JP2008196992A - Electric field enhancing structure of surface plasmon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric field enhancing structure of surface plasmon, capable of properly and fully providing electric field enhancement of the surface plasmon, in the traveling direction of exciting light, in the surface of the neighborhood of the top end of a metal body in which a tapered hollow space is formed in the depth direction. <P>SOLUTION: The electric field enhancing structure 100 of surface plasmon comprises an optical coupling unit 17, configured so that radial polarization light 15 is entered in dielectric 12 in a tapered hollow space 14 in the depth direction divided by the metal body 11 and can be optical coupled with the surface plasmon SP that is localized in the metal body 11, in which the surface plasmon SP is excited by the optical coupling between the radial polarization light 15 and the plasmon itself; and an electric field so as to be enhanced in the traveling direction of light 15 in the surface of a metal body 11, corresponding to the neighborhood of the top end of the dielectric 12. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electric field enhancement structure for surface plasmons.

  A surface plasmon polariton wave (hereinafter abbreviated as “surface plasmon”) is a collective oscillation of free electrons localized on the surface of a metal. When a metal is irradiated with light, a wave accompanied by electric polarization is generated. It is a coupled wave formed by combining with the electric field of light. Light that propagates in space usually does not interact (couple) with surface plasmons because the phase velocities do not match, whereas it does not couple with near-field light that is non-propagating light that exists near the boundary surface of matter. To do.

  When surface plasmon resonance excitation occurs based on the coupling between surface plasmons and electromagnetic waves (light) due to special conditions on the metal surface, the electric field strength near the surface is enhanced. The enhanced Raman scattering (SERS) phenomenon has been studied. For example, local surface plasmons are excited by a microstructure on the metal surface (for example, a concavo-convex structure on the metal surface), which is considered to be one of the causes of surface-enhanced Raman scattering.

  However, in this case, the place where the electric field enhancement due to the surface plasmon occurs is left to chance, and such an accidental electric field enhancement due to the surface plasmon is caused by an optical device such as an optical coupler or an optical integrated circuit. However, it is not suitable for the application of the technology on the premise that the electric field enhancement of the surface plasmon is spatially controlled.

  Therefore, the surface of the metal body corresponding to the vicinity of the tip of the hollow space (metal surface) is formed by a metal body (hereinafter abbreviated as “tapered metal structure”) in which a hollow space tapered (for example, tapered) in the depth direction is formed. ) Is theoretically predicted that surface plasmon electric field enhancement will occur, and as part of an attempt to spatially control the surface plasmon electric field enhancement, we aim to enhance the surface plasmon electric field by such a tapered metal structure. Efforts are being made.

  For example, as an example of such a tapered metal structure, there is a conventional structure in which a metal thin film such as Au or Ag is vapor-deposited on the side surface of a conical light prism (hereinafter referred to as “conical prism”) (Patent Document). 1). In other words, this conventional structure includes a metal body (metal thin film) in which a conical recess space tapered in the depth direction is formed, and a conical prism disposed in the recess space.

In addition, for the optical excitation of the surface plasmon of this conventional structure (coupling between the laser beam and the surface plasmon), the light is totally reflected at the boundary between the conical prism and the metal thin film, and is reflected in the metal thin film at the boundary surface. A Kretschmann optical arrangement that forms an evanescent wave is used. Therefore, according to this conventional structure, the surface plasmon is localized at the apex of the conical prism by causing the laser light to enter the conical prism (in the hollow space) from the bottom surface of the conical prism. It is said that the electric field enhancement of surface plasmons can be obtained at the apex.
JP 2002-116149 A (FIG. 2)

  By the way, according to the above-mentioned Patent Document 1, linearly polarized laser light (excitation light) is incident on the conical prism from the bottom surface of the conical prism for the purpose of exciting surface plasmons by the conical prism ( For example, see paragraph [0014] of Patent Document 1. Note that linearly polarized light refers to light in which the vibration direction of the electric field of the light wave is one direction.

  On the other hand, the inventors of the present invention appropriately increase the electric field in the light traveling direction on the surface of the metal thin film in the vicinity of the tip of the tapered metal structure (for example, the structure in which the metal thin film is deposited on the side surface of the conical prism). We believe that using a radially polarized light is an indispensable requirement in order to get enough. The reason for this will be described later. Note that the radially polarized light refers to light whose electric field vibration direction is a radial distribution.

  Therefore, in short, the conventional structure described in Patent Document 1 is not an optimal design from the viewpoint of enlarging a large electric field by surface plasmons, is a halfway technology, and there is still room for improvement. There is.

  The present invention has been made in view of such circumstances, and appropriately enhances the electric field enhancement of the surface plasmon in the light traveling direction on the surface near the tip of the metal body in which the tapered hollow space is formed in the depth direction. An object of the present invention is to provide a surface plasmon electric field enhancement structure which can be sufficiently performed.

In order to solve the above problems, the present invention is directed to a method in which a radially polarized light incident in a depth direction on a dielectric in a hollow space tapered in a depth direction defined by a metal body is the metal body. Comprising optical coupling means configured to be capable of optical coupling with surface plasmons localized at
The surface plasmon is excited by the optical coupling with the radially polarized light, and the electric field is enhanced in the traveling direction of the light on the surface of the metal body corresponding to the vicinity of the tip of the dielectric. Provides an electric field enhancement structure of plasmons.

  That is, in the electric field enhancement structure of the surface plasmon according to the present invention, the vibration components of the electric field in the traveling direction of the radially polarized light overlap, and a light field having a large vibration component of the electric field is formed in this direction.

  Therefore, when the surface plasmon is excited by such optical coupling with the radially polarized light, the surface plasmon is appropriate in the light traveling direction on the surface of the metal body corresponding to the vicinity of the tip of the dielectric. The electric field can be sufficiently enhanced.

  Here, the opposing portions of the dielectric may be inclined linearly symmetrically with respect to an imaginary straight line that passes through the apex of the dielectric and is perpendicular to the bottom surface of the dielectric.

  Thereby, excellent energy efficiency is exhibited when surface plasmon resonance excitation occurs, which is preferable.

  The opposing portions of the dielectric may be inclined in an arcuate shape symmetrically with respect to an imaginary straight line that passes through the apex of the dielectric and is perpendicular to the bottom surface of the dielectric.

  Thereby, the angle dependency of the incident direction of the light of the radially polarized light in which the resonance excitation of the surface plasmon occurs is relaxed, and there is an advantage that the adjustment operation of the incident direction of the light can be made efficient. Therefore, the electric field enhancement structure of the surface plasmon described above is useful when it is desired to easily achieve resonance excitation of the surface plasmon.

  Further, the light is preferably a Laguerre Gaussian beam in which light intensity peaks are distributed in a donut shape. In this case, it is preferable that the dielectric has a conical shape, and the light intensity of the Laguerre Gaussian beam is distributed in an annular shape so as to follow the circumference of the bottom surface of the dielectric.

  In this way, when the radially polarized Laguerre Gaussian beam is condensed so that the distribution of the annular light intensity peak is along the circumference of the bottom surface of the dielectric, the light energy of this beam is converted to the surface plasmon. It is preferable that it can be used efficiently by being concentrated by the focusing effect.

Here, the optical coupling means using the Kretschmann optical arrangement includes an optical prism as the dielectric, and a metal thin film as the metal body that is in direct contact with a side surface of the optical prism,
The surface plasmon may be optically coupled with the radially polarized light by totally reflecting the radially polarized light at a boundary between the optical prism and the metal thin film.

  Accordingly, it is possible to realize an optical coupling unit having a simpler configuration than an optical coupling unit using an Otto optical arrangement described later.

On the other hand, the optical coupling means using an Otto optical arrangement is opposed to the metal body with a gap therebetween, and the optical prism as the dielectric in the hollow space and the dielectric existing in the gap. A dielectric layer having a lower refractive index than the body,
The surface plasmon may be optically coupled with the radially polarized light by totally reflecting the radially polarized light at a boundary between the optical prism and the dielectric layer.

  Thereby, not only a thin-film metal but a bulk metal body can excite surface plasmons and may be beneficial.

  The dielectric layer may be a layer composed of air.

  Thereby, a dielectric layer can be formed easily.

Further, the electric field enhancement structure of the surface plasmon according to the present invention is incident in the depth direction into the hollow space tapered in the depth direction defined by the metal body, and the radially polarized light is incident on the metal body. Comprising optical coupling means configured to be capable of optical coupling with localized surface plasmons,
The optical coupling means further includes a diffraction grating formed on the metal body,
The surface plasmon is excited by optical coupling with the radially polarized light based on the selection by the diffraction grating of the wavelength of the radially polarized light, and the surface of the metal body corresponding to the vicinity of the tip of the dielectric The electric field may be enhanced in the traveling direction of the light.

  By adopting such a diffraction grating, not only a thin film metal but also a bulk metal body can excite surface plasmons and does not require an optical prism, which may be beneficial.

  Also in this case, as described above, the light is preferably a Laguerre Gaussian beam in which light intensity peaks are distributed in a donut shape.

  Moreover, it is preferable that the said hollow space is divided into cones and the light intensity of the said Laguerre Gaussian beam is distributed circularly so that the circumference of the bottom of the said hollow space may be followed.

  In this way, when the radially polarized Laguerre Gaussian beam is condensed so that the distribution of the annular light intensity peak is along the circumference of the bottom of the hollow space, the light energy of this beam is converted to the surface plasmon. It is preferable that it can be used efficiently by being concentrated by the focusing effect.

  According to the present invention, there is provided a surface plasmon electric field enhancement structure capable of adequately and sufficiently enhancing the electric field of surface plasmons in the light traveling direction on the surface near the tip of the metal body in which a tapered hollow space is formed in the depth direction. can get.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

(First embodiment)
First, the configuration of the electric field enhancement structure for surface plasmons according to the first embodiment of the present invention will be described.

  FIG. 1 is a cross-sectional view schematically showing a configuration example of an electric field enhancing structure for surface plasmons according to the present embodiment. For the convenience of the following description, in FIG. 1 (the same applies to FIG. 4 described later), a radially polarized donut-shaped (annular) Laguerre Gaussian beam 15 (Laguerre-Gaussian Beam; The advancing direction of the radially polarized LG beam 15) is defined as “Z direction”, and the direction perpendicular to the “Z direction” and along the paper surface is defined as “X direction”. The detailed contents of the radially polarized LG beam 15 will be described later.

  As shown in FIG. 1, the electric field enhancement structure 100 of the surface plasmon SP has a known radial polarization LG beam formation in which the optical coupling means 17 and the radially polarized LG beam 15 are incident on the optical coupling means 17. Optical system 16.

  The optical coupling means 17 constitutes the main structure of the electric field enhancement structure 100 of the surface plasmon SP, which is a tapered (for example, tapered) depression in the depth direction (Z direction) as shown in FIG. In the inclined inner surface 11a of the metal body 11 so as to occupy the whole area of the hollow space 14 defined by the metal body 11 formed of the metal thin film in which the space 14 is formed and the inclined inner surface 11a (metal surface) of the metal body 11. And a dielectric 12 made of a high refractive index glass provided so as to be in contact with each other.

  The external region 13 outside the metal body 11 is normally an atmospheric space (air layer), but the external region 13 may be formed of a dielectric member having a refractive index lower than that of the dielectric 12.

  In this way, the optical coupling means 17 is incident in the depth direction (Z direction) into the dielectric 12 occupying the hollow space 14 tapered in the depth direction defined by the inclined inner surface 11a, and The radially polarized LG beam 15 is configured to be capable of optical coupling with the surface plasmon SP localized by the metal body 11.

  In the method of exciting the surface plasmon SP using an optical prism (here, the dielectric 12), the dielectric and the metal thin film are opposed to each other with a dielectric layer (for example, an air layer) present in a slight gap. In this embodiment, as described above, a Kretschmann optical arrangement in which a metal thin film is directly attached to the side surface 12a is used. Thereby, it is possible to realize an optical coupling means having a simpler configuration than the optical coupling means using the Otto optical arrangement, which is preferable.

  The electric field enhancement structure for surface plasmons using the Otto optical arrangement will be described in a second embodiment to be described later.

  The metal body 11 described above is manufactured using a material capable of forming a thin film that can form the surface plasmon SP. Examples of the material of the metal body 11 include gold, silver, aluminum, copper, and platinum. Among these, gold is suitable because it is difficult to be oxidized in the air.

  The dielectric 12 is, for example, an optical prism made of a known optical glass material, and its three-dimensional shape is any shape as long as it sharpens (angle θ) in the depth direction. However, here, as shown in FIG. 1, with respect to an imaginary straight line L drawn by the opposite side surface 12 a of the dielectric 12 through the apex P of the dielectric 12 and perpendicular to the bottom surface of the dielectric 12. The taper is inclined in a symmetrical manner. Examples of such a three-dimensional shape include a conical shape and a regular quadrangular pyramid shape. However, the present invention is not limited to this, and the dielectric 12 may be a quadrangular pyramid shape other than a triangular pyramid shape or a regular quadrangular pyramid shape, and may be a polygonal pyramid shape having five or more corners. However, when the doughnut-shaped (annular) Laguerre Gaussian beam is used as excitation light for exciting the surface plasmon SP on the surface of the metal body 11 as in this embodiment, the inclined inner surface 11a (dielectric) of the metal body 11 is used. The shape of the dielectric 12 is preferably a cone so that the periphery of the bottom surface of the body 12 follows the donut-shaped light intensity peak of the Laguerre Gaussian beam. In this case, the electric field enhancing structure 100 of the surface plasmon SP is formed by appropriately forming the above-described conical dielectric 12 by etching and substantially the entire side surface 12a (conical surface) of the dielectric 12. It can be manufactured very simply by coating the metal body 11 made of a metal thin film based on a vacuum process (vacuum deposition or sputtering).

  Therefore, in the following description, it is assumed that the hollow space 14 is partitioned into a conical shape, and this is referred to as a conical space 14 as necessary, and the dielectric 12 has a conical shape that fits closely into the conical hollow space 14. It is assumed that this is called a conical prism 12 (conical apex angle θ) as necessary.

  Next, the contents of the radially polarized LG beam 15 that excites the surface plasmon SP, which is a feature of the present embodiment, will be described.

  FIG. 2 is a diagram schematically showing the light intensity distribution and the polarization of the radially polarized LG beam.

  The hatched portion in FIG. 2 indicates a portion where the light intensity of the radially polarized LG beam 15 is high. Moreover, the arrow of FIG. 2 has shown the polarization direction (vibration direction of an electric field) of the radially polarized LG beam 15. FIG. In FIG. 2, the state of the radially polarized LG beam 15 having the largest electric field vector on the outside and the state of the radially polarized LG beam 15 having the largest electric field vector on the inside are representatively illustrated. Yes.

  As shown in the hatched portion of FIG. 2, the light intensity peak of the radially polarized LG beam 15 is distributed in an annular shape (doughnut shape). Therefore, if the light intensity distribution of the radially polarized LG beam 15 is adjusted so that the peak of the light intensity of the radially polarized LG beam 15 incident on the conical prism 12 is along the outer periphery of the bottom surface of the conical prism 12, It is preferable because the light energy of the radially polarized LG beam 15 can be efficiently applied to the surface plasmon SP.

  As indicated by the arrows in FIG. 2, the vibration direction of the electric field of the radially polarized LG beam 15 is the radial direction of the ring, and the magnitudes of the electric field vectors are the same. That is, the radial polarization LG beam 15 is a radial direction (radial) polarization beam that is symmetrical with respect to the azimuth angle (azimuthal).

  Such a radially polarized beam, when combined with the above-described optical coupling means 17, has the following special effects compared to the linearly polarized beam used in the conventional structure (Patent Document 1). Demonstrate.

  A preferred example of the radially polarized beam is the above-described radially polarized LG beam 15. However, the effects described below are not limited to the Laguerre Gaussian beam (donut-shaped beam), and other radial polarized beams may be used. Even a beam, for example, a radially polarized beam having a Gaussian distribution, can be obtained.

  FIG. 3 is a diagram schematically illustrating an electric field vector during focusing of light that excites surface plasmons. 3A is a diagram showing an electric field vector of a radially polarized beam used in the present embodiment, and FIG. 3B is a linearly polarized beam used in the conventional structure (Patent Document 1). It is the figure which showed electric field vector.

  As shown in FIG. 3A, by combining the electric field vectors of the radially polarized beam, the vibration components of the electric field in the Z direction (light traveling direction) of the radially polarized beam overlap (the vibration component of the electric field in the X direction is A field of light having a large electric field vibration component in the Z direction is created. Therefore, the surface plasmon SP excited by the radially polarized beam is enhanced in the electric field in the Z direction on the surface of the metal body 11 (inclined outer surface 11 b) near the tip of the conical prism 12.

  FIG. 7 is a diagram showing the electric field enhancement of the surface plasmon on the surface of the metal body in the vicinity of the tip of the conical dielectric. Note that the scale in the figure is normalized by the wavelength dimension.

  The electric lines of force shown in FIG. 7 are obtained when the dielectric constant of air is adopted as the physical property condition (dielectric constant) of the dielectric, and the dielectric constant of gold is adopted as the physical property condition (dielectric constant) of the metal body. For the case where the cone apex angle of the body 12 is set to 10 °, the Maxwell equation of the magnetic field is obtained by the partial differential equation method developed to analytically understand the phenomenon of the tapered surface plasmon called the quasi-variable separation method. It is the result obtained by solving by numerical calculation. Here, detailed description of the above analysis method is omitted.

  According to FIG. 7, it can be seen that the electric lines of force become denser toward the tip of the conical dielectric, and this confirms that the electric field is enhanced at the tip of the dielectric. It can also be understood from the state of the electric lines of force at the tip of the dielectric that the electric field here is only the component in the Z direction.

  On the other hand, as shown in FIG. 3B, the vibration component of the electric field in the X direction (direction perpendicular to the traveling direction of the excitation light) overlaps (Z The vibration component of the electric field in the direction cancels out, and a light field having a large vibration component of the electric field in the X direction is created. For this reason, the surface plasmon SP excited by the linearly polarized beam is enhanced in the electric field in the X direction on the surface (inclined outer surface 11 b) of the metal body 11 near the tip of the conical prism 12.

  By the way, since the direction in which the electric fields due to the radially polarized beam overlap (Z direction) is the traveling direction of the light, the vibration of the electric field of the surface plasmon SP enhanced by such a radially polarized beam is a longitudinal wave. Although it is difficult to re-couple with transverse light, the direction in which the electric field of the linearly polarized beam overlaps (X direction) is perpendicular to the traveling direction of the light. The vibration of the electric field of the surface plasmon SP is a transverse wave, and is easily coupled again with the light of the transverse wave.

  Then, the vibration energy of the electric field of the surface plasmon SP enhanced based on the linearly polarized beam as in the above-described conventional configuration is easily emitted to the outside as a light emission mode, and the surface plasmon SP excited by the linearly polarized beam. Is feared that the electric field cannot be increased appropriately and sufficiently.

  On the other hand, the vibration energy of the electric field of the surface plasmon SP enhanced based on the radially polarized beam of this embodiment is unlikely to be emitted to the outside as a light emission mode, and the surface plasmon SP excited by the radially polarized beam is It is expected that the electric field can be appropriately and sufficiently enhanced.

  Further, when a radially polarized beam is used, if an appropriate metal member (not shown) is arranged with a gap in the Z direction from the surface of the metal body 11 near the tip of the conical prism 12, an electric field is enhanced in the Z direction. The surface plasmon SP thus obtained is further enhanced (gap mode excitation) by the interaction with the metal member, and this is preferable because the electric field of the surface plasmon SP can be more actively enhanced.

  Next, an example of the operation of the electric field enhancement structure 100 of the surface plasmon SP of the present embodiment will be described with reference to FIG.

  First, the radially polarized LG beam 15 (see FIG. 2) emitted from the optical system 16 is condensed so that the distribution of the annular light intensity peak is along the circumference of the bottom surface of the conical prism 12, and a predetermined incident angle is obtained. The light enters the conical prism 12 of the optical coupling means 17 at an angle. As a result, the light energy of the radially polarized LG beam 15 can be concentrated and concentrated due to the focusing effect of the surface plasmon SP.

  The incident angle of the radially polarized LG beam 15 is set to a resonance angle at which resonance excitation of the surface plasmon SP occurs from the relationship with the cone apex angle θ of the cone prism 12. The resonance angle exists within the range of the angle at which the radially polarized LG beam 15 can be totally reflected at the boundary between the side surface 12a of the conical prism 12 and the inclined inner surface 11a of the metal body 11. Are well known and will not be described in detail here.

  When the radially polarized LG beam 15 is totally reflected at the above-described boundary, an evanescent wave (not shown) generated thereby oozes out to the metal body 11, and as a result, the surface plasmon SP is resonantly excited.

  The surface plasmon SP is a wave having an electric field vibration main component in a direction perpendicular to the surface in the outer region 13 in the vicinity of the surface of the metal body 11 (inclined outer surface 11b). Polarized light) functions as the excitation light of the surface plasmon SP. As described above, the radially polarized LG beam 15 satisfies this condition.

  In this way, the surface plasmon SP can be optically coupled with the radial polarization beam LG using the localized evanescent wave that satisfies the phase matching condition with the surface plasmon SP. As a result, the surface plasmon SP excited by the radially polarized LG beam 15 is appropriately and sufficiently enhanced in the light traveling direction (Z direction) on the surface of the metal body 11 (inclined outer surface 11b) in the vicinity of the tip of the conical prism 12. Is done.

  As described above, the surface plasmon electric field enhancement structure 100 of the present embodiment is directed toward the depth direction in the dielectric 12 occupying the tapered conical space 14 defined by the inclined inner surface 11 a of the metal body 11. The incident radial polarization LG beam 15 includes optical coupling means 17 configured to be capable of optical coupling with the surface plasmon SP localized by the metal body 11. For this reason, the vibration components of the electric field in the Z direction (light traveling direction) of the radially polarized LG beam 15 overlap, and a light field having a large electric field vibration component in the Z direction is formed. Therefore, when this surface plasmon SP is excited by such optical coupling with the radially polarized LG beam 15, the surface of the metal body 11 corresponding to the vicinity of the tip of the dielectric 12 (the inclined outer surface 11b). The electric field can be appropriately and sufficiently enhanced in the light traveling direction (Z direction).

  In the surface plasmon electric field enhancement structure 100 of the present embodiment, a donut-shaped (annular) Laguerre Gaussian beam is used as excitation light for exciting the surface plasmon SP on the surface of the metal body 11. Therefore, when the radially polarized LG beam 15 is condensed so that the distribution of the annular light intensity peak is along the circumference of the bottom surface of the conical prism 12, the light energy of the radially polarized LG beam 15 is converted to surface plasmon. It can be used efficiently by being concentrated by the SP focusing effect.

[Modification]
Up to this point, in the conical dielectric 12 of the coupling means 17, as shown in FIG. 1, the part of the opposite side surface 12 a of the dielectric 12 passes through the apex P of the dielectric 12 and the bottom surface of the dielectric 12. Although an example in which the line is symmetrically inclined linearly with respect to the imaginary straight line L drawn perpendicularly to the above, the configuration of the tapered dielectric is not limited thereto.

  FIG. 4 is a cross-sectional view schematically showing a configuration example of an electric field enhancement structure for surface plasmons according to a modification of the first embodiment.

  That is, as shown in FIG. 4, as a modified example of the tapered dielectric, the portion of the opposite side surface 112 a of the dielectric 112 passes through the apex P of the dielectric 112 and is perpendicular to the bottom surface of the dielectric 112. With respect to the drawn virtual straight line L, it may be inclined symmetrically so as to be gently bowed in the direction of the hollow space 114 so that the apex angle θ ′ satisfies θ ′ <θ. In this case, the metal body 111 covering the side surface of the dielectric 112 is also curved with the same degree of curvature as the side surface 112a, for example, the same curvature. With such a configuration of the dielectric 112, the electric field enhancement structure 110 of the surface plasmon SP of the present modification has the following effects.

  The cone apex angle θ of the conical prism 12 described in the present embodiment (FIG. 1) is an angle that governs the above-described resonance angle, and the relationship with the cone apex angle θ so that the resonance excitation of the surface plasmon SP does not deviate. Therefore, it is necessary to appropriately adjust the incident direction of the radially polarized LG beam 15. However, if this adjustment work is unfamiliar, it may be difficult to adjust the optimal incident direction of the radially polarized LG beam 15 by taking time for the adjustment work.

  Therefore, in the electric field enhancement structure 110 of the surface plasmon SP of the present modification example, as shown in FIG. 4, the side surface 112a of the dielectric 112 is gently bent in an arcuate shape toward the hollow space 114, thereby causing radial polarization LG The crossing angle between the beam 15 (incident light) and the metal body 111 can be continuously changed along the depth direction of the hollow space 114. Therefore, in the electric field enhancement structure 110 of the surface plasmon SP of this modification, the angle dependency of the incident direction of the radially polarized LG beam 15 in which resonance excitation of the surface plasmon SP occurs is relaxed, and the incident direction of the radially polarized LG beam 15 is reduced. There is an advantage that the adjustment work can be made efficient. On the other hand, in this case, the energy efficiency when the resonance excitation of the surface plasmon SP occurs is slightly inferior to the conical prism 12 described in the present embodiment (FIG. 1).

That is, the electric field enhancement structure 110 of the surface plasmon SP according to the present modification is useful when it is desired to easily perform the resonance excitation of the surface plasmon SP even if the energy efficiency of the resonance excitation of the surface plasmon SP is slightly sacrificed. .
(Second Embodiment)
In the electric field enhancement structure 100 of the surface plasmon SP of the first embodiment, the Kretschmann optical arrangement is adopted as the optical coupling means 17. In the second embodiment, instead of this, an example using an otto optical arrangement will be described.

  The configuration of the electric field enhancement structure of the surface plasmon SP of the present embodiment other than the configuration of the optical coupling means is the same as the configuration of the electric field enhancement structure 100 of the surface plasmon SP of the first embodiment. The description of the configuration and operation common to these is omitted.

  FIG. 5 is a cross-sectional view schematically showing the optical coupling means of the surface plasmon electric field enhancement structure of the present embodiment. In FIG. 5, the illustration of the configuration other than the optical coupling means 217 is omitted.

  As shown in FIG. 5, the optical coupling unit 217 includes a metal body in which a hollow space 214 that is tapered (for example, tapered) in the depth direction is formed.

  As shown in FIG. 5, the metal body may be a metal substrate 211. In this case, the hollow space 214 is formed from the surface side of the metal substrate 211 by an appropriate drilling method (for example, a focus ion beam method). It is formed. The hollow space 214 has an inclined surface 211a of the tapered metal substrate 211 that is inclined so that a pair of angles φ with the back surface of the metal substrate 211 are substantially the same (preferably the same) in a cross-sectional view. It is divided by.

  In addition, the metal body may have a dielectric substrate (not shown) as a base material. In this case, the hollow space 214 is formed on the dielectric substrate by an appropriate drilling method (for example, a focus ion beam method). After the same kind of hole (not shown) is formed, a metal thin film (not shown) functioning as a metal in which the surface plasmon SP is localized covers the surface of the dielectric substrate that defines the hole. Vacuum deposition formation or vacuum sputtering formation is performed.

  Examples of materials for the metal substrate 211 and the metal thin film include gold, silver, aluminum, copper, and platinum. Among these, gold is suitable because it is difficult to be oxidized in air.

  The three-dimensional shape of the hollow space 214 may be any shape as long as it is sharpened (angle θ) in the depth direction, but as described in the first embodiment, the excitation light of the surface plasmon SP When a doughnut-shaped (annular) Laguerre Gaussian beam is used, a conical shape is preferable.

  The optical coupling means 217 further includes a dielectric 212 that is opposed to the inclined surface 211a of the metal body 211 with a dielectric layer 213 present in a slight gap therebetween and embedded in the hollow space 214. .

  The dielectric 212 is made of, for example, a known optical glass material, whereby a portion of the dielectric 212 embedded in the hollow space 214 functions as an optical prism (hereinafter, this portion is referred to as “dielectric”). Abbreviated as "optical prism portion of the body 212"). That is, as shown in FIG. 5, the optical prism portion of the dielectric 212 is such that the portion of the opposite side surface 212a passes through the apex P of the optical prism portion of the dielectric 212 and is perpendicular to the surface of the metal plate 211. The taper is symmetrically inclined with respect to the drawn imaginary straight line L. Here, as described above, the optical prism portion of the dielectric 212 fits into the hollow space 214 via the dielectric layer 213. It is preferable to be formed in a conical shape.

  The dielectric layer 213 may be formed of any material as long as it has a lower refractive index than that of the dielectric 212. However, it is most convenient and preferable to use air as such a dielectric material. .

  Next, an example of the optical coupling operation of the optical coupling means 217 of this embodiment will be described with reference to FIG.

  First, the radially polarized LG beam 15 (see FIG. 2) emitted from an optical system (not shown) follows the circular light intensity peak distribution along the circumference of the bottom surface of the conical hollow space 214. And is incident on the optical prism portion of the dielectric 212 of the optical coupling means 217 at a predetermined incident angle. As a result, the light energy of the radially polarized LG beam 15 can be concentrated and concentrated due to the focusing effect of the surface plasmon SP.

  The incident angle of the radially polarized LG beam 15 is set to a resonance angle at which resonance excitation of the surface plasmon SP occurs from the relationship with the cone apex angle θ of the hollow space 214 (the optical prism portion of the dielectric 212). The resonance angle exists within an angle range in which the radially polarized LG beam 15 can be totally reflected at the boundary between the optical prism portion of the dielectric 212 and the dielectric layer 213. Such a resonance angle is known in the art. Therefore, detailed description is omitted here.

  When the radially polarized LG beam 15 is totally reflected at the above-described boundary, an evanescent wave (not shown) generated thereby oozes out to the dielectric layer 213, and as a result, the surface plasmon SP is resonantly excited. The surface plasmon SP is a wave having a main component of electric field vibration in a direction perpendicular to the surface of the dielectric layer 213 in the vicinity of the surface (inclined surface 211a) of the metal body 211. Polarized light) functions as excitation light for the surface plasmon SP. As described above, the radially polarized LG beam 15 (see FIG. 2) satisfies this condition.

In this manner, the surface plasmon SP can be optically coupled with the radially polarized LG beam using the localized evanescent wave that satisfies the phase matching condition with the surface plasmon SP. By adopting the Otto optical arrangement described above as the optical coupling means 217, the surface plasmon SP may be beneficial not only for the thin film metal but also for the bulk metal substrate 211.
(Third embodiment)
In the electric field enhancement structure of the surface plasmon SP of the first and second embodiments described above, the surface plasmon SP is excited using the optical prism (here, the dielectrics 12 and 212). In the third embodiment, instead of this, a method of exciting the surface plasmon SP using a diffraction grating will be described.

  The configuration of the electric field enhancement structure of the surface plasmon SP of the present embodiment other than the configuration of the optical coupling means is the same as the configuration of the electric field enhancement structure 100 of the surface plasmon SP of the first embodiment. The description of the configuration and operation common to these is omitted.

  FIG. 6 is a cross-sectional view schematically showing the optical coupling means of the surface plasmon electric field enhancement structure of the present embodiment. In FIG. 6, the illustration of components other than the optical coupling means 317 is omitted.

  As shown in FIG. 6, the optical coupling means 317 includes a metal body in which a hollow space 314 that is tapered (for example, tapered) in the depth direction is formed.

  The metal body may be a metal substrate 311 as shown in FIG. 6. In this case, the hollow space 314 is formed from the surface side of the metal substrate 311 by an appropriate drilling method (for example, a focus ion beam method). It is formed. The hollow space 314 has an inclined surface 311a of the tapered metal substrate 311 that is inclined so that a pair of angles φ formed with the back surface of the metal substrate 211 are approximately the same (preferably the same) in a cross-sectional view. It is divided by.

  In addition, the metal body may have a dielectric substrate (not shown) as a base material. In this case, the metal substrate is provided with a hollow space 314 by an appropriate drilling method (for example, a focus ion beam method). After the same kind of hole (not shown) is formed, a metal thin film (not shown) functioning as a metal in which the surface plasmon SP is localized covers the surface of the dielectric substrate that defines the hole. Vacuum deposition formation or vacuum sputtering formation is performed.

  Examples of materials for the metal substrate 311 and the metal thin film include gold, silver, aluminum, copper, and platinum. Among them, gold is preferable because it is difficult to be oxidized in air.

  The three-dimensional shape of the hollow space 314 may be divided into any shape as long as it has a wedge shape that sharpens (angle θ) in the depth direction. As described in the first embodiment, the excitation of the surface plasmon SP is performed. When a donut-shaped (annular) Laguerre Gaussian beam is used as the light, it is preferably partitioned in a conical shape.

  The optical coupling means 317 further includes a plurality of diffraction gratings 301 formed integrally with the inclined surface 311a of the metal body 311. Although the diffraction grating 301 illustrated in FIG. 6 is an annular protrusion protruding from the inclined surface 311a, this may be an annular groove. The arrangement and shape of the diffraction grating 301 are designed so that the wavelength of the light of the radially polarized LG beam can be appropriately selected. However, such a design technique is well known, and detailed description thereof is omitted here. .

  Next, an example of the optical coupling operation of the optical coupling means 317 of this embodiment will be described with reference to FIG.

  First, a radially polarized LG beam 15 (see FIG. 2) emitted from an optical system (not shown) has a circular light intensity peak distribution in the circumference of the bottom of a hollow space 314 partitioned conically. And is incident on the diffraction grating 301 in the hollow space 314 of the optical coupling means 317 at a predetermined incident angle. As a result, the light energy of the radially polarized LG beam 15 can be concentrated and concentrated due to the focusing effect of the surface plasmon SP. The surface plasmon SP is a wave having an electric field vibration principal component in a direction perpendicular to the surface in the external space near the surface (inclined surface 311a) of the metal body 311 and light that vibrates in that direction (P-polarized light). ) Functions as the excitation light of the surface plasmon SP. As described above, the radially polarized LG beam 15 (see FIG. 2) satisfies this condition.

  In this way, the surface plasmon SP can be optically coupled with the radially polarized LG beam 15 by selecting the wavelength of the radially polarized LG beam 15 by the diffraction grating 301. By adopting the diffraction grating 301 described above as the optical coupling means 317, the surface plasmon SP can be excited not only for the thin film metal but also for the bulk metal substrate 311 and an optical prism is required. Sometimes it is beneficial.

[Application example]
The technology described in each of the above embodiments is assumed to be used for various purposes. Here, as an example, the technology described in the first embodiment is applied to the optical coupler for the electric field enhancement structure 100 of the surface plasmon SP. An application example will be described.

  In the electric field enhancing structure 100 of the surface plasmon SP shown in FIG. 1, nanotips (for example, a diameter of 1/10 wavelength of light; for example, 600 nm red visible light) If there is, a metal wire of about 60 nm) can be connected to transmit light to the metal wire. Thereby, there is a possibility of realizing an ultra-small optical device that breaks the limit of the existing optical technology that it is difficult to make the size of the optical waveguide smaller than the optical wavelength. In this case, the electric field enhancement structure 100 of the surface plasmon SP described above is used as an optical coupler that injects light energy into a nano-sized metal wire. In addition, it replaces with such a metal wire, and the application to the molecular device of arrange | positioning a molecule | numerator at the above-mentioned front-end | tip part and exciting this molecule | numerator with light energy is also assumed.

  Further, as another application form of the electric field enhancement structure 100 of the surface plasmon SP shown in FIG. 1, after slightly rounding the tip portion to be enhanced in the electric field enhancement structure 100 of the surface plasmon SP without completely sharpening, You may modify | change so that it may extend in a Z direction in the state. For example, when the tip of the metal body 11 is viewed as a width dimension in the X direction, when rounded to about 1/3 wavelength of light (for example, about 200 nm in the case of 600 nm red visible light), The tip is considered to reach a state where the electric field enhancement of the surface plasmon SP begins to occur.

  Therefore, the tip of the metal body 11 is rounded so as to have a width in the X direction of about 1/3 wavelength of light, and the metal body 11 is extended in the Z direction by a predetermined distance by the width. The tip may be completely sharpened. Then, the portion where the metal body extends can function as a light transmission path based on the beginning of the electric field enhancement of the surface plasmon SP, and the electric field enhancement is again performed at the completely sharp tip of the metal body 11. Is made.

  With such a configuration, it is considered that the electric field enhancement structure 100 of the surface plasmon SP can be easily formed as a device on the substrate.

  According to the present invention, there is provided a surface plasmon electric field enhancement structure capable of adequately and sufficiently enhancing the electric field of surface plasmons in the light traveling direction on the surface near the tip of the metal body in which a tapered hollow space is formed in the depth direction. Thus, the electric field enhancement structure of the surface plasmon of the present invention can be used for an optical device such as an optical coupler.

It is sectional drawing which showed typically the example of 1 structure of the electric field enhancement structure of the surface plasmon by 1st Embodiment. It is the figure which showed typically about the light intensity distribution and polarization of a radial polarization LG beam. It is the figure which drawn typically the mode of the electric field vector at the time of focusing of the laser beam which excites surface plasmon. It is sectional drawing which showed typically the example of 1 structure of the electric field reinforcement structure of the surface plasmon by the modification of 1st Embodiment. It is sectional drawing which showed typically the optical coupling means of the electric field enhancement structure of the surface plasmon of 2nd Embodiment. It is sectional drawing which showed typically the optical coupling means of the electric field enhancement structure of the surface plasmon of 3rd Embodiment. It is the figure which represented the mode of the electric field reinforcement | strengthening of the surface plasmon in the surface of the metal body near the front-end | tip of a cone-shaped dielectric material with the electric force line.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Metal body 11a Inclined inner surface 11b Inclined outer surface 12, 112, 212 Dielectric body 12a, 112a Side surface 13 External region 14, 114, 214 Indentation space 15 Radially polarized LG beam 16 Optical system 17, 217, 317 Optical coupling means 100, 110 Surface plasmon electric field enhancement structure 211, 311 Metal substrate 211a, 311a Inclined surface 301 Diffraction grating SP Surface plasmon P Vertex L Virtual straight line θ Conical apex angle φ Angle formed

Claims (11)

Radially polarized light incident in the depth direction on the dielectric in the hollow space tapered in the depth direction defined by the metal body is localized on the surface plasmon and the optical cup. An optical coupling means configured to be ringable,
The surface plasmon is excited by the optical coupling with the radially polarized light, and the electric field is enhanced in the traveling direction of the light on the surface of the metal body corresponding to the vicinity of the tip of the dielectric. Plasmon electric field enhancement structure.   2. The surface plasmon according to claim 1, wherein the opposing portions of the dielectric are linearly inclined symmetrically with respect to an imaginary straight line that passes through the apex of the dielectric and is perpendicular to the bottom surface of the dielectric. Electric field enhancement structure.   2. The surface plasmon according to claim 1, wherein opposing portions of the dielectric are inclined in an arcuate shape symmetrically with respect to an imaginary straight line that passes through the top of the dielectric and is perpendicular to the bottom surface of the dielectric. Electric field enhancement structure.   2. The surface plasmon electric field enhancement structure according to claim 1, wherein the light is a Laguerre Gaussian beam in which light intensity peaks are distributed in a donut shape.   5. The electric field enhancement structure for surface plasmons according to claim 4, wherein the dielectric is conical, and the light intensity of the Laguerre Gaussian beam is distributed in an annular shape along the circumference of the bottom surface of the dielectric. The optical coupling means includes an optical prism as the dielectric,
A metal thin film as the metal body in direct contact with the side surface of the optical prism;
With
6. The surface plasmon is optically coupled with the radially polarized light by total reflection of the radially polarized light at a boundary between the optical prism and the metal thin film. The electric field enhancing structure for surface plasmons according to any one of the above. The optical coupling means is opposed to the metal body with a gap therebetween, and an optical prism as the dielectric in the hollow space;
A dielectric layer that is present in the gap and has a lower refractive index than the dielectric;
With
6. The surface plasmon is optically coupled with the radially polarized light by total reflection of the radially polarized light at a boundary between the optical prism and the dielectric layer. The electric field enhancing structure for surface plasmons according to any one of the above.   8. The surface plasmon electric field enhancement structure according to claim 7, wherein the dielectric layer is a layer made of air. Light that is incident in the depth direction into the hollow space tapered in the depth direction defined by the metal body and that is radially polarized can be optically coupled to the surface plasmon localized in the metal body. A surface plasmon electric field enhancement structure with optical coupling means configured as follows:
The optical coupling means further includes a diffraction grating formed on the metal body,
The surface plasmon is excited by optical coupling with the radially polarized light based on the selection by the diffraction grating of the wavelength of the radially polarized light, and the surface of the metal body corresponding to the vicinity of the tip of the dielectric The electric field enhancement structure of surface plasmon, wherein the electric field is enhanced in the light traveling direction.   10. The surface plasmon electric field enhancement structure according to claim 9, wherein the light is a Laguerre Gaussian beam in which light intensity peaks are distributed in a donut shape.   11. The surface plasmon electric field enhancement structure according to claim 10, wherein the hollow space is partitioned into cones, and the light intensity of the Laguerre Gaussian beam is distributed in an annular shape along the circumference of the bottom of the hollow space.

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