JP2015018095A - Variable focus mirror - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable focus mirror capable of quickly changing a focal distance.SOLUTION: A variable focus mirror 10 includes: a substrate 1 formed of crystal materials in which physical distortion occurs when a voltage is applied; a plurality of electrodes 2 and 3 for applying a voltage to the substrate 1; and a light reflection film 4 configured on the electrodes 2 for reflecting incident light, therein when the voltage is applied to the substrate 1, the surface of the substrate 1 is changed by the physical distortion of the crystal materials such that the curvature of the light reflection film 4 is varied.

Description

  The present invention relates to a variable focus mirror capable of adjusting a focal length.

  Conventionally, optical components such as optical lenses and prisms are optical devices (cameras, microscopes, telescopes, etc.), electrophotographic recording devices (printers, copiers, etc.), optical recording devices such as DVDs, communication and industrial use. It is used for optical devices. Such an apparatus often uses a lens whose focal length can be adjusted according to the situation, so-called variable focus.

  The conventional variable focus lens mechanically adjusts the focal length by combining a plurality of lenses. However, such a mechanical variable focus lens has a limit in expanding the application range in terms of response speed, manufacturing cost, miniaturization, power consumption, and the like.

  On the other hand, a variable focus concave mirror can be considered as an optical element capable of adjusting the focal length. In recent years, an optical element has been developed in which a plurality of actuators are attached to the back surface of a thin and deformable mirror, and the curvature of the mirror can be changed by operating the actuator (non-patent document). Since the curvature of the mirror can be changed, it can also be used as a concave mirror having a variable focal length. Here, such an element is called a variable mirror.

Gleb Vdovin et al., Deformable mirror with thermal actuators, May 1, 2002 / Vol. 27, No. 9 / OPTICS LETTERS

  However, the conventional variable mirror has a problem that it cannot be applied to a high-speed response of 1 ms or less because there is a limit to the response speed required to change the focal length by changing the curvature of the mirror.

  An object of the present invention is to provide a variable focus mirror that can change the focal length at high speed.

  In order to solve the above problems, the present invention provides a substrate formed of a crystal material that causes physical strain when a voltage is applied thereto, a plurality of electrodes for applying a voltage to the substrate, and the plurality of electrodes A mirror configured to reflect incident light, and when the voltage is applied to the substrate, due to physical distortion of the crystal material, The curvature of the mirror is varied by changing the surface.

  Here, the electrode may include the electro-optic material and a material in which an ohmic junction is formed.

  The crystal material may have an oxygen octahedral structure.

  The crystal material may be a perovskite single crystal material having inversion symmetry.

  In the electro-optic material, the main component of the crystal may be composed of Group Ia and Group Va of the periodic table, Group Ia may be potassium, and Group Va may include at least one of niobium or tantalum.

  According to the present invention, the focal length can be changed at high speed.

It is a figure which shows the structural example of the variable focus mirror in one Embodiment of this invention. It is a figure which shows the relationship between the electric flux density at the time of applying a voltage to a variable focus mirror, and the distortion by an electrostrictive effect.

  Hereinafter, an embodiment of the variable focus mirror of the present invention will be described. The variable focus mirror of this embodiment uses the piezo effect or the electrostrictive effect to obtain a higher response speed than a conventional variable focus lens or variable focus mirror. Therefore, a stable operation can be performed even at a repetition frequency of 100 kHz or more, for example.

  FIG. 1 is a diagram illustrating a configuration example of a variable focus mirror 10 according to the present embodiment, where (a) is a perspective view of the variable focus mirror 10, and (b) is a cross-sectional view of the variable focus mirror 10.

  A variable focus mirror 10 shown in FIG. 1 includes a substrate 1, an upper electrode 2 formed on the upper surface of the substrate 1, a lower electrode 3 formed on the lower surface of the substrate 1, and a light reflecting film ( Mirror) 4.

  The substrate 1 is made of, for example, a crystal material having an electrostrictive effect. In this embodiment, the substrate 1 is formed in a disc shape, for example, but the substrate 1 is not limited to the shape shown in FIG. 1, and other shapes known to those skilled in the art can also be applied.

[Operation Principle of Variable Focus Mirror 10]
Next, the operating principle of the variable focus mirror 10 will be described.

  When the electric field applied to the substance is E and the strain proportional to the square of the electric field is e due to the electrostrictive effect, the application of E causes the distortion of e. This is represented by the following formula (1).

  In the equation (1), ε: dielectric constant, D: electric flux density, Q: electrostriction coefficient.

  In general, there are electrons, holes, and protons (hydrogen ions) that are injected into the electrode. In this embodiment, for example, electrons are used.

  In the variable focus mirror 10 of the present embodiment, when the substrate 1 has a thickness of 1 mm, the upper electrode 2 is an anode, the lower electrode 3 is a cathode, and a high voltage of several hundred volts is applied, electrons are injected from the lower electrode 3. The As a result, space charges are formed inside the substrate 1.

  There may be various forms of formation of this space charge. In this embodiment, it is assumed that space charges having a uniform charge density ρ are formed in the substrate 1. The charge density ρ assumes a negative value because charge injection by electrons is assumed. From this charge density ρ, the electric flux density D can be obtained according to Gauss's law of the following equation (2).

  As will be described later, a crystal material having an electrostrictive effect usually has a very large dielectric constant. In this case, since the electric field E and the electric flux density D are both perpendicular to the electrode inside the substrate 1, only the y-axis component in the direction perpendicular to the electrode 1 is considered for the electric flux density D. That is, the above formula (2) is given by the following formula (3).

From the above formula (3), the electric flux density D _y of the y-axis component is expressed by the following formula (4).

  In the formula (4), V represents an applied voltage applied between the upper electrode 2 and the lower electrode 3, and d represents the thickness of the substrate 1.

In Expression (4), D _y changes along the function of the position coordinate y, that is, the y direction (up and down method) in FIG.

  Accordingly, the strain e also changes along the y direction (up and down method) in FIG. 1B according to the following equation (5) from the equation (1).

  In the formula (5), V: applied voltage, d: thickness of the substrate 1 are shown.

  Next, the relationship between the electric flux density D and the distortion due to the electrostrictive effect when a voltage is applied to the varifocal mirror 10 will be described with reference to FIGS.

FIG. 2 is a diagram showing the relationship between the electric flux density D and the strain due to the electrostrictive effect, wherein (a) is the relationship between D _y shown in the above equation (4) and the position in the y direction shown in FIG. (B) shows the state of distortion when the distortion coefficient is a positive value.

  In FIG. 2, the space charge is fixed inside the substrate 1 and is constant regardless of the applied voltage. 2A and 2B, the horizontal axis represents the position in the vertical direction (the y direction in FIG. 1) inside the substrate 1. Here, the right end of the various distributions (D1 to D3, e1 to e3) in FIGS. 2A and 2B is the position of the upper electrode 2, and the left end is the position of the lower electrode 3.

  The electric flux density D shown in the above equation (4) has a distribution as shown in FIG. 2A according to the applied voltage (V in FIG. 2). That is, the electric flux density distributions D1, D2, and D3 all have a linear change whose slope is ρ.

For example, in the case of the electric flux density distribution D3, V = −d ² · ρ / (2ε) (V is a positive voltage because ρ is negative). Therefore, at the position of the lower electrode 3, D = 0. On the other hand, when V is lowered, the electric flux density distribution D3 is translated upward sequentially as indicated by D2 and D1.

In the case of D2, when V = 0, D = 0 at the center of the electric flux density distribution of D2. When the voltage V is further reduced to V = d ² · ρ / (2ε) (where ρ is negative, V is a negative voltage), as shown in the electric flux density distribution D1, the upper electrode At the position of 2, D = 0.

The distortion when the electrostriction coefficient is a positive value takes a value as shown in FIG. That is, as shown in the strain distribution e3, when V = −d ² · ρ / (2ε), the absolute value of D is small in the vicinity of the lower electrode 3, so that the strain is also small. On the other hand, the strain increases as it approaches the upper electrode 2.

  When the voltage = 0, as shown in the strain distribution e2, the strain = 0 at the center of e2, and the strain distribution is symmetric.

When V = d ² · ρ / (2ε), the strain distribution e1 is a distribution in which the strain distribution of e3 is reversed left and right.

  In general, due to the above-described electrostrictive effect, the substance extends in the direction of the electric field, and the electrostriction coefficient often has a positive value depending on this orientation. In addition, when a substance extends in the direction of the electric field, the substance usually contracts in a direction perpendicular to that direction.

  In the varifocal mirror 10 of the present embodiment, the direction of the electric field is the y direction, so that the substrate 1 extends in the y direction and the left and right direction of the paper surface of FIG.

In the example of FIG. 2B, when V = −d ² · ρ / (2ε) is given as a positive voltage, in the vicinity of the upper electrode 2 serving as the anode, the horizontal direction of the paper surface of FIG. The shrinkage rate in the inward direction becomes high, and the shrinkage hardly occurs near the lower electrode 3 serving as the cathode. As a result, the substrate 1 is warped and deformed to have a convex curved shape in the downward direction (the direction of the starting point in the y direction). In this case, the light reflecting film 4 is deformed in a round shape simultaneously with the deformation action, and becomes a concave mirror.

  In FIG. 2B, when V = 0, the distortion is symmetrical. In FIG. 2, the horizontal axis is the y direction (vertical direction) in FIG.

  Since the vicinity of the upper electrode 2 contracts in the same manner as the vicinity of the lower electrode 3, the substrate 1 does not warp and maintains a normal shape.

  Here, when the voltage V is increased in the positive direction, the contraction of the upper electrode 2 is gradually increased and the contraction of the lower electrode 3 is decreased, so that the balance of stress is lost and the curvature of the concave mirror is increased.

  From this principle, the varifocal mirror 10 can change the curvature of the light reflecting film 4 by the applied voltage by utilizing the electric charge injected into the substrate 1. Thereby, for example, the variable focus mirror 10 can be made to function as a concave mirror, and the focus of the variable focus mirror 10 can be changed.

  As shown in FIG. 2B, when a negative voltage V is applied, the strain distribution is opposite to the positive voltage, so that the variable focus mirror 10 functions as a convex mirror. Also in this case, the focal point of the variable focus mirror 10 can be changed by causing the variable focus mirror 10 to function as a convex mirror.

[Space charge]
As a form of current that flows when electrons are injected into a substance from an electrode, there is a space charge limited current. This current is a current that flows into the material by the electrons when excessive electrons are injected into the material. When electrons are injected excessively, it seems that there is no limit to the current, but a spatial distribution is automatically formed in the electron concentration in the material. This increases the electron concentration near the cathode, thus limiting the current. The charge density when the space charge limited current flows is expressed by the following formula (6).

  From equation (6), it can be seen that the charge density increases near the cathode (y is small). In the explanation of the principle described above, the explanation was made on the assumption that the space charge is fixed inside the substrate 1, but in the space charge limited current mode, the space charge is generated by the free electrons contributing to the current. The space charge is not fixed.

  Here, some of the crystal materials used for the substrate 1 have an energy level for capturing electrons. This local energy level is called a trap.

  When there is a trap, most of the injected electrons are captured by the trap and do not become free electrons. The trap has a low probability of emitting electrons, and in this case, it is considered that the space charge is substantially fixed to the substrate 1.

  Even if the voltage is returned to V = 0 after applying a high voltage for electron injection, the space charge is stored for a long time. When using traps, the charge density ρ can be made spatially uniform. The above description of the principle is based on this mode.

[Crystal material with electrostrictive effect]
As a phenomenon in which a material is deformed when an electric field is applied, there are a piezo effect and an electrostrictive effect. The piezo effect is a phenomenon in which electricity is generated inside a material when the material is distorted. In a material that generates such a phenomenon, distortion occurs when an electric field is applied. This is known as an inverse piezo effect, but is often simply a piezo effect. In the piezo effect, the strain is proportional to the applied electric field.

  The electrostrictive effect refers to a phenomenon in which strain is proportional to the square of an applied electric field. This is a secondary effect. As a substance whose strain is induced by an electric field, for example, a crystal material having an oxygen octahedral structure is used.

  In the oxygen octahedron, one oxygen ion is arranged at each of the eight vertex positions of the regular octahedron, and one metal element ion is surrounded by these oxygen ions to have a fine structure. The octahedron is not a perfect regular octahedron and may have some “distortion”, for example, extending in one direction.

Some crystalline materials have a primary effect and some have a secondary effect, but materials having an effect of inducing strain by an electric field generally have an oxygen octahedron as the basis of the crystal structure. Yes. The space inside the oxygen octahedron is larger than the size of the metal element ions. For this reason, the fact that the metal element ions are easy to move causes the distortion due to the electric field to increase. Typical examples include PbTiO ₃ and PZT (PZT in which PbTiO ₃ and PbZrO ₃ are mixed). Ti or Zr ions are inside the oxygen octahedron.

  Among the above-described piezo effect and electrostrictive effect, the piezo effect, which is a primary effect, is widely used in the industry, but in this embodiment, any effect can be used, and the crystalline material is A material having an oxygen octahedral structure can be used. However, in practice, it is preferable to use a material having an electrostrictive effect. This is because, when the piezo effect is used, the mode using the trap cannot be used out of the two types of space charge modes described above, and the effect becomes small in the space charge limited current mode. is there.

  The mode that utilizes traps when using the piezo effect cannot be used because the strain is not proportional to the electric flux density or the square of the electric field. Since strain is proportional to the first power of the electric flux density, the strain distribution shown in FIG. 2B represents the same characteristics as the electric flux density distribution shown in FIG. In this embodiment, the shape of the substrate 1 changes and the substrate 1 is warped because there is a difference in strain between the upper electrode 2 side and the lower electrode 3 side. The difference in the amount of distortion is proportional to the warp of the light reflecting surface.

  Therefore, even in the case of the piezo effect, the substrate 1 is warped. In the case of the piezo effect, as shown in FIG. 2A, when the applied voltage is changed, the overall distribution of the electric flux density fluctuates up and down, but the upper electrode 2 side and the lower electrode 3 side. The difference in the electric flux density at is not changed. That is, the difference in distortion amount does not change. For this reason, even if the voltage is changed, the curvature of the warp of the substrate 1 remains constant, and the curvature cannot be changed at high speed by the voltage.

  It is the charge density that determines the curvature, that is, the focal length, and it is difficult to change the density of the space charge once formed by injecting electrons at a high speed.

In the mode using the space charge limited current, the space charge is proportional to the dielectric constant as shown in the above formula (6), and therefore it is desirable that the dielectric constant is high. In the case of a material having a piezo effect, although the dielectric constant is higher than that of a general dielectric, compared to the material that exhibits an electrostrictive effect often has a relative dielectric constant of tens of thousands, In many cases, a sufficient dielectric constant cannot be obtained. However, in the case of a material having a tungsten bronze type crystal structure, there are some which show a dielectric constant of several tens of thousands while having a piezo effect. Therefore, such a material is preferable. Examples of such materials include Sr _x Ba _1-x Nb ₂ O ₆ , Ba ₄ Na ₂ Nb ₁₀ O ₃₀ , K ₃ Li ₂ Nb ₅ O ₁₅ , or some or all of these constituent elements, Examples include those substituted with another element within a range in which the crystal structure is not changed. These materials also have an oxygen octahedron containing Nb ions as a basic structure.

  On the other hand, when a material having an electrostrictive effect is used, a mode utilizing a trap can be used as described above.

  Further, a perovskite crystal material having inversion symmetry can have a huge dielectric constant. For example, the relative dielectric constant reaches several tens of thousands.

  Next, a material having a perovskite structure having inversion symmetry will be described.

  In general, when an electric field is applied from the outside to the dielectric, polarization proportional to the electric field is generated. However, when the electric field is removed, the polarization becomes zero. However, there are substances that retain finite polarization even when the electric field is removed. Having polarization in the absence of an external electric field is called spontaneous polarization.

  For spontaneous polarization, there are substances whose direction can be reversed by an external electric field. This is called a ferroelectric.

  A single crystal having inversion symmetry refers to a crystal that has the same arrangement as the original arrangement of atoms when inverted in the x, y, z coordinate system around a certain origin. Since a crystal having spontaneous polarization is inverted in the direction of spontaneous polarization when inverted in the above-described coordinate system, it cannot be said that such a crystal has inversion symmetry. From this, the ferroelectric has spontaneous polarization and therefore does not have inversion symmetry.

  On the other hand, there are substances that have spontaneous polarization but cannot be inverted by an external electric field. Such a material does not have inversion symmetry, but is not a ferroelectric material. Therefore, not all materials that do not have inversion symmetry are ferroelectric materials.

In addition, it is not possible to be a ferroelectric and have inversion symmetry. The aforementioned PZT, Sr _x Ba _1-x Nb ₂ O ₆ , Ba ₄ Na ₂ Nb ₁₀ O ₃₀ , K ₃ Li ₂ Nb ₅ O ₁₅ etc. are all ferroelectrics and have no inversion symmetry, Has a piezo effect.

  As a material having inversion symmetry, for example, there is a single crystal material having a perovskite crystal structure. The perovskite type single crystal material is a typical crystal material based on an oxygen octahedron. This crystal material becomes a cubic phase having inversion symmetry in the use state if the use temperature is appropriately selected. In the cubic phase, there is no piezo effect, and the electrostrictive effect is dominant.

For example, in the case of the well-known barium titanate (BaTiO ₃ , hereinafter referred to as BT), a temperature exceeding the temperature at which the phase transition from the tetragonal phase to the cubic phase (hereinafter referred to as the phase transition temperature) is around 120 ° C. In some cases, it becomes a cubic phase and has an electrostrictive effect.

  Further, as shown in Expression (1), the amount of strain increases as the dielectric constant increases even for the same applied electric field. In perovskite crystal materials, it is known that the temperature dependence of the dielectric constant follows the law called the Curie-Weiss law. According to the Curie-Weiss law, the dielectric constant becomes huge near the phase transition temperature. The relative dielectric constant usually reaches several tens of thousands. For this reason, it is preferable to use the perovskite crystal having inversion symmetry near the phase transition temperature.

A single crystal material mainly composed of potassium tantalate niobate (KTN: KTa _1-x Nb _x O ₃ , 0 <x <1) has more preferable characteristics. BT has a predetermined phase transition temperature, whereas KTN can select a phase transition temperature depending on the composition ratio of tantalum and niobium. Thereby, the phase transition temperature can be set near room temperature, and the giant electrostrictive effect can be easily realized as described above.

  Furthermore, as the single crystal material related to KTN, a material in which the main component of the crystal is composed of the Ia group and the Va group of the periodic table can be used. In this case, the group Ia is, for example, potassium, and the group Va is a material containing at least one of, for example, niobium and tantalum.

Moreover, 1 type or multiple types of IIa group (for example, lithium) or IIa group of the periodic table except potassium as an additional impurity can also be included. For example, a cubic phase KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ , 0 <x <1, 0 <y <1) crystal may be used.

Similarly, as a material having both inversion symmetry and high dielectric constant at a temperature near room temperature, a crystal material in which the above-mentioned BT and SrTiO ₃ are mixed, PZT in which La is added to the above-mentioned PZT or PZT, etc. Is also a material that can be suitably used in the present invention.

[Electrode material]
In the present embodiment, by applying a high voltage between the upper electrode 2 and the lower electrode 3, charges are injected from the electrodes 2 and 3 into the substrate 1 to generate space charges in the crystal material of the substrate 1. Let In this case, it is necessary to inject charges from the electrodes 2 and 3 efficiently.

  In the substrate 1, when the carrier contributing to electrical conduction is an electron, the energy barrier at the interface with the substrate 1 becomes lower as the work function of the electrode material becomes smaller. This approaches the ohmic junction, and as a result, the carrier injection efficiency increases. Therefore, the electrodes 2 and 3 are preferably formed of a substrate material and a material for forming an ohmic junction. Specifically, in the crystal of the substrate 1, when the carrier contributing to electrical conduction is an electron, the work function of the material of the electrodes 2 and 3 is preferably less than 5.0 eV.

  For example, Ti (3.84) or the like can be used as an electrode material having a work function of less than 5.0 eV. The value in parentheses indicates the work function, and the unit is eV.

  Since the Ti single-layer electrode is oxidized and becomes high resistance, generally, the Ti layer and the substrate crystal are bonded using an electrode in which Ti / Pt / Au is sequentially laminated. Alternatively, a transparent electrode such as ITO (Indium Tin Oxide) or ZnO can be used.

  When the carrier contributing to electrical conduction in the substrate crystal is a hole, the work function of the electrode material is preferably less than 5.0 eV in order to suppress hole injection. For example, as an electrode material having a work function of 5.0 eV or more, Co (5.0), Ge (5.0), Au (5.1), Pd (5.12), Ni (5.15), Ir (5.27), Pt (5.65), Se (5.9) can be used.

  Next, a method for manufacturing the variable focus mirror 10 will be described with reference to FIG. 1 again.

  An upper electrode 2 and a lower electrode 3 are formed on the upper and lower surfaces of the substrate 1 obtained by processing the electrostrictive material into a plate shape, respectively. Further, a light reflecting film 4 is formed on the upper electrode 2.

  In this example, the substrate 1 is made of KTN single crystal, the diameter of the substrate 1 is 10 mm, and the thickness of the substrate 1 is 1 mm. The thickness direction is the <100> direction of the crystal. Since this KTN single crystal had a phase transition temperature of 35 ° C., it is used at 40 ° C., which is slightly higher than this. The relative dielectric constant at this temperature was set to 20000, for example. Both electrodes 2 and 3 were formed by sequentially depositing Ti / Pt / Au.

The light reflecting film 4 was formed as a dielectric multilayer film by alternately laminating SiO ₂ and TiO ₂ on the upper electrode 2.

  Collimated laser light is incident on the varifocal mirror 10 thus manufactured while the temperature is controlled at 40 ° C.

  When a positive voltage of 500 V, for example, is applied to the lower electrode 3 and the upper electrode 2, electrons are injected into the substrate 1 through this electrode, and subsequently the substrate 1 is warped, and the reflected light from the reflective film 4 is condensed. And function as a concave mirror. The focal length in this case was 20 cm.

  On the other hand, when no voltage is applied, there is no light collecting effect and the focal length is infinite. From this, the focal length can be changed in the range from infinity to 20 cm by changing the applied voltage from 0V to 500V.

  When a negative voltage was applied to the lower electrode 3 and the upper electrode 2, the variable focus mirror 10 functioned as a convex mirror. Since changing the focal length only changes the applied voltage, the response time was 1 μs or less. This indicates that the response time of the conventional variable focus mirror is improved by three orders of magnitude or more.

  As described above, in the varifocal mirror 10 of the present embodiment, the substrate 1 is formed of a crystal material that causes physical distortion when a voltage is applied. Due to the physical strain of the crystal material, the surface of the substrate 1 changes (curves) (for example, uneven shape) and the curvature of the light reflecting film 4 changes. Thereby, the focus of the light incident on the light reflection film 4 and reflected can be changed.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Upper electrode 3 Lower electrode 4 Light reflection film 10 Variable focus mirror

Claims (5)

A substrate formed of a crystalline material that undergoes physical strain when a voltage is applied;
A plurality of electrodes for applying a voltage to the substrate;
A mirror configured on any one of the plurality of electrodes to reflect incident light;
With
A variable focus mirror characterized in that, when the voltage is applied to the substrate, the curvature of the mirror is varied by changing the surface of the substrate due to physical strain of the crystal material.   The variable focus mirror according to claim 1, wherein the electrode includes the electro-optic material and a material in which an ohmic junction is formed.   The variable focus mirror according to claim 1, wherein the crystal material has an oxygen octahedral structure.   4. The variable focus mirror according to claim 3, wherein the crystal material is a perovskite single crystal material having inversion symmetry in which a temperature is set in the vicinity of a phase transition temperature.   The electro-optic material is characterized in that the main component of the crystal is composed of a group Ia and a group Va in the periodic table, the group Ia is potassium, and the group Va includes at least one of niobium or tantalum. Item 4. The variable focus mirror according to Item 3.

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