Topological surface-emitting quantum cascade laser and method of fabrication

Abstract

The present disclosure provides a topological surface emitting quantum cascade laser and a preparation method thereof. The preparation method of the topological surface emitting quantum cascade laser comprises: sequentially forming a metal bonding layer, an active layer and an upper contact layer on a substrate; performing photoetching development on the upper contact layer by using a mask, coating photoresist at the patterned position of the mask, and then performing electron beam evaporation of the metal with the photoresist on the upper contact layer to form a metal layer; stripping the metal layer with the photoresist to obtain a metal topological pattern layer; and etching the upper contact layer to form an absorption boundary at the edge of the metal topological pattern layer, so as to enhance the absorption loss of the whispering gallery mode, and keep the topological surface emitting quantum cascade laser in a working state of the topological mode.

Description

Technical Field

[0001] This disclosure relates to the technical field of optoelectronic devices, and more specifically, to a topological surface-emitting quantum cascade laser and its fabrication method. Background Technology

[0002] Terahertz waves refer to electromagnetic waves with frequencies ranging from 0.1THz to 10THz (wavelengths from 3000μm to 30μm). They are key to next-generation high-speed wireless communication (6G) and have important application prospects in many aspects. Surface-emitting single-mode terahertz photonic crystal lasers have always attracted much attention.

[0003] Currently, quantum cascade lasers, due to their tunable wavelength, offer several methods for achieving single-mode surface emission, such as vertical-cavity surface-emitting lasers (VCSELs), second-order DFB gratings, and photonic crystals (quasi-crystals). VCSELs are widely used in short-wavelength and near-infrared bands, but their application is limited to other ranges. Longer wavelengths are unsuitable for VCSELs due to the predominantly TM polarization characteristic of commonly used quantum cascade laser sources. Second-order DFB gratings are not limited by wavelength range, but their far-field divergence angle is large due to ridge width constraints to maintain fundamental mode operation, and their device size cannot be further increased due to coupling strength considerations, thus limiting their surface emission power. Compared to conventional methods for achieving single-mode surface emission (second-order DFB gratings), photonic crystals can achieve better far-field patterns and compress divergence angles in both directions while maintaining single-mode operation. However, achieving stable single-mode operation has always been difficult for photonic crystals, ultimately limiting their single-mode stability and surface emission power. Summary of the Invention

[0004] To address at least one technical problem mentioned above and in other aspects in the prior art, this disclosure provides a method for fabricating a topological surface-emitting quantum cascade laser, which can enhance the absorption loss of the whispering gallery mode and keep the topological surface-emitting quantum cascade laser in the topological mode operating state.

[0005] The embodiments of this disclosure provide a method for fabricating a topological surface-emitting quantum cascade laser, comprising: sequentially forming a metal bonding layer, an active layer, and an upper contact layer on a substrate; photolithographically developing the upper contact layer using a mask, coating photoresist at patterned locations on the mask, then performing electron beam evaporation of the metal with photoresist on the upper contact layer to form a metal layer; peeling off the metal with photoresist to obtain a metal topological pattern layer; and etching the upper contact layer to form absorption boundaries at the edges of the metal topological pattern layer to enhance the absorption loss of the whispering-gallery mode, thereby maintaining the topological surface-emitting quantum cascade laser in the topological mode operating state.

[0006] According to some embodiments of this disclosure, the aforementioned metal topological pattern layer includes a plurality of hexagonal topological photonic crystals. The hexagons are divided into six triangular sub-regions based on their six side lengths. In each of the six sub-regions, three first triangular sub-units and three second triangular sub-units are formed at alternating intervals. The geometric center of the first triangular sub-unit coincides with the geometric center of the sub-region in which it is located, and the geometric center of the second triangular sub-unit is offset relative to the geometric center of the sub-region in which it is located.

[0007] According to some embodiments of this disclosure, the aforementioned topological photonic crystal is designed to have different sizes based on the gain of the matched active layer, so that the aforementioned topological surface-emitting quantum cascade laser emits laser beams in different terahertz bands, the terahertz bands ranging from 0.1THz to 10THz.

[0008] According to some embodiments of this disclosure, the second triangular sub-unit is displaced relative to the geometric center of the sub-region along a preset displacement vector.

[0009] According to some embodiments of this disclosure, the displacement vector represents the line connecting the geometric center of the second triangular sub-unit and the geometric center of the sub-region, relative to the phase angle of the line connecting the geometric center of the sub-region where the second triangular sub-unit is located and the center of the regular hexagon, and the length of the line connecting the geometric center of the second triangular sub-unit and the geometric center of the sub-region where it is located. The phase angle ranges from 0 to 2π, so that the metal topology pattern layer forms a vortex cavity that varies from 0 to 2π.

[0010] According to some embodiments of this disclosure, the vortex cavity is adjusted according to a modulation function to transform the phase angle into a nonlinear change, thereby altering the symmetry of the metal topology layer and thus changing the far-field symmetry of the output laser beam.

[0011] According to some embodiments of this disclosure, the width of the absorption boundary ranges from 10 μm to 20 μm.

[0012] According to some embodiments of this disclosure, after forming the absorption boundary, the process includes: etching the body of the topological surface-emitting quantum cascade laser to maintain the overall morphology of the topological surface-emitting quantum cascade laser in a regular hexagonal shape, so as to reduce the gain of the whispering gallery mode.

[0013] According to some embodiments of this disclosure, after etching the body of the aforementioned topological surface-emitting quantum cascade laser, externally charged strip regions are drawn out at the three 120° spaced vertices of the aforementioned topological surface-emitting quantum cascade laser, which has a regular hexagonal shape. The strip regions maintain the symmetry of C3V, so that the current flows in uniformly.

[0014] Another aspect of the embodiments of this disclosure provides a topological surface-emitting quantum cascade laser, comprising: a substrate; an epitaxial wafer disposed on the substrate, the epitaxial wafer including a metal bonding layer, an active layer and an upper contact layer, wherein an absorption boundary is formed at the edge of the upper contact layer to enhance the absorption loss of the whispering-gallery mode, thereby maintaining the topological surface-emitting quantum cascade laser in a topological mode operating state; a metal topological pattern layer disposed on the epitaxial wafer; and a lower electrode layer disposed on the lower surface of the substrate, wherein the metal topological pattern layer and the lower electrode layer cooperate to provide electrical energy to the active layer.

[0015] According to an embodiment of this disclosure, a topological surface-emitting quantum cascade laser and its fabrication method are described. A photolithography method is used to develop the upper contact layer using a mask. Photoresist is then coated at the patterned locations on the mask. Electron beam evaporation of the metal with photoresist onto the upper contact layer is performed to form a metal layer. The metal layer with photoresist is then peeled off to obtain a metal topological pattern layer. The upper contact layer is etched to form absorption boundaries at the edges of the metal topological pattern layer. This enhances the absorption loss of the whispering-gallery mode, enabling the topological surface-emitting quantum cascade laser to maintain its topological mode operation. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating a method for fabricating a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure;

[0017] Figure 2 This is a top view of a metal topology graphic layer according to an illustrative embodiment of the present disclosure;

[0018] Figure 3 yes Figure 2 An enlarged schematic diagram of part A in the middle;

[0019] Figure 4 This is a schematic diagram of the fabrication process of a method for preparing a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure;

[0020] Figure 5 This is a flowchart illustrating the sequential formation of a metal bonding layer, an active layer, and an upper contact layer on a substrate of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0021] Figure 6 This is an initial morphological diagram of a topological photonic crystal according to an illustrative embodiment of the present disclosure;

[0022] Figure 7 This is a morphological diagram of the second triangular subunit of a topological photonic crystal according to an illustrative embodiment of the present disclosure after displacement along the displacement vector relative to the geometric center of the subregion it belongs to;

[0023] Figure 8 This is a schematic diagram of the phase angle in a metal topology pattern layer according to an illustrative embodiment of the present disclosure;

[0024] Figure 9 This is an energy band diagram of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure;

[0025] Figure 10 This is a morphological diagram of a vortex cavity after modulation function adjustment according to an illustrative embodiment of the present disclosure;

[0026] Figure 11 This is a partially enlarged schematic diagram of a metal topology pattern layer adjusted by a modulation function according to an illustrative embodiment of the present disclosure;

[0027] Figure 12 This is a far-field symmetry diagram of the output laser beam of a topological surface-emitting quantum cascade laser (before modulation) according to an illustrative embodiment of the present disclosure;

[0028] Figure 13 This is a far-field symmetry diagram of the output laser beam of a topological surface-emitting quantum cascade laser (modulated) according to an illustrative embodiment of the present disclosure;

[0029] Figure 14 This is a spectral diagram of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure;

[0030] Figure 15 This is a power comparison diagram of a topological surface-emitting quantum cascade laser and a non-topological photonic crystal device according to an illustrative embodiment of the present disclosure; and

[0031] Figure 16 This is a schematic cross-sectional view of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0032] In the accompanying drawings, the meanings of the reference numerals are as follows:

[0033] 1. Substrate;

[0034] 2. Epitaxial wafer;

[0035] 3. Metal topology graphic layer;

[0036] a. First triangular subunit;

[0037] b. The second triangular subunit;

[0038] 4. Lower electrode layer;

[0039] 5. Metal bonding layer;

[0040] 6. Active layer;

[0041] 7. Upper contact layer;

[0042] 8. Groove;

[0043] 9. Absorption boundary. Detailed Implementation

[0044] The embodiments of the present disclosure will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the disclosure. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the present disclosure for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of the present disclosure.

[0045] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0046] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0047] When using expressions such as "at least one of A, B, and C," the expression should generally be interpreted in accordance with the meaning commonly understood by a person skilled in the art (e.g., "a system having at least one of A, B, and C" should include, but is not limited to, systems having A alone, having B alone, having C alone, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.). Similarly, when using expressions such as "at least one of A, B, or C," the expression should generally be interpreted in accordance with the meaning commonly understood by a person skilled in the art (e.g., "a system having at least one of A, B, or C" should include, but is not limited to, systems having A alone, having B alone, having C alone, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.).

[0048] According to one aspect of the inventive concept of this disclosure, photonic crystals are difficult to achieve stable single-mode operation because there are two identical high-gain band-edge modes competing with each other in band-edge photonic crystals. In order to achieve single-mode stability of the device and improve the surface emission power, this disclosure uses a photomask to perform photolithography on the upper contact layer, coats photoresist at the patterned position of the photomask, then performs electron beam evaporation of the metal with photoresist on the upper contact layer to form a metal layer, peels off the metal layer with photoresist to obtain a metal topological pattern layer, and etches the upper contact layer to form an absorption boundary at the edge of the metal topological pattern layer, which can enhance the absorption loss of the whispering-gallery mode and keep the topological surface emission quantum cascade laser in the topological mode operating state.

[0049] Figure 1 This is a flowchart illustrating a method for fabricating a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure. Figure 4 This is a schematic diagram illustrating the fabrication process of a method for preparing a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure. Figure 16 This is a schematic cross-sectional view of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0050] According to embodiments of this disclosure, such as Figure 1 , Figure 4 and Figure 16 As shown, a method for fabricating a topological surface-emitting quantum cascade laser includes the following steps S1-S4.

[0051] Step S1: Sequentially form a metal bonding layer 5, an active layer 6, and an upper contact layer 7 on substrate 1 (see below for details). Figure 4 (See (a) to (d) for a detailed description).

[0052] According to embodiments of this disclosure, substrate 1 may be an N-type GaAs substrate.

[0053] According to embodiments of this disclosure, the upper contact layer 7 is a highly doped layer with a high Al composition.

[0054] Step S2: As Figure 4 As shown in sections (e) to (f), the upper contact layer 7 is photolithographically developed using a mask, photoresist is coated at the patterned positions on the mask, and then the upper contact layer 7 is subjected to electron beam evaporation of the metal with photoresist to form a metal layer.

[0055] According to an embodiment of this disclosure, the upper contact layer 7 is subjected to electron beam evaporation of a metal with adhesive, and the film thickness of the evaporated metal layer is Ti / Au: 40nm / 300nm.

[0056] Step S3: As Figure 4As shown in part (g), the adhesive-coated metal layer is peeled off, and the peeled portion of the metal layer forms a groove 8, resulting in a metal topology pattern layer 3 formed by multiple grooves 8.

[0057] Step S4: As Figure 4 As shown in sections (h) to (i), the upper contact layer 7 is etched to form an absorption boundary 9 at the edge of the metal topology pattern layer 3 to enhance the absorption loss of the whispering gallery mode, thereby keeping the topology surface-emitting quantum cascade laser in the topology mode operating state.

[0058] Figure 2 This is a top view of a metal topology graphic layer according to an illustrative embodiment of the present disclosure. Figure 3 yes Figure 2 An enlarged schematic diagram of part A in the middle.

[0059] According to embodiments of this disclosure, such as Figure 2 and Figure 3 As shown, the shaded area is the groove 8 formed after the adhesive metal layer is peeled off, that is, the part without the metal layer, and the unshaded area is the part where the metal layer was not peeled off, that is, the part with the metal layer.

[0060] According to embodiments of this disclosure, after obtaining the metal topological pattern layer 3, the upper contact layer 7 at the lower part of the groove 8 is etched, and the etching depth is greater than the thickness of the upper contact layer 7. This can reduce the influence of the upper contact layer 7 on light (such as...). Figure 3 Absorption at the shaded area (position in the middle) improves the efficiency of surface emission of the topological surface-emitting quantum cascade laser.

[0061] According to embodiments of this disclosure, the width of the absorbing boundary 9 ranges from 10 μm to 20 μm.

[0062] According to embodiments of this disclosure, an absorption boundary 9 is formed at the edge of the metal topological pattern layer 3, which can enhance the absorption loss of the whispering-gallery mode caused by the limited area of ​​the electric pump electrode, enabling the topological surface-emitting quantum cascade laser to achieve stable single-mode surface emission and operate in a topological mode with a threshold much lower than that of the band-edge mode, thereby obtaining high single-mode stability.

[0063] According to embodiments of this disclosure, a whispering gallery mode refers to a boundary resonance of a quantum cascade laser emitted along a topological surface, similar to a whispering gallery propagation mode, in which the light field is concentrated at the boundary.

[0064] According to embodiments of this disclosure, by peeling off the adhesive-coated metal layer, two regions—one with metal and one without—are formed at the center of the topological surface-emitting quantum cascade laser. The purpose of creating these two regions is to construct two regions with different refractive indices. If the topological surface-emitting quantum cascade laser operates in a whispering-gallery mode and is concentrated at the boundary, then the two constructed regions have no effect on this mode, and stable single-mode surface emission cannot be achieved. By retaining an upper contact layer 7 as an absorption boundary 9 at the edge of the metal topological pattern layer 3, the whispering-gallery mode loss is increased, and the light field is concentrated at the center of the topological surface-emitting quantum cascade laser, thereby operating in a topological mode controlled by the two designed regions with different refractive indices, achieving stable single-mode surface emission.

[0065] Figure 4 This is a schematic diagram illustrating the fabrication process of a method for preparing a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure. Figure 5 This is a flowchart illustrating the sequential formation of a metal bonding layer 5, an active layer 6, and an upper contact layer 7 on a substrate of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0066] According to embodiments of this disclosure, such as Figure 4 , Figure 5 and Figure 16 As shown, the sequential formation of a metal bonding layer, an active layer, and an upper contact layer on the substrate includes the following steps S101-S105:

[0067] Step S101: As Figure 4 As shown in sections (a) to (b), Ti / Au was evaporated by electron beam on the initial epitaxial wafer and the N-type GaAs substrate respectively, and In / Au was evaporated by electron beam on the N-type GaAs substrate.

[0068] According to embodiments of the present disclosure, the initial epitaxial wafer includes an upper contact layer 7, an active layer 6, a lower contact layer, and a semi-insulating GaAs substrate.

[0069] According to embodiments of this disclosure, the front-side electron beam evaporation conditions for the initial epitaxial wafer are Ti / Au: 40nm / 1000nm, and the front-side electron beam evaporation conditions for the N-type GaAs substrate are Ti / Au: 40nm / 250nm.

[0070] Step S102: As Figure 4 As shown in sections (b) to (c), the initial epitaxial wafer that underwent front-side electron beam evaporation was flipped and subjected to low-temperature In-Au metal bonding with an N-type GaAs substrate to obtain metal bonding layer 5.

[0071] According to embodiments of this disclosure, the In-Au metal bonding layer thickness is In / Au: 1700nm / 100nm.

[0072] According to embodiments of this disclosure, the metal bonding method can also be Cu-Cu, Au-Au bonding, etc.

[0073] According to embodiments of this disclosure, metallic bonding can improve heat dissipation in topological surface-emitting quantum cascade lasers and enhance contact between interfaces.

[0074] Step S103: As Figure 4 As shown in sections (c) to (d), the semi-insulating GaAs substrate of the initial epitaxial wafer after metal bonding is mechanically thinned.

[0075] Step S104: As Figure 4 As shown in sections (d) to (e), the remaining semi-insulating GaAs substrate after mechanical thinning was removed using the etching solution NH4OH:H2O2.

[0076] According to embodiments of this disclosure, the concentration ratio of the etching solution NH4OH:H2O2 is 1:19, which passes through the Al in the initial epitaxial wafer. 0.5 Ga 0.5 As a stop layer to control the corrosion process, that is, the NH4OH:H2O2 corrosion solution on Al 0.5 Ga 0.5 The As barrier layer is not corroded.

[0077] Step S105: Remove Al using HF etching solution 0.5 Ga 0.5 As the cutoff layer, until the upper contact layer 7 on the surface of the epitaxial wafer is obtained.

[0078] According to embodiments of this disclosure, the epitaxial wafer treated with an etchant includes a metal bonding layer 5, an active layer 6, and an upper contact layer 7.

[0079] According to embodiments of this disclosure, see Figure 2 , Figure 3 , Figures 6 to 8 As shown, the metal topological pattern layer 3 includes multiple hexagonal topological photonic crystals, each hexagon being divided into six triangular sub-regions based on its six sides. Within each of the six sub-regions, three first triangular sub-units a and three second triangular sub-units b are formed at alternating intervals. The geometric center O2 of the first triangular sub-unit a coincides with the geometric center of its sub-region, while the geometric center O1 of the second triangular sub-unit is offset relative to the geometric center of its sub-region.

[0080] Figure 6 This is an initial morphological diagram of a topological photonic crystal according to an illustrative embodiment of the present disclosure.

[0081] According to embodiments of this disclosure, such as Figure 6As shown, three first triangular sub-units a and three second triangular sub-units b are formed alternately in six sub-regions, respectively. R represents the circumcircle radius of a single first triangular sub-unit a or second triangular sub-unit b, θ0 represents the angle between two adjacent lines connecting the geometric center of the second triangular sub-unit b in the three initial forms and the center O of the regular hexagonal topological photonic crystal, and c represents the side length of the topological photonic crystal.

[0082] According to embodiments of this disclosure, the topological photonic crystal is designed with different sizes based on the gain of the matched active layer 6, enabling the topological surface-emitting quantum cascade laser to emit laser beams in different terahertz bands, ranging from 0.1 THz to 10 THz.

[0083] According to embodiments of this disclosure, the topological photonic crystal is designed to different sizes based on the gain of the active layer 6 so that the topological surface-emitting quantum cascade laser emits laser beams in different terahertz bands, which can achieve a better surface emission effect and overcome the long strip-shaped far field defect of the second-order grating surface-emitting terahertz laser.

[0084] According to an embodiment of this disclosure, the second triangular sub-unit b is displaced relative to the geometric center of the sub-region along a preset displacement vector.

[0085] According to an embodiment of this disclosure, the displacement vector represents the line connecting the geometric center O1 of the second triangular subunit b to the geometric center O2 of the sub-region it is located in, relative to the phase angle of the line connecting the geometric center O2 of the sub-region where the second triangular subunit b is located to the center O of the regular hexagon, and the length of the line connecting the geometric center O1 of the second triangular subunit b to the geometric center O2 of the sub-region it is located in, the phase angle ranges from 0 to 2π, so that the metal topology pattern layer 3 forms a vortex cavity that varies from 0 to 2π.

[0086] According to embodiments of this disclosure, a vortex cavity refers to a vortex that changes along the center of the cavity.

[0087] Figure 7 This is a morphological diagram of the second triangular subunit b of a topological photonic crystal according to an illustrative embodiment of the present disclosure after displacement along the displacement vector relative to the geometric center O2 of the sub-region.

[0088] According to embodiments of this disclosure, such as Figure 7 As shown, the center of the regular hexagon is represented by O, the geometric center of the second triangular subunit b is represented by O1, the geometric center of the subregion containing the second triangular subunit b is represented by O2, and the phase angle is represented by θ. rThe displacement vector is represented by r, and the length of the line connecting the geometric center O1 of the second triangular sub-unit b and the geometric center O2 of the sub-region is represented by r0. r0 can be designed to be 0.16c, 0.2c, 0.22c, etc.

[0089] According to embodiments of this disclosure, the displacement vector can be represented by formula (1):

[0090]

[0091] Figure 8 The phase angle θ in the metal topology pattern layer 3 according to an illustrative embodiment of this disclosure. r A schematic diagram.

[0092] According to embodiments of this disclosure, the phase angle θ r It can be expressed by formula (2):

[0093]

[0094] Where x represents the x and y coordinates of the intersection point O of the geometric center extensions of the three first triangular subunits a in the metal topological pattern layer 3 relative to the three first triangular subunits a in the other topological photonic crystal, and y represents the y coordinate of the intersection point O of the geometric center extensions of the three first triangular subunits a in the metal topological pattern layer 3 relative to the three first triangular subunits a in the other topological photonic crystal.

[0095] According to an embodiment of this disclosure, the center O of the regular hexagon is chosen as the intersection of the geometric center extensions of the three first triangular subunits a, which enables the vortex cavity to maintain C3V symmetry.

[0096] According to embodiments of this disclosure, such as Figure 8 As shown, the phase angle θ r The x-axis changes linearly from the positive half-axis (0) to 2π, and the angle of the rotation vector increases linearly from 0 to 2π.

[0097] Figure 9 This is an energy band diagram of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0098] According to embodiments of this disclosure, Figure 9 The abscissas of parts (a) and (b) represent different locations in the Brillouin zone. The second triangular subunit is displaced relative to the geometric center of its subregion along a preset displacement vector. This can fold the bandgap point originally located at point K in the Brillouin zone to point Γ, forming a doubly degenerate Dirac cone. Different displacement directions correspond to different degrees of bandgap opening, which can improve the surface emission extraction efficiency of the topological surface emission quantum cascade laser.

[0099] Figure 10 This is a morphological diagram of a vortex cavity after modulation function adjustment according to an illustrative embodiment of the present disclosure.

[0100] According to embodiments of this disclosure, see Figure 10 The vortex cavity is adjusted according to the modulation function, and the phase angle is transformed into a nonlinear change to change the symmetry of the metal topology layer 3, thereby changing the far-field symmetry of the output laser beam.

[0101] According to embodiments of this disclosure, the modulation function can be expressed by formula (3):

[0102]

[0103] Where, θ r ′ represents the phase angle after the nonlinear change of the phase angle after modulation.

[0104] According to an embodiment of this disclosure, when q is 3, the symmetry of the metal topology pattern layer 3 will change. Before modulation, the metal topology pattern layer 3 maintains C3V symmetry, and after modulation, the metal topology pattern layer 3 only retains mirror symmetry, thereby changing the far-field symmetry of the output laser beam, enabling the topological surface emission quantum cascade laser to achieve single-lobe far-field emission.

[0105] According to another embodiment of this disclosure, q can also be 1.5.

[0106] According to embodiments of this disclosure, the modulated displacement vector can be expressed by formula (4):

[0107]

[0108] According to embodiments of this disclosure, each topological photonic crystal in the metal topological pattern layer 3 undergoes the following process: Figure 10 The figure shown is derived from θ r to θ r The nonlinear change of θ′ r The value increases non-linearly from 0 to 2π and θ r ′≤θ r .

[0109] Figure 11 This is a partially enlarged schematic diagram of a metal topology pattern layer 3 adjusted by a modulation function according to an illustrative embodiment of the present disclosure.

[0110] According to embodiments of this disclosure, such as Figure 11 As shown, the second triangular subunit b within each topological photonic crystal varies linearly according to its position, and also exhibits nonlinear variations adjusted by the modulation function.

[0111] Figure 12This is a far-field symmetry diagram of the output laser beam of a topological surface-emitting quantum cascade laser (before modulation) according to an illustrative embodiment of the present disclosure.

[0112] According to embodiments of this disclosure, such as Figure 12 As shown, (a) is an experimental diagram of the far-field symmetry of the output laser beam of the topological surface-emitting quantum cascade laser (before modulation), and (b) is a simulation diagram of the far-field symmetry of the output laser beam of the topological surface-emitting quantum cascade laser (before modulation). It can be seen that the far-field of the output laser beam of the topological surface-emitting quantum cascade laser before modulation has three lobes.

[0113] Figure 13 This is a far-field symmetry diagram of the output laser beam of a topological surface-emitting quantum cascade laser (modulated) according to an illustrative embodiment of the present disclosure.

[0114] According to embodiments of this disclosure, such as Figure 13 As shown, (a) is a graph showing the phase angle changing from linear to nonlinear when q=3 in a topological surface-emitting quantum cascade laser (after modulation), and (b) is an experimental diagram showing the far-field symmetry of the output laser beam of the topological surface-emitting quantum cascade laser (after modulation).

[0115] According to embodiments of this disclosure, by adjusting the vortex cavity according to the modulation function, the phase angle is transformed into a nonlinear change, which enables far-field emission of a single lobe of a topological surface-emitting quantum cascade laser, while also enabling the topological surface-emitting quantum cascade laser to maintain stable single-mode operation.

[0116] According to an embodiment of the present disclosure, after forming the absorption boundary 9, the process includes: etching the body of the topological surface-emitting quantum cascade laser to maintain the overall morphology of the topological surface-emitting quantum cascade laser in a regular hexagonal shape, so as to reduce the gain of the whispering gallery mode.

[0117] According to embodiments of this disclosure, the overall morphology of the topological surface cascaded laser maintains a regular hexagonal shape and has the same orientation as the topological photonic crystal.

[0118] According to embodiments of this disclosure, the overall morphology of the topological surface-emitting quantum cascade laser maintains a regular hexagonal shape. While reducing the gain of the whispering-gallery mode, it can also improve the coupling effect between the active layer of the topological surface-emitting quantum cascade laser and the topological photonic crystal, thereby enhancing the surface emission extraction effect.

[0119] According to an embodiment of this disclosure, after etching the topological surface of the quantum cascade laser body, externally charged strip regions are drawn out at the three 120° spaced vertices of the hexagonal topological surface of the quantum cascade laser. The strip regions maintain the symmetry of C3V, so that the current flows in uniformly.

[0120] According to an embodiment of this disclosure, the strip region has a size of 100μm×100μm, which is beneficial for gold wire bonding. The strip region maintains the symmetry of C3V, allowing current to flow in evenly without disrupting the symmetry of the metal topology layer.

[0121] According to embodiments of this disclosure, such as Figure 4 As shown in sections (g) to (h), after forming the absorption boundary, a hard mask material, which can be silicon oxide, is grown on the surface of the topologically emitting quantum cascade laser, and then photolithography is performed.

[0122] According to embodiments of this disclosure, the overall morphology of the hexagonal shape of the topological surface-emitting quantum cascade laser and the externally charged strip region are transferred to the silicon oxide layer using dry etching. Then, the silicon oxide layer is used as a mask to perform dry etching on the body. ICP dry etching can be selected.

[0123] According to embodiments of this disclosure, the epitaxial wafer after dry etching is cleaned and the remaining silicon oxide on the surface is removed by HF etching solution.

[0124] According to embodiments of this disclosure, the epitaxial wafer is thinned and polished to a reasonable thickness to facilitate heat dissipation of the topologically oriented quantum cascade laser.

[0125] According to embodiments of this disclosure, such as Figure 4 As shown in sections (h) to (i), the back metal is evaporated using electron beam evaporation to obtain the lower electrode layer, which improves the heat dissipation of the topological surface-emitting quantum cascade laser and enables back ohmic contact. The lower electrode layer can be Ti / Au with a thickness of 40nm / 300nm.

[0126] According to embodiments of this disclosure, the lower electrode layer can also be made of GeAu / NiAu alloy annealed, which can effectively reduce the threshold voltage of the topological surface-emitting quantum cascade laser.

[0127] According to embodiments of this disclosure, after fabrication, the topological surface-emitting quantum cascade laser can be welded onto a diamond heat sink and a large copper heat sink to improve heat dissipation, while achieving electrical isolation between the metal topological pattern layer and the lower electrode layer.

[0128] Figure 14 This is a spectral diagram of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0129] According to embodiments of this disclosure, such as Figure 14 As shown, it can be seen that topological surface-emitting quantum cascade lasers can achieve stable single-mode surface emission under high current.

[0130] Figure 15This is a power comparison diagram of a topological surface-emitting quantum cascade laser and a non-topological photonic crystal device according to an illustrative embodiment of the present disclosure.

[0131] According to embodiments of this disclosure, such as Figure 15 As shown, compared with non-topological photonic crystal devices of the same diameter of 800 μm, the topological surface-emitting quantum cascade laser disclosed herein improves the extraction efficiency and output power of surface emission, while avoiding the elongated far-field problem caused by methods such as second-order gratings. The surface power of the topological surface-emitting quantum cascade laser disclosed herein can reach up to 50 mW.

[0132] Figure 16 This is a structural diagram of a topological surface-emitting quantum cascade laser according to an illustrative embodiment of the present disclosure.

[0133] According to another aspect of the embodiments of this disclosure, a topological surface-emitting quantum cascade laser is provided, such as... Figure 16 As shown, the topological surface-emitting quantum cascade laser includes a substrate 1, an epitaxial wafer 2, a metal topological pattern layer 3, and a lower electrode layer 4. The epitaxial wafer 2 is disposed on the substrate 1 and includes a metal bonding layer 5, an active layer 6, and an upper contact layer 7. An absorption boundary 9 is formed at the edge of the upper contact layer 7 to enhance the absorption loss of the whispering-gallery mode, enabling the topological surface-emitting quantum cascade laser to maintain its topological mode operation. The metal topological pattern layer 3 is disposed on the epitaxial wafer 2. The lower electrode layer 4 is disposed on the lower surface of the substrate 1. The metal topological pattern layer 3 and the lower electrode layer 4 cooperate to provide electrical energy to the active layer 6.

[0134] According to embodiments of this disclosure, a topological surface-emitting quantum cascade laser operates at a single laser wavelength, and the wavelength of the output laser beam depends on the period within the topological surface-emitting quantum cascade laser. Therefore, multiple topological surface-emitting quantum cascade lasers with different periods can output laser beams of different wavelengths, thus realizing surface-emitting array laser output.

[0135] The embodiments of this disclosure have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of this disclosure. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. The scope of this disclosure is defined by the appended claims and their equivalents. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of this disclosure, and all such substitutions and modifications should fall within the scope of this disclosure.

Claims

1. A method for fabricating a topological surface-emitting quantum cascade laser, comprising: A metal bonding layer, an active layer, and an upper contact layer are sequentially formed on the substrate; The upper contact layer is photolithographically developed using a photomask. Photoresist is then applied to the patterned areas of the photomask. Finally, the upper contact layer is subjected to electron beam evaporation of the metal with photoresist to form a metal layer. The adhesive-coated metal layer is peeled off to obtain a metal topology pattern layer; as well as The upper contact layer is etched to form an absorption boundary at the edge of the metal topological pattern layer, thereby enhancing the absorption loss of the whispering-gallery mode and keeping the topological surface-emitting quantum cascade laser in the topological mode operating state. The metal topological pattern layer includes multiple hexagonal topological photonic crystals. The hexagon is divided into six triangular sub-regions based on its six side lengths. Within each of the six sub-regions, three first triangular sub-units and three second triangular sub-units are formed at alternating intervals. The geometric center of the first triangular sub-unit coincides with the geometric center of its sub-region, and the geometric center of the second triangular sub-unit is offset relative to the geometric center of its sub-region along a preset displacement vector. The displacement vector represents the line connecting the geometric center of the second triangular sub-unit to the geometric center of the sub-region it belongs to, relative to the phase angle of the line connecting the geometric center of the sub-region it belongs to and the center of the regular hexagon, and the length of the line connecting the geometric center of the second triangular sub-unit to the geometric center of the sub-region it belongs to. The phase angle ranges from 0 to 2π, so that the metal topology pattern layer forms a vortex cavity that varies from 0 to 2π. The vortex cavity is adjusted according to the modulation function to transform the phase angle into a nonlinear change, thereby changing the symmetry of the metal topology pattern layer and thus changing the far-field symmetry of the output laser beam.

2. The method for fabricating a topological surface-emitting quantum cascade laser according to claim 1, wherein, The topological photonic crystal is designed with different sizes according to the gain of the matched active layer, so that the topological surface-emitting quantum cascade laser emits laser beams in different terahertz bands, the terahertz bands being 0.1THz to 10THz.

3. The method for fabricating a topological surface-emitting quantum cascade laser according to claim 1, wherein, The width of the absorption boundary ranges from 10 μm to 20 μm.

4. The method for fabricating a topological surface-emitting quantum cascade laser according to claim 1, after forming the absorption boundary, includes: The body of the topological surface-emitting quantum cascade laser is etched to maintain the overall shape of the topological surface-emitting quantum cascade laser as a regular hexagon, thereby reducing the gain of the whispering gallery mode.

5. The method for fabricating a topological surface-emitting quantum cascade laser according to claim 4, wherein, After etching the body of the topological surface-emitting quantum cascade laser, externally charged strip regions are drawn out at the three 120° spaced vertices of the hexagonal topological surface-emitting quantum cascade laser. The strip regions maintain the symmetry of C3V, so that the current flows in uniformly.

6. A topological surface-emitting quantum cascade laser, comprising: Substrate; An epitaxial wafer is disposed on the substrate. The epitaxial wafer includes a metal bonding layer, an active layer and an upper contact layer. An absorption boundary is formed at the edge of the upper contact layer to enhance the absorption loss of the whispering-gallery mode and keep the topological surface-emitting quantum cascade laser in the topological mode operating state. A metal topology pattern layer is disposed on the epitaxial wafer; A lower electrode layer is disposed on the lower surface of the substrate, and the metal topology pattern layer and the lower electrode layer cooperate to provide electrical energy to the active layer; The metal topological pattern layer includes multiple hexagonal topological photonic crystals. The hexagon is divided into six triangular sub-regions based on its six side lengths. Within each of the six sub-regions, three first triangular sub-units and three second triangular sub-units are formed at alternating intervals. The geometric center of the first triangular sub-unit coincides with the geometric center of its sub-region, and the geometric center of the second triangular sub-unit is offset relative to the geometric center of its sub-region along a preset displacement vector. The displacement vector represents the line connecting the geometric center of the second triangular sub-unit to the geometric center of the sub-region it belongs to, relative to the phase angle of the line connecting the geometric center of the sub-region it belongs to and the center of the regular hexagon, and the length of the line connecting the geometric center of the second triangular sub-unit to the geometric center of the sub-region it belongs to. The phase angle ranges from 0 to 2π, so that the metal topology pattern layer forms a vortex cavity that varies from 0 to 2π. The vortex cavity is adjusted according to the modulation function to transform the phase angle into a nonlinear change, thereby changing the symmetry of the metal topology pattern layer and thus changing the far-field symmetry of the output laser beam.

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