A two-dimensional photonic crystal surface emitting laser

Abstract

The present invention discloses a two-dimensional photonic crystal surface emitting laser, which relates to the technical field of semiconductor lasers. The surface emitting laser comprises an n-doped substrate, an n-doped cladding, an active layer, a photonic crystal layer, a p-doped cladding and a p-doped contact layer arranged in sequence from top to bottom; the photonic crystal layer is formed by periodic arrangement of the same unit cells, and each unit cell comprises a first different refractive index region, a second different refractive index region and a third different refractive index region having a different refractive index from that of the parent material. The surface emitting laser based on the photonic crystal structure can maintain the single-mode working characteristics of the device under a large mode field area, thereby obtaining a high-power single-mode surface emitting laser.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor lasers, and in particular to a two-dimensional photonic crystal surface emitting laser. Background Art

[0002] Semiconductor lasers have the advantages of small size, high electro-optical conversion efficiency and low cost, so they are widely used in laser pumping, laser processing, lidar, laser lighting and other fields. Generally, these application scenarios also have high requirements for the output power, beam divergence or brightness of the laser. Since the single-mode light-emitting area of ​​the semiconductor laser is small, the beam divergence angle is large, which has become a major shortcoming. For example, the fast-axis divergence angle of the edge-emitting semiconductor laser is about 20°, and the divergence angle of the vertical-cavity surface-emitting laser is also about 10°. In order to meet application requirements, it is often necessary to add an additional lens system for collimation, which increases the application cost and reduces the reliability of the system.

[0003] The emergence of photonic crystal surface emitting lasers (PCSELs) has changed this situation. It can achieve single-mode operation with a large mode field area, and thus obtain a small divergence angle. This is because, compared with edge-emitting lasers and vertical cavity surface emitting lasers that use Fabry-Perot cavities as resonant cavities, the distributed feedback mechanism of the photonic crystal surface emitting laser resonant cavity has better selectivity for resonant modes. By adjusting the parameters of the photonic crystal, the lasing thresholds of different resonant modes in the cavity can be changed. The greater the difference between the fundamental mode and the higher-order mode thresholds, the better the single-mode operation stability of the laser. In addition, the second-order Bragg diffraction can be used to extract the light field energy oscillating parallel to the junction plane in a direction perpendicular to the junction plane, thereby achieving vertical output of the laser.

[0004] At present, there are two main routes to increase the threshold difference between the fundamental mode and the higher-order mode: 1. Use a double-lattice photonic crystal cavity, change the spacing between the two nested lattices, and adjust the shape of the photonic crystal hole. This method has a relatively simple structure, but it requires the two sets of lattices to have different hole shapes. For example, the "ellipse + circle" configuration is currently mainly used. This difference in pattern brings great challenges to the subsequent pattern exposure and hole etching process, and increases the preparation cost. This is because at the micro-nano scale, under the same etching conditions, the shape and size of the hole have a great influence on the depth of the etched hole. This means that there will be a large difference in depth between the two etched holes, which will bring difficulties to batch preparation processes such as nanoimprinting. 2. Use a topological photonic crystal cavity and use the bandgap mode as the working mode of the laser. This method can also obtain a large threshold difference, and the shape of the holes is the same, and there is no problem of uneven etching depth. However, compared with the square lattice photonic crystal cavity (the aforementioned double-lattice photonic crystal cavity is a type of square lattice photonic crystal cavity), the in-plane pattern of the photonic crystal is more complex, and the requirements for electron beam exposure are higher. In addition, there are problems such as difficulty in electrical pumping and complex far-field patterns. Summary of the invention

[0005] In view of this, the main purpose of the present invention is to provide a two-dimensional photonic crystal surface emitting laser. Based on the photonic crystal surface emitting laser, the single-mode working characteristics of the device can be maintained under a large mode field area, thereby obtaining a high-power single-mode surface emitting laser.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme:

[0007] The present invention provides a two-dimensional photonic crystal surface emitting laser, comprising an n-doped substrate, an n-doped cladding layer, an active layer, a photonic crystal layer, a p-doped cladding layer and a p-doped contact layer arranged in sequence from top to bottom;

[0008] The photonic crystal layer is formed by periodically arranging the same unit cells, and each unit cell comprises a first different refractive index region, a second different refractive index region and a third different refractive index region, which have refractive indices different from those of the base material.

[0009] Preferably, a square Ni / Au-Ge / Ni / Au metal film and a circular Ti / Pt / Au metal film 206b are deposited on the surfaces of the n-doped substrate and the p-doped contact layer, respectively, and circular light exit holes are left at the center of the square metal films.

[0010] Preferably, the distance between the center of gravity of the second different refractive index region and the center of gravity of the first different refractive index region in the x direction is 0.31a to 0.37a, and the distance between the center of gravity of the second different refractive index region and the first different refractive index region in the y direction is 0.14a to 0.2a;

[0011] The distance between the center of gravity of the third different refractive index region and the center of gravity of the first different refractive index region in the x direction is 0.14a-0.2a, and the distance between the center of gravity of the third different refractive index region and the first different refractive index region in the y direction is 0.31a-0.37a, wherein a is a lattice constant.

[0012] Preferably, the shapes of the first different refractive index region, the second different refractive index region and the third different refractive index region in the periodic plane are the same or different.

[0013] Preferably, the shapes of the first different refractive index region, the second different refractive index region and the third different refractive index region in the periodic plane are circular, rectangular, elliptical or triangular.

[0014] Preferably, the total area of ​​the first different refractive index region, the second different refractive index region, and the third different refractive index region accounts for 5% to 20% of the total area of ​​all regions including the base material region and the different refractive index regions.

[0015] Preferably, the thicknesses of the first different refractive index region, the second different refractive index region, and the third different refractive index region are the same or different, and the thicknesses are all within the range of 0.5a to 5a.

[0016] Preferably, the cross-sectional shape of the first different refractive index region, the second different refractive index region, and the third different refractive index region in a direction perpendicular to the periodic plane is rectangular, conical, trapezoidal, or teardrop-shaped.

[0017] Preferably, the first different refractive index region, the second different refractive index region and the third different refractive index region are made of the same material, and are one or two of semiconductor materials, gases, insulators or metals as the base material.

[0018] Preferably, the semiconductor material is any single substance or alloy material of group III-V, group II-VI or group IV.

[0019] Beneficial effects of the present invention

[0020] The invention discloses a two-dimensional photonic crystal surface emitting laser, wherein the two-dimensional photonic crystal is formed by three independent photonic crystal structures having the same lattice constant and being nested with each other according to a preset lattice offset, thereby realizing the separate regulation of the mode coupling coefficient in the photonic crystal in the direction parallel to and perpendicular to the lattice period. Each unit cell of the photonic crystal constructed by this method contains three discrete regions with a different refractive index from that of a parent material (101). The surface emitting laser based on the photonic crystal structure can maintain the single-mode working characteristics of the device under a large mode field area, thereby obtaining a high-power single-mode surface emitting laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 A schematic diagram of the structure of a two-dimensional photonic crystal in a two-dimensional photonic crystal surface emitting laser according to the present invention;

[0023] Figure 2 A schematic diagram of the three-dimensional structure of a two-dimensional photonic crystal surface emitting laser according to the present invention;

[0024] Figure 3 for Figure 1 The principle diagram of the two-dimensional photonic crystal for increasing the difference in mode thresholds;

[0025] Figure 4 for Figure 1 The two-dimensional photonic crystal parameter coupling coefficient control diagram;

[0026] Figure 5 : is the resonance mode distribution corresponding to the embodiment of the present invention and the comparative example;

[0027] Figure 6 1 and 2 are far-field and near-field simulation results of an embodiment of the present invention.

[0028] Among them, 201, n-doped substrate, 202, n-doped cladding, 203, active layer, 101, photonic crystal layer, 102a, the first different refractive index region, 102b, the second different refractive index region, 102c, the third different refractive index region, 103, unit cell, 204, p-doped cladding, 205, p-doped contact layer, 206a, Ni / Au-Ge / Ni / Au metal film, 206b, Ti / Pt / Au metal film. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0030] As mentioned above, in the actual application of lasers, we hope that the device always works in the fundamental mode. However, this is usually difficult to meet, especially when the area of ​​the photonic crystal device is large, the threshold difference between the fundamental mode and the higher-order mode is often small. At this time, under the condition of large injection current, the laser will show a multi-mode working state. In addition, the threshold difference between different band edge modes will also affect the stability of the device's single-mode operation.

[0031] To this end, we need to increase the threshold gain difference between the fundamental mode and the higher-order modes inside the resonant cavity. The photonic crystal cavity corresponds to four band edges at the second-order Γ point. When designing the cavity, we selectively reduce the mode loss of one of the band edges so that it has priority over the other three band edges for lasing. However, the lasing band edge still has more intrinsic transverse modes. Therefore, we increase the loss difference between the fundamental transverse mode and other modes by precisely controlling the coupling coefficient of the photonic crystal cavity, so that the device can still work stably in the lowest threshold mode when the size of the photonic crystal resonant cavity is increased, thereby obtaining a single-mode laser.

[0032] The two-dimensional photonic crystal in this embodiment has Figure 1 The structure shown in FIG. 1 is a top view and FIG. 1b is a main view. The lattice structure of the photonic crystal is a square lattice, that is, the two periodic arrangement directions perpendicular to each other have the same lattice constant a, and the lattice constant is numerically the same as the lasing wavelength λ of the laser involved in this embodiment in the material. n In this embodiment, the lattice constant is set to 280 nm.

[0033] Each unit cell 103 of the two-dimensional photonic crystal includes three regions whose refractive index is different from that of the photonic crystal layer 101 (parent material), namely the first different refractive index region 102a, the second different refractive index region 102b and the third different refractive index region 102c. In this embodiment, these three regions have the same refractive index.

[0034] In this embodiment, the first different refractive index region 102a, the second different refractive index region 102b and the third different refractive index region 102c have the same or different shapes in the periodic plane, and can be circular, rectangular, elliptical or triangular, preferably circular. The cross-sectional shapes of the first different refractive index region, the second different refractive index region and the third different refractive index region in the direction perpendicular to the periodic plane are rectangular, conical, trapezoidal or teardrop-shaped.

[0035] In this embodiment, the total area of ​​the first different refractive index region 102a, the second different refractive index region 102b and the third different refractive index region 102c accounts for 5% to 20% of the total area of ​​all regions including the base material region and the different refractive index region (the duty cycle, i.e., the ratio of the region area to the unit cell area), and preferably is 3.3%.

[0036] In this embodiment, the thickness h of the first different refractive index region 102a, the second different refractive index region 102b and the third different refractive index region 102c are the same or different, and the thickness is in the range of 0.5a to 5a, preferably 200nm.

[0037] The centroid distance between the first different refractive index region 102a and the second different refractive index region 102b in the x direction is denoted by d. 1x , the centroid distance between region 102a and region 102c along the x direction is denoted as d 2x , whose values ​​are 97nm and 48nm respectively. The photonic crystal structure in this embodiment is symmetrical about y=x, and the diameter of the photonic crystal resonant cavity is 500μm.

[0038] In this embodiment, the first different refractive index region 102a, the second different refractive index region 102b and the third different refractive index region 102c are made of the same material, and are made of one or two of semiconductor materials, gases, insulators or metals as the base material. The semiconductor material is any single substance or alloy material of group III-V, group II-VI or group IV.

[0039] The overall structure of the photonic crystal surface emitting laser constructed with the two-dimensional photonic crystal as the resonant cavity described in this embodiment is as follows Figure 2 The laser has multiple functional layers in the direction perpendicular to the junction plane, which are n-doped substrate 201, n-doped cladding 202, active layer 203, photonic crystal layer 101, p-doped cladding 204 and p-doped contact layer 205, and the material components of these six functional layers are GaAs, Al 0.7 Ga 0.3 As, InGaAs multiple quantum wells, GaAs, Al 0.4 Ga 0.6 As, GaAs. The main function of the photonic crystal layer 101 is to regulate the light field in the laser and construct the resonant cavity mode. The purpose of the active layer is to continuously amplify the light in the resonant mode to achieve laser lasing. In addition, in order 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 surface of the n-doped substrate 201 and the p-doped contact layer 205, respectively, and a circular light exit hole is left in the center of the square metal film.

[0040] Figure 3 The relative magnitudes of the second-order and first-order expansion coefficients and d after Fourier expansion of the refractive index distribution of the photonic crystal are given. 1x and d 2x The functional relationship between them is shown in 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).], in order to achieve a single-mode laser, it is necessary to maintain a large mode threshold difference, which requires maintaining a low one-dimensional coupling intensity (positively correlated with the second-order Fourier coefficient of the refractive index distribution). According to Figure 3 (a), the second-order Fourier coefficient has a minimum value at points 301a and 301b, so taking values ​​near them is conducive to obtaining a single-mode laser. Figure 3 (b), these two points are far away from the minimum value of the first-order coefficient, thus ensuring that the laser has sufficient vertical light output. Figure 3 Based on the design shown, the d 1x Further optimization of Figure 4 Shows the coupling coefficient R and μ vs. d 1x The functional relationship between 2x is fixed at 0.17a). 1x When the value is 0.348a, R≈μ, and the laser constructed based on this will obtain the maximum threshold gain difference, Figure 1 The photonic crystal structure parameters shown in also correspond to this point.

[0041] Figure 5 (a) shows the mode distribution diagram of this embodiment, where the threshold difference Δα between the fundamental mode and the higher-order mode is 4.65 cm -1 , which meets the resonant cavity requirements of single-mode lasers. In comparison, Figure 5 (b) shows the mode distribution of the comparative example, in which the hole shape in the unit cell is a single circular hole with a duty cycle of 10%. The threshold difference of this structure is only 0.026 cm -1 . Figure 6 (a) and Figure 6 (b) shows the far-field and near-field simulation results corresponding to this embodiment. It can be seen that the laser has a near-Gaussian distribution in the near field and a single-lobe far field with a divergence angle less than 0.3°.

[0042] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0043] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A two-dimensional photonic crystal surface emitting laser, comprising an n-doped substrate, an n-doped cladding layer, an active layer, a photonic crystal layer, a p-doped cladding layer and a p-doped contact layer arranged in sequence from top to bottom; It is characterized in that The photonic crystal layer is formed by periodically arranging the same 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 having a refractive index different from that of the base material; The distance between the center of gravity of the second different refractive index region and the center of gravity of the first different refractive index region in the x direction is 0.31a to 0.37a, and the distance between the center of gravity of the second different refractive index region and the first different refractive index region in the y direction is 0.14a to 0.2a; The distance between the center of gravity of the third different refractive index region and the center of gravity of the first different refractive index region in the x direction is 0.14a-0.2a, and the distance between the center of gravity of the third different refractive index region and the first different refractive index region in the y direction is 0.31a-0.37a, wherein a is a lattice constant.

2. A two-dimensional photonic crystal surface emitting laser according to claim 1, characterized in that: 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 circular light-emitting holes are left at the center of the square metal films.

3. A two-dimensional photonic crystal surface emitting laser according to claim 1, characterized in that: The shapes of the first different refractive index region, the second different refractive index region and the third different refractive index region in the periodic plane are the same or different.

4. A two-dimensional photonic crystal surface emitting laser according to claim 3, characterized in that: The shapes of the first different refractive index region, the second different refractive index region and the third different refractive index region in the periodic plane are circular, rectangular, elliptical or triangular.

5. The two-dimensional photonic crystal surface emitting laser according to claim 1, characterized in that: The total area of ​​the first different refractive index region, the second different refractive index region, and the third different refractive index region accounts for 5% to 20% of the total area of ​​all regions including the base material region and the different refractive index regions.

6. A two-dimensional photonic crystal surface emitting laser according to claim 1, characterized in that: The thicknesses of the first different refractive index region, the second different refractive index region, and the third different refractive index region are the same or different, and the thicknesses are all within the range of 0.5a to 5a, wherein a is a lattice constant.

7. The two-dimensional photonic crystal surface emitting laser according to claim 1, characterized in that: The cross-sectional shapes of the first different refractive index region, the second different refractive index region, and the third different refractive index region in the direction perpendicular to the periodic plane are rectangular, conical, trapezoidal, or teardrop-shaped.

8. The two-dimensional photonic crystal surface emitting laser according to claim 1, characterized in that: The first different refractive index region, the second different refractive index region, and the third different refractive index region are made of the same material, and are made of one or two of semiconductor materials, gas, insulator or metal as the base material.

9. A two-dimensional photonic crystal surface emitting laser according to claim 8, characterized in that: The semiconductor material is any single substance or alloy material of group III-V, group II-VI or group IV.

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