RU2703941C1 - Apparatus for converting infrared radiation into a surface electromagnetic wave on a flat face of a conductive body - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: invention relates to investigation of metal surfaces and semiconductors by measuring characteristics of surface electromagnetic waves (SEW) directed to it and can be used in sensor devices, absorption spectrometers and interferometers which use surface plasmon-polaritons (SPP) as a data medium, which are a variety SEW. Device for converting IR radiation into SEW on a flat face of the conductive body comprises a source of p-polarized monochromatic radiation, an optical lens, cylindrical segment, convex surface of which is able to direct SEW, is coated with dielectric layer of subwavelength thickness, has axial line perpendicular to radiation incidence plane, is limited by two acute-angled ribs in direction perpendicular to incidence plane, has line of intersection with this plane shorter than propagation length SEW and is conjugated by one of ribs with flat face of body, absorbing screen located above track SEW outside its field and oriented perpendicular both to the face and the plane of incidence, the lens is selected as collimating. Convex surface of the segment is equipped with a planar diffraction grating, the strokes of which are perpendicular to the plane of incidence. Source and the lens are mounted on a platform capable of moving along an arc whose axis coincides with the center line of the grid.

EFFECT: high efficiency of converting monochromatic infrared radiation into a surface electromagnetic wave on a flat face of a conducting body.

1 cl, 1 dwg

Description

The invention relates to the field of research of the surface of metals and semiconductors by measuring the characteristics of the surface electromagnetic waves (SEW) of the infrared (IR) or terahertz (THz) range directed to it, and can find application in sensor devices, absorption spectrometers and interferometers using surface information plasmon polaritons (SPP), which are a type of SEW [1].

SPP generation is carried out, most often, acting on the test surface of the sample with a body wave of a radiation source. In this case, to match the phase velocities of the radiation of the source and the IR IFR, as well as their wave vectors, the phenomenon of plane wave diffraction by one or another diffraction element (edge of the screen, edge of a transparent prism, planar diffraction grating, or simply inhomogeneity on the surface) is used, placed within the depth of penetration of the SPP field into the environment [2]. A negative aspect of this method of converting infrared or THz radiation into SPP is the generation of a wide fan of stray body waves accompanying this effect, some of which are near-surface and therefore difficult to distinguish from SPP [3]. The use of the method of impaired total internal reflection (ATR) for converting source radiation into IFR of the middle IR and THz ranges is rarely practiced, due to the macroscopic propagation length of such SEWs, significantly exceeding the diameter of the incident radiation beam, which leads to a large value of their radiation losses towards the ATR prism [2 ].

A known aperture device for exciting SEWs on a flat face of a conducting body, containing a p-polarized radiation source, a focusing lens, an opaque screen oriented perpendicular to both the plane of incidence and the face plane, the edge of which is within the depth of penetration of the SEW field into the environment [4 ]. The main disadvantages of such a device are: 1) low efficiency (fraction of a percent) of the conversion of the radiation of the source into the SEW; 2) a wide fan of intense body waves generated by radiation diffraction at the edge of the screen and creating a spurious background for the photodetector; 3) reflection of the highlighted edge of the screen in the mirror surface of the face, which creates the conditions for observing Lloyd interference in the area of the photodetector.

More widely, to convert the radiation of a source into SPP of the middle IR and THz ranges, planar diffraction gratings formed on the surface guiding the SEW are used [5]. Such transformation elements have not only the advantage of planarity, but also higher efficiency (tens of percent) [6]. However, the use of such devices for converting a body wave to a surface one is associated with the need to modify the surface of the sample, which is not always acceptable, and also with the formation of a fan of intense body waves that impede the correct detection of SEW.

The closest in technical essence to the claimed device is a device for converting terahertz radiation of a free electron laser (FEL) into surface plasmon polaritons (SPP) on a flat metallized substrate containing a radiation source, a polarizer, a cylindrical focusing lens, a cylindrical segment, the convex surface of which capable of guiding SPP, covered with a dielectric layer of subwavelength thickness, has an axial line perpendicular to the plane of incidence of radiation, limited to two I have acute-angled edges in the direction perpendicular to the plane of incidence, has a line of intersection with this plane shorter than the SPP propagation length and is conjugated by one of the ribs with a flat surface of the substrate, an absorbing screen located above the SPP track outside their field and oriented perpendicular to both the substrate and the plane falls [7]. The transformation of a body wave into an SPP occurs as a result of diffraction of focused FEL radiation on a free edge of a segment. A distinctive advantage of such a device (in comparison with the ones described above) is the deep screening of the radiation receiver in the form of SPP from spurious body waves generated as a result of diffraction of FEL radiation on the segment edge. However, the efficiency of converting bulk radiation into SPP using a device such as its aperture counterpart is low (a fraction of a percent).

The technical result to which the invention is directed is to increase the efficiency of converting monochromatic infrared radiation to a surface electromagnetic wave on a flat face of a conducting body while retaining such an important advantage of the prototype device as deep screening of the receiver from spurious radiation emanating from the diffraction element matching the body and surface waves .

The technical result is achieved by the fact that in the device for converting infrared radiation to a surface electromagnetic wave (SEW) on a flat face of a conducting body containing a source of p-polarized monochromatic radiation, an optical lens, a cylindrical segment whose convex surface is capable of directing SEW, is covered with a subwavelectric dielectric layer thickness, has an axial line perpendicular to the plane of radiation incidence, bounded by two acute-angled ribs in the direction perpendicular to the plane fall velocity, has a line of intersection with this plane shorter than the SEW propagation length and is conjugated by one of the ribs with a flat face of the body, an absorbing screen located above the SEW track outside its field and oriented perpendicular to both the face and the plane of incidence, the lens is chosen as collimating, the convex surface of the segment is equipped with a planar diffraction grating, the strokes of which are perpendicular to the plane of incidence, the source and lens being mounted on a platform capable of moving along an arc whose axis coincides with the price sweeping stroke of the lattice.

An increase in the efficiency of conversion of radiation into SEW is achieved by using not a linear diffraction element (the edges of a cylindrical segment) to match the volume and surface waves, but a diffraction grating distributed over the convex surface. In addition, in order to increase the conversion efficiency, the radiation incident on the diffraction element is not focused (as in the prototype device), but is formed by the collimator into a beam of parallel rays incident on the grating at an angle that matches the phase velocities and the tangential components of the radiation wave vectors and SEW . Moreover, unlike the prototype device, the conversion of radiation into SEW in the inventive device takes place not only for a narrow sector of the beam rays [8], but for all the rays (due to their collimation) incident on the grating with almost the same efficiency.

In FIG. 1 shows a diagram of the inventive device, where the numbers denote: 1 - the source of p-polarized monochromatic infrared radiation; 2 - collimation lens; 3 - planar diffraction grating, whose strokes are perpendicular to the plane of incidence; 4 - a cylindrical segment, the convex surface of which contains a lattice 3; 5 is a platform containing aligned elements 1 and 2, which can move along an arc with an axis coinciding with the center stroke of the grating 3; 6 - a flat face of a conducting body adjacent to the edge of the convex surface of segment 4; 7 - a flat absorbing screen located above the SEW track outside its field and oriented perpendicular to both the track and the plane of incidence.

The inventive device operates as follows. The radiation of the source 1 is collimated by the lens 2 and the resulting light beam is directed to the grating 3 formed on the convex surface of the segment 4. Turning the platform 5 with the source 1 and lens 2 fixed on it, change the angle of incidence φ of radiation on the grating 3 and achieve equality:

here κ 'is the real part of the refractive index of the SEW; λ is the radiation wavelength in vacuum; Λ is the period of the diffraction grating. When condition (1) is fulfilled, the phase velocities and wave vectors of radiation of the first order of diffraction and SEW are matched, directed by the surface of segment 4. As a result, radiation with an efficiency of tens of percent is converted to SEW, while radiation of other diffraction orders retains the shape of body waves (OM) emitted by the grating 3 into the surrounding space at various angles to the surface of segment 4. These spurious OM are blocked on the way to the photodetector detecting the SEW on the flat face 6 of the conducting body, both the horizon line of the convex surface of segment 4, and the screen 7.

As an example of the application of the inventive device, we consider the possibility of converting the radiation of FEL with λ = 130 μm into SPP, directed by a flat face of a gold sample placed in the air and containing a coating layer of zinc sulfide 0.4 μm thick; the propagation length of such SPPs L is 13 cm, and the real part of the refractive index is κ'≈1.005 [7]. The cross-sectional diameter of the collimated radiation beam d of the source 1 will be chosen equal to 10 mm, and as the radiation conversion element in the SPP, we will choose a cylindrical segment 4 with a generatrix equal to 60 mm, on whose convex surface a planar diffraction grating 3 with a period Λ = 300 μm, length and whose width is not less than d. Assuming the central angle between the lattice 3 and the edge of segment 4 adjacent to the flat face of body 7 to be equal to 45 °, we find that the length of the SPP track on the convex surface of segment 4 is approximately 47 mm (significantly less than L), which causes the SPP to lose 36% of its energy at this distance (regardless of the type of transformation element). Substituting the indicated values κ 'λ and Λ into equation (1), we find that the matching of phase velocities and wave vectors of first-order diffraction radiation and SPP is achieved when the angle of incidence ϕ of the radiation of the source (FEL) 1 onto the grating 3 is equal to 35 ° 06'.

Thus, the use of the conversion of a body wave into a surface planar diffraction grating deposited on the convex surface of a cylindrical segment, combined with the collimation of the radiation incident at a certain angle on the grating, allows not only to retain such an important advantage of the prototype device as deep shielding of the SPT detector on a flat face of a conducting body from their spurious diffraction satellites, but also to raise (not less than an order of magnitude) conversion efficiency, h This will lead to an additional increase in the signal-to-noise ratio in measurements using surface plasmon polaritons in the infrared and terahertz ranges.

Sources of information taken into account when preparing the application: 1. Surface polaritons. Electromagnetic waves on surfaces and media interfaces / Ed. V.M. Agranovich and D.L. Mills. - M: Nauka, 1985 .-- 525 p.

2. Vaicikauskas V., Antanavicius R., and Januskevicius R. Efficiency of FIR SEW excitation by aperture, prism and mesh methods // Intern. J. of Infrared and Millimeter Waves, 1999, v. 20, No. 3, p. 447-452.

3. Zhizhin G.N., Parker S.F., Chester M.A., Yakovlev V.A. The effectiveness of aperture excitation in the spectroscopy of surface electromagnetic waves // Optics and Spectroscopy, 1988, V. 65, Issue. 2, p. 371-375.

4. Gerasimov V.V., Knyazev B.A., Nikitin A.K. A method for indicating diffraction satellites of surface plasmons of the terahertz range // Letters in ZhTF, 2010, Volume 36, no. 21, p. 93-101.

5. Geary J.W., Medhi G., Peale R.E., and Buchwald W.R. Long-wave infrared surface plasmon grating coupler // Applied Optics, 2010, v. 49, No. 16, p. 3102-3110.

6. Gaborit G., Armand D., Coutaz J.-L., Nazarov M., Shkurinov A. Excitation and focusing of terahertz surface plasmons using a grating coupler with elliptically curved grooves // Applied Physics Letters, 2009, v. 94, 231108.

7. Gerasimov V.V., Knyazev B.A., Lemzyakov A.G., Nikitin A.K., Zhizhin G.N. Growth of terahertz surface plasmon propagation length due to thin-layer dielectric coating // JOSA (B), 2016, v. 33, No. l11 p. 2196-2203. DOI: 10.1364 / JOSAB.33.002196 (prototype).

8. Gerasimov V.V., Knyazev B.A., Kotelnikov I.A., Nikitin A.K. 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, No. 8, p. 2182-2190. DOI: 10.1364 / JOSAB.30.002182.

Claims (1)

A device for converting infrared radiation to a surface electromagnetic wave (SEW) on a flat face of a conductive body, containing a source of p-polarized monochromatic radiation, an optical lens, a cylindrical segment whose convex surface is capable of directing a SEW, is covered with a dielectric layer of subwavelength thickness, has a perpendicular plane of radiation incidence center line, bounded by two acute-angled ribs in the direction perpendicular to the plane of incidence, has a line of intersection with this the plane is shorter than the propagation length of the SEW and is conjugated by one of the ribs with a flat face of the body, an absorbing screen located above the SEW track outside its field and oriented perpendicular to both the face and the plane of incidence, characterized in that its lens is chosen as collimating, a convex surface the segment is equipped with a planar diffraction grating, whose strokes are perpendicular to the plane of incidence, and the source and lens are mounted on a platform capable of moving along an arc whose axis coincides with the central stroke of etki.

Images