JP2013257371A - Photonic crystal element having polarization conversion function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photonic crystal element having a polarization conversion function which can serve as a minute transmission type optical element by using interference of light.SOLUTION: A photonic crystal element includes two kinds of optical elements 11,12 each of which has a width of 200nm or more and 1 μm or less alternately, wherein the photonic crystal element has a thickness of 2 μm or more and 20 μm or less. In the photonic crystal element, light incident upon one optical element enters the other optical element, and interferes with light propagating through the other optical element, so that a polarization plane of the incident light can be converted. This invention also relates to a polarization plane conversion method using the photonic crystal element.

Description

  The present invention relates to a photonic crystal element having a polarization conversion function. More specifically, the present invention relates to an optical element having a photonic crystal that can convert and output the polarization plane of input light by converting the polarization plane using interference of light entering from an adjacent region. .

  Japanese Patent No. 4777064 (Patent Document 1) discloses a plate-like optical element in which quarter-wave plate portions and -1 / 4-wave plate portions are alternately formed via joint portions. Yes. The optical element disclosed in this publication is a self-cloning photonic crystal.

Japanese Patent No. 4777064

  The optical element disclosed in Patent Document 1 has been adjusted so that light in each region does not interfere with each other. For this reason, the light leaking between the regions has been reduced as much as possible.

  An object of the present invention is to provide a photonic crystal element having a polarization conversion function that can function as a minute transmission optical element by utilizing interference of light.

  The present invention is basically a photonic crystal element having a polarization conversion function that can function as a minute transmissive optical element by utilizing interference of light. This photonic crystal element reduces the region width of the self-cloning photonic crystal including two regions alternately and increases the thickness of the photonic crystal. Then, light leaks to an adjacent region, and an interference phenomenon due to this leaked light occurs, so that the photonic crystal element has a polarization conversion function.

  The first aspect of the present invention relates to a photonic crystal element. This photonic crystal element is adjacent to the first optical element 11 and the first optical element 11, and the direction of the slow axis is different from that of the first optical element 11 (the sign of the slow wave axis is the opposite sign). There is a photonic crystal 13 in which second optical elements 12 are alternately present. Then, when the direction in which the first optical element 11 and the second optical element 12 are alternately present is the x-axis direction, the width of the first optical element 11 in the x-axis direction and the second optical element 12 The width in the x-axis direction is 200 nm or more and 1 μm or less, respectively. The slow wave axis of the first optical element (11) and the slow wave axis of the second optical element (12) are opposite in sign and form 1 degree or more and 40 degrees or less with respect to the x axis. Further, the thickness of the photonic crystal 13 is 2 μm or more and 20 μm or less.

  The second aspect of the present invention relates to a polarization conversion method using the above-described photonic crystal element. Light enters the first optical element 11 and the second optical element 12. The light incident on the first optical element 11 enters the second optical element 12. Then, the light that enters the second optical element 12 and propagates through the second optical element 12 interferes with the light that has entered the second optical element from the first optical element 11. The reverse is also true. Then, as demonstrated by the examples, the plane of polarization of the light output from the photonic crystal element can be converted.

  More generally, it can be said that the photonic crystal element and the polarization conversion method are as follows. The photonic crystal element has a large number of optical elements (i = 1, 2,..., N, n + 1,...). For example, the (n−1) th optical element and the (n + 1) th optical element are adjacent to the nth optical element. Thus, the nth and (n + 1) th optical elements are sequentially present in a certain direction. This direction is the x-axis direction. Then, the nth and (n + 1) th optical elements have different slow axis directions (that is, the signs of the slowwave axes), and the slow wave axes of the nth and (n + 1) th optical elements are x An angle of 1 degree to 40 degrees with respect to the axis. The nth and (n + 1) th optical elements have a width in the x-axis direction of 200 nm to 1 μm and a thickness of 2 μm to 20 μm. Light enters the nth and (n + 1) th optical elements. The light that has entered the nth optical element enters the (n−1) th and (n + 1) th optical elements, and the (n−1) th and (n + 1) th optical elements respectively enter. Interferes with propagating light. On the other hand, the light incident on the (n + 1) th optical element enters the nth and (n + 2) th optical elements and propagates through the nth and (n + 2) th optical elements, respectively. have a finger in the pie. As a result, the polarized light in the y direction incident on the photonic crystal element can be efficiently converted into the polarized light in the x direction on the output surface.

  Conventionally, a photonic crystal has prevented light from entering between adjacent optical elements as much as possible. The present invention actively utilizes light interference caused by light entering from adjacent optical elements. Thus, the present invention can provide a photonic crystal element having a polarization conversion function that can function as a minute transmission optical element.

FIG. 1 is a conceptual diagram of a photonic crystal element of the present invention. FIG. 2 is a diagram illustrating a structure example of a photonic crystal element. FIG. 3 is a diagram for explaining the concept of the present invention. FIG. 4 is a graph replaced with a drawing showing the amplitude distribution of the y-polarized component and the x-polarized component when Y excitation is performed. FIG. 5 is a graph instead of a drawing showing the amplitude distribution of the y-polarized component and the x-polarized component in the basic and second modes. FIG. 6 is a graph instead of a drawing showing the amplitude distribution of the y-polarized component and the x-polarized component when X excitation is performed.

  FIG. 1 is a conceptual diagram of a photonic crystal element of the present invention. As shown in FIG. 1, the photonic crystal element of the present invention is adjacent to the first optical element 11 and the first optical element 11, and the direction of the slow wave axis is different from that of the first optical element 11. It has the photonic crystal 13 with which the 2nd optical element 12 exists alternately. In the example of FIG. 1, layers made of two different substances are alternately accumulated in the height direction (z-axis direction). On the other hand, in the example of FIG. 1, two types of regions extending in the y-axis direction are alternately formed in the x-axis direction. In each region, peaks and valleys are regularly formed.

  In this photonic crystal element, light incident on the first optical element 11 region propagates to the adjacent second optical element 12 due to diffraction spread. Then, interference with the incident light to the second optical element 12 occurs. By utilizing this phenomenon, the photonic crystal element of the present invention can output by controlling the polarization plane of incident light. For example, in the optical element manufactured in the embodiment, when a plane wave is incident from the −z direction, the x-polarized light component passes through the + z direction almost as it is. On the other hand, the y-polarized component is converted into x-polarized light, diffracted to +/− 1 order, and output. That is, the optical element obtained in the embodiment can convert both the x-polarized component and the y-polarized component of incident light into the x-polarized component with high efficiency and output.

  As shown in FIG. 1, the photonic crystal element of the present invention is adjacent to the first optical element 11 and the first optical element 11, and the direction of the slow axis is different from that of the first optical element 11 ( It has photonic crystals 13 in which second optical elements 12 (that is, opposite signs) exist alternately. A “photonic crystal element” is an optical element for adjusting the phase or optical path of light. The slow axis of the first optical element 11 and the slow axis of the second optical element 12 are 1 degree or more and 40 degrees or less with respect to the x axis. This angle may be 1 degree or more and 20 degrees or less, 2 degrees or more and 15 degrees or less, 5 degrees or more and 15 degrees or less, 15 degrees or more and 40 degrees or less, or 20 degrees or more and 35 degrees or less. It may be 20 degrees or more and 30 degrees or less, or 25 degrees or more and 40 degrees.

  A photonic crystal is a micro periodic structure that functions as an optical element. Such a photonic crystal can be manufactured in accordance with a known photonic crystal manufacturing method. As an example of a method for producing a photonic crystal, a method disclosed in International Publication No. WO 2004/008196 pamphlet can be mentioned. On the other hand, as a more specific method for producing a photonic crystal, two or more kinds of substances are periodically and sequentially deposited on a substrate having two-dimensional periodic irregularities disclosed in Japanese Patent Laid-Open No. 10-335758. There is a method of manufacturing an optical element by stacking and using sputter etching alone or simultaneously with film formation on at least a part of the stack. This method is also called a self-cloning photonic crystal production method.

  The first optical element 11 and the second optical element 12 mean optical element regions having different slow axis directions. Each optical element functions as a wave plate. In order for an optical element to function as a wave plate, the period of peaks (valleys) of each region, the thickness of each layer constituting each region, the material constituting each layer constituting each region, and the input light The wavelength may be controlled. A technique for constructing a wave plate using a self-cloning crystal is known (for example, JP 2008-197399 A). Usually, the first optical element 11 and the second optical element 12 are manufactured as one photonic crystal. The first optical element 11 and the second optical element 12 may have a strip shape as shown in FIG. In the following description, the direction in which the first optical element 11 and the second optical element 12 are alternately present as shown in FIG. For example, when viewing the first optical element 11 and the second optical element 12 from the −z direction, the first optical element 11 and the second optical element 12 are formed by rectangular regions extending in the y-axis direction. May be. The number of the first optical element 11 and the second optical element 12 included in one optical element is arbitrary. An example of the number of the first optical element 11 and the second optical element 12 included in one optical element is 5 or more and 100000 or less, may be 10 or more and 10,000 or less, and may be 20 or more and 1000 or less.

  When the photonic crystal is manufactured with regularity, it has a function as an optical element such as a wave plate or a polarizer. In the present invention, the first optical element 11 and the second optical element 12 preferably function as wave plates, respectively. Since the first optical element 11 and the second optical element 12 have different physical patterns, the directions of the slow axis are different. Examples of physical patterns are mountain and valley directions. The first optical element 11 and the second optical element 12 are each formed from a plurality of peaks and valleys, and the peak directions are preferably θ and −θ from the x-axis. In this case, an example of θ is 10 ° to 44 °, may be 10 ° to 30 °, and may be 15 ° to 30 °. θ may be 46 ° or more and 89 ° or less. The regularity of the photonic crystal depends on the periodic pattern formed on the substrate.

  The period on the substrate also affects the retardation of the wave plate and the extinction ratio of the polarizer. In order to function as a self-cloning photonic crystal in the ultraviolet region, it is preferable that the period in the periodic structure formed on the substrate is 130 nm or less. The period is preferably 90 nm to 130 nm. In the present invention, this period may be not less than 50 nm and not more than 85 nm.

  A preferable photonic crystal may be designed so that the sum of the optical thickness (thickness × refractive index) of the high refractive index layer and the optical thickness of the low refractive index layer is less than or equal to the transmission wavelength. Examples of the refractive index are 1.1 or more and 2.5 or less. That is, the preferred wavelength plate according to the second aspect of the present invention is that the thickness of the high refractive index layer is 70 nm or less. The thickness of the low refractive index layer is 80 nm or less. The sum of the optical thickness of one high refractive index layer and the optical thickness of one low refractive index layer is 300 nm or less. It is assumed that the high refractive index layer is made of hafnium oxide and the low refractive index layer is made of silicon oxide. For example, when designing a wavelength plate for light of 300 nm, if the thickness of the low refractive index layer is 0, the thickness of the high refractive index layer containing hafnium oxide may be 150 nm or less. On the other hand, when the thickness of the high-refractive index layer is almost 0, and the refractive index of the low-refractive index layer is 1.6, when designing a wavelength plate for light of 300 nm, it is sufficient that the thickness is 188 nm or less. When the thickness of the high refractive index layer is 70 nm or less, the optical thickness of the high refractive layer is about 140 nm or less. On the other hand, the optical thickness of the low refractive index layer is about 128 nm or less. Therefore, the optical thickness of one high refractive index layer and the optical thickness of one low refractive index layer is preferably 268 nm or less. Furthermore, the refractive index of silicon oxide may be 1.5, for example. In this case, the optical thickness of the low refractive index layer is 120 nm or less. Therefore, 260 nm or less is preferable as the optical thickness of one high refractive index layer and the optical thickness of one low refractive index layer. On the other hand, in the example, the thickness of the high refractive index layer was 20 nm, and the thickness of the low refractive index layer was 20 nm. The optical thickness of the high refractive index layer in this example is 38 nm to 42 nm. On the other hand, the optical thickness of the low refractive index layer in this embodiment is 30 nm to 32 nm. Therefore, even if the optical thickness of one high refractive index layer and the optical thickness of one low refractive index layer is in the range of 68 nm to 74 nm, it functions well as a wave plate.

Width 2d ₂ in the x-axis direction of the x-axis direction of the width 2d ₁ and the second optical element 12 of the first optical element 11 may be the same or different. An example of d ₁ / d ₂ is 0.8 or more and 1.2 or less, and may be 0.9 or more and 1.1 or less. 2d ₁ and 2d ₂ are preferably designed in the same order as the wavelength of the transmitted light. For this reason, examples of 2d ₁ and 2d ₂ are 200 nm to 1 μm, respectively, and may be 400 nm to 800 nm. When the refractive index parallel to the slow wave axis is n _s and the refractive index parallel to the fast wave axis is n _f for the first optical element 11 or the second optical element 12, 2d ₁ and 2d ₂ are It is preferably designed to be 0.8 times or more and 1.2 times or less, or 0.9 times or more and 1.1 times or less of 2d.

_{^{2d = vλπ -1 (n s 2}} -n f 2) -1/2

  In the above formula, v is the speed of light, and λ means the wavelength of light. The wavelength of light is, for example, 200 nm or more and 1500 nm or less, 250 nm or more and 1100 nm or less, or 300 nm or more and 800 nm or less. The light is preferably visible light. The period (width) of the first optical element and the second optical element may be adjusted according to the wavelength of the light for adjusting the polarization plane. An example of the period is from 1/5 to 5 times the wavelength, from 1/3 to 3 times the wavelength, and from 1/2 to 2 times the wavelength.

  In Japanese Patent No. 4777064, quarter-wave plate portions and -1 / 4-wave plate portions are alternately formed via joint portions. As described above, in the conventional photonic crystal having two or more regions, it is desirable to form a junction portion so that light does not enter from adjacent regions. On the other hand, in the photonic crystal element of the present invention, it is preferable to smooth the boundary between the first optical element 11 and the second optical element 12 in order to actively leak light from adjacent regions. . More specifically, as shown in FIG. 1, the first optical element 11 and the second optical element 12 are designed to be crystals having a plurality of peaks and valleys having the same pitch, The interface between the optical element 11 and the second optical element 12 is a continuous mountain and valley. By doing so, it is possible to prevent a situation where the light travels in an unintended direction when light enters from an adjacent region.

  The photonic crystal element of the present invention controls polarization by causing light to enter from adjacent regions and causing interference. For this reason, the optical element preferably has a thickness. Examples of the thickness of the optical element are 2 μm or more and 20 μm or less, 3 μm or more and 15 μm or less, or 4 μm or more and 10 μm or less.

As described above, since the self-cloning method is already known, the photonic crystal element of the present invention can be produced using a known method. At that time, unlike the case of manufacturing a normal self-cloning photonic crystal, the width of the adjacent region is designed to be small, and the pattern (groove) on the substrate is designed so that the regular pattern in the adjacent region is continuous.

Polarization conversion method of the present invention The polarization conversion method of the present invention uses the photonic crystal element described above. Then, light enters the first optical element 11 and the second optical element 12. Thereafter, the light incident on the first optical element 11 enters the second optical element 12 and interferes with the light propagating through the second optical element 12. Similarly, the light incident on the second optical element 12 enters the first optical element 11 and interferes with the light propagating through the first optical element 11. Thereby, the polarization plane of the light output from the photonic crystal element can be converted.

Principle of Polarization Control by Photonic Crystal Element An example of the structure of a photonic crystal element is shown in FIG. On the other hand, FIG. 3 is a diagram for explaining the concept of the present invention. In the photonic crystal element of the present invention, the slow wave axis and the fast wave axis are +/− θ; π / 2 +/− θ in each strip region. The following describes the physical picture of the motion and how it is analyzed.

In FIG. 3, consider the case of excitation with uniform y-polarized light. In the wavelength plate (birefringent medium), if when the area is large enough, at a phase velocity when the electric field is parallel to the slow wave axis corresponding to the large refractive index n _s, if parallel to the fast wave axis refractive index less Propagate independently at a phase velocity corresponding to n _f . As shown in FIG. 3, when each region has a periodic structure and the width of the strip is on the order of wavelength, the slow wave and the fast wave are combined. In other words, the slow wave and the fast wave in the strip propagate in the z direction and “cross-border” to the adjacent strip due to diffraction spread. Then, because the main axis is different in the adjacent region, the slow wave and fast wave are coordinate-transformed in a new environment and are forcibly polarized. As a result, a constant pattern is formed, and the slow wave and the fast wave are superimposed at a constant ratio and propagate in the z direction at a constant phase velocity. This is called a mode (Y mode). There is not only one Y mode, but second, third,. Due to symmetry, the same phenomenon occurs when the structure of FIG. 3 is excited with x-polarized light (X mode; the first, second,... Are the same).

  The above is summarized. When a uniform plane wave of arbitrary polarization enters the structure of FIG. 3 vertically, it is a superposition of x-polarized light and y-polarized light. x-polarized light is expressed as X-mode superposition, and y-polarized light is expressed as Y-mode superposition. Each excited mode propagates with a unique phase velocity. The distribution of the electromagnetic field in a cross section at a distance from the excitation point is expressed as a superposition of them.

  First, the state of polarization merging, and the shape of the mode that forms the basis of it will be described. In order to facilitate the analysis, the number of modes used for the expansion is limited to 2 (which may be sufficient accuracy). Excitation is performed with uniform y-polarized light, and is propagated (developed) to a cross section where the propagation phase difference between the first Y mode and the second Y mode is π. FIG. 4 shows amplitude distributions of the y-polarized component of the first Y mode and the x-polarized component of the second Y mode at the excitation point and the surface after propagation. From FIG. 4, it is observed that according to the present invention, polarization conversion from y-polarized light to x-polarized light is performed with high efficiency.

  In order for such a phenomenon to be possible, the first and second Y modes are (1) the power of each x component is about the same as the power of the y component (2) the sum of the x components at the excitation point It is preferable that there is a point where the y component becomes zero and the x component becomes maximum due to (3) evolution which is substantially zero.

  FIG. 5 shows the x and y components of the minimum mode and the second smallest mode. FIG. 5 shows that the first and second modes of Y have such properties. Next, consider what happens when excited by x-polarized light. In conclusion, the X mode does not have such a remarkable property. Since the first mode does not have a large y component, even if the first and second modes propagate while transitioning between the sum, difference, and other states, the x-polarized light is predominant and no conversion occurs. . FIG. 6 shows the state of (small) polarization conversion in the cross section of the propagation phase difference π.

3. Experiments We designed and prototyped a self-cloning photonic crystal waveplate array based on the above polarization conversion and the resulting confluence of polarized light. The specifications are shown in the table below.

  The polarization conversion characteristics verified by the propagation experiment are shown below.

  From the table, it can be seen that when X-polarized light is incident, the zero-order light of X-polarized light is the main output. On the other hand, when Y-polarized light is incident, it is understood that the +/− 1 order light of X-polarized light (component in a direction slightly shifted from the straight traveling direction) is the main. That is, the table shows that in this example, the polarization plane of incident light is controlled and output as X-polarized light.

  The present invention can be used in the field of optical instruments.

11 First optical element 12 Second optical element 13 Photonic crystal

Claims (4)

A polarization conversion method using a photonic crystal element,
The photonic crystal element is
A first optical element (11), a second optical element (12) adjacent to the first optical element (11) and having a slow axis direction different from that of the first optical element (11); Have photonic crystals (13) alternately present,
When the direction in which the first optical element (11) and the second optical element (12) exist alternately is the x-axis direction, the slow wave axis of the first optical element (11) The slow wave axis of the second optical element (12) has the opposite sign, and is 1 degree or more and 40 degrees or less with respect to the x axis,
The width of the first optical element (11) in the x-axis direction and the width of the second optical element (12) in the x-axis direction are 200 nm to 1 μm, respectively.
The photonic crystal (13) has a thickness of 2 μm or more and 20 μm or less;
A region where light is incident on the first optical element (11) and the second optical element (12);
Light incident on the first optical element (11) enters the second optical element (12) and interferes with light propagating in the second optical element (12);
The light incident on the second optical element (12) enters the first optical element (11) and interferes with the light propagating through the first optical element (11);
Thus, a polarization conversion method for efficiently converting polarized light in the y direction incident on the photonic crystal element into polarized light in the x direction on the output surface. A first optical element (11), a second optical element (12) adjacent to the first optical element (11) and having a slow axis direction different from that of the first optical element (11); A photonic crystal element having photonic crystals (13) alternately present,
When the direction in which the first optical element (11) and the second optical element (12) are alternately present is the x-axis direction,
The width of the first optical element (11) in the x-axis direction and the width of the second optical element (12) in the x-axis direction are 200 nm to 1 μm, respectively.
The photonic crystal (13) has a thickness of 2 μm or more and 20 μm or less;
When the direction in which the first optical element (11) and the second optical element (12) exist alternately is the x-axis direction, the slow wave axis of the first optical element (11) The slow wave axis of the second optical element (12) has an opposite sign, and forms 1 degree or more and 40 degrees or less with respect to the x axis.
Photonic crystal element.
A photonic crystal element having a large number of continuous optical elements,
When the (n−1) th optical element and the (n + 1) th optical element are adjacent to the nth optical element and the direction in which the optical elements sequentially exist is the x-axis direction, the nth optical element And the (n + 1) th optical element has different slow axis directions, and the slow axis of the nth and (n + 1) th optical elements has an angle of 1 degree to 40 degrees with respect to the x axis. None,
The nth and (n + 1) th optical elements have a width in the x-axis direction of 200 nm to 1 μm and a thickness of 2 μm to 20 μm.
Photonic crystal element.
A polarization conversion method using a photonic crystal element,
The photonic crystal element is a photonic crystal element having a large number of continuous optical elements,
When the (n−1) th optical element and the (n + 1) th optical element are adjacent to the nth optical element and the direction in which the optical elements sequentially exist is the x-axis direction, the nth optical element And the (n + 1) th optical element has different slow axis directions, and the slow axis of the nth and (n + 1) th optical elements has an angle of 1 degree to 40 degrees with respect to the x axis. None,
The nth and (n + 1) th optical elements have a width in the x-axis direction of 200 nm to 1 μm, a thickness of 2 μm to 20 μm,
Light is incident on the nth and (n + 1) th optical elements;
The light incident on the nth optical element enters the (n-1) th and (n + 1) th optical elements, and the (n-1) th and (n + 1) th optical elements respectively. Interfering with light propagating through the optical element;
The light incident on the (n + 1) th optical element enters the nth and (n + 2) th optical elements and propagates through the nth and (n + 2) th optical elements, respectively. Interfering steps,
A polarization conversion method that converts polarized light in the y direction incident on the photonic crystal element into polarized light in the x direction on the output surface.

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