JP2022135684A5 - - Google Patents

Description

The present invention relates to a circular polarizer, and particularly to a broadband circular polarizer that can be used over a wide band and a measuring device equipped with the broadband circular polarizer.

A wave plate is an optical element that changes the state of incident polarized light by giving a predetermined phase difference (optical path difference) to two orthogonal polarized light components. Generally, two types of wavelength plates are commonly used: a half-wave plate (λ/2 plate) and a quarter-wave plate (λ/4 plate).

A half-wave plate (HWP) is a wave plate that gives a phase difference of 1/2 to an incident light beam. Specifically, a half-wave plate (HWP) is a wave plate that gives a phase difference of 1/2 to an incident light beam. This is an optical element that provides a phase difference of /2 (180°). The 1/2 wavelength plate is used to give a phase difference of λ/2 (180°) to the incident light beam, rotate linearly polarized light, and emit the light.

A quarter-wave plate (QWP) is a wave plate that gives a phase difference of 1/4 to an incident light beam. Specifically, a quarter-wave plate (QWP) is a wave plate that gives a phase difference of 1/4 to an incident light beam. This is an optical element that provides a phase difference of /4 (90°). A quarter-wave plate is used to give a phase difference of λ/4 (90°) to an incident light beam, and to change linearly polarized light into circularly polarized light or circularly polarized light into linearly polarized light.

Patent Document 1 discloses that by using a retardation plate in which a 1/4 wavelength plate and a 1/2 wavelength plate are bonded together at a pre-designed angle, the wavelength dispersion of retardation can be controlled. A technique is described in which the ratio of retardation (Δnd/λ) to ) is kept approximately constant.

Patent Document 2 discloses a laminated wave plate in which two wave plates are laminated so that their respective optical axes intersect at an angle of 60°, and the whole functions as a quarter-wave plate at wavelengths of 400 nm, 650 nm, and 785 nm. is listed.

Japanese Patent Application Publication No. 10-68816 Patent No. 4825951

In recent years, light sources in which the polarization state of emitted light is circularly polarized light have been developed, but at present there are generally few light sources that emit circularly polarized light. Therefore, a specific optical element is required to create a circularly polarized state.

Using circularly polarized light makes the optical system less susceptible to the effects of instrumental polarization, and is also useful for improving analysis accuracy in polarization analysis. Circularly polarized light is also necessary for circular dichroism spectroscopy, which is useful for analyzing organic substances and liquid crystals. Therefore, there has been a need for an optical element that can create a circularly polarized state over a wide wavelength band.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an optical element that can be used in the wavelength range from vacuum ultraviolet to near-infrared and that can produce highly accurate circularly polarized light. and to provide a measurement device equipped with the optical photons.

As a result of intensive research to solve the above problems, the inventors of the present invention have developed a broadband wavelength plate that combines a conventionally known birefringent λ/2 plate and a λ/4 plate, and a broadband wave plate known as a broadband retarder. The present invention was completed by discovering the combination of Fresnel ROMs.

According to the broadband circular polarizer of the present invention, the above-mentioned problem is solved by the broadband circular polarizer of the present invention. and a λ/4 part that provides a phase difference of 90 ° ±10% between p-waves and s-waves having wavelengths of at least 200 to 2000 nm by total reflection, with the axis of the incident light beam as the axis of rotation, The incidence plane of the λ/4 part is rotated by 60 ° ±10% with respect to the incidence plane of the λ/2 part, and the λ/2 part and the λ/4 part are made of Fresnel ROM. This can be solved by converting linearly polarized light having a wavelength of at least 200 to 2000 nm into circularly polarized light.

In this way, by combining broadband Fresnel-Rom retarders with a broadband structure, we have created an ultraviolet retarder with a phase difference of λ/4 (90°) that can be used in the wavelength range from vacuum ultraviolet to near-infrared. It is possible to provide a broadband circular polarizer that has broadband characteristics and can produce highly accurate circularly polarized light.

At this time, the Fresnel ROM constituting the λ/2 part and the λ/4 part is preferably formed of at least one material selected from the group consisting of quartz, _CaF2, and LiF.
At this time, it is preferable that the Fresnel ROM constituting the λ/2 part and the λ/4 part have a multilayer film formed on at least one total reflection surface.
At this time, the λ/2 section has a first Fresnel ROM and a second Fresnel ROM that provide a phase difference of 90 ° ±10% between a p-wave and an s-wave having a wavelength of at least 200 to 2000 nm due to total reflection. The λ/4 section has a third Fresnel ROM that provides a phase difference of 90 ° ±10% between the p-wave and the s-wave having a wavelength of at least 200 to 2000 nm through total reflection, and The incidence surface of the first Fresnel ROM is aligned with the incidence surface of the second Fresnel ROM, with the axis of rotation being the axis of rotation, and the incidence surface of the third Fresnel ROM is aligned with the incidence surface of the second Fresnel ROM, with the axis of the incident light beam being the axis of rotation. , the first Fresnel ROM and the second Fresnel ROM may be rotated by 60° ±10% with respect to the incident surfaces of the first Fresnel ROM and the second Fresnel ROM.
At this time, the Fresnel ROM preferably provides a phase difference of 90 ° ±10% between p-waves and s-waves having wavelengths of at least 200 to 2000 nm through two total reflections.
At this time, it is preferable that the Fresnel ROM provides a phase difference of 90 ° ±10% between p-waves and s-waves having wavelengths of at least 200 to 2000 nm through three total reflections.

Further, the above-mentioned problem is solved by the measuring device of the present invention including the broadband circular polarizer described above.

According to the broadband circular polarizer of the present invention, the phase difference is λ/4 (90°), it can be used in the wavelength range from vacuum ultraviolet to near-infrared (for example, 200 to 2600 nm), and it has a highly accurate circular polarizer. It is possible to create polarized light.

In addition, since the broadband circular polarizer of the present invention has the above-mentioned characteristics, it can be applied to measurement devices including various measurement devices, analysis devices, inspection devices, and observation devices, so that it can be used for inspection of minute areas, It becomes possible to observe and perform various polarization measurements.

FIG. 2 is a schematic diagram of an H+Q structure broadband wave plate. 2 is a graph showing phase difference characteristics of a λ/4 plate (QWP) constituting an H+Q structure broadband wave plate. 2 is a graph showing the phase difference characteristics of a λ/2 plate (HWP) constituting the H+Q structure broadband wave plate. It is a graph which shows the phase difference characteristic of the H+Q structure broadband wave plate. It is a graph which shows the ellipticity characteristic of the H+Q structure broadband wave plate. It is a graph which shows the polarization axis direction of the H+Q structure broadband wavelength plate. It is a schematic diagram of a standard λ/4 Fresnel ROM (manufactured by BK-7). It is a schematic diagram of a standard λ/2 Fresnel ROM (manufactured by BK-7). It is a graph showing the relationship between the angle of incidence from BK-7 to the air and the phase difference. It is a graph showing the relationship between the angle of incidence from quartz to air and the phase difference. It is a graph showing the wavelength dependence of the phase difference of a Fresnel ROM (λ/4) manufactured by BK-7. It is a graph showing the wavelength dependence of the phase difference of a Fresnel ROM (λ/2) manufactured by BK-7. It is a graph showing the wavelength dependence of the phase difference of a Fresnel ROM made of quartz (λ/4). It is a graph showing the wavelength dependence of the phase difference of a Fresnel ROM made of quartz (λ/2). 1 is a schematic diagram showing the structure of an H+Q type Fresnel ROM according to an embodiment of the present invention. It is a graph showing the phase difference characteristics of an H+Q type quartz Fresnel ROM. It is a graph which shows the ellipticity characteristic of H+Q type quartz Fresnel ROM. It is a graph showing the polarization axis direction of an H+Q type quartz Fresnel ROM. It is a graph showing the phase difference characteristics (fine adjustment of the combination angle) of the H+Q type quartz Fresnel ROM. It is a graph showing the ellipticity characteristics (fine adjustment of the combination angle) of the H+Q type quartz Fresnel ROM. It is a graph showing the polarization axis direction (fine adjustment of the combination angle) of the H+Q type quartz Fresnel ROM. It is a graph showing a comparison between an example of the present invention and a King type Fresnel ROM with an optical film. FIG. 2 is a schematic diagram showing the structure of a King-type Fresnel ROM with an H+Q-type optical film. It is a graph showing the retardation characteristics of a King type quartz Fresnel ROM with an H+Q type optical film. It is a graph showing the ellipticity characteristics of a King type quartz Fresnel ROM with an H+Q type optical film. It is a graph showing the polarization axis direction of a King type quartz Fresnel ROM with an H+Q type optical film. FIG. 2 is a schematic diagram showing the structure of a crystal Fresnel prism. It is a graph showing an example of the circularly polarized light separation angle of a Fresnel prism made of quartz. FIG. 2 is a schematic diagram showing the structure of an H+Q type quartz Fresnel ROM (Example 1). 3 is a graph showing the phase difference characteristics of the element of Example 1. 3 is a graph showing the ellipticity characteristics of the element of Example 1. 3 is a graph showing the polarization axis direction of the element of Example 1. FIG. 3 is a graph showing the phase difference characteristics of the element of Example 1-2. 3 is a graph showing the ellipticity characteristics of the element of Example 1-2. 3 is a graph showing the polarization axis direction of the element of Example 1-2. 7 is a graph showing the phase difference characteristics of the element of Example 2. 7 is a graph showing the ellipticity characteristics of the element of Example 2. 7 is a graph showing the polarization axis direction of the element of Example 2. FIG. 3 is a schematic diagram showing the structure of an H+Q type LiF Fresnel ROM (Example 3). 3 is a graph showing the phase difference characteristics of the element of Example 3. 3 is a graph showing the ellipticity characteristics of the element of Example 3. 3 is a graph showing the polarization axis direction of the element of Example 3. It is a schematic diagram which shows the structure of H+Q type quartz Fresnel ROM with a multilayer film (Example 4). 12 is a graph showing the phase difference characteristics of the element of Example 4. 12 is a graph showing the ellipticity characteristics of the element of Example 4. 12 is a graph showing the polarization axis direction of the element of Example 4. FIG. 3 is a schematic diagram showing the structure of a King type H+Q type quartz Fresnel ROM with an optical film (Example 5). 12 is a graph showing the phase difference characteristics of the element of Example 5. 12 is a graph showing the ellipticity characteristics of the element of Example 5. 7 is a graph showing the polarization axis direction of the element of Example 5.

Hereinafter, a broadband circular polarizer according to an embodiment of the present invention (hereinafter referred to as the present embodiment) will be described with reference to FIGS. 1 to 24C. The broadband circular polarizer according to this embodiment is a broadband circular polarizer in which the wavelength dependence of the phase difference is small in the wavelength range from vacuum ultraviolet to near infrared.

In this specification, ○nm to Δnm means ○nm or more and Δnm or less.
In this specification, approximately means a numerical value of ±10%, preferably ±5%, more preferably ±3%, still more preferably ±2%, particularly preferably ±1%.
In this specification, the term "contact" refers to a pair of adjacent Fresnel ROMs (prism elements, rhombohedrons) that are placed in contact with each other, including optical contacts that are directly bonded, as well as optical contacts that are bonded directly. Also includes joining.
In addition, in this specification, Fresnel ROMs (prism elements, rhombohedrons) "opposing" refer to two cases: optical contacts that are directly bonded, and two cases where they are bonded with something interposed, such as an adhesive. This includes cases where an air layer is interposed.

[1. Basic idea]
The broadband circular polarizer according to this embodiment is a combination of a broadband wavelength plate that is a combination of a conventionally known birefringent λ/2 plate and a λ/4 plate, and a Fresnel ROM that is known as a broadband retarder. It is something. First, a conventionally known broadband wavelength plate that is a combination of a birefringent λ/2 plate and a λ/4 plate will be described.

(1) Broadband wavelength plate that combines a birefringent λ/2 plate and a λ/4 plate (H+Q structure wavelength plate)
FIG. 1 shows a structural diagram (layout diagram) of the H+Q structure wave plate HQP. As shown in Figure 1, first, a λ/2 plate (HWP) is placed so that the optical axis Ha is rotated by 15 degrees with respect to the incident linearly polarized light, and then the optical axis Qa is rotated with respect to the incident linearly polarized light. It has a structure in which a λ/4 plate (QWP) is arranged in a direction rotated by 75 degrees. In this element, it is necessary to use a HWP on the incident side and a QWP on the output side. The HWP and QWP may be a single birefringent plate or a wavelength plate that is a combination of a plurality of birefringent plates.

FIGS. 2A and 2B show the wavelength dependence of the phase difference between a λ/2 plate (HWP) and a λ/4 plate (QWP), which are a combination of quartz crystal and MgF2 crystal, which constitute the H+Q structure broadband wave plate. The optical characteristics of the H+Q structure broadband wave plate constructed by combining these are shown in FIGS. 2C to 2E. From FIG. 2C, it can be seen that the wavelength band in which the phase difference is within 90°±5% is λ=350 to 1500 nm.

(2) Fresnel Rom Figures 3A and 3B show the shape of a standard Fresnel Rom. FIG. 3A is a schematic diagram of a λ/4 Fresnel ROM (manufactured by BK-7), and FIG. 3B is a schematic diagram of a λ/2 Fresnel ROM (manufactured by BK-7).

As shown in FIG. 3A, the first rhombohedral body 10 constituting the λ/4 Fresnel ROM has a first entrance end surface 11, a first exit end surface 12 arranged parallel to the first entrance end surface 11, and a first entrance end surface. 11 and the first total reflection surface 12 , and a second total reflection surface 14 arranged parallel to the first total reflection surface 13 .

In the first rhombohedron 10, the first incident end surface 11 and the first exit end surface 12 are parallel to each other, and the first total reflection surface 13 and the second total reflection surface 14 are parallel to each other. Further, in the first rhombohedron 10, the distance between the first incident end surface 11 and the first total reflection surface 13 and between the first output end surface 12 and the second total reflection surface 14 is less than 90 degrees with respect to each other. They intersect at an angle (wedge angle α). Furthermore, in the first rhombohedron 10, the distance between the first incident end surface 11 and the second total reflection surface 14 and between the first output end surface 12 and the first total reflection surface 13 is greater than 90 degrees with respect to each other. intersect at an angle.

As shown in FIG. 3B, the λ/2 Fresnel ROM has a structure (roof-type Fresnel ROM) in which two parallelogram-shaped prism elements are combined. Specifically, the λ/2 Fresnel ROM has a parallelogram shape in which the cross section (a cross section parallel to the plane of FIG. 3B, that is, a cross section perpendicular to each incident end face, output end face, and total reflection surface described later) is a parallelogram. A first rhombohedron 10 and a second rhombohedron 20 are provided. The first rhombohedron 10 and the second rhombohedron 20 have the same shape and are made of isotropic material.

The second rhombohedral body 20 includes a second incident end surface 21 , a second output end surface 22 arranged parallel to the second incident end surface 21 , and a third total reflection surface that intersects with the second incident end surface 21 and the second output end surface 22 . 23, and a fourth total reflection surface 24 arranged parallel to the third total reflection surface 23.

In the second rhombohedral body 20, the second incident end surface 21 and the second exit end surface 22 are parallel to each other, and the third total reflection surface 23 and the fourth total reflection surface 24 are parallel to each other. Further, in the second rhombohedral body 20, the distance between the second incident end surface 21 and the third total reflection surface 23 and between the second exit end surface 22 and the fourth total reflection surface 24 is less than 90 degrees with respect to each other. They intersect at an angle (wedge angle α). Furthermore, in the second rhombohedral body 20, the distance between the second incident end surface 21 and the fourth total reflection surface 24 and between the second exit end surface 22 and the third total reflection surface 23 is greater than 90 degrees with respect to each other. intersect at an angle.

The first rhombohedral 10 and the second rhombohedral 20 are arranged to face each other such that the first output end surface 12 of the first rhombohedral 10 and the second entrance end surface 21 of the second rhombohedral 20 are parallel to each other. ing. It is preferable that the first output end face 12 and the second input end face 21 be directly joined by optical contact, but it is also possible to use ultraviolet-transmissive adhesive to adhesively fix the first output end face 12 and the second entrance end face 21, or to leave a gap without joining. It is also possible to arrange the

As described above, the first incident end surface 11 and the first total reflection surface 13 (the first output end surface 12 and the second total reflection surface 14) of the first rhombohedron 10 form a wedge angle α, and similarly, the second total reflection surface 13 forms a wedge angle α. The second incident end surface 21 and the third total reflection surface 23 (the second output end surface 22 and the fourth total reflection surface 24) of the rhombohedral body 20 also form a wedge angle α. Here, the wedge angle α can be set appropriately depending on the type of isotropic material that constitutes the first rhombohedron 10 and the second rhombohedron 20, and when the isotropic material is BK-7. In the case of quartz, the wedge angle α is 54°, and when the isotropic material is _CaF2 , the wedge angle α is 54°.

As mentioned above, the Fresnel ROM is made of an isotropic material that does not have birefringence, but is a retarder that utilizes the phase difference between the p-wave and the s-wave generated during total internal reflection. Total internal reflection is a phenomenon that occurs when light is incident from a high refractive material to a low refractive material at an angle equal to or greater than a critical angle. FIGS. 4A and 4B show the relationship between the angle of incidence on the reflecting surface and the generated phase difference when the materials are BK-7, which is a typical optical glass, and quartz.

It can be seen from FIGS. 4A and 4B that no phase difference occurs in normal reflection below the critical angle. Furthermore, it can be seen that the generated phase difference differs depending on the angle even at or above the critical angle. It can also be seen that the maximum value of the generated phase difference differs depending on the refractive index of the ROM material (the difference in refractive index between the incident and exit media).

When the ROM material is BK-7, the phase difference is about 45 degrees when the incident angle is about 55 degrees. Therefore, by total reflection twice, a phase difference of 90° can be achieved, and by total reflection four times, a phase difference of 180° can be created.

FIGS. 5A and 5B show the wavelength dependence of the phase difference of the Fresnel ROM made of BK-7. Further, FIGS. 6A and 6B show the wavelength dependence of the phase difference of the quartz Fresnel ROM. It can be seen from FIGS. 5A and 5B, and FIGS. 6A and 6B that the wavelength dependence of the Fresnel ROM phase difference is small. However, it can be seen that the quartz Fresnel ROM has a larger phase difference deviation from 90° and 180° than the BK7 Fresnel ROM.

(3) H+Q type Fresnel ROM (broadband circular polarizer of this embodiment)
The broadband circular polarizer according to this embodiment has a structure in which the above two broadband retarders are combined. By configuring a broadband Fresnel ROM retarder with a broadband structure, unprecedented ultra-wideband characteristics can be obtained.

As an example of the broadband circular polarizer 1 according to this embodiment, FIG. 7 shows an H+Q type Fresnel ROM made of quartz. Further, output polarization parameters such as phase difference are shown in FIGS. 8A to 8C. The broadband circular polarizer according to this embodiment has the characteristics of a general λ/4 plate (QWP), as is the case with a wave plate with an H+Q structure, and has excellent performance as an element that converts linearly polarized light into circularly polarized light. Demonstrate.

The basic structure is as shown in Figure 7, which includes a phase shifter (λ/2 part: H part) that has a phase difference of 180° between p-wave and s-wave by total reflection, and a total reflection The retarder (λ/4 part: Q part) which has a phase difference of 90° between the p-wave and the s-wave is rotated by about 60° (θ2-θ1 =75°-15°).

It is not necessary to generate a phase difference of about 180° or about 90° between the p-wave and the s-wave in one total reflection, and in the example of Fig. 7, the phase difference generated in one total reflection is , about 45 degrees, total reflection is repeated four times to generate a phase difference of about 180 degrees, and total reflection is repeated twice to generate a phase difference of about 90 degrees.

In FIG. 7, the ROMs (rhombohedral blocks) are connected, but they do not need to be connected. Also, when bonding each ROM, an adhesive may be used as long as it is transparent in the area of use. However, since adhesives that are transparent in the deep ultraviolet region are rare, it is preferable to use optical contact for bonding. When joining, care must be taken to avoid stress being applied to the isotropic ROM material and anisotropy occurring.

An AR film (antireflection film) may be applied to the end face, and it is preferable that the transmittance can be improved over the entire usage band. However, for example, when the material of the element of the present invention is quartz, the wavelength band to be used is λ = 180 to 2600 nm, and it is generally difficult to form an effective anti-reflection film over this entire band, so it is difficult to reduce the reflectance. Although it cannot be significantly reduced, there is a method using a MgF2 single layer film. Another method is to apply a special anti-reflection film with a moth-eye structure. Naturally, it is not necessary to apply an antireflection film. In this case as well, there is no significant effect on the output polarization. The term "end face" refers not only to the entrance/exit surface, but also to the entrance/exit surface of each ROM when the ROMs are not joined together.

In Fig. 7, the three ROMs have the same shape, but they do not necessarily have to be the same, but they can be combined with retarders (λ/2 part H and λ/4 part Q) with a phase difference of about 180° and about 90°. For example, there is an effect of widening the band. Further, the number of reflections does not need to be a combination of four and two. By applying an optical multilayer film to the total reflection surface and using a broadband ROM, the band becomes wider and the accuracy of the output circularly polarized light increases.

In the retarder of this structure, as shown in FIGS. 8C and 9C, the direction of the polarization axis of the emitted light changes depending on the wavelength. This also applies to the H+Q structure using birefringent materials. However, since the output polarized light is circularly polarized light, this does not pose a particular problem. However, in order to obtain linearly polarized light from circularly polarized light, it is necessary to change the optical axis of the incident circularly polarized light for each wavelength as shown in FIGS. (side), but this is not realistic. Therefore, this element is mainly effective in creating circularly polarized light from linearly polarized light, and it is necessary to input the light from the ROM side (λ/2 part H side) with a phase difference of 180°.

From FIG. 8B, it can be seen that the ellipticity is close to 1 and the output polarized light is almost circularly polarized. However, it deviates from 1 in the infrared region. By finely adjusting the angle 60° between the two ROMs in FIG. 7 to 59.6°, the characteristics shown in FIGS. 8A to 8C can be adjusted as shown in FIGS. 9A to 9C.

(4) Advantages and disadvantages of the broadband circular polarizer according to this embodiment The advantages of the broadband circular polarizer according to this embodiment are that it can convert linearly polarized light into circularly polarized light over a wide wavelength range, and that it is easy to manufacture. One thing can be mentioned. Further, the disadvantages of the broadband circular polarizer according to this embodiment include that the incident light and the output light are not on the same axis, that when changing circularly polarized light to linearly polarized light, the direction of linearly polarized light differs depending on the wavelength, and that the general It is not suitable for use in changing the polarization state from linearly polarized light to elliptically polarized light to circularly polarized light like a λ/4 plate, and it is relatively large. In this way, the broadband circular polarizer according to this embodiment has many disadvantages compared to its advantages, but it is an element that is unique in its ability to convert linearly polarized light into highly accurate circularly polarized light over a wide wavelength band. be.

[2. Conventional technology]
A broadband λ/4 phase shifter and a circular polarizer are shown below as conventional techniques. As described above, the H+Q structure has better broadband properties than the original λ/2 retarder or λ/4 retarder, and is a structure that can produce a retarder with an ellipticity close to 1. Therefore, if the original retarder has less wavelength dependence and an ellipticity close to 1 (close to circularly polarized light), it is possible to create a circular polarizer with less wavelength dependence and an ellipticity close to 1.

As is conventionally known, Fresnel ROMs have less wavelength dependence than λ/2 plates and λ/4 plates that use birefringent materials. It is clear that a circular polarizer with a wider band can be obtained than a circular polarizer that combines the following.

Furthermore, even if a circular polarizer with an H+Q structure is made using a birefringent film to broaden the wavelength band, it will not be transparent in the ultraviolet region as long as it is made using organic materials. , there are characteristic absorption lines even in the infrared region, so it cannot be used as a circular polarizer that covers the range from ultraviolet to infrared.

Next, a comparison will be made with a King-type Fresnel ROM with an optical thin film, which is an element with the widest wavelength band and an ellipticity close to 1 among conventional Fresnel ROMs. FIG. 10 shows a comparison of the ellipticities of the King-type Fresnel ROM with an optical film and the Fresnel ROM (broadband circular polarizer according to this embodiment) shown in FIG. From FIG. 10, it can be seen that the broadband properties and ellipticity characteristics are almost the same. Here, the compared King-type Fresnel ROM with an optical thin film is a λ/4 retarder, so by arranging two of them in series, it becomes a λ/2 retarder, and another By arranging one, the King type Fresnel ROM with optical thin film can be arranged in an H+Q type arrangement. By arranging them in this manner, it is possible to further bring the ellipticity closer to 1.

11 and 12A to 12C show the structure and optical characteristics of a King type H+Q type Fresnel ROM with an optical film. Therefore, it is meaningless to compare the present invention with conventional Fresnel ROMs. From FIG. 12C, it can be seen that in this example, the wavelength dependence of the polarization axis is also small, and therefore it is also possible to change circularly polarized light to linearly polarized light.

A Fresnel prism is a broadband circular polarizer with a completely different structure. This is, for example, a circular polarizer that combines a right-handed crystal and a left-handed crystal to separate left and right circularly polarized light at the joint surface.

FIG. 13 shows a schematic diagram of a quartz Fresnel prism. Further, FIG. 14 shows an example of the wavelength dependence of the separation angle of clockwise and counterclockwise circularly polarized light. A Fresnel prism is an element that can obtain nearly perfect circularly polarized light over a wide band, but a material that can obtain a large separation angle has not been found, making it an element with little practical use. The present invention is a practical circular polarizer because any isotropic material with a wide transmission band can be used.

[3. Detailed explanation of the structure of broadband circular polarizer 1 of this embodiment]
The broadband circular polarizer 1 of this embodiment has a structure that combines a λ/4 Fresnel ROM (Fig. 3A, λ/4 part Q) and a λ/2 Fresnel ROM (Fig. 3B, λ/2 part H). This is an optical element that converts light into circularly polarized light. Specifically, as shown in FIG. 15, the broadband circular polarizer 1 provides a phase difference of approximately 180° (specifically, 180°±10%) between the p-wave and the s-wave due to total internal reflection. It includes a λ/2 part H and a λ/4 part Q that provides a phase difference of approximately 90° (specifically, 90°±10%) between the p-wave and the s-wave due to total reflection, and With the axis as the rotation axis, the entrance plane of λ/4 section Q is rotated by approximately 60° (specifically, 60° ± 10%) (θ2 - θ1) with respect to the entrance plane of λ/2 section H. has been done.

Each Fresnel ROM constituting the λ/2 part H and the λ/4 part Q is formed from an isotropic material, and FIG. 7 above shows the case where quartz is used as the isotropic material (wedge). angle α = 54°). The isotropic material may be any material that transmits wavelengths from vacuum ultraviolet to near infrared, and from the viewpoint of availability, quartz (fused silica: refractive index n = 1.46 @ 550 nm), calcium fluoride ( It is preferable to use at least one material selected from the group consisting of CaF2: refractive index n=1.44@546 nm) and lithium fluoride (LiF _: refractive index n=1.39@600 nm). Regarding the isotropic material used, it is also important that there are no defects or distortions in the material that would degrade the phase difference, and it is preferable to use quartz (fused silica) rather than _CaF2 .

As shown in FIG. 7, the λ/2 part H is a first Fresnel ROM FR1 that provides a phase difference of approximately 90° (specifically, 90°±10%) between the p-wave and the s-wave due to total reflection. and a second Fresnel ROM FR2, and the λ/4 section Q is a third Fresnel ROM that provides a phase difference of approximately 90° (specifically, 90°±10%) between the p-wave and the s-wave due to total reflection. It has FR3. The entrance surface of the first Fresnel ROM FR1 is arranged to coincide with the entrance surface of the second Fresnel ROM FR2, with the incident light beam axis serving as the rotation axis. The entrance surface of the third Fresnel ROM FR3 is rotated by approximately 60° (specifically, 60°±10%) with respect to the entrance surfaces of the first Fresnel ROM FR1 and the second Fresnel ROM FR2, with the incident light beam axis as the rotation axis. It is located.

The incident light ray I that entered the first incident end surface 11 of the first Fresnel ROM FR1 (first rhombohedral body 10) constituting the λ/2 part H is totally reflected four times (reflected ray R) and becomes λ/4. The light beam E is emitted from the third Fresnel ROM FR3 forming part Q. The broadband circular polarizer 1 has an angle of about 90° (specifically, 90°±5 % , preferably 90°±3 % , more preferably 90°) with respect to the incident light I in the vacuum ultraviolet to near-infrared wavelength region. The output beam E is emitted with a phase difference of 90°±2 % , more preferably 90°±1 % .

When using the broadband circular polarizer 1, the incident light ray I made from the outside substantially perpendicular to the first incident end surface 11 of the first Fresnel ROM FR1 (first rhombohedron 10) is as follows. Proceed to. The incident light ray I is internally reflected at the first total reflection surface 13 and the second total reflection surface 14 of the first Fresnel ROM FR1 (first rhombohedral 10), and the reflected ray R is internally reflected at the first Fresnel ROM FR1 (first rhombohedral 10). ) is emitted from the first output end face 12 of the second rhombohedral body 20 to the second entrance end face 21 of the second rhombohedral body 20 . Then, the reflected light ray R incident from the first output end face 12 of the first Fresnel ROM FR1 (first rhombohedral body 10) to the second incidence end face 21 of the second Fresnel ROM FR2 (second rhombohedral body 20) is reflected by the third total reflection. The reflected light beam R is internally reflected at the surface 23 and the fourth total reflection surface 24, and enters the third Fresnel ROM FR3 from the second output end surface 22 of the second Fresnel ROM FR2 (of the second rhombohedral body 20), and is internally reflected twice. It is reflected and emitted as an emitted light beam E.

(Multilayer film M)
The Fresnel ROM constituting the λ/2 part H and the λ/4 part Q preferably has a multilayer film M formed on at least one total reflection surface. Specifically, the first total reflection surface 13 of the first rhombohedron 10, the second total reflection surface 14, and the third total reflection surface of the second rhombohedron 20 as Fresnel ROMs constituting the λ/2 part H and the λ/4 part Q. The total reflection surface 23 and the fourth total reflection surface 24 are coated with a multilayer film M having a refractive index different from that of the isotropic material constituting the first rhombohedron 10 and the second rhombohedron 20. and is suitable.

Here, the multilayer film M includes a high refractive index film M _H formed of a high refractive index material having a higher refractive index than the isotropic material constituting the first rhombohedron 10 and the second rhombohedron 20; Low refractive index films M _L made of a low refractive index material having a lower refractive index than the isotropic material constituting the one rhombohedron 10 and the second rhombohedron 20 are alternately laminated. Even if the high refractive index film M _H and the low refractive index film M _L are laminated in the order of the high refractive index film M _H and the low refractive index film M _L on the isotropic material serving as the substrate, Alternatively, the low refractive index film M _L and the high refractive index film M _H may be laminated in this order on the isotropic material serving as the substrate.

The incident light beam I is totally reflected by each total reflection surface, and at the same time, a phase difference occurs between the p-polarized light and the s-polarized light. Normally, the phase difference caused by total internal reflection becomes larger as the wavelength becomes shorter. Therefore, in each Fresnel ROM, a high refractive index film M H having a larger refractive index than the isotropic material (quartz or CaF ₂ ) constituting the first rhombohedral 10 and the second rhombohedral 20 and a low refractive index film M _H having a smaller refractive index are used. It is preferable to apply a multilayer film M in which two types of film materials consisting of a reflective film M _{and L} are alternately laminated on the total reflection surface.

High refractive index materials constituting the high refractive index film _MH include gadolinium fluoride (GdF3: refractive index n=1.59@550 nm), lanthanum fluoride (LaF3: refractive index n=1.59@550 nm), and An example is neodymium fluoride (NdF3: 1.61@550 nm), but the material is not limited to these materials. Further, as a low refractive index material constituting the low refractive index film M _L , magnesium fluoride (MgF2: refractive index n = 1.38 to 1.40 @ 550 nm) is exemplified, but is not limited thereto. isn't it.

The high refractive index film M _H and the low refractive index film M _L can be formed by methods such as vacuum deposition, CVD, and sputtering. The film thicknesses of the high refractive index film M _H and the low refractive index film M _L depend on the type of material and may be, for example, 100 Å or more and 650 Å or less, but are not limited to this range.

In the broadband circular polarizer 1 of this embodiment, the λ/4 section Q and the λ/2 section H are not limited to the configuration shown in FIG. 7 described above. For example, as shown in Figure 19, to have a phase difference of 180°, there are eight reflections at λ/2 part H, and to have a phase difference of 90°, there are four reflections at λ/4 part Q. It is also possible to adopt a structure in which three roof-type Fresnel ROMs are arranged side by side. In this structure, two roof-type Fresnel ROMs with a phase difference of 180° and one roof-type Fresnel ROM with a phase difference of 90° rotate the incident ray axis by 59.5° (θ2-θ1 = 74.5°-15°). be placed.

Furthermore, as shown in Fig. 23, three king-type Fresnel ROMs with a phase difference of 90° are arranged in the direction of the optical axis, and the two on the incident side are regarded as Fresnel ROMs with a phase difference of 180° (λ/2 part H), and the two on the exit side are One of them is regarded as a Fresnel ROM with a phase difference of 90° (λ/4 part Q), and the Fresnel ROM with a phase difference of 180° and the Fresnel ROM with a phase difference of 90° are set at 60° (θ2-θ1=75°- It is also possible to have a structure rotated by 15°).

[4. Application example of broadband circular polarizer of this embodiment]
Application examples of the broadband circular polarizer of this embodiment are shown below. The broadband circular polarizer of this embodiment can be applied to measurement devices. Here, the term "measuring device" includes various measuring devices, analysis devices, inspection devices, and observation devices.

Semiconductor inspection equipment is a device that inspects minute areas and actively uses ultraviolet light, so it is suitable to use the broadband circular polarizer of this embodiment, which can be used in the wavelength range from vacuum ultraviolet to near infrared. It is. The broadband circular polarizer of this embodiment can be used in any semiconductor inspection device that uses polarized white light.

A spectroscopic ellipsometer is exemplified as a specific target device to which the broadband circular polarizer of this embodiment is applied. Specifically, a broadband circular polarizer can be used as a compensation element in a spectroscopic ellipsometer.

Further, the broadband circular polarizer of this embodiment can also be applied to an observation device for observing a minute area. Contrast can sometimes be improved by controlling polarization. In order to perform finer observation, it is necessary to perform observation using shorter wavelengths, so there is an advantage in using the broadband circular polarizer of this embodiment.

Furthermore, in order to detect foreign objects, we use observation and inspection equipment that utilizes the characteristics of the polarization state of scattered light from foreign objects that differs from normal parts, specifically observing the polarization state of wiring patterns on semiconductor wafers. The broadband circular polarizer of this embodiment can be applied to a device for detecting foreign objects. The method of using polarized light differs depending on the device, but when discovering abnormalities (defects) that occur in various semiconductor processes from polarized light information, the broadband circular polarizer of this embodiment is used with a phase difference of 180° (λ/2). By combining it with a phase shifter, various polarization measurements become possible.

Furthermore, the broadband circular polarizer of this embodiment can be applied to a film thickness meter. Film thickness gauges are mainly used for inspections during semiconductor processes, but they are also used to inspect other film-like objects, such as measuring film thickness and coating thickness. The measurement principles of film thickness meters vary, but there are also devices that measure film thickness using polarization analysis similar to an ellipsometer.

Additionally, the broadband circular polarizer of this embodiment can be used in place of a depolarizing element to reduce instrument polarization. In a reflective optical system, the reflectance of p-polarized light and s-polarized light is different, so if the polarization state of the incident light beam is different or fluctuates, the transmittance will be different or fluctuate. To prevent this, the polarization state of the incident light beam may be made constant, or the polarization state of the output light beam from the optical system may be made constant. In the case of a device that performs precise measurements, it is necessary to keep the polarization state of the incident light beam and the output light beam constant. By combining the broadband circular polarizer of this embodiment with a retarder with a phase difference of 180° (λ/2), any polarization state can be created, so it is possible to keep the polarization state of the incident light beam and the output light beam constant. It becomes possible.

Methods for analyzing the three-dimensional structures of polymers such as proteins, pharmaceuticals, and foods include optical rotation dispersion (ORD) and circular dichroism (CD) spectroscopy, which examine optical isomers of polymers. These measurements are methods of measuring the difference in refractive index and absorption between left and right circularly polarized light, and require a circular polarizer with high accuracy and a wide wavelength band. The broadband circular polarizer of the present embodiment meets these demands and is capable of improving the accuracy of optical rotation dispersion measurement and circular dichroism spectroscopy measurement.

Hereinafter, the broadband circular polarizer of the present invention will be described in more detail based on Examples, but the present invention is not limited to the following Examples.

<Example 1: H+Q type quartz Fresnel ROM>
The H+Q type quartz Fresnel ROM of Example 1 consists of a roof-type Fresnel ROM (λ/2 part: H part) made of general quartz with a phase difference of 180°, and a parallelogram-shaped Fresnel ROM (λ/4 part) with a phase difference of 90°. The structure is such that the Q part) is rotated by 60° (θ2-θ1=75°-15°) using the incident light beam axis as the rotation axis (FIG. 15, Table 1).

The roof-type Fresnel ROM (λ/2 part: H part) with a phase difference of 180° is arranged by arranging two parallelogram Fresnel ROMs with a phase difference of 90°, so the parallelogram Fresnel ROM 3 with a phase difference of 90° It can also be regarded as a structure in which the two elements are rotated by 15 degrees, 15 degrees, and 75 degrees with respect to the incident polarized light, respectively.

The parallelogram Fresnel ROMs may be placed apart from each other, or they may be joined together. Optical contact is preferable because the joining has a wide wavelength band, but the phase difference due to stress in the parallelogram Fresnel ROMs is A method such as adhesion or fusion may be used as long as no light absorption occurs and there is no light absorption. Although it is preferable to apply an antireflection film to the light-transmitting surface of each parallelogram Fresnel ROM, it is not necessary to provide an antireflection film. 16A to 16C show the optical characteristics of the H+Q type quartz Fresnel ROM of Example 1. From FIG. 16A, it can be seen that the phase difference is 90° in the wavelength range of λ=200 nm to 2600 nm. Further, from FIG. 16B, it can be seen that the ellipticity is close to 1 in the above wavelength band, and circularly polarized light is emitted.

<Example 1-2: H+Q type quartz Fresnel ROM optimized type>
Example 1-2 has a structure in which the rotation angle of the Fresnel ROM with a phase difference of 180° and the Fresnel ROM with a phase difference of 90° in Example 1 is set to 59.6° (θ2-θ1=74.6°-15°). Table 2). As can be seen from the comparison between FIGS. 16A to 16C and FIGS. 17A to 17C, by adjusting the rotation angles of the λ/2 section (H section) and λ/4 section (Q section) to the optimum , it is possible to bring the ellipticity closer to 1 over the entire used wavelength band (200 nm to 2600 nm). Note that the bonding method and antireflection film are the same as in Example 1.

<Example 2: H+Q type CaF ₂ Fresnel ROM optimized type>
CaF2 crystal is an isotropic material that is transparent from λ=130 to 6000 nm. A Fresnel ROM using _CaF2 is also a broadband retarder, but a Fresnel ROM with a phase difference of 180° and a Fresnel ROM with a phase difference of 90° made of _CaF2 are rotated by 59° (θ2-θ1 = 74°-15°). By adopting a structure in which the ellipticity is close to 1, an element having a wider band and an ellipticity close to 1 can be obtained (Table 3, FIGS. 18A to 18C). Note that the bonding method and antireflection film are the same as in Example 1. The H+Q type CaF2 Fresnel ROM optimized type (Example 2) whose ellipticity characteristics are shown in FIG. 18B is better than the H+ _Q type quartz Fresnel ROM optimized type (Example 1) whose ellipticity characteristics are shown in FIG. 17B. It can be seen that circularly polarized light with an ellipticity close to 1 can be obtained in a wide wavelength band of wavelength λ=130 nm to 6000 nm .

<Example 3: H+Q type LiF Fresnel ROM optimized type>
LiF crystal is an isotropic crystal that is transparent in a wavelength range of about λ=110 to 9000 nm. Therefore, if a Fresnel ROM is manufactured using LiF, there is a possibility that a retarder with a very wide band can be manufactured. However, the refractive index is relatively small, and the difference in refractive index is large between the vacuum ultraviolet region and the mid-infrared region, making it difficult to manufacture a retarder that takes advantage of the transparent band.

Example 3 of an H+Q type LiF Fresnel ROM using LiF as a material is shown below. Since the refractive index is relatively small and it is difficult to obtain a large phase difference in one total reflection, the phase difference in one reflection is set to approximately 22.5°, and in order to have a phase difference of 180°, a λ/2 part ( The configuration is such that the λ/4 section (Q section) undergoes 8 reflections and 4 reflections at the λ/4 section (Q section) to provide a phase difference of 90° (FIG. 19, Table 4). In other words, it has a structure in which three roof-shaped Fresnel ROMs are arranged side by side. Two roof-type Fresnel ROMs with a phase difference of 180° and one roof-type Fresnel ROM with a phase difference of 90° were arranged with the incident light axis rotated by 59.5° (θ2-θ1 = 74.5°-15°). It is a structure. Note that the bonding method and antireflection film are the same as in Example 1.

20A to 20C show the optical characteristics of the H+Q type LiF Fresnel ROM optimized type of Example 3. From FIGS. 20B and 20C, it can be seen that although the performance deteriorates to some extent on the long wavelength side, the performance is sufficiently high compared to the conventional retarder. From FIG. 20A, it can be seen that the phase difference is 90° in the wavelength range of λ=110 nm to 6000 nm. Further, from FIG. 20B, it can be seen that the ellipticity is close to 1 in the above wavelength band, and circularly polarized light is emitted.

<Example 4: H+Q type quartz Fresnel ROM with small multilayer film>
In Example 4, three small Fresnel ROMs with a multilayer film with a phase difference of 90° are arranged in the direction of the light beam, two on the incident side are regarded as Fresnel ROMs with a phase difference of 180° (λ/2 part: H part), and one on the output side is is regarded as a Fresnel ROM with a phase difference of 90° (λ/4 part: Q part), and a Fresnel ROM with a phase difference of 180° and a Fresnel ROM with a phase difference of 90° are 59.8° (θ2-θ1=74 .8°-15°) (Figure 21 and Table 5).

Since input and output light are coaxial in the original small-sized Fresnel ROM with multilayer film, the output light and the input light are on the same straight line in this embodiment as well. Note that the bonding method and antireflection film are the same as in Example 1. 22A to 22C show the optical characteristics of the H+Q type quartz Fresnel ROM with a small multilayer film of Example 4. From FIG. 22B, it can be seen that highly accurate circularly polarized light with an ellipticity close to 1 is emitted especially when λ=200 to 2000 nm. Furthermore, from FIG. 22A, it can be seen that the phase difference is 90° at the above wavelength. Among the examples, the invention product with the configuration of this example has the narrowest wavelength range, but it can be seen that the wavelength band is wider than the conventional product.

<Example 5: King type H+Q type quartz Fresnel ROM with optical film>
A king-type Fresnel ROM with a retardation of 90° and a 90° phase difference made of quartz with an ellipticity close to 1 and a total internal reflection angle of 72° is used. Three are arranged in the axial direction, and the two on the input side are considered as Fresnel ROMs with a phase difference of 180° (λ/2 part: H part), and the one on the output side is considered as a Fresnel ROM with a phase difference of 90° (λ/4 part: Q part). This is a structure in which a Fresnel ROM with a phase difference of 180° and a Fresnel ROM with a phase difference of 90° are rotated by 60° (θ2-θ1 = 75°-15°) with the incident light beam axis as the rotation axis (Fig. 23 and Table 6). ).

As in Example 1, each king type Fresnel ROM may or may not be bonded. When bonded, if the Fresnel ROM is not strained to cause optical anisotropy and there is no light absorption, the bonding is possible. I don't choose the method. Although it is preferable to apply an antireflection film to the surface through which light rays pass, it is not necessary to apply an antireflection film.

24A to 24C show the optical characteristics of the King type H+Q type quartz Fresnel ROM with optical film of Example 5. From FIGS. 24A to 24C, the output polarization characteristics are superior to any conventional Fresnel ROM, and also exhibit characteristics superior to any retarder. Since this structure is a structure that further enhances the performance of the retarder of the component, it can be seen that when the performance of the component is high, higher characteristics are exhibited.

In this example, as the retardation and ellipticity characteristics increase, the wavelength dependence of the polarization axis also decreases. Therefore, like a general λ/4 plate, the incident polarization is changed from linear polarization to elliptic polarization. It is possible to freely change the polarized light to circularly polarized light. Further, good polarization characteristics can be obtained even when the light beam is incident from the phase difference side of 90° (λ/4 portion side). However, better performance is obtained when the light beam is incident from the phase difference side of 180° (λ/2 part side).

1 Broadband circular polarizer H λ/2 part (H part)
Q λ/4 part (Q part)
FR1 First Fresnel ROM FR2 Second Fresnel ROM FR3 Third Fresnel ROM 10 First rhombohedron 11 First incidence end face 12 First output end face 13 First total reflection surface 14 Second total reflection surface 20 Second rhombohedron 21 Second incidence end face 22 Second output end face 23 Third total reflection surface 24 Fourth total reflection surface M Multilayer film θ Incident angle α Wedge angle α1 Total reflection angle HQP H+Q structure wave plate HWP λ/2 plate Ha λ/2 optical axis of plate QWP λ/ 4 plates Qa Optical axis I of λ/2 plate Incident ray Ia Incident polarization direction R Reflected ray E Outgoing ray X Optical axis

Claims (7)

a λ/2 part that provides a phase difference of 180 ° ±10% between p-waves and s-waves having a wavelength of at least 200 to 2000 nm due to total internal reflection;
a λ/4 part that provides a phase difference of 90 ° ±10% between p-waves and s-waves having a wavelength of at least 200 to 2000 nm by total reflection,
The incidence plane of the λ/4 part is rotated by 60 ° ±10% with respect to the incidence plane of the λ/2 part, with the incident light beam axis as the rotation axis,
The λ/2 part and the λ/4 part are made of Fresnel ROM,
A broadband circular polarizer, characterized in that it converts linearly polarized light having a wavelength of at least 200 to 2000 nm into circularly polarized light. 2. The Fresnel ROM constituting the λ/2 part and the λ/4 part is made of at least one material selected from the group consisting of quartz, CaF2, and LiF. Broadband circular polarizer. The broadband circular polarizer according to claim 1 or 2, wherein the Fresnel ROM constituting the λ/2 part and the λ/4 part has a multilayer film formed on at least one total reflection surface. . The λ/2 section has a first Fresnel ROM and a second Fresnel ROM that provide a phase difference of 90 ° ±10% between a p-wave and an s-wave having a wavelength of at least 200 to 2000 nm due to total reflection,
The λ/4 section has a third Fresnel ROM that provides a phase difference of 90 ° ±10% between p-waves and s-waves having wavelengths of at least 200 to 2000 nm due to total reflection,
The incidence surface of the first Fresnel ROM is arranged to coincide with the incidence surface of the second Fresnel ROM, with the incident light beam axis as the rotation axis,
The incident surface of the third Fresnel ROM is arranged to be rotated by 60 ° ±10% with respect to the incident surfaces of the first Fresnel ROM and the second Fresnel ROM, with the incident light beam axis as a rotation axis. The broadband circular polarizer according to any one of claims 1 to 3. 1. The Fresnel ROM provides a phase difference of 90 ° ±10% between a p-wave and an s-wave having a wavelength of at least 200 to 2000 nm by two total reflections. 4. Broadband circular polarizer according to any one of 4. The Fresnel ROM provides a phase difference of 90 ° ±10% between p-waves and s-waves having wavelengths of at least 200 to 2000 nm through three total reflections. 4. Broadband circular polarizer according to any one of 4. A measuring device comprising the broadband circular polarizer according to any one of claims 1 to 6.