Method for separation of combined surface and volume electromagnet waves of terahertz range

FIELD: physics.

SUBSTANCE: method for separation of combined surface and volume electromagnet waves of terahertz range, which includes preliminary shaping of groove with smoothened edges on sample surface, at that groove axis is perpendicular to plane of incidence that crosses track of surface electromagnet wave (SEW) rays bundle and having size along track that is less that SEW spread length, and further direction of combined waves to groove, differs by the fact that groove is shaped in the form of regular cone half, axis of which lies in the plane of sample surface, at that angle of SEW deviation from incidence plane that contains volume wave, is equal to the following: γ=arcsin[tg(α)-(π-2)-k'], where α is angle between generatrix and cone axis, k' is actual part of SEW refraction index.

EFFECT: provision of spatial separation of SEW and volume wave by means of SEW direction variation.

3 dwg

 

The invention relates to the field of transmitting and receiving information by means of surface electromagnetic waves (sew) terahertz (THz) range (frequency from 0.1 to 10 THz) and may find application in spectroscopy of solid surfaces, in electro-optical devices for transmission and processing of information in the infrared (IR) technology.

With the creation of tunable (including in the THz range) of free electron lasers and pulsed lasers, generating femtosecond pulses with a spectral width of up to 3000 cm-1began intensive development of THz region of the spectrum [1]. One of the important applications of THz radiation spectroscopy is a solid surface, and the transmission of information via sew, to the class which includes the surface plasmons on the border "metal-insulator" [2].

In devices such as spectrometers, refractometers, sensors), in which media use THz sew, a complex problem is not found until its effective resolution is the separation sew and body waves (S)generated by the incident radiation on the conversion element S falling in sew [3].

The known method of separation of combined surface and volume electromagnetic waves in the THz range, consisting in that the conversion element is of S falling in the sew and the analyzed surface is placed on adjacent sides of the sample, separated rounded (to reduce radiation losses sew) edge [4]. The main disadvantage of this method is the availability of over the edge combined with sew secondary body waves propagating in the plane of incidence and is caused by diffraction at the edge of the primary body waves, generated in the conversion element.

The known method of separation of combined surface and volume electromagnetic waves in the THz range, consisting in that the conversion element S falling in sew and analyzed surface is placed on one face of the sample, but separate them by an opaque screen, perpendicular to the plane of incidence and separated from the surface of the gap value in (10÷20)·λ, where λ is the wavelength of the incident radiation [5]. The main disadvantage of this method is the generation on the edge of the screen a new body waves propagating in the plane of incidence and spatial combined with sew.

The closest to the technical nature of the claimed method is a method of separating a combined surface and bulk electromagnetic waves in the THz range, consisting in the fact that on the sample surface shape oriented with its axis perpendicular to the direction of propagation of the beam of parallel rays sew and transverse groove (heterogeneous the spine) with a cylindrical surface and smoothed edges, and over the groove, at a distance of not less than the depth of penetration of the field sew into the environment, placing an opaque screen, oriented along the axis of the groove [6]. The main disadvantage of this method is the generation on the edge of the screen (due to diffraction) new body wave, propagating, and sew in the plane of incidence.

The technical result of the invention is a complete spatial separation sew and body waves (caused either as a result of diffraction on the element of conversion of incident radiation to sew, or as a result of diffraction sew on the edge of the screen, separating the conversion element S in the sew and the photodetector) by changing the direction sew.

The technical result is achieved by the method of separation of combined surface and volume electromagnetic waves in the terahertz range, including pre-formation on the sample surface grooves with smooth edges and an axis perpendicular to the plane of incidence, crossing the track of the beam surface electromagnetic waves (sew) and has a size along the track is less than the length distribution sew, and the subsequent direction of the combined waves on the groove, the groove is formed in the shape of half right cone whose axis lies in the plane of the sample surface, and the deflection angle sew from plose the spine of the fall, containing body wave, equal to:

γ=arcsin[tg(α)·(π-2)·κ'],

where α is the angle between the generatrix and the axis of the cone, κ' is the real part of the refractive index sew.

The method is illustrated using three drawings. Figure 1 shows the General scheme of the heterogeneity of the sample surface, providing turn wavefront sew on the angle γ, in figure 2 - scheme of the groove of the conical shape in the surface of the sample, providing turn sew on the angle γ in figure 3 - calculated dependence of the angle γ is the angle α between the generatrix and the axis of the cone to sew with λ=100 μm on the surface of aluminum, bordering air.

The effect of separating the combined sew and S is achieved by rotation of the wave front sew on the angle γ in overcoming the various rays of the beam sew created heterogeneity in different areas.

Here is the substantiation of this claim. Let it sew, characterized by a certain refractive index, κ, is spread on the flat surface of the sample in the form of a bundle of parallel rays of width L and perpendicular to the direction of propagation sew on the surface created heterogeneity in the form of a black rectangle with sides L and a, providing a linear dependence of the optical path of rays sew from the coordinates of the beam on the x-axis, perpendicular to the direction sew (figure 1).

Let the optical path of rays sew Δl, when preharden and their heterogeneity, is determined by the linear equation: Δl=[(L-x)/L)·a·K', where K' is the real part of the complex refractive index of sew κ.

Then the difference of optical paths of extreme rays sew ΔS=Δl(0)-Δl(L)=a·K'. Therefore, the upper (1) beam sew reaches the edge of heterogeneity in the point And before the lower beam - in point In the time interval Δt=ΔS/ϑ=ΔS/(C/κ')=a·(K')2/C, where ϑ is the phase velocity sew, C is the speed of light in vacuum.

Then, according to the principle of Huygens - fundamentals of wave theory of light, the point a becomes a source of secondary waves with circular front at time Δt earlier than the point of the Century, But for the time Δt secondary waves emitted by a point And will pass the distance AC=ϑ· ∆ T=(/κ')·[a·(K')2/S]=a·K'.

And, finally, of a right-angled triangle ABC, we have: sin(γ)=AC/L=a·κ'/L. Where the deflection angle sew from the direction of propagation of S is equal to: γ=arcsin(a·K'/L).

Note that the angle γ depends on the ratio a/L (size heterogeneity along and across the direction of propagation of the combined waves). Therefore, from the point of view of applicability, the inventive method is limited to the condition that the length distribution sew exceed the longitudinal (relative to the direction of wave propagation) size and heterogeneity, otherwise sew just will not come until the second (in the direction of radiation) region heterogeneity, and the problem of separating the waves will lose their act is a reality due to the disappearance of one of the shared objects. This condition can be easily done for surface plasmons in the THz region of the spectrum, because of their length distribution in the tens and hundreds of centimeters [3-6].

Let us prove that the groove 3, made in the form of half of the right cone whose axis lies in the plane of the sample surface, provides a linear dependence of the optical path of rays sew from the coordinates of the beam on the axis perpendicular to the direction of propagation of the combined waves (i.e. such that the groove is, in fact, surveying prism [7]), and therefore can perform prescribed by the claims function.

Let sew with a refractive index κ is distributed on the flat surface of the sample in the form of a bundle of parallel rays of width L and perpendicular to the direction of propagation sew groove formed conical shape, the axis of which lies in the plane of the sample surface (figure 2).

Calculate the difference of the geometric path ΔSoextreme rays of a beam sew incident on the tapered groove. We introduce the following notation: Rois the radius of the "base" of the cone, R is the current radius of the surface grooves, L-cone height (equal to the width of the beam sew), x is the coordinate axis along the axis of the right cone. Scroll to the surface of the sample rectangle with sides 2Roand L covering the groove.

Then zavisimosti geometric path of an arbitrary beam sew from the x coordinate has the form: S o(x)=2·(Ro-R)+π·R. But R(x)=Ro-(Ro/L)-(L-x). Therefore: So(x)=Ro·[(x/L)·(2-π)+π]. From this expression it is seen that the value of S depends on the coordinates of x in a linear manner.

Next, the geometric path difference of the extreme rays sew (with coordinates x=0 and x=L) is equal to: ∆ So=So(0)-So(L)=Ro·(π-2)and the optical path difference of these rays ΔS=ΔSo·κ'=Ro·(π-2)·κ', respectively. Moreover, the time Δt for which the bottom (figure 2) beam sew will be the distance ΔS, is: Δt=ΔS/ϑ=[Ro·(π-2)·κ']/(C/κ'), where ϑ is the phase velocity sew, C is the speed of light in vacuum.

Then, according to the Huygens principle, point a, to which the upper beam sew came on time Δt earlier than the lower beam to the point b becomes a source of secondary waves with circular front. During the time Δt these secondary waves will pass the distance AC=ϑ· ∆ T=(C/κ')·{[Ro·(π-2)·κ']/(C/κ')}=Ro·(π-2)·κ'.

And finally, for a right triangle ABC, we have: sin(γ)=AC/L=[Ro·(π-2)·κ']/L=tg(α)·(π-2)·κ'. Thus, the formula for calculating the angle sew a tapered groove from the original direction of propagation is: γ=arcsin[tg(α)·(π-2)·κ'].

Note that if the axis of the cone lies in the plane of the sample surface, the dependence of S(x) is not linear, and this leads to the difference of the directions of rays of the beam sew, past the groove. In the wave front of the beam is EV distorted, what is unacceptable in terms of the task. This fact explains the necessity of the conditions of belonging to the axis of the cone to the plane of the sample surface.

The condition of finding the "top" of the cone on the surface of the sample is not required. Indeed, in the case of finding the "top" of the outside surface (but its plane) the formula for the angle γ becomes:

where R1and R2the radii of the cross-section of the cone on the side (relative to the track sew the sides of the sample; L - the width of the beam sew equal to the width of the sample surface. Expressing R1and R2through the angle α at the "top" of the cone, we will obtain the expression: γ=arcsin[tg(α)·(π-2)·κ'].

The method is as follows. A beam of monochromatic radiation with a non-zero p-component falls on the element of transformation, and with some effectiveness converted into THz sew, at the same time, as a result of diffraction of the radiation conversion element, is generated by near-surface bulk wave (S). Combined in space and having almost the same phase velocity of the beam sew and S reach grooves and here their trajectories in the plane of incidence differ: S rays continue to propagate rectilinearly, while rays sew direct on the surface of the trenches and, passing polukrugom trajectory, the length of which is directly proportional to the distance from the "top" of the cone. As a result, the corresponding rays beams sew and S reaches the second rounded edges of the grooves simultaneously: OB - in the past, sew later. Moreover, the delay for closer to the "bottom" of the cone of rays sew will be greater than for rays close to the "top" of the cone. The difference lag rays in the beam sew, by virtue of the principle of Huygens, and leads to a rotation of the wave vector sew on the angle γ.

As an example application of the proposed method will calculate the value of angle γ to sew, excited by radiation with λ=100 μm on the surface of aluminum, bordering air, after passing sew tapered grooves with an angle α at the "top" of the cone's surface. In this case, the length distribution sew obtained using the Drude model for the dielectric constant of aluminum, is 685 cm (with a large margin satisfies the imposed above condition on the ratio of the length distribution sew and radius of the base of the conical surface, which cannot be greater than the thickness of the substrate and is usually less than 10 cm).

Figure 3 shows the calculated dependence of γ(α). From the graph we see that deviations sew from the plane of incidence, for example, 30° to the need on the sample surface to produce a tapered groove with ug is ω α≈24°40'.

Thus, the inventive method allows complete spatial separation of the combined surface and volume electromagnetic waves in the terahertz range by changing the direction of propagation of sew regarding body wave.

Sources of information

1. P.H. Siegel Terahertz technology // IEEE Transactions on Microwave Theory and Techniques. - 2002. - v.50. - No.3. - p.910-955.

2. Csurgay, A.I., W. Porod Surface plasmon waves in nanoelectronic circuits // Intern. J. of Circuit Theory and Applications. - 2004. - v.32. - p.339-361.

3. Klopfleisch M., Schellenberger, U. Experimental determination of the attenuation coefficient of surface electromagnetic waves // Journal of Applied Physics. - 1991. - V.70. - No.2. - p.930-934.

4. E.S. Koteles, W.H. McNeill Far infrared surface plasmon propagation // International Journal on Infrared and Millimeter Waves. - 1981. - V.2. - No.2. - p.361-371.

5. Silin V.I., Voronov S.A., Yakovlev V.A., Zhizhin G.N. IR surface plasmon (polariton) phase spectroscopy// Intern. J. Infrared and Millimeter Waves. - 1989. - v.10. - No.1. - p.101-120.

6. Jeon, T.-I., D. Grischkowsky THz Zenneck surface wave (THz surface plasmon) propagation on a metal sheet // Applied Physics Letters. - 2006. - v.88. - Article No.061113 (prototype).

7. Hensinger R. Integrated optics. Theory and technology // M.: Mir, 1985. - c.321.

The method of separation of combined surface and volume electromagnetic waves in the terahertz range, including pre-formation on the sample surface grooves with smooth edges and an axis perpendicular to the plane of incidence, crossing the track of the beam surface electromagnetic waves (sew) and has a size along the track of men is higher length distribution sew, and the subsequent direction of the combined waves on the groove, wherein the groove is formed in the shape of half right cone whose axis lies in the plane of the sample surface, and the deflection angle sew from the plane of incidence containing body wave, equal
γ=arcsin[tg(α)·(π-2)·κ'],
where α is the angle between the generatrix and the axis of the cone, k' is the real part of the refractive index sew.



 

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