Optical device
A semiconductor nanowire with an asymmetric cross-sectional shape and light-loss-inducing structure generates OAM light efficiently, addressing the complexity and size issues of existing nanostructure-based devices, enabling miniaturization and effective OAM light extraction.
Patent Information
- Application Number
- JP2024031186
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-03-01
- Publication Date
- 2025-09-11
AI Technical Summary
Existing OAM light generation technologies using nanostructures like metasurfaces and ring resonators are complex and require bulk optical elements, making them large and power-consuming, and there is a need for a simpler structure to generate OAM light directly from a light source.
An optical device comprising a semiconductor nanowire with a symmetrical cross-section and an asymmetric cross-sectional shape, incorporating a structure that causes light loss, allowing OAM light generation without a complex structure.
The device generates OAM light efficiently and miniaturizes the device size by utilizing the nanowire's propagation mode, enabling extraction of OAM light without degeneracy and reducing unwanted modes.
Smart Images
Figure 2025133310000001_ABST
Abstract
Description
[Technical Field]
[0001] The present invention relates to an optical device for generating OAM light. [Background technology]
[0002] Light with vector beams or orbital angular momentum (OAM) is known as a topological light wave, and is known to have vortices in its polarization and phase. Both types of light have singularities in phase and polarization at the optical axis. Utilizing this unique optical property enables new device applications. For example, applications such as nanomaterial capture, laser processing, microscopy, and quantum information have been proposed. Summary of the Invention [Problem to be solved by the invention]
[0003] Until now, topological light waves have been generated using holograms, crystals with refractive index gradients, multiple wave plates, etc., but in most cases, bulk optical elements are combined, making the entire device large. In order to miniaturize and reduce the power consumption of devices that generate topological light waves, it is important that OAM light can be generated directly from the light source, and that the light source itself is small. Against this background, OAM light generation technologies have been developed in recent years using nanostructures such as metasurfaces and ring resonators. Although these are much smaller than devices that combine bulk optical elements, the complexity of their structure has been an issue.
[0004] The present invention has been made to solve the above problems, and has an object to make it possible to generate light having OAM without using a complicated structure. [Means for solving the problem]
[0005] The optical device of the present invention comprises a nanowire made of semiconductor, having a symmetrical cross-section and a normalized frequency of 7 or more, and a structure arranged near the nanowire to impart loss to light guided through the nanowire, and the cross-sectional shape of the nanowire including the structure is asymmetric. [Effects of the Invention]
[0006] As described above, according to the present invention, a structure that causes loss in light is placed near a nanowire, and the cross-sectional shape of the nanowire including the structure is made asymmetric, so that light with OAM can be generated without using a complex structure. [Brief explanation of the drawings]
[0007] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of an optical device according to an embodiment of the present invention. [Figure 2A] FIG. 2A is a distribution diagram showing the distribution of the electric field in the nanowire obtained by 2D electromagnetic field analysis simulation. [Figure 2B] FIG. 2B is a characteristic diagram showing the relationship between the azimuthal quantum number l in the nanowire and the obtained OAM. [Figure 3A] FIG. 3A is a distribution diagram showing the distribution of the electric field in a nanowire in the vicinity of which a structure is arranged, obtained by a 2D electromagnetic field analysis simulation. [Figure 3B] FIG. 3B is a characteristic diagram showing the relationship between the azimuthal quantum number l and the obtained OAM in a nanowire with a structure disposed nearby. [Figure 4] FIG. 4 is a characteristic diagram showing the dispersion relationship of the modes of the nanowire. [Figure 5A] FIG. 5A is a distribution diagram showing the distribution of the electric field obtained by a 2D electromagnetic field analysis simulation when structures are placed at multiple locations near a nanowire. [Figure 5B] FIG. 5B is a characteristic diagram showing the relationship between the azimuthal quantum number l and the obtained OAM when structures are arranged at multiple locations near the nanowire. [Figure 6]FIG. 6 is a distribution diagram showing the distribution of the electric field obtained by a 2D electromagnetic field analysis simulation when structures are placed at multiple locations near a nanowire. [Figure 7A] FIG. 7A is a distribution diagram showing the distribution of the electric field in a nanowire with a square cross section obtained by D electromagnetic field analysis simulation. [Figure 7B] FIG. 7B is a distribution diagram showing the distribution of the electric field obtained by a 2D electromagnetic field analysis simulation when structures are placed at multiple locations near a nanowire with a square cross section. [Figure 8A] FIG. 8A is an explanatory diagram for explaining a method for manufacturing an optical device according to an embodiment of the present invention. [Figure 8B] FIG. 8B is an explanatory diagram for explaining a method of manufacturing an optical device according to an embodiment of the present invention. [Figure 8C] FIG. 8C is an explanatory diagram for explaining a method of manufacturing an optical device according to an embodiment of the present invention. [Figure 9A] FIG. 9A is an explanatory diagram for explaining a method for manufacturing an optical device according to an embodiment of the present invention. [Figure 9B] FIG. 9B is an explanatory diagram for explaining a method for manufacturing an optical device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION
[0008] An optical device according to an embodiment of the present invention will now be described with reference to Fig. 1. This optical device comprises a nanowire 101 and a structure 102 arranged in the vicinity of the nanowire 101.
[0009] The nanowire 101 is made of a semiconductor and has a symmetrical cross section. Furthermore, the nanowire 101 has a normalized frequency of 7 or more. For example, the nanowire 101 has a point-symmetrical cross section or an axis-symmetrical cross section. For example, the nanowire 101 can be made of GaN, in which case the cross section of the nanowire 101 can be a regular hexagon. Furthermore, the nanowire 101 can have a hollow structure. For example, the cross section of the nanowire 101 can have a regular hexagonal outer shape and can have a hollow structure with a cylindrical cavity 103 centered on the axis of the nanowire 101.
[0010] The structure 102 is disposed near the nanowire 101 and causes a loss in light guided through the nanowire 101. The structure 102 affects light leaking from the nanowire 101 and causes a loss in light guided through the nanowire 101. The structure 102 can be made of, for example, a metal or a semiconductor that induces scattering loss of light. The structure 102 can also be disposed at multiple locations near the nanowire 101. For example, the structure 102 can be disposed near the peripheral surface (side surface) of the nanowire 101. For example, the structure 102 can be disposed inside a cavity 103 of the hollow structure of the nanowire 101 that has a hollow structure.
[0011] Furthermore, in this optical device, the cross-sectional shape of nanowire 101 including structure 102 is asymmetric. The cross-sectional shape (external shape of the cross section) including nanowire 101 perpendicular to the axis of nanowire 101 at the location where structure 102 is arranged does not have a center or axis of symmetry (structural symmetry is broken).
[0012] For example, if the cross-sectional shape of nanowire 101 has an outer shape of a regular hexagon and further has a hollow structure with cylindrical cavity 103 centered on the axis of nanowire 101, the distance R from the axis of nanowire 101 to the vertex of the regular hexagonal cross section can be 240 nm, and the radius r of the cross-sectional shape of cavity 103 can be 130 nm. Furthermore, structure 102 can be a sphere with a radius rp of 55 nm. Furthermore, the distance D from the axis of nanowire 101 to the center of structure 102 can be 330 nm.
[0013] For example, the angle formed by a reference line 121 connecting the axis of nanowire 101 to one of the vertices of the regular hexagonal cross section and a line 122 connecting the axis of nanowire 101 to the center of structure 102 can be set to 55.3°. Note that reference line 121 and line 122 are imaginary lines. By setting this state, the cross-sectional shape (external shape of the cross section) including nanowire 101 perpendicular to the axis of nanowire 101 at the location where structure 102 is arranged will have no center of symmetry or axis of symmetry.
[0014] The optical device according to the above-described embodiment can generate light having orbital angular momentum (OAM light) without using a complex structure. This optical device can be, for example, a light-emitting device with a semiconductor nanowire 101 as an active region. Furthermore, this optical device can be, for example, an optical waveguide device using the nanowire 101.
[0015] For example, semiconductor nanowires are extremely small nanomaterials with diameters ranging from tens of nanometers to several micrometers and lengths of several micrometers (Reference 1). Semiconductor nanowires can be mass-produced at once by growing them from a substrate (bottom-up) or by etching (top-down). Semiconductor nanowires made of III-V semiconductors or nitride semiconductors can also be integrated on silicon substrates. Using these, it is possible to mass-produce very simple and small OAM light generation devices.
[0016] Unlike devices that use whispering gallery modes (WGMs), such as ring resonators, OAM generation using nanowires is possible because the propagation mode of the nanowire optical waveguide itself has OAM, making it possible to reduce the device size and useful for trapping nanomaterials in a small area and efficiently extracting OAM light in the perpendicular direction.
[0017] Nanowires can be used as optical waveguides, and by adjusting the size of the system, various propagation modes can be created. Among these propagation modes, there are modes with OAM. However, because these modes are usually degenerate, it is difficult to extract a single mode with a specific OAM. In contrast, according to the embodiment, by placing a structure near the nanowire, a change is made in the cross-sectional shape of this part that breaks the structural symmetry, making it possible to extract OAM light, which is one of the degenerate modes of the nanowire.
[0018] In general, nanowires can reduce unwanted modes by adopting a hollow structure. By making the nanowire hollow and giving it a circular cross-sectional shape, it becomes easier to selectively generate a circular (doughnut-shaped) mode.
[0019] By placing a structure (made of the same material as the nanowire) that provides absorbing material such as metal or optical scattering (scattering loss) near such a nanowire and breaking the symmetry of the cross-sectional shape at this location, degenerate OAM light can be extracted. Thus, according to the embodiment, OAM is generated from the propagation mode rather than the WGM. Furthermore, when the nanowire is cylindrical, if the structure has a center or axis of symmetry, the structural symmetry cannot be broken no matter where the structure is placed. In this case, by using a structure that does not have a center or axis of symmetry, the structural symmetry of the cross-sectional shape (external cross-section) including the nanowire perpendicular to the nanowire axis can be broken.
[0020] Next, the electric field (norm) and OAM values when a spherical metal structure 102 is placed around the nanowire 101 and when it is not placed will be described. As an example, the distance R from the axis of the nanowire 101 to the vertex of the regular hexagonal cross section can be set to 240 nm, and the radius r of the cross-sectional shape of the cavity 103 can be set to 130 nm. The structure 102 can be a sphere with a radius rp of 55 nm. The distance D from the axis of the nanowire 101 to the center of the structure 102 is set to 330 nm.
[0021] First, the normalized OAM can be written as follows:
[0022]
number
[0023] Here, W represents the intensity of the energy input to the nanowire (input intensity), and E represents the electric field in polar coordinates (r, φ, z).
[0024] When the structure 102 is not placed, two modes are degenerated as shown in Figures 2A and 2B. Figure 2A shows the distribution of the electric field obtained by a 2D electromagnetic field analysis simulation. Also, Figure 2B shows the relationship between the azimuthal quantum number l and the obtained OAM. In Figure 2B, the magnitude of the OAM on the vertical axis is a normalized value. In this example, it can be confirmed that OAM exists at the locations where the azimuthal quantum number l = 5 and the azimuthal quantum number l = -5. The effective refractive index (effective refractive index) n eff However, since both are the same, 1.262, we can confirm the degeneracy.
[0025] Next, Figures 3A and 3B show the case where structure 102 is arranged. Figure 3A shows the distribution of the electric field obtained by 2D electromagnetic field analysis simulation. Figure 3B shows the relationship between the azimuthal quantum number l and the obtained OAM. In Figure 2B, the vertical axis shows the normalized value of the OAM magnitude. In this case, as shown in Figure 3B, it can be confirmed that OAM is present at the point where azimuthal quantum number l = 5, and it can be seen that only OAM from one of the two degenerate modes can be extracted. The magnitude of the OAM in this case was approximately 4. As such, according to this embodiment, it is possible to extract OAM light from nanowire 101.
[0026] Next, the dispersion relationship of the modes of a nanowire will be explained with reference to Figure 4. In Figure 4, the horizontal axis represents the distance from the axis of nanowire 101 to the vertex of the regular hexagonal cross section (outer diameter of the nanowire). Also, in Figure 4, the vertical axis represents the effective refractive index. As shown in Figure 4, the 15th and 16th degenerate modes from the base of mode 1 have OAM. Therefore, for these modes to exist, it is important that the diameter (outer diameter) of this nanowire is at least 440 nm or more (radius r = 220 nm).
[0027] The outer diameter of the nanowire mentioned above can be generalized using an index called the normalized frequency (V parameter), which is used to evaluate the mode of an optical waveguide. When this value is approximately 7 or greater, it can be confirmed that a wavelength with OAM exists. Since the region where a single mode normally occurs is approximately V=2.4, using this value as a reference, we can estimate that V=7 from the region where the 15th and 16th modes exist in Figure 4.
[0028] Next, we will explain the case where structures are placed at multiple locations near a nanowire with reference to Figures 5A and 5B. Figure 5A shows the distribution of the electric field obtained by 2D electromagnetic field analysis simulation. Figure 5B shows the relationship between the azimuthal quantum number l and the resulting OAM. As shown in Figure 5A, structures are placed at six locations (each side) in the circumferential direction on the side of a nanowire with a hollow structure and a regular hexagonal cross-sectional shape. The structures are placed in contact with the side of the nanowire. Even with this configuration, it can be confirmed that the OAM degeneracy is resolved and OAM is only present at the azimuthal quantum number l = 5.
[0029] Next, a case where the structure to be placed near the nanowire is a triangular prism will be described with reference to Fig. 6. As shown in Fig. 6, a structure with a regular triangular cross section can be placed on the side of a nanowire with a hollow structure having a cylindrical cavity with a regular hexagonal cross section. In this example, the structures are placed at multiple locations (six locations) at approximately equal intervals around the circumferential direction of the nanowire on the same plane perpendicular to the axis of the nanowire.
[0030] 7A, structures with rectangular cross sections can be arranged on the side of a hollow nanowire with a square cross section and a cylindrical cavity. Also, as shown in FIG. 7B, structures with rectangular cross sections can be arranged at multiple locations on the side of a hollow nanowire with a square cross section and a cylindrical cavity. In this example, structures are arranged at multiple locations (four locations) at approximately equal intervals around the circumferential direction of the nanowire on the same plane perpendicular to the nanowire axis.
[0031] Next, a brief description will be given of a method for manufacturing an optical device according to an embodiment. For example, a GaN layer is grown on a sapphire substrate 111 and patterned by selective etching using a well-known thermal sublimation method, thereby producing nanowires 101 as shown in FIG. 8A. The resulting nanowires 101 may have a regular hexagonal cross-sectional shape and a hollow structure with a cylindrical cavity 103 centered on the axis. The axis of the resulting nanowires 101 is normal to the surface of the sapphire substrate 111. After the nanowires 101 are produced in this manner, a metal layer is patterned by a known lift-off process to form cylindrical structures 102a on the sapphire substrate 111 near the nanowires 101.
[0032] Furthermore, after nanowire 101 is fabricated in the same manner as described above, as shown in Fig. 8B, structure 102b made of metal nanowire can be placed on sapphire substrate 111 near nanowire 101 using a micromanipulator. Similarly, as shown in Fig. 8C, spherical structure 102 made of Au can be placed on sapphire substrate 111 near nanowire 101.
[0033] 9A, nanowires 101a made of Si and having a square cross section can be formed on a silicon substrate 112 via a silicon oxide layer 113. For example, nanowires 101a can be formed by using a well-known SOI (Silicon on Insulator) substrate and patterning the surface silicon layer on the silicon oxide layer 113, which is a buried insulating layer. In this example, the axis of nanowire 101a is parallel to the substrate plane.
[0034] After forming the nanowire 101a in this manner, the metal layer is patterned by a known lift-off process to form the structure 102b on the top surface of the nanowire 101a. The structure 102b can be a rectangular pillar extending in the axial direction of the nanowire 101a. Furthermore, as shown in FIG. 9B, the metal layer is patterned by a known lift-off process to form the structure 102b on the silicon oxide layer 113 near the nanowire 101a.
[0035] As described above, according to the present invention, a structure that causes loss in light is placed near a nanowire, and the cross-sectional shape of the nanowire including the structure is made asymmetric, so that light with OAM can be generated without using a complex structure.
[0036] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.
[0037] [Reference] M. Notomi, M. Takiguchi, S. Sergent, G. Zhang, and H. Sumikura, "Nanowire photonics toward wide wavelengthrange and subwavelength confinement [Invited]", Optical Materials Express, vol. 10, no. 10, pp. 2560-2596, 2020. [Explanation of symbols]
[0038] 101... nanowire, 102... structure, 103... cavity, 121... reference line, 122... line.
Claims
1. a nanowire made of a semiconductor, having a symmetrical cross section, and having a normalized frequency of 7 or more; a structure disposed near the nanowire to impart loss to light guided through the nanowire; Equipped with An optical device in which the cross-sectional shape of the nanowire including the structure is asymmetric.
2. 2. The optical device according to claim 1, The nanowire is an optical device having a hollow structure.
3. 2. The optical device according to claim 1, An optical device having the structure disposed at multiple locations near the nanowire.
4. The optical device according to any one of claims 1 to 3, The structure is an optical device made of a metal or a semiconductor that induces scattering loss of light.