RU2491533C1 - Method to determine depth of penetration of field of terahertz surface plasmons into environment - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to optical methods of metal and semiconductor surface control in terahertz range of the spectrum and may find application in technological processes for control of thickness and homogeneity of thin-layer coatings of metallised items and semiconductor substrates, in the methods for detection of irregularities on/of conducting surface, in infrared (IR) refractometry of metals for determination of their dielectric permeability, in IR sensor devices and inspection technology. The method includes measurement of intensity of the field of surface plasmons (SP) in the plane of drop of radiation generating a bundle of SP rays, and calculation of the value 5 by results of measurements, for this purpose the SPs are converted into a volume wave on the front line, which belongs to the selected plane of the bundle cross section, the wave is focused into a line lying in the drop plane, and distribution of radiation intensity is measured on this line, and the angle of inclination of wave rays to the surface, guiding the SPs.

EFFECT: invention makes it possible to reduce time of measurements.

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Description

The invention relates to optical methods for monitoring the surface of metals and semiconductors in the terahertz (THz) range of the spectrum and can be used in technological processes for controlling the thickness and uniformity of thin-layer coatings of metallized products and semiconductor substrates, in methods for detecting inhomogeneities of (on) a conductive surface, in infrared (IR) refractometry of metals to determine their dielectric constant, in IR sensor devices and instrumentation.

- расстояние, на котором интенсивность поля уменьшается в e≈2,718 раз; здесь $$k=k_0\cdot \sqrt{k^2-\epsilon }$$

- нормальная (относительно поверхности) компонента волнового числа ПП, k_o=2π/λ, k - комплексный показатель преломления ПП, ε - диэлектрическая проницаемость окружающей среды, λ - длина волны излучения, генерирующего ПП. Измеряя δ, можно не только обнаруживать (по вариациям δ вдоль трека ПП) неоднородности на поверхности, но и определять диэлектрическую проницаемость материала проводящего образца или толщину и показатель преломления переходного слоя поверхности [Gerasimov V.V., Knyazev B.A., Nikitin A.K., Zhizhin G.N. A way to determine the permittivity of metallized surfaces at terahertz frequencies // Applied Physics Letters, 2011, v.98, 171912].Surface plasmons (PP) - are a type of surface electromagnetic waves directed by a conductive surface, and are widely used for its control and spectroscopy [Surface polaritons. Electromagnetic waves on surfaces and media interfaces / Ed. V.M. Agranovich and D.L. Mills. - M .: Nauka, 1985. - 525 p.]. One of the most important characteristics of PP, depending on the optical properties of the surface and its coating, is the depth of penetration of the PP field into the environment (in particular, air or vacuum) $$\delta ={[Re(k)]}^{-\mathrm{one}}$$

- the distance at which the field intensity decreases by e≈2.718 times; here $$k={\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="00000004.png"\; state="\{\{state\}\}">k</figure-callout>}}_0\cdot \sqrt{k^2-\epsilon }$$

is the normal (relative to the surface) component of the PP wave number, k _o = 2π / λ, k is the complex refractive index of the PP, ε is the dielectric constant of the environment, λ is the wavelength of the radiation generating the PP. By measuring δ, one can not only detect (by variations of δ along the PP track) inhomogeneities on the surface, but also determine the dielectric constant of the material of the conducting sample or the thickness and refractive index of the transition layer of the surface [Gerasimov VV, Knyazev BA, Nikitin AK, Zhizhin GN A way to determine the permittivity of metallized surfaces at terahertz frequencies // Applied Physics Letters, 2011, v. 98, 171912].

There is a method of determining the penetration depth of the THz field of the PP in the environment δ, including measuring the field intensity along the normal to the track of the PP in the plane of incidence of the radiation generating the PP, introducing into the PP field the tip of a fiber optic probe connected to a photo detector connected to a galvanometer, measuring the intensity dependence the light signal entering the photodetector from the distance separating the tip from the surface guiding the PP and calculating the value of δ from the measurement results [Mueckstein R., Mitrofanov O. Imaging of terahertz surface pla smon waves excited on a gold surface by a focused beam // Optics Express, 2011, v.19, No.4, p.3212-3217]. The main disadvantages of the method are the perturbation of the probe field PP, which distorts the measurement results, and the long duration of the sensing procedure.

The closest in technical essence and the achieved result to the claimed one is a method for determining the penetration depth of the THz PP field into the environment δ, including measuring the field intensity along the normal to the PP track in the plane of incidence of the radiation generating the PP, introducing into the field a photodetector connected to a galvanometer equipped with a slot a diaphragm oriented parallel to the guide of the PP surface and perpendicular to the track, measuring at the end of the sample the dependence of the field intensity on the distance separating a gap from the surface, and the calculation values from the measurements δ [Gerasimov V.V., Knyazev VA, Nikitin AK, Zhizhin G.N. A way to determine the permittivity of metallized surfaces at terahertz frequencies // Applied Physics Letters, 2011, v. 98, 171912].

The main disadvantages of the method are the distortion of the PP field reflected from the diaphragm radiation, the inability to perform measurements at an arbitrary point on the track and their long duration.

The technical result to which the invention is directed is to reduce the measurement time.

The technical result is achieved by the fact that in the known method for determining the penetration depth of the THz PP field into the environment δ, including measuring the intensity of the PP field in the plane of incidence of the radiation generating the PP beam, and calculating the δ value from the measurement results, according to the invention, the PP is converted into volumetric wave on the front line belonging to the selected plane of the beam cross section, focus the wave in a line lying in the plane of incidence, and measure the distribution of radiation intensity on this line and the angle of incidence the bosom of the rays of the wave to the surface directing the PP.

The reduction of the measurement time is achieved as a result of the simultaneous measurement of the PP field intensity by a line of photodetectors at a number of environmental points above the controlled track point.

The invention is illustrated by drawings: in Fig. 1 is a diagram of a device that implements the method; in fig. 2 - dependence of the PP field intensity in the considered example on the distance separating a given point of the field from the sample surface.

The proposed method can be implemented using a device, the circuit of which is shown in Fig. 1, where the numbers denote: 1 - source of monochromatic radiation; 2 - polarizer; 3 - a flat mirror; 4 - concave mirror with a cylindrical reflective surface; 5 - conductive sample; 6 - prism with a metallized flat base oriented parallel to the surface of sample 5; 7 - an absorbing flat screen, oriented perpendicular to the plane of incidence of radiation and the edge of which is removed from the sample 5 by a distance of several millimeters; 8 - oblique movable flat mirror that converts the PP into a body wave; 9 - a lens with a cylindrical focusing surface, 10 - a line of photo detectors located in the plane of incidence and located in the focus of the lens 9; 11 - a set of galvanometers separately connected to the detectors of line 10; 12 - information processing device; 13 - movable platform with elements 8, 9 and 10 installed on it.

The method is as follows.

The radiation of the source 1 is directed to the polarizer 2, which emits the p-component from the electromagnetic wave. Using mirrors 3 and 4, polarized radiation is directed into the gap between the conducting surface of sample 5 and the metallized base of prism 6. In the gap, radiation is converted into TM modes of a hollow metal waveguide formed by the base of prism 6 and the surface of sample 5. Diffracting at the edge of prism 6, the modes with some efficiency, they transform into PPs and generate a fan of spurious body waves absorbed by the screen 7 [Gong M., Jeon T.-L, Grischkowsky D. THz surface wave collapse on coated metal surfaces // Optics Express, 2009, v.1 7 ( 19), 17088]. The PP beam passes under the screen 8 and propagates in the plane of incidence along the surface of the sample 5. Having reached the front edge of the mirror 8, perpendicular to the wave front of the beam, the PP are transformed into a body wave (OB). The conversion of the PP into volume radiation occurs as a result of the transmission of a negative impulse by the mirror (with respect to the propagation direction of the PP) by the mirror. Which ensures the fulfillment of the inequality k _PP <k _o (where k _PP and k _o are the moduli of the wave vectors of the PP and the plane wave in the environment, respectively), which is necessary for the transformation of the PP into a volume wave [Surface polaritons. Electromagnetic waves on surfaces and media interfaces / Ed. V.M. Agranovich and D.L. Mills. - M .: Nauka, 1985. - 525 p.]. Note that the intensity distribution in the organic matter propagating parallel to the surface of sample 5 is identical to the distribution of the PP field intensity at the edge of the mirror 8. This OB is directed by the mirror 8 through the lens 9 to the array 10. The electrical signals from the outputs of the detectors of the array 10 are measured by the corresponding galvanometers of set 11 and arrive at the device 12, which normalizes them to the maximum signal and, taking into account the angle of inclination of the OB rays to the surface of the sample 5, calculates the desired value of the penetration depth of the PP field into the environment δ. Moving the platform 13 along the direction of the THz SP track, one can similarly determine the value of δ at any point on the track (if there is heterogeneity on the surface of sample 5, the value of δ can change accordingly).

As an example of the application of the proposed method, we consider the possibility of determining the value of δ for THz PP generated by free-electron laser radiation with a wavelength of λ = 130 μm [Knyazev V.A., Kulipanov GN, Vinokurov NA. Novosibirsk terahertz free electron laser: instrumentation development and experimental achievements // Meas. Sci. & Techn., 2010, v.21, 054017] on a flat surface of a 20 cm long gold sample placed in air. In this case, the complex refractive index of PP calculated using the dispersion equation of the PP for a two-layer structure and the Drude model for gold dielectric constant [Ordal M.A., Bell RJ, Alexander RW, Long LL, and Querry MR Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag , Ti, V, and W // Applied Optics, 1985, v. 24, No.24, p. 4493-4499], is equal to κ = 1,000273 + i · 5,6 · 10 ^-7 , which corresponds to the propagation length of PP equal to 180 cm. We set the angle of inclination of mirror 8 to the surface of sample 5 equal to 45 °, which ensures the distribution of OB generated by the PP on the leading edge of the mirror 8, perpendicular to the track PP. As a photodetector, we choose a bolometric matrix consisting of 320 × 240 pixels, 51 × 51 microns in size each [Demyanenko MA, Yesaev D.G., Ovsyuk V.N., Fomin B.I., Aseev A.L. ., Knyazev B.A., Kulipanov G.N., Vinokurov N.A. Matrix microbolometric receivers for IR and THz ranges // Optical Journal, 2009, v. 76, No. 12, pp. 5-11]. In fig. Figure 2 shows the dependence of the normalized PP field intensity I / I ₀ (where I ₀ is the PP field intensity on the surface) in the considered example on the distance r separating a given point of the field from the sample surface, taking into account the distortion of the I / I ₀ (z) distribution due to the slope mirrors 8 to 45 °. From this dependence it follows that the value of δ in this example is 15.3 mm. The PP field can be completely (within the range of δ) almost instantly detected by the matrix column, provided that its side is 320 pixels parallel to the plane of incidence, and the value of δ is quantified by the information processing device with an accuracy of 0.3% (pixel size to δ).

Thus, the given example clearly demonstrates the possibility of almost instantly determining the depth of penetration of the THz PP field into the environment δ, which ensures the achievement of the goal of the invention — reducing the measurement time of the value δ.

Claims (1)

A method for determining the penetration depth of the field of terahertz surface plasmons into the environment, including measuring the intensity of the field of surface plasmons in the plane of incidence of the radiation generating a beam of surface plasmons, and calculating the depth of penetration of the field of terahertz surface plasmons into the environment according to the measurement results, characterized in that the surface plasmons are converted into a body wave on the front line belonging to the selected plane of the beam cross section, focus They wave the wave into a line lying in the plane of incidence, and measure the distribution of the radiation intensity on this line and the angle of inclination of the wave's rays to the surface directing the surface plasmons.

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