JP4427026B2 - Polarizer and polarization separation element - Google Patents

Description

  The present invention is used in an optical apparatus that utilizes polarization control of light, and provides a function of transmitting only a linearly polarized component in a specific direction of incident light, and two orthogonal linearly polarized components of incident light. The present invention relates to a polarization separation element that provides a function of separating light.

The polarizer is an element for converting non-polarized light or elliptically polarized light whose electromagnetic field vibrates in an unspecified direction into linearly polarized light by transmitting only a vibration component in a specific direction. This is one of the most basic optical elements, and is widely used in optical communication devices, optical disk pickups, liquid crystal projectors, liquid crystal displays, optical applied measurements, and the like. Polarizers are roughly divided into two types, (1) those that absorb unnecessary polarization, and (2) those that split two orthogonal polarization components incident on the same optical path into separate optical paths. Is done. (2) is also used as a polarization separation element. Here, both are collectively referred to as a polarizer.
Polarizers currently in practical use that operate in the above (1) are those in which a dichroic molecule such as iodine is placed in a polymer film, glass in which needle-shaped metal particles are arranged in one direction, etc. There is.
On the other hand, as the polarization separation element that performs the operation (2), there are an element using a Brewster angle by forming a multilayer film on the slope of the prism (so-called PBS), a polarization prism made of birefringent crystals such as calcite, and the like. .
Recently, in the field of optical recording and display, a polarizer that operates in the visible range is regarded as important. In particular, in a liquid crystal projector, linearly polarized light is incident on each of the liquid crystal elements of RGB. However, in order to achieve high brightness and miniaturization, light of high power density is often handled. Since it is absorbed, the element generates heat. This is a problem that causes deterioration in reliability. In the type {circle around (2)}, PBS is not suitable for miniaturization because it has a cubic shape, and birefringent crystal prisms are not generally used because of their high cost.
By the way, as a new polarization separation element belonging to (2), a photonic crystal polarization separation element according to Patent Document 1 (Japanese Patent Laid-Open No. 2001-83321) has been proposed. This has a structure in which wavy thin films are stacked in multiple layers, and can be separated into a polarization component parallel to the groove direction and a polarization component perpendicular to the groove direction. Since a lossless material is used, there is no internal absorption, and there is no problem of heat generation even with high light density light.
In the optical communication wavelength region, this can be realized by configuring the structure using Si and SiO ₂ . For example, Non-Patent Document 1 (T. Kawashima, et al., “Photonic crystal polarization beams and therir applications,” Optical Fiber Communication Confections, AFC3, OF200p. In the vicinity of 0.5 μm, a transmission loss of 0.2 dB or less and a transmission quenching non-40 dB or more are obtained.
On the other hand, Si absorbs at wavelengths shorter than about 1 μm and is not suitable for use. In such a wavelength region, a transparent dielectric material Ta ₂ O ₅ , Nb ₂ O ₅ , TiO ₂ , ZrO ₂ or the like having a high refractive index may be used instead of Si. However, the refractive index of these materials is 2 to 2.4, which is not as great as 3.5 of Si. Accordingly, there is a problem that the cutoff wavelength region in the multilayer film combined with the low refractive index material is narrowed, and the operating band of the polarizer is narrowed. For example, it is difficult to cover R (red), G (green), and B (blue) wavelength bands (for example, 400 nm to 500 nm, 500 nm to 600 nm, and 600 nm to 700 nm) that are often used in display devices. Further, when light is incident obliquely on the surface of the polarizer, the band is further narrowed, so it cannot be said that it is suitable for practical use.

The present invention is to solve the above-mentioned problem of widening the bandwidth, and an object of the present invention is to expand the band by changing the laminated film thickness or providing a plurality of regions having different periods. . At the same time, it is intended to increase the manufacturing tolerance and suppress the incident angle tolerance by suppressing the band shift due to the film thickness variation.
First, an outline of the photonic crystal polarizer will be described. A transparent high-refractive-index medium 102 and a low-refractive-index medium 103 are alternately stacked on a transparent material substrate 101 having a periodic groove array as shown in FIG. 1 while preserving the shape of the interface. Each layer has periodicity in the x direction, but the y direction may be uniform, or may have a periodic or aperiodic structure having a length sufficiently larger than the x-axis direction. Although it may be periodic in the z-axis direction, it may be a combination of a plurality of periods in which the stacking period gradually changes or changes stepwise as shown in FIG.
Such a technique for forming a periodic structure is called a self-cloning technique described in Patent Document 2 (Japanese Patent Laid-Open No. 10-335758), and has a high reproducibility and uniformity, and an industrially fine periodic structure ( This is an excellent method for producing a photonic crystal.
When non-polarized light or elliptically polarized light is incident on the periodic structure thus obtained perpendicularly or obliquely to the xy plane, the polarization is parallel to the groove row, that is, y-polarized light, and x-polarized light orthogonal thereto. On the other hand, TE mode light and TM mode light are excited inside the periodic structure. Usually, a multilayer film has a wavelength region in which light can propagate and a wavelength region in which light is reflected and blocked. In the case where there is an uneven period in the plane as shown in FIG. 1, the cutoff region can be polarized. For example, it can be designed such that TM waves are transmitted and TE waves are reflected.
As the low refractive index medium, a material mainly composed of SiO ₂ is the most common, has a wide transparent wavelength region, is stable chemically, thermally and mechanically, and can be easily formed. However, other optical glass may be used, and a material having a lower refractive index such as MgF ₂ may be used. High refractive index materials include semiconductors such as Si and Ge, oxides and nitrides such as Ta ₂ O ₅ , TiO ₂ , Nb ₂ O ₅ , HfO ₂ , Al ₂ O ₃ , and Si ₃ N ₄ , or those Can be used, has a wide transparent wavelength range, and can also be used in the visible light region. On the other hand, a semiconductor is limited to the near infrared region, but has an advantage of a large refractive index.
FIG. 2A is an example of a dispersion curve used in the design of a polarizer. The vertical axis represents the value obtained by normalizing the reciprocal of the wavelength by the lamination period, and the horizontal axis represents the value obtained by normalizing the phase change amount when propagating one period by π. White circles indicate TE waves and black circles indicate TM waves. Here, assuming that the in-plane period is Lx and the stacking period is L _z , L _z / L _x = 1 in (a). In the hatched band, the TE wave is reflected as a band gap, and the TM wave is transmitted because it is a propagation band, and thus operates as a polarization separation element.
Parameters for controlling this operating wavelength range are the refractive index, filling factor, groove row cycle L _x , stacking cycle cycle L _z , and the like. Among these, L _z can be arbitrarily changed during lamination. For example, when Lz is increased, the wavelength band for blocking the TE wave is shifted to the longer wavelength side. FIG. 3 shows the concept. A solid line 301 indicates the transmittance of a TE wave having a periodic structure with a small Lz, and the transmission is blocked in the wavelength region indicated by A. On the other hand, the broken line 302 is the transmittance of the TE wave having a periodic structure with a large Lz, and is blocked in the region B on the slightly longer wavelength side. A solid line 303 indicates the transmittance of TM waves of both structures. Therefore, in the wavelength region indicated by C, the TM wave is a propagation region in all the stacking directions, and the TE wave is at least partially included in the cutoff region. By changing the stacking period in this way, it is possible to expand the wavelength region that operates as a polarizer.

FIG. 1 is a diagram illustrating a structure of a polarization separation element.
FIG. 2 is a diagram illustrating a dispersion relation of light propagating through the periodic structure.
FIG. 3 is a diagram showing the concept of expansion of the cut-off area by multiplexing the stacking periods.
FIG. 4 is a diagram showing a cross-sectional structure of the polarization separation element.
FIG. 5 is a diagram showing a transmission spectrum of a polarizer.
FIG. 6 is a diagram showing a cross-sectional structure of the polarization beam splitting element.
FIG. 7 is a diagram illustrating one embodiment.
FIG. 8 is a diagram illustrating one of the embodiments.

FIG. 4 is a diagram showing the structure of the embodiment of the present invention. In this figure, reference numeral 401 is an amorphous Ta ₂ O ₅ layer, and reference numeral 402 is a SiO ₂ layer. Each layer has a wave shape, and the period Lx in the x-axis direction is 0.20 μm. The layers are alternately stacked in the z-axis direction. Immediately after the substrate, the layers are stacked with a period of 0.20 μm, and then the period is gradually increased, and the period is 0.22 μm in the second half of the multilayer film. By combining multilayer films with different periods, the wavelength range covered by one period can be synthesized while being shifted little by little, so that the operating wavelength range of the entire element can be expanded.
The manufacturing method is as follows. First, periodic grooves as shown in FIG. 4 are formed on the substrate 101 by electron beam lithography and dry etching on the substrate. Other photolithography, interference exposure, and stamping techniques using a mold may be used. The cross-sectional shape of the groove is rectangular, but may be other shapes such as a triangle. Here, quartz glass was used for the substrate. The width of the groove is 0.1 μm, and the depth of the groove is 0.1 μm. In order to prevent reflection on the boundary surface, an antireflection film 403 is attached. On this substrate, Ta ₂ O ₅ and SiO ₂ targets are used, and alternate multilayer films are laminated by combining sputtering deposition and bias sputtering. At this time, it is important to appropriately set the bias condition so that the periodic uneven shape of each layer is preserved in the x-axis direction. The conditions were as follows. In the film formation of the Ta ₂ O ₅ layer, the gas pressure is 2 mTorr, the target applied high frequency power 300 W, and in the film formation of the SiO ₂ layer, the gas pressure 6 mTorr, the target applied high frequency power 300 W, sputter etching is performed after the SiO ₂ layer is formed. The pressure is 2 mTorr and the substrate applied high frequency power is 90 W. The ratio of the thickness of the Ta ₂ O ₅ layer and the SiO ₂ layer was 43:57.
In order to expand the operating band of the polarizer, the period in the stacking direction is changed. Specifically, immediately after the substrate, three cycles are stacked with a cycle of 0.20 μm, and thereafter the cycle is gradually increased by 2%, and the center portion of the multilayer film is 0.22 μm. Three cycles are stacked in this cycle.
FIGS. 2A and 2B show a band structure having a simple repetition period in the stacking direction (L _x = 0.2 μm, L _z = 0.22 μm). The calculation used the FDTD method. The operating range in which the TE wave (polarized wave parallel to the groove) becomes the stopband and the TM wave (polarized wave perpendicular to the groove) becomes the passband is 384 nm to 408 nm (L _x /λ=0.52 in FIG. 0.49) and (b) are 393 to 433 nm (L _x /λ=0.51 to 0.462). By combining these, the operating range of the polarizer can be from 383 nm to 433 nm. The purpose of this design was to extend the operating wavelength range, and as shown in FIG. 4, the stacking period was linearly chirped from 200 nm to 220 nm. FIG. 5 shows transmission spectra of TE and TM waves calculated by the FDTD method. The propagation mode near the wavelength of 390 nm is reduced and the substantial band is widened.
Depending on parameters such as the refractive index, the operating band, and the incident angle of the constituent materials, the rate of change of the stacking period and the width of the change can be other values. For example, the same effect can be realized by gradually reducing the film thickness according to the lamination. Further, as shown in FIG. 6, the stacking period may be increased and may be decreased from the middle. Moreover, it may be changed as thick-thin-thick as shown in FIG. 7A, or the period may be changed stepwise as shown in FIG. 7B.
Fabrication was performed with a lamination period of 23 periods (4 mm square, lamination thickness of 5 μm or less). In the transmission spectrum of the polarizer measured with the spectrophotometer, the TE wave is blocked from a wavelength of 350 nm to 418 nm, and operates as a polarizer. The insertion loss at the flat part of the transmission band excluding Fresnel reflection at the substrate boundary was about 0.1 dB, and the TE wave extinction ratio measured with a laser beam having a wavelength of 400 nm was 40 dB or more.

Another embodiment of the present invention is shown in FIG. As an example of a polarizer operating at an oblique incidence, a case where the incident angle of light is 45 degrees with respect to the substrate plane is shown. The direction of the groove formed on the substrate 101 is perpendicular to the paper surface. In this case, the reflected light is an S wave and the transmitted light is a P wave. A photonic crystal polarizer 801 made of a multilayer film is formed on such a substrate by a self-cloning method. The film forming method is the same as that in the previous embodiment.
The thickness of the laminated film needs to be designed according to the incident angle. In this structure, the operating band shifts to the long wavelength side compared to the case of normal incidence. For example, in order to operate at a wavelength of 490 nm to 520 nm, it is necessary to chirp the pitch of the substrate unevenness to 0.2 μm and the stacking period from 0.20 μm to 0.22 μm. In addition to the above example, the operating wavelength range can be arbitrarily selected within the range where the material is transparent. In accordance with the wavelength to be operated, the pitch of the substrate unevenness and the lamination period are set. For example, for a wavelength of 1064 nm, an example of a design in which the uneven pitch of the substrate is modulated to 560 nm and the lamination period is changed from 0.30 μm to 0.34 μm.
Also, a design in which the direction of the groove is parallel to the paper surface is possible. In this case, the reflected light is a P wave and the transmitted light is an S wave. In this case, it is known from the calculation that when the incident angle is changed, the band is narrowed, but the shift of the center wavelength is small.
By using the oblique incidence as in this embodiment, the reflection component can be largely shifted from the axis of the incident light. Therefore, unnecessary polarization components can be separated and removed by an absorber that is separately installed. Alternatively, both polarization components can be used by separating them into two polarizations.

  The polarization separation element having the structure of the present invention is made of an inorganic material thin film that is thermally and chemically stable, and can cope with the visible ultraviolet region to the near infrared region. In particular, since it operates in the visible range, it is useful for optical parts such as liquid crystal projectors, optical pickups, and laser printers that handle high output light. Further, this structure is suitable for mass production by a technique based on a bias sputtering method called a self-cloning method, and can replace a conventional element.

Claims (6)

A stacking direction in which a high refractive index medium layer made of a transparent and high refractive index medium and a low refractive index medium layer made of a transparent and low refractive index medium in the three-dimensional orthogonal coordinates x, y, z are along the z-axis direction. A multi-layer structure alternately laminated to each other,
Each medium layer has a periodic concavo-convex structure having a wavelength equal to or less than the wavelength of light used in the x-axis direction, a uniform structure in the y-axis direction, or a periodic length longer than the x-axis direction. Has an aperiodic uneven structure,
The laminating period in the z-axis direction of the alternating layers composed of two medium layers laminated adjacent to each other is different in at least a part of the laminating direction,
For light that is incident on the xy plane perpendicularly or obliquely from the z-axis direction surface of the multilayer structure, either the polarization of the electric field orthogonal to the y-axis or the polarization orthogonal to the x-axis has a period in the stacking direction. The period of the periodic concavo-convex structure in the x-axis direction and the z-axis direction are included so that they are commonly included in the propagation region in all different regions, and the other polarization is included in the cutoff region in at least a part of the region in the stacking direction A polarization separation element, wherein a lamination period is selected. The high refractive index medium layer is made of Si, Ge, TiO _2, Ta ₂ O _5, Nb ₂ O _5, HfO _2, Al ₂ O _3, Si ₃ N ₄ or a mixture thereof, and the low refractive index medium layer is The polarization separation element according to claim 1, wherein the polarization separation element is made of SiO ₂ or MgF ₂ .   The polarized light separating element according to claim 1, wherein the polarization separating element can be used in a visible ultraviolet region to a near infrared region. The high refractive index medium layer is made of TiO _2, Ta ₂ O _5, Nb ₂ O _5, HfO _2, Al ₂ O _3, Si ₃ N ₄ or a mixture thereof, and operates in the visible light region. The polarization separation element according to claim 2.   5. The polarization separation element according to claim 1, wherein an incident angle of light is 45 degrees or less with respect to the xy plane.   The thickness of the alternating layers is increased or decreased as it goes to the air side, so that the stacking period of the alternating layers in the z-axis direction is different in the stacking direction. The polarization separation element according to claim 1, 2, 3, 4, or 5.

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