Reflective polarizing light separating diffractive element and optical measuring device having the same

Abstract

This invention provides a reflective polarization-separating diffraction element usable over a wide wavelength range including the ultraviolet region, and an optical measuring apparatus incorporating the element. The reflective polarization-separating diffraction element comprises: a substrate (1); a reflective surface (2) formed on the surface of the substrate (1); and a grating-like structure (3) disposed on the reflective surface (2), exhibiting structural birefringence (Δn). * The grating structure (3) consists of four patterned grating structures (3A, 3B, 3C, and 3D), each with a different azimuth angle. Multiple patterned grating structures (3A, 3B, 3C, and 3D) are arranged along a predetermined direction on the reflecting surface 2 in a manner that causes the azimuth angle of the grating structure to change periodically.

Description

Technical Field

[0001] This invention relates to a reflective polarization-separating diffraction element, and also to an optical measuring device such as a dichroism measuring device and a birefringence measuring device incorporating the element. Background Technology

[0002] Conventional methods for measuring birefringence of an object include the orthogonal Nicol method, but these require the polarizer and analyzer to be rotated relative to the object. In contrast, the birefringence measuring apparatus described in Patent Document 1, which utilizes a transmission-type polarization-separating diffraction grating, has attracted attention because it does not require the rotating mechanism described above.

[0003] In a transmission-type polarization-separating diffraction grating, when right-circularly polarized light is incident on the incident surface of the grating, the right-circularly polarized light emerges from the opposite exit surface along the direction of the +1st order diffraction. Conversely, when the incident light is left-circularly polarized, the left-circularly polarized light emerges from the reflecting surface along the direction of the -1st order diffraction. In other words, the transmission-type polarization-separating diffraction grating has the characteristic of distributing right-circularly polarized light and left-circularly polarized light into the directions of ±1st order diffraction. (Patent Document 1) Figure 3 It plots the polarization state (ellipticity tan) of the incident light incident on a diffraction grating that separates polarized light from other light sources. -1 (b / a) Charts showing the intensities of ±1st order diffracted light under various variations.

[0004] When the incident light is elliptically polarized light (ellipticity = +15°, +30°) which is close to right circularly polarized light (ellipticity = +45°), the intensity of the +1st order diffracted light is greater than that of the -1st order diffracted light.

[0005] Conversely, when the incident light is elliptically polarized light (ellipticity = -15°, -30°) which is close to left circularly polarized light (ellipticity = -45°), the intensity of the -1st order diffracted light is greater than the intensity of the +1st order diffracted light.

[0006] When the incident light is linearly polarized (ellipticity = 0°), the intensities of the ±1st order diffracted light are equal. Linearly polarized light can be considered as polarized light obtained by superimposing phase-aligned left and right circularly polarized light. Therefore, it can be said that by using a transmission-type polarization-separating diffraction grating, the right circularly polarized component of the linearly polarized light is allocated to the direction of the +1st order diffracted light, and the left circularly polarized component is allocated to the direction of the -1st order diffracted light.

[0007] In the birefringence measuring apparatus of Patent Document 1, when right-circularly polarized light passes through the object being measured, the polarization state (ellipticity) of the right-circularly polarized light changes according to the birefringence of the object. When this transmitted light is incident on a polarization-separating diffraction grating, the intensities of the ±1st order diffracted light corresponding to the polarization state of the transmitted light are detected. Therefore, the polarization state (ellipticity) of the transmitted light can be determined based on the balance of the two intensity detection values, and as a result, the birefringence of the object being measured can be determined.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2016 / 031567 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] Regarding the development of polarization-separated diffraction gratings as described above, progress is being made in the development of transmission-type polarization-separated diffraction gratings, but reflection-type polarization-separated diffraction gratings have not been developed. One reason for this is that transmission-type polarization-separated diffraction gratings exhibit what is known as structural birefringence (Δn). * The periodic microgroove structure of the light transmitted through the structure has the property of imparting a phase difference to the light, thereby generating diffracted light. Therefore, even if it is desired to construct a reflective polarization-separated diffraction grating based on it, the specific mechanism by which the reflective properties will be imparted has not yet been determined.

[0013] On the other hand, the inventors believe that the reflective polarization-separating diffraction grating is extremely advantageous for miniaturizing the optical path configuration of various measuring devices that utilize the reflective polarization-separating diffraction grating, and is very helpful for the miniaturization of various measuring devices.

[0014] The purpose of this invention is to provide a miniaturized reflective polarization-separating diffraction grating suitable for various measuring devices, the specific mechanism of which is known, and an optical measuring device using the reflective polarization-separating diffraction grating.

[0015] Solution for solving the problem

[0016] That is, the reflective polarization-separating diffraction element involved in this invention comprises:

[0017] substrate;

[0018] A reflective surface, which is formed on the surface of the substrate; and

[0019] A grating-like structure, disposed on the reflective surface, exhibits structural birefringence (Δn). * ),

[0020] The grating-like structure is composed of multiple patterned grating-like structures, each with a different azimuth angle.

[0021] These multiple patterned grating structures are arranged along a predetermined direction on the reflective surface in a manner that causes the azimuth angle of the grating structure to change periodically.

[0022] According to this structure, multiple patterned grating-like structures are arranged in a certain regularity on the reflecting surface. Therefore, for incident light, firstly, a "phase impartation" is generated by the grating-like structures when passing through them, and simultaneously, "different directions of diffracted light" are generated due to the periodic changes in the pattern structure. This characteristic is denoted as "T(x)".

[0023] Next, regarding the transmitted light, the effect of the "complex refractive index" at the reflecting surface during reflection is considered. This characteristic is denoted as "R". M ".

[0024] Its reflected light passes through a grating structure with multiple patterns, and is therefore again affected by the aforementioned property "T(x)".

[0025] Thus, it can be said that the reflective polarization-separating diffraction element of the present invention has the characteristics expressed as "T(x)·R". M The inventors calculated based on this matrix model and proved that, using the reflective polarization-separating diffraction element of this invention, the right-circularly polarized component originally contained in the incident light is assigned to the direction of the +1st order diffraction light, and the left-circularly polarized component is assigned to the direction of the -1st order diffraction light. Therefore, if the reflective polarization-separating diffraction element of this invention is used, the polarization state of the incident light can be determined based on the intensity of each reflected light that becomes ±1st order diffraction light.

[0026] Furthermore, if a reflective polarization-separating diffraction element with this characteristic is used, it is possible to make measuring devices such as circular dichroism measuring devices and birefringence measuring devices very compact.

[0027] Furthermore, preferably, the surface of the substrate is formed in a stepped shape along the predetermined direction.

[0028] Matching the stepped shape, the multiple patterned grating structures are also arranged in a stepped manner.

[0029] The stepped shape of the substrate surface repeats in a manner that matches the periodic changes in the azimuth angle structure.

[0030] In addition, it is preferable that the reflective surface and the grating structure are formed on both surfaces of the substrate, so that the reflective polarization-separating diffraction element can be used in reverse.

[0031] The reflective polarization-separating diffraction element involved in this invention comprises:

[0032] Substrate; and

[0033] A grating-like uneven structure is formed on the surface of the substrate, exhibiting structural birefringence (Δn). * ),

[0034] The concave and convex surfaces of the grating-shaped concave and convex structure are reflective surfaces.

[0035] The grating-shaped concave-convex structure is composed of multiple patterned grating-shaped concave-convex structures, each with a different azimuth angle.

[0036] These multiple patterned grating-like convex and concave structures are arranged along a predetermined direction on the surface of the substrate in such a way that the azimuth angle of the grating-like convex and concave structures changes periodically.

[0037] According to this structure, multiple patterned grating-like concave-convex structures are arranged in a certain regularity, and their concave-convex surfaces also become reflecting surfaces. Therefore, when incident light is reflected at the concave-convex surfaces, the effect of the "complex refractive index" at the reflecting surfaces occurs. This characteristic is denoted as "R". M ".

[0038] Furthermore, for its reflected light, a "phase impartation" is generated by the grating-like uneven structure of multiple patterns, and the "different directions of diffracted light" are produced due to the periodic changes in the structure of the patterns. This characteristic is denoted as "T(x)".

[0039] Thus, it can be said that the reflective polarization-separating diffraction element of the present invention has the characteristic expressed as "R". M The inventors calculated based on this matrix model and proved that, using the reflective polarization-separating diffraction element of this invention, the right-circularly polarized component originally contained in the incident light is assigned to the direction of the +1st order diffraction light, and the left-circularly polarized component is assigned to the direction of the -1st order diffraction light. Therefore, if the reflective polarization-separating diffraction element of this invention is used, the polarization state of the incident light can be determined based on the intensity of each reflected light that becomes ±1st order diffraction light.

[0040] Furthermore, if a reflective polarization-separating diffraction element with this characteristic is used, it is possible to make measuring devices such as circular dichroism measuring devices and birefringence measuring devices very compact.

[0041] Furthermore, preferably, the surface of the substrate is formed in a stepped shape along the predetermined direction.

[0042] In accordance with the stepped shape, the multiple patterned grating-like convex and concave structures are also formed in a stepped shape, and the stepped shape of the substrate surface repeats in accordance with the periodic changes in the azimuth angle structure.

[0043] In addition, preferably, the grating-shaped uneven structure and the reflective surface are formed on both surfaces of the substrate, thereby enabling the reflective polarization-separating diffraction element to be used in reverse.

[0044] The optical measuring device involved in this invention comprises:

[0045] Any of the above-mentioned reflective polarization-separating diffraction elements;

[0046] An incident optical unit that directs the measurement light toward the reflective polarization-separating diffraction element; and

[0047] The detection optical unit detects light in at least one specific direction from the reflected light after it has been diffracted by the reflective polarization-separating diffraction element.

[0048] The optical measuring device measures the optical properties of the object being measured that is placed in the optical path of the incident optical unit or the detection optical unit.

[0049] As described above, using a reflective polarization-separating diffraction element facilitates the miniaturization of the optical path layout, enabling highly compact optical measurement devices such as circular dichroism measuring apparatuses and birefringence measuring apparatuses. In particular, in a circular dichroism measuring apparatus, the left and right circularly polarized components of the incident light incident on the reflective polarization-separating diffraction element are distributed to the ±1st order diffraction directions, thus allowing for the "simultaneous" detection of two circularly polarized components. Consequently, the measurement time for circular dichroism can be significantly reduced. Attached Figure Description

[0050] Figure 1 (A) is an overall structural diagram of the reflective polarization-separating diffraction grating according to the first embodiment, and (B) is a diagram of the reflective polarization-separating diffraction grating viewed from the front.

[0051] Figure 2(A) to (F) are diagrams showing the polarization-separated diffraction grating viewed from the front, and are diagrams showing the deformation of the polarization-separated diffraction grating with a grating-like structure formed on one side.

[0052] Figure 3 Figures (A) to (F) are diagrams of a polarization-separated diffraction grating viewed from the front, showing a deformation of the polarization-separated diffraction grating with grating-like structures formed on both sides.

[0053] Figure 4 This is a front view and a top view of a portion thereof of the reflective polarization-separating diffraction grating with a blazed structure according to the second embodiment.

[0054] Figure 5 This is a three-dimensional diagram showing a liquid crystal-type birefringent structure that can be used in a reflective polarization-separating diffraction element involved in the reference method.

[0055] Figure 6 (A) is an overall structural diagram of the optical measuring device according to the fourth embodiment, and (B) is an overall structural diagram of the optical measuring device according to the fifth embodiment.

[0056] Figure 7 (A) is an overall structural diagram of the optical measuring device according to the sixth embodiment, and (B) is an overall structural diagram of the optical measuring device according to the seventh embodiment.

[0057] Figure 8 (A) is an overall structural diagram of the optical measuring device according to the eighth embodiment, and (B) is an overall structural diagram of the optical measuring device according to the ninth embodiment.

[0058] Figure 9 This is an overall structural diagram of the optical measuring device according to the tenth embodiment.

[0059] Figure 10 (A) is an overall structural diagram of the circular dichroism measuring device according to the eleventh embodiment, and (B) is a diagram showing the signal processing unit that calculates the CD signal based on its detection signal.

[0060] Figure 11 The graph showing the characteristics of a polarization-separated diffraction grating is a graph that indicates that the detection sensitivity of the polarization state of a reflective polarization-separated diffraction grating is higher than that of a transmissive polarization-separated diffraction grating. Detailed Implementation

[0061] The structure of the reflective polarization-separating diffraction element and the optical measuring device of the present invention will be described below using the accompanying drawings. Figure 1(A) is an overall structural diagram of the reflective polarization separation diffraction grating according to the first embodiment. Figure 1 (B) is a diagram of the reflective polarization-separated diffraction grating viewed from the front.

[0062] A reflective polarization-separating diffraction grating comprises: a substrate 1 made of a metallic material; a reflective surface 2 formed on the surface of the substrate 1; and a grating-like structure 3 disposed on the reflective surface 2, exhibiting structural birefringence (Δn). * ).

[0063] The grating structure 3 is composed of multiple patterned grating structures (3A, 3B, 3C, and 3D) with different azimuth angles (0°, +45°, +90°, and +135°). These multiple patterned grating structures (3A, 3B, 3C, and 3D) are arranged along the X-axis on the reflecting surface 2 in a way that the azimuth angles of the grating structures change periodically. The periodicity of the azimuth angles along the X-axis can be either discontinuous as shown in the figure, or continuous. For example, a continuous periodicity using a trigonometric function can be achieved using a liquid crystal or similar device.

[0064] In this embodiment, the grating structure of each pattern is composed of parallel slots. That is, multiple grating elements 4 of the light-transmitting dielectric are arranged in parallel with equal intervals, for example, 100 nm. Preferably, the spacing of the grating elements 4 is less than half the wavelength of the measurement light (from ultraviolet to infrared). Furthermore, the width of the grating structure of each pattern in the X-axis direction is set to 1 μm, but this width can also be set to be in the range of approximately 0.5 μm to 2 μm. The height h of the grating structure of the present invention is here equivalent to... Figure 1 The depth and height h of the groove shown in (B) are 100 nm or more, preferably 300 nm or more. The height h is preferably more than half the wavelength of the measuring light used.

[0065] In this specification, the direction of the long side of the grating member 4 is referred to as the "azimuth angle". Based on the azimuth angle (0°) of the grating structure of pattern 3A, the azimuth angle of pattern 3B is +45°, the azimuth angle of pattern 3C is +90°, and the azimuth angle of pattern 3D is +135°. Furthermore, regarding the grating member 4, all patterns have the same shape and are arranged in the same way; only the azimuth angle differs according to the pattern.

[0066] exist Figure 1The example shown is a structural period (Λ) of a grating structure with a total of four patterns in the order of 0°→+45°→+90°→+135°→0°. However, it can also be set to three patterns, or even five or more patterns. In addition, the structural period Λ can be arranged in a way that repeats.

[0067] Figure 2 Figures (A) to (F) are diagrams showing the polarization-separating diffraction grating viewed from the front, illustrating a deformation of the polarization-separating diffraction grating with a grating-like structure formed on one side. The component forming the reflecting surface 2 is marked with a solid shading line.

[0068] Figure 2 (A) is: a metal film 2a is formed on the surface of the substrate 1a, and grating components 4 are arranged on the metal film 2a. Figure 2 (B) is: a concave-convex structure 4a made of a light-transmitting dielectric is formed on the reflective surface 2 of the metal substrate 1. Figure 2 (C) means that a grating-like structure is formed by alternately arranging two light-transmitting dielectrics (4b, 4c) with different refractive indices on the substrate 1, rather than having a concave-convex structure. Figure 2 (D) is: A surface with a surface that is similar to... Figure 2 The same grating-like structure as (C) is formed thereon, and a layer of one of the light-transmitting dielectrics 4d is formed thereon. That is, a grating-like structure is provided inside the diffraction grating.

[0069] Figure 2 (E) means that the surface of the metal substrate 11 is formed in an uneven shape, and the uneven surface becomes a reflective surface. That is to say, Figure 2 The (E) reflective polarization-separating diffraction grating comprises a substrate 11 and a grating-shaped uneven structure 12 formed on the surface of the substrate 11, exhibiting structural birefringence (Δn). * The grating-shaped uneven structure 12 has a reflective surface. The grating-shaped uneven structure 12 is composed of multiple patterns of grating-shaped uneven structures with different azimuth angles. These multiple patterns of grating-shaped uneven structures are arranged along a predetermined direction on the surface of the substrate 11 in a manner that causes the azimuth angle of the grating-shaped uneven structure to change periodically. Furthermore, in Figure 2 In (E), a substrate obtained by laminating a translucent dielectric to form an uneven surface can also be used as substrate 11. Figure 2 (F) is: the surface of substrate 11a and Figure 2 (E) also has an uneven topography, but a metal film 12a is formed on the uneven surface.

[0070] Figure 3Figures (A) to (F) are diagrams showing the polarization-separating diffraction grating viewed from the front, illustrating a deformation of the polarization-separating diffraction grating with grating-like structures formed on both sides. For comparison with... Figure 2 The common markings of (A) to (F) are marked with the same markings.

[0071] The optical characteristics of the reflective polarization-separated diffraction grating of this embodiment will now be explained using numerical formulas.

[0072] <Previous transmission-type polarization-splitting diffraction elements>

[0073] First, the previous transmission-type polarization-separating diffraction elements are described. Transmission-type polarization-separating diffraction elements consist of structural birefringence (Δn). * It is composed of a grating structure with a structural height h and a structural period (Λ), and has the following characteristics: when any polarized light is incident, it is separated into right circularly polarized light and left circularly polarized light, so that the right circularly polarized light and the left circularly polarized light are diffracted to the +1 order and -1 order directions, respectively.

[0074] The 2×2 matrix (Jones matrix) exhibiting the characteristics of conventional transmission-type polarization-separating diffraction elements is written as follows.

[0075] [Number 1]

[0076]

[0077] Here,

[0078] Equation (1) means that the fast axis component of the incident light is given a phase of +ζ, the slow axis component is given a phase of -ζ, and the azimuth angle of the fast axis of the element varies with respect to the optical axis of the incident light in the manner of -kx / 2 (rotating 180° with x=Λ).

[0079] Here, when considering right-handed circularly polarized light as the incident light E in When incident on the element, T(x)·E in It will become like this.

[0080] [Number 2]

[0081]

[0082] Conversely, when considering left-handed circularly polarized light incident on the element, T(x)·E in It will become like this.

[0083] [Number 3]

[0084]

[0085] Here, any polarized light can be represented as the superposition of left and right circularly polarized light. Therefore, when the amplitude of the right circularly polarized light is written as 'a' and the amplitude of the left circularly polarized light is written as 'b', it becomes the following equation. Here, 'a' and 'b' are defined to satisfy 'a'. 2 +b 2 =1.

[0086] [Number 4]

[0087]

[0088] Therefore, when using equations (2) and (3) as references and equation (4) to calculate T(x)·E in At that time, it becomes like this.

[0089] [Number 5]

[0090]

[0091] Here, the m-th order diffracted light vector D m It can be represented by the following formula.

[0092] [Number 6]

[0093]

[0094] When equation (6) is substituted into equation (5) for calculation, the vector of the diffracted light in the case of arbitrary polarized light incident on the polarization-separated diffraction element is obtained as follows.

[0095] [Number 7]

[0096] D0=cosζ (7)

[0097]

[0098]

[0099] Here, according to equation (8), the +1st order diffracted light is right-handed circularly polarized light with amplitude b·sinζ, and according to equation (9), the -1st order diffracted light is left-handed circularly polarized light with amplitude a·sinζ. The diffracted light intensity η m Provided as η m =|D m | 2 / |E in | 2 According to equation (4), |E in | 2 =1, therefore the following can be obtained.

[0100] [Number 8]

[0101]

[0102]

[0103]

[0104] Therefore, as long as η is measured ±1 The intensity of the incident light can be used to determine the ellipticity (e = b / a) or Stokes parameter of the incident light according to the relationship in equation (13). <s3>.

[0105] [Number 9]

[0106]

[0107] As will be described later Figure 6 (B) Figure 10 As shown, rotating optical elements such as diffraction gratings at a specific frequency is also an effective means to improve the S / N ratio. The transfer matrix T(x) when the optical element is rotated becomes as follows. T(x) in equation (14) is obtained by multiplying the T(x) in equation (1) above by the rotation matrix R(θ) of angle θ from the front and by the rotation matrix R(-θ) of angle -θ from the back.

[0108] [Number 10]

[0109]

[0110] First, when considering right-handed circularly polarized light incident on the element, T(x)·E in It will become like this.

[0111] [Number 11]

[0112]

[0113] Similarly, when considering left-handed circularly polarized light incident on the element, T(x)·E in It will become like this.

[0114] [Number 12]

[0115]

[0116] Here, similarly to equation (4), any polarized light is written as a superposition of left and right circularly polarized light (let the amplitude of the right circularly polarized light be a, and the amplitude of the left circularly polarized light be b: a 2 +b 2 =1: The following formula is used to calculate T(x)·E in At that time, it becomes like this.

[0117] [Number 13]

[0118]

[0119] [Number 14]

[0120]

[0121] As in equation (6), the m-th order diffraction vector D m It can be represented by the following formula.

[0122] [Number 15]

[0123]

[0124] Therefore, the vector of the diffracted light when any polarized light is incident on a rotating polarization-separating diffraction element can be calculated as follows.

[0125] [Number 16]

[0126] D O =cosζ (18)

[0127]

[0128]

[0129] Here, according to equation (19), the +1st order diffracted light has an amplitude of b·sinζ·e. i2θ According to equation (20), the right-hand circularly polarized light has an amplitude of a·sinζ·e. -i2θ Left-handed circularly polarized light. Diffracted light intensity η m Provided as η m =|D m | 2 / |E in | 2 According to equation (4), |E in | 2 =1, and also, due to the following formula,

[0130] [Number 17]

[0131]

[0132] Therefore, as in equations (21) to (23), we obtain the same results as in equations (10) to (12).

[0133] Find the following.

[0134] [Number 18]

[0135]

[0136]

[0137]

[0138] Therefore, as long as η is measured ±1 The intensity of the incident light can be used to determine the ellipticity (e = b / a) or Stokes parameter of the incident light according to the relationship in equation (13). <s3>.

[0139] <Reflective polarization-splitting diffraction element>

[0140] Based on the above considerations regarding the characteristics of transmission-type diffraction gratings (polarization-separating diffraction elements), a polarization-separating diffraction element that functions as a reflective configuration can be arranged as follows. The function of the reflective polarization-separating diffraction element will be explained below, including its differences from transmission-type elements.

[0141] (A) The case where a reflective metal surface is placed directly below the structure of a transparent dielectric. :

[0142] The following is an example of how to represent a metal surface incident at 0° (perpendicularly) (e.g., Figure 2 The transfer matrix of light reflection of the metal film of the polarization separation diffraction grating (A) 2a).

[0143] [Number 19]

[0144]

[0145] Here, These are the components of the complex refractive index of the transparent dielectric (grating component 4) (in the structurally parallel and perpendicular directions). It is the complex refractive index of a metal.

[0146] m-order diffraction vector D' m It can be represented by the following formula.

[0147] [Number 20]

[0148]

[0149] Here, when considering the propagation of light in (A) "a polarization-separating diffraction element with a reflective metallic surface disposed directly below a structure of transparent dielectric", the transmission of light through the element is T. in (x)”→"Metal reflection R M → Component transmission T out (x)”, therefore T'(x) is provided by the following formula.

[0150] [Number 21]

[0151] T'(x)=T(x)·R M ·T(x) (26)

[0152] Therefore, when any polarized light is treated as a superposition of left and right circularly polarized light in the same way as in equation (4) (let the amplitude of the right circularly polarized light be a, and the amplitude of the left circularly polarized light be b: a) 2 +b 2 =1) to find T'(x)·E in At that time, it becomes like this.

[0153] [Number 22]

[0154]

[0155] When equation (27) is substituted into equation (25) to obtain their respective D' m η' m First, the 0th order light is as follows.

[0156] [Number 23]

[0157]

[0158] It can be confirmed that the polarization state of the 0th-order diffracted light is the same as that of the incident light. Furthermore, the diffraction efficiency of the 0th-order light is given by the following formula.

[0159] [Number 24]

[0160]

[0161] Next, the vector of the +1 order diffracted light is as follows.

[0162] [Number 25]

[0163]

[0164] It can be confirmed that the polarization state of the +1st order diffracted light is a superposition of left-circularly polarized light and right-circularly polarized light. Furthermore, the diffraction efficiency becomes as follows.

[0165] [Number 26]

[0166]

[0167] In addition, the vector of the -1 order light is as follows.

[0168] [Number 27]

[0169]

[0170] It can be confirmed that the polarization state of the -1st order diffracted light is a superposition of right-circularly polarized light and left-circularly polarized light. Furthermore, the diffraction efficiency becomes as follows.

[0171] [Number 28]

[0172]

[0173] Therefore, it is believed that as long as η is measured... ±1 The intensity can be determined by the following equation (34) to find the ellipticity (e = b / a) or Stokes parameter of the incident light, similar to the case of a transmission element. <s3>This calculation requires the characteristic values ​​of r, which are the metal layer and the phase-assigning layer. TE r TM 、ζ.

[0174] [Number 29]

[0175]

[0176] (B) The case where the structure of the grating-like structure is formed of metal. :

[0177] Consider again the case where the structure of the grating is formed of metal (e.g., Figure 2 The grating-like uneven structure 12 of (E) is observed. When considering the propagation of light in the element of (B), the phase imparted by the unevenness is "metallic reflection", and thus T(x) is provided by the following formula.

[0178] [Number 30]

[0179] T(x)=R M ·T(x) (35)

[0180] Therefore, T(x)·E can be calculated in the same way as in (A) for any polarized light incident on the element. in When, it becomes as follows.

[0181] [Number 31]

[0182]

[0183] When using equation (36) to calculate their respective D' m η' m At that time, the 0th order light is as follows.

[0184] [Number 32]

[0185]

[0186] It can be confirmed that the polarization state of the 0th-order diffracted light is the same as that of the incident light. Furthermore, the diffraction efficiency of the 0th-order light is expressed by the following formula.

[0187] [Number 33]

[0188]

[0189] Next, the vector of the +1 order diffracted light is as follows.

[0190] [Number 34]

[0191]

[0192] It can be confirmed that the polarization state of the +1st order diffracted light is right-circularly polarized. Furthermore, the diffraction efficiency becomes as follows.

[0193] [Number 35]

[0194]

[0195] The vector of the -1st order light is as follows.

[0196] [Number 36]

[0197]

[0198] It can be confirmed that the polarization state of the -1st order diffracted light is left-circularly polarized. Furthermore, the diffraction efficiency becomes as follows.

[0199] [Number 37]

[0200]

[0201] Therefore, as long as η is measured ±1 Given the intensity, the case of element (B) is similar to that of the transmission element; the ellipticity (e = b / a) or Stokes parameter of the incident light can be determined according to the relationship in equation (13). <s3>.

[0202] Based on the explanation using equations (1) to (42) above,

[0203] (1) When polarized light is incident on a polarization-splitting diffraction element made of a transparent dielectric, the transmitted diffracted light is separated into right-hand circularly polarized light (+1st order diffracted light) and left-hand circularly polarized light (-1st order diffracted light). Therefore, by measuring the intensity of the ±1st order diffracted light, the ellipticity of the incident light and the Stokes parameter representing the difference in amplitude between the left and right circularly polarized light can be determined. <s3>This characteristic remains unchanged even when the element is rotated. This means that as long as phase-locked detection can be performed in sync with the element's rotation, a further improvement in the signal-to-noise ratio (S / N) can be expected, but the types of information obtained through the element's rotation will not increase. In this sense, the same effect can be achieved by simply cutting off the incident light without rotating the element.

[0204] (2) When a device with a fine structure of a transparent dielectric on a metal thin film, or a device with a fine structure directly fabricated on a metal thin film, is used as a reflective device, the ellipticity and Stokes parameter of the incident light can be determined by measuring the intensity of the ±1st order diffracted light, just as in the case of the transmissive device described in (1). <s3>.

[0205] (3) In the case of the reflective element described in (2) above, as long as phase-locked detection can be performed in sync with the rotation of the element, a further improvement in the S / N ratio can be expected. The same effect can be obtained by simply cutting off the incident light without rotating the element.

[0206] Figure 4 This is an overall structural diagram of the reflective polarization-separating diffraction grating with a blazed structure according to the second embodiment, including its front view and partial top view.

[0207] The polarization-separating diffraction grating in this embodiment and Figure 1 The polarization-separated diffraction grating also includes: a substrate 13 made of a metallic material; a reflective surface formed on the surface of the substrate 13; and four patterned grating-like structures (3A, 3B, 3C, and 3D) disposed on the reflective surface, exhibiting structural birefringence (Δn). * In other words, such as Figure 4 As shown in the top view above, on the grating surface of this polarization-separating diffraction grating, and... Figure 1 Similarly, the polarization-separated diffraction grating has four grating-like structures (3A, 3B, 3C and 3D) arranged periodically along the X-axis.

[0208] On the other hand, with Figure 1 The difference between the polarization-separated diffraction grating and the polarization-separated diffraction grating is that, Figure 4 As shown in the lower front view, the surface of the substrate 13 is formed into a stepped shape with four steps along the X-axis, and each step is provided with a grating structure (3A, 3B, 3C or 3D) with a different pattern. Thus, in this embodiment, the four patterned grating structures (3A, 3B, 3C or 3D) are matched with the stepped structure of the surface of the substrate 13 to form a stepped shape.

[0209] Furthermore, the polarization-separating diffraction gratings, with each unit consisting of a stepped structure formed by four steps, are repeatedly arranged in a planar manner along the X-axis, thereby forming a reflective polarization-separating diffraction grating with a blazed structure. In other words, the reflective polarization-separating diffraction grating of this embodiment is formed in such a way that the stepped structure on the surface of the substrate 13 is repeated in a periodic manner to match the structural changes in the azimuth angles of the four patterned grating structures (3A, 3B, 3C, and 3D).

[0210] Furthermore, the height h (depth of the groove) of the grating structure is the same for all patterns of the grating structure (3A, 3B, 3C and 3D).

[0211] The reflective surface 2 in this embodiment is not limited to... Figure 1 The structure enables applications such as Figure 2 The various structures shown. Additionally, in Figure 4 In this configuration, the incident side surface of each substrate 13 is formed in a stepped shape, and the bottom surface of the substrate 13 is a flat plane. However, this is not a limitation; stepped grating structures (3A, 3B, 3C, and 3D) can also be used, such as... Figure 3 As shown, it is formed on both sides of the substrate 13.

[0212] According to this embodiment, the reflective polarization-separating diffraction grating, with its blazed structure, can weaken the 0th-order diffraction light and enhance the intensity of diffraction light of desired diffraction orders such as +1 (or -1). Therefore, if a measuring device equipped with such a reflective polarization-separating diffraction grating is used, weaker signals can be detected, and the signal-to-noise ratio (SN ratio) of the detected signal can be improved.

[0213] Figure 5 This is a three-dimensional diagram showing a liquid crystal-type birefringent structure that can be used in a reflective polarization-separating diffraction element involved in the reference method.

[0214] Although not shown in the figure, the reference-type reflective polarization-separating diffraction element includes a substrate, a reflective surface formed on the surface of the substrate, and a plurality of liquid crystal birefringent structures 51 arranged in a predetermined direction on the reflective surface.

[0215] The liquid crystal birefringent structure 51 is as follows Figure 5 The plate-like structure shown has a reflective surface formed on either its surface or its back surface (XY plane). This birefringent structure 51 has a large number of liquid crystal molecules (also called birefringent regions) disposed within a segment. These liquid crystal molecules are arranged along the X-axis in such a manner that the direction of the birefringence optical axis of each liquid crystal molecule continuously varies 360 degrees within a plane parallel to the reflective surface. Here, the direction of the optical axis represents the direction of the molecular orientation of the liquid crystal molecules, and the periodic change in the molecular orientation of the birefringent structure 51 is as follows: Figure 1 The structural period (Λ) of such a grating structure is twice that of the structure.

[0216] The symbols in the diagram indicate the orientation of the optical axes of the liquid crystal molecules. When described along the X-axis, in the region at the left end of the X-axis, the optical axes of the liquid crystal molecules within the segment point towards the positive direction of the Y-axis (upward); in the center of the X-axis, the optical axes point towards the negative direction of the Y-axis (downward); and in the region at the right end of the X-axis, the optical axes again point towards the positive direction of the Y-axis (upward). In other words, the liquid crystal molecules at both ends of the segment point in the same direction.

[0217] Furthermore, the optical axes of the liquid crystal molecules distributed in the YZ plane perpendicular to the X-axis all face the same direction.

[0218] Furthermore, in the region from the left end to the center of the X-axis, the direction of the optical axis continuously changes from the upward direction to the downward direction. Then, in the region from the center to the right end of the X-axis, the direction of the optical axis continuously changes from the downward direction to the upward direction. The thickness d of the birefringent structure 51 ranges from a few micrometers to tens of micrometers.

[0219] As a characteristic of the 51 monomer, a birefringent structure, such as Figure 5 In this way, the incident light from the back passes through the birefringent structure 51, and the right circularly polarized light contained in the incident light becomes +1 order diffraction light, and the left circularly polarized light contained in the incident light becomes -1 order diffraction light, which has good polarization separation characteristics.

[0220] Thus, in the reference-mode reflective polarization-separating diffraction element, multiple birefringent structures 51 are arranged on the reflective surface. A large number of liquid crystal molecules (birefringent portions) are arranged within one segment of each birefringent structure 51. The direction of the optical axis of the birefringence of the liquid crystal molecules changes continuously along the X-axis. Therefore, if a measuring device using a reference-mode reflective polarization-separating diffraction element is employed, its SN ratio is significantly improved. This is because the smaller the difference in the direction of the optical axis of the birefringence of adjacent liquid crystal elements, the greater the improvement in the polarization separation characteristics of the diffraction element.

[0221] Furthermore, it is not limited to polarization-separating diffraction gratings that utilize molecular orientation. The same effect can be obtained even with polarization-separating diffraction elements that use birefringent regions other than liquid crystal molecules and are configured in a way that continuously changes the direction of their birefringence optical axes.

[0222] Next, the structure of an optical measuring apparatus (e.g., a circular dichroism measuring apparatus, a birefringence measuring apparatus) equipped with a reflective polarization-separating diffraction grating according to this embodiment will be described. Figure 6 (A) is an overall structural diagram of the optical measuring device according to the fourth embodiment.

[0223] like Figure 6 As shown in (A), the optical measurement apparatus 10 includes a reflective polarization-separating diffraction grating 20, an incident optical unit 30, and a detection optical unit 40. The incident optical unit 30 is an optical system including a pre-splitter 31, which selects light of a predetermined wavelength from the measurement light from a light source (e.g., a deuterium lamp emitting ultraviolet light in the wavelength range of 185 nm to 360 nm), and incident this light as the phase-aligned measurement light of the predetermined wavelength onto the reflective polarization-separating diffraction grating 20. When the object to be measured is positioned in the optical path between the pre-splitter 31 and the polarization-separating diffraction grating 20, the measurement light from the pre-splitter 31, after its polarization state changes due to the optical characteristics of the object to be measured (circular dichroism (CD), linear dichroism (LD), circularly polarized birefringence (CB), linearly polarized birefringence (LB), etc.), is incident onto the reflective polarization-separating diffraction grating 20.

[0224] The detection optical unit 40 includes two detectors 41, which are positioned relative to the ±1st order diffracted light direction with respect to the reflective polarization-separating diffraction grating 20, and detect the ±1st order diffracted light. When the object to be measured is positioned in the optical path of the detection optical unit 40, the ±1st order diffracted light is detected after its polarization state changes due to the birefringence of the object to be measured.

[0225] Figure 6 (B) is an overall structural diagram of the optical measuring apparatus according to the fifth embodiment. By providing a unit that rotates or vibrates the reflective polarization-separating diffraction grating 20, the detection signal changes periodically. Therefore, even minute signals such as circular dichroism can be measured with high sensitivity using a detection unit such as a lock-in amplifier.

[0226] in addition, Figure 6 The (B) type reflective polarization-separating diffraction grating 20 has a grating-like structure formed on one side, but it can also be replaced with a reflective polarization-separating diffraction grating with grating-like structures formed on both sides. For example, the surface of the rotating polarization-separating diffraction grating can be oriented towards... Figure 6 In the optical measurement apparatus of (B), left-circularly polarized light is incident on the detector 41 when the detector 41 is positioned facing the +1st order diffraction direction, and right-circularly polarized light is incident on the detector 41 when the back of the rotating polarization-separating diffraction grating faces the front of the detector 41. By using the two surfaces of the reflective polarization-separating diffraction grating separately in this way, optical properties such as circular dichroism can be measured with high sensitivity and high speed. Furthermore, when measuring linear dichroism, P-polarized light can also be incident on the detector 41 from the surface of the polarization-separating diffraction grating, and S-polarized light can be incident on the detector 41 from the back of the polarization-separating diffraction grating.

[0227] Figure 7 (A) is an overall structural diagram of the optical measurement apparatus according to the sixth embodiment. A chopper 32 is arranged in the optical path between the pre-splitter 31 and the reflective polarization-separating diffraction grating 20. The measurement light is repeatedly irradiated and cut off by the operation of the chopper 32. In addition, Figure 7 (B) is an overall structural diagram of the optical measurement apparatus according to the seventh embodiment. A sample holding unit 33, which can be removed and placed, is arranged in the optical path between the pre-beam splitter 31 and the reflective polarization-separating diffraction grating 20. The sample and background measurements are repeatedly performed by the operation of the sample holding unit 33. Through these units, highly sensitive measurements of optical properties can be performed.

[0228] Furthermore, regarding the high-sensitivity measurement of optical properties, through methods such as... Figure 8 As in the eighth embodiment of (A), the optical measurement apparatus arranging the polarizer 34 orthogonally to the central axis of the optical path of the measurement light and providing a unit that allows the polarizer 34 to rotate around the central axis of the optical path enables high-sensitivity measurement. Furthermore, by means of... Figure 8 The optical measuring apparatus according to the ninth embodiment of (B) can also perform high-sensitivity measurements by arranging a PEM (photoelastic modulator) 35 in the optical path of the measuring light.

[0229] Figure 9 This is an overall structural diagram of the optical measuring device according to the tenth embodiment. Figure 9 The optical measurement apparatus includes a reflective polarization-separating diffraction grating 20, an incident optical unit 30 including a light source 36, and a detection optical unit 40 including an array detector 42 such as a CCD. Measurement light, including measurement light within a specified wavelength range, is emitted from the light source 36. Figures 6 to 8 Similarly, the object to be measured can be placed either in the optical path of the incident optical unit 30 or in the optical path between the reflective polarization-separating diffraction grating 20 and the array detector 42. With this optical measurement device, optical properties of multiple wavelengths (circular dichroism (CD), linear dichroism (LD), circularly polarized birefringence (CB), linearly polarized birefringence (LB), etc.) can be measured simultaneously.

[0230] Figure 10 (A) is an overall structural diagram of the circular dichroism measuring apparatus according to the eleventh embodiment. Figure 10 (B) is a diagram showing the signal processing unit that calculates the CD signal based on its detection signal. Label 43 indicates the aperture. Label 20 indicates a polarization-separating diffraction grating with a grating-like structure formed on both sides. In this circular dichroism measuring apparatus, by rotating or vibrating the polarization-separating diffraction grating 20, right-hand circularly polarized light and left-hand circularly polarized light are alternately irradiated toward the object being measured. Therefore, as... Figure 10 As shown in (B), it is possible to acquire detection signals with periodic variations in both sample-containing and sample-free conditions, and to perform high-sensitivity measurement of circular dichroism using waveform forming unit 61 (BPF, etc.) and lock-in amplifier 62.

[0231] Figure 11 A characteristic graph of a polarization-separated diffraction grating is shown, illustrating that the detection sensitivity of the polarization state at a specific ellipticity angle of the reflective polarization-separated diffraction grating is higher than that of the transmission polarization-separated diffraction grating at the same ellipticity angle. The horizontal axis of the graph represents the ellipticity angle, and the vertical axis represents the intensity of the diffracted light of the +1st order. In other words, by using the reflective polarization-separated diffraction grating of this embodiment, the distribution of diffracted light intensity relative to the ellipticity angle can be manipulated when the incident angle is other than 0 degrees by sequentially selecting the material, thickness, and coating method such as tilted evaporation of the reflective film (including the reflective surface), thereby achieving an improved sensitivity (S / N) for a specific ellipticity angle.

[0232] The optical measurement apparatus according to this embodiment includes a reflective polarization-separating diffraction grating, which allows for optical path refraction, resulting in a compact configuration. Furthermore, because it is a reflective polarization-separating diffraction grating, compared to a transmissive one, it suppresses light loss due to absorption in the ultraviolet region, enabling the use of measurement light across a wide wavelength range from the ultraviolet to the infrared region for optical measurements such as circular dichroism and polarized birefringence. Additionally, the reflective polarization-separating diffraction grating distributes the left and right circularly polarized light contained in the incident light in different directions, thus enabling high-speed optical measurements.

[0233] exist Figure 11 In the example, coating the outermost surface with silver improves the linearity (linearity) of the diffraction intensity relative to the ellipticity angle. Therefore,

[0234] 1) It enables the slope of the diffraction intensity relative to the ellipticity angle (-45° and +45°) of the incident circularly polarized light to be larger than the typical (transmission type) small slope. Therefore, the rate of change of diffraction intensity near -45° and +45° is greater, thus effectively improving the sensitivity of circularly polarized light detection. Furthermore,

[0235] 2) It enables the slope to be more fixed across the entire angular range, independent of the ellipticity angle. Thus, regardless of the polarization state (ellipticity angle) of the incident light, it is possible to measure with a fixed sensitivity throughout the entire region.

[0236] Explanation of Figures and Tables

[0237] 1: Substrate; 2: Reflective surface; 3: Grating-like structure; 11, 13: Substrate; 12: Grating-like concave-convex structure; 10: Optical measuring device; 20: Polarization separation diffraction grating; 30: Incident optical unit; 40: Detection optical unit; 51: Birefringent structure.

Claims

1. A reflective polarization-separating diffraction element, characterized in that, have: substrate; A reflective surface, which is formed on the surface of the substrate; and A grating-like structure, disposed on the reflective surface, exhibits structural birefringence Δn * , The grating-like structure is composed of multiple patterned grating-like structures, each with a different azimuth angle. These multiple patterned grating-like structures are arranged along a predetermined direction on the reflective surface in a manner that causes the azimuth angle of the grating-like structure to change periodically. The surface of the substrate is formed in a stepped shape along the predetermined direction, and the stepped shape of the substrate surface repeats periodically to match the structural changes in the azimuth angle. Each step forming the stepped shape is provided with a grating structure with a different pattern, so that the multiple patterned grating structures are also arranged in a stepped shape to match the stepped shape of the surface of the substrate.

2. The reflective polarization-separating diffraction element according to claim 1, characterized in that, The reflective surface and the grating structure are formed on both sides of the substrate.

3. A reflective polarization-separating diffraction element, characterized in that, have: Substrate; and A grating-like uneven structure is formed on the surface of the substrate, exhibiting structural birefringence Δn. * , The concave and convex surfaces of the grating-shaped concave and convex structure are reflective surfaces. The grating-shaped concave-convex structure is composed of multiple patterned grating-shaped concave-convex structures, each with a different azimuth angle. These multiple patterned grating-like convex and concave structures are arranged along a predetermined direction on the surface of the substrate in a manner that causes the azimuth angle of the grating-like convex and concave structures to change periodically. The surface of the substrate is formed in a stepped shape along the predetermined direction, and the stepped shape of the substrate surface repeats periodically to match the structural changes in the azimuth angle. Each step forming the stepped shape is provided with a grating-shaped convex-concave structure with a different pattern, so that the plurality of patterned grating-shaped convex-concave structures are also formed in a stepped shape to match the stepped shape of the surface of the substrate.

4. The reflective polarization-separating diffraction element according to claim 3, characterized in that, The grating-shaped uneven structure and the reflective surface are formed on both sides of the substrate.

5. An optical measuring device, characterized in that, have: The reflective polarization-separating diffraction element according to any one of claims 1 to 4; An incident optical unit that directs the measurement light toward the reflective polarization-separating diffraction element; and The detection optical unit detects light in at least one specific direction from the reflected light after it has been diffracted by the reflective polarization-separating diffraction element. The optical measuring device measures the optical properties of the object being measured that is placed in the optical path of the incident optical unit or the detection optical unit.

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