Nanowire laser and method of manufacturing same

Abstract

This nanowire laser is constituted by: a nanowire (102) formed on top of a substrate (101); and a one-dimensional photonic crystal (103a) and a one-dimensional photonic crystal (103b) formed respectively at one end side and another end side of the nanowire (102). A relief part (101a) with a flat upper surface is formed on top of the substrate (101). The nanowire (102) is disposed on top of the relief part (101a). The cross-section shape of the nanowire (102) is configured to be an ellipse the lengthwise direction of which is parallel to the plane of the substrate (101). The one-dimensional photonic crystal (103a) and the one-dimensional photonic crystal (103b) are formed spaced apart from each other and constitute a resonator.

Description

[Technical field]

[0001] The present invention relates to nanowire lasers and methods for making the same. [Background technology]

[0002] In recent years, there has been much research into optical elements using semiconductor nanowires. Nanowires are made of semiconductors and are very small structures with diameters ranging from tens of nm to several μm and lengths of several μm. Nanowires can be grown in large quantities from a substrate at once, and under certain conditions, they can also be grown directly on a silicon substrate. For this reason, nanowires are considered to be an important nanomaterial that can be used not only in future semiconductor elements, but also in quantum information devices and photonics-electronics hybrid elements (Non-Patent Document 1).

[0003] Nanowires have been researched so far for their use in electronic devices such as transistors and for sensing applications, but by changing the composition of the semiconductor that makes up the nanowire, it is possible to make it emit light in a variety of wavelength ranges, and it is thought that they may also be promising for use as lasers.

[0004] Nanowire lasers have been demonstrated in various wavelength bands. In particular, nanowire lasers in the telecommunication wavelength band, which are transparent to silicon, can be integrated and operated in silicon photonic circuits, making them promising candidates for use as light sources in future photonics-electronics convergence chips. Among these, when integrated into silicon photonic circuits for use as devices, there is a demand for nanowire lasers that selectively oscillate in the TE polarization, allowing efficient coupling to optical waveguides. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] M. Notomi et al., "Nanowire photonics toward wide wavelength range and subwavelength confinement [Invited]", Optical Materials Express, vol. 10, no. 10, pp. 2560-2596, 2020. [Non-Patent Document 2] M. Takiguchi et al., "Thermal effect of InP / InAs nanowire lasers integrated on different optical platforms", OSA Continuum, vol. 4, no. 6, pp. 1838-1845, 2021. Summary of the Invention [Problem to be solved by the invention]

[0006] However, previous nanowire lasers operated in multimode, oscillating in both TE and TM modes, which made it difficult to efficiently couple to optical waveguides.

[0007] The present invention has been made to solve the above problems, and has an object to provide a nanowire laser that can selectively oscillate TE polarized waves and efficiently couple to an optical waveguide. [Means for solving the problem]

[0008] The nanowire laser according to the present invention comprises a nanowire formed on a substrate, the nanowire having an elliptical cross-sectional shape with its long dimension parallel to the plane of the substrate, and a resonator.

[0009] The method for producing a nanowire laser according to the present invention includes a step of arranging a nanowire having a circular cross section on a substrate, and a step of etching the nanowire perpendicular to the plane of the substrate by a dry etching method having perpendicular anisotropy. UpwardThe method includes a processing step of etching the nanowire to make the cross-sectional shape of the nanowire into an ellipse whose long dimension is parallel to the plane of the substrate, and a resonator step of forming a resonator. Effect of the Invention

[0010] As described above, according to the present invention, a nanowire having an elliptical cross-sectional shape is used, so that a nanowire laser can be provided that selectively oscillates TE polarized waves and efficiently couples to an optical waveguide. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view showing a configuration of a nanowire laser according to a first embodiment of the present invention. [Figure 2A] FIG. 2A is a perspective view showing a state of a nanowire laser in the middle of a process, illustrating a method for manufacturing a nanowire laser according to the first embodiment of the present invention. [Figure 2B] FIG. 2B is a perspective view showing a state of the nanowire laser in the middle of a process to explain the method for manufacturing the nanowire laser according to the first embodiment of the present invention. [Figure 2C] FIG. 2C is a perspective view showing a state of the nanowire laser in the middle of a process to explain the method for manufacturing the nanowire laser according to the first embodiment of the present invention. [Diagram 3] FIG. 3 is a distribution diagram showing an electromagnetic field simulation of the photonic crystal structure of one-dimensional photonic crystals 103a and 103b of the nanowire laser according to the first embodiment. [Figure 4A] FIG. 4A is a photograph showing an electron microscope image of the actually fabricated nanowire laser according to the first embodiment. [Figure 4B] FIG. 4B is a photograph showing an electron microscope image of the actually fabricated nanowire laser according to the first embodiment. [Figure 5A] FIG. 5A is a characteristic diagram showing the change in effective refractive index (equivalent refractive index) with respect to the change in diameter of a nanowire having a circular cross section. [Figure 5B]FIG. 5B is a characteristic diagram showing the change in effective refractive index (equivalent refractive index) with respect to the change in the minor axis of a nanowire having an elliptical cross section. [Figure 6] FIG. 6 is a characteristic diagram showing the change in emission intensity of the nanowire laser before and after dry etching to remove the damaged portion and to form an ellipse. [Figure 7] FIG. 7 is a perspective view showing a configuration of a nanowire laser according to the second embodiment of the present invention. [Figure 8A] FIG. 8A is a perspective view showing a state of a nanowire laser in the middle of a process to explain a method for manufacturing a nanowire laser according to a second embodiment of the present invention. [Figure 8B] FIG. 8B is a perspective view showing a state of the nanowire laser in the middle of a process to explain the method for manufacturing the nanowire laser according to the second embodiment of the present invention. [Figure 9] FIG. 9 is a characteristic diagram showing optical confinement in the fundamental mode of a nanowire laser in which a nanowire is disposed on a metal substrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, a nanowire laser according to an embodiment of the present invention will be described.

[0013] [Embodiment 1] First, a nanowire laser according to a first embodiment of the present invention will be described with reference to Fig. 1. This nanowire laser is composed of a nanowire 102 formed on a substrate 101, and one-dimensional photonic crystals 103a and 103b formed on one end and the other end of the nanowire 102, respectively.

[0014] In this example, a convex portion 101a with a flat upper surface is formed on a substrate 101, and a nanowire 102 is disposed on the convex portion 101a. The upper surface of the convex portion 101a is formed parallel to the plane of the substrate 101. Moreover, the convex portion 101a has approximately the same area as the nanowire 102 when viewed from above. The cross-sectional shape of the nanowire 102 is an ellipse whose long axis direction is parallel to the plane of the convex portion 101a (substrate 101). Moreover, the nanowire 102 has a thickness that eliminates the degeneracy of the fundamental mode.

[0015] The one-dimensional photonic crystals 103a and 103b are composed of lattice elements 104 that are periodically arranged in a straight line at a predetermined interval. The lattice elements 104 are portions that have a refractive index different from that of the nanowire 102, and are columnar (e.g., cylindrical) holes. The one-dimensional photonic crystals 103a and 103b thus configured are formed at a distance from each other to form a resonator.

[0016] With this nanowire laser, laser oscillation can be obtained by irradiating the nanowire 102 with excitation light. Laser oscillation can also be achieved by injecting carriers by current injection rather than optical excitation. In this case, an active layer is formed in the nanowire 102, with a p-type region and an n-type region sandwiching the active layer. Laser oscillation can be achieved by injecting a current into the p-type region and the n-type region.

[0017] Next, a method for manufacturing the nanowire laser according to the first embodiment will be described with reference to FIGS. 2A, 2B, and 2C.

[0018] First, as shown in FIG. 2A, a nanowire 121 having a circular cross section and a predetermined diameter is prepared, and the prepared nanowire 121 is placed (transferred) on a substrate 101 (placement step). Next, as shown in FIG. 1B, an oxide film 122 is formed on the surface of the nanowire 121. For example, by using a well-known atomic layer deposition (ALD) device, the oxide film 122 can be formed so as to cover the nanowire 121. Furthermore, by forming the oxide film 122 in this manner, the adhesion between the substrate 101 and the nanowire 121 is improved, and it is expected that the nanowire will be prevented from being blown away when the substrate 101 is washed in a later step.

[0019] 2C, one-dimensional photonic crystals 103a and 103b are formed at a distance from each other on one end side and the other end side of nanowire 121, respectively, to form a resonator (resonator process). For example, openings are formed in oxide film 122 at locations where lattice elements 104 are to be formed by electron beam lithography or the like. Next, using oxide film 122 with the openings formed as a mask, nanowire 121 is etched by dry etching to form lattice elements 104, thereby forming one-dimensional photonic crystals 103a and 103b.

[0020] Also, the lattice elements 104 can be formed by processing using a focused ion beam (FIB). FIB processing can reduce the difficulty of fabrication compared to the above-mentioned methods. However, in FIB processing, it is desirable that the thickness of the oxide film 122 is at most about 100 nm in order to suppress the occurrence of charge-up during observation.

[0021] Next, the nanowire 121 is etched from above perpendicular to the plane of the substrate 101 using a dry etching method with vertical anisotropy, to form the nanowire 102 with an elliptical cross-sectional shape whose long dimension is parallel to the plane of the substrate 101 (processing step). This process changes the aspect ratio of the diameter of the nanowire 121, and the nanowire 102 with an elliptical cross-section is formed. This process also etches the substrate 101 around the nanowire 102, forming a protrusion 101a, with the nanowire 102 positioned on the protrusion 101a (see FIG. 1).

[0022] The FIB processing described above has a very high processing accuracy and can form any structure, but the ions used in the processing (Ga, Ne ions, etc.) can damage the nanowire 121, and the shot ions can act as an absorption medium, significantly degrading the light emission characteristics. In response to this, by performing dry etching with vertical anisotropy to change the aspect ratio of the diameter described above, it is possible to make the cross section elliptical and remove the damage caused by the FIB processing.

[0023] According to the above-mentioned first embodiment, the nanowire 102 has an elliptical cross-sectional shape in an anisotropic shape, which improves mode selectivity and enables selective oscillation of TE polarized waves. In addition, by making the nanowire 102 thick enough to release the degeneracy of the fundamental mode, it is possible to selectively oscillate TE polarized waves in a single mode.

[0024] Furthermore, the resonator made of the one-dimensional photonic crystals 103a and 103b can obtain a higher reflectance and lower the oscillation threshold compared to a Fabry-Perot resonator made of nanowire end face reflection. As shown in the electromagnetic field simulation in FIG. 3, this photonic crystal structure creates a resonator in the nanowire 102 with an elliptical cross section, making it possible to strongly confine light. End face reflection is usually about 30%, but by using a photonic crystal structure, it is possible to increase the reflectance to 90% or more. The reflectance of the photonic crystal structure can be controlled by increasing the number of lattice elements or by fine-tuning the diameter of each lattice element.

[0025] As described above, according to the first embodiment in which the resonator has a high reflectivity, the laser oscillation threshold can be lowered and continuous oscillation can be achieved at room temperature. Nanowire lasers to date have not been able to achieve continuous oscillation at room temperature, and this is due to the high laser oscillation threshold. It is believed that when the laser oscillation threshold is high, gain saturation and gain broadening due to heat affect continuous oscillation, resulting in pulsed oscillation instead of continuous oscillation (Non-Patent Document 2). In contrast, in the first embodiment, a high reflectivity can be obtained in the resonator, so the laser oscillation threshold can be lowered, and as a result, continuous oscillation at room temperature is possible.

[0026] Figure 4A shows an electron microscope image of a nanowire laser actually fabricated using FIB. Periodic air holes have been created in the nanowire. Figure 4B shows an electron microscope image of a nanowire observed at a 45-degree angle after the surface layer has been removed by dry etching in a direction perpendicular to the substrate surface. The cross section of the nanowire is elliptical, demonstrating that the structure of the present invention can be fabricated.

[0027] For example, in the case of materials in the communication wavelength band, in a nanowire with a diameter of about 400-500 nm, light propagates in a single mode as shown in Figure 5A. In Figure 5A, the numbers in the legend indicate the mode order. Here, Figure 5A shows that the mode of order 1 and the mode of order 2 overlap, indicating degeneracy.

[0028] As described above, when processing with FIB, the nanowire is damaged by ions implanted by FIB up to a depth of about several hundred nm from the top surface. Therefore, this portion needs to be removed. Therefore, in the process described with reference to FIG. 2A, a nanowire 121 having a diameter several hundred nm thicker than the diameter of the target is prepared and placed (transferred) on the substrate 101.

[0029] Nanowire 102, which is formed by etching thick nanowire 121 described above from above perpendicular to the plane of substrate 101 using a dry etching method with vertical anisotropy, has a smaller diameter overall and a different aspect ratio of the diameter. Figure 5B shows the waveguide mode of nanowire 102 obtained by processing nanowire 121, which has an initial diameter of 1 μm, using the dry etching method described above. In Figure 5B, the diameter on the horizontal axis is the minor axis. Also in Figure 5B, the numbers in the legend indicate the mode order.

[0030] In Figure 5B, the first order mode (fundamental mode) and second order mode are separated, and this separation becomes clearer as the minor axis becomes smaller, and it can be seen that the degeneracy between the first order mode and the second order mode is resolved (the degeneracy of the fundamental mode is resolved). When the axis becomes smaller than 0.4 μm, the waveguiding becomes single mode, and if the degeneracy is resolved, the TM mode can be excluded and the TE mode can be selectively extracted.

[0031] Figure 6 shows the change in emission intensity before and after dry etching to remove the damaged areas and form an ellipse for a nanowire laser in which a resonator was formed using a one-dimensional photonic crystal processed by FIB. In Figure 6, (a) shows the emission intensity before the above-mentioned dry etching process, and (b) shows the emission intensity after the above-mentioned dry etching process. As shown in Figure 6, the emission intensity was improved by removing the damaged areas.

[0032] [Embodiment 2] Next, a nanowire 102 laser according to a second embodiment of the present invention will be described with reference to Fig. 7. This nanowire laser is composed of nanowire 102 formed on substrate 101, and metal layer 105. Metal layer 105 is formed on substrate 101 in contact with nanowire 102.

[0033] In this example, a convex portion 101a having a flat upper surface is formed on a substrate 101, and a metal layer 105 is formed on the upper surface of the convex portion 101a. A nanowire 102 is disposed on and in contact with the metal layer 105. The upper surface of the convex portion 101a is formed parallel to the plane of the substrate 101. The convex portion 101a and the metal layer 105 have approximately the same area as the nanowire 102 when viewed from above. The cross-sectional shape of the nanowire 102 is an ellipse whose long axis is parallel to the plane of the convex portion 101a (substrate 101). The nanowire 102 has a thickness that eliminates the degeneracy of the fundamental mode.

[0034] In the second embodiment, a resonator is formed by nanowire 102 and metal layer 105 formed in contact with nanowire 102. A plasmonic waveguide is formed at the location of metal layer 105 in contact with nanowire 102.

[0035] In the portion of metal layer 105 that contacts nanowire 102, the light protruding from nanowire 102 induces a surface plasmon polariton on the surface of metal layer 105. The surface plasmon polariton is a type of elementary excitation that is formed by coupling a surface plasmon induced on the surface of metal layer 105 having free electrons with light irradiated onto metal layer 105. The surface plasmon polariton induced in this way is guided through the above-mentioned plasmonic waveguide.

[0036] In the second embodiment, a Fabry-Perot resonator is formed by end face reflection of nanowire 102 constituting the above-mentioned plasmonic waveguide. With this configuration, since a plasmonic waveguide structure is used, a higher light confinement effect can be obtained due to the light confinement effect of plasmons compared to end face reflection of a normal nanowire. In addition, in the second embodiment, the cross-sectional shape of nanowire 102 is elliptical, so an even higher light confinement effect can be obtained. As a result, according to the second embodiment, the oscillation threshold can be further lowered.

[0037] Next, a method for manufacturing a nanowire laser according to the second embodiment will be described with reference to FIGS. 8A, 8B, and 8C.

[0038] First, as shown in Fig. 8A, a metal layer 105 is formed on a substrate 101. For example, by depositing gold (Au) by a well-known vapor deposition method, the metal layer 105 made of Au can be formed on the substrate 101. Next, as shown in Fig. 8B, a nanowire 121 having a circular cross section and a predetermined diameter (e.g., a diameter of 1 µm) is prepared, and the prepared nanowire 121 is placed (transferred) on the substrate 101 on which the metal layer 105 has been formed (placement step).

[0039] Next, the nanowire 121 is etched from above perpendicular to the plane of the substrate 101 by a dry etching method having vertical anisotropy, to form the nanowire 102 having an elliptical cross-sectional shape with the long dimension parallel to the plane of the substrate 101 (processing step). This process changes the aspect ratio of the diameter of the nanowire 121, and the nanowire 102 having an elliptical cross-sectional shape is formed. In this process, for example, the short dimension of the nanowire 102 can be set to 0.4 μm. This process also etches the metal layer 105 and the substrate 101 around the nanowire 102, forming a protrusion 101a, and the nanowire 102 is placed on the protrusion 101a via the metal layer 105 (see FIG. 7).

[0040] In the second embodiment described above, the nanowire 102 has an elliptical cross-sectional shape in an anisotropic shape, which improves mode selectivity and enables selective oscillation of TE polarized waves. Furthermore, by making the nanowire 102 thick enough to release the degeneracy of the fundamental mode, it is possible to selectively oscillate TE polarized waves in a single mode.

[0041] Figure 9 shows the optical confinement in the fundamental mode of a nanowire laser in which a nanowire with an elliptical cross-section obtained by processing a nanowire with a diameter of 1 μm by the above-mentioned manufacturing method is disposed on a metal substrate, and a nanowire laser in which a nanowire with a diameter of 1 μm and a circular cross-section is disposed on a metal substrate. The optical confinement in the fundamental mode of the nanowire laser with a nanowire with an elliptical cross-section is shown by the black circles in Figure 9. The optical confinement in the fundamental mode of the nanowire laser with a nanowire with a circular cross-section is shown by the black squares in Figure 9. As shown in Figure 9, the nanowire laser in which a nanowire with an elliptical cross-section is disposed on a metal substrate achieves higher optical confinement.

[0042] As described above, according to the present invention, a nanowire having an elliptical cross-sectional shape is used, making it possible to provide a nanowire laser that selectively oscillates TE polarized waves and can efficiently couple to an optical waveguide.

[0043] It should be noted that the present invention is not limited to the above-described embodiments, and it is apparent 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. [Explanation of symbols]

[0044] 101...substrate, 101a...protrusion, 102...nanowire, 103a, 103b...one-dimensional photonic crystal, 104...lattice element.

Claims

1. A nanowire formed on a substrate, the cross-sectional shape of which is an ellipse with the long dimension direction being parallel to the plane of the substrate; Resonator and Equipped with The resonator is composed of one-dimensional photonic crystals formed at one end side and the other end side of the nanowire, the one-dimensional photonic crystals being spaced apart from each other. A nanowire laser comprising:

2. A nanowire formed on a substrate, the cross-sectional shape of which is an ellipse with the long dimension direction being parallel to the plane of the substrate; Resonator and Equipped with The nanowire laser, wherein the resonator is composed of a metal layer formed on the substrate in contact with the nanowire.

3. 3. The nanowire laser according to claim 1, A nanowire laser, wherein the nanowire has a thickness that eliminates the degeneracy of the fundamental mode.

4. A step of disposing a nanowire having a circular cross section on a substrate; a processing step of etching the nanowire from above perpendicular to the plane of the substrate by a dry etching method having vertical anisotropy to make the cross-sectional shape of the nanowire into an ellipse whose long axis is parallel to the plane of the substrate; a resonator process for forming a resonator; A method for manufacturing a nanowire laser comprising:

5. 5. The method for producing a nanowire laser according to claim 4, The resonator step includes a step of forming one-dimensional photonic crystals at each of one end side and the other end side of the nanowire, the one-dimensional photonic crystals being spaced apart from each other. A method for manufacturing a nanowire laser comprising the steps of:

6. 5. The method for producing a nanowire laser according to claim 4, The method for manufacturing a nanowire laser, wherein the resonator step includes a step of forming a metal layer on a surface of the substrate between the substrate and the nanowire.

7. The method for producing a nanowire laser according to any one of claims 4 to 6, A method for manufacturing a nanowire laser, wherein the processing step includes a step of forming the nanowire to a thickness that removes degeneracy of a fundamental mode.

Images