JP2014021200A - Optical switch and reflective display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical switch that has a fast speed of changing displays, can display without restricted by the environment of usage and has high reliability.SOLUTION: A reflective display device 2 is constituted by combining a substrate 103 on which an actuator 104 comprising a thin film PZT is formed and a transparent cover member 101 on which an inverse opal structure 102 is formed as an optical switch. The inverse opal structure 102 comprises a material having flexibility produced by using a close-packed structure of spherical fine particles as a template, and has a structure having a photonic band gap function to reflect light at a desired wavelength by the structural period. A switching function can be obtained by deforming the inverse opal structure by applying force by the actuator 104. The paired combination of the inverse opal structure and the actuator constitutes a display part of one pixel.

Description

  The present invention relates to an optical switch that reflects light of a specific wavelength, and a reflective display device that includes the optical switch as a display member.

Under strong light rays such as sunlight, the self-luminous display has poor visibility.
For this reason, there is a high demand for the realization of a display that can exhibit high visibility even in a bright environment, and reflective displays by various methods have been realized so far.
Among them, those using structural colors that exhibit regular periodic structures such as opal crystals in which fine particles of uniform particle size are arranged in a close-packed structure can be driven with relatively low energy consumption, so they are actively researched and developed. Has been done.

There has been proposed a technique for realizing an optical switch or a display using a principle that a periodic structure is changed by compressive stress or tensile stress and a structural color changes accordingly.
A conventional display using a structural color of a periodic structure change type based on an opal crystal or an aggregate of fine particles utilizes a structural color change of a mixed system composed of fine particles and a matrix enclosed in a cell.
For this reason, there are problems that it takes a long time to switch the display due to the slow reaction speed, and that the cell containing the solution makes it difficult to reduce the size and form an array.

  Further, since it is a mixed system of solid fine particles and liquid, there is a problem that it is easily affected by gravity, acceleration, and environmental temperature, and is restricted by the environment and use posture.

Patent Document 1 aims to realize a display color obtained by changing a structural color in response to an external stimulus at a high response speed, and to reduce variation due to environmental changes such as aging and temperature. A display member having the following has been proposed.
This display member is composed of a sphere and a matrix, and has a structure enclosed in a cell frame. The display member is deformed by receiving an external stimulus, and a structural color change is displayed accordingly.
The matrix is characterized by comprising an ionic liquid having a melting point of 50 ° C. or lower.

  However, since the technique described in Patent Document 1 has a structure in which a liquid is sealed in a cell, it is difficult to obtain a sufficient speed for switching the display, and furthermore, the drive system is complicated and the size and array can be reduced. The above problems such as difficulty are still not solved.

  The present invention was devised in view of such a current situation. The main feature of the present invention is to provide an optical switch that has a high display reliability and a display switching speed that can be displayed without being restricted by a use environment. Purpose.

  In order to achieve the above object, the present invention is composed of a flexible material using a close-packed structure of spherical fine particles as a mold, and has a photonic band gap function in which the structure period reflects a desired wavelength. An optical switch having an inverse opal structure having a structure and obtaining a switching function by deforming the inverse opal structure.

According to the present invention, the inverse opal structure is formed of a flexible material, and the inverse opal structure itself is deformed to express a change in structural color. As a result, the display switching speed is high, and there are no restrictions on the use environment or use posture.
In addition, the driving method is simple, and miniaturization and arraying become easy.

It is principal part sectional drawing of the reflection type display apparatus which concerns on one Embodiment of this invention. FIG. 5 is a diagram showing a method for forming an inverse opal structure, where (a) shows a state in which a dispersion liquid containing spherical fine particles is introduced into each cell, and (b) shows a state where the dispersion liquid is evaporated and spherical fine particles are closely packed. The figure which shows the state which filled the flexible material (ultraviolet curable epoxy resin) in the space | gap of a close-packed structure after forming a structure, (c) is a figure which shows the state which removed only spherical fine particles, d) is a view showing a state in which a separation wall forming a cell is removed.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Before describing the specific configuration, the gist of the present invention will be described. A structure having a periodic structure close to the wavelength of light has a characteristic of exhibiting a characteristic behavior with respect to light.
Specifically, in a structure whose refractive index changes periodically in three dimensions, when the period is close to the order of the wavelength of light, the difference between the two types of refractive indices and the wavelength determined by the period size The phenomenon that light cannot propagate occurs.
This phenomenon is called a photonic band gap in the photonic crystal.

That is, it is possible to control the wavelength of the photonic band gap by changing the refractive index or the period size of the two types of substances constituting the photonic crystal.
The fact that the wavelength corresponding to the photonic band gap cannot propagate through the photonic crystal indicates that light of that wavelength is selectively reflected.
Therefore, if the wavelength of the photonic band gap is changed, the wavelength of the reflected light will change. By controlling the photonic band gap of the pixels formed in an array individually, a reflective display It becomes possible to function as.

In the present invention, since this periodic structure is realized by an inverse opal structure using a regular array of fine particles as a template, an inverse opal structure can be formed by a manufacturing method with excellent dimensional controllability.
Furthermore, since the inverse opal structure is a so-called skeleton structure made of a flexible material, the force required for deformation can be reduced, and as a result, an optical switch (change in structural color) with high reaction speed can be achieved even with energy-saving drive. Realization is possible.

The present invention is an optical switch using a photonic band gap of a photonic crystal. Furthermore, a reflective display is realized by forming optical switches in an array and individually controlling them to change the wavelength of the photonic band gap.
The greatest feature of the present invention is that an inverse opal structure is used as a member that exhibits the function of a photonic band gap.

In general, a regular array in which spherical fine particles having a uniform particle diameter are arranged in a self-organizing manner has the property of a photonic crystal.
It is known that an inverted opal structure formed using this as a template, that is, what is generally called an inverse opal structure, also exhibits a photonic band gap function.
In the present invention, it is utilized that the regular array formed by self-organizing spherical fine particles having a uniform particle diameter has a communicating void.

An inverse opal structure can be obtained by selectively removing only the fine particles after filling and solidifying a material having a certain fluidity in the voids.
By using a flexible material as the filling material, the inverse opal structure can be easily deformed.
When forming such an inverse opal structure, a substrate is prepared so that individual regions corresponding to the pixels are formed, and an actuator that deforms each pixel is individually provided so that only the target pixel is provided. Can be deformed.

A pixel having a deformed inverse opal structure has a phenomenon that a wavelength corresponding to the photonic band gap is different from a pixel whose periodic structure is not deformed.
This means that the wavelength of light reflected by the deformed pixel and the non-deformed pixel is different, that is, it functions as a reflective display.
The use of thin film PZT generally used in MEMS (Micro Electro Mechanical Systems) technology as an actuator for deforming the inverse opal structure is also a major feature of the present invention.

Forming the thin film PZT as an actuator that can be individually driven in a desired region by using a semiconductor manufacturing process can be easily achieved by using a general technique.
Further, since the thin film PZT is a voltage-driven actuator, it has a feature of low power consumption, which contributes to energy-saving driving.

The present invention is characterized in that an actuator made of such a thin film PZT is formed with an inverse opal structure functioning as a display pixel with high dimensional accuracy.
That is, the main feature is that both the deforming inverse opal structure and the actuator that exerts the deforming force can be formed with solid constituent requirements (solid state).
It is not a complex system of mixed matrix of fine particles and solution as in the past, but it can be formed with solid components, so it is not affected by gravity and acceleration, and the environment of use and posture of use are flexible. Can be maintained in a high state.

Hereinafter, it demonstrates concretely using drawing.
FIG. 1 is a cross-sectional view of the main part of the reflective display device 2. A substrate 103, an actuator 104 made of a thin film PZT (hereinafter also simply referred to as “PZT”), an inverse opal structure 102 as an optical switch, a transparent cover member 101, and a control (not shown) that selectively drives the actuator 104 It consists of parts.
An inverse opal structure 102 is provided in contact with the transparent cover member 102.
An actuator 104 that applies a force to the inverse opal structure 102 is formed on the substrate 103.

These are combined to constitute the reflective display device 2.
The inverse opal structure 102 provided independently is an inverse opal structure formed using a regular array of spherical fine particles selected to reflect a certain wavelength as a template.
Each inverse opal structure 102 and each actuator 104 have a one-to-one correspondence, and a set of the display unit that displays one pixel.

Hereinafter, the expression and control of the structural color in the present invention will be described in detail.
Consider a case where incident light having a certain wavelength is incident on a structure of spherical fine particles, called a so-called opal crystal, having a photonic crystal characteristic. In the present invention, an inverse opal structure that finally develops a structural color can be calculated by inverting the volume fraction.
In order to qualitatively calculate the position of the photonic band gap, which is a significant characteristic of the photonic crystal, the following Bragg equation is effective.

  Here, when light is incident on the (111) plane of the opal crystal having the fcc structure, the wavelength (λ) of the photonic band gap is expressed by the following equation (1).

Here, n _a is expressed by the following equation (2).

Where d is the distance between the fine particles, θ is the incident angle of light, n _a is the average refractive index, n _sphere and n _void are the refractive index of the fine particles, and the refractive index of the voids, respectively, and f is the volume fraction of the fine particles. .
When considering vertically incident light, since θ = 0 °, sin θ = 0. Further, the volume fraction f of the fine particles in the close-packed fcc structure can be obtained by calculation as 0.74, and from this, the value of 1-f is obtained as 0.26.
Therefore, if the refractive index of the spherical void and the refractive index of the material constituting the skeleton are known, the wavelength (λ) at the position of the photonic band gap can be obtained by calculation using Equations (1) and (2). .

Here, an example of calculating a photonic band gap by using a specific material and utilizing its refractive index is shown.
A case where an ultraviolet curable epoxy resin is used as a flexible material forming a skeleton in the inverse opal structure will be described.
In addition, since it is an inverse opal structure, the substance which occupies a spherical space | gap is air.

Spherical void: Air Refractive index n _sphere = 1
Volume fraction = 0.74
Particle size = 300nm
Skeleton: UV curable epoxy resin Refractive index n _void = 1.55
Volume fraction = 0.26
Substituting these values into the above formulas (1) and (2) to obtain the wavelength (λ) at the photonic band gap position,
λ = 572.3 (nm) is obtained.

Next, the case where an ultraviolet curable epoxy resin is used as the flexible material forming the skeleton will be described.
Spherical void: Air Refractive index n _sphere = 1
Volume fraction = 0.74
Particle size = 300nm
Skeleton; Low density polyethylene Refractive index n _void = 1.51
Volume fraction = 0.26
Substituting these values into the above formulas (1) and (2) to obtain the wavelength (λ) at the photonic band gap position,
λ = 565.6 (nm) is obtained.

Thus, even when the size of the same spherical void is 300 nm, it can be seen that the wavelength of the photonic band gap varies depending on the refractive index of the material forming the skeleton.
In addition, since the particle size of the spherical fine particle used first determines the wavelength of the photonic band gap of the final inverse opal structure, the dimensional accuracy and size variation of the spherical fine particle size determine the quality of the inverse opal structure. To do.
In this respect, recently, fine particles having excellent particle size accuracy and dimensional variation, such as silica fine particles and polystyrene fine particles, can be said to be an effective production method because they are easily available.

Although it is possible to form a three-dimensional regular periodic structure other than the inverse opal structure by making full use of a semiconductor manufacturing process, there is a problem that the manufacturing process is complicated and the cost is very high.
From this aspect, it can be said that the present invention using the inverse opal structure using the fine particle aggregate as a template has a great feature.

Next, a design guideline for actual implementation as a reflective display will be described.
An explanation will be given of a case where an ultraviolet curable epoxy resin (refractive index = 1.55) is used as a flexible material forming a skeleton, and a green structural color is exhibited in a non-deformed state and the color disappears when deformed. .
Select a wavelength of 524 nm as the green wavelength.
Using the above formulas (1) and (2), the period of the inverse opal structure where the photonic band gap is 524 nm, that is, the diameter of the spherical fine particle used as a template for forming the inverse opal structure is obtained. And 280.0 nm.

Spherical fine particles having such a particle size can be easily obtained, and have a size that is easy to handle even when forming a regular array, so that there is no problem in the manufacturing process.
It is assumed that a compressive force is applied to the inverse opal structure having a periodic structure of this size, and the period dimension is reduced to 254.7 nm, for example.
In the configuration of the present invention, isotropic compression is not achieved, but at least shrinkage in the film thickness direction can be achieved.

For ease of calculation, if the isotropic contraction occurs and the period dimension changes in this way, the wavelength of the photonic band gap is 480 nm, and the green structural color no longer exists.
As described above, since the color of the structural color to be presented differs between the area where the compressive force is applied to the inverse opal structure and the area where the compression force is not applied, a reflective display can be realized by controlling these colors.

Next, a case where a low-density polyethylene (refractive index = 1.51) is used as a flexible material forming a skeleton, and a red structural color is exhibited without deformation, and the color disappears when deformed. To do.
Select the wavelength of 564 nm as the red wavelength.
Using the above formulas (1) and (2), the period of the inverse opal structure with a photonic band gap of 564 nm, that is, the diameter of the spherical fine particle used as a template for forming the inverse opal structure is obtained. And 295.7 nm.

Applying a compressive force to an inverse opal structure having a periodic structure of this size, for example, if the period dimension is reduced to 262.1 nm, the wavelength of the photonic band gap is 500 nm, and the structure color of red is no longer It will disappear.
In this case also, it can be realized as a reflective display.
Similarly, for blue, since it is possible to control the case where the structural color is exhibited and the case where the structural color disappears, a color display can be realized.

  The change in structure described above can be realized with a dimensional change of about 10% in any case, so if it is an inverse opal structure made of a flexible material, it can be deformed without difficulty, It can cope with repeated use.

Next, the actuator will be described. The actuator of the present invention can achieve the object of the present invention as long as it is an actuator that can apply a force to the above-mentioned inverse opal structure, but it is preferable to adopt a method of applying a compression force using PZT.
The reason is that PZT can be formed by a so-called semiconductor process, can be patterned, and can be aligned with a wiring member without any problem.

Specifically, a PZT actuator having a desired shape can be easily obtained by forming PZT by a sputtering method, a vapor deposition method, or the like, and patterning by a general semiconductor process such as photolithography or dry etching.
If designed and formed so as to correspond to the inverse opal structure, a force can be individually applied to the independent inverse opal structure.
Further, since PZT is a voltage-driven actuator, it consumes less current, and as a result, energy-saving driving is possible. Also, the reaction rate is sufficiently fast and there is no problem at all.

Based on FIG. 2, the formation method of the inverse opal structure as a structural color expression member (optical switch) is demonstrated.
First, as shown in FIG. 2A, a plurality of separation walls 105 are formed on the transparent cover member 101 to form separated cells 106.
Each separation wall 105 is designed and formed so as to finally determine a region corresponding to each pixel.

Since the separation wall 105 is finally removed, the separation wall 105 should be appropriately selected in consideration of the combination with the material of the transparent cover member 101.
It is also desirable to be able to pattern to form each cell 106.
Specifically, when the transparent cover member 101 is glass, a photoresist or dry film resist of a type that can be formed thick is a suitable material.

A dispersion liquid containing spherical fine particles (here, silica) 107 is introduced into the cell 106 formed by such a separation wall 105 and allowed to stand.
As the solvent of the dispersion liquid evaporates, spherical fine particles accumulate in a close-packed structure. This process can be easily realized by using a generally known technique.
This point will be further described. The present invention is characterized by using an array technique that utilizes the self-organization phenomenon of fine particles.
In general, it is known that fine particles have a close-packed structure if conditions are adjusted by supplying and drying a dispersion liquid in which fine particles are dispersed to a substrate.

This phenomenon is interpreted as a transverse capillary force acting on the fine particles when the solvent of the dispersion is dried. As a result, it is considered that the fine particles subjected to the transverse capillary force are arranged in a close-packed structure with drying.
In the present invention, this phenomenon is actively used.
Although it is conceivable to arrange the fine particles one by one at the target position by manipulation operation, such a method requires enormous time and cost, and it is unrealistic as an actual device creation technique.

The aggregates of fine particles stacked in a close-packed structure are in close contact with each other but have voids that communicate with each other at the same time.
As shown in FIG. 2B, the filler 108 having fluidity is injected into the void, impregnated and solidified.
Specifically, the ultraviolet curable epoxy resin is filled in the gaps, and when the resin is sufficiently spread in all the gaps, the ultraviolet curable epoxy resin is solidified by irradiating ultraviolet rays.

At this time, when the space is filled with the ultraviolet curable epoxy resin, in order to prevent the silica aggregate from collapsing, the fine particle aggregate is subjected to a short heat treatment before the ultraviolet curable epoxy resin is filled. It is also recommended.
As a recommended condition in the case of a silica fine particle aggregate, it is sufficient to perform a heat treatment at 300 ° C. for about 30 minutes.

Next, as shown in FIG. 2C, only spherical fine particles are selectively removed.
As described above, since the constituent material of the fine particles is silica and the resin filled in the voids is an ultraviolet curable epoxy resin, only the silica fine particles are dissolved and removed using dilute hydrofluoric acid.
As mentioned earlier, since the fine particles are closely gathered and accumulated, dilute hydrofluoric acid penetrates into the solution while dissolving the fine silica particles, and finally all the fine silica particles can be completely removed. It becomes.
Since the ultraviolet curable epoxy resin is resistant to dilute hydrofluoric acid, the shape of the inverse opal structure 102 can be obtained as a skeleton without any damage.

Finally, as shown in FIG. 2D, the separation wall 105 existing for separating each cell is removed, and the inverse opal structure 102 independently formed on the transparent cover member 101 is removed. A member in which only is present is obtained.
The separation wall 105 can be completely removed by using an organic solvent such as acetone in the above-described photoresist or dry film resist.

  The transparent cover member 101 on which the inverse opal structure 102 is formed and the substrate 103 on which the PZT pattern and wiring member are formed are combined and integrated to connect necessary peripheral mechanisms such as a separately provided control circuit and power supply. Thus, the reflective display 2 is completed.

One feature of the present invention is that a highly regular fine particle array structure is realized by using a solution system.
That is, even in the dry state, even ultrafine particles that easily aggregate can be prevented from being aggregated by utilizing the advantages of the state of a solution system to improve the dispersibility.
By controlling the pH, for example, appropriately selecting and controlling the ion species to be added, the isoelectric point relationship can be utilized. As a result, a highly regular fine particle array can be obtained.

This point will be described in more detail. In general, when fine particles made of, for example, a metal oxide are immersed in water, the fine particles have a positive or negative charge, and move in a direction having an opposing electric field when an electric field is present.
This phenomenon is an electrophoresis phenomenon. By this electrophoresis phenomenon, it is possible to know the charge of fine particles in water, that is, the presence of an interface potential (zeta potential). This interfacial potential varies greatly depending on the pH of the fine particle-water system.
In general, when the horizontal axis represents the pH of the aqueous system and the vertical axis represents the interfacial potential, the interfacial potential varies depending on the pH of the aqueous system, and the pH of the aqueous system at which the interfacial potential “0” is cut is defined as the “isoelectric point”. .

From this phenomenon, the interface potential on the surface of the metal oxide fine particles generally has a positive polarity on the acidic side and a negative polarity on the alkaline side. However, this isoelectric point varies greatly depending on the material. For example, a value of “2.0” is introduced for colloidal silica, “9.0” for α-alumina, and “6.7” for hematite.
That is, the interface potential increases as the distance from the isoelectric point increases, and the value of the interface potential increases toward the positive side as it moves toward the acidic side.
On the other hand, the value of the interfacial potential is more negative as it goes to the alkali side. This means that it can be controlled by pH.

The pH can be controlled with good controllability by adding acid or alkali. In the present invention, this phenomenon is actively utilized, and aggregation of fine particles can be effectively prevented in the state of a dispersion.
As a result, even when the dispersion liquid is dropped into the cell, the fine particles are present in a non-aggregated state. Therefore, a high-quality array state can be easily realized in the subsequent array process.
This phenomenon is an advantage that cannot be obtained by a dry process.

2 Reflective display device 102 Inverse opal structure 104 PZT as actuator
107 Silica as Spherical Fine Particle 108 UV-curable Epoxy Resin as Flexible Material

JP 2009-216964 A

Claims (8)

It has an inverse opal structure of a structure having a photonic band gap function that is composed of a material having flexibility using a close-packed structure of spherical fine particles as a mold, and whose structure period reflects a desired wavelength,
An optical switch characterized in that a switching function is obtained by deforming the inverse opal structure. The optical switch according to claim 1,
An optical switch, wherein the flexible material is an ultraviolet curable epoxy resin. The optical switch according to claim 1,
The optical switch, wherein the flexible material is low density polyethylene. A reflective display device comprising: an optical switch; and an actuator that applies a force to the inverse opal structure of the optical switch to exhibit a switching function,
The reflection type display device according to claim 1, wherein the optical switch is one according to claim 1. The reflective display device according to claim 4,
A plurality of the inverse opal structures are arranged in an array, and an actuator that can be individually driven for each inverse opal structure is arranged in a pair, and a display unit that displays one pixel as a set is configured. A reflective display device, characterized in that The reflection type display device according to claim 4 or 5,
The reflective display device, wherein the actuator is PZT.   The dispersion liquid in which the spherical fine particles are dispersed is placed in a cell partitioned as a raw material, the solvent of the dispersion liquid is evaporated to arrange the spherical fine particles in a close-packed structure, and then the flexible material in the cell The optical switch according to any one of claims 1 to 3, wherein after the solidification of the flexible material, the spherical fine particles are removed to form the inverse opal structure. Production method. In the manufacturing method of the optical switch according to claim 7,
A manufacturing method of an optical switch, wherein a dispersion liquid in which the spherical fine particles are dispersed by controlling the pH value according to the material of the spherical fine particles is used when producing the close packed structure of the spherical fine particles. .

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