Surface-emitting quantum cascade laser and its control method

Abstract

To provide a surface emitting quantum cascade laser that oscillates in a single mode, and a control method thereof.SOLUTION: A surface emitting quantum cascade laser includes: a first surface that emits laser light; a second surface opposite to the first surface; an active layer; a photonic crystal; and first to third electrodes. The active layer is provided between the first surface and the second surface. The photonic crystal is provided between the active layer and the first or second surface, and is located between the first surface and the second electrode. The first electrode is provided on the first surface and is located outside a region where the laser light is emitted. The second electrode is provided on the second surface. The third electrode is provided on the second surface and separated from the second electrode. The active layer extends between the first surface and the second electrode and between the first surface and the third electrode.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The embodiments relate to a surface-emitting quantum cascade laser and a control method therefor.

Background Art

[0002] A surface-emitting quantum cascade laser includes a photonic crystal that controls laser oscillation and can achieve single-mode oscillation. However, sub-modes that do not depend on the photonic crystal may also occur.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The embodiments provide a surface-emitting quantum cascade laser that performs single-mode oscillation and a control method therefor.

Means for Solving the Problems

[0005] The surface-emitting quantum cascade laser according to the embodiments includes a first surface that emits laser light, a second surface opposite to the first surface, an active layer, a photonic crystal, and first to third electrodes. The active layer is provided between the first surface and the second surface. The photonic crystal is provided between the active layer and the first surface or between the active layer and the second surface. The first electrode is provided on the first surface and is located outside a region of the first surface that emits the laser light. The second electrode is provided on the second surface, and the photonic crystal is located between the first surface and the second electrode. The third electrode is provided on the second surface and is spaced apart from the second electrode. The active layer extends between the first surface and the second electrode and between the first surface and the third electrode. [Brief explanation of the drawing]

[0006] [Figure 1] This is a schematic cross-sectional view showing a quantum cascade laser according to an embodiment. [Figure 2] This is a schematic cross-sectional view showing the active layer of a quantum cascade laser according to an embodiment. [Figure 3] This is a schematic diagram showing the energy bands of the active layer according to the embodiment. [Figure 4] This is a schematic diagram showing the oscillation spectrum of a quantum cascade laser according to the embodiment. [Figure 5] This is a schematic plan view showing the reflective side electrode of a quantum cascade laser according to the embodiment. [Figure 6] This is a schematic plan view showing the reflective surface side electrode according to a modified embodiment. [Figure 7] This is a schematic plan view showing the reflective surface side electrode according to another modified embodiment. [Figure 8] This is a schematic plan view showing the light-emitting surface of a quantum cascade laser according to the embodiment. [Figure 9] This is a schematic plan view showing the light-emitting surface of a quantum cascade laser according to a modified embodiment. [Modes for carrying out the invention]

[0007] The embodiments will be described below with reference to the drawings. Identical parts in the drawings will be numbered the same, and detailed explanations of those parts will be omitted as appropriate, while different parts will be described. Note that the drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the ratio of sizes between parts, etc., are not necessarily the same as in reality. Furthermore, even when representing the same part, the dimensions and ratios may be depicted differently in different drawings.

[0008] Furthermore, the arrangement and configuration of each part will be explained using the X, Y, and Z axes shown in each figure. The X, Y, and Z axes are mutually orthogonal and represent the X, Y, and Z directions, respectively. In some cases, the Z direction is described as upward and the opposite direction as downward.

[0009] Figure 1 is a schematic cross-sectional view showing a quantum cascade laser 1 according to an embodiment. The quantum cascade laser 1 is a surface-emitting quantum cascade laser. Here, surface emission is a concept that distinguishes it from edge-emitting type, which emits laser light from the edge of the active layer. In surface emission, the laser light propagates, for example, in the direction from the active layer toward the semiconductor substrate, or in the reverse direction, and is emitted from the back surface of the semiconductor substrate or the surface of the growth layer on the semiconductor substrate.

[0010] The quantum cascade laser 1 comprises a semiconductor substrate 10, a first semiconductor layer 20, an active layer 30, a second semiconductor layer 40, and a contact layer 50. The first semiconductor layer 20, the active layer 30, the second semiconductor layer 40, and the contact layer 50 are epitaxially grown on the semiconductor substrate 10, for example, using MBE (Molecular Beam Epitaxy).

[0011] The semiconductor substrate 10 is, for example, an n-type InP substrate. The semiconductor substrate 10 has an emissive surface LS and an epitaxial growth surface GS. The epitaxial growth surface GS is located on the opposite side of the emissive surface LS.

[0012] The first semiconductor layer 20 is, for example, an n-type InP layer. The first semiconductor layer 20 is provided on the epitaxial growth surface GS of the semiconductor substrate 10.

[0013] The active layer 30 is provided on the first semiconductor layer 20. The active layer 30 has, for example, a multiple quantum well structure.

[0014] The second semiconductor layer 40 is provided on the active layer 30. The second semiconductor layer 40 includes a photonic crystal PC.

[0015] The second semiconductor layer 40 includes, for example, a first layer 43 and a second layer 45. The first layer 43 is provided on the active layer 30, and the second layer 45 is provided on the first layer 43. The first layer 43 includes, for example, a ternary compound InGaAs represented by the composition formula In x Ga 1-x As (0 < x < 1). The first layer 43 is an n-type InGaAs layer. The InGaAs layer has a composition x that is lattice-matched to InP, for example. The second layer 45 is, for example, an n-type InP layer.

[0016] The first layer 43 has, for example, a plurality of protrusions provided periodically on the side of the second layer 45, and the second layer 45 is provided so as to embed the plurality of protrusions of the first layer 43. The photonic crystal PC is composed of the protrusions of the first layer 43 and the second layer 45. The photonic crystal PC has a structure in which the refractive index changes periodically in a direction along the interface between the active layer 30 and the second semiconductor layer 40.

[0017] The contact layer 50 is provided on the second semiconductor layer 40. The contact layer 50 is, for example, an n-type InGaAs layer. The contact layer 50 has an energy bandgap narrower than the energy bandgap of the second layer 45 of the second semiconductor layer 40.

[0018] The quantum cascade laser 1 further includes a first electrode 60, a second electrode 70, and a third electrode 80. The first electrode 60 is provided on the light emitting surface LS of the semiconductor substrate 10. The second electrode 70 and the third electrode 80 are provided on the reflection surface RS on the side opposite to the light emitting surface LS. Here, the reflection surface RS is the surface of the contact layer 50. The active layer 30 is located between the light emitting surface LS (the first surface) and the reflection surface RS (the second surface).

[0019] The first electrode 60 is provided, for example, outside the light emitting region on the light emitting surface LS. The first electrode 60 has, for example, a two-layer structure including titanium (Ti) and gold (Au).

[0020] The second electrode 70 is provided on the contact layer 50. The second electrode 70 is provided so as to be in contact with the contact layer 50, for example, through a contact hole in an insulating film 73 provided on the contact layer 50. The second electrode 70 includes a portion that is in contact with the contact layer 50 and a portion that is provided on the insulating film 73. The insulating film 73 is, for example, a silicon oxide film. The second electrode 70 includes, for example, Au and is provided so as to reflect light emitted from the active layer 30.

[0021] The second electrode 70 is provided, for example, in a plane parallel to the reflective surface RS, such that it has a smaller area than the area of ​​the photonic crystal PC. That is, in a plan view parallel to the reflective surface RS, the photonic crystal PC is provided such that it has a portion located directly below the second electrode 70 and a portion located outside the second electrode 70.

[0022] The third electrode 80 is positioned on the contact layer 50, spaced apart from the second electrode 70. The third electrode 80, for example, contains the same material as the second electrode 70 and reflects light emitted from the active layer 30.

[0023] The active layer 30 extends between the light-emitting surface LS and the second electrode 70, and between the light-emitting surface LS and the third electrode 80. The photonic crystal PC is provided between the light-emitting surface LS and the second electrode 70. The photonic crystal PC does not necessarily have to be provided between, for example, the light-emitting surface LS and the third electrode 80.

[0024] The embodiments are not limited to the examples described above. For example, the photonic crystal PC may be provided in the first semiconductor layer 20. Alternatively, the laser light may be emitted from the surface side of the contact layer 50. In that case, the first electrode 60 is provided on the surface of the contact layer 50, and the second electrode 70 and the third electrode 80 are provided on the back surface of the semiconductor substrate 10.

[0025] FIG. 2 is a schematic cross-sectional view showing an active layer 30 of the quantum cascade laser 1 according to the embodiment. The active layer 30 includes a plurality of barrier layers 33 and a plurality of quantum well layers 35. The barrier layers 33 and the quantum well layers 35 are alternately stacked in a direction from the first semiconductor layer 20 to the second semiconductor layer 40, for example, the Z direction. The quantum well layer 35 is located between adjacent barrier layers 33. The barrier layer 33 includes, for example, a ternary compound AlInAs represented by the composition formula Al y In 1-y As (0 < y < 1). The quantum well layer 35 includes InGaAs.

[0026] FIGS. 3(a) to (c) are schematic diagrams showing the energy bands of the active layer 30 according to the embodiment. FIG. 3(a) represents the energy band structure of the active layer 30 in the thermal equilibrium state. FIGS. 3(b) and (c) represent the conduction band Ec of the active layer 30 under a predetermined bias.

[0027] As shown in FIG. 3(a), the barrier layer 33 has an energy band gap Egb between the valence band Ev and the conduction band Ec. On the other hand, the quantum well layer 35 has an energy band gap Egw narrower than the energy band gap Egb. The valence band Ev and the conduction band Ec each have a quantum well QW caused by the energy difference between the energy band gap Egb and the energy band gap Egw.

[0028] FIG. 3(b) represents the conduction band Ec of the active layer 30 biased by a voltage lower than the threshold voltage for laser oscillation. Electrons injected into the active layer 30 are distributed in the quantum well QW of the conduction band Ec. The electrons in the active layer 30 are distributed, for example, in the subband Esl in the ground state.

[0029] FIG. 3(c) represents the conduction band Ec of the active layer 30 biased by a voltage higher than the threshold voltage. As shown in FIG. 3(c), the active layer 30 includes a light-emitting quantum well QWE and an injection quantum well QWI.

[0030] Electrons injected into the active layer 30 are accelerated by the electric field and excited to the subband Esh, which has a higher energy than the subband Esl. Furthermore, the excited electrons transition from subband Esh to subband Esl in the emission quantum well QWE, and light is emitted. The electrons that have transitioned to subband Esl move through the injection quantum well QWI due to the electric field, are excited along the way, and transition from subband Esh to subband Esl in the next emission quantum well QWE. As a result, light is emitted again. By repeating this process in the active layer 30, light is emitted efficiently, leading to laser oscillation.

[0031] In quantum cascade laser 1, a voltage higher than the laser oscillation threshold voltage is applied between the first electrode 60 and the second electrode 70. On the other hand, a voltage lower than the threshold voltage is applied between the first electrode 60 and the third electrode 80.

[0032] In the active layer 30 between the first electrode 60 and the second electrode 70, laser oscillation controlled by the photonic crystal PC occurs, and the laser light propagates in the direction of the light-emitting surface LS and the reflection surface RS (see Figure 1). The laser light propagating in the direction of the reflection surface RS is reflected by the second electrode 70, and its propagation direction is reversed toward the light-emitting surface. As a result, the laser light is emitted outward from the light-emitting surface LS.

[0033] In the active layer 30 located between the first electrode 60 and the third electrode 80, laser oscillation does not occur, and a light absorption region with a high density of electrons is formed. Therefore, for example, Fabry-Perot mode laser oscillation caused by end-face reflection of light propagating in a direction parallel to the interface between the active layer 30 and the second semiconductor layer 40 can be suppressed.

[0034] Figures 4(a) and (b) are schematic diagrams showing the oscillation spectrum of the quantum cascade laser 1 according to the embodiment.

[0035] Figure 4(a) shows the oscillation spectrum when a voltage higher than the threshold voltage is applied between the first electrode 60 and the second electrode 70, and no voltage is applied between the first electrode 60 and the third electrode 80.

[0036] Figure 4(b) shows the oscillation spectrum when a voltage higher than the threshold voltage is applied between the first electrode 60 and the second electrode 70, and a voltage lower than the threshold voltage is applied between the first electrode 60 and the third electrode 80.

[0037] The oscillation spectrum shown in Figure 4(a) includes the main mode MS and submodes SS1 and SS2. The main mode MS is the oscillation mode controlled by the photonic crystal PC. Submodes SS1 and SS2 are Fabry-Perot modes resulting from end-face reflections of the active layer 30.

[0038] On the other hand, in the oscillation spectrum shown in Figure 4(b), submodes SS1 and SS2 disappear, and the main mode MS remains. In other words, single-mode oscillation controlled by the photonic crystal PC is obtained.

[0039] Thus, in the quantum cascade laser 1, by applying a voltage lower than the laser oscillation threshold voltage between the first electrode 60 and the third electrode 80, a light absorption region can be formed inside the active layer 30, thereby suppressing Fabry-Perot mode oscillation.

[0040] For example, by increasing the distance between the portion of the active layer 30 located beneath the photonic crystal PC and the end face of the active layer 30, the light absorption in the active layer 30 can be increased without providing a third electrode 80. This also makes it possible to suppress the Fabry-Perot mode.

[0041] For example, in lasers that use light emission due to interband transitions between the valence band and the conduction band, the active layer acts as a light-emitting absorber, but the light absorption of the active layer 30 in quantum cascade lasers is negligible. However, even in quantum cascade lasers, there is light absorption that is usually negligible, such as light absorption due to impurity levels. Therefore, as the propagation distance of light in the active layer 30 increases, the attenuation of light becomes a level that cannot be ignored. For example, if the separation distance between the portion of the active layer 30 located beneath the photonic crystal PC and the edge face of the active layer 30 is 100 μm or more, the Fabry-Perot mode can be suppressed by light absorption due to impurities contained in that space. Furthermore, as the separation distance increases, the light absorption becomes greater, and the suppression effect of the Fabry-Perot mode increases. For example, by setting a separation distance of 500 μm or more, it is possible to suppress the multimode formation of laser oscillation near the oscillation threshold current. Furthermore, by setting the separation distance to 1000 μm or more, an even greater effect can be expected, making it possible to suppress the Fabry-Perot mode even in the injection current region which far exceeds the oscillation threshold current.

[0042] On the other hand, such a configuration has the disadvantage of increasing the chip size and thus the manufacturing cost. Furthermore, the size of the device incorporating the quantum cascade laser would also increase. Therefore, in the quantum cascade laser 1 according to this embodiment, a third electrode 80 is provided to reduce the size of the laser chip while achieving single-mode oscillation.

[0043] Figures 5(a) to 5(c) are schematic plan views showing the reflective side electrodes of the quantum cascade laser 1 according to the embodiment. The second electrode 70 is located in the center of the laser chip LC. The third electrode 80 is spaced apart from the second electrode 70 and is positioned to surround the second electrode 70.

[0044] In the example shown in Figure 5(a), the third electrode 80 is provided so as to cover the entire surface of the chip, excluding the laser oscillation region, in a plan view. This maximizes the area of ​​the light absorption region formed in the active layer 30.

[0045] In the example shown in Figure 5(b), the third electrode 80 is not provided on the dicing region along the outer edge of the laser chip LC. This facilitates the cutting of the laser chip from the wafer. Furthermore, it prevents the third electrode 80 from peeling off during the dicing process of the laser chip LC, thereby improving the manufacturing yield.

[0046] In the example shown in Figure 5(c), the third electrode 80 is not positioned on the diagonal connecting two of the four corners of the laser chip LC. Fabry-Perot mode oscillation is caused by multiple reflections between opposing end faces of the active layer 30. Therefore, Fabry-Perot modes resulting from light propagating diagonally across the laser chip LC can be ignored. Consequently, it is possible to remove the portion of the third electrode 80 located on the diagonal. Such a third electrode 80 can be formed, for example, by a deposition method using a metal mask.

[0047] Figures 6(a) to 6(c) are schematic plan views showing the reflective surface side electrode according to a modified embodiment. In these examples, the second electrode 70 is located in the center of the laser chip LC and has, for example, a circular shape. The planar shape of the second electrode 70 is arbitrary and may be, for example, polygonal. The third electrode 80 is spaced apart from the second electrode 70 and is provided so as to surround the second electrode 70.

[0048] As shown in Figure 6(a), the third electrode 80 is provided so as to cover the entire surface of the chip, excluding the laser oscillation area, in a plan view. The third electrode 80 is provided so as to be equally spaced from the second electrode 70.

[0049] As shown in Figure 6(b), the third electrode 80 is not provided on the dicing region along the outer edge of the laser chip LC.

[0050] As shown in Figure 6(c), the third electrode 80 is provided such that it has a constant width in the direction from the center of the laser tip LC toward the outer edge.

[0051] Figures 7(a) and (b) are schematic plan views showing the reflective surface side electrode according to another modified embodiment. The second electrode 70 is located in the center of the laser chip LC. The third electrode 80 is spaced apart from the second electrode 70 and is located surrounding the second electrode 70.

[0052] In the example shown in Figure 7(a), the second electrode 70 has a circular planar shape. The third electrode 80 is provided so as to have a constant width in the direction from the center of the laser chip LC toward the outer edge. Furthermore, the third electrode 80 is not provided on the diagonal line connecting two of the four corners of the laser chip LC.

[0053] In the example shown in Figure 7(b), the second electrode 70 has a rectangular planar shape. The third electrode 80 has a circular outer edge and is provided with a constant width in the direction from the center of the laser tip LC toward the outer edge.

[0054] Figures 8(a) to 8(c) are schematic plan views showing the light-emitting surface LS of the quantum cascade laser 1 according to the embodiment. The first electrode 60 is provided on the light-emitting surface LS so as to surround the light-emitting region LER.

[0055] In the example shown in Figure 8(a), the first electrode 60 is provided so as to cover the entire light-emitting surface LS, excluding the light-emitting region LER.

[0056] As shown in Figure 8(b), the first electrode 60 is not located on the dicing region along the outer edge of the laser chip LC.

[0057] As shown in Figure 8(c), the first electrode 60 may be provided with multiple portions that are spaced apart from each other. This allows the first electrode 60 to be formed, for example, by a vapor deposition method using a metal mask.

[0058] Figures 9(a) to 9(c) are schematic plan views showing the light-emitting surface LS of a quantum cascade laser 1 according to a modified embodiment. The first electrode 60 is provided on the light-emitting surface LS so as to surround a circular light-emitting region LER. The planar shape of the light-emitting region LER is arbitrary and may be, for example, a polygon.

[0059] As shown in Figure 9(a), the first electrode 60 is provided so as to cover the entire light-emitting surface LS, excluding the circular light-emitting region LER.

[0060] As shown in Figure 9(b), the first electrode 60 is not located on the dicing region along the outer edge of the laser chip LC.

[0061] As shown in Figure 9(b), the first electrode 60 includes a fine wire electrode 60f provided in the light-emitting region LER. By providing the fine wire electrode 60f, the electrons injected into the active layer 30 can be made more uniform, thereby improving the light emission efficiency.

[0062] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0063] (Note 1) The first surface emits laser light, The second surface is on the opposite side from the first surface, An active layer provided between the first surface and the second surface, A photonic crystal having a predetermined periodicity is provided between the active layer and the first surface, or between the active layer and the second surface. On the first surface, a first electrode located outside the region that emits the laser light, A second electrode provided on the second surface, wherein the photonic crystal is located between the first surface and the second electrode, A third electrode is provided on the second surface, spaced apart from the second electrode, and the active layer extends between the first surface and the second electrode, and between the first surface and the third electrode, A surface-emitting quantum cascade laser equipped with [a specific feature]. (Note 2) The surface-emitting quantum cascade laser according to Appendix 1, wherein the photonic crystal is provided in a direction parallel to the surface of the active layer facing the photonic crystal such that its refractive index with respect to the laser light changes with the periodicity described above. (Note 3) The photonic crystal is not provided between the first surface and the third electrode in the surface-emitting quantum cascade laser according to Appendix 1 or 2. (Note 4) A first semiconductor layer is provided between the first surface and the active layer, A second semiconductor layer is provided between the second surface and the active layer, Furthermore, The photonic crystal is provided in either the first semiconductor layer or the second semiconductor layer, and is a surface-emitting quantum cascade laser according to any one of appendices 1 to 3. (Note 5) The photonic crystal has a first area in a plane parallel to the boundary between the first semiconductor layer and the active layer. The surface-emitting quantum cascade laser according to claim 4, wherein the second electrode has a second area smaller than the first area in the plane. [Explanation of Symbols]

[0064] 1…Quantum cascade laser, 10…Semiconductor substrate, 20…First semiconductor layer, 30…Active layer, 33…Barrier layer, 35…Quantum well layer, 40…Second semiconductor layer, 43…First layer, 45…Second layer, 50…Contact layer, 60…First electrode, 60f…Non-wire electrode, 70…Second electrode, 73…Insulating film, 80…Third electrode, LS…Emitting surface, LER…Emitting region, RS…Reflecting surface, GS…Epitaxial growth surface, LC…Laser chip, PC…Photonic crystal, Ec…Conduction band, Ev…Valence band, Egb, Egw…Energy band gap, Esh, Esl…Subband, QW…Quantum well, QWI…Injection quantum well, QWE…Emitting quantum well, MS…Main mode, SS1, SS2…Submode

Claims

1. The first surface emits laser light, The second surface is on the opposite side from the first surface, An active layer provided between the first surface and the second surface, A photonic crystal having a predetermined periodicity is provided between the active layer and the first surface, or between the active layer and the second surface. On the first surface, a first electrode located outside the region that emits the laser light, A second electrode provided on the second surface, wherein the photonic crystal is located between the first surface and the second electrode, A third electrode is provided on the second surface, spaced apart from the second electrode, and the active layer extends between the first surface and the second electrode, and between the first surface and the third electrode, A surface-emitting quantum cascade laser equipped with [a specific feature].

2. The surface-emitting quantum cascade laser according to claim 1, wherein the photonic crystal is provided in a direction parallel to the surface of the active layer facing the photonic crystal such that its refractive index with respect to the laser light changes with the periodicity described above.

3. The surface-emitting quantum cascade laser according to claim 1, wherein the photonic crystal is not provided between the first surface and the third electrode.

4. A first semiconductor layer is provided between the first surface and the active layer, A second semiconductor layer is provided between the second surface and the active layer, Furthermore, The surface-emitting quantum cascade laser according to claim 1, wherein the photonic crystal is provided in either the first semiconductor layer or the second semiconductor layer.

5. The photonic crystal has a first area in a plane parallel to the boundary between the first semiconductor layer and the active layer. The surface-emitting quantum cascade laser according to claim 4, wherein the second electrode has a second area smaller than the first area in the plane.

6. A method for controlling a surface-emitting quantum cascade laser according to any one of claims 1 to 5, A voltage higher than the oscillation threshold voltage of the surface-emitting quantum cascade laser is applied between the first electrode and the second electrode. A control method comprising applying a voltage lower than the oscillation threshold voltage between the first electrode and the third electrode.

7. The first surface emits laser light, The second surface is on the opposite side from the first surface, An active layer provided between the first surface and the second surface, A photonic crystal having a predetermined periodicity is provided between the active layer and the first surface, or between the active layer and the second surface. On the first surface, a first electrode located outside the region that emits the laser light, A second electrode provided on the second surface, wherein the photonic crystal is located between the first surface and the second electrode, Equipped with, A surface-emitting quantum cascade laser, wherein, in a direction along the surface of the active layer facing the photonic crystal, the photonic crystal is spaced apart from the edge of the active layer, and the spacing is sufficient to suppress Fabry-Perot mode oscillation.

Images