JP5668110B2 - Optical element and optical device - Google Patents

Description

  The present invention relates to a reflective optical element having a sub-wavelength metal structure and exhibiting birefringence, and an optical apparatus using the same.

  Optical devices are widely used in general, and for example, optical elements that control light are often used in optical information communication devices, displays, optical pickups, optical sensors, and the like. As these devices have higher functions, optical elements are also required to have higher functions, higher added values, and lower costs.

  As an example of such an optical device, “Patent Document 1” describes a demodulator technique that uses a phase compensation element to reduce signal quality degradation due to Polarization Dependent Frequency Shift (PDFS) that occurs during demodulation of a phase modulation method as a receiver for optical information communication. Is disclosed. “Patent Document 2” discloses a technique for improving the light utilization efficiency of a light source for a projector (display) by a metal structure smaller than a wavelength. “Patent Document 3” discloses an optical pickup technique that improves the S / N ratio by causing a reference light and a signal light to interfere with each other using a homodyne system. These optical devices realize a desired function by combining a plurality of reflecting mirrors, a beam splitter that switches an optical path according to the polarization state of light, a wavelength plate that converts the polarization state of light, and the like.

  As a beam splitter using the difference in polarization state of light, a polarization beam splitter using an optical multilayer film, a wire grid having a comb-like lattice structure of metal wires with a smaller interval than a wavelength, and the like are known. As the wave plate, those using an optically anisotropic crystal typified by quartz or calcite, or a dielectric having an interval smaller than the wavelength as disclosed in “Non-patent Document 1” and “Patent Document 4” are used. Some have a comb-like lattice structure. “Non-Patent Document 2” describes a metamaterial in which a refractive index is artificially controlled by a structure mainly composed of metal smaller than a wavelength and a technique related to negative refraction. Further, “Patent Document 5” discloses a technique relating to a polymer film having a wavelength plate function mainly for a display. Further, “Patent Document 6” discloses a technique related to a retardation plate using a polymer film.

JP 2008-224313 A (corresponding US 2008/0218836) JP 2008-122618 A JP 2008-65961 A (corresponding US 2008/0067321) International Publication WO2007-055245 (corresponding to US2009 / 0128908) JP 2001-215462 A JP 2004-170623 A

Applied Optics, 41, 3558 (2002) Science, 305, 788 (2004)

  In each of “Patent Document 1” to “Patent Document 3”, a plurality of combinations of a reflecting mirror, an optical element having a polarization beam splitter function that switches an optical path according to a polarization state, an optical element having a wavelength plate function, and the like are used as each optical device. The function is realized. Here, for example, if a small and inexpensive new optical element integrating the reflecting mirror and the wave plate function is realized, it goes without saying that these optical devices can be reduced in size and cost.

  Among the optical elements described above, the most expensive one is a wave plate. A conventional wave plate is obtained by processing an optically anisotropic crystal having birefringence into a predetermined thickness. Optically anisotropic crystals have different refractive indices for specific polarized light (ordinary light) and polarized light perpendicular to it (extraordinary light). As a typical example, calcite has a refractive index difference Δn of 0.17 at a wavelength of 633 nm. . On the other hand, in “Non-patent Document 1” and “Patent Document 4”, a wavelength is obtained without using an expensive optically anisotropic crystal by performing fine processing on a dielectric material such as glass using a semiconductor process. A plate (described as a polarization separation element in “Patent Document 4”) is realized. The pitch of the dielectric microstructure needs to be smaller than the wavelength to avoid branching of incident light due to diffraction. Further, since the refractive index difference Δn is about 0.2, it is said that the aspect ratio of the comb structure needs to be 7 or more. If the comb-like structure has an aspect ratio of 1 or less, it can be manufactured at low cost by an injection molding process used in CDs, DVDs, etc. without using a semiconductor process that requires a large-scale manufacturing apparatus. This cannot be realized only by the techniques disclosed in “Non-patent Document 1” and “Patent Document 4”. Compared to these, the refractive index difference Δn between orthogonally polarized light is about 0.2 to 0.3 for a liquid crystal material.

On the other hand, the wave plate using the polymer material described in “Patent Document 5” and “Patent Document 6” is mainly suitable for display applications because it can provide a large-area member at low cost. However, since these use polymer materials, their performance and environmental performance are not as good as those of elements using inorganic materials, and optical information communication devices and optical devices described in "Patent Document 1" and "Patent Document 3" are used. It was difficult to cope with the use of the pickup.

  In view of the problems of the conventional optical elements described above, an object of the present invention is to provide a small and inexpensive new optical element that integrates a reflecting mirror, a wave plate function, and the like.

  In order to solve the problems of the present invention, it is premised to provide a novel optical material in which the refractive index difference Δn between orthogonally polarized light is greatly increased as compared with the conventional art. Such an optical material does not exist in nature, but in the present invention, based on the concept of metamaterial, it is artificially created by a structure in which a metal comb-like structure and a mirror are arranged close to each other (hereinafter referred to as a metal groove). Experiments and simulations show that the generated refractive index difference Δn is 5 or more. Based on this result, the function and shape of the optical element and the configuration of the optical device using these will be specifically shown.

  Hereinafter, in the present invention, a coordinate system having an x-axis and a z-axis on the paper surface will be described in a unified manner. The polarization direction of light is unified to TE polarization and TM polarization. At this time, TE polarized light is light having a magnetic field vibration component in the x direction, and TM polarized light is light having an electric field vibration component in the x direction. As a numerical solution of Maxwell's equations, FDTD (Finite Differential Time Domain) method is used. As for the refractive index of metals and semiconductor materials, the Palik handbook (Palik E.D. (ed.) (1991) Handbook of Optical Constants of Solids II. Academic Press, New York.) Is referred to unless otherwise specified. In addition, a detailed description of Maxwell's equations, metal optics, plasmonics, optical and electrical properties of amorphous materials, and operating principles of individual optical devices is beyond the scope of the present invention, and will be omitted.

First, the origin of the refractive index difference obtained by the dielectric comb structure disclosed in “Non-patent Document 1” and “Patent Document 4” will be described using simulation results. FIG. 2 is a calculation result regarding optical anisotropy of a comb structure using SiO ₂ (silica glass). Here, the calculation was performed for the case where the wavelength of incident light is 700 nm, the pitch of the comb is 200 nm, the width of the comb is 100 nm, the height of the comb is 4000 nm, and light is incident from the air region on the upper side in the z direction. The refractive index of SiO ₂ was 1.47. If only one period of the comb structure is calculated according to the periodic boundary condition in the x direction, the interaction between the infinitely spread structure and the plane wave can be calculated. In the figure, the absolute value of the magnitude of the electric field amplitude is represented by shading, and the interval between adjacent bright stripes corresponds to ½ wavelength. When the incident light reaches the lower end of the comb-like structure, the phase difference between the TM polarized light and the TE polarized light is shifted by an adjustment of ½ wavelength, and it can be seen that this element functions as a ½ wavelength plate. In the case of TE polarized light, the number of fringes of electric field strength is one, so the refractive index of TE polarized light is larger, and the refractive index difference Δn is 0.0875 (1/15) from the ratio of the number of fringes of electric field strength. I know that there is. The difference between the two is caused by the difference in the boundary conditions on the side of the comb, that is, the difference in the continuous conditions of the electric flux density D and the electric field strength E. In the case of TM polarized light, the brightness of the stripes is different between the inside (SiO ₂ ) and the outside (air) of the comb because the electric flux density D = εEx (ε is a dielectric constant and Ex is an electric field component in the x direction) is constant. This reflects the boundary condition of On the other hand, in the case of TE polarization, the electric field strength is uniform inside and outside the comb because the electric field strength Ey (Ey is the electric field component in the direction perpendicular to the paper surface) on the side wall of the comb is constant. Is reflected. Since the light energy (photon density) is εE ^2/2, towards the case of TM polarized light, it can be seen that the photons are abundant in the interior of large comb refractive index. It can be seen that such a difference in the photon density distribution is the origin of the refractive index difference Δn. Therefore, as the difference between the refractive index of the dielectric material forming the comb structure and the refractive index between the surroundings (air) increases, Δn obtained also increases. Dielectrics having a large refractive index include SiN and diamond, but since the upper limit is about 2.5, a large increase in Δn cannot be expected. Further, when a dielectric having a large refractive index is used, loss due to reflection at the interface with air cannot be ignored.

  Next, the operation principle of the wire grid will be described. As schematically shown in FIG. 3, the wire grid can be considered as a comb-like structure having a period smaller than the wavelength. As shown in Fig. 3 (a), for TM polarized light, the free electrons in the metal gather only on one side of the comb in accordance with the direction of vibration of the electric field and polarization occurs, so that light passes through the comb structure. Can do. On the other hand, as shown in FIG. 3B, for TE-polarized light, free electrons in the metal vibrate without being restricted by the side walls of the comb, so that the light is reflected in the same manner as a continuous metal film. . When the comb height is higher than the thickness that allows light to penetrate into the metal (Skin Depth), the wire grid transmits TM polarized light and has a high polarization separation function (extinction ratio) that reflects TE polarized light It becomes an element. The behavior of free electrons in metals in TM polarization is equivalent to plasmons in a broad sense, and light is attenuated slightly if the matching conditions with the surrounding dielectric (same dielectric constant and opposite sign) are prepared. In addition, it is possible to propagate light over a very long distance (a few millimeters at the maximum). Further, as described in “Non-patent Document 2”, the magnitude of the polarization generated at this time is very large, so the polarizability χ of this space is much larger than that of the dielectric. That is, a large refractive index can be artificially obtained by the metal microstructure.

  FIG. 4 is an example of a wire grid simulation result. Here, the wavelength of incident light was 700 nm, the metal material was Ag, the wire pitch was 200 nm, the wire width was 100 nm, and the wire height was 100 nm. As can be seen from the figure, the wire grid has an excellent polarizing filter function that transmits only TM polarized light.

  As described above, it is difficult to increase the refractive index difference Δn obtained from the dielectric comb-like structure, and the wire grid has a large refractive index and provides a polarizing filter function, but functions as a wave plate. It turns out that it does not have.

  Here, if the giant refractive index of the wire grid can be used as the refractive index difference Δn between orthogonal polarized lights, an excellent wave plate should be able to be created. FIG. 5 is a schematic diagram showing a basic structure of the element (metal groove). A metal groove is an integrated wire grid and metal mirror. In the figure, TE polarized light is reflected by the surface of the metal comb-like structure on the surface of the metal groove, and TM polarized light is reflected after reaching the mirror part after passing through the comb-like structure. Both of them interfere after being reflected by the metal groove, and are converted into a polarization state different from the incident light. Since TM polarized light reciprocates in the giant refractive index space of the metal comb structure, a large phase difference can be obtained. That is, the metal groove can have a function of a reflective wave plate having a low aspect ratio and good performance. In the present invention, for the sake of simplification of description, the cross-sectional shape of the metal comb-like structure is limited to a rectangular shape. However, as described above, if a good conductor typified by metal is a wire shape, Similar effects can be obtained. Therefore, the cross-sectional shape is not limited to a rectangle, but may be a trapezoid or a triangle. Further, the convex portion of the comb-like structure is provided to extend in the y-axis direction in the example of FIG. 5, but it may be three-dimensionally shaped like a sword mountain. The size of the optical element of the present invention should be at least several times the wavelength of light in the medium in both the x-axis direction and the y-axis direction in order to obtain the desired reflected light by reducing the influence of diffraction and scattering. That's fine. The relationship between the size of the element and the size of diffraction and scattering is obtained by solving the Maxwell equation, but the specific content will be omitted. As a result, the result became common sense in line with the wave nature of light. Including this structure, it is called a comb structure in the present application. In addition, the material of the comb-like structure is not limited to metal as long as it has sufficient conductivity for the wavelength of the light used (electromagnetic waves and radio waves are acceptable), composite materials of metal / organic matter, graphite, carbon nanotubes Etc. can be used.

  Next, the simulation of metal grooves and the results of trial manufacture / experiment will be described.

  FIG. 6 is a sectional view summarizing parameters for determining the characteristics of the metal groove. As shown in the figure, the metal groove is basically composed of a substrate material and a metal material, and the pitch p, the width w, the height h, and the thickness d of the mirror portion are the main structural parameters. .

FIG. 7 is an example of a simulation result. Here, the substrate material is SiO ₂ , the metal material is Ag, the pitch is p = 200 nm, the width is w = 100 nm, the height is h = 100 nm, the thickness of the mirror part is d = 300 nm, and the wavelength of incident light is 633 nm. The comb structure has an aspect ratio (h / w) of 1. As shown in the figure, TM polarized light and TE polarized light are shifted by about 1/2 of the interval between the fringes of the electric field intensity where the incident light and reflected light interfere, and this has a function as a quarter wavelength plate. I understand. Hereinafter, in the present invention, a simulation result in which the substrate material is SiO ₂ unless otherwise specified is shown.

  FIG. 8 is an example of a simulation result showing the wavelength dependence of the metal groove. Here, the metal material is Ag, pitch p = 200 nm, width w = 80 nm, height h = 40 nm, and mirror part thickness d = 300 nm. The comb-like structure has an aspect ratio (h / w) of 0.5. Further, the orientation of the metal groove was arranged to be rotated by 45 degrees in the xy plane, and the intensity of each polarization component contained in the reflected light was shown with respect to the incident light of TE polarized light. In the figure, it is shown that if the intensity of the TE polarization component is sufficiently small at the wavelength where the intensity of the TM polarization component is maximum (hereinafter referred to as peak wavelength), the metal groove functions as a good half-wave plate. In the figure, TE is indicated by a solid line and TM is indicated by a dotted line. Hereinafter, in the present invention, unless otherwise specified, the orientation of the metal groove is an arrangement rotated by 45 degrees in the xy plane, and the case where TE polarized light is incident is the standard condition for simulation and experiment. As seen in FIG. 8A, it can be seen that the metal groove having the parameters shown here functions as a good half-wave plate at a wavelength of about 580 nm. For comparison, FIG. 8B shows a calculation result in the case of a half-wave plate made of an optically anisotropic crystal (the same applies to a dielectric comb-like structure). Here, the dispersion of the optically anisotropic crystal is sufficiently small. As the difference is clear when the two are compared, the wavelength range in which the metal groove functions as a wave plate can be narrowed. In principle, an optically anisotropic crystal inevitably functions as a wave plate even at a wavelength that is an integral multiple of the peak wavelength. Compared to conventional wave plates when you want to selectively control the polarization state for a specific wavelength from multiple wavelengths of light, for example, when reducing the number of parts in an optical pickup that supports CD / DVD / BD. Metal grooves have excellent characteristics. This is an effect brought about by the fact that the imaginary part of the complex refractive index of the metal material changes greatly with respect to the wavelength, that is, the wavelength dispersion is large. The complex refractive index or complex dielectric constant of these metals is explained by a model that approximates the refractive index of metals as the motion of free electrons that are weakly bound to the nucleus, as represented by the Drude model. As each metal has a different color, the dispersion characteristics in the visible light region are different for each metal material. By utilizing this, various designs with different wavelength dependence are possible for the metal groove. Specific examples are shown in the examples.

  Here, I will add the restrictions of FDTD simulation. In the FDTD simulation, reflection on the surface of the substrate cannot be handled due to restrictions on the memory used, etc., so that the light intensity is inevitably increased by about 4% compared to the actual measurement.

  An example of the peak wavelength design method is shown in FIG. This is a simulation result focusing on the relationship between the height h of the comb structure and the peak wavelength. Here, the calculation was performed for three cases where the metal material is Ag, the pitch p = 200 nm, the width w = 80 nm, the thickness d = 300 nm of the mirror portion, and the height h = 20, 60, 100 nm. As can be seen from the figure, the peak wavelength increases as the height h of the comb structure increases. Also, if we call the maximum value of the ratio of light converted to TM polarized light and the conversion efficiency, the conversion efficiency is about 60% when the height h = 20 nm, and almost 100% when the height h = 60 and 100 nm. Become. In contrast to the conventional half-wave plate, the conversion efficiency is fixed at 100%, whereas in the metal groove, the conversion efficiency is variable depending on the aspect ratio (h / w). Examples of the optical element utilizing this feature will be described later. The lower limit of the metal groove aspect ratio will be described. In the metal groove, a large change in conversion efficiency is obtained near the condition shown in FIG. 9 (a), h / w = 20 nm / 80 nm = 0.25, and it does not function at an aspect ratio of zero. Therefore, the lower limit of the aspect ratio at which the metal groove functions effectively as an optical element is about 0.1.

  Next, the results of trial manufacture and evaluation of metal grooves are shown. FIG. 10 is a schematic diagram showing the configuration of an experimental apparatus for evaluating a prototype device. A spectrophotometer (Hitachi, U4100) was used to perform the measurement under the standard conditions described above. Since the element size is 4mm x 3mm, use a spectrophotometer with a measuring area of 2mmφ, 5 ° specular reflection measuring jig, and attach two Lambert Gran-Taylar prisms, respectively, an analyzer and a polarizer. It was.

  FIG. 11 shows the evaluation results of the prototype device. Here, an element pattern with a pitch of p = 200 nm fabricated on a glass substrate using an electron beam drawing apparatus was fabricated, and a metal groove having a comb-shaped structure with a different height h was fabricated using an imprint method and a sputtering method. . In the figure, an electron micrograph of the glass substrate is shown. AgPdCu alloy was selected as the metal material. As can be seen from the figure, it was confirmed that the peak wavelength shifted to the longer wavelength side as h increased to 60 nm, 100 nm, and 180 nm, as in the simulation results. Further, good results were obtained in which the maximum value of the light intensity at the peak wavelength was 80% or more.

  FIG. 12 compares the evaluation results of the prototype device with the simulation results. It can be seen that the relationship between the height h of the comb-like structure, which is one of the performance control parameters of the metal groove, and the peak wavelength shows good agreement between the experiment and the simulation. From this result, it was found that the refractive index difference Δn between the orthogonally polarized light obtained by the metal groove is about 6, which is about 20 times larger than the conventional one.

  Here, as a configuration example of the metal groove, a configuration in which a wire grid and a metal mirror are integrated has been described. However, they are not necessarily integrated, and may be separated as long as they are within the coherence length of the light used (equivalent to the reciprocal of the spread of the wavelength spectrum). If the distance between the two is longer than the coherence length of the light, the TE polarization component and the TM polarization component do not interfere with each other. As a result, the metal groove becomes a wave plate and the polarization state cannot be manipulated. is there. In general, the coherence length in the case of a non-polarized light source such as a mercury lamp, a cold cathode tube, or sunlight is about several wavelengths, and in the case of a semiconductor laser used for an optical pickup, about several tens of wavelengths, In the case of a gas laser, the wavelength is 100 or more. Therefore, an element in which a wire grid element and a mirror are formed on both sides of a glass substrate having a thickness of about 1 mm does not function as a wave plate from the viewpoint of coherence with a mercury lamp or an optical pickup semiconductor laser. In addition, such an element is inferior to the present invention from the viewpoint of stability and reliability when the influence of the accuracy of the thickness of the glass substrate and the temperature change of the refractive index is considered. As described above, the present invention has an inventive step that cannot be realized by a simple combination of conventional techniques.

  As described above, the metal groove of the present invention can provide a novel optical element that combines the functions of a reflecting mirror and a wave plate, has a small aspect ratio, and excellent workability. Hereinafter, in the present invention, the technical contents relating to an optical element and an optical device having a function of performing any one of waveguide, modulation, or detection of light (electromagnetic wave, radio wave) by applying a metal groove will be disclosed.

  According to the present invention, it is possible to provide a novel optical element that combines at least the functions of a reflecting mirror and a wave plate, can set optical characteristics with a structure, has good workability, and can be reduced in cost. An optical device using this element will be described in Examples.

The figure which shows the structure of the optical element of this invention. Simulation results on optical anisotropy of the comb structure. FIG. 6 is a schematic diagram illustrating the operation of a wire grid. Wire grid simulation results. The figure which shows the basic composition of the optical element of this invention. The figure which shows the main parameters which determine the characteristic of the optical element of this invention. The example of the simulation result of the optical element of this invention. The example of the simulation result regarding the wavelength dependence of the optical element of this invention. The example of the simulation result regarding the wavelength dependence of the optical element of this invention. The schematic diagram which shows the structure of the experimental apparatus for evaluating the optical element of this invention. The figure which shows the result of trial manufacture and evaluation of the optical element of this invention. The figure which shows the comparison result of experiment and simulation of the optical element of this invention. FIG. 3 is a schematic diagram showing a method for producing an optical element of the present invention. The example of the simulation result which shows the thickness of the interlayer dielectric of the optical element of this invention, and the relationship of wavelength dependence. The example of the simulation result which shows the wavelength dependence of the optical element of this invention. The example of the simulation result which shows the difference by the wavelength-dependent metal material of the optical element of this invention. The example of the simulation result at the time of using Al as a metal material for the optical element of this invention. The Example of the light source optical system using the optical element of this invention. 6 is another example of a light source optical system using the optical element of the present invention. An example of an optical pickup having a homodyne detection system using the optical element of the present invention. The Example which shows the structure of the optical system of the detector module for optical communications using the optical element of this invention. Measurement results of changes in resistivity of various chalcogenide thin films. FIG. 3 is a schematic diagram showing a configuration of an active optical element of the present invention. FIG. 3 is a schematic diagram showing a configuration of an active optical element of the present invention. The schematic diagram which shows the electrode structure of the active type optical element of this invention. Simulation results on the phase of light. FIG. 4 is an embodiment showing a configuration of a three-wavelength compatible optical pickup using the phase control element of the present invention. FIG. 4 is an embodiment showing the configuration of a millimeter-wave microstrip antenna using the phase control element of the present invention. Simulation results showing improved antenna detection with metal grooves.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<How to create>
FIG. 13 shows the most inexpensive manufacturing method for metal grooves. As described above, it is well known that a transparent plastic substrate having a predetermined uneven pattern can be formed by an injection molding method applied to a CD or DVD. Since metal grooves have a small aspect, this manufacturing method can be used. Information pits and guide grooves are formed concentrically on CDs and DVDs. However, metal grooves need only have a linear groove pattern. A metal layer is formed on this plastic substrate by sputtering. As shown in FIGS. 13 (a) to 13 (c), the surface irregularities are gradually smoothed as the thickness of the metal layer to be formed increases. Go. At this time, in order to improve the adhesion between the plastic substrate and the metal layer, an adhesive layer as shown in the figure may be formed therebetween. As the material for the adhesive layer, metals such as Mo, Cr and Ta and oxides thereof are effective. When these materials are used, the standard value of the thickness of the adhesive layer is about 0.5 to 2 nm. The shape shown in FIG. 13C is the same as the element structure shown in FIG. In practice, when high performance is required depending on the application, in order to reduce the birefringence of the plastic substrate, it can be produced on a glass substrate by applying a lithography process as described above. It is better to use a nanoimprint method using a cured resin. Further, as the metal material, in addition to the aforementioned Ag and Ag alloy, metals such as Au, Cu, Pt, Fe, Cr, Mo, and W, alloys thereof, and the like can be used. In addition, a metal groove can be formed on the Si substrate without using a transparent substrate such as glass or plastic. In this case, one method with a small number of steps is to leave a comb-like structure by etching a metal that has been formed thick in advance.

<Optical element>
FIG. 1 is a schematic diagram showing another configuration of the metal groove. This is a structure having a dielectric between a comb-shaped wire grid and a mirror, and is the most common structure of a metal groove. Here, s is the thickness of the interlayer dielectric between the wire grid portion and the mirror 1 portion. FIG. 14 is a simulation result showing the relationship between the thickness s of the interlayer dielectric and the wavelength dependence of the metal groove. Here, the metal material is Ag, the interlayer dielectric material is SiO ₂ , the pitch p = 200 nm of the comb-like structure, the width w = 80 nm, the height h = 80 nm, and the thickness d = 300 nm of the mirror part. Calculations were made for three cases of thickness s = 20, 50, and 100 nm. As can be seen from the figure, the peak wavelength can also be adjusted by changing the thickness s of the interlayer dielectric. This is because the optical path length of TM polarized light can be increased as the dielectric becomes thicker. In the above description, the peak wavelength is changed depending on the height of the comb structure, that is, the aspect ratio. On the other hand, if the height of the comb-like structure is changed, the TE polarized light reflectance and erasure ratio and Joule heat loss also change at the same time, but the wire grid part and mirror part can be integrated, so the cost is low. An element can be manufactured. On the other hand, the method shown here is the same method as changing the peak wavelength depending on the thickness of the optically anisotropic crystal. Although the structure is complicated, the peak wavelength can be adjusted more precisely and linearly. Therefore, the adjustment method and the device structure are suitable for applications requiring high performance. Here, an example in which SiO ₂ is used as an interlayer dielectric material has been shown, but this is not limited to a dielectric material, as long as a necessary and sufficient transmittance is obtained with respect to the wavelength of light (electromagnetic waves) to be used. A resin material or the like can be used. For example, crystalline Si is sufficiently transmissive to infrared light, and polyimide resin, polystyrene foam, rubber, etc. can be used if millimeter wave band light is used.

  FIG. 15 shows a simulation result when the thickness of the interlayer dielectric is s = 150 nm. As can be seen from the figure, there are high-order interference peaks at wavelengths substantially corresponding to R, G, and B. This is because a change in s has the same effect as a change in the thickness of a conventional element, and thus a high-order interference peak appeared. If such an element is used, the metal groove can be operated at a plurality of wavelengths for display applications and the like.

  Next, the effect when the metal material is changed will be described. FIG. 16 shows a simulation result when the metal material is compared between Ag and Al in the case of the integrated element structure of FIG. Here, the pitch p of the comb structure is 200 nm, the width is w = 60 nm, the height is h = 80 nm, and the thickness d of the mirror portion is d = 300 nm. As seen in FIG. 16A, when the metal material is Ag, there is a region where the intensity of TM light is large in a wavelength region of 500 to 700 nm. On the other hand, as shown in FIG. 16B, when the metal material is Al, there is a region where the intensity of TM light is large in a wavelength region of 350 to 500 nm. As described above, it can be seen that, depending on the wavelength dependency of the complex refractive index of the metal material, different wavelength dependency can be obtained even if the shape is the same. This indicates that elements having different wavelength dependencies can be manufactured using the same mold by changing the composition of an appropriate alloy material. Moreover, even if it is an element of a uniform pattern, it shows that desired wavelength dependence is acquired by changing the kind of metal material formed into a film according to a place.

  FIG. 17 shows a simulation result when Al is used as the metal material. Here, in the case where the pitch p of the comb structure is 200 nm and the width is 80, 60, and 40 nm, the height h is 60, 80, and 120 nm, respectively. As can be seen from the figure, when the width w is reduced and the height h is increased at the same time, the wavelength range in which good half-wave plate performance can be obtained is widened. In particular, in the case of FIG. 17C, a half-wave plate performance can be obtained over almost the entire visible light region. In the above example, it has been shown that an element having peaks at a plurality of wavelengths can be realized by high-order interference, but in this example, an element that obtains broad wavelength characteristics can be realized. These may be appropriately selected depending on the application.

  In the example of FIG. 1, all the mirror portions are made of metal, but only the reflection surface may be made of metal, and other portions may be made of other materials.

<Optical device>
Hereinafter, an optical device to which the optical element of the present invention is applied will be described with reference to the drawings.

  FIG. 18 is a schematic diagram showing the configuration of a light source optical system using the optical element of the present invention. This is an example of an optical device that guides light with high efficiency by applying a metal groove. In the figure, a non-polarized light beam emitted from the light source 10 enters the light guide plate 110 from the incident window. A metal groove of the present invention is formed on the light incident side of the light guide plate 110, and a polarization filter 120 such as a wire grid that transmits TM polarized light and reflects TE polarized light is formed on the light emitting side. In such a configuration, the TM polarization component of the light emitted from the light source is transmitted, the TE polarization component is reflected by the polarization filter 120, is reflected by the metal groove 100 and is simultaneously converted to TM polarization, and is transmitted through the polarization filter. And exit. With such a configuration, light use efficiency in a projector or a display optical system using a liquid crystal element (not shown) is improved. This is because, as is well known, the liquid crystal element has an intensity modulation function only for polarized light in a specific direction. Here, as the light source, a non-polarized light source such as an LED, a cold cathode tube, a high-pressure mercury lamp or the like is used. Further, the optical system can be integrally formed by creating a predetermined pattern on both surfaces of the waveguide plate 120 by injection molding and forming a metal thin film by sputtering or the like. When a wire grid is used as the polarizing filter, the mirror portion may be removed by polishing using a CMP method or the like after forming the wire grid and mirror by the same method as described above.

  FIG. 19 shows another embodiment of a light source optical system using the optical element of the present invention. In this example, it corresponds to the case where the light source is incident from the side surface of the element including the metal groove, and is assumed to be applied to an illumination optical system of a small liquid crystal display for a mobile phone or the like. In this case, it is necessary to extract light with uniform intensity upward in the figure. In order to realize this, in this embodiment, the metal groove 100 is divided into areas 1 to 3 to form comb-like structures having different pitches, widths, and heights. For example, as shown in FIG. 9, in the metal groove, the peak wavelength and the conversion efficiency can be set by changing the parameters of the comb-like structure. Further, as shown in FIGS. 14 to 17, it is possible to design a peak wavelength and a wavelength range that functions as a wavelength plate by selecting a comb-like parameter or a metal material. Depending on the type and requirements of the light source, such as when a white LED or the like is used as the light source, or when an RGB independent LED is used, such area division is one of the effective means. Even if the area is divided, only the pattern of the substrate changes, so the merit of the metal groove that it can be manufactured at a low cost is not lost.

  FIG. 20 is a schematic diagram showing the configuration of an optical pickup having a homodyne detection system using the optical element of the present invention. In the optical pickup having the homodyne detection system described in “Patent Document 3”, the signal light reflected from the optical disk medium and the reference light are made to interfere with each other, thereby amplifying the signal amplitude and improving the quality of the reproduced signal. The operation principle will be briefly described with reference to the drawings. The light emitted from the semiconductor laser 301 is transmitted through the half-wave plate 321 so that the polarization direction is rotated by 45 degrees. The polarized light is separated into two linearly polarized light beams orthogonal to each other by the polarization beam splitter 341, and one polarized light beam (reproduction light) is reflected and converted to circularly polarized light by passing through the quarter-wave plate 322. After that, the light is condensed by the objective lens 311 and irradiated onto the optical disk 4. Reflected light (signal light) from the optical disk 4 rotated by the spindle motor 77 is returned to parallel light again by the objective lens 311, and linearly polarized light whose polarization direction is orthogonal to the original light by the quarter wavelength plate 322. Is converted to For this reason, the signal light passes through the polarization beam splitter 341 and travels in the direction of the beam splitter 342. The component called reference light (reference light) that first passes through the polarizing beam splitter 341 is reflected by the metal groove 100 of the present invention after being converted into polarized light whose polarization state is orthogonal, reflected by the polarizing beam splitter 341, and reflected by the signal light. It is combined and headed in the direction of the beam splitter 342. At this time, the signal light and the reference light are combined with their polarization directions orthogonal to each other.

  One of the combined lights is transmitted through the beam splitter 342 which is a half mirror, the polarization direction is rotated 45 degrees by the half-wave plate 324, and then separated into orthogonal linearly polarized light by the polarization beam splitter 343, and the detector 361 (PD1) and photodetector 362 (PD2). Similarly, the other of the combined light is reflected by the beam splitter 342 which is a half mirror, and after a phase difference of π / 2 is given between the signal light and the reference light by the quarter wavelength plate 325, The polarization direction is rotated by 45 degrees by the half-wave plate 326, and the linearly polarized light is orthogonally separated by the beam splitter 344 and detected by the detector 363 (PD3) and the photodetector 364 (PD4). By performing phase diversity detection using the four photodetectors PD1 to PD4, it is possible to cancel out the influence of fluctuations in the optical path difference and obtain a good reproduction signal. Here, in “Patent Document 3”, a quarter wavelength plate and a mirror are mounted instead of the metal groove element. By using the metal groove of the present invention, the number of parts can be reduced, and the optical pickup can be reduced in size and cost. Not limited to this example, in general, an optical system including a unit that rotates a polarization direction of reflected light by 90 degrees by a combination of a 1/4 wavelength plate and a mirror, or an optical system including a 1/2 wavelength plate, By replacing these with the metal groove of the present invention, it is possible to reduce the number of parts and reduce the cost.

  FIG. 21 shows a configuration of an optical system of a detector module for optical communication using the optical element of the present invention. This is a configuration of a demodulator of a differential phase shift keying signal. The differential phase shift keyed signal light transmitted from the optical fiber 801 is converted into parallel light by the collimator 802, enters the half beam splitter 402, and the intensity ratio between the first branched light 403 and the second branched light 404 Separate one-on-one. The first branched light 403 is incident on the metal groove element 101 at an angle close to vertical, so that the polarization direction of the reflected light is rotated by 90 degrees and is incident on the beam splitter 402 again. As a result, among the polarization components of the first branched light 403, the TE polarization component is converted to TM polarization at the time when it again enters the half beam splitter 402, and similarly, the TM polarization component is converted to the TE polarization component. Similarly, the second branched light 404 is incident on the metal groove element 102 at an angle close to vertical, whereby the polarization direction of the reflected light is rotated by 90 degrees and is incident on the beam splitter 402 again. The first branched light 403 and the second branched light 404 are combined when entering the half beam splitter 402 again, and the first interference light 409 and the second interference light 410 are generated. Here, the metal groove elements 101 and 102 are arranged so that the difference in optical path length between the first branched light 403 and the second branched light 404 corresponds to one bit of the modulated light. For example, when the modulation frequency is 40 Gb / s, the difference in optical path length is about 7.5 mm. For this reason, the first interference light 409 and the second interference light 410 are in a state of constructive interference or destructive interference depending on whether the phase shift amount between adjacent bits of the light to be measured is 0 or π. Thus, the phase modulation signal is converted into a light intensity signal. These interference lights are condensed by the condenser lenses 803 and 804 on the two light receiving portions of the balanced photodetector 805, respectively. The parallel photodetector 805 outputs a current signal corresponding to the intensity difference between these interference lights, and this output is converted into a voltage signal by the transimpedance amplifier 806 to obtain a final output 807. In such a configuration, the relative phase difference PDFS generated when the first branched light 403 and the second branched light 404 are branched and combined by the half beam splitter 402 is caused by the action of the metal groove element of the present invention. Since the polarization components are exchanged, they cancel each other, and as a result, they are not affected by PDFS, so that it is possible to demodulate information regardless of the polarization state of the signal light.

<Active optical element>
The optical element of the present invention can have a comb-like structure using metal and a mirror. By applying a voltage between them and disposing a material whose optical characteristics change according to the applied voltage between them, an optical element whose characteristics can be actively changed by voltage control can be realized. . As an optical material whose characteristics change depending on an applied voltage or an applied electric field strength, for example, there is an amorphous chalcogenide thin film. An amorphous thin film such as Ge—Sb—Te used as a recording material for an optical disk reversibly changes between an amorphous state and a crystalline state depending on the irradiation condition of the laser beam. On the other hand, such an amorphous thin film exhibits high resistance when the applied electric field strength is small, but the resistivity starts to decrease when the applied electric field strength is on the order of 0.01 V / nm, and is about 0.1 V / nm. Shows almost the same resistivity as the crystalline state. The change in resistivity at this time is about one million times. FIG. 22 shows the results of measuring the change in resistivity in a state in which various chalcogenide thin films are formed (amorphous) and in a state in which annealing is performed at about 300 ° C. for 30 minutes (crystal). Some show a large change in resistivity. In the Ge ₁ Sb ₂ Te ₄ and InSb thin films, the change in resistivity is small because the film forming conditions and annealing conditions are not appropriate. These materials include the fact that the resistivity changes from an amorphous state to a crystalline state by applying a voltage, and that the imaginary part (extinction coefficient) of the complex refractive index is expressed by the resistivity and the frequency of light. The refractive index should change with voltage application. Here, a change in the refractive index of the thin film material could not be measured while applying a voltage, but the refractive index in the amorphous state and the crystalline state can be measured using a spectrophotometer or an ellipso. Here, the simulation is performed assuming that the refractive index becomes the same as the crystalline state by applying a voltage to the amorphous thin film.

FIG. 23 is an example showing the configuration of the active optical element of the present invention. In this example, an amorphous thin film / dielectric / metal cathode is formed below the metal groove. With such a configuration, an etalon structure is formed by the metal groove and the metal cathode, and a large change in reflectance and phase can be obtained by changing the refractive index of the amorphous material. For example, Al is selected as the metal material, Bi (4at%)-Ge ₂ Sb ₂ Te ₅ as the amorphous material, and SiO ₂ as the dielectric material, the pitch p = 200 nm, the width w = 40 nm, the height h = 120 nm, the mirror When the thickness of the part d = 20 nm, the thickness of the amorphous layer d1 = 15 nm, and the thickness of the dielectric layer d2 = 120 nm, the reflectance from 2% before and after the voltage application is applied to the TM polarized light having a wavelength of 405 nm. It was found from the simulation results that it changed to 45%.

FIG. 24 shows another embodiment showing the configuration of the active optical element of the present invention. In this example, an amorphous material is arranged between metal groove comb structures, and a voltage can be applied between adjacent comb structures. In this case, by using a strong evanescent field generated along the side wall of the comb structure, a large interaction can be obtained even for an amorphous material that is optically nearly transparent. In this case, if a comb-like structure is formed as shown in FIG. 25, an optical element in which the function as a metal groove and the function as an electrode coexist can be configured. In this configuration, when a voltage is applied, a current flows directly through the amorphous material. As a result, the power consumption increases, but a large refractive index difference can be obtained by using both the temperature change and the electric field intensity change due to Joule heat generation. The power consumption can be reduced by covering the comb structure with a dielectric insulating material such as SiO ₂ as in the case of the gate insulating film of the MOS transistor and reducing the current flowing through the amorphous material.

  The active optical element shown here requires a voltage control switch, a clock source, etc. in addition to a power supply, so these circuit elements and metal groove elements are integrated on a chip using a semiconductor process on a Si substrate. Forming is suitable as a manufacturing method.

  Moreover, although the case where a chalcogenide amorphous thin film was used was demonstrated, an amorphous semiconductor can also be used as an amorphous material. Amorphous silicon is known to have a refractive index different from that of crystalline silicon, and exhibits a particularly large refractive index change in the wavelength band of 400 to 450 nm. Therefore, it is an excellent material when using a blue light source. Similarly, any material whose refractive index changes with application of voltage can be used for the active optical element of the present invention. If an inorganic material is used as such a material, an optical switch or a phase modulator having a faster response speed than a liquid crystal material can be realized. In addition, if a heater mechanism serving as a heat source is provided between the electrodes using a material that can obtain a large change in dielectric constant (refractive index change) due to a temperature change near the Curie point, such as barium titanate, It is also possible to control the phase and intensity.

  Applications of the active optical element of the present invention include (1) replacement of a spatial phase modulator in an optical system for hologram recording, and (2) two lights in an optical pickup using the aforementioned homodyne method and a detection module for optical communication. (3) Replacement of phase modulator in transmitter for optical communication, (4) Correlation crosstalk reduction element in optical pickup corresponding to multilayer optical disk, (5) Speckle pattern in laser projector It can be used as an element for suppressing the above. (1) to (3) can be easily understood. For (4) and (5), interference between multiple light beams is averaged by applying high-frequency phase modulation to the light emitted from the light source at a frequency (several tens of MHz to several GHz) that exceeds the band of the detection system. It is realized by doing.

<Phase control element>
Here, an embodiment of an optical element using the phase difference of light obtained by a metal groove will be shown.

FIG. 26 shows a simulation result regarding the phase difference of light obtained by the metal groove. Here, in the structure shown in FIG. 5, the light when the pitch p is changed with the substrate material SiO ₂ , the metal material Al, the ratio of pitch p to width w w / p = 0.4, and the height h = 300 nm. The phase of was calculated. The wavelength of the light source is 780 nm. As can be seen from the figure, the phase of the reflected light can be controlled by changing the pitch. In the region where the pitch is about 520 nm or more, it is a condition that the first-order diffracted light is generated. For example, in the case of TM polarized light, it is possible to control the phase from 0.3λ to 0.4λ in a region where the pitch is 400 nm or less.

  FIG. 27 shows an example of a configuration of a three-wavelength compatible optical pickup that applies a phase difference obtained by a metal groove. In the figure, laser light emitted from a BD (Blu-ray (registered trademark) Disc) laser 301 (λ = 405 nm), a DVD laser 302 (λ = 660 nm), and a CD laser 303 (λ = 780 nm) has a wavelength of The selection beam splitters 346 and 347 travel on the same optical path, are reflected by the metal groove 100, and are collected on the optical disk 4 by the objective lens 311. Here, when light of three different wavelengths is condensed on the corresponding disks by one objective lens, it is well known that residual spherical aberration becomes a problem. At this time, the wavelength of the BD and the wavelength of the DVD are approximately 1.5 times different from each other. Therefore, a technique for correcting spherical aberration by using a wavelength separation type diffraction grating using this is well known. However, since the wavelength of BD and the wavelength of CD are almost doubled, there is a problem that the wavelength separation type diffraction grating does not work effectively. In general, since optical design is performed in accordance with a BD having a high recording density, a decrease in light utilization efficiency and residual spherical aberration become technical problems for a CD. On the other hand, in the metal groove, the selection of the operating wavelength and the control of the phase difference shown in FIG. 27 can be controlled by the material of the comb-like structure and the shape parameters as shown in FIG. In the drawing, the central portion of the metal groove is shown in an enlarged manner, but this indicates that a metal groove having a ring-like shape is formed. Using the optical phase difference generated by the metal groove element having such a structure, spherical aberration remaining in the CD laser light can be corrected. Due to the wavelength selectivity described above, in BD and DVD, the metal groove can function as a reflector. With such a configuration, it is possible to improve the performance of the three-wavelength compatible optical pickup.

  The optical element shown here can be considered as a combination of a kind of diffraction grating and a metal groove. Since the grating pitch of diffraction gratings used for optical pickups is generally 10 μm or more, a hybrid diffraction grating in which the convex part of the diffraction grating is formed by a metal groove to improve polarization-dependent performance, wavelength selection performance, and phase control performance. Can also be provided.

  Although the above has been described mainly as a corresponding element from visible light to near infrared light, it can also be applied to radio waves which are a kind of light, and will be described subsequently. FIG. 28 shows an example of the configuration of a millimeter-wave microstrip antenna to which the phase control element of the present invention is applied. A microstrip antenna is a type of antenna and is also called a patch antenna. The band is narrow and has a wide directivity, and the antenna element can be manufactured at low cost by metal etching. Microstrip antennas are sized by the wavelength at the resonant frequency, so they are typically used at ultra-high frequency (UHF), microwave, and millimeter-wave frequencies, and are wireless communications that are installed outside an aircraft or spacecraft or inside an automobile. Used in equipment, in-vehicle millimeter wave radar, and the like. FIG. 28A schematically shows the configuration of a conventional microstrip antenna. In this case, the radio wave (= light) output from the antenna is distributed vertically with respect to the antenna surface due to symmetry. On the other hand, as shown in FIG. 28 (b), when a metal groove is disposed at the lower part of the antenna, the radio wave emitted downward is reflected by the metal groove and interfered and added to the radio wave emitted upward. It can be taken out as a double output. As a result, the directivity and output of the antenna can be improved. This characteristic is effective for an in-vehicle millimeter wave antenna or the like. FIG. 29 is a simulation result showing the improvement of the antenna output by the metal groove. Here, in the configuration shown in FIG. 28B, assuming that the wavelength is 4 mm (77 GH), the width of the comb structure is 0.05 mm, and the pitch is 0.5 mm, the relationship between the height h and the intensity of the output radio wave is calculated. The metal material was treated as a perfect conductor according to a general calculation method. As shown in the figure, it can be seen that if the height of the metal groove comb-like structure is about 1 mm, the sensitivity is improved about twice. At the same time, the polarization perpendicular to the direction of the output polarization (= vibration direction of the electric field) becomes the condition of zero height in FIG. 29 according to the principle shown in FIG. 5, and the radio wave is canceled by interference with the reflected wave. It is. The effects obtained by this are (1) the gain in the antenna upward direction is doubled, and (2) the orthogonal polarization is suppressed to almost zero. By utilizing these effects, it is possible to improve the output of the in-vehicle millimeter wave laser and increase the S / N ratio. This can be used for both transmitting and receiving antennas.

  The configuration of the antenna described here is not limited to millimeter waves but can be similarly used for microwaves. As a metal groove manufacturing method corresponding to millimeter waves and microwaves, press working, grinding, etching, plating and the like are preferable.

  According to the present invention, an optical device for optical communication, an optical device for optical recording, a display device, a device for wireless communication, and the like can be configured in a small size and at low cost.

100, 101, 102: Metal groove element,
10: Light source
110: Light guide plate,
120: Polarizing filter
130: Entrance window.

Claims (5)

A cross-sectional comb-like structure , a comb-like structure portion made of a metal material ,
A mirror structure part that is disposed within the coherence length of the light to be controlled from the comb structure and reflects incident light ;
The period of the comb-like structure is smaller than the wavelength of the light to be controlled,
The optical element characterized in that the comb-like structure extends in a direction crossing the cross section. The optical element according to claim 1,
An optical element, wherein the comb-shaped comb has conductivity, and a dielectric is provided between the comb-shaped combs. The optical element according to claim 1,
The optical element, wherein the comb-shaped structure portion and the mirror structure portion are integrated. The optical element according to claim 1,
The optical element, wherein the light is a radio wave.   An optical device comprising the optical element according to claim 1.

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