JP2015121726A - Diffraction grating and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction grating that is manufactured as one lattice structure by one-step exposure, that generates Bragg diffraction light of two wavelengths with one incident angle, and controls and switches them between the two wavelengths.SOLUTION: The diffraction grating has a periodic phase separation structure of a liquid crystal and polymer formed by one-step holographic exposure, can cause Bragg diffraction to light incident at one incident angle and having two or more different wavelengths, and can switch between diffraction/non-diffraction states by changing a polarization light direction of incident light. Polarization light (p polarization light) in a plane including an incident beam and a diffraction beam can perform switching between light of one wavelength being in a diffraction state and light of the other wavelength being in a non-diffraction state, from light of two different wavelengths incident at one incident angle. On the other hand, when a polarization light direction is switched to polarization light (s polarization light ) perpendicular to the plane, light of the former wavelength is switched to the non-diffraction state, and light of the latter wavelength is switched to the diffraction state. Thus, dual wavelength interchangeable switching is allowed by which the switching can be performed reversibly in the polarization light direction.

Description

  The present invention relates to a diffraction grating and a manufacturing method thereof. More specifically, the present invention relates to various types of optical switches, fiber switches, wavelength selectors, spectrophotometers (diffraction gratings), polarization analyzers, polarization conversion elements, polarization beam splitters, polarization selection filters, optical pickups, optical disk devices, and the like. The present invention relates to a diffraction grating useful as a wavelength switching element used in optical devices and optical devices, and a manufacturing method thereof.

The optical wavelength selection function is required in various optical controls, and elements and devices for optical wavelength selection in various scenes have been developed. For example, in an optical pickup system, writing / reading of an optical disk is performed by lasers having different wavelengths, but it is necessary to control propagation light such as optical path coupling and optical path branching. Further, in the spectroscopic device, it is necessary to cause diffraction in a wavelength-selective manner according to the incident angle of light incident on a built-in diffraction grating, scan the incident angle, shift the diffraction wavelength, and perform spectroscopy. On the other hand, in the expression of short-wavelength light due to the SHG effect, the second, third,... Second harmonics (wavelengths λ _1/2, respectively) generated when light of the fundamental wave (having wavelength λ ₁ ) is incident λ _1/3, ···) it is necessary to divide the control by the wavelength selection element for processing by dividing the emitted light. An optical element having a wavelength selection function and the like in these various scenes is required to be as simple and compact as possible, and it is required to reduce the size and weight of the entire apparatus and to reduce the manufacturing cost.

  As one of the conventional types, there is a small optical head (for example, Patent Document 1) in which a dichroic mirror or a trichromatic mirror for splitting light for each wavelength is combined with a beam splitter or the like to form a compact optical system. However, in this case, since the optical components are individually combined, there is a limit in reducing the size, and considering the number of components, it is not easy to improve the manufacturing cost. As a solution, an element that can selectively split light at two or more wavelengths has been developed. Among them, an element capable of selecting a wavelength depending on the polarization direction is an effective technique, and many types have been reported so far. For example, a periodic grating groove is formed in a substrate of optically anisotropic material having birefringence, such as lithium niobate, and the groove is filled with an isotropic material having a refractive index matching that of one of the optically anisotropic materials. A diffraction grating has been proposed (Patent Document 2). This element is designed so that when light of a certain wavelength is incident, diffraction occurs in a specific polarization direction, and light in a polarization direction perpendicular to the polarization direction travels straight. On the other hand, when light of different wavelengths is incident on this element, the relationship between the refractive indices of these two materials in contact with each other changes due to the wavelength dispersion of the material, and spatial refractive index modulation is maintained regardless of the polarization direction, resulting in diffraction. It becomes a state and can be used as a wavelength selector. In addition, a composite diffraction grating that is selectively diffracted at a plurality of wavelengths by forming a plurality of transmission diffraction gratings having different grating pitches into a multilayered structure is proposed (Patent Document 3). In this element, the diffraction grating is made of a combination of a plurality of anisotropic diffraction gratings made of birefringent materials, each of which is designed to diffract with different polarizations. Can have sex.

  These conventional examples have been made compact by integrating a plurality of wavelength selection functions in the element, but the process is likely to be complicated because the structure for selectively diffracting for each desired wavelength is individually produced, and the cost is low. It can be said that there is still room for improvement in terms of reduction.

  As a diffraction grating forming technique, a simple method using a kind of self-organization has been reported. For example, a refractive index modulation type transmission diffraction grating composed of a periodic structure of a liquid crystal and a polymer is called a holographic polymer dispersed liquid crystal (HPDLC) diffraction element, and many techniques have been reported so far. This is because the raw material, which is a mixture of liquid crystal and photopolymerizable monomer added with an additive such as a polymerization initiator, is thinly sandwiched between transparent substrates, and this is subjected to non-uniform exposure such as two-beam interference exposure, resulting in spatial light during exposure. By locally polymerizing depending on the strength, the diffraction grating is formed by separating the liquid crystal and the polymer into two phases in a spatially periodic manner. At this time, a Bragg type diffraction grating having self-organized liquid crystal orientation with higher order in a specific direction and optical anisotropy can be obtained. When light enters the diffraction grating, only a specific polarization direction is selectively diffracted, and the diffracted light is turned on / off by changing the polarization direction (Patent Document 4). Thus, in HPDLC, it is known that light of one wavelength satisfying the Bragg condition is selectively diffracted for a specific incident angle, and this diffracted light is turned on / off by changing the polarization direction. ing.

JP2008-181637 WO2004-97819 Patent 4735749 JP 2006-134504 A

  HPDLC is a technology that attracts attention because it can be fabricated by one-step (one-time) two-beam interference exposure as a simple technique. However, in the conventional HPDLC diffractive element as in the above example, it is not realized to generate Bragg diffracted light of two wavelengths at one incident angle and to switch and control them according to the polarization direction.

  The present invention eliminates such conventional problems and can realize wavelength switching by polarization control with a single grating structure fabricated by one-step exposure, making elements and devices compact, expanding the range of applications, and versatility. It is an object of the present invention to provide a new diffraction grating that can be improved and a method for manufacturing the same.

The present inventor has examined the HPDLC diffractive element, and as described above, the HPDLC diffractive element is formed by polymerization, phase separation, and liquid crystal alignment by two-beam interference exposure (interference fringe spacing Λ), and interference at the time of exposure. Depending on the light intensity distribution, a refractive index modulation distribution with a pitch Λ is formed, and the Bragg condition expressed by this periodic structure
Bragg diffraction occurs at a specific wavelength λ and an incident angle θ satisfying the above, and from equation (1), by forming a grating structure in which the grating pitch Λ reversibly changes with switching of the polarization direction, the same incident angle θ It was found that the selection of Bragg diffraction wavelength λ can be controlled by polarization. It was also confirmed that this lattice structure can be realized by designing a refractive index modulation distribution including birefringence formed by one-step exposure by selecting a starting material in production and conditions at the time of exposure.

  The present invention has been completed based on these findings and verifications. More specifically, the present invention is characterized by the following.

  (1) A periodic phase separation structure of liquid crystal and polymer formed by one-step holographic exposure, which causes Bragg diffraction for light having two or more different wavelengths incident at one incident angle. A diffraction grating that can be switched between diffracted and non-diffracted states by changing the polarization direction of incident light.

  (2) In-plane polarized light containing incident and diffracted rays (p-polarized light), one of the two different wavelengths incident at one incident angle is in the diffracted state and the other in the wavelength Is in the non-diffractive state, but if the polarization direction is switched to the polarized light (s-polarized light) perpendicular to the plane, the light of the former wavelength is switched to the non-diffracted state and the latter wavelength is switched to the diffracted state. The diffraction grating according to the above (1), which is reversible in the polarization direction and capable of switching between two wavelengths.

  (3) When light including two different wavelengths is incident, light of one wavelength is switched between a diffracted state with p-polarized light and a non-diffracted state with s-polarized light, whereas light with the other wavelength is p-polarized. The diffraction grating of (1), which maintains a non-diffracting state regardless of polarization and s-polarization.

  (4) When light containing two different wavelengths is incident, the light of one wavelength maintains the diffracted state regardless of the polarization, and the light of the other wavelength becomes the diffracted state by p-polarized light and the non-diffracted state by s-polarized light. The diffraction grating according to (1), wherein the diffraction grating is switched to

  (5) The diffraction grating according to (1), wherein when light including two different wavelengths is incident, the light of the two wavelengths is aligned and switched to a diffracted state with p-polarized light and a non-diffracted state with s-polarized light.

  (6) When light including two different wavelengths is incident, light of one wavelength is switched between a s-polarized and diffracted state and a p-polarized light and a non-diffracted state. The diffraction grating according to (1), wherein the diffraction grating maintains a non-diffracting state regardless of s-polarized light.

  (7) When light including two different wavelengths is incident, the light of one wavelength maintains the diffracted state regardless of the polarization, and the light of the other wavelength becomes the diffracted state by s-polarized light and the non-diffracted state by p-polarized light. The diffraction grating according to (1), wherein the diffraction grating is switched to

  (8) The diffraction grating according to (1), wherein when light including two different wavelengths is incident, the light of these two wavelengths is switched to a diffracted state with s-polarized light and a non-diffracted state with p-polarized light.

(9) All wavelengths λ that are Bragg diffracted with respect to the incident angle θ
The diffraction grating according to any one of (1) to (8), wherein N is a natural number and Λ is a grating pitch.

  (10) A diffraction grating corresponding to any one of the above (1) to (9), wherein the diffraction order is reversibly generated / disappeared by temperature change by utilizing alignment order transition of liquid crystal.

  (11) With respect to the spatial periodic concentration distribution of the liquid crystal and the polymer, the orientation order of the liquid crystal molecules changes spatially with the concentration change, so that any of the features described in the above (1) to (10) Appearing diffraction grating.

  (12) The diffraction grating according to any one of (1) to (11), having a thickness of 5 to 50 μm and a grating pitch of 0.5 to 2 μm sandwiched between transparent substrates.

  (13) The method for producing a diffraction grating according to any one of (1) to (12), wherein the monomer has a small average functional group number, more specifically, the average functional group number is in the range of 1.14 to 1.2. A method for producing a diffraction grating, which is produced by interference exposure using a prepared monomer as a starting material.

  (14) While maintaining the composition ratio of liquid crystal concentration X wt%, functional group number 1 monomer (90-X) wt%, functional group number 2 monomer 10 wt%, X in the range of 30-50% Mixing, further adding a photopolymerization initiator and a sensitizer (pigment) as a starting material, the diffraction grating according to the above (13), which is kept constant at any temperature in the range of 25 to 70 ° C. and subjected to interference exposure Production method.

  According to the present invention, wavelength switching by polarization control can be realized with a single grating structure manufactured by one-step exposure, so that the device and thus the apparatus can be made more compact, and the application range can be expanded and versatility can be improved. Moreover, since this lattice structure can be easily formed in a self-organized manner by collective holographic exposure, the manufacturing cost can be reduced.

The production procedure, diffraction, and structure scheme of the diffraction element of this invention. (a) Basic optical system for batch holographic exposure and scheme of HPDLC formation process. (b) Structure of diffraction grating and polarization diffraction. (c) A cross-sectional scheme that is closer to the actual lattice structure. Distribution of the liquid crystal phase in the liquid crystal / polymer diffraction grating (c) and the distribution of the liquid crystal orientation order (S) in the lattice vector direction (x) (above), and the anisotropic refractive index determined by them Distribution (n _x , n _yz ). In (a) and (b), for example, the calculation was performed based on the difference in whether the degree of order S is constant or sinusoidal. Anisotropic refractive index distribution (n _x , n _yz ) in the lattice structure. (a) and (b) are the results under different production conditions. The background is displayed by superimposing SEM cross-sectional images of the lattice. Bragg diffraction intensity by polarization, including higher-order modes for incident angle (in medium) and wavelength. Expressed by a three-dimensional plot, the horizontal, vertical, and height correspond to the incident angle (θ ′), wavelength (λ), and diffraction efficiency (ηp or ηs) in the medium, respectively. In this study, Bragg diffraction intensities up to order N = 1-7 were observed as indicated by a plurality of real curves in the θ′-λ plane. This plot shows the results for the sample in Fig. 3 (a), where (a) is p-polarized light and (b) is s-polarized light measured at 20 ° C. The measured diffraction efficiency was expressed by the height of the bar. Bragg diffraction intensity by polarization, including higher-order modes for incident angle (in medium) and wavelength. This plot shows the results for the sample in Fig. 3 (b), where (a) is the p-polarized light and (b) is the s-polarized light measured at 20 ° C. See Figure 4 for a description of the figure. Bragg diffraction intensity by polarization, including higher-order modes for incident angle (in medium) and wavelength. This plot shows the results for the sample in Fig. 3 (b), where (a) is p-polarized light and (b) is s-polarized light measured at 50 ° C. See Figure 4 for a description of the figure. Various types of polarization-controlled wavelength selective switches. The upper figure of each A to G is p-polarized light, and the lower figure is s-polarized light. A solid line and a broken line indicate different wavelengths. Dependence of Bragg diffraction efficiency on X, Y and exposure temperature T. The diffraction efficiency of p-polarized light with respect to the incident angle and wavelength. The solid curve on the horizontal plane in the three-dimensional display shows the Bragg relationship. Refer to the explanation of FIG. 4 for how to read the figure. Dependence of Bragg diffraction efficiency on X, Y and exposure temperature T. Diffraction efficiency of s-polarized light with respect to incident angle and wavelength. The solid curve on the horizontal plane in the three-dimensional display shows the Bragg relationship. Refer to the explanation of FIG. 4 for how to read the figure. Dependence of Bragg diffraction efficiency on X, Y and exposure temperature T. Primary and secondary modes at incident angle θ = 30 ° (θ '= 19.5 ° in the medium) (N = 1 and 2 respectively) with Bragg wavelengths of 1.0 μm (solid line with white circles) and 0.5 μm (solid line without markers) Dependence of diffraction efficiency on polarization angle. Polarization angles of 0 ° and 90 ° correspond to p and s polarized light, respectively. A to D described in each graph indicate that they correspond to respective operation schemes of the element types A to D shown in FIG.

  Embodiments of the present invention will be described below.

  As shown in the manufacturing procedure of the diffraction grating of the present invention in FIG. 1 (a), the liquid crystal / monomer mixture injected between two transparent substrates according to the intensity of light generated by holographic exposure (two-beam interference exposure). The raw material periodically undergoes photopolymerization and phase separation, resulting in the formation of a holographic structure. Furthermore, in our material system of the present invention, liquid crystals are aligned in a self-organized manner with phase separation. With such a grating structure having optical anisotropy, Bragg diffraction with high polarization can be obtained as schematically shown in FIG. 1 (b). In fact, as shown in detail in FIG. 1 (c), the liquid crystal droplets have a phase separation structure in which the liquid crystal droplets are continuously and periodically distributed in the polymer, and the liquid crystal droplets generated when this distribution is formed By controlling the orientation, various polarization diffraction patterns can be obtained.

FIGS. 2 (a) and 2 (b) show two examples of lattice structures designed by simulation. The horizontal axis is the position inside the lattice along the lattice vector direction (here, set to the x-axis) and normalized by the lattice pitch Λ. The upper diagram shows the spatial distribution along the x-axis of the concentration c of liquid crystal molecules and the degree of orientation order S. Here, the degree of orientation order S is an index from 0 to 1 from the complete disorder to the complete order of the alignment of the liquid crystal molecules, and the relationship with the birefringence is an equation including the first type perfect elliptic integral function. Although expressed, the present invention can be approximated by a linear relationship (F. Basile, et al., Phys. Rev. E, 48 (1993) 432). The figure below shows the refractive index distribution determined by c and S in the figure above. Here, there is linearity (additiveness) in the relationship between the concentration of liquid crystal and polymer and the refractive index, and the relationship between the alignment order of the liquid crystal and the birefringence. Under the assumption that the component is parallel to the x direction in consideration of optical anisotropy
And the component perpendicular to the x-axis

(H. Kakiuchida, et al., Phys. Rev. E, 86 (2012) 061701). Here, n _e and n _o is the extraordinary light and the refractive index of the liquid crystal with respect to ordinary light, the liquid crystal of the values used in this embodiment as an example, are assumed to be of 1.7288,1.5331. N _ply is the refractive index of the polymer, which is 1.5258 here. FIG. 2 (a) shows a case where the LC concentration c is distributed in a sine wave shape in the x direction, and the degree of orientation order S maintains a spatially constant value. In this case, as shown in the figure below, the refractive index distributions n _x and n _yz are distributed in a sinusoidal shape with different amplitudes and a pitch Λ. On the other hand, FIG. 2 (b) shows the case where the degree of orientation order S changes in a sinusoidal shape with the LC concentration c. In this case, as shown below, the refractive index distribution of the n _x is slightly offset from the sine wave, for n _yz modulation component in the half pitch (lambda / 2) appears remarkably. Thus, according to the simulation, since the alignment order S of the liquid crystal is not constant in the lattice vector direction, anisotropy can be generated in the spatial periodic distribution of the refractive index. In particular, while the n _x is substantially maintained sinusoidal distribution of the pitch lambda, n _yz showed that sinusoidal distribution of a half pitch (lambda / 2) appears remarkably. This is the refractive index of pitches other than Λ, even if the liquid crystal (or polymer) concentration distribution of pitch Λ is formed by interference exposure with interference fringe spacing Λ, etc. This means that the modulation can be anisotropic. By realizing such an optically anisotropic structure, the object of the present invention can be achieved in which the grating pitch is switched by changing the polarization direction of incident light. The inventor searched for materials and exposure conditions to be used in order to form this structure by one-step (one-time) two-beam interference exposure.

  The composition of the liquid crystal and polymer phase in the HPDLC lattice and the distribution of the molecular orientation order in the liquid crystal droplets in the direction of the lattice vector are the liquid crystal / monomer mixture ratio in the starting material, the number of monomer functional groups, and the temperature during phase separation in the preparation process. It is reported that it changes by several factors. For example, Pogue et al. Prepared HPDLC by adjusting the number of functional groups of the monomer mixed with the liquid crystal, investigated the shape of the liquid crystal droplets, and used five types of monomers having different average functional groups in the range of 2 to 5, The relationship between the size and distribution of liquid crystal droplets and the shape anisotropy and the number of functional groups has been reported (RT Pogue, Polymer, 41 (2000) 733). In addition, Sarkar et al. Investigated the influence on the liquid crystal droplet shape using monomers having different average functional group numbers in the range of 1.3 to 3.5 (M. D. Sarkar, Macromolecules, 36 (2003) 630). According to their report, the liquid crystal droplet size in HPDLC produced by phase separation tends to increase as the number of monomer functional groups in the starting material increases. On the other hand, Kyu et al. Reported that the distribution and orientation order of the liquid crystal phase accompanying the phase separation in the HPDLC stripe pattern was reproduced by simulation, and that the phase separation morphology changed depending on the liquid crystal concentration (T. Kyu, Phys. Rev. E, 63 (2001) 061802).

  On the other hand, as a physical interpretation, the phase separation mechanism is often explained by a change in the free energy of the system. According to this, in the system of liquid crystal and monomer, the sum of the free energy generated by mixing them and the free energy due to the alignment order of the liquid crystal phase dominates the phase separation condition, and as the polymerization proceeds further, the liquid crystal / monomer / polymer mix. Since the ratio changes one by one, the behavior becomes more complicated. The molecular orientation order inside the liquid crystal droplets is closely related to the phase separation structure such as the size and distribution of the liquid crystal droplets, and the formation process is as follows: (1) ratio of liquid crystal to monomer, (2) number of monomer functional groups, (3) exposure It is influenced by factors such as temperature and (4) viscosity. More specifically, in (1), the liquid crystal ratio increases and the liquid crystal rich region during phase separation increases. Regarding (2), when the number of functional groups of the monomer is increased, the size of the liquid crystal droplets tends to decrease due to the network structure of the polymer. (3) and (4) are due to the fact that the phase separation mechanism is deeply related to temperature and viscosity.

  In Example 1 of the present invention, a search was performed based on the following concept with the aim of changing the spatial distribution of the alignment order of liquid crystal molecules under the influence of the shape and distribution of liquid crystal droplets present in the polymer matrix. (i) Aiming for a polymer network structure that promotes self-organizing orientation of liquid crystals, the effects of monomers with a small number of functional groups are explored. (ii) Increase (or decrease) the liquid crystal concentration and simultaneously increase (decrease) the number of functional groups of the monomer to maintain the structural stability of the device. (iii) The relationship between the number of monomer functional groups and the liquid crystal concentration is fixed, and the introduction effect of monomers having different reactivity is investigated. (iv) With the material system as it is, the effect of the change of the liquid crystal droplet shape by the exposure temperature is investigated.

  Here, an example of an actual manufacturing procedure is shown, but the present invention is not limited to this in order to realize the structure of the present invention. First, as starting materials, liquid crystals, monomers, polymerization initiators, and dyes (sensitizers) described in Table 1 were mixed at a predetermined weight ratio and injected into a 10 μm gap between two transparent substrates.

  4-Cyano-4'-pentylbiphenyl (so-called 5CB) (Merck) in the liquid crystal, 2-hydroxy-3-phenoxy propyl acrylate (Kyoeisha Chemical), dimethylol tricyclo decane diacrylate (Kyoeisha Chemical), 1-Vinyl-2-Pyrrolidinone as monomer (Aldrich), 2-hydroxyethyl methacrylate (Kyoeisha Chemical), N-Phenylglycine (Tokyo Kasei) as a starting material, and Dibromofluoroscein (Tokyo Kasei) as a dye (sensitizer) were added. The weight ratio of monomer 1 with liquid crystal and one functional group is X% and (85-X)% respectively, monomer 2 with two functional groups is 10%, and different types of monomers that serve as crosslinkers 3 and 4 are Y% and (5-Y)%, respectively, for a total of 100%, further adding an initiator and a dye (sensitizer), X = 30-50%, Y = 1-3% Shake the amount in the range. The change in X corresponds to the above operations (i) and (ii), and by increasing X from 30% to 50%, the network structure is strengthened by increasing the number of functional groups, and the shape of liquid crystal aggregation droplets is increased by increasing the liquid crystal concentration. Attempts were made to form a distribution and thus a spatial distribution of orientational order. On the other hand, the change in Y corresponds to (iii), and the effects of methacrylic and vinyl monomers with different reaction rates and the like were compared, maintaining the number of functional groups constant. Holographic exposure was performed at any temperature in the range of 25 to 70 ° C., and as described in (iv), the effect of temperature change was investigated separately from the effect of monomer / liquid crystal composition.

In this embodiment, the laser beam emitted from a single mode oscillation laser light source (Showa Optronics, J150GS) with a wavelength of 532 nm is divided into two light beams, and the beam is directed to the sample substrate so that the grating pitch is 1 μm and the grating slant angle is 0 °. Interference fringes were formed by incidence at ± 15 ° from the vertical axis of the surface, and irradiation was performed for 5 minutes with a light intensity of 40 mW / cm ² .

The lattice structure of the sample was examined by scanning electron microscope (SEM) and optical spectroscopic diffraction. FIG. 3 shows the refractive index distributions of two examples of diffraction gratings manufactured as examples. FIGS. 3 (a) and 3 (b) are the results of measuring a sample prepared at an exposure temperature of 40 ° C. with X = 40% and 50%, respectively, at a temperature of 20 ° C., and 2 along the lattice vector direction (x-axis). Two refractive index modulation distributions Δn _x and Δn _yz (white solid line and broken line) are shown. The background of the figure shows an SEM cross-sectional image with the scale adjusted to match the refractive index modulation pitch (produced with a 1 μm design, but actually Λ = 1.03 μm). From this, it can be seen that the phase separation structure changes depending on the manufacturing conditions, and the refractive index distribution changes greatly. In particular, appears clearly that modulation component of the [Delta] n _yz half pitch (lambda / 2), the optical structure obtained by the present study, namely that the spatial modulator having anisotropy [Delta] n _x and [Delta] n _yz are formed confirmed. The temperature of these HPDLC devices was raised, and the refractive index distribution was examined at 50 ° C. higher than the nematic-isotropic phase transition point of the liquid crystal (35 ° C. in this material) (black thin solid line and thick broken line). As a result, the birefringence in the periodic structure disappeared, as seen in the black solid line and the thick broken line, and it was concluded that this optical anisotropy was caused by the orientational order distribution of the liquid crystal. Attached. Thus, the refractive index modulation distribution of HPDLC produced in this study shows different pitch components depending on the polarization direction, and forms a grating with such optical anisotropy, which is the subject of the present invention, the polarization direction. Can realize a wavelength selector that can switch Bragg diffraction wavelength.

FIGS. 4 and 5 show the incident angle and wavelength dependence of the Bragg diffraction spectrum generated in the refractive index modulation distribution shown in FIGS. 3 (a) and 3 (b), respectively, in a three-dimensional graph. The horizontal axis is the incident angle θ ′ in the medium, the vertical axis is the wavelength λ, the height axis is the diffraction efficiency η _p or η _s , and each bar graph plotted in the height direction is the measurement data. The solid line curve with N = 1 to 7 in the θ'-λ plane is the Bragg relational expression corresponding to the lattice pitch (Λ / N)
In the present invention, it is expressed as an Nth-order Bragg mode.

The refractive index modulation Δn _{x at} low temperature shown in FIG. 3 (a) shows a distribution closer to a sine wave of the basic pitch Λ than that in FIG. 3 (b), and Δn _yz has an Nth order pitch (Λ As shown in FIG. 4, such a grating structure has a diffraction efficiency of N = 1 for p-polarized light and a diffraction efficiency of N = 2 for s-polarized light. On the other hand, in FIG. 3 (b), the refractive index at low temperature is modulated in a non-sinusoidal _shape , and in Δn _yz , the change of the half pitch (Λ / 2) becomes significant, and as shown in FIG. In N = 1, 2 and a plurality of Bragg modes appear, and in s-polarized light, N = 2 mode shows high diffraction efficiency. 4 and 5 show the results at 20 ° C., and when this is raised to 50 ° C., which is higher than the nematic-isotropic phase transition point, as shown in FIG. Both p, s and s polarized light are weakly diffracted and have the same value, resulting in an isotropic diffraction grating structure.

  The operation scheme of the polarization control type wavelength switching element according to the present invention is summarized in FIG.

  Type A is polarized light in the plane containing incident light and diffracted light (p-polarized light). Of light of two different wavelengths incident at one incident angle, light of one wavelength (solid line) is in the diffracted state and the other The light of the wavelength (broken line) is in the non-diffracted state, but when the polarization direction is switched to the polarized light (s-polarized light) perpendicular to the plane, the light of the former wavelength is in the non-diffracted state and the latter wavelength is in the diffracted state In other words, this switching is reversibly performed in the polarization direction, so-called switching switching between two wavelengths.

  In Type B, when light that includes two different wavelengths is incident, one of the wavelengths (solid line) is switched to a diffracted state for p-polarized light and a non-diffracted state for s-polarized light, whereas the other wavelength is switched. The light (dashed line) has a characteristic of maintaining a non-diffracting state regardless of p and s polarized light.

  In Type C, when light containing two different wavelengths is incident, light of one wavelength (broken line) maintains the diffracted state regardless of polarization, and light of the other wavelength (solid line) is diffracted by p-polarized light. And s-polarized light that switches to a non-diffracting state.

  Type D has the characteristic that when light containing two different wavelengths is incident, the light of these two wavelengths is aligned and switched to a diffracted state with p-polarized light and a non-diffracted state with s-polarized light.

  In type E, when light containing two different wavelengths is incident, one of the wavelengths (solid line) switches from s-polarized to diffracted state and p-polarized to non-diffracted state, whereas the other wavelength The light (dashed line) has a characteristic of maintaining a non-diffracting state regardless of p and s polarized light.

  In Type F, when light containing two different wavelengths is incident, light of one wavelength (broken line) maintains the diffracted state regardless of polarization, and light of the other wavelength (solid line) is diffracted by s-polarized light. And has the characteristic of switching to a non-diffracting state with p-polarized light.

  Type G has the characteristic that when light including two different wavelengths is incident, the light of these two wavelengths is aligned and switched to a diffracted state with s-polarized light and a non-diffracted state with p-polarized light. These operation schemes shown in FIG. 7 are realized by changing the material composition and manufacturing conditions as described above.

FIGS. 8, 9 and 10 show the diffraction characteristics of the HPDLC elements prototyped under various conditions. FIGS. 8 and 9 show the diffraction efficiencies η _p and η _s for p and s polarized light, respectively, in the same format as FIGS. Within the combination of the raw material weight ratio X and exposure temperature, not only the primary Bragg mode (N = 1) but also multiple modes
The diffraction efficiency of each mode changes when the polarization direction is switched. Fig. 10 shows the incident angle θ = 30 ° in Fig. 8 and Fig. 9 (corresponding to θ '= 19.2 ° in the medium), and the primary and secondary Bragg modes (N = 1 and 2, respectively, with wavelength This shows the polarization angle dependence of the diffraction efficiency (corresponding to 1.0 and 0.5 μm). Here, the result when Y is shaken is also shown. The alphabets A to D described in the respective results of FIG. 10 correspond to the operation schemes of the types A to D shown in FIG. 7, and it can be seen that various wavelength selective switches are realized depending on the manufacturing conditions. When X and temperature are shaken in combination, there is a strong tendency to become type D in the range where X is relatively small, and as X increases, it gradually becomes type C, and it can be seen that it further shifts to type B and A . Looking at the change in exposure temperature, when X is relatively small, type C and D have no dependence, whereas when X is large, it can be seen that the type B or C changes to A. On the other hand, types E to G correspond to replacing the behavior of p and s polarized light with type s and p polarized light in types B to D, respectively. In principle, the positive polarizabilities used in types B to D are changed. For example, N- (p-methoxy benzylidene) -p'-butyl aniline (MBBA) is used by changing the raw material to a liquid crystal having negative polarizability.

  In the above explanation, we focused on monomers with a small number of functional groups, and showed the controllability of the distribution of liquid crystal droplets and the alignment order of liquid crystal molecules in the droplets while the polymer was relatively loose to form a network structure. There is a high possibility that the control by the self-organization of the distribution of the orientational order in the structure can be realized by various combinations of material systems and fabrication conditions, and is not limited to this embodiment.

  The diffraction grating of the present invention includes various optical switches, fiber switches, wavelength selectors, spectrophotometers (diffraction gratings), polarization analyzers, polarization conversion elements, polarization beam splitters, optical pickup devices, optical disk devices, polarization selection filters, and the like. It will be useful in applications.

Claims (14)

  A periodic phase separation structure of liquid crystal and polymer formed by one-step holographic exposure, which can cause Bragg diffraction for light having two or more different wavelengths incident at one incident angle, A diffraction grating characterized in that it can be switched between a diffracted state and a non-diffracted state by changing the polarization direction of incident light.   In-plane polarized light (p-polarized light) that includes incident light and diffracted light. Of light of two different wavelengths incident at one incident angle, light of one wavelength is diffracted, and light of the other wavelength is non-diffracted. In contrast, when the polarization direction is switched to polarized light (s-polarized light) perpendicular to the plane, the former wavelength light switches to the non-diffracted state and the latter wavelength switches to the diffracted state. 2. The diffraction grating according to claim 1, wherein exchangeable switching between two wavelengths can be performed reversibly.   When light including two different wavelengths is incident, light of one wavelength is switched to a diffracted state by p-polarized light and a non-diffracted state by s-polarized light, whereas light of the other wavelength is switched to p-polarized light and s-polarized light. 2. The diffraction grating according to claim 1, wherein the non-diffractive state is maintained regardless of polarization.   When light containing two different wavelengths is incident, the light of one wavelength maintains the diffracted state regardless of the polarization, and the light of the other wavelength switches to the diffracted state with p-polarized light and switches to the non-diffracted state with s-polarized light. The diffraction grating according to claim 1.   2. The diffraction grating according to claim 1, wherein when light including two different wavelengths is incident, the lights of the two wavelengths are switched to a diffracted state with p-polarized light and a non-diffracted state with s-polarized light.   When light including two different wavelengths is incident, light of one wavelength is switched between a s-polarized and diffracted state and a p-polarized and non-diffracted state, whereas the other wavelength of light is p and s-polarized. 2. The diffraction grating according to claim 1, wherein the diffraction grating is maintained in a non-diffracting state.   When light containing two different wavelengths is incident, the light of one wavelength maintains the diffracted state regardless of the polarization, and the light of the other wavelength switches to the diffracted state with s-polarized light and switches to the non-diffracted state with p-polarized light. The diffraction grating according to claim 1.   2. The diffraction grating according to claim 1, wherein when light including two different wavelengths is incident, the lights of the two wavelengths are switched to a s-polarized diffraction state and a p-polarized light non-diffractive state. All wavelengths λ that are Bragg diffracted with respect to the incident angle θ
The diffraction grating according to claim 1, wherein N is a natural number and Λ is a grating pitch.   The diffraction grating according to any one of claims 1 to 9, wherein the diffraction order is reversibly generated / disappeared by temperature change by utilizing alignment order transition of liquid crystal.   The diffraction grating according to any one of claims 1 to 10, wherein the orientational order of liquid crystal molecules changes spatially periodically as the concentration changes with respect to the spatial periodic concentration distribution of liquid crystal and polymer. .   The diffraction grating according to any one of claims 1 to 11, wherein the diffraction grating has a thickness of 5 to 50 µm and a grating pitch of 0.5 to 2 µm sandwiched between transparent substrates.   A method for producing a diffraction grating according to any one of claims 1 to 12, wherein a monomer prepared such that the mass ratio of monomers having a small average number of functional groups is in the range of 1.14 to 1.2 is used as a starting material. A method of manufacturing a diffraction grating, wherein interference exposure is performed using the method.   While maintaining the composition ratio of liquid crystal concentration X wt%, monomer with 1 functional group (90-X) wt%, monomer with 2 functional groups 10 wt%, X is mixed in the range of 30-50%, 14. The diffraction grating according to claim 13, wherein a photopolymerization initiator and a sensitizer (pigment) are further added to form a starting material, which is held at a constant temperature of 25 to 70 ° C. and subjected to interference exposure. Manufacturing method.