JP5725211B2 - Variable optical attenuator - Google Patents

Description

  The present invention relates to a variable optical attenuator. More particularly, the present invention relates to a variable optical attenuator using non-reciprocity of light due to an electromagnetic effect.

  In recent years, sensing technology utilizing non-invasiveness of light and spectroscopy has been attracting attention. For example, in fundus examination devices that measure the oxygen saturation of hemoglobin and cancer tests using fluorescent proteins bound to specific parts of the body, a spectroscopic imaging technology that uses laser light in the visible to near-infrared wavelength range is used. Development is underway. These imaging devices and fluorescence microscopes require a multi-wavelength light source according to the application, and white laser light called supercontinuum light generated by irradiating a nonlinear optical crystal with ultrashort pulse laser light must be used. There are many. What is important here is a variable wavelength filter that selects a required wavelength from continuous wavelengths and a variable optical attenuator that adjusts the intensity thereof. Of these, many of the currently used variable optical attenuators are of the waveguide type, and it is difficult to increase the area.

On the other hand, a completely different operating principle has been reported recently. This is due to the fact that a directional dichroism property develops due to nonreciprocity in a substance where magnetization that breaks time reversal symmetry and polarization that breaks space reversal symmetry coexist. To do. This directional dichroism is based on a so-called electromagnetic effect, that is, an effect in which magnetization is induced by an electric field and polarization is induced by a magnetic field. For example, a conventional optical isolator such as an optical switch that forms a diffraction grating in a GaFeO ₃ crystal and changes the first-order diffracted light according to the direction of magnetization (Patent Document 1) or an optical fiber such as an Er-doped BaTiO ₃ single crystal is used. It can be omitted (Patent Document 2).

JP 2007-034093 A International Publication No. 2006/129453

However, GaFeO ₃ has a problem that the function expression temperature of directional dichroism is lower than room temperature (for example, 300 K). In addition, since the Er-doped BaTiO ₃ single crystal is paramagnetic, there is a problem that a magnetic field must always be applied to obtain directional dichroism.

  The present invention has been made in view of the above problems. The present invention contributes to manufacturing a variable optical attenuator that is easy to increase in area, requires no external field except when changing the transmittance, and can operate at room temperature. It is.

  As a result of examining the above problems, the inventors of the present application have found that in a strongly correlated oxide thin film formed on a surface of a high index plane substrate, the polarization in the crystal axis direction induced by the substrate strain is caused by the magnetic properties of the substance. Regardless, for example, it was noted that it continues to exist even at room temperature above 300K. And the specific means which can solve at least any one of the said subject based on the completely new principle was discovered.

As a specific solution, in one embodiment of the present invention, a La _1-x Sr _x MnO ₃ film (LSMO film, provided that 0.1 <x <0) is formed on a (210) -oriented substrate. 7) and magnetization control means for controlling the magnetization in the film surface of the LSMO film, and by controlling the magnetization of the LSMO film, the variable light whose transmittance with respect to the transmitted light of the LSMO film is controlled An attenuator is provided.

  According to the above aspect, it is possible to generate polarization having a component in the [−120] axial direction as an in-plane component at room temperature in the LSMO film. Moreover, the in-plane magnetization of the LSMO film can be generated, for example, in the [001] axis or [00-1] axis direction regardless of the presence or absence of the polarization. That is, in the above embodiment, polarization and magnetization orthogonal to each other can coexist in the LSMO film at room temperature. As a result, in the above-described aspect, the electromagnetic effect at room temperature can be used, and a variable optical attenuator using the directional dichroism as an operating principle can be realized. That is, in this configuration, for example, light can be incident from the front or back surface of the configuration (light modulation element) including the variable optical attenuator substrate and the LSMO film formed in a generally planar shape. Unlike the above, it is easy to increase the area. Moreover, since the LSMO film is ferromagnetic, it is not necessary to apply an external field (external magnetic field or injection current) in order to maintain the modulated transmittance. Note that in the present application in general, light that is controlled by attenuation or the like when passing through the variable optical attenuator is not only visible light, but also, for example, near-infrared light and infrared light for optical communication, and the LSMO film. It contains electromagnetic waves of any wavelength that can exhibit directional dichroism.

  In the above aspect, the magnetization control means includes any physical means for controlling at least one of the direction and strength of magnetization of the LSMO film. A typical magnetization control means includes a magnetic field application means that can apply a magnetic field to the LSMO film from the outside and change the strength and direction of the magnetic field. Further, a combination of a pinning portion and a current unit for creating a current with a biased spin distribution according to the principle of spin injection magnetization reversal is an example of such a magnetization control unit.

  In the variable optical attenuator of the above aspect, the direction of magnetization is the in-plane [001] axis direction of the LSMO film, and the light linearly polarized parallel to the [001] axis of the LSMO film is on the substrate side. To the LSMO film. In this aspect, it is more preferable that the magnetization control means controls the magnetic field strength applied in the [00-1] axis direction of the LSMO film, and the transmittance is controlled thereby.

  Further, in the variable optical attenuator of the above aspect, the magnetization direction is the in-plane [00-1] axis direction of the LSMO film, and the linearly polarized light parallel to the [001] axis of the LSMO film is the substrate. The light enters the LSMO film from the side opposite to the LSMO film. In this aspect, it is preferable that the magnetization control means controls the magnetic field strength applied in the [001] axis direction of the LSMO film, and thereby the transmittance is controlled.

  According to each of the above aspects, when a magnetic field is not applied, light is not transmitted or sufficiently attenuated, and the transmittance can be variably controlled so that the transmittance is increased according to the intensity by applying the magnetic field. Furthermore, according to each of the preferred embodiments, light is not transmitted when a magnetic field is not applied, and the transmittance can be variably controlled according to the intensity by applying the magnetic field.

  In addition, according to an aspect of the present invention, in the variable optical attenuator according to any of the above aspects, the magnetization control unit is applied in the [001] axial direction or the [00-1] axial direction of the LSMO film. Current density is controlled, whereby the transmittance is controlled. According to this aspect, a high-speed response is possible in which the transmittance change is executed in a short time of about nanoseconds.

In one embodiment of the present invention, it is preferable that the substrate has a composition of SrTiO ₃ . By using SrTiO ₃ , the polarization direction in the LSMO film grown epitaxially with respect to the substrate can be directed to an effective direction with respect to the transmitted light transmitted in the direction perpendicular to the substrate.

  Since the variable optical attenuator of the present invention is based on the directional dichroism due to the electromagnetic effect of the film formed on the substrate, it is easy to increase the area, and external field application is possible except when changing the transmittance. A variable optical attenuator which can be operated at room temperature without being required is realized.

It is explanatory drawing which shows the structure at the time of use of the structure of the variable optical attenuator of embodiment with this invention, and a light modulation element. 1A shows the first arrangement, and FIG. 1B shows the second arrangement. It is explanatory drawing which shows the arrangement | positioning of magnetization and polarization of an LSMO film | membrane by the direction seen from the film surface of the light modulation element in the light modulation element contained in the variable optical attenuator of embodiment with this invention. FIGS. 2A and 2B show the first and second arrangements, respectively. FIG. 3A is a schematic cross-sectional view of a light modulation element according to an embodiment of the present invention, FIG. 3A is an overall view, and FIG. 3B is an enlarged view showing a crystal structure of an LSMO film. In the variable optical attenuator of an embodiment of the present invention, a schematic diagram (FIG. 4A) showing a state of magnetic field application in the first arrangement, and a graph showing a state of modulation of the transmittance of the LSMO film (FIG. 4). (B)).

  Hereinafter, embodiments of a variable optical attenuator according to the present invention will be described with reference to the drawings. In the description, unless otherwise specified, common parts or elements are denoted by common reference numerals throughout the drawings. In the drawings, each element of each embodiment is not necessarily shown in a scale ratio.

<First Embodiment>
Hereinafter, embodiments of a variable optical attenuator according to the present invention will be described with reference to FIGS. FIG. 1 is an explanatory diagram showing the configuration of the variable optical attenuator 100 and the arrangement when the light modulation element 10 is used. The variable optical attenuator 100 includes an optical modulation element 10 and a magnetization control means 50. The magnetization control means 50 is typically any means that can adjust the direction and strength of the magnetic field 4. In the light modulation element 10, a La _1-x Sr _x MnO ₃ film 2 (hereinafter referred to as “LSMO film 2”) is formed on the surface of the substrate 1 having the (210) plane orientation of SrTiO ₃ . Here, the specific composition of the LSMO film 2 will be described as an example where x = 0.3. The crystal structure of SrTiO ₃ of the substrate 1 is produced in the (210) plane orientation and is a perovskite structure. The LSMO film 2 is formed on the (210) surface of the substrate 1 by growing it coherently with respect to the crystal structure of the substrate 1.

  The light modulation element 10 included in the variable optical attenuator 100 is used by the first and second configurations. 1A shows a first arrangement in which incident light 31A is incident from the substrate 1 side, and FIG. 1B shows a second arrangement in which incident light 31B is incident from the surface side (film surface side) of the film 2. It is shown. The outgoing lights in the first and second arrangements are outgoing lights 32A and 32B, respectively. Both the incident light 31 </ b> A and the incident light 31 </ b> B are light linearly polarized in the [001] axis direction of the crystal structure of the LSMO film 2. In these arrangements, the direction of magnetization in the LSMO film 2 is different in addition to the incident direction of light. That is, in the first and second arrangements, the combination of the magnetization and polarization in the LSMO film 2, the light incident direction, and the magnetic field application direction is as shown in FIGS. Is set.

FIG. 2 is an explanatory diagram showing the arrangement of the magnetization 5 and the polarization 6 of the LSMO film 2 in the light modulation element 10 of the present embodiment as viewed from the film surface of the light modulation element 10. (B) shows the arrangement of the magnetization 5 and the polarization 6 in the first and second arrangements corresponding to FIGS. 1 (a) and 1 (b), respectively. In the LSMO film 2, the magnetization direction is in the film plane. (210) In the LSMO film 2 formed on the substrate 1, the polarization 6 is generated in the [−120] axial direction as described later. This polarization 6 is drawn from the front to the back in FIG. 1 and from the left to the right on the page in FIG. Note that the Curie temperature (T _C ) of the LSMO film 2 is 360 K and exhibits ferromagnetism at room temperature. The coercive force is about 20 Oe (about 0.25 A / m).

  Next, the directional dichroism used in the LSMO film 2 of the variable optical attenuator 100 of the present embodiment will be described in the case of the first arrangement (FIGS. 1A and 2A). In the first arrangement, the magnetization 5 is set upward in the figure ([001] axis direction). Therefore, in FIG. 2A, the magnetization 5 is upward with respect to the polarization 6 directed to the right, and the magnetization 5 and the polarization 6 are orthogonal to each other in the film plane. In the first arrangement, the outgoing light 32A is weakened by the electromagnetic effect of the LSMO film 2 with respect to the incident light 31A incident from the substrate 1 side, so that the transmittance is lowered. On the other hand, the electromagnetic effect of the LSMO film 2 does not act on reverse-direction incident light (not shown) incident from the film surface side in this first arrangement, and the emitted light (not shown) is not easily weakened. This is because reverse-direction incident light is transmitted without being attenuated except for normal reflection on the film surface and absorption on the film itself. Directional dichroism is a property in which the transmittance varies depending on the propagation direction.

  Directional dichroism is also exhibited in the second arrangement (FIGS. 1B and 2B). The second arrangement differs from the first arrangement in that the magnetization 5 is directed downward (opposite the [001] axis) in the figure with respect to the polarization 6 directed to the right as shown in FIG. 2B. It is to be. That is, the magnetization 5 is set in the direction opposite to the [001] axis. Therefore, in the second arrangement, as shown in the arrangement shown in FIG. 2B, the downward magnetization 5 and the right polarization 6 are obtained. In this case, the electromagnetic effect of the LSMO film 2 acts on the incident light 31B incident from the LSMO film 2 side, and the emitted light 32B is weakened and the transmittance is lowered. On the other hand, the electromagnetism effect of the LSMO film 2 does not act on the reverse direction incident light (not shown) incident from the substrate 1 side, and the emitted light (not shown) is not easily weakened.

  The cause of directional dichroism seen in both the first and second arrangements is asymmetry or non-reciprocity of the magnetization 5 and polarization 6 with respect to the light traveling direction. For this reason, the arrangement | positioning of the initial state which uses the light modulation element 10 of this embodiment is made into either the 1st or 2nd arrangement | positioning mentioned above. That is, the arrangement of the incident light 31A and 31B, the magnetization 5, and the polarization 6 sets a combination in which the outgoing light 32A or 32B is weakened and the transmittance of the LSMO film 2 is lowered to the initial state. The initial state is maintained by the magnetization 5 of the LSMO film 2 itself.

  A typical modulation operation of the variable optical attenuator 100 is to modulate the transmittance from the initial state using an external magnetic field 4 (hereinafter referred to as “magnetic field 4”) by the magnetization control means 50. When the magnetic field 4 is applied in the direction opposite to the magnetization 5 from the initial state, the magnetization 5 is reduced or reversed in any of the first and second arrangements described above, and as a result, the attenuation of the transmitted light is weakened. 32A or 32B increases. For example, in the first arrangement, the magnetization 5 in the [001] direction in the initial state in FIG. 1A is weakened or remains in the [00-1] direction, and the emitted light 32A is compared with the initial state. Increase. Here, when the magnitude of the magnetic field 4 applied in order to modulate the transmittance is set to be equal to or less than the coercive force of the LSMO film 2, the magnitude of the magnetization 5 changes although the direction does not change. Therefore, transmittance modulation is realized in which the magnitude of the magnetization 5 is controlled by applying the magnetic field 4 and the intensity of the emitted light 32 </ b> A is controlled according to the magnitude of the magnetization 5. Moreover, even when the application of the magnetic field 4 is stopped, the magnetization 5 is maintained by the properties of the LSMO film 2. For this reason, the variable optical attenuator 100 does not require application of a magnetic field except when modulating the transmittance.

Next, the polarization 6 in the LSMO film 2 formed on the substrate 1 having the (210) plane orientation will be briefly described. FIG. 3 is a schematic cross-sectional view of the light modulation element 10 with the LSMO film 2 facing upward on the paper surface of the substrate 1, FIG. 3 (a) shows the whole image, and FIG. 3 (b) shows the LSMO film. It is an enlarged view which shows the crystal structure of 2. FIG. In the film of the LSMO film 2, as shown by a slightly upward right arrow in FIG. 3A, the cation is displaced relative to the anion of the oxygen atom, and as a result, polarization also occurs. The component in the film plane of this polarization is the [−120] axial direction. The cause of this will be described by paying attention to the crystal lattice orientation of the substrate 1 and the LSMO film 2. The substrate 210 having the (210) plane orientation has a crystal lattice in which the (010) plane is inclined by about 26.6 degrees. For this reason, as shown in FIG. 3B, the LSMO film 2 formed coherently on the surface of the substrate 1 is also (La, Sr) O—MnO ₂ — (La, Sr) O—MnO ₂ —. As described above, the atomic planes of (La, Sr) O and MnO ₂ atomic planes containing Mn are alternately stacked. Note that (La, Sr) means that La or Sr randomly occupies the A site of the perovskite. Further, the above alternate lamination is carried out in the normal direction of the surface of the LSMO film 2 (right direction of the substrate surface). In-plane symmetry is broken in such a crystal of the LSMO film 2. As a result, (La, Sr) and Mn are spatially displaced as indicated by the arrows attached to each of (La, Sr) and Mn on the atomic layer plane in FIG. And the polarization 6 occurs in the same direction as this displacement, that is, in a direction slightly inclined from the [−120] axis in the stacking direction, that is, in the [−110] axis direction. In the variable optical attenuator 100 of the present embodiment, the polarization 6 is determined mainly corresponding to the orientation of the crystal lattice of the substrate 1.

Here, the material that can be used as the (210) substrate 1 is any material suitable for epitaxial growth of the crystal of the LSMO film 2. Examples of such materials include (LaAlO ₃ ) _0.3 (SrAl _0.5 Ta _0.5 O ₃ ) _0.7 (LSAT) and SrTiO ₃ . In particular, the use of SrTiO ₃ is advantageous in that the polarization 6 generated in the LSMO film 2 is effectively used for transmitted light in the vertical direction. That is, when the extension strain from the substrate 1 affects the LSMO film 2, the angle of the polarization 6 from the substrate surface is about 18.4 degrees. As a result, the projection component onto the film surface of the polarization 6 acting on the transmitted light propagating in the direction perpendicular to the light modulation element 10 is increased to about 95% of the size of the polarization 6 and generated. Most of the polarization 6 has a suitable configuration that contributes to nonreciprocity to transmitted light. When the angle of the light modulation element with respect to the incident light can be arbitrarily set, the plane formed by the magnetization in the (210) plane and the polarization 6 tilted by about 18.4 degrees from the substrate surface (ie, (110) plane) On the other hand, by setting the incident angle so that the transmitted light propagates perpendicularly, the intensity change due to nonreciprocity can be utilized without any reduction.

  Incidentally, it should be noted that the polarization such as the polarization 6 cannot be obtained by a thin film formed coherently on the surface of the (100) plane substrate, for example, in which the in-plane is fourfold symmetrical. Furthermore, even if spin injection magnetization reversal, which will be described later, is used as a modification, it is possible to perform intermediate control of the transmittance described above with reference to FIG.

  Next, taking the first arrangement (FIG. 1 (a) and FIG. 2 (a)) as an example, how the magnetic field 4 is applied in the direction opposite to the magnetization to modulate the transmittance will be described with reference to FIG. . FIG. 4 is a schematic diagram (FIG. 4A) showing a state of magnetic field application in the first arrangement in the variable optical attenuator 100, and a graph showing a state of modulation of the transmittance of the LSMO film (FIG. 4 (FIG. 4). b)). As shown in FIG. 4A, application of the magnetic field 4 reduces the strength of the magnetization 5 while maintaining its direction. That is, the magnetization 5 and the polarization 6 become smaller without changing the direction as the magnetic field 4 is increased from (a2), (a3)... Before the application of the magnetic field 4 shown at the left end (a1). The magnetic field 4 applied here is not more than the coercive force of the LSMO film 2. Corresponding to the change of the magnetization 5, the transmittance of the LSMO film 2 changes. That is, the transmittance increases as schematically shown in FIG. 4B according to the increase of the magnetic field 4. As described above, in the operation of the variable optical attenuator 100, the magnitude of the magnetization 5 is controlled by applying the magnetic field 4 opposite to the magnetization 5, and finally transmitted through the light modulation element 10 having the LSMO film 2. The rate can be changed.

  As described above, the operation of the magnetization control means such as application of the magnetic field 4 is required only at the timing when the transmittance needs to be changed from the previous value. When the magnetization control means controls the external magnetic field, since the LSMO film 2 is a ferromagnetic material that retains the magnetization 5, if the application of the magnetic field 4 is stopped in a range weaker than the coercive force, The strength of the magnetization 5 is maintained. Accordingly, the transmittance is also maintained after the applied magnetic field is reduced to zero as indicated by the arrow in FIG. This transmittance can be modulated in multiple steps or continuously. Further, the description related to FIG. 4 is shown only in the case of the first arrangement, but the transmittance can be controlled by the same mechanism in the second arrangement.

A method for manufacturing the light modulation element 10 included in the variable optical attenuator 100 of this embodiment will be described. The light modulation element 10 of this embodiment can be manufactured by depositing the LSMO film 2 using the SrTiO ₃ (210) plane orientation substrate as the substrate 1. Specifically, the LSMO film 2 is formed by a laser ablation method. As the target material for the LSMO film 2, a polycrystalline material of LSMO produced by a solid phase reaction method is employed. First, an SrTiO ₃ (210) substrate is attached in the vacuum chamber, and then evacuated to 3 × 10 ⁻⁹ Torr (4 × 10 ⁻⁷ Pa) or less, for example. Thereafter, high-purity oxygen gas is introduced at 1 mTorr (0.133 Pa), and the substrate is heated to a temperature of about 900 ° C., for example. Subsequently, by irradiating the target of LSMO with a laser such as a KrF excimer laser having a wavelength of 248 nm, LSMO can be formed as an LSMO film 2 only in an arbitrary atomic layer. Here, the control of the film thickness of the LSMO, that is, the number of atomic layers, can be determined based on the relationship between the number of shots of the laser pulse and the number of atomic layers studied in advance. The light modulation element 10 can be formed by such a film forming process.

<Modification of this embodiment: Other magnetization control means>
In the description of the above embodiment, an external magnetic field by the magnetization control means 50 is used to control the magnetization 5 of the variable optical attenuator 100. In the embodiment of the present invention, unlike the variable optical attenuator 100 described above, the same operation can be performed by a magnetization control means other than an external magnetic field. A typical modification of this embodiment is to control the magnetization 5 and the transmittance by utilizing spin injection magnetization reversal.

Since the LSMO film 2 is also a so-called half metal having a polarized spin, magnetization reversal by spin injection can also be used. That is, the direction or strength of the magnetization 5 is controlled by injecting a current with an uneven spin distribution into the LSMO film 2. However, in order to cause spin injection magnetization reversal, the current density must be increased above a certain threshold. In order to perform spin injection magnetization reversal by sufficiently increasing the current density, it is effective to limit, for example, a region where magnetization is reversed. As an example, it is assumed that the film thickness is 100 nm and the LSMO film 2 is formed in a stripe pattern having a width of 10 μm extending in the [−120] axis direction. In this case, the cross-sectional area of the LSMO film 2 is 100 nm × 10 μm and the area is 1 μm ² . With such a cross-sectional area, it is possible to apply a current density exceeding the threshold value for magnetization reversal by current injection.

  Specifically, a pair of electrodes spaced apart in the [001] axial direction of the LSMO film 2 are provided. At that time, in order to bring the pair of electrodes into contact with the LSMO film 2 having the stripe pattern, one electrode is connected via an antiferromagnetic pinning layer (pinning portion) such as IrMn. The other electrode is directly connected. With such a configuration, it is possible to cause spin injection magnetization reversal using magnetization pinned in the opposite direction to the magnetization of the LSMO film 2. That is, by applying a current from one electrode from the other electrode, that is, by passing electrons from the one electrode, the LSMO2 layer that is not in contact with the pinned layer is changed from the LSMO2 layer that is in contact with the pinned layer. An electric current (spin current) having a spin opposite to the spin flows, and the magnetization of the LSMO film 2 not in contact with the pinned layer can be reversed. In order to align the magnetization direction of the film in the initial state, a current having a different polarity may be applied. By using the magnetization control means based on the principle of spin injection magnetization reversal, it becomes possible to control the light transmittance at high speed.

<Modification Example of Embodiment: Composition Range of LSMO Film>
Further, La _1-x Sr _x MnO ₃ of the LSMO film 2 may be in a range other than x = 0.3 described above. Specifically, a composition range of 0.1 <x <0.7 can be used. This is because La _1-x Sr _x MnO ₃ exhibits a ferromagnetic phase in the above composition range. Furthermore, in the composition range of 0.1 <x <0.17, La _1-x Sr _x MnO ₃ is preferable because it exhibits a ferromagnetic insulating phase. This is because the non-modulable absorption that increases when the LSMO film 2 has high conductivity can be reduced.

  As described above, the variable optical attenuator 100 of the present embodiment can be easily increased in area, and a variable optical attenuator that can operate at room temperature without applying an external field can be realized except when changing the transmittance. Become. Each of the embodiments described above has been described in order to explain the invention. The material of the thin film or the substrate, the composition, the film thickness, the forming method, the specific type of the magnetic field applying means, the forming method, etc. are not limited to the above embodiment. Rather, the scope of the invention of the present application should be determined based on the description of the claims. In addition, modifications that exist within the scope of the present invention including other combinations of the embodiments are also included in the scope of the claims.

  The present invention is applicable to any apparatus that uses a variable optical attenuator.

DESCRIPTION OF SYMBOLS 100 Variable optical attenuator 10 Light modulation element 1 (210) Plane orientation board | substrate 2 LSMO film | membrane 31A, 31B Incident light 32A, 32B Output light 4 Magnetic field (external magnetic field)
5 Magnetization 6 Polarization 50 Magnetization control means

Claims (7)

(210) La _1-x Sr _x MnO ₃ film (LSMO film, where 0.1 <x <0.7) formed on a substrate having a plane orientation;
Magnetization control means for controlling the magnetization in the film surface of the LSMO film,
A variable optical attenuator in which the transmittance of the LSMO film is controlled by controlling the magnetization of the LSMO film. The magnetization direction is the in-plane [001] axis direction of the LSMO film;
The variable optical attenuator according to claim 1, wherein light linearly polarized parallel to the [001] axis of the LSMO film is incident on the LSMO film from the substrate side. The magnetization direction is the in-plane [00-1] axis direction of the LSMO film;
The variable optical attenuator according to claim 1, wherein light linearly polarized parallel to the [001] axis of the LSMO film is incident on the LSMO film from the side of the LSMO film opposite to the side of the substrate. The variable optical attenuator according to claim 2, wherein the magnetization control means controls the magnetic field strength applied in the [00-1] axis direction of the LSMO film, whereby the transmittance is controlled. The variable optical attenuator according to claim 3, wherein the magnetization control means controls a magnetic field strength applied in the [001] axis direction of the LSMO film, and thereby the transmittance is controlled. The magnetization control unit controls a current density applied in the [001] axis direction or the [00-1] axis direction of the LSMO film, and thereby the transmittance is controlled. 4. The variable optical attenuator according to any one of 3 above. The variable optical attenuator according to claim 1, wherein the substrate has a composition of SrTiO ₃ .

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