Two-dimensional photonic crystal surface-emitting laser

Abstract

To provide a single-mode surface emitting laser of high output.SOLUTION: A surface emitting laser includes an N-doped substrate, an N-doped clad layer, an active layer, a photonic crystal layer, a P-doped clad layer, and a P-doped contact layer formed in sequence from top to bottom. The photonic crystal layer includes a base material and a plurality of periodically arranged unit cells, and each unit cell includes a first different refractive index region, a second different refractive index region, and a third different refractive index region, all of which have a refractive index different from that of the base material. The surface emitting laser based on the photonic crystal structure is capable of maintaining a single mode operation property of the device even with a large mode field area. As a result, a single-mode surface emitting laser with high output can be obtained.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to the technical field of semiconductor lasers, and more particularly to two-dimensional photonic crystal surface-emitting lasers. [Background technology]

[0002] Due to their small size, high electrical-to-optical conversion efficiency, and low cost, semiconductor lasers are widely used in fields such as laser pumping, laser processing, LIDAR, and laser illumination. These application scenarios typically place high requirements on the laser's output power, beam divergence, and brightness. Because the area of ​​the single-mode light-emitting region is small, semiconductor lasers have a large beam divergence, which is a major drawback. For example, the fast axis divergence angle of an edge-emitting semiconductor laser is approximately 20°, while that of a vertical-cavity surface-emitting laser is approximately 10°. To meet these requirements, additional collimation lens systems are often required, which increases operating costs and reduces system reliability.

[0003] This situation has changed with the advent of photonic crystal surface-emitting lasers (PCSELs). Photonic crystal surface-emitting lasers (PCSELs) can achieve single-mode operation even with a large mode-field area, resulting in a small divergence angle. This is because the distributed feedback mechanism of the cavity in PCSELs provides superior resonant mode selectivity compared to edge-emitting lasers and vertical-cavity surface-emitting lasers, which use a Fabry-Perot cavity as the resonator. By adjusting the parameters of the photonic crystal, the lasing thresholds of various resonant modes within the cavity can be altered. The larger the threshold difference between the fundamental mode and higher-order modes, the more stable the single-mode operation of the laser. Furthermore, second-order Bragg diffraction can be utilized to extract the light field energy oscillating parallel to the junction plane in a direction perpendicular to the junction plane, thereby achieving vertical laser output.

[0004] Currently, there are two main methods for increasing the threshold difference between the fundamental and higher-order modes. 1. Using a double-lattice photonic crystal cavity, the spacing between two nested lattices is altered to adjust the hole shape of the photonic crystal. While this method has a relatively simple structure, it requires two sets of lattices with different hole shapes. For example, the "ellipse + circle" configuration is currently the most commonly used. This pattern difference poses significant challenges for the subsequent pattern exposure and hole etching processes, increasing manufacturing costs. This is because, at the micro-nanoscale, the shape and size of the holes have a significant impact on the etched hole depth under the same etching conditions. This results in a large difference in the depth of the two etched holes, which in turn creates difficulties in batch manufacturing processes such as nanoimprinting. 2. Using a topological photonic crystal cavity, the bandgap mode is used as the laser operating mode. This method also achieves a large threshold difference, all of the holes have the same shape, and does not suffer from uneven etching depth. However, compared with square lattice photonic crystal cavities (the aforementioned double lattice photonic crystal cavities are also a type of square lattice photonic crystal cavities), the in-plane pattern of photonic crystals is more complex, which imposes stricter requirements on electron beam lithography, and also poses problems such as the difficulty of electrical pumping and the complexity of the far-field pattern. Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above, a main object of the present invention is to provide a two-dimensional photonic crystal surface-emitting laser that can maintain the single-mode operation characteristics of the device even with a large mode field area, thereby providing a high-output single-mode surface-emitting laser. [Means for solving the problem]

[0006] To achieve the above objectives, the present invention adopts the following technical solutions:

[0007] The present invention provides a two-dimensional photonic crystal surface-emitting laser including, in order from top to bottom, an n-type doped substrate, an n-type doped cladding layer, an active layer, a photonic crystal layer, a p-type doped cladding layer, and a p-type doped contact layer, The photonic crystal layer includes a base material and a plurality of periodically arranged unit cells, and each unit cell includes a first modified refractive index area, a second modified refractive index area, and a third modified refractive index area, each having a refractive index different from that of the base material, to provide a two-dimensional photonic crystal surface-emitting laser.

[0008] Preferably, a square Ni / Au-Ge / Ni / Au metal film and a circular Ti / Pt / Au metal film are deposited on the surface of the n-doped substrate and the p-doped contact layer, respectively, leaving a circular exit aperture in the center of the square metal film.

[0009] Preferably, the distance in the x direction between the center of gravity of the second modified refractive index area and the center of gravity of the first modified refractive index area is 0.31a to 0.37a, and the distance in the y direction is 0.14a to 0.2a; The distance in the x direction between the center of gravity of the third modified refractive index area and the center of gravity of the first modified refractive index area is 0.14a to 0.2a, and the distance in the y direction is 0.31a to 0.37a, where a is a lattice constant.

[0010] Preferably, the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area have the same shape or different shapes within the periodic surface.

[0011] Preferably, the shapes of the first modified refractive index area, the second modified refractive index area and the third modified refractive index area within the periodic plane are circular, rectangular, elliptical or triangular.

[0012] Preferably, the total area of ​​the first modified refractive index area, the second modified refractive index area and the third modified refractive index area occupies 5% to 20% of the total area of ​​the photonic crystal layer.

[0013] Preferably, the thicknesses of the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area are the same or different and are within the range of 0.5a to 5a.

[0014] Preferably, the first modified refractive index area, the second modified refractive index area and the third modified refractive index area have a cross-sectional shape in a direction perpendicular to the periodic surface that is rectangular, conical, trapezoidal or droplet-shaped.

[0015] Preferably, the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area are made of the same material, and these materials, together with the base material, are one or two of a semiconductor material, a gas, an insulator, or a metal.

[0016] Preferably, the semiconductor material is a single element or alloy material of any of Group III-V, Group II-VI or Group IV. [Effects of the Invention]

[0017] The present invention discloses a two-dimensional photonic crystal surface-emitting laser. The two-dimensional photonic crystal is constructed by nesting three independent photonic crystal structures with the same lattice constant according to a preset lattice offset, allowing for independent control of the mode coupling coefficient within the photonic crystal in directions parallel and perpendicular to the lattice period. In a photonic crystal constructed in this manner, each unit cell contains three discrete regions with refractive indices different from that of the host material (101). A surface-emitting laser based on this photonic crystal structure can maintain the single-mode operation characteristics of the device even with a large mode field area, thereby achieving a high-power single-mode surface-emitting laser. [Brief explanation of the drawings]

[0018] In order to more clearly describe the embodiments of the present invention or the technical solutions of the prior art, the following will briefly describe the drawings necessary for describing the embodiments or the prior art. However, the drawings in the following description are only some embodiments of the present invention, and it is obvious to those skilled in the art that other drawings can be obtained based on these drawings without any creative efforts. [Figure 1]1 is a schematic diagram of the structure of a two-dimensional photonic crystal in a two-dimensional photonic crystal surface-emitting laser of the present invention. [Figure 2] 1 is a schematic diagram of a three-dimensional structure of a two-dimensional photonic crystal surface-emitting laser of the present invention. [Figure 3] FIG. 2 is a diagram illustrating the principle of increasing the difference in mode thresholds of the two-dimensional photonic crystal of FIG. 1. [Figure 4] FIG. 2 is a control diagram of the coupling coefficient of the two-dimensional photonic crystal parameter of FIG. [Figure 5] 10 shows resonance mode distributions corresponding to an example of the present invention and a comparative example. [Figure 6] 10 shows simulation results of the far field and near field of an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] In order to allow those skilled in the art to fully understand the aspects of the present invention, the present invention will be described in more detail below with reference to the drawings and specific embodiments. It is clear that the described embodiments are only a part of the embodiments of the present invention, and do not represent all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without any creative work fall within the scope of protection of the present invention.

[0020] As mentioned above, in practical applications of lasers, it is desirable for the device to always operate in the fundamental mode. However, this is generally difficult, especially when the area of ​​a photonic crystal device is large, as the threshold difference between the fundamental mode and higher-order modes is often small. In this case, under conditions of large injection current, the laser will exhibit multimode operation. Furthermore, the threshold difference between different band-edge modes also affects the stability of the device's single-mode operation.

[0021] Therefore, it is necessary to increase the threshold gain difference between the fundamental mode and higher-order modes within the cavity. A photonic crystal cavity corresponds to four band edges at the quadratic Γ point. When designing the cavity, the mode loss of one of the band edges is selectively reduced to make it lasing preferentially over the other three band edges. However, the lasing band edge contains many inherent transverse modes. Therefore, by precisely controlling the coupling coefficient of the photonic crystal cavity, the loss difference between the fundamental transverse mode and other modes can be increased, allowing the device to operate stably even at the lowest threshold mode while increasing the size of the photonic crystal cavity, thereby achieving a single-mode laser.

[0022] The two-dimensional photonic crystal in this embodiment has a structure shown in Figure 1, where Figure 1a is a top view and Figure 1b is a front view. The lattice structure of the photonic crystal is a square lattice, i.e., it has the same lattice constant a in two mutually orthogonal periodic arrangement directions, and the value of this lattice constant is the lasing wavelength λ in the material of the laser according to this embodiment. n In this example, the lattice constant is set to 280 nm.

[0023] The photonic crystal layer 101 includes a base material 101b and a plurality of periodically arranged unit cells 103, and each unit cell 103 of the two-dimensional photonic crystal includes three regions having a refractive index different from that of the base material 101b, namely, a first modified refractive index region 102a, a second modified refractive index region 102b, and a third modified refractive index region 102c, and in this embodiment, these three regions have the same refractive index.

[0024] In this embodiment, the first modified refractive index area 102a, the second modified refractive index area 102b, and the third modified refractive index area 102c have the same or different shapes within the periodic plane, and this shape may be circular, rectangular, elliptical, or triangular, but is preferably circular. The cross-sectional shape of the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area in the direction perpendicular to the periodic plane is rectangular, conical, trapezoidal, or droplet-shaped.

[0025] In this embodiment, the total area of ​​the first modified refractive index area 102a, the second modified refractive index area 102b, and the third modified refractive index area 102c accounts for 5% to 20%, preferably 3.3%, of the total area of ​​all areas including the base material area and the modified refractive index areas (duty ratio, i.e., the ratio between the area of ​​the unit cell and the total area of ​​the photonic crystal layer).

[0026] In this embodiment, the thickness h of the first modified refractive index area 102a, the second modified refractive index area 102b, and the third modified refractive index area 102c may be the same or different, and is within the range of 0.5a to 5a, and is preferably 200 nm.

[0027] In this embodiment, the distance between the centers of gravity of the first modified refractive index area 102a and the second modified refractive index area 102b in the x direction is d 1x The distance between the centers of gravity of the region 102a and the region 102c in the x direction is d 2x These values ​​are 97 nm and 48 nm, respectively. The photonic crystal structure in this example is symmetric with respect to y=x, and the diameter of the photonic crystal resonator is 500 μm. The photonic crystal resonator refers to the region in photonic crystal layer 101 where unit cells 103 are distributed.

[0028] In this embodiment, the first modified refractive index area 102a, the second modified refractive index area 102b, and the third modified refractive index area 102c are made of the same material, and these materials, together with the base material, are one or two of a semiconductor material, a gas, an insulator, or a metal. The semiconductor material is any element or alloy material of Group III-V, Group II-VI, or Group IV.

[0029] In this embodiment, the overall structure of the photonic crystal surface-emitting laser constructed using the two-dimensional photonic crystal as a resonator is shown in Figure 2. This laser has multiple functional layers in the direction perpendicular to the junction surface, which are, in order, an n-doped substrate 201, an n-doped cladding layer 202, an active layer 203, a photonic crystal layer 101, a p-doped cladding layer 204, and a p-doped contact layer 205. The material components of these six functional layers are GaAs, Al, and GaAs, respectively. 0.7 Ga 0.3 As, InGaAs multi-quantum well, GaAs, Al 0.4 Ga 0.6 The photonic crystal layer 101 is made of As and GaAs. The main function of the photonic crystal layer 101 is to adjust the optical field within the laser and establish the resonator mode. The active layer continuously amplifies the resonator mode to realize laser lasing. Furthermore, to achieve effective electrical injection, a square Ni / Au-Ge / Ni / Au metal film 206a and a circular Ti / Pt / Au metal film 206b are deposited on the surfaces of the n-type doped substrate 201 and the p-type doped substrate 201, respectively, leaving a circular emission opening in the center of the square metal film.

[0030] Figure 3 shows the relative magnitudes of the second- and first-order expansion coefficients after Fourier expansion of the refractive index distribution of a photonic crystal, and the d 1x and d 2x The functional relationship between the points 301a and 301b is shown in Figure 3(a) and 3(b), respectively. 1x ,d 2x) are (0.17a, 0.33a) and (0.33a, 0.17a), respectively. According to the literature [Inoue, T. et al. Nat. Commun. 13, 3262 (2022)], to realize a single-mode laser, it is necessary to maintain a large difference in mode thresholds, which in turn requires maintaining a low one-dimensional coupling strength (positively correlated with the second-order Fourier coefficient of the refractive index profile). Also, according to Figure 3(a), the second-order Fourier coefficient reaches its minimum value at points 301a and 301b, so taking values ​​close to these points is advantageous for achieving a single-mode laser. Also, according to Figure 3(b), these two points are far from the minimum point of the first-order coefficient, ensuring sufficient vertical optical output for the laser. Based on the design shown in Figure 3, d 1x is further optimized. Figure 4 shows the relationship between the coupling coefficient R and μ and d 1x The functional relationship between (d 2x is fixed at 0.17a). 1x When the value of is 0.348a and R≈μ, the laser constructed based on this has the largest threshold gain difference, and the photonic crystal structure parameters shown in Figure 1 also correspond to this point.

[0031] FIG. 5(a) shows the mode distribution diagram of this example, and the difference in threshold Δα between the fundamental mode and the higher-order mode is 4.65 cm -1 This satisfies the requirements for a single-mode laser cavity. For comparison, Fig. 5(b) shows the mode distribution of a comparative example, in which the hole shape of the unit cell is a single circular hole and the duty ratio is 10%. In this structure, the difference in threshold is only 0.026 cm. -1 6(a) and 6(b) show the far-field and near-field simulation results corresponding to this example, which show that the laser has a nearly Gaussian near-field distribution and a single-lobe far-field with a divergence angle of less than 0.3°.

[0032] The technical features of the above-mentioned embodiments may be combined in any manner. For the sake of brevity, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, all combinations of these technical features should be considered within the scope of this specification unless there is a contradiction.

[0033] The above examples merely illustrate some embodiments of the present invention, and the descriptions are relatively specific and detailed, but this should not be understood as limiting the scope of the invention patent. It should be noted that those skilled in the art can make some modifications and improvements without departing from the concept of the present invention, and all of these fall within the scope of protection of the present invention. Therefore, the scope of protection of the invention patent should be governed by the appended claims. [Explanation of symbols]

[0034] 201 n-type doped substrate 202 n-type doped cladding layer 203 Active layer 101 Photonic crystal layer 102a First modified refractive index area 102b Second modified refractive index area 102c Third modified refractive index area 103 unit cells 204 p-doped cladding layer 205 p-doped contact layer 206a Ni / Au-Ge / Ni / Au metal film 206b Ti / Pt / Au metal film

Claims

1. A two-dimensional photonic crystal surface-emitting laser including, in order from top to bottom, an n-type doped substrate, an n-type doped cladding layer, an active layer, a photonic crystal layer, a p-type doped cladding layer, and a p-type doped contact layer, the photonic crystal layer includes a base material and a plurality of periodically arranged unit cells, each unit cell including a first modified refractive index area, a second modified refractive index area, and a third modified refractive index area having a refractive index different from that of the base material; a square Ni / Au-Ge / Ni / Au metal film and a circular Ti / Pt / Au metal film are deposited on the surface of the n-doped substrate and the p-doped contact layer, respectively, and a circular emission aperture is left in the center of the square metal film; the distance in the x direction between the center of gravity of the second modified refractive index area and the center of gravity of the first modified refractive index area is 0.31a to 0.37a, and the distance in the y direction between the center of gravity of the second modified refractive index area and the center of gravity of the first modified refractive index area is 0.14a to 0.2a; The distance in the x direction between the center of gravity of the third modified refractive index area and the center of gravity of the first modified refractive index area is 0.14a to 0.2a, and the distance in the y direction between the center of gravity of the third modified refractive index area and the center of gravity of the first modified refractive index area is 0.31a to 0.37a, where a is a lattice constant. A two-dimensional photonic crystal surface-emitting laser.

2. 2. The two-dimensional photonic crystal surface-emitting laser according to claim 1, wherein the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area have the same shape or different shapes within the periodic plane.

3. 3. The two-dimensional photonic crystal surface-emitting laser according to claim 2, wherein the shapes of the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area within the periodic plane are circular, rectangular, elliptical, or triangular.

4. 2. The two-dimensional photonic crystal surface-emitting laser according to claim 1, wherein the total area of ​​the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area accounts for 5% to 20% of the total area of ​​the photonic crystal layer.

5. The two-dimensional photonic crystal surface-emitting laser according to claim 1, characterized in that the thicknesses of the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area are the same or different and are in the range of 0.5a to 5a.

6. 2. The two-dimensional photonic crystal surface-emitting laser according to claim 1, wherein the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area have a cross-sectional shape perpendicular to the periodic plane that is rectangular, conical, trapezoidal, or droplet-shaped.

7. 2. The two-dimensional photonic crystal surface-emitting laser according to claim 1, wherein the first modified refractive index area, the second modified refractive index area, and the third modified refractive index area are made of the same material, and these materials, together with the base material, are one or two of a semiconductor material, a gas, an insulator, or a metal.

8. 8. The two-dimensional photonic crystal surface-emitting laser according to claim 7, wherein the semiconductor material is a single element selected from the group III-V, group II-VI, and group IV.

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