JP2011242308A - Near-field light probe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost, simple structured near-field light probe which does not require the preparation of a complicated structured probe in order to generate large intensity near-field light and with which linearly polarized light can also be used.SOLUTION: A near-field light probe for generating near-field light includes a cone body or a polygonal pyramid-shape body that is made of a dielectric and that is coated with a plasmon active material. Linearly polarized light enters from the bottom surface of the probe and near-field light with large intensity electric field vector generates at the probe tip. The probe meets one or more demands described below: (a) the vertex of the cone body or the polygonal pyramid-shape body is displaced from the central axis of the bottom surface; (b) the incident light enters in an oblique direction; (c) one portion of the bottom surface is protected from light; (d) the plasmon active material is asymmetrically coated; and (e) one portion of the cone body or the polygonal pyramid-shape body is unitarily formed on a dielectric substrate such that the probe tip of the cone body or the polygonal pyramid-shape body is placed over the dielectric substrate.

Description

  The present invention relates to a near-field optical probe that irradiates a sharpened probe tip with excitation light and generates near-field light in the vicinity of the probe tip by surface plasmon resonance. The present invention also relates to a method for effectively increasing the intensity of near-field light.

  Plasmon refers to a state in which free electrons in a metal vibrate collectively and behave as pseudo particles. The vibration of the electromagnetic field is induced by the vibration of the electric charge, and the vibration of the electromagnetic field affects the vibration of the electric charge, so that a system in which both vibrations are combined is formed. On the solid surface, unlike plasmons in the solid, there is another collective oscillation that satisfies the boundary condition at the surface (Tsukada Jun: Surface electronic excitation (Maruzen, 1996)). This is called surface plasmon. In particular, metal nanoparticles such as gold colloids have a vivid color tone due to light absorption caused by coupling between a visible to near-infrared photoelectric field and plasmons. This phenomenon is surface plasmon resonance (SPR), and an electric field that is significantly enhanced locally is also generated. That is, by converting light energy into surface plasmons, not only is light energy stored on the surface of the metal nanoparticles, but also light control in a region smaller than the light diffraction limit becomes possible. There is also a resonance wavelength depending on the particle shape and the dielectric constant of the surrounding medium. Such interaction between metal nanoparticles and light has attracted attention in the field of optical science and technology in recent years (Wikipedia Free Encyclopedia).

  By the way, in order to excite surface plasmons with electromagnetic waves, the phase velocity of electromagnetic waves must match the phase velocity of surface plasmons. However, even if an electromagnetic wave propagating through a normal medium is incident on the surface, the condition that the phase velocities of the two coincide with each other cannot be obtained. Therefore, an evanescent wave generated when the electromagnetic wave is totally reflected at the boundary surface is used. When an electromagnetic wave is incident on a low medium from a medium with a high refractive index, the electromagnetic wave is totally reflected when the incident angle is set to a certain critical angle or more. In this case, the vertical component of the wave number boundary surface is an imaginary number. Electromagnetic waves penetrate inside the low medium up to about one wavelength. An electromagnetic wave that has penetrated into the medium under the above conditions is called an evanescent field, and an electromagnetic wave emitted therefrom is called an evanescent wave. If this evanescent wave is used, its phase velocity can be matched with that of surface plasmons, and surface plasmons can be excited resonantly (Keisuke Nagashima: Surface plasmon fundamentals and applications (J. Plasma Fusion Res Vol.84, No.1 2008)).

  By using such surface plasmons, near-field light having a high intensity and a nanometer-sized spread can be generated at the tip of a metal cone (hereinafter referred to as “probe”). Near-field light is expected to be used in many applications such as a scanning near-field optical microscope (hereinafter referred to as “SNOM”), a nano-area optical sensor, a DVD optical head, and the like. For example, with SNOM, an object smaller than the wavelength of light can be seen with light. By generating near-field light in the vicinity of the sample and operating it with a probe, it can be imaged. Optical observation beyond the limit of optical resolution is possible, so observation in a vacuum or sample No metal foil vapor deposition is required, and non-conductors can be observed non-destructively in the atmosphere. Moreover, if near-field light is applied to a DVD optical head, the storage capacity may be increased 10,000 times.

  Various proposals have been made as a method for generating high-intensity near-field light having a nanometer-sized micro spot size at the probe tip using surface plasmons. For example, it consists of a light-transmitting protrusion in the shape of a polygonal pyramid, one of the side surfaces is covered with a metal film, and the other surface is covered with a metal film except for the region below the light wavelength at the tip. Near field optical probe (Patent Document 1) characterized by having a metal region in the vicinity of the most advanced surface of the sharpened probe, and the metal region is exposed to the most advanced surface or outside the metal region A near-field optical probe (Patent Document 2) in which a dielectric thin layer having a thickness of 1/20 or less of the excitation light wavelength is formed, the outer surface of the protrusion is covered with a metal film, and the metal film is not formed at the apex portion. There are a near-field optical probe having an opening (Patent Document 3), a probe in which a groove having an I-shaped narrow portion from the top is formed in a metal pyramid structure (Patent Document 4). With respect to these probes, it is necessary to make a metal-coated region and a region not covered with metal in a very small region, and there is a problem that a manufacturing method is difficult.

  In addition, a probe that is currently considered promising is a probe in which a surface of a dielectric cone having rotational symmetry about a central axis is coated with a plasmon active substance such as gold. This is because surface plasmons can be efficiently excited in the coated gold thin film by utilizing total reflection at the side surface of the light incident on the inside of the dielectric. However, in order to generate a high-intensity near field with a small (nanometer size) spot size at the probe tip, it is necessary to use a special light beam polarized in the radial direction as the incident light. Could not be used directly. Therefore, a configuration for converting linearly polarized light into radially polarized light is required.

JP 2002-221478 A JP 2005-195500 A Japanese Patent Laid-Open No. 2003-14608 JP 2004-109965 A

  As described above, low-cost, simple-structure near-field light that can use linearly polarized light without preparing a probe with a complicated structure to generate high-intensity near-field light with a small spot size. An object is to provide a probe.

  As a result of intensive studies to solve the above-mentioned problems and achieve the intended purpose, it is noted that conventional probes are used to maintain symmetry, and this is intentionally made asymmetric. As a result, it was found that linearly polarized light can be used.

  That is, the present invention provides a high-intensity electric field having a minute spot size at the probe tip by coating a plasmon active substance on a cone-polyhedral structure composed of a dielectric, and making linearly polarized light incident from the bottom surface. A probe for generating near-field light having a vector, characterized in that the apex of the cone / polyhedron is deviated from the center axis of the bottom surface. A conventional near-field optical probe in which a metal thin film is coated on a conical dielectric has a cone apex on the central axis of the bottom surface. Therefore, it was necessary to use a special light beam, but by deviating this apex position, it became possible to use linearly polarized light. Specifically, the apex position is preferably at a position having an inclination of about 10 ° with respect to the vertical (center axis) direction from the center of the bottom surface.

  In addition, even when the linearly polarized light is used, a large spot size can be obtained by making the light incident at a predetermined angle with respect to the bottom of the cone / polygonal body. Near-field light having a strong electric field vector can be generated. When linearly polarized light is directly incident on a conventional cone, the electric field in the z direction is canceled at the probe tip. Therefore, specifically, it may be incident obliquely at an angle of about 25 ° with respect to the vertical direction of the bottom surface.

  Furthermore, it is possible to generate near-field light having a high-intensity electric field vector with a minute spot size by shielding part of the bottom surface of the cone / polygonal cone and partially intercepting linearly polarized light. it can. For example, in the case of the bottom of the cone, the light shielding portion can be shielded in a semicircular shape.

  In addition, even with cones and polygonal cones having rotational symmetry, if the plasmon active substance is coated on the side surface in an asymmetric distribution, near-field light having a high intensity electric field vector with a minute spot size can be generated. it can. For example, if a portion of the cone side is not coated, an asymmetric coating distribution can be created.

  In addition, if a part of a cone / polygonal body is made so that the tip thereof exists on the dielectric substrate, and the plasmon active substance is coated on the whole / part of the cone / polygonal body, a minute spot Near-field light having a large size electric field vector can be generated on a dielectric substrate. For example, if a half of a cone is created on a dielectric substrate and a plasmon active material is coated on the cone, a near field with a small spot size and a high-intensity electric field vector on the substrate where the cone apex and the dielectric substrate intersect. Light can be generated. In this way, the object to be measured can be placed at a specified position on the dielectric substrate, and the distance between the probe and the non-measuring object can be easily controlled.

  It is also possible to configure a probe by combining any two or more of the above. For example, the apex of the cone probe is formed at a position having an inclination of about 10 ° with respect to the vertical direction from the center of the bottom surface, and is incident obliquely at an angle of about 15 ° with respect to the vertical direction of the bottom surface. It can also be configured as follows. Furthermore, if the probe shape, coating distribution, and incident conditions as described above are asymmetric, the same effect can be obtained not only when the incident wave is linearly polarized, but also for light polarized in an arbitrary direction. Obtainable. That is, any polarized light can be handled.

  Since the probe of the present invention has a simple configuration, it can be manufactured at low cost and without requiring complicated equipment. Further, since linearly polarized light can be used, for example, the configuration of the entire apparatus of a scanning near-field light microscope including a probe can be simplified. In addition, since a probe that expresses high-intensity near-field light having a fine spot size with high efficiency can be provided, the development of various inspection devices and recording devices using near-field light to new applications is further accelerated.

FIG. 1 is a diagram schematically showing a cross section and a surface plasmon of a conventional probe. FIG. 2 is a diagram showing an electric field vector distribution of a polarized light beam. FIG. 3 is a diagram schematically showing an embodiment of the probe of the present invention. FIG. 4 is a diagram schematically showing the probe shape design of FIG. FIG. 5 is a diagram schematically showing a cross section and a surface plasmon of the probe of FIG. FIG. 6 is a diagram showing the relationship between the near-field light intensity at the probe tip and the angle A in the embodiment of FIG. FIG. 7 is a diagram schematically showing another embodiment of the probe of the present invention. FIG. 8 is a diagram schematically showing a cross section and a surface plasmon of the probe shown in FIG. FIG. 9 is a diagram showing the relationship between the near-field light intensity at the probe tip and the incident angle B in the embodiment of FIG. FIG. 10 is a diagram schematically showing another embodiment of the probe of the present invention. FIG. 11 is a diagram schematically showing a cross section and a surface plasmon of the probe shown in FIG. FIG. 12 is a diagram schematically showing another embodiment of the probe of the present invention. FIG. 13 is a diagram schematically showing a cross section and a surface plasmon of the probe shown in FIG. FIG. 14 is a diagram schematically showing another embodiment of the probe of the present invention. FIG. 15 is a diagram schematically showing a cross section and a surface plasmon of the probe shown in FIG. FIG. 16 is a conceptual diagram showing a state in which the probe of the present invention is used in a scanning near-field light microscope. FIG. 17 is a conceptual diagram showing a state in which the probe of the present invention is used in an optical recording / reproducing head. FIG. 18 is a diagram simulating generation of near-field light in Example 1 of the present invention and a comparative example. FIG. 19 is a diagram simulating the generation of near-field light in Example 2 of the present invention. FIG. 20 is a diagram simulating generation of near-field light in Example 3 of the present invention.

Hereinafter, the probe of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a conventional cone probe 10 having rotational symmetry about the central axis (Z axis) of the bottom surface. The cross-sectional shape of this probe is an isosceles triangle, and assuming that the base direction is the X axis and the height direction is the Z axis, in FIG. 1, light (incident light) 20 linearly polarized in the X axis direction is a dielectric in the Z axis direction. 30 shows a state of being incident on the light beam 30, that is, a state in which the traveling direction of incident light is perpendicular to the bottom surface. Since the probe is coated with the metal thin film 40 on the hypotenuse of the isosceles triangle, the incident light is totally reflected, and the surface plasmon 50 indicated by the arrow is excited on the metal thin film (the hypotenuse). In FIG. 1, surface plasmons are excited by the left side metal thin film and the right side metal thin film, and the electric field vectors have the same magnitude and the same direction. Since this surface plasmon propagates the same distance on the left and right sides of the probe, when the surface plasmon reaches the probe tip, the electric field vector in the Z-axis direction has a magnitude at the probe tip due to the symmetry of the probe structure. Because they are the same and opposite directions, they will cancel each other. Therefore, high intensity near-field light having a Z-direction electric field vector is not generated at the probe tip.

  In order to generate high-intensity near-field light having a Z-axis direction electric field vector in the conventional probe shown in FIG. 1, the electric field vector of the surface plasmon excited to that of the left side metal thin film and the right side metal thin film on the bottom surface of the probe Need to have the opposite direction. Therefore, the incident light must be a special light beam that is polarized in the radial direction as shown in FIG. That is, even when a linearly polarized light beam as shown in FIG. 2B is made incident, the electric field vectors in the Z direction cancel each other at the tip of the probe as described above. (The straight arrows shown in the figure schematically represent the electric field vectors.)

  Thus, the linearly polarized light emitted from the normal laser light source (shown in FIG. 2B) cannot be used as it is, and the beam center and the probe center axis (Z shown in FIG. 2A) are also used. There is a problem that it is costly because alignment with the shaft) is necessary.

  Therefore, the present invention has proposed a shape that does not give symmetry to the probe 11 as shown in FIG. The probe 11 has a structure in which a conical dielectric structure that is asymmetric about the center axis of the bottom surface is formed on a dielectric substrate 32, and the surface is coated with a plasmon active medium (for example, a metal such as gold). Then, linearly polarized light enters the probe from below the dielectric 31.

  The probe 11 can be manufactured as follows, for example. A dielectric is poured in a molten state into a mold having a recess for forming a probe shape as shown in FIG. 3 and cooled. The solidified dielectric is removed from the mold and the surface is coated with a plasmon active material. At this time, if necessary to sharpen the probe tip, it may be sharpened with an electron beam, an ion beam or the like. Examples of the dielectric include glass, plastic, transparent semiconductor, and the like, preferably glass. As the plasmon active substance, metals such as gold, silver, copper, aluminum, platinum and palladium are used. Further, these active substances can be coated by a film forming method such as sputtering, vapor deposition, CVD, or plating.

FIG. 4 shows an explanation of the probe shape design in which linearly polarized light incident in parallel to the Z axis from the bottom surface of the probe shown in FIG. 3 causes total reflection on the side surface of the probe coated with the metal thin film 41. Yes. The phase velocity V _{p in} the probe side direction of the incident wave and the reflected wave is given by V _p = c ÷ (n × cos θ) from FIG. 4 (n: refractive index of glass inside the probe, c: light velocity, θ: The angle between the incident direction and the side of the cone. The thickness of the coating is 10 to 50 nm, the height of the probe is 1000 to 2000 nm, and the radius of the bottom surface is 2000 to 4000 nm. The angle of the tip 15 is designed to be 20 to 60 °.

Phase velocity V _plasmon surface plasmon traveling coated metal thin film probe side to the probe tip direction is smaller than the speed of light c in vacuum. Therefore, it is possible to adjust V _p to V _plasmon at a part of the probe side surface by adjusting the angle of the probe side surface. When V _p matches the V _plasmon, it is possible to excite surface plasmons at a high efficiency as shown in FIG. Further, surface plasmons are excited along the metal thin film 41 also from the edge boundary between the metal thin film 41 coated with the cone and the dielectric substrate (FIGS. 3 and 32). As surface plasmons propagate along the probe side in the tip direction, the cross section perpendicular to the propagation direction decreases, so the intensity of the surface plasmon is greatly enhanced, and high-intensity near-field light with a small spot size is generated at the probe tip. is there.

  FIG. 5 is a sectional view showing a state in which linearly polarized light 20 in the X-axis direction similar to that shown in FIG. 1 is incident on the dielectric 31 in the Z-axis direction. . In this probe, the apex 15 of the cone is shifted from the center axis (Z axis) of the bottom surface to the right side in the figure by an angle A, and the gold thin film 41 is coated on the hypotenuse of the triangle so that the incident light is totally reflected and the gold is reflected. Surface plasmons 51 indicated by straight arrows are excited around the thin film. Since the surface plasmon propagation distance to the probe tip is different between the probe left side and the right side, the Z direction electric field vector at the probe tip is different between the surface plasmon propagating on the right side and the surface plasmon propagating on the left side. They are different in size and direction and do not cancel each other as shown in FIG. Therefore, if the probe shape is adjusted so that the Z direction electric field vector of the surface plasmon propagating on the left and right side surfaces at the probe tip is maximum and in the same direction, a minute spot size high intensity near-field light having a Z direction electric field at the probe tip It can be generated.

  For the probe 11, it is necessary to shift the apex position from the center axis of the bottom surface in a similar manner, not only in the case of a cone but also in a polygonal pyramid such as a triangular pyramid and a quadrangular pyramid. As shown, it is 4 to 30 °, preferably 8 to 15 °, and most preferably 10 °. FIG. 6 shows the relationship between the near-field light intensity at the probe tip and the angle A.

  Next, a probe configured to generate a minute spot size high-intensity near-field light from linearly polarized light by adjusting incident light even when the probe has symmetry will be described. 7 is configured so that linearly polarized light 21 having a finite angle B with the vertical direction is incident on the bottom surface of the conventional probe 10 described in FIG. FIG. 8 schematically shows a state where the Z-direction electric field is enhanced at the probe tip in the case of such oblique incidence. As shown in FIG. 8, the surface plasmon 52 generated in the vicinity of the metal thin film 40 does not have symmetry with respect to the Z axis as shown in FIG. 1 on the left side surface and the right side surface. It can be done.

  FIG. 9 shows the relationship between the near-field light intensity at the probe tip and the incident angle B in the embodiment. More specifically, the angle of oblique incidence at this time is 10 to 50 °, preferably 20 to 40 °, and most preferably 25 ° at an incident angle B shown in FIG.

  Furthermore, even when the probe has symmetry as described above, high-intensity near-field light can be generated by blocking a part of incident light. In FIG. 10, an opaque material 17 that blocks incident light 20 is provided on the bottom surface of the probe. By blocking a part of the incident light, a high-intensity near field is generated at the probe tip without canceling out the Z-direction electric field vector as shown in FIG. FIG. 11 shows this state. As shown in FIG. 11, the surface plasmon 53 generated in the vicinity of the metal thin film 40 has no symmetry between the left side surface and the right side surface in the cross section shown in the figure. In the state where the section is rotated by 90 °, the situation as shown in FIG. There will be no cancellation.

  The degree of such light shielding is 5 to 20% of the area of the bottom surface, and light is not shielded symmetrically. For example, in the case of a cone, the light is shielded so that the light shielding part is semicircular or fan-shaped. It is preferable to block.

  Even when the probe has symmetry as described above, if a plasmon active substance is coated on the side of the probe in an asymmetric distribution as shown in FIG. 12, near-field light having a high intensity electric field spectrum with a minute spot size is generated. can do. This state is shown in FIG. As shown in FIG. 12, in the probe 13, a part of the surface of the dielectric cone 30 is not coated with the metal thin film 40, that is, the metal thin film is uncoated on the surface of the dielectric cone. Then, as shown in FIG. 13, the surface plasmon 54 generated in the vicinity of the metal thin film 40 has no symmetry between the left side surface and the right side surface in the cross section shown in the figure. Therefore, high-intensity near-field light can be generated without the surface plasmons canceling each other at the probe tip.

  In FIG. 14, half of the cone probe 14 is prepared so that its tip is on the dielectric substrate 33, and the dielectric cone 34 divided in half is coated with the plasmon active material 40. In this way, near-field light having a small spot size and high electric field strength can be generated on the dielectric substrate. This is shown in FIG. A conical half 34 is formed on the dielectric substrate 33, the surface of which is coated with a metal thin film 40. Light 22 is incident on the bottom surface of the probe so as to form an angle C with the conical central axis (Z axis) placed on the surface of the dielectric substrate 33. At this time, the surface plasmon 55 is excited along the metal thin film 40 coated on the surface of the cone 34. This structure is also an asymmetric probe, and it is possible to generate high intensity near-field light with a small spot size without the surface plasmons canceling each other at the probe tip. Since the positional relationship between a predetermined position of the dielectric substrate 33 and the probe tip can be clarified from the structure shown in FIG. 15, a non-measurement object (for example, a DNA structure) is placed close to the probe tip at a minute spot size with high intensity. It can be easily irradiated with field light. With this configuration, optical measurement of only a part of the non-measurement object becomes possible.

  A simple conceptual diagram in the case of using the probe of the present invention as described above to constitute a near-field microscope (FIG. 16) and an optical recording / reproducing head (FIG. 17) is shown. FIGS. 16 and 17 are examples in which light is incident from an oblique direction as shown in FIG. 7 on a probe whose apex position is deviated as shown in FIG. These can also be used in combination.

  In order to clarify the present invention more specifically, some examples according to the present invention are shown below.

  A probe as shown in FIG. 3 was prepared. The probe has a height of 1870 nm, a bottom surface radius of 1000 nm, a tip angle of about 30 °, and a coating thickness of about 20 nm. Glass was used as the dielectric, its refractive index was 1.5, and gold (Au) was used as the coated plasmon active material. On the other hand, for comparison, except that the probe was made with a height of 1870 nm, a bottom surface radius of 1000 nm, and a tip angle of about 32 °, a symmetrical probe as shown in FIG. FIG. 18 shows a state in which the light intensity distribution of the near-field light when it is vertically incident is simulated.

  FIG. 18A shows an example of the present invention, and FIG. 18B shows a comparative example. In the figure, the larger the light intensity is, the more white it is shown. As is apparent from the figure, it can be seen that in the example of the present invention, near-field light with high intensity at the probe tip is generated. In the comparative example, the light intensity at the probe tip is about 0.5 times the incident intensity, whereas in the example of the present invention, the light intensity at the probe tip is about 1600 times.

  A probe having the same outer shape as that of the comparative example was prepared. As shown in FIG. 7, when the incident light is incident on the bottom surface of the probe with an inclination of about 25 ° (FIG. 19A), a semicircular shape as shown in FIG. 10 so that the area of the bottom surface becomes 20%. This shows a state in which the light intensity distribution of near-field light is simulated when the light shielding portion is formed on the bottom surface of the probe (FIG. 19B).

  In the figure, the larger the light intensity is, the more white the same as described above. As is apparent from the figure, near-field light having a large light intensity is formed at the probe tip both when obliquely incident and when the light shielding portion is formed. The light intensity at the probe tip when obliquely incident was about 6300 times that of the incident light, and the light intensity when the light shielding portion was provided was about 1962 times.

  A probe having the same outer shape as that of the comparative example was prepared. As shown in FIG. 12, about 30% of the side surface of the probe is not coated with metal and incident light is incident on the bottom surface of the probe (FIG. 20A), and half on the dielectric substrate as shown in FIG. A state in which the light intensity distribution of the near-field light is simulated when a metal coating is applied to the conical probe divided into two and incident on the bottom surface of the probe with an inclination of about 20 ° (FIG. 20B) is shown.

  In the figure, the larger the light intensity is, the more white the same as described above. As is clear from the figure, both the asymmetrical coating of the metal thin film and the metal-coated conical probe divided in half on the dielectric substrate, both have a small spot size and a high light intensity at the probe tip. Field light is formed. In the case of FIG. 20A, the light intensity at the probe tip was about 1494 times that of the incident light, and in the case of FIG. 20B, it was 876 times.

  Since the near-field optical probe of the present invention can be easily manufactured with a simple structure, many applications such as a scanning near-field optical microscope, a nano-region optical sensor, and a DVD optical head are expected.

10, 11, 12, 13, 14 Near-field probe 20, 21, 22 Incident light 30, 31, 34 Dielectric (probe)
32, 33 Dielectric (Base)
40, 41 Metal thin film 50, 51, 52, 53, 54, 55 Surface plasmon

Claims (4)

A plasmon active material is coated on a cone-polyhedral structure composed of dielectric material, and light polarized in an arbitrary direction is incident from the bottom to generate near-field light having a high-intensity electric field vector at the probe tip. A probe for,
(A) The apex of the cone / polyhedron is displaced from the central axis of the bottom surface,
(B) The light is configured to enter the bottom surface at a finite angle with the vertical direction;
(C) that a part of incident light on the bottom surface is blocked;
(D) the plasmon active substance is asymmetrically coated;
(E) A part of the cone / polyhedral structure is formed integrally with the dielectric base so that the probe tip of the cone / polyhedral structure is placed on the dielectric base. thing,
A probe characterized by being at least one selected from any one of the above.   2. The probe according to claim 1, wherein in the case of (a), the apexes of the cone and the polygonal pyramid are displaced with an inclination of 4 to 30 ° with respect to the perpendicular from the bottom surface.   2. The probe according to claim 1, wherein in the case of (b), the incident angle is 10 to 50 °.   The probe according to claim 1, wherein in the case of (c), 5 to 20% is shielded from light.