Circularly polarized light modulation device

Abstract

This circularly polarized light modulating device is configured from: a laminated structure (1) that includes at least an n-side electrode (20), an active layer (21), and a p-side electrode (22); a current injection circuit (2) that causes a current to flow from the p-side electrode (22) through the active layer (21) to the n-side electrode (20); and a spin drive circuit (3) that causes a current to flow to at least one among the p-side electrode (22) and the n-side electrode (20) in a direction perpendicular to the current injected from the current injection circuit (2) into the laminated structure (1).

Description

[Technical field]

[0001] The present invention relates to a light source that emits circularly polarized light, and more particularly to a circularly polarized light modulation device that is capable of modulating the rotation direction of circularly polarized light. [Background technology]

[0002] In optical devices, a technology has been developed to control the state of circularly polarized light by controlling the electron spin in the device. Since the spin of the carriers used for radiative recombination corresponds to the emitted circularly polarized light, a technology for emitting circularly polarized light has been realized, for example, as disclosed in Non-Patent Document 1. In most devices that emit circularly polarized light, as in the example of Non-Patent Document 1, spin is injected by magnetizing a ferromagnetic material such as Fe and using it as an electrode. The magnetization of a ferromagnetic material has hysteresis characteristics. For this reason, in a structure in which a ferromagnetic material is magnetized and used as an electrode, there is a problem that the modulation of the rotation direction (right-handed / left-handed) of the circularly polarized light cannot be accelerated.

[0003] To solve this problem, for example, the technology disclosed in Non-Patent Document 2 realizes modulation using an optical device equipped with a pair of ferromagnetic electrodes magnetized in opposite directions. However, the technology disclosed in Non-Patent Document 2 can only achieve a modulation speed of about several hundred kHz.

[0004] Non-Patent Document 3 discloses the results of a simulation of the frequency response of the modulation of circularly polarized light. According to Non-Patent Document 3, it is shown that a high modulation speed of 200 to 300 GHz can be achieved by the modulation of circularly polarized light. However, in conventional technology, a device that controls spins at high speed has not been realized, and therefore only a modulation speed of about several hundred kHz has been achieved, and high-speed switching of the injected spins has been an issue. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] N. Nishizawa et al., “Pure circular polarization electroluminescence at room temperature with spin-polarized light-emitting diodes”, PNAS, vol. 114, no. 8, pp. 1783-1788, 2017 [Non-Patent Document 2] N. Nishizawa et al., “Arbitrary helicity control of circularly polarized light from lateral-type spin-polarized light-emitting diodes at room temperature”, Applied Physics Express, vol. 11, no. 5, pp. 053003, 2018 [Non-Patent Document 3] M. Lindemann et al., “Ultrafast spin-lasers”, Nature, vol. 568, pp. 212-215, 2019 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above problems, and has an object to provide a circular polarization modulation device capable of high-speed modulation of circularly polarized light. [Means for solving the problem]

[0007] The circularly polarized light modulation device of the present invention comprises an n-side electrode, an active layer, and a p-side electrode. Consists of a first intermediate layer and a second intermediate layer A laminated structure, the p-side electrode to the active layer and the first intermediate layer and the second intermediate layer. and a spin drive circuit configured to pass a current through the p-side electrode and the n-side electrode in a direction perpendicular to the current injected from the injection current circuit into the stacked structure. The first intermediate layer is an n-type semiconductor layer inserted between the n-side electrode and the active layer, and the second intermediate layer is a p-type semiconductor layer inserted between the active layer and the p-side electrode. It is characterized by the above. AlsoIn one configuration example of the circular polarization modulation device of the present invention, the first intermediate layer of the laminated structure further includes a tunnel insulating layer. Furthermore, the circular polarization modulation device of the present invention is characterized in that it comprises a layered structure including at least an n-side electrode, an active layer, and a p-side electrode, an injection current circuit configured to pass a current from the p-side electrode through the active layer to the n-side electrode, and two spin drive circuits configured to pass a current through each of the p-side electrode and the n-side electrode in a direction perpendicular to the current injected from the injection current circuit into the layered structure. In one configuration example of the circular polarization modulation device of the present invention, the electrode through which the current from the spin drive circuit flows out of the p-side electrode and the n-side electrode is made of a heavy metal. In one configuration example of the circular polarization modulation device of the present invention, the spin drive circuit is characterized in that it is possible to control the direction of a current in response to an electrical signal input from the outside. Effect of the Invention

[0008] According to the present invention, in addition to the injection current circuit of a general device, a spin drive circuit is provided in at least one of the p-side electrode and the n-side electrode, which supplies a current in a direction perpendicular to the current injected from the injection current circuit into the stacked structure. This makes it possible to utilize the spin Hall effect in the metal to rapidly switch the direction of rotation of the circularly polarized light, thereby expanding the modulation bandwidth. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating the spin Hall effect. [Diagram 2] FIG. 2 is a diagram showing a configuration of a circular polarization modulation device according to a first embodiment of the present invention. [Diagram 3] FIG. 3 is a diagram showing the configuration of a circular polarization modulation device according to a second embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing another configuration of the circular polarization modulation device according to the first embodiment of the present invention. [Diagram 5] FIG. 5 is a diagram showing another configuration of a circular polarization modulation device according to the second embodiment of the present invention. [Figure 6] FIG. 6 is a diagram showing a model of a circularly polarized light modulation device created using an electromagnetic field simulator. [Figure 7A-7B]7A and 7B are diagrams showing an example of frequency characteristics of the reflection coefficient of a spin drive circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [First Example] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the present invention, a phenomenon called the spin Hall effect, as shown in FIG. 1, is used. The spin Hall effect is a phenomenon in which, when a current I is passed through a heavy metal 10 such as Pt, Ta, or W, spin dependence occurs in the scattering direction of electrons 11, so that a spin current is generated in a direction perpendicular to the current I. In FIG. 1, 12 indicates the flow of upward spin electrons, and 13 indicates the flow of downward spin electrons. The spin Hall effect is a phenomenon that occurs in most solids (metals and semiconductors), but the effect is prominent in heavy metals, particularly Pt, Ta, W, and Au-Pt alloys, and therefore it is desirable to use these materials as electrodes for devices.

[0011] 2 is a diagram showing the configuration of a circularly polarized light modulation device according to a first embodiment of the present invention. The circularly polarized light modulation device is composed of a stacked structure 1 including at least an n-side electrode 20, an active layer 21, and a p-side electrode 22, an injection current circuit 2 that passes a current from the p-side electrode 22 through the active layer 21 to the n-side electrode 20, and a spin drive circuit 3 that passes a current to at least one of the p-side electrode 22 and the n-side electrode 20 in a direction perpendicular to the current injected from the injection current circuit 2 to the stacked structure 1.

[0012] The laminated structure 1 is composed of an n-side electrode 20, an active layer 21, a p-side electrode 22, an intermediate layer 23 inserted between the n-side electrode 20 and the active layer 21, and an intermediate layer 24 inserted between the active layer 21 and the p-side electrode 22. The intermediate layers 23, 24 may be conductive and may comprise, for example, an n-doped semiconductor, or may comprise a thin insulating layer.

[0013] This embodiment shows an example in which the spin drive circuit 3 passes a current only through the n-side electrode 20. Therefore, it is desirable to use a heavy metal such as Pt, Ta, W, or an Au-Pt alloy as the material for the n-side electrode 20, but the p-side electrode 22 may be made of a material other than a heavy metal.

[0014] In this embodiment, a spin current is generated in a direction parallel to the injection current injected from the injection current circuit 2 to the stacked structure 1 by the current (referred to as the spin drive current) flowing from the spin drive circuit 3 to the n-side electrode 20, and the spin current is injected into the active layer 21 together with the injection current, so that circularly polarized light is emitted from the active layer 21.

[0015] The spin drive circuit 3 can control the direction of the current according to an externally input electric signal. If the direction of the spin drive current is reversed, the direction of the spin current is also reversed, and the circularly polarized light is also reversed, so that the circularly polarized light can be modulated by the spin drive current.

[0016] As described above, in this embodiment, when injecting spin-polarized electrons into the active layer 21, a ferromagnetic material such as Fe is not used, but the spin Hall effect in a metal is used, thereby making it possible to control the direction of the spin current and realize high-speed modulation of circularly polarized light by an electrical signal.

[0017] [Second Example] Next, a second embodiment of the present invention will be described. Fig. 3 is a diagram showing the configuration of a circularly polarized light modulation device according to a second embodiment of the present invention. In this embodiment, the present invention is applied to a stacked structure 1a constituting a pin junction LED.

[0018] The laminated structure 1a is composed of a p-side electrode 35, a p-type semiconductor layer 34 formed on the p-side electrode 35, an active layer 33 formed on the p-type semiconductor layer 34, an n-type semiconductor layer 32 formed on the active layer 33, a tunnel insulating layer 31 formed on the n-type semiconductor layer 32, and an n-side electrode 30 made of Pt formed on the tunnel insulating layer 31. The n-type semiconductor layer 32 and the tunnel insulating layer 31 correspond to the intermediate layer 23 in FIG. 2, and the p-type semiconductor layer 34 corresponds to the intermediate layer 24.

[0019] In this embodiment, since the spin drive circuit 3 passes a spin drive current through the n-side electrode 30, Pt is used as the material of the n-side electrode 30, but the p-side electrode 35 may be made of a material other than a heavy metal. The tunnel insulating layer 31 is intended to reduce spin relaxation that occurs when spins are injected from the n-side electrode 30 into the active layer 33 .

[0020] As in the first embodiment, a spin current is generated in a direction parallel to the injection current injected from the injection current circuit 2 into the stacked structure 1a by the spin drive current flowing from the spin drive circuit 3 to the n-side electrode 30, and the spin current is injected into the active layer 33 together with the injection current, so that circularly polarized light is emitted from the active layer 33. If the direction of the spin drive current is reversed, the direction of the spin current is also reversed and the circularly polarized light also rotates in the opposite direction, so that the circularly polarized light can be modulated by the spin drive current.

[0021] In this embodiment, the laminate structure 1a is an LED, but the present invention can also be applied to devices that emit and amplify light by radiative recombination of carriers, such as lasers and semiconductor optical amplifiers.

[0022] As described above, it is also possible to pass a spin drive current through the p-side electrode by the spin drive circuit. In the first embodiment, a configuration in which a spin drive current is passed through both the n-side electrode 20 and the p-side electrode 22 is shown in Fig. 4, and in the second embodiment, a configuration in which a spin drive current is passed through both the n-side electrode 30 and the p-side electrode 35 is shown in Fig. 5.

[0023] Since the direction of the spin current is determined by the difference between the flow of upward spin electrons and the flow of downward spin electrons, the direction of the spin current can be controlled by adjusting the balance between the spin drive current flowing from the spin drive circuit 3-1 to the n-side electrodes 20, 30 and the spin drive current flowing from the spin drive circuit 3-2 to the p-side electrodes 22, 35. Moreover, it goes without saying that when a spin drive current is applied only to the p-side electrodes 22 and 35, only the spin drive circuit 3-2 may be provided.

[0024] [Third Example] Next, a third embodiment of the present invention will be described. In this embodiment, the effect of the present invention is verified using a circuit simulator. In the device shown in Fig. 3, the two factors that are considered to limit the modulation speed are (1) the frequency of the spin driving circuit and (2) the time constant of the spin Hall effect.

[0025] First, we investigate the effect of the frequency characteristics of the spin drive circuit on the modulation speed of the circular polarization modulation device. When the response of the spin drive current to the voltage applied to the spin drive circuit is slow, the spin drive circuit becomes the rate-limiting factor of the modulation speed.

[0026] Here, using the electromagnetic field simulator SONNET (registered trademark) ver.16.56, a model of a circularly polarized light modulation device as shown in Fig. 6 was created, and the frequency characteristics of the spin drive circuit were calculated. In the example of Fig. 6, a model of a laminated structure was created consisting of a GaAs substrate 41, an AlGaAs / GaAs-LED 42 grown on the GaAs substrate 41, a Pt channel 40 serving as an n-side electrode, and a p-side electrode 43 on the back surface of the substrate. The GaAs substrate 41 and the AlGaAs / GaAs-LED 42 correspond to the active layer 21 and intermediate layers 23 and 24 in Fig. 2.

[0027] The thickness of the Pt channel 40 is 50 nm, the thickness of the GaAs substrate 41 is 100 μm, the thickness of the AlGaAs / GaAs-LED 42 is 1.5 μm, and the thickness of the p-side electrode 43 is 1 μm. The dielectric constant of the GaAs substrate 41 and the AlGaAs / GaAs-LED 42 is 9.0, and the resistivity of the Pt channel 40 is 1.05×10 -7 Ωm, and the resistivity of the p-side electrode 43 is 1×10 -7 The spin drive circuit 3 was an AC power source with an output impedance of 50 Ω.

[0028] 7A and 7B show the frequency characteristics of the reflection coefficient of the spin drive circuit 3 calculated under the above conditions. Fig. 7A shows the frequency characteristics of the real part Re(S) of the reflection coefficient, and Fig. 7B shows the frequency characteristics of the imaginary part Im(S) of the reflection coefficient.

[0029] 7A and 7B, a resonance peak was confirmed near 19 GHz, but S=-1 at other frequencies, indicating a near-short circuit. In the present invention, it is important whether the spin drive current output from the spin drive circuit 3 follows the AC power supply voltage input to the spin drive circuit 3. Therefore, the results in FIGS. 7A and 7B show that the AC power supply can operate as the spin drive circuit 3 up to near 250 GHz, except near the resonance point.

[0030] Next, we will examine the effect of the time constant of the response due to the spin Hall effect on the modulation speed of the circular polarization modulation device. In order to investigate the dynamic behavior of the spin Hall effect, it has been proposed to treat electrons as particles that obey a one-dimensional diffusion equation and solve an equation that includes a term that represents the spin Hall effect (NPStern et al., "Time-resolved dynamics of the spin Hall effect", Nature Physics, vol. 4, pp. 843-846, 2008).

[0031] Using a similar model, the time constant of the response due to the spin Hall effect, τ SH can be expressed as follows:

[0032]

number

[0033] In formula (1), τ s is the spin relaxation time, L s is the spin diffusion length, and d is the thickness of the Pt channel 40. The thickness d of the Pt channel 40 is set to 50 nm, the same as in the model shown in FIG. 6, and the spin relaxation time τ s , the spin diffusion length L sIf we use the value at room temperature disclosed in the literature "R. Freeman et al., "Evidence for Dyakonov-Perel-like Spin Relaxation in Pt", Physical Review Letters, vol. 120, pp. 067204, 2018", the time constant τ SH = 194 fs. The spin relaxation time τ s The value of is 0.2 ps, and the spin diffusion length L s The value of is 3.7 nm. SH = 194fs is converted to a 3dB cutoff frequency, f 3dB =819 GHz, which is equal to or higher than the operable frequency of the spin drive circuit 3.

[0034] From the rough calculation results of the frequency characteristics of the spin drive circuit 3 and the rough calculation results of the time constant of the spin Hall effect, it can be said that a modulation speed of about 250 GHz can be expected. [Industrial Applicability]

[0035] The present invention is applicable to light sources that emit circularly polarized light. [Explanation of symbols]

[0036] Reference Signs List 1,1a...laminated structure, 2...injection current circuit, 3,3-1,3-2...spin drive circuit, 20,30...n-side electrode, 21,33...active layer, 22,35,43...p-side electrode, 23,24...intermediate layer, 31...tunnel insulating layer, 32...n-type semiconductor layer, 34...p-type semiconductor layer, 40...Pt channel, 41...GaAs substrate, 42...AlGaAs / GaAs-LED.

Claims

1. a laminated structure including an n-side electrode, an active layer, a p-side electrode, a first intermediate layer, and a second intermediate layer; an injection current circuit configured to pass a current from the p-side electrode through the active layer, the first intermediate layer, and the second intermediate layer to the n-side electrode; a spin drive circuit configured to pass a current through at least one of the p-side electrode and the n-side electrode in a direction perpendicular to a current injected from the current injection circuit into the stacked structure; the first intermediate layer is an n-type semiconductor layer inserted between the n-side electrode and the active layer, The circularly polarized light modulation device, wherein the second intermediate layer is a p-type semiconductor layer inserted between the active layer and the p-side electrode.

2. 2. The circular polarization modulation device according to claim 1, The circular polarization modulation device according to claim 1, wherein the first intermediate layer of the laminated structure further includes a tunnel insulating layer.

3. A laminate structure including at least an n-side electrode, an active layer, and a p-side electrode; an injection current circuit configured to inject a current from the p-side electrode through the active layer to the n-side electrode; A circular polarization modulation device comprising two spin drive circuits configured to pass a current through each of the p-side electrode and the n-side electrode in a direction perpendicular to the current injected from the injection current circuit into the stacked structure.

4. 4. The circular polarization modulation device according to claim 1, The circular polarization modulation device, wherein one of the p-side electrode and the n-side electrode through which a current from the spin drive circuit flows is made of a heavy metal.

5. 5. The circular polarization modulation device according to claim 1, The circular polarization modulation device is characterized in that the spin drive circuit is capable of controlling the direction of current in response to an electrical signal input from the outside.

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