JP6521289B2 - Retardation plate in which asymmetric openings are periodically arranged in metal film - Google Patents

Description

  The present invention relates to a retardation plate for electromagnetic waves in the light wave band in which a plurality of asymmetric apertures are periodically arranged in a metal film and surface plasmons are excited, or terahertz bands in which pseudo surface plasmons occur, and gigahertz bands. It is a thing.

A conventional retardation plate that converts linearly polarized light into circularly polarized light has a structure in which different dielectrics proposed in Non-Patent Document 1 are alternately stacked. FIG. 17 is a perspective view of a conventional retardation plate disclosed in Non-Patent Document 1. As shown in FIG. The retardation plate has a periodic structure in which a high-refractive-index dielectric having high transparency and a low-refractive-index dielectric (in the embodiment, an air layer) are alternately stacked. In the figure, L is the width of the high refractive index dielectric, P is the period length, L / P is the filling factor f, space occupancy in one period, H is the dielectric thickness, n ₁ and n ₂ are the respective dielectrics. Represents the refractive index.

As another structure based on the same principle, there is a retardation plate in which a periodic sequence of triangular holes is provided on the dielectric substrate shown in FIG. 18 proposed in Non-Patent Document 2. In the figure, w represents the length of one side of a triangle, Λ represents the period length, h represents the thickness of the dielectric substrate, n _H and n _L represent the refractive index of each dielectric.

  On the other hand, as a structure utilizing optical anisotropy generated in a photonic crystal metal film, there is a retardation plate proposed in Patent Document 1. FIG. 19 is a perspective view of a conventional retardation plate disclosed in Patent Document 1. As shown in FIG. A plurality of circular openings are periodically arranged in the metal film. In this structure, the thickness of the metal film needs to be 0.3 to 2.0 times the wavelength of the electromagnetic wave passing through the opening.

On the other hand, a retardation plate in a light wave band using surface plasmon induced on a metal surface has been proposed based on a principle different from the above. As a configuration example, there is a phase difference plate disclosed in Non-Patent Document 3 and provided with a plurality of cross-shaped slots. FIG. 20 is a perspective view of the conventional retardation plate disclosed in Non-Patent Document 3. As shown in FIG. This retardation plate has a configuration in which linear slots slightly different in length are formed in a cross shape in a metal film made of silver, and a plurality of slots of this shape are periodically disposed. W represents the width of the metal slot, l _x and l _y represent the slot length, and d represents the period length.

  Further, as a retardation plate in the light wave band based on the similar principle, there is one using an L-shaped metal element proposed in Non-Patent Document 4. FIG. 21 is a configuration diagram of a conventional retardation plate disclosed in Non-Patent Document 4. As shown in FIG. The retardation plate has a configuration in which an element made of L-shaped gold is periodically wired on a dielectric substrate, and utilizes surface plasmons generated between metal elements. In the figure, H represents the thickness of the metal, W represents the width of the metal, L represents the length of the metal, D represents the thickness of the dielectric substrate, and P represents the periodic length.

  Further, as a retardation plate of a light wave band based on the similar principle, there is a polarization control device which is proposed in Patent Document 2 and which comprises a plurality of metal structures and a metal film having a plurality of openings. FIG. 22 is a block diagram of a conventional polarization controller (retardation plate) disclosed in Patent Document 2. As shown in FIG. In this retardation plate, an opening of a metal film and a unit composed of two or more metal structures are two-dimensionally arranged, and polarization control is performed using surface plasmons generated in the metal portion. .

JP 2004-117703 A JP, 2009-223123, A

Saito Makiko et al. "Optimal design of broadband quarter-wave plate by RCWA method", OPTRONICS, pp. 179-186, No. 12, 2009 Junji Yamauchi et al., “Polarization Converters Using Sub-Wavelength Triangle Hole Arrays”, 2013 Electronics Society Conference of the Institute of Electronics, Information and Communication Engineers, p. 181 (C-3-59) A. Roberts and L. Lin "Plassonic quarter-wave plate", Optics Letters, vol. 37, No. 11, pp. 1820-1822, 2012 B. Yang et al. Design of ultrathin plasmonic quarter-wave plate based on period coupling ", Optics Letters, vol. 38, No. 5, pp. 679-681, 2013

A retardation plate represented by a quarter-wave plate can be used for on / off switching of pixels such as liquid crystal and organic electroluminescence display, and is also applied to optical instruments and measuring instruments such as laser interferometers. ing. A thin retardation plate is desired from the viewpoint of miniaturizing individual parts with a limited occupied volume. However, in the structure of Non-Patent Document 1 shown in FIG. 17, the thickness is about 2 μm (about 3 wavelengths in the visible light band). In the structure shown in FIG. 17, when an electromagnetic wave polarized in a direction inclined 45 degrees from the x-axis is incident, the first mode and the second mode are generated with equal amplitude in the x-axis and y-axis directions. If the propagation constants of the first mode and the second mode are expressed as β ₁ and β ₂ , the conversion length L _c to circularly polarized light is determined by L _c = π / 2 (β ₁ −β ₂ ). The thickness of the retardation plate is determined by the propagation constant difference. In general, the retardation plate becomes thicker because the propagation constant difference can not be increased.

  Further, in the structure of Non-Patent Document 2 shown in FIG. 18, the periodic array of triangular holes is disposed in the dielectric substrate, and polarization conversion is obtained by the same effect as Non-Patent Document 1. However, its thickness is 2.36 μm (about 2 wavelengths in the optical communication wavelength band), and it is a half-wave plate that converts horizontal linear polarization to vertical linear polarization. In addition, there are fabrication difficulties in drilling small triangular holes periodically and deeply.

  The structure of Patent Document 1 shown in FIG. 19 utilizes a metal film and one or more openings penetrating the surface area, and performs mode coupling when two orthogonal polarizations have the same parity. We are using. For this reason, the incident wave needs to be slightly tilted with respect to the metal film, and does not operate at normal incidence. Also, in order to obtain parity matching, the metal needs to be at least 0.3 times the thickness of the wavelength.

  On the other hand, the structure of Non-Patent Document 3 shown in FIG. 20 is made of only a metal film and does not require a dielectric substrate, thus achieving thinness and slightly changing the slot length in the horizontal and vertical directions. By adjusting the resonance wavelength of the surface plasmon mode, the phase difference of 90 degrees is obtained. However, since the aperture area obtained by the slot portion of the cross structure is small, the transmitted light wave can not be increased, and the transmittance is about 40%.

Also, in the conventional retardation plate of Non-Patent Document 4 shown in FIG. 21 utilizing the surface plasmon mode, it is necessary to dispose an L-shaped metal element on a dielectric substrate, and the transmittance is also about 40%. It is.

  On the other hand, in the structure of Patent Document 2 shown in FIG. 22, an opening of a metal film and a unit consisting of two or more metal structures are two-dimensionally arranged, and a surface plasmon mode is generated by this combination. Optical anisotropy is obtained. In addition to the increase in thickness associated with the two-dimensional arrangement, the production is complicated.

  In addition, as in Non-Patent Document 3 and Non-Patent Document 4, when a phase difference plate is configured using a metal film or a metal element by using a surface plasmon mode, the thickness can be reduced and the device operates with vertical incidence of electromagnetic waves. Although there is an advantage, there is a problem that it is difficult to increase the transmittance.

  The present invention has been made in view of the circumstances of the prior art, and it is an object of the present invention to provide a retardation plate which is easy to manufacture, has an extremely thin film thickness, and operates with normal incidence of electromagnetic waves and has high transmittance. It will be an issue.

According to the present invention, in order to solve the above-mentioned problems, first , a plurality of openings which are a right-angled isosceles triangle in plan view are periodically arranged in a metal film, and each opening is an incident electromagnetic wave linear polarization plane against a point-asymmetrical shape with long sides forming a constant slope angle, the two orthogonal surface plasmon mode or pseudo-surface plasmon modes based on periodic coupling in opening is excited, the 90 degree phase A phase difference plate characterized in that linearly polarized light is converted into circularly polarized light by using orthogonal field components different from each other, and right-handed circularly polarized light and left-handed circularly polarized light are obtained by the wavelength band of incident electromagnetic waves. provide.

Second , in the first aspect of the present invention, there is provided a retardation plate characterized in that the inclination angle of the long side portion is 43 degrees to 47 degrees with respect to the plane of linearly polarized incident electromagnetic waves.

Thirdly, in the first or second invention, the thickness of the metal film is 0.1 to 0.3 times the operating wavelength of the electromagnetic wave, and the incident angle of the electromagnetic wave includes vertical incidence, and ± Provided is a retardation plate characterized by being in a range of 10 degrees.

Fourth , in any one of the first to third inventions, the periodic length at which the opening is disposed is smaller than the operating wavelength of the electromagnetic wave, and the filling factor is set to 0.7 to 0.9. The present invention provides a phase difference plate characterized by

Fifth , in the invention according to any one of the first to fourth inventions, the material of the metal film is one selected from the group consisting of silver, gold, aluminum, nickel and copper when the electromagnetic wave is in the light wave band. In the case of the terahertz band or gigahertz band, it is one kind selected from the group consisting of aluminum, copper, nickel and iron.

Sixth , in any one of the first to fifth inventions, the metal film in which a plurality of openings are periodically disposed is integrated by being sandwiched between a pair of translucent dielectric substrates. Provided is a retardation plate characterized by having a configuration.

  According to the present invention, a retardation plate operating at a specific wavelength band in a light wave band, a terahertz band, and a gigahertz band, which is easy to manufacture, is made of an ultrathin metal film, and operates with normal incidence of electromagnetic waves. Can be provided.

It is a figure which shows the structure of the phase difference plate of the 1st structural example by this invention, (a) is a perspective view, (b) is a top view, (c) is sectional drawing. It is a figure showing two examples of arrangement of a triangular opening. It is a figure of the eigenmode field distribution which becomes the principle of this invention, (a) is a symmetric mode, (b) is a field distribution diagram of an asymmetrical mode. Is a diagram to decompose field distribution E _x component and E _y component, (a) E _x component of the symmetric mode, (b) E _y component of the symmetric mode, (c) the E _x component of the asymmetric mode, (D) is a field distribution diagram of the E _y component of the asymmetric mode. It is a figure of the propagation field which shows an example of polarization conversion operation | movement. It is a figure which shows the wavelength characteristic of the transmittance | permeability of the Example in an optical communication wavelength range, a polarization angle, and an ellipticity. It is a figure which shows the wavelength characteristic of the transmittance | permeability of the Example in a visible light zone, a polarization angle, and an ellipticity. It is a figure which shows the wavelength characteristic of the transmittance | permeability of the Example in the visible light band contrasted with the form of the conventional nonpatent literature 3, a polarization angle, and an ellipticity. It is a figure which shows the influence which the side length of a triangular aperture gives to the wavelength characteristic of the transmittance | permeability, a polarization angle, and an ellipticity. It is a figure which shows the influence which the thickness of a metal film gives to the wavelength characteristic of the transmittance | permeability, a polarization angle, and an ellipticity. It is a figure which shows the structure of the phase difference plate which becomes another embodiment of this invention, (a) is a perspective view, (b) top view, (c) is sectional drawing. It is a figure which shows the wavelength characteristic of the transmittance | permeability in another embodiment of this invention, a polarization angle, and an ellipticity. It is a figure which shows the structure of the phase difference plate of the 2nd structural example by this invention, (a) is a perspective view, (b) is a top view, (c) is sectional drawing. It is a figure which shows two examples of arrangement of a rectangular opening. It is a figure which shows another form of a 2nd structural example. It is a figure which shows the transmittance | permeability of the phase difference plate of a 2nd structural example, a polarization angle, and the wavelength characteristic of an ellipticity. It is a perspective view of the phase difference plate using the conventional dielectric. It is a perspective view of the phase difference plate which provided the triangular perforation part in the conventional dielectric. It is a perspective view of the phase difference plate which consists of the conventional metal film and its opening which penetrates. It is a perspective view of the cross slot type phase difference plate which used the conventional metal film. It is a perspective view of the phase difference plate which arranged the conventional L character metal element in the dielectric substrate. It is a perspective view of the polarization control element which consists of the conventional several metal structure and several metal opening part.

Hereinafter, the phase difference plate according to the embodiment of the present invention will be described in detail.
In the retardation plate of the present invention, a plurality of openings are periodically arranged in a metal film, and each opening has an asymmetrical shape having long sides forming a predetermined inclination angle with respect to the incident electromagnetic wave linear polarization plane. Is a big feature. Here, asymmetry means that the shape of the aperture is not symmetrical with respect to the incident electromagnetic wave linear polarization plane.

  In the retardation plate of the present invention, in particular, the long side of the asymmetric aperture periodically formed in the metal film is cut off to be 45 degrees with respect to the incident electromagnetic wave linear polarization plane, that is, the incident electromagnetic wave linear polarization axis In this case, two orthogonal surface plasmon modes (when the incident electromagnetic wave is in the light wave band) or pseudo surface plasmon modes (when the incident electromagnetic wave is in the terahertz band or the gigahertz band) are excited. Linear polarization is converted to circular polarization by using orthogonal field components different in phase. The two types of surface plasmon modes or pseudo surface plasmon modes are referred to as symmetric mode and asymmetric mode.

  In the retardation plate of the present invention, in particular, the long side portion of the asymmetric aperture periodically formed in the metal film is cut off at 45 degrees with respect to the incident electromagnetic wave linear polarization plane, that is, the incident electromagnetic wave linear polarization axis. Have a simple structure that excites two orthogonal surface plasmon modes (when the incident electromagnetic wave is in the light wave band) or pseudo surface plasmon modes (when the incident electromagnetic wave is in the terahertz band or gigahertz band) [symmetrical mode and asymmetric mode]. Operates as a quarter-wave plate capable of converting the incident linearly polarized electromagnetic wave into circularly polarized light. In order to excite the symmetric mode and the asymmetric mode equally, the inclination angle of the long side to the incident electromagnetic wave linear polarization axis is preferably in the range of 43 to 47 degrees, more preferably in the range of 44 to 46 degrees, particularly preferably By setting the tilt angle to such a value, the symmetric mode and the asymmetric mode are equally excited, and good polarization conversion action is maintained. Equal excitation of both modes most preferably occurs at normal incidence of the electromagnetic wave, but acts as a retarder if the angle of incidence is in the range of ± 10 degrees with respect to the metal film.

  In the present invention, the thickness of the metal film can be set to 0.1 to 0.3 times the operating wavelength of the electromagnetic wave. When the thickness of the metal film is in such a range, it is possible to maintain a good polarization conversion action and to maintain the transmittance of the incident electromagnetic wave at a high level.

  In the present invention, from the viewpoint of enabling the good generation of surface plasmons or pseudo surface plasmons in a state where the aperture area is increased and further improving the transmittance of incident electromagnetic waves, filling that is the ratio of the opening base length to the periodic length Λ It is preferable to set the factor f in the range of 0.7 to 0.9.

  In the present invention, the period length 開口 of the aperture is preferably smaller than the operating wavelength λ because the effect of the higher order diffracted wave is generated when the operating wavelength λ is equal to about 0.5λ to about 0.9λ.

  The shape of the opening of the metal film of the present invention can typically be exemplified by a right triangle, but in principle, it only needs to have an asymmetric structure with respect to the incident electromagnetic wave linear polarization plane, and the apex angle and the bottom The corner may be rounded, trapezoidal, rectangular or the like. In the case of the trapezoidal shape, the holes are formed such that the long side portions have the above-described inclination angles, and in the case of the rectangular shape, the holes are formed such that the long side portions have the above-described inclination angles. To arrange the openings of.

In the present invention, the arrangement number of the openings is nine in FIG. 1 for the sake of simplicity, but it can be an appropriate number that can perform good polarization conversion.
The size of the polarization conversion portion of the retardation plate of the present invention may be wide enough to irradiate the linearly polarized light to be converted.

  The metal film constituting the retardation plate of the present invention is preferably a material capable of favorably generating surface plasmon when the incident electromagnetic wave is in the light wave band, and examples of such a material include silver and gold. , Aluminum, nickel, copper and the like. In the case where the incident electromagnetic wave is in the terahertz band or gigahertz band, it is preferably a material that can satisfactorily generate pseudo surface plasmons. Examples of such materials include aluminum, copper, nickel, iron and the like. it can.

  The metal film of the present invention can stand alone without providing a support when the incident electromagnetic wave is in the terahertz band or gigahertz band, but a support may be provided, and when the incident electromagnetic wave is a light wave band, glass or It can be formed on a transparent substrate such as a polymer.

  The metal film of the present invention can be formed to have a desired opening by using a known film forming technique such as vapor deposition or chemical plating.

  In addition, the metal film of the present invention may have a structure in which it is sandwiched between a pair of light transmitting dielectric substrates as described later.

  In order to operate the retardation plate of the present invention as a half-wave plate, two retardation plates may be arranged in the incident direction of the electromagnetic wave.

  In the case of using, for example, silver for the metal film and air for the opening as the retardation plate of the present invention and forming the opening as shown in FIG. Polarization conversion can be achieved with a thickness of about 1/2 compared to Document 1. Further, as compared with the cruciform slot type phase difference plate disclosed in the above-mentioned Non-Patent Document 3, the aperture area can be made larger, so that the transmittance can be improved by about 50%. Furthermore, unlike the non-patent document 4 in which the metal element is divided and disposed, in addition to the fact that the thickness can be reduced and the manufacture becomes easy, since the metal film is continuous, it is electrically conductive. Specific electromagnetic waves can be transmitted while being maintained.

Hereinafter, the present invention will be described more specifically based on the retardation plate of the first configuration example shown in FIGS. 1 (a), (b) and (c). Here, although the arrangement of the triangular openings is described in the form of (a) of FIG. 2, the form as shown in (b) of FIG. 2 can be employed.
The retardation plate is for converting the incident linearly polarized electromagnetic wave into circularly polarized light. In FIG. 1A, reference numeral 1 denotes a flat metal film having a thickness t, and a plurality of openings 3 of an isosceles right triangle are formed periodically in the x direction and the y direction in plan view. In this example, nine openings 3 are provided to simplify the explanation, but the number can be set to an appropriate number according to the incident electromagnetic wave to be polarization-converted. FIG. 1 (b) is a plan view of the polarization conversion unit of one unit (one element), and in a square block of vertical and horizontal ridges, a long side along a diagonal indicated by a broken line (hereinafter, also referred to as an inclined portion) 2. A right triangular opening 3 is formed which is formed by a short side of width w along the x direction and a short side of width w along the y direction. The dielectric constant of the metal film is _nm , and the opening 3 is _air having a dielectric constant n _air . The x direction is the incident linear polarization axis, and the inclined portion 2 forms an inclination angle θ (45 degrees in this example) with the incident linear polarization axis. The retardation plate of this example has a configuration in which one unit of polarization conversion unit is arranged three each in the x direction and the y direction.

  Next, the operation principle of the retardation plate having the structure exemplified in FIG. 1 will be described. Here, the electromagnetic wave entering the retardation plate is described as horizontal linearly polarized light incident, but the operation principle is the same even when vertical linearly polarized light is incident. Further, silver is used as an example of the material of the metal film 1. Furthermore, unlike in Patent Document 1, the electromagnetic wave is assumed to be incident on the metal film 1 from the perpendicular (normal) direction.

As shown in FIGS. 3A and 3B, the electromagnetic wave that has entered the metal film 1 has an oblique polarization axis (S) at each of the periodic asymmetric openings 3 having the inclined portions 2. It is decomposed into mode and asymmetric (A) mode. The names of symmetry and asymmetry herein are referred to as whether or not the shape of the aperture is symmetrical with respect to the oblique polarization axis. Here, both modes are excited with substantially equal phase and equal amplitude, but x component (E _x ) and y component (E _y ) of the electric field of each mode are excited with the same phase. As an example, the case where silver is used for the metal film and the operating wavelength λ = 1.5 μm is described. In the analysis of the eigenmodes induced in each asymmetric aperture, the FDTD method based on Yee lattice considering periodic boundary conditions is used, and an infinite period is assumed.

The field distribution of the symmetric mode decomposed into E _x and E _y components observed by one element is shown in FIGS. 4A and 4B, and the field distribution of the asymmetric mode is shown in FIGS. 4C and 4D, respectively. According to calculation results, the E _x component and the E _y component in the symmetric mode are strongly excited in the direction normal to the metal surface, which is a feature of the surface plasmon mode. In the asymmetric mode, the division of the field which is not in the symmetric mode is observed. As a result, the field amplitude is about twice as large as that of the symmetric mode, and a phase difference of 180 degrees occurs. According to calculations, the symmetric mode and the asymmetric mode have a phase difference of about 90 degrees.

  When horizontal linearly polarized light is actually incident on the retardation plate, a field distribution is formed in the form of a combination of symmetry and asymmetry shown in FIG. The field distribution of each component at the time of incidence of horizontal linearly polarized light calculated by the FDTD method is shown in FIG. The places where the field amplitude is large are on the horizontal side and the vertical side, the amplitudes of the two are approximately equal, and the phase difference is about 90 degrees. As a result, the incident horizontal linearly polarized light is converted to circularly polarized light. The same effect can be obtained from the symmetry of the structure also when vertical linearly polarized light is incident. However, the rotational direction of circularly polarized light is opposite to that of horizontal linearly polarized light.

FIG. 6 shows wavelength characteristics when t = 0.36 μm, w = 0.82 μm, m = 1 μm, f = w / == 0.82, θ = 45 degrees. It is calculated by pulse wave analysis of the FDTD method. FIG. 6 shows the transmittance, the polarization angle, and the ellipticity. When the ellipticity is 0.7 or more or -0.7 or less, the transmitted wave can be regarded as circularly polarized light within 3 dB. When the ellipticity is 0.7 or more, it is right-handed circularly polarized, and when it is -0.7 or less left-handed circularly polarized. From the data of wavelength-ellipticity in FIG. 6, the transmitted wave can be regarded as right-handed circularly polarized wave in a band of λ = 1.458 to 1.515 μm. If the 3 dB circular polarization bandwidth is defined as 2 (λ _L −λ _S ) / (λ _L + λ _S ), the bandwidth is calculated to be 3.8%. In this band, the transmittance is 48% or more, and the maximum transmittance is as high as 68%. In particular, the highest ellipticity of 0.99 is observed at λ = 1.483 μm, and a transmission of 57% is obtained. Here, λ _L and λ _S are respectively the wavelengths on the long wavelength side and the short wavelength side where the ellipticity is 3 dB. In addition, in the band of λ = 1.104 to 1.113 μm, the transmitted wave can be regarded as left-handed circular polarization. Although the band for circular polarization is narrow, high transmittance of 70% to 83% can be obtained. In particular, the highest ellipticity -0.9 is observed at λ = 1.109 μm, and a transmission of 78% is obtained. FIG. 6 also shows the result when metal is regarded as a perfect conductor. Even in the case of a perfect conductor, the operating wavelength shifts about 15% to the low band, but it can be seen that it operates as a retardation plate like silver. From this, it can be understood that a normal conductive metal film is used to operate as a retardation plate in the terahertz band and the gigahertz band.

  Here, the reason why the transmitted wave is anti-circularly polarized is considered. The linearly polarized light entering the metal plate is decomposed into a symmetric mode and an asymmetric mode having polarization axes perpendicular or horizontal to the oblique side of the triangular hole. According to the calculation, it is confirmed that the ellipticity has a peak at λ = 1.109 μm and 1.483 μm, and that both modes have equal amplitude and the phase difference is 90 degrees, and further that an asymmetric mode with λ = 1.109 μm. It is also confirmed that the phase difference with the asymmetric mode of λ = 1.483 μm is 180 degrees, as a result, the rotation directions of the polarized waves of the transmitted waves are reversed at both wavelengths.

Since the embodiment of Non-Patent Document 3 is directed to the visible light band, an embodiment according to the present invention in the visible light band will be shown next in order to clarify the comparison. FIG. 7 shows wavelength characteristics for t = 0.14 μm, w = 0.28 μm, Λ = 0.4 μm, and f = w / Λ = 0.82. The transmitted wave can be regarded as right-handed circularly polarized wave in a band of λ = 0.692 to 0.717 μm. The 3 dB circular polarization bandwidth is 3.6%, and the transmission in this range is at least 44% with a maximum of 60%. These values are higher than the maximum value of 47% at a transmittance of 41% or more and a 3 dB circular polarization bandwidth of 2.4% obtained in the example of Non-Patent Document 3 shown in FIG. On the other hand, from the data of wavelength-ellipticity in FIG. 7, the transmitted wave can be regarded as left-handed circularly polarized wave in a band of λ = 0.506 to 0.509 μm. The 3 dB circular polarization bandwidth is 0.59% and the transmission in this range is at least 48% with a maximum of 54%. In FIG. 8, the anti-circular polarization characteristic is not generated.
FIGS. 6 and 7 show that this embodiment can be implemented by the selection of dimensions in a wide band of the lightwave band.

The following example shows the case where the design wavelength is the optical communication wavelength band λ = 1.5 μm band.
In order to clarify the influence of changes in structural parameters, the influence on wavelength characteristics when w and t are changed is examined. In order to study the characteristics in the optical communication wavelength band, Λ = 1 μm, f = 0.82 is fixed.

  FIG. 9 shows the wavelength characteristics with the change of w when t is fixed at 0.36 μm. As w becomes smaller, the obtainable band of circularly polarized light shifts to the short wavelength side, and conversely, as w is increased, the circular polarized band shifts to the long wavelength side. Thus, by changing the size of w, it is possible to design a retardation plate operating at a desired wavelength. It is understood from the data in FIG. 9 that the anti-circular polarization characteristic is generated.

  FIG. 10 shows wavelength characteristics when w is fixed to 0.82 μm and t is changed. The wavelength characteristic is insensitive to the change of t, and it is demonstrated that the degree of freedom is large in the thinning as the object of the present embodiment. The thickness is in the range of 0.1 times to 0.3 times the wavelength, and in particular, it is characterized in that it operates with an ultra thin film of 0.1 times the wavelength. From the data in FIG. 10, it can be seen that anti-circular polarization characteristics are generated.

  If the present invention is applied, as shown in FIG. 11, a configuration in which the phase difference contributing portion 11 is sandwiched between a pair of dielectric substrates 12 is possible. In this case, although the thin film is lost as compared with the case of using only the metal film, the durability against impact and the like can be improved.

FIG. 12 shows wavelength characteristics of transmittance, polarization angle, and ellipticity in the case of the dielectric substrate thickness t _d = 0.7 μm in the configuration of FIG. The refractive index of the dielectric substrate is 1.34. The addition of the dielectric substrate shortens the wavelength of the plasmon mode, so the operating wavelength shifts to the long wavelength side. The 3 dB circular polarization bandwidth is calculated to be 2.6%. In this zone, the transmittance is 48% or more, the maximum transmittance is 67%, and high transmittance is maintained. From the data in FIG. 12, it can be seen that anti-circular polarization characteristics are generated.

Next, a retardation plate of a second configuration example according to the present invention will be described.
FIG. 13 shows the structure of the retardation plate of this configuration example. This retardation plate also converts the incident linearly polarized electromagnetic wave into circularly polarized light. In FIG. 13A, reference numeral 21 denotes a flat metal film having a thickness t, and a plurality of rectangular openings 23 are periodically formed in a direction orthogonal to each other as shown in the drawing in plan view. In this example, nine openings 23 are provided to simplify the description, but the number can be set to an appropriate number that can perform good polarization conversion. FIG. 13 (b) is a plan view of the polarization conversion unit of one unit (one element), and in a square block of longitudinal and lateral sides, two long sides parallel to a diagonal connecting the upper left corner to the lower right corner of the unit (hereinafter also referred to as the inclined portion) 22a, 22b, these long side portions 22a, rectangular opening 23 formed by the two short sides of the rectangular width l _s to 22b are formed. The dielectric constant of the metal film is _nm , and the opening 23 is _air having a dielectric constant n _air . The x direction is the incident linear polarization axis, and the inclined portions 22a and 22b form an inclination angle θ _c (45 degrees in this example) with respect to the incident linear polarization axis in order to have asymmetry with respect to the incident electromagnetic wave linear polarization plane. There is. The retardation plate of this example has a configuration in which nine polarization conversion portions of one unit are arranged in a direction perpendicular to each other, three each.
Here, although the arrangement of the rectangular openings is described in the form of FIG. 14 (b), the form may be as shown in FIG. 14 (a). A diagram corresponding to FIG. 13 in that case is as shown in FIG.

  The operation principle of the retardation plate of this configuration example is the same as that of the retardation plate of the first configuration example.

Ag is used for the metal film 21 and treated as a Drude dispersive medium. The medium inside the upper, lower, and rectangular openings 23 of the retardation plate is air. The thickness t _m of the metal film 21 is 0.36 μm, the long side length l _{L of the} rectangular opening is 0.88 μm, the short side length l _{s is} 0.60 μm, and the unit period Λ = 1.0 μm Set to In order to make the structure asymmetric with respect to the incident polarization axis, the opening is inclined at θ _c = 45 degrees.

The wavelength characteristics of the transmitted wave were evaluated in terms of transmittance, polarization angle, and ellipticity. A linearly polarized wave (E _x ) having a uniform amplitude was incident from the air layer under the retardation plate, and a transmitted wave was observed in the air layer above the retardation plate. The analysis used the FDTD method which applied periodic boundary conditions. The PLRC method (Piecewise Linear Recursive Convolution method) was applied to handle structures containing metal. The step size used for analysis was selected as Δx = Δy = Δz = 0.01 μm.

  FIG. 16 shows wavelength characteristics of transmittance, polarization angle, and ellipticity. Further, FIG. 16 also shows the result when the metal is regarded as a perfect conductor. Here, at a wavelength at which the absolute value of the ellipticity is 0.7 or more, the transmitted wave can be regarded as circular polarization of 3 dB or less. Peaks of ellipticity are observed at two points from the data of wavelength-ellipticity of FIG. Circularly polarized light can be obtained in a wide band of λ = 1.046 to 1.281 μm. Furthermore, the transmittance in this band is 61% or more, and the maximum value increases to 99%. In addition, an ellipticity of 0.99 is obtained at two peaks of λ = 1.080 and 1.164 μm. Therefore, it is understood that the retardation plate of this configuration example operates in a wide band and has a high transmittance.

  The present invention can be preferably applied to an optical circuit having a limitation in occupied thickness in the visible light band to the optical communication wavelength band. Furthermore, the present invention can be applied not only to the lightwave band but also to the terahertz band and gigahertz band where pseudo surface plasmon modes associated with the periodic structure formed of the conductor plate can occur, and the heat resistance is excellent. Can provide a thin retardation plate.

1, 21 metal film 2, 22a, 22b long side (inclined portion)
3, 23 Asymmetrical Opening 11 Retardation Contribution 12 Dielectric Substrate

Claims (6)

A plurality of openings is a rectangular equilateral triangle in plan view the metal film is being periodically arranged, each opening point-asymmetrical having a long side portion which forms a constant inclination angle with respect to the incident electromagnetic wave linearly polarized light plane Two orthogonal surface plasmon modes or pseudo surface plasmon modes based on the shape and periodic coupling based on periodic coupling are excited, and the linearly polarized light is converted to circularly polarized light by using orthogonal field components different in phase by 90 degrees. A phase difference plate characterized in that it is converted, and right-handed circular polarization and left-handed circular polarization are obtained by the wavelength band of an incident electromagnetic wave .   The retardation plate according to claim 1, wherein the inclination angle of the long side portion is 43 degrees to 47 degrees with respect to the incident electromagnetic wave linearly polarized plane.   The thickness of the metal film is 0.1 times to 0.3 times the operating wavelength of the electromagnetic wave, and the incident angle of the electromagnetic wave is in the range of ± 10 degrees including the vertical incidence. Retardation plate described in.   The period length in which the said opening is arrange | positioned is smaller than the operating wavelength of electromagnetic waves, and the filling factor is set to 0.7 to 0.9, The position according to any one of Claim 1 to 3 characterized by the above-mentioned. Phase plate.   The material of the metal film is one selected from the group consisting of silver, gold, aluminum, nickel and copper when the electromagnetic wave is in the light wave band, and in the case of the terahertz band or gigahertz band, it is made of aluminum, copper, nickel, iron The retardation plate according to any one of claims 1 to 4, which is one selected from the group consisting of The structure according to any one of claims 1 to 5 , characterized in that the metal film in which a plurality of openings are periodically disposed is integrated by being sandwiched between a pair of light transmitting dielectric substrates . Retardation plate.