Method for generating vortex beam based on GaN optical micro-ring resonant cavity

Abstract

The invention discloses a method for generating a vortex beam based on a GaN optical micro-ring resonant cavity, and particularly relates to the technical field of optical micro-cavities, and the method comprises the steps: S1, building a simulation model, S2, building a critical coupling state, S3, etching an angular grating array, S4, building a mapping relation between a wavelength and an OAM mode, and S5, judging the OAM mode and outputting the vortex beam. A micron-sized integrated structure is adopted, vortex light beams with l = 1 to l = 4 can be stably output in a critical coupling state by means of high stability and high nonlinearity characteristics of GaN materials, all modes are independently transmitted through wavelength decoupling, high integration level and high reliability are achieved, and meanwhile, the GaN-based vortex laser has a wide application prospect. In combination with an optical micro-comb technology, the device can be expanded to an application scene in which multiple frequency components carry independent orbital angular momentum, a core device with excellent performance is provided for the fields of high-capacity optical communication, high-precision optical control and the like, and the device has remarkable practical value and application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of optical microcavities, and more particularly to a method for generating a vortex beam based on a GaN optical microring resonator. Background Art

[0002] With the rapid development of optical communications, quantum information, and other fields, the demand for large-capacity information transmission and high-precision optical manipulation is becoming increasingly urgent. Vortex beams, which carry unique orbital angular momentum (OAM) and have a hollow ring-shaped phase distribution, have become one of the key technologies to meet these needs. In 1992, Allen et al. demonstrated that the orbital angular momentum of a photon is directly related to the topological charge l of the vortex phase. The phase singularity at its center causes the beam to have a ring-shaped distribution. This property has made it possible to exponentially increase the capacity of optical communication channels and has also promoted technological breakthroughs in optical micromanipulation, high-resolution imaging, and other fields. However, the efficient generation and integration of high-quality vortex beams have always been a core issue restricting their application. Traditional vortex beam generation methods rely on discrete devices such as spiral phase plates and spatial light modulators, which have drawbacks such as large size, low integration, and high loss. In contrast, optical microcavities, with their compact structure, high quality factor, and ease of on-chip integration, have become an ideal platform for the next generation of vortex beam emitters, among which whispering gallery mode microcavities are particularly prominent. Whispering gallery modes originate from the "whispering gallery" phenomenon. In 1912, Lord Rayleigh discovered at St. Paul's Cathedral in London that sound can be transmitted over long distances through multiple reflections along the walls of a wall. This principle, extended to the field of optics, led to the formation of whispering gallery modes, where light undergoes total internal reflection at the edge of a circular microcavity, forming stable resonances. These modes confine the optical field intensity to the microcavity edge, providing a strong field environment for the excitation of vortex beams. Furthermore, microcavities based on whispering gallery modes have been demonstrated to function as on-chip emitters of monochromatic optical vortices and as efficient nonlinear optical platforms for generating multi-frequency light (i.e., microcombs), laying the foundation for the integrated generation of multiple OAM modes. In terms of material selection, third-generation semiconductor materials have become the preferred choice for optical microcavities. Among them, GaN, a new type of semiconductor following Ge and Si (first generation) and GaAs and InP (second generation), features a wide direct bandgap, strong atomic bonds, high thermal conductivity, and excellent chemical stability (almost impervious to acid corrosion). It also possesses strong radiation resistance and can operate stably in high-temperature, high-power environments, making it ideal for constructing high-performance optical microcavities. Combining GaN with whispering gallery modes is expected to overcome the limitations of traditional materials in nonlinear optical effects and light field confinement. Whispering gallery mode-based microring resonators have been used to generate vortex beams, but they still face problems such as low integration of multiple OAM modes and inter-mode coupling interference. In existing technologies, a single microcavity can usually only output a vortex beam with a single topological charge. Combining a microring resonator with microcomb technology requires complex structural design to achieve matching of multiple frequency components with multiple OAM modes. Therefore, how to utilize the characteristics of GaN materials to design integrated optical devices with compact structures that can simultaneously generate multi-modal vortex beams has become a technical problem that needs to be urgently solved in this field. Summary of the Invention

[0003] In order to overcome the above-mentioned defects of the prior art, the present invention provides a method for generating a vortex beam based on a GaN optical microring resonator to solve the problems raised in the above-mentioned background technology.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a method for generating a vortex beam based on a GaN optical microring resonator, comprising the following steps: S1. Construct a simulation model, construct a physical model and simulate it using the FDTD method. The model includes a GaN micro-ring resonator, a rectangular input waveguide and a TM point light source, wherein: The outer diameter of the GaN microring resonator is 5±0.1μm, the inner diameter is 4.3±0.1μm, and the ring thickness is 0.7±0.05μm; The rectangular input waveguide has a length of 30±1μm, a height of 0.7±0.05μm, and a spacing of 0.2±0.02μm from the GaN microring; The wavelength of the TM point light source is 1.52174 ± 0.01 μm, and the horizontal distance from the GaN microring is 7 ± 0.5 μm; S2. Establish a critical coupling state by injecting a TM point light source into the rectangular input waveguide and adjusting the distance between the rectangular input waveguide and the GaN microring until the valley depth of the transmission spectrum at 1.52174 μm is greater than 20 dB, thus establishing a critical coupling state. S3. Etch an angular grating array to observe the distribution of the electric field Ez inside the GaN microring. Etch an angular grating at the eight peaks of the whispering gallery mode. The width of a single grating is 0.16±0.01μm and the height is 0.08±0.005μm. S4. Establish a mapping relationship between wavelength and OAM mode, identify the split resonance peak in the transmission spectrum, and establish a mapping relationship between wavelength and orbital angular momentum mode, specifically: 1.55568±0.01μm corresponds to topological charge l=1; 1.58978±0.01μm corresponds to topological charge l=2; 1.62592±0.01μm corresponds to topological charge number l=3; 1.66331±0.01μm corresponds to topological charge number l=4; S5, determine the OAM mode and output the vortex beam, extract the phase distribution of each wavelength, and calculate the total value of the phase jump. Determine the OAM mode and output vortex beams from l=1 to l=4.

[0005] Preferably, in step S2, the establishment of the critical coupling state needs to satisfy the following requirements: the resonance valley depth of the transmission spectrum at 1.52174 μm is greater than 20 dB and the full width at half maximum is less than or equal to 0.1 nm.

[0006] Preferably, in step S4, the adjacent wavelength intervals of the split resonance peaks are ≥34 nm, and the specific intervals satisfy: Interval between l=1 and l=2: 34.1±0.5nm; Interval between l=2 and l=3: 36.14±0.5nm; The interval between l=3 and l=4 is 37.39±0.5nm.

[0007] Preferably, in step S3, the number of the angular gratings is 8, exciting 4 OAM modes, l=1~4, and the grating positions strictly correspond to the peaks of the whispering gallery modes.

[0008] Preferably, in step S5, the total phase jump value is obtained by The phase change verification is as follows: When l=1, the phase jumps from 0→2π; When l=2, the phase jumps from 0→4π; When l=3, the phase jumps from 0→6π; When l=4, the phase jumps from 0→8π.

[0009] The present invention also provides a vortex beam generating device for implementing the above-mentioned method for generating a vortex beam based on a GaN optical microring resonator, comprising: GaN micro-ring resonator with an outer diameter of 5±0.1μm, an inner diameter of 4.3±0.1μm, and a ring thickness of 0.7±0.05μm; Rectangular input waveguide with a length of 30 ± 1 μm, a height of 0.7 ± 0.05 μm, and a spacing of 0.2 ± 0.02 μm from the microring; An angular grating array is etched on the inner wall of the microring at eight points with the maximum electric field Ez intensity. The grating has a width of 0.16±0.01μm and a depth of 0.08±0.005μm. TM mode point light source, wavelength is 1.52174±0.01μm, and the distance between the position and the center of the microring is 7±0.5μm Preferably, the third-order nonlinear coefficient X of the GaN material is 3 is 10 -19 to 10 -18 m 2 / W, microring quality factor Q>10 4 , used to generate optical microcombs carrying independent OAM.

[0010] The technical effects and advantages of the present invention are as follows: 1. By using GaN material to construct an integrated structure of a micron-scale microring resonator and waveguide, the overall size is small (the outer diameter of the microring is only 5μm), which can be integrated on a chip and is suitable for large-scale optoelectronic device arrays. At the same time, GaN material has high thermal conductivity, strong radiation resistance and chemical stability, and can operate stably in high-power light fields, reducing the impact of heat loss on beam quality and ensuring long-term stable output of the vortex beam; 2. By etching an angular grating at the whispering gallery mode peak, a multimodal vortex beam with l=1 to l=4 can be excited in a single shot. Each mode is independently transmitted through wavelength decoupling (adjacent spacing ≥34nm). Combined with a critical coupling design (resonance valley depth >20dB), the optical field coupling efficiency is high, and the topological charge and wavelength have a clear mapping relationship, facilitating the rapid identification and control of OAM modes through spectral characteristics. 3. Through the high nonlinear characteristics of GaN materials (third-order nonlinear coefficient 10 -19 to 10 -18 m 2 / W) and micro-ring high Q value (> 10 4 ) can be used as an efficient nonlinear platform to generate optical micro-combs, so that each frequency component carries an independent OAM value, providing core device support for large-capacity optical communications and high-precision optical micro-manipulation that integrate wavelength division multiplexing and orbital angular momentum multiplexing, and significantly improving information transmission capacity and control accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is the overall flow chart of the present invention.

[0012] Figure 2 Schematic diagram of the GaN microring resonator and waveguide coupling structure of the present invention.

[0013] Figure 3 This is the transmission spectrum diagram of point A at the output end of the waveguide of the present invention.

[0014] Figure 4 This is a distribution cloud diagram of the electric field Ez component in the GaN microring and waveguide of the present invention.

[0015] Figure 5 This is a transmission spectrum diagram of point A at the output end of the waveguide after etching the angular grating of the present invention; Figure 6For the present invention Figure 4 A magnified view of the transmission spectrum in the 1.52-1.525 μm band; Figure 7 This is the phase distribution cloud diagram of the present invention at a wavelength of 1.55568 μm; Figure 8 This is the phase distribution cloud diagram of the present invention at a wavelength of 1.58978 μm; Figure 9 This is the phase distribution cloud diagram of the present invention at a wavelength of 1.62592 μm; Figure 10 This is the phase distribution cloud diagram of the present invention at a wavelength of 1.66331 μm. DETAILED DESCRIPTION

[0016] This application provides a method for generating vortex beams based on a GaN optical microring resonator. By constructing a coupling structure between a GaN microring and a waveguide, and utilizing the synergistic effect of whispering gallery modes and angular gratings, efficient generation of multimodal vortex beams is achieved. The specific implementation steps are as follows: S1. Build a simulation model As attached Figure 2 As shown, a physical model is constructed using 3D modeling software (such as Lumerical FDTDSolutions), and a 2D simulation is performed in the xy plane based on the finite-difference time-domain (FDTD) method, focusing on monitoring the distribution characteristics of the electric field component Ez in the z-axis direction. The specific model parameters are as follows: The GaN microring resonator is made of the third-generation semiconductor GaN, with an outer diameter of 5±0.1μm, an inner diameter of 4.3±0.1μm, and a ring thickness of 0.7±0.05μm. GaN was chosen for its wide direct bandgap (~3.4eV), high thermal conductivity, and strong nonlinear properties, which support high-power optical field transmission and reduce heat loss. Rectangular input waveguide, length 30±1μm, height 0.7±0.05μm (consistent with the thickness of the microring to ensure mode matching), spacing from the outer edge of the microring is 0.2±0.02μm (initial setting, which can be adjusted later); The TM mode point light source is set at 7±0.5μm on the left side of the waveguide and outputs TM polarized light with a wavelength of 1.52174±0.01μm (the electric field direction is perpendicular to the propagation plane). This wavelength is in the optical communication band and is suitable for subsequent application scenarios.

[0017] The simulation boundary conditions are set as follows: perfectly matched layer (PML) absorption boundaries are used in the x and y directions, periodic boundaries are used in the z direction, and the grid accuracy is set to 20 nm × 20 nm to ensure the calculation accuracy of the electric field distribution.

[0018] S2. Establishing critical coupling state The TM mode light is injected into the GaN microring resonator through the input waveguide, and the transmission spectrum of the waveguide output end is monitored in real time (e.g. Figure 3 As shown in the figure, by fine-tuning the spacing between the waveguide and the microring (within the range of 0.2±0.02μm), a resonance valley with a depth greater than 20dB appears at 1.52174μm in the spectrum, and the full width at half maximum (FWHM) is ≤0.1nm. At this time, the system reaches the critical coupling state.

[0019] The significance of critical coupling: In this state, the light field energy can be efficiently coupled from the waveguide into the microring, reducing reflection losses and providing a strongly confined light field for the subsequent excitation of the whispering gallery mode.

[0020] S3, Etching Angular Grating Array The Ez field distribution inside the GaN microring was observed in the FDTD simulation (e.g. Figure 4 As shown in the figure, the peak position of the whispering gallery mode (the point with the maximum electric field intensity) is identified. Since the microring has a circular structure, the whispering gallery mode is periodically distributed along the ring direction, with a total of 8 evenly spaced peaks. Angular gratings were etched at the eight peaks using electron beam etching. The dimensions of a single grating are 0.16 ± 0.01 μm in width and 0.08 ± 0.005 μm in height. The gratings break the mode degeneracy of the microring, splitting the single resonance peak into multiple modes carrying different orbital angular momentum (OAM).

[0021] Accuracy of grating position: The grating must be strictly aligned with the wave crest (deviation ≤ 5nm), otherwise the mode coupling efficiency will decrease and the vortex beam quality will be affected.

[0022] S4. Establishing the mapping relationship between wavelength and OAM mode After etching the grating, the transmission spectrum was monitored again and it was observed that the original single peak at 1.52174 μm was split into five resonance peaks (such as Figure 5 、 Figure 6 ), where: The central peak still corresponds to the l = 0 mode (no orbital angular momentum); The four split peaks on the right correspond to OAM modes from l=1 to l=4, and the mapping relationship between wavelength and topological charge is: l = 1 → 1.55568 ± 0.01 μm (peak spacing with l = 0 34.1 ± 0.5 nm); l = 2 → 1.58978 ± 0.01 μm (peak spacing with l = 1 36.14 ± 0.5 nm); l = 3 → 1.62592 ± 0.01 μm (peak spacing with l = 2 37.39 ± 0.5 nm); l=4→1.66331±0.01μm (peak distance with l=3 is 37.39±0.5nm).

[0023] The wavelength interval between adjacent peaks is ≥34 nm and shows an increasing trend. This is because the effective optical path lengths of modes with different l values ​​in the microring are different, resulting in different resonant wavelengths. This interval is sufficient to achieve wavelength decoupling of each mode.

[0024] S5. Determine the OAM mode and output the vortex beam Extract the electric field phase distribution at the wavelength corresponding to the above four split peaks (such as Figure 7-10 As shown in Figure 2), by observing the change of phase along the azimuth angle φ (with the center of the microring as the pole, 0≤φ≤2π), the topological charge l of each beam is determined: When it goes around the azimuth angle φ once, the phase jumps from 0 to 2π → corresponding to l=1 (single spiral phase structure); The phase jumps from 0 to 4π → corresponding to l=2 (double helix phase structure); Phase jumps from 0 to 6π → corresponding to l=3 (triple spiral phase structure); The phase jumps from 0 to 8π → corresponding to l=4 (quadruple spiral phase structure).

[0025] The phase jump pattern described above conforms to the characteristics of the spiral phase front exp(ilφ), and the presence of a phase singularity (a hollow region with zero light intensity) at the center of the beam confirms that the generated beam is a vortex beam carrying orbital angular momentum. The final output contains a multimodal vortex beam with l=1 to l=4 modes, each of which can be distinguished by wavelength and transmitted independently.

[0026] The present invention also provides a vortex beam generating device for implementing the above-mentioned method for generating a vortex beam based on a GaN optical microring resonator, comprising: The GaN micro-ring resonator has the same size parameters as in step S1. It is formed by epitaxially growing a GaN film on a sapphire substrate using MOCVD (metal organic chemical vapor deposition) technology and then etching it using ICP. The rectangular input waveguide is integrally formed with the microring using the same GaN material to ensure low-loss coupling. The spacing between the waveguide and the microring is precisely controlled by photolithography. The angular grating array is formed by electron beam etching at the eight Ez field intensity maxima (whispering gallery mode peaks) on the inner wall of the microring. The grating size is 0.16±0.01μm in width and 0.08±0.005μm in depth.

[0027] The TM mode point light source adopts a distributed feedback laser (DFB), with an output wavelength stabilized at 1.52174±0.01μm, and is coupled to the waveguide input end through an optical fiber.

[0028] In this device, the third-order nonlinear coefficient X of the GaN material is 3 is 10 -19 to 10 -18 m 2 / W, microring quality factor Q>10 4 , optical micro-combs can be generated through nonlinear effects such as four-wave mixing, so that each frequency component carries an independent OAM value, further expanding to the field of large-capacity optical communications.

[0029] In summary: The present invention proposes a method for generating vortex beams based on a GaN optical microring resonator. This method achieves precise control of orbital angular momentum (OAM) modes through the synergistic effect of whispering gallery modes and angular gratings. First, a microring resonator and a rectangular waveguide of a specific size are constructed using GaN material. By adjusting the spacing between the waveguide and the microring, a critical coupling state is established, allowing TM mode light to efficiently couple into the microring and excite whispering gallery modes. This is achieved by forming a stable resonant optical field distribution through multiple total reflections of light at the microring edge. Subsequently, an angular grating array was etched at the electric field peak of the whispering gallery mode. The grating's phase modulation of the optical field broke the mode degeneracy, splitting the single peak in the transmission spectrum into multiple resonant peaks. These split peaks form a one-to-one mapping with OAM modes of different topological charges l (l = 1 to 4). The correspondence is distinguished by wavelength differences, and the wavelength interval between adjacent peaks remains stable. Finally, by observing the phase distribution corresponding to each wavelength, the OAM mode is determined based on the total value of the phase jump along the azimuth angle (l=1 corresponds to 2π, l=2 corresponds to 4π, and so on), and a vortex beam carrying a specific orbital angular momentum is output. The high nonlinear characteristics of the GaN material and the high quality factor of the microring further support the generation of optical microcombs, allowing multiple frequency components to carry independent OAM values, laying the foundation for applications such as high-capacity optical communications.

[0030] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for generating a vortex beam based on a GaN optical microring resonator, characterized by: The following steps are involved: S1. Construct a simulation model, construct a physical model and simulate it using the FDTD method. The model includes a GaN micro-ring resonator, a rectangular input waveguide and a TM point light source, wherein: The outer diameter of the GaN microring resonator is 5±0.1μm, the inner diameter is 4.3±0.1μm, and the ring thickness is 0.7±0.05μm; The rectangular input waveguide has a length of 30±1μm, a height of 0.7±0.05μm, and a spacing of 0.2±0.02μm from the GaN microring; The wavelength of the TM point light source is 1.52174 ± 0.01 μm, and the horizontal distance from the GaN microring is 7 ± 0.5 μm; S2. Establish a critical coupling state by injecting a TM point light source into the rectangular input waveguide and adjusting the distance between the rectangular input waveguide and the GaN microring until the valley depth of the transmission spectrum at 1.52174 μm is greater than 20 dB, thus establishing a critical coupling state. S3. Etch an angular grating array to observe the distribution of the electric field Ez inside the GaN microring. Etch an angular grating at the eight peaks of the whispering gallery mode. The width of a single grating is 0.16±0.01μm and the height is 0.08±0.005μm. S4. Establish a mapping relationship between wavelength and OAM mode, identify the split resonance peak in the transmission spectrum, and establish a mapping relationship between wavelength and orbital angular momentum mode, specifically: 1.55568±0.01μm corresponds to topological charge l=1; 1.58978±0.01μm corresponds to topological charge l=2; 1.62592±0.01μm corresponds to topological charge number l=3; 1.66331±0.01μm corresponds to topological charge number l=4; S5, determine the OAM mode and output the vortex beam, extract the phase distribution of each wavelength, and calculate the total value of the phase jump. Determine the OAM mode and output vortex beams from l=1 to l=4.

2. The method for generating a vortex beam based on a GaN optical microring resonator according to claim 1, characterized in that: In step S2 , the establishment of the critical coupling state must satisfy the following requirements: the resonance valley depth of the transmission spectrum at 1.52174 μm is greater than 20 dB and the full width at half maximum is less than or equal to 0.1 nm.

3. The method for generating a vortex beam based on a GaN optical microring resonator according to claim 2, characterized in that: In step S4, the adjacent wavelength intervals of the split resonance peaks are ≥34 nm, and the specific intervals satisfy: Interval between l=1 and l=2: 34.1±0.5nm; Interval between l=2 and l=3: 36.14±0.5nm; The interval between l=3 and l=4 is 37.39±0.5nm.

4. The method for generating a vortex beam based on a GaN optical microring resonator according to claim 3, characterized in that: In step S3, the number of the angular gratings is 8, which excite 4 OAM modes, l=1~4, and the grating positions strictly correspond to the peaks of the whispering gallery modes.

5. The method for generating a vortex beam based on a GaN optical microring resonator according to claim 4, characterized in that: In step S5, the total phase jump value is calculated by The phase change verification is as follows: When l=1, the phase jumps from 0→2π; When l=2, the phase jumps from 0→4π; When l=3, the phase jumps from 0→6π; When l=4, the phase jumps from 0→8π.

6. A vortex beam generating device, used to implement the method for generating a vortex beam based on a GaN optical microring resonator as claimed in claim 5, characterized in that: include: GaN micro-ring resonator with an outer diameter of 5±0.1μm, an inner diameter of 4.3±0.1μm, and a ring thickness of 0.7±0.05μm; Rectangular input waveguide with a length of 30 ± 1 μm, a height of 0.7 ± 0.05 μm, and a spacing of 0.2 ± 0.02 μm from the microring; An angular grating array is etched on the inner wall of the microring at eight points with the maximum electric field Ez intensity. The grating has a width of 0.16±0.01μm and a depth of 0.08±0.005μm. TM mode point light source, wavelength is 1.52174±0.01μm, and the distance between its position and the center of the microring is 7±0.5μm.

7. The vortex beam generating device according to claim 6, characterized in that: The third-order nonlinear coefficient X of the GaN material 3 is 10 -19 to 10 -18 m 2 / W, microring quality factor Q>10 4 , used to generate optical microcombs carrying independent OAM.

Images