JP6919549B2 - Nanowire optical device - Google Patents

Description

The present invention relates to a nanowire optical device including a nanowire portion made of a semiconductor.

Semiconductor nanowires are attracting attention as new optical elements. Nanowires have functionality such as quantum dots and quantum wells, and since nanowires have a very small capacitance, they have excellent optical and electrical characteristics. In addition, nanowires enable mass production of nanodevices because nanostructures can be uniformly and mass-produced on a substrate. In recent years, techniques for producing desired compound nanowires at arbitrary positions on a silicon substrate have been advanced, and it can be said that they are promising nanomaterials for future optical chips and integrated circuits.

Efforts to introduce such nanowires into optical devices and chips are extremely important in terms of application. However, in general, nanowires are sufficiently small in terms of the wavelength size of light, so that the nanowires alone cannot strengthen the light confinement and cannot fully exhibit the characteristics of the nanowires. Therefore, in order to use nanowires as an optical device, some kind of optical confinement structure is required. For example, it is important to design a photonic crystal, which is a dielectric periodic structure, and a plasmonic structure, which is a metal nanostructure, in combination with nanowires.

For example, when combined with a two-dimensional photonic crystal, by transferring the nanowire to a photonic crystal substrate and arranging it at a desired location, light can be confined in the nanowire by the structure outside the nanowire. This technique has achieved some success in that it has made it possible to observe laser oscillations and quantum optics effects (see Non-Patent Document 1).

On the other hand, when combined with a metal structure, the structure is simpler than that of a photonic crystal structure, which is promising in terms of application. The most famous method is a plasmon structure in which nanowires are placed on a metal surface (or metal coated with a thin insulating layer). Further, as a metal structure, a method of arranging a bow tie antenna (see Non-Patent Document 2), a metal-insulating layer-metal structure (MIM) structure, or the like is considered.

M. Takiguchi et al., "Continuous-wave operation and 10-Gb / s direct modulation of InAsP / InP sub-wavelength nanowire laser on silicon photonic crystal", APL Photonics, vol. 2. no. 4, 046106, 2017. M. Ono et al., "Nanowire-nanoantenna coupled system incorporated by nanomanipulation", Optics Express, vol. 24, no. 8, pp. 8647-8659, 2016.

By the way, the conventional plasmon structure is a system in which the optical absorption loss (α) is large in principle, and in a device such as a laser, the loss is a fatal problem because it raises the threshold value. rice field. Therefore, in the conventional plasmonic nanowire lasers, as shown in FIGS. 12A and 12B, the nanowire portion 502 is separated from the metal layer 501 by sandwiching the dielectric layer 503 between the metal layer 501 and the nanowire portion 502. , I have tried to reduce the loss. However, there is a trade-off relationship between the absorption loss and the light confinement coefficient, and the above-mentioned technique has had to sacrifice the light confinement effect (confinement coefficient: Γ).

The present invention has been made to solve the above problems, and an object of the present invention is to make it possible to improve the light confinement coefficient in a plasmonic structure without increasing the loss.

The nanowire optical device according to the present invention is a structure other than a structure made of metal, a nanowire portion made of a semiconductor arranged with side surfaces in contact with the structure, and a contact region between the structure and the nanowire portion. It is provided with a dielectric layer formed on the surface of the nanowire portion facing the nanowire portion, and the nanowire portion is thick enough to allow a plasmonic mode of light propagating through the nanowire portion to appear.

In the nanowire light devices, structures, Ru grating structure der the plurality of structures, each insulated separated are arranged.

In the nanowire optical device, the structure may form a groove along the extending direction of the nanowire portion, and the nanowire portion may be arranged in the groove.

The nanowire optical device may be provided with an excitation means for exciting the nanowire portion, and the nanowire portion may be configured to oscillate a laser beam having a predetermined wavelength.

The nanowire optical device may include a current injection unit that injects a current into the nanowire unit, and the nanowire unit may be configured to emit light having a predetermined wavelength.

In the nanowire optical device, the nanowire portion may have a substantially circular cross section, and the thickness d of the nanowire portion may be in a range satisfying the following equation.

As described above, according to the present invention, since the dielectric layer is formed on the surface of the nanowire portion facing the structure other than the contact region between the structure and the nanowire portion, the loss is not increased in the plasmonic structure. In addition, an excellent effect that the light confinement coefficient can be improved can be obtained.

FIG. 1A is a perspective view showing a configuration of a nanowire optical device according to an embodiment of the present invention. FIG. 1B is a cross-sectional view showing the configuration of a nanowire optical device according to an embodiment of the present invention. FIG. 2 is a characteristic diagram showing a dispersion relationship regarding the diameter of the nanowire portion 102 and the effective refractive index when the nanowire portion 102 is alone. FIG. 3 is a characteristic diagram showing a dispersion relationship regarding the diameter of the nanowire portion 102 and the effective refractive index when the nanowire portion 102 is arranged on the structure 101. FIG. 4 is a characteristic diagram showing the result of simulating the confinement coefficient with the diameter of the nanowire portion 102 set to 300 nm. FIG. 5 is a characteristic diagram showing a comparison of confinement coefficients between the nanowire optical device according to the embodiment and the conventional nanowire optical device. FIG. 6 is a characteristic diagram showing the relationship between the thickness of the dielectric layer and Γ / α as a result of comparing the nanowire optical device in the embodiment with the conventional nanowire optical device. FIG. 7 is a cross-sectional view showing the configuration of another nanowire optical device according to the embodiment of the present invention. FIG. 8 is a perspective view showing the configuration of another nanowire optical device according to the embodiment of the present invention. FIG. 9A is a perspective view showing the configuration of another nanowire optical device according to the embodiment of the present invention. FIG. 9B is a characteristic diagram showing a change in the confinement coefficient with respect to the thickness of the dielectric layer 103 in the nanowire optical device provided with the groove 141. FIG. 9C is a characteristic diagram showing a change in the confinement coefficient with respect to the thickness of the dielectric layer 103 in the nanowire optical device provided with the groove 141. FIG. 10 is a perspective view for explaining the arrangement of the light source 105 as an excitation means for the nanowire optical device according to the embodiment of the present invention. FIG. 11 is a perspective view showing the configuration of a nanowire optical device including the current injection means according to the embodiment of the present invention. FIG. 12A is a perspective view showing the configuration of a conventional nanowire optical device. FIG. 12B is a cross-sectional view showing the configuration of a conventional nanowire optical device.

Hereinafter, the nanowire optical device according to the embodiment of the present invention will be described with reference to FIGS. 1A and 1B. This nanowire optical device includes a structure 101 made of metal, a nanowire portion 102 made of a semiconductor arranged on the structure 101, and a dielectric layer 103 formed on the surface of the nanowire portion 102. The structure 101 is formed in a plate shape, for example. Further, the structure 101 may be composed of a metal layer formed on an insulating substrate.

The side surface of the nanowire portion 102 is in contact with the structure 101 in the contact region 121. The dielectric layer 103 may be formed on the surface of the nanowire portion 102 facing the structure 101 other than the contact region 121 between the structure 101 and the nanowire portion 102. The structure 101 may be composed of, for example, Au. The nanowire portion 102 may be made of a compound semiconductor such as InP. The dielectric layer 103 may be composed of, for example, a dielectric (insulator) such as _{Al 2} O _3.

Further, the nanowire portion 102 has a thickness (diameter) at which a plasmonic mode (hybrid mode) of light propagating through the nanowire portion 102 can appear. Here, the plasmonic mode is a mode in which the dielectric mode cannot exist, and the hybrid mode is the lowest degenerate mode in the air, which is degenerated by the metal structure 101. Refers to the mode. Therefore, the condition for the plasmonic mode to appear in the nanowire portion 102 must be at least finer than the single mode condition. The thickness (diameter) d of the nanowire portion 102 having a substantially circular cross section may be within a range satisfying the following equation (1).

The dispersion relation between the diameter of the nanowire portion 102 and the effective refractive index will be described with reference to FIGS. 2 and 3. In FIGS. 2 and 3, the horizontal axis is the diameter of the nanowire portion 102 having a circular cross section, the vertical axis is the effective refractive index, and the wavelength of light propagating through the nanowire portion 102 is assumed to be 1,55 μm. The nanowire portion 102 is composed of InP. With the nanowire unit 102 alone, as shown in FIG. 2, in the region satisfying the equation (1), the mode is degenerate and there is only one. On the other hand, when the nanowire portion 102 is arranged on the structure 101, the degeneracy of the mode can be solved by the effect of the metal as shown in FIG. This region is the so-called hybrid mode (mixed state of plasmon mode and photonic mode).

In general, when the electric field is concentrated on the metal surface in the plasmonic mode, the loss increases due to metal absorption. In order to suppress this loss, in the conventional structure, as shown in FIG. 12B, a structure in which the dielectric layer 503 is sandwiched between the nanowire portion 502 and the metal layer 501 is adopted. However, in the conventional structure, if the light exuding into the gap (in the air) between the nanowire portion 502 and the metal layer 501 can be pushed into the wire, the confinement coefficient can be increased.

In the present invention, by forming the dielectric layer 103 on the surface of the nanowire portion 102 arranged in contact with the surface of the structure 101, the light leaked into the air is taken into the 102 portion in the nanowire without increasing the loss. , The light confinement coefficient was improved, and the relationship of α / Γ was improved compared to the conventional structure. The result of simulating this state with the diameter of the nanowire portion 102 set to 300 nm is shown in FIG. 4A. The confinement coefficient was 44%. The results of the same simulation for the case where the dielectric layer 103 is not formed are shown in FIG. 4 (b). The electric field exuded into the air, and the confinement coefficient was 26%. As described above, according to the embodiment, the light confinement coefficient is improved, for example, the threshold value of the nanowire laser can be lowered, and the characteristics of the light emitting device using the nanowire can be improved.

As a method of forming the dielectric layer 103 on the surface of the nanowire portion 102 in the gap between the nanowire portion 102 and the structure 101, for example, a well-known atomic layer deposition method (ALD) or other deposition method is used. Just do it. According to the ALD method, the dielectric layer 103 can be formed at the atomic layer level even in a very small gap.

According to the above-described embodiment, since the nanowire portion 102 and the structure 101 are in contact with each other, the effect of metal absorption cannot be suppressed, but when considering a laser element, the threshold value is α / Γ (absorption coefficient is This value is important because α and Γ are proportional to the confinement coefficient). FIG. 5 shows a comparison of the confinement coefficients of the nanowire optical device in the embodiment and the conventional nanowire optical device. In FIG. 5, for convenience, 1 / α is set as the propagation distance. Further, the relationship between the thickness of the dielectric layer and Γ / α is shown in FIG. 6 as a result of comparing the nanowire optical device in the embodiment with the conventional nanowire optical device. In each case, (a) is the result of the embodiment, and (b) is the result of the conventional method.

As shown in FIGS. 5 and 6, by appropriately setting the thickness of the dielectric layer 103, Γ is improved and Γ / α is improved by about 1.4 times as compared with the conventional case. Such characteristics are not limited to the case where the nanowire portion 102 is composed of InP, and the same tendency is obtained even when the nanowire portion 102 is composed of other semiconductors such as InAs, GaAs, ZnO, and CdSe. Further, the structure 101 is not limited to Au, and is the same even if it is composed of Ag and Al. Further, the dielectric layer 103 is not limited to _{Al 2} O _3, and may be composed of other dielectrics such as _{SiO 2.}

By the way, the entire surface of the nanowire portion 102 may not be covered with the dielectric layer. For example, as shown in FIG. 7A, the nanowire portion 102 is arranged on the structure 101, and then Al ₂ O ₃ is deposited by the ALD method to form the dielectric layer 103. After that, the dielectric layer 103 is etched by a well-known etching process having high vertical anisotropy such as reactive ion etching. As a result, as shown in FIG. 7B, the dielectric layer 131 is formed on the surface of the nanowire portion 102 facing the structure 101 other than the contact region 121 between the structure 101 and the nanowire portion 102. In the present invention, the dielectric layer 131 may be arranged between the structure 101 and the nanowire portion 102.

By the way, when the nanowire optical device is a laser, as shown in FIG. 8, a dielectric layer 304 is coated on a grating structure 301 made of metal and in which a plurality of structures 303 each insulated and separated are arranged. A configuration is conceivable in which the nanowire portion 302 is arranged. The grating structure 301 acts as a mirror for conditions that satisfy Bragg reflection in order to provide a periodic structure of refractive index. By irradiating the nanowire portion 302 with excitation light from a predetermined light source and exciting the grating structure 301 as a mirror in this way, laser light having a predetermined wavelength can be oscillated.

In this case, the structure of the portion where the nanowire portion 302 is placed on the structure 303 and the portion placed in the air are periodically arranged, and there are a plasmonic mode (hybrid mode) and a photonic mode, respectively. It is important to make the conditions possible. It suffices that the diameter of the nanowire portion 302 satisfies the equation (1) and propagates under the single mode condition.

Further, as shown in FIG. 9A, the nanowire portion 102 may be arranged in the groove 141 formed by the two metal structures 104 arranged on the substrate 111. The groove 141 is formed along the extending direction of the nanowire portion 102. The substrate 111 is made of, for example, an insulating material such as _{SiO 2.} This structure is called a MIM (Metal-Insulator-Metal) waveguide. Since there is no photonic mode in such a MIM waveguide, the diameter of the nanowire portion 102 may be smaller than the range satisfying the equation (1). In such a MIM waveguide, by forming the dielectric layer 103 on the surface of the nanowire portion 102, the confinement coefficient can be improved by several times.

For example, assuming that the width of the groove 141 is 100 nm and the diameter of the nanowire portion 102 is 70 nm, the confinement coefficient can be increased by forming the dielectric layer 103, as shown in FIG. 9B. Further, assuming that the width of the groove 141 is 100 nm and the diameter of the nanowire portion 102 is 110 nm, the confinement coefficient can be increased by forming the dielectric layer 103, as shown in FIG. 9C.

By the way, as shown in FIG. 10, a light source 105 for exciting the nanowire portion 102 is arranged, and the excitation light from the light source 105 is collected by the lens 151 from the upper part in the radiation direction away from the plane of the structure 101. It may be configured to irradiate the nanowire portion 102. Laser oscillation is possible by such an excitation means.

Further, a laser may be configured by injecting a carrier by injecting an electric current, or a nanowire optical device may be configured as an LED (Light Emitting Diode) by injecting a carrier by injecting an electric current from the nanowire unit 102. Light of a predetermined wavelength may be emitted.

For example, as shown in FIG. 11, p-type regions 106a and n-type regions 106b are formed in the nanowire portion 102. The wiring 107a and the wiring 107b are connected to the p-type region 106a and the n-type region 106b. A power supply (not shown) is connected to the wiring 107a and the wiring 107b, and a current is injected into the p-type region 106a and the n-type region 106b. The p-type region 106a and the n-type region 106b may be formed by introducing p-type impurities and n-type impurities.

Further, by forming the impurity region in this way, the nanowire optical device can be operated as a receiver. If an optical excitation light source is incorporated into a device, it can be used as a wavelength conversion element in a chip and can also operate as an optical switch.

As described above, according to the present invention, since the dielectric layer is formed on the surface of the nanowire portion facing the structure other than the contact region between the structure and the nanowire portion, the loss is increased in the plasmonic structure. The light confinement coefficient can be improved without this.

The present invention is not limited to the embodiments described above, and many modifications and combinations can be carried out by a person having ordinary knowledge in the art within the technical idea of the present invention. That is clear.

101 ... structure, 102 ... nanowire portion, 103 ... dielectric layer, 121 ... contact region.

Claims (5)

A structure made of metal and
A nanowire portion made of a semiconductor whose side surfaces are in contact with each other on the structure,
A dielectric layer formed on the surface of the nanowire portion facing the structure other than the contact region between the structure and the nanowire portion is provided.
The nanowire portion has a thickness that allows the plasmonic mode of light propagating through the nanowire portion to appear .
The structure nanowire light device in which a plurality of metal structures which are each insulated separation and wherein the Ru grating structure der arranged. In nanowire optical device according to claim 1 Symbol placement,
The structure constitutes a groove along the extending direction of the nanowire portion.
The nanowire optical device is characterized in that the nanowire portion is arranged in the groove. In the nanowire optical device according to claim 1 or 2.
An excitation means for exciting the nanowire portion is provided.
The nanowire unit is a nanowire optical device characterized in that it oscillates a laser beam having a predetermined wavelength. In the nanowire optical device according to claim 1 or 2.
A current injection unit for injecting a current into the nanowire unit is provided.
The nanowire unit is a nanowire optical device characterized in that it emits light having a predetermined wavelength. In the nanowire optical device according to any one of claims 1 to 4.
The nanowire portion has a substantially circular cross section.
A nanowire optical device, wherein the thickness d of the nanowire portion is within a range satisfying the following equation.

n _core : Refractive index of the nanowire part n _clad : Refractive index around the nanowire part λ: Wavelength of light propagating in the nanowire part

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