RU2589465C1 - Method of increasing propagation length of infrared monochromatic surface electromagnetic waves on flat metal surface - Google Patents

Abstract

FIELD: information technology.

SUBSTANCE: invention relates to information-communication technologies, particularly, to increase in length of propagation of infrared monochromatic surface electromagnetic waves (SEW) along flat metal surface. Method includes application of non-absorbing dielectric layer on surface. Before layer application direction of maximum beam pattern of bulk electromagnetic waves (BEW) emitted by SEW from their track is determined. Layer thickness and refraction index of its material is selected so that presence of layer provides increment of real part of module of SEW vector by value

where k_o = 2π/λ - wave number BEW in surrounding surface medium; λ is wavelength of radiation in environment; φ_Max is angle of deviation of maximum beam pattern from plane surface.

EFFECT: technical result consists in increase of propagation length (SEW) and providing its protection against external effects.

1 cl, 2 dwg

Description

The invention relates to the field of information and communication technologies, in which the collection, processing and transfer of information is carried out by infrared (IR) surface electromagnetic waves (SEW), in particular surface plasmons directed by a flat metal surface, and can be used in optical communication channels, interferometers, spectrometers and other devices for the collection, processing and transmission of information using IR PEV.

The main applications of IR PEV are optical methods for studying the surface of a solid (spectroscopy, microscopy, ellipsometry, refractometry), as well as devices for sensory and information and communication purposes [Maier SA Plasmonics: Fundamentals and Applications // Springer Science + Business Media LLC, 2007 . p. 89-106]. The main advantage of using IR surface plasmons in communication channels and information processing devices is their high phase velocity, the magnitude of which is only slightly less than the speed of light in the environment. The wider distribution of plasmon information lines is hindered by the fact that the propagation length of IR SEWs over a real metal surface, although reaching 10 ³ wavelengths (k), is nevertheless approximately two orders of magnitude smaller than the value calculated using the Drude model for permittivity metal [Ordal M.A., Long LL, Bell RJ, et al. Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared // Applied Optics, 1985, v.22, Is.7, p. 1099-1120]. The main reason for this phenomenon is the radiation losses of surface electromagnetic waves on the inhomogeneities (roughness and foreign inclusions) of the real metal surface [Gerasimov VV, Knyazev VA, Kotelnikov IA, Nikitin AK, et al. Surface plasmon polaritons launched using a terahertz free electron laser: propagating along a gold-ZnS-air interface and decoupling to free waves at the surface tail end // JOSA (B), 2013, v. 30, Is. 8, p. 2182-2190]. The claimed invention describes a method of reducing such losses by applying to the surface of a layer of non-absorbing dielectric subwavelength thickness.

There is a method of compensating for the loss of monochromatic IR SEWs during their propagation over a flat metal surface, including applying a dielectric layer with a thickness of the order of λ to the surface and creating an inverted population layer with energy levels of matter particles [Zhang X., Liu N., and Zhong Y. Compensation of propagation loss of surface plasmon polaritons with a finite-thickness dielectric gain layer // J. Optics, 2012, v. 14, 125003]. The main disadvantage of this method is the need to create and constantly maintain an inverse population in the material of the layer, which suggests the presence of a pumping device and additional energy costs in the process of implementing the method.

The closest in technical essence and the achieved result to the claimed one is a method of increasing the propagation length of monochromatic IR SEWs on the surface of a flat transparent metal film, including applying a layer of non-absorbing dielectric to the film surface with a thickness exceeding the depth of penetration of the SEW field into the layer material and having a refractive index close to the refractive index of the substrate [Vaicikauskas V. Fourier transform infrared analysis of long-range surface polaritons excited by the end-fire method // Thin Solid Films, 2005, v. 493, p. 288-292]. The main disadvantage of the known method is the limited class of objects to which the method is applicable only with transparent metal films, while in plasmon communication channels and information devices, IR PEVs are often directed by the surface of opaque metal bodies.

The technical result of the invention is to increase the propagation length of monochromatic IR sew on a real flat metal surface, and also to protect it from external influences.

The technical result is achieved by the fact that in the known method of increasing the propagation length of monochromatic IR SEWs on a flat metal surface, including applying a non-absorbing dielectric to the surface of the layer, before applying the layer, the direction of the maximum radiation pattern of volume electromagnetic waves (OEW) emitted by SEWs from their track is determined, and the thickness of the layer and the dielectric constant of its material is chosen such that the presence of the layer provides a negative increment of the real part of the module in SEW new vector the amount Δk '= k _o · [1-cos (φ _max)], where k _o = 2π / λ - EIA wavenumber in the surrounding medium surface; λ is the radiation wavelength in the environment; φ _max - the angle of deviation of the maximum radiation pattern from the surface plane.

The theoretical basis of the claimed invention are the following facts:

(где i - мнимая единица) относительно его значения

для идеально плоской и однородной границы «металл-диэлектрик» [Raether Н. Surface plasmons on smooth and rough surfaces and on gratings // Springer Tracts in Modern Physics, 1988, v. 111, Springer-Verlag, Berlin. - 530 р.]. Отрицательное приращение Δk=Δk′+i·Δk″ (зависящее от характеристик неоднородностей) к модулю вектора k_ПЭВ обуславливает частичное преобразование ПЭВ в объемное излучение, поскольку становится возможным выполнение неравенства

, являющегося условием преобразования ПЭВ в ОЭВ, излучаемые с трека ПЭВ в окружающую среду. Совокупность таких ОЭВ и представляет собой радиационные потери ПЭВ, дополнительные к их тепловым потерям в металле.1) the presence of inhomogeneities on the surface of the SEW guide in the form of roughnesses and variations in the dielectric constant determines the “blurring” (as a rule, of a Gaussian shape) of the SEW wave vector module

(where i is the imaginary unit) relative to its value

for a perfectly flat and uniform metal-insulator interface [Raether N. Surface plasmons on smooth and rough surfaces and on gratings // Springer Tracts in Modern Physics, 1988, v. 111, Springer-Verlag, Berlin. - 530 p.]. The negative increment Δk = Δk ′ + i · Δk ″ (depending on the characteristics of the inhomogeneities) to the modulus of the _SEW vector k determines the partial transformation of the SEW into volume radiation, since it becomes possible to fulfill the inequality

, which is a condition for the conversion of SEW into EIA, emitted from the SEW track into the environment. The totality of such EIAs is the radiation losses of SEWs, additional to their thermal losses in the metal.

2) Since the emission of the EIA occurs predominantly at an angle φ _max , then for the tangential component k _{ox of} the wave vector module of such EIA the relation is true: k _ox = k _o · cos (φ _max ). Then, by virtue of the law of conservation of the tangential component of the wave vector, we obtain:

превышает k_o всего на сотые доли процента, можно утверждать, что $$k_{oПЭВ}^{\text{'}}\approx k_o$$

. Тогда равенство (1) примет вид:where Δk ′ is the real part of the increment Δk = Δk ′ + i · Δk ″ reported by surface inhomogeneities to the SEW wave vector. Given the fact that in the IR range, the value

exceeds k _o only by hundredths of a percent, it can be argued that $$k_{oPE\mathrm{AT}}^{\text{'}\text{'}}\approx k_o$$

. Then equality (1) takes the form:

Whence we get the formula for estimating Δk ′:

на величину

, пропорциональную толщине слоя [Жижин Г.Н., Никитин А.К., Богомолов Г.Д. и др. Поглощение поверхностных плазмонов ТГц диапазона в структуре "металл-покровный слой-воздух" // Оптика и спектроскопия, 2006, Т. 100, №5, с. 798-802]. Увеличение

на приращение

приводит к тому, что условие преобразования ПЭВ в ОЭВ

при наличии слоя на поверхности выполняется для значительно меньшего числа гармонических составляющих фурье-спектра неоднородностей, чем в отсутствие слоя. Это и обеспечивает достижение основной цели изобретения - уменьшение радиационных потерь и увеличение длины распространения ИК ПЭВ. Кроме того, наличие слоя на металлической поверхности, направляющей ПЭВ, обеспечит ее защиту от внешних воздействий.3) The application of a dielectric layer to the surface directing the SEW leads to an increase in the modulus of the SEV wave vector

by the amount

proportional to the thickness of the layer [Zhizhin GN, Nikitin AK, Bogomolov GD et al. Absorption of surface plasmons of the THz range in the structure “metal-coating layer-air” // Optics and Spectroscopy, 2006, vol. 100, No. 5, p. 798-802]. Increase

increment

leads to the fact that the condition for the conversion of SEW to EIA

in the presence of a layer on the surface, it is performed for a significantly smaller number of harmonic components of the Fourier spectrum of inhomogeneities than in the absence of a layer. This ensures the achievement of the main objective of the invention is to reduce radiation losses and increase the propagation length of IR-SEW. In addition, the presence of a layer on the metal surface guiding the SEW will ensure its protection from external influences.

The invention is illustrated by drawings: in Fig. 1 shows a diagram of a device that implements the method; in fig. 2 - calculated dependence of the increment of the real part of the refractive index of the SEW Δκ ′ on the thickness d of the ZnS layer for SEW with λ = 130 μm in the structure "gold - ZnS layer - air".

The proposed method can be implemented using a device, a diagram of which is shown in Fig. 1, where the numbers denote: 1 - a source of monochromatic IR radiation; 2 - a sample placed in the environment 3 and having a flat metal face capable of directing SEW; 4 - a flat rotary mirror; 5 - a cylindrical mirror; 6 - an element for converting the radiation of source 1 into a SEW, having a cylindrical face capable of guiding the SEW and the length of which in the plane of radiation incidence is less than the length of the SEW propagation through sample 2; 7 - photodetector; 8 - a flat screen placed in the environment 3 and oriented perpendicular to the SEW track, and the lower edge of the screen is removed from the edge of the sample 2 at a distance not less than the depth of penetration of the SEW field into the environment 3; 9 - an optical bench having a rotation axis belonging to the plane of the face of sample 2 and intersecting the SEW track at right angles; 10 - focusing lens mounted on the bench 9 and in the focus of which is placed the receiver 7, also fixed on the bench 9; 11 - adjustable diaphragm located in front of the inlet of the receiver 7; 12 - galvanometer connected to the receiver 7; 13 - device for the accumulation and processing of information.

The method is implemented as follows. First, radiation from the source 1 generates SEWs on a flat metal face of sample 2, adjacent to the environment 3. For this purpose, the radiation 4 is directed to a cylindrical mirror 5, focusing it on the edge of the cylindrical face of element 6. As a result of diffraction of radiation on the edge, it is converted with some efficiency in sew [Stegeman GI, Wallis RF, Maradudin AA Excitation of surface polaritons by end-fire coupling // Optics Letters, 1983, v. 8 (7), p. 386-388]. The SEW generated in this way propagates along the cylindrical surface of element 6 and passes from it to the flat face of sample 2 adjacent to the second edge of the cylindrical face. Note that element 6 is introduced into the circuit of the device for shielding the photodetector 7 from spurious EIA arising from the conversion of the radiation of source 1 into SEW by diffraction. A similar function is performed by the screen 8, which protects the receiver 7 from parasitic EIA arising as a result of diffraction of SEWs at the junction of element 6 and sample 2. Propagating along the flat face of sample 2, SEWs are diffracted on the inhomogeneities of its surface and partially emitted in the form of EIA into the environment 3 ; moreover, the directivity diagram of these EIA has a maximum that is at an angle φ _max [Knyazev B.A., Nikitin A.K., Zhizhin G.N. A method for measuring the propagation length of infrared surface plasmons over a real surface // RF patent for the invention No. 2512659, Bull. No. 1 dated January 10, 2014]. Having reached the opposite edge of the face, SEWs are diffracted on it and converted into EIA propagating in the environment 3 [Gerasimov VV, Knyazev VA, Kotelnikov IA, Nikitin AK, et al. Surface plasmon polaritons launched using a terahertz free electron laser: propagating along a gold-ZnS-air interface and decoupling to free waves at the surface tail end // JOSA (B), 2013, v. 30, Is. 8, p. 2182-2190].

At the first stage of the method, the direction of the maximum of the directivity pattern of the EIA of the emitted SEW from the track is determined. To do this, an optical bench 9 with a lens 10 mounted on it and a photodetector 7 equipped with an adjustable diaphragm 11 is placed so that its axis of rotation passes through the center of the section of the SEW track bounded by the screen 8 and the distant (along the SEW) edge of the flat face of sample 2 By smoothly varying the angle φ between the optical axis of the lens 10 and the flat face of sample 2, the signal I from the receiver 7 is measured with a galvanometer 12. Moreover, to ensure a constant size Δl of the observed section of the track, the size D of the diaphragm 11 is changed depending ing on the angle φ in accordance with the formula D = Δl · sin (φ). Based on the combination of the corresponding values of φ and I, the device 13 determines the direction φ _{max of the} maximum radiation pattern of the EIA emitted by the SEW from the track.

At the second stage of the method implementation according to the formula (3), using the measured φ _{max, the} value Δk ′ is calculated. Next, determine the thickness d of the dielectric layer at a given dielectric constant ε of the _{layer of} its material. For this, the dispersion equation of the SEW for a three-layer structure is solved (with respect to d) [Maier SA Plasmonics: Fundamentals and Applications // Springer Science + Business Media LLC, 2007. p. 89-106; Zhizhin G.N., Moskaleva MA, Shomina EB, Yakovlev V.A. Distribution of sew on metal surfaces // Ch. 3 (p. 70-104) in the collection “Surface polaritons. Surface electromagnetic waves at the boundaries of media "ed. V.M. Agranovich and D.L. Mills. M .: Nauka, 1985], substituting into it the values of Δk ′, λ and dielectric permittivities of the metal ε ₁ and the environment ε ₂ .

At the third stage of the method implementation, a dielectric layer with an ε _{layer of} thickness d ₀ corresponding to φ _max is applied to the flat face of sample 2. The presence of such a layer on the surface of the face ensures a decrease in the radiation losses of the surface electromagnetic waves, and, therefore, an increase in the length of the propagation of the surface electromagnetic waves, by about a factor of two, compared with sample 2, which does not contain a dielectric layer. Such an estimate of the magnitude of the reduction in radiation losses is based on the fact that in the IR range the fraction of the SEW field energy transferred in the metal is hundredths of a percent [Zhizhin GN, Moskaleva MA, Shomina EV, Yakovlev V. BUT. Distribution of sew on metal surfaces // Ch. 3 (p. 70-104) in the collection “Surface polaritons. Surface electromagnetic waves at the boundaries of media "ed. V.M. Agranovich and D.L. Mills. M .: Nauka, 1985]; on the other hand, the real length of the SEW propagation is two orders of magnitude less than that calculated for an ideal (non-heterogeneous) metal surface. Consequently, the SEW losses on a real surface are mainly of a radiation nature. Therefore, the cutoff by applying a layer of half of the radiation OM (due to the Gaussian form of “blurring” the value of the SEW wave vector on the non-ideal metal surface and the connection of the Δk ′ value with the angle φ _max ) due to surface inhomogeneities should lead to a decrease in the radiation losses of the SEW by half and, accordingly, a twofold increase in the length of their distribution.

, путем нанесения на нее слоя сульфида цинка (ZnS) с ε_слоя=8.70. Известно [Gerasimov V.V., Knyazev В.А., Nikitin А.К., Zhizhin G.N. // A way to determine the permittivity of metallized surfaces at terahertz frequencies // Applied Physics Letters, 2011, v. 98, 171912], что показатель преломления

таких ПЭВ на не содержащей слоя плоской поверхности напыленного золота равен κ_o=1.000657+i·0.00027, что соответствует их длине распространения

мм. Пусть максимум диаграммы направленности ОЭВ, излучаемых ПЭВ с их трека, направлен под углом φ_max=5° к поверхности золота. Это означает, что неоднородности поверхности сообщают волновому вектору ПЭВ отрицательное приращение Δk′, которое, согласно (3), для данных λ и φ_max равно 1.84 см^-1, что соответствует приращению Δκ′ к показателю преломления ПЭВ, равному Δκ′=Δk′/k_o=3.81·10^-3. Определим толщину слоя ZnS, которую необходимо нанести на поверхность золота, чтобы увеличить L примерно в два раза. Для этого, используя дисперсионное уравнение ПЭВ для трехслойной структуры [Жижин Г.Н., Москалева М.А., Шомина Е.В., Яковлев В.А. Распространение ПЭВ по металлическим поверхностям // Гл. 3 (с. 70-104) в сборнике «Поверхностные поляритоны. Поверхностные электромагнитные волны на границах сред» под ред. В.М. Аграновича и Д.Л. Миллса. М.: Наука, 1985], рассчитаем зависимость показателя преломления ПЭВ к в структуре "золото - слой ZnS - воздух" от толщины d слоя. Диэлектрическую проницаемость золота рассчитаем по модели Друде, подставляя в нее столкновительную частоту ν_τ=215 см^-1 и плазменную частоту ν_p=72800 см^-1 [Ordal М.А., Long L.L., Bell R.J., et al. Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared // Applied Optics, 1985, v. 22, Is. 7, p. 1099-1120]. На рис. 2 приведен график расчетной зависимости Δκ′ (d); из нее видно, что значение Δκ′=3.81·10^-3 достигается при d=d_o≈1.96 мкм. При наличии такого слоя ZnS на поверхности золота длина распространения ПЭВ увеличится примерно в два раза и достигнет значения 77 мм. В то же время, такой слой ZnS надежно защищает золото от внешних (механических и химических) воздействий.As an example of the application of the proposed method, we consider the possibility of increasing the propagation length of the surface electromagnetic waves with λ = 130 μm, guided by a flat surface of gold in air with a refractive index

, by applying a layer of zinc sulfide (ZnS) on it with ε _layer = 8.70. It is known [Gerasimov VV, Knyazev V.A., Nikitin A.K., Zhizhin GN // A way to determine the permittivity of metallized surfaces at terahertz frequencies // Applied Physics Letters, 2011, v. 98, 171912] that the refractive index

of such SEWs on a layer of a deposited gold not containing a flat surface is κ _o = 1.000657 + i · 0.00027, which corresponds to their propagation length

mm Let the maximum radiation pattern of the EIA emitted by the SEW from their track be directed at an angle φ _max = 5 ° to the gold surface. This means that the surface inhomogeneities give the SEW wave vector a negative increment Δk ′, which, according to (3), for given λ and φ _max is 1.84 cm ^-1 , which corresponds to the increment Δκ ′ to the SEI refractive index equal to Δκ ′ = Δk ′ / k _o = 3.81 · 10 ^-3 . Let us determine the thickness of the ZnS layer, which must be applied to the surface of gold in order to increase L by approximately two times. For this, using the dispersion equation of the SEW for a three-layer structure [Zhizhin GN, Moskaleva MA, Shomina EV, Yakovlev VA Distribution of sew on metal surfaces // Ch. 3 (p. 70-104) in the collection “Surface polaritons. Surface electromagnetic waves at the boundaries of media "ed. V.M. Agranovich and D.L. Mills. M .: Nauka, 1985], we calculate the dependence of the refractive index of SEW k in the structure "gold - ZnS layer - air" on the thickness d of the layer. We calculate the dielectric constant of gold according to the Drude model, substituting the collisional frequency ν _τ = 215 cm ^-1 and the plasma frequency ν _p = 72800 cm ^-1 [Ordal MA, Long LL, Bell RJ, et al. Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared // Applied Optics, 1985, v. 22, Is. 7, p. 1099-1120]. In fig. 2 shows a graph of the calculated dependence Δκ ′ (d); it can be seen that the value Δκ ′ = 3.81 · 10 ^{-3 is} achieved at d = d _o ≈1.96 μm. In the presence of such a ZnS layer on the surface of gold, the propagation length of the SEW will increase by approximately two times and reach a value of 77 mm. At the same time, such a ZnS layer reliably protects gold from external (mechanical and chemical) influences.

The considered example clearly demonstrates the possibility of increasing the propagation length of monochromatic IR SEWs by reducing their radiation losses by applying a layer of non-absorbing dielectric thickness of a subwavelength to the waveguide surface, which also performs the function of a surface protector from external influences.

The application of the proposed method will allow more efficient use of IR PEV in optical communication lines and sensor devices, and also will increase the reliability of their operation and increase the service life.

Claims (1)

A method for increasing the propagation length of infrared monochromatic surface electromagnetic waves (SEWs) over a flat metal surface, comprising applying a non-absorbing dielectric layer to the surface, characterized in that prior to applying the layer, the direction of the maximum radiation pattern of volume electromagnetic waves (OEW) emitted by SEWs from their track is determined, and the layer thickness and the refractive index of its material are chosen such that the presence of the layer provides an increment of the real part of the wave modulus SEW vectors by the amount of:

where k _o = 2π / λ is the wave number of the EIA in the environment surrounding the surface; λ is the radiation wavelength in the environment; φ _max - the angle of deviation of the maximum radiation pattern from the surface plane.

Images