angles. It has also been shown that the combination of MIM structures can significantly increase the width of near-perfect absorption band, and, in addition, the structure calculated in this way is a good starting point for numerical optimization of the thicknesses of a multilayer metal-insulator structure in order to extend the high absorption band to the visible and also to partially cover the near ultraviolet wavelength range. The resonant nature of the high absorption effect in the considered metal-insulator structures was also shown. The authors believe that this structure is promising for various practical applications, since layered structures, in contrast to structures with patterned layers (diffraction gratings and metasurfaces), are relatively easy to fabricate. The obtained results can be applied to solar energy harvesting, photovoltaics and thermophotovoltaics, photodetection, absorption filtering and sensing.

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THE AUTHORS

KASHAPOV Artem I.

GOLOVASTIKOV Nikita V.

nikita.golovastikov@gmail.com

ar.kashapov@outlook.com ORCID: 0000-0002-7367-6277

ORCID: 0000-0002-0123-252X

BEZUS Evgeni A.

evgeni.bezus@gmail.com ORCID: 0000-0001-7496-8960

leonid@ipsiras.ru

ORCID: 0000-0001-8649-028X

DOSKOLOVICH Leonid L.

BYKOV Dmitry A.

bykovd@gmail.com ORCID: 0000-0002-9576-2360

Received 14.07.2022. Approved after reviewing 17.07.2022. Accepted 18.07.2022.

© Peter the Great St. Petersburg Polytechnic University, 2022