CN110021878B - Quantum Cascade Laser - Google Patents
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- CN110021878B CN110021878B CN201810014182.6A CN201810014182A CN110021878B CN 110021878 B CN110021878 B CN 110021878B CN 201810014182 A CN201810014182 A CN 201810014182A CN 110021878 B CN110021878 B CN 110021878B
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
- H01S5/34346—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser characterised by the materials of the barrier layers
- H01S5/34366—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser characterised by the materials of the barrier layers based on InGa(Al)AS
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Abstract
本发明提供一种量子级联激光器。量子级联激光器具有活性层、第1包层和第2包层、以及光引导层。活性层具有多个注入量子阱区域和多个发光量子阱区域。各注入量子阱区域和各发光量子阱区域交替地层叠。所述第1包层和第2包层以从两侧夹持所述活性层的方式设置，具有比所述各发光量子阱区域的实效折射率低的折射率。所述光引导层以将所述活性层沿着层叠方向一分为二的方式配置。所述光引导层具有比所述各发光量子阱区域的所述实效折射率高的折射率，具有比所述各发光量子阱区域的量子阱层的全部的阱层的厚度大的厚度。
The present invention provides a quantum cascade laser. The quantum cascade laser has an active layer, a first cladding layer and a second cladding layer, and a light guide layer. The active layer has a plurality of implanted quantum well regions and a plurality of light emitting quantum well regions. The injection quantum well regions and the light emitting quantum well regions are alternately stacked. The first cladding layer and the second cladding layer are provided so as to sandwich the active layer from both sides, and have a refractive index lower than the effective refractive index of each of the light-emitting quantum well regions. The light guide layer is arranged so as to divide the active layer into two along the stacking direction. The light guide layer has a refractive index higher than the effective refractive index of each light-emitting quantum well region, and has a thickness larger than a thickness of all well layers of the quantum well layers of each light-emitting quantum well region.
Description
Technical Field
Embodiments of the present invention relate to quantum cascade lasers.
Background
Quantum Cascade lasers (QCL: Quantum Cascade lasers) emit infrared Laser light.
When the threshold current of the quantum cascade laser is reduced, the quantum efficiency and the optical output can be improved.
However, the quantum cascade laser has an active layer in which a plurality of light-emitting quantum well regions are cascade-connected. Therefore, light confinement in the vertical direction of the thick active layer is not sufficient, and the threshold current is not easily reduced.
Disclosure of Invention
The present invention provides a quantum cascade laser, comprising:
an active layer including a plurality of injection quantum well regions and a plurality of light-emitting quantum well regions, each of the light-emitting quantum well regions emitting laser light by intersubband optical transition of carriers, the injection quantum well regions relaxing the carriers after the intersubband optical transition to a microstrip level and injecting the carriers into a downstream light-emitting quantum well region, the active layer being formed by alternately laminating the injection quantum well regions and the light-emitting quantum well regions;
a 1 st cladding layer and a 2 nd cladding layer which are provided so as to sandwich the active layer from both sides and have a refractive index lower than an effective refractive index of each of the light-emitting quantum well regions; and
the optical guiding layer is configured to divide the active layer into two parts along the stacking direction, has a refractive index higher than the effective refractive index of each light-emitting quantum well region, and has a thickness larger than the thickness of all the quantum well layers of each light-emitting quantum well region.
Drawings
Fig. 1(a) is a graph illustrating the optical confinement in the vertical direction of the quantum cascade laser according to embodiment 1, and fig. 1(b) is a graph illustrating the energy level of the conduction band.
Fig. 2(a) is a graph illustrating the optical confinement in the vertical direction of the quantum cascade laser according to comparative example 1, and fig. 2(b) is a graph illustrating the optical confinement in the vertical direction of the quantum cascade laser according to comparative example 2.
Fig. 3(a) is a schematic perspective view of the quantum cascade laser according to embodiment 1, and fig. 3(b) is a schematic cross-sectional view taken along line a-a.
Fig. 4(a) is a graph showing a simulation result of a relative light intensity distribution of specific example 1 of embodiment 1, and fig. 4(b) is a graph showing a simulation result of a dependency of a threshold current on a thickness of an optical guiding layer.
Fig. 5 is a graph showing simulation results of a relative light intensity distribution of a specific example of comparative example 2.
Fig. 6(a) is a graph showing a simulation result of a relative light intensity distribution of specific example 2 of embodiment 1, and fig. 6(b) is a graph showing a simulation result of a dependency of a threshold current on a thickness of an optical guiding layer.
Fig. 7(a) is a graph showing a simulation result of a relative light intensity distribution of specific example 3 of embodiment 1, and fig. 7(b) is a graph showing a simulation result of a dependency of a threshold current on a thickness of an optical guiding layer.
Detailed Description
The quantum cascade laser of the embodiment has an active layer, 1 st and 2 nd cladding layers, and a light guiding layer. The active layer has a plurality of implant quantum well regions and a plurality of light emitting quantum well regions. The implanted quantum well regions and the light-emitting quantum well regions are alternately stacked. The light-emitting quantum well regions emit laser light by optical transition between sub-bands of carriers. Each of the injected quantum well layers relaxes the carriers after the optical transition between the sub-bands to a microstrip level and injects the carriers to a downstream light emitting quantum well region. The 1 st cladding layer and the 2 nd cladding layer are provided so as to sandwich the active layer from both sides, and have a refractive index lower than an effective refractive index of each of the light emitting quantum well regions. The light guide layer is disposed so as to divide the active layer into two parts along the stacking direction. The light guiding layer has a refractive index higher than the effective refractive index of each of the light emitting quantum well regions, and has a thickness larger than the thickness of all of the quantum well layers of each of the light emitting quantum well regions.
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Fig. 1(a) is a graph illustrating light confinement in the vertical direction with respect to the active layer of the quantum cascade laser according to embodiment 1, and fig. 1(b) is a graph illustrating the energy level of the conduction band.
In fig. 1(a), the vertical axis represents the refractive index n (solid line) and the light intensity IL(dotted line), and the horizontal axis Z represents the vertical (stacking direction) position of the semiconductor layers. As shown in fig. 1(a), the quantum cascade laser of embodiment 1 includes an active layer 24(24a, 24b), a 1 st cladding layer 23, a 2 nd cladding layer 26, and a light guiding layer 25.
In fig. 1(b), the vertical axis represents the conduction band energy level, and the horizontal axis Z represents the vertical position. As shown in fig. 1(b), the active layer 24 has a plurality of injection quantum well regions 90 and a plurality of light-emitting quantum well regions 80. The active layer 24 includes a laminate in which the implanted quantum well regions 90 and the light-emitting quantum well layers 80 are alternately laminated. That is, the unit stacked body 100 including a pair of 1 injection quantum well region 90 and 1 light-emitting quantum well region 80 is arranged. That is, in fig. 1(a), the regions sandwiched by the broken lines correspond to the unit laminated bodies 100, respectively.
The number of the arrays is, for example, 30 to 200. Each of the light-emitting quantum well regions 80 emits laser light by optical transition between sub-bands of carriers 102 such as electrons. Each injection quantum well region 90 relaxes the carriers 102 after optical transition between the sub-bands to the microstrip level M, and further injects them into the downstream light-emitting quantum well region 80. When the carriers 102 are electrons as shown in fig. 1B, the electrons are relaxed (level a → B) at the microstrip M injected into the quantum well region 90, emit laser light by intersubband transition (level B → C) at the luminescent quantum well region 80, and are relaxed (level D) after being injected into the injected quantum well region 90 downstream.
The light guiding layer 25 has a refractive index higher than the effective refractive index of each light-emitting quantum well region 80. The thickness of the well layer of the light-emitting quantum well region 80 and the thickness of the well layer of the barrier layer are sufficiently shorter than the wavelength of laser light. Therefore, the effective refractive index of the light-emitting quantum well region 80 can be regarded as a refractive index that is intermediate between the refractive index of the well layer and the refractive index of the barrier layer lower than the refractive index of the well layer, that is, a refractive index that is medium-uniformly distributed. In general, a substance having a higher refractive index tends to have a smaller band gap energy Eg. In this system, the band gap energy of the optical guiding layer 25 is also smaller than that of the quantum well layer. In the pn junction laser diode, since a substance having a small band gap energy Eg serves as a light absorbing layer, a light guiding layer is not provided in the center of the active layer. However, since the quantum cascade laser emits light with energy smaller than the band gap energy, even if a substance having a high refractive index is provided in the central portion of the active layer 25, light absorption is suppressed.
Preferably, the optical guiding layer 25 does not contain quantum wells that cause interband optical transitions. Therefore, the optical guiding layer 25 has a thickness TG larger than the thickness of all the quantum well layers of each light emitting quantum well region 80, respectively. In fig. 1(b), 1 light-emitting quantum well region 80 has two quantum well layers, and TG > TW represents the thickness of a larger well layer as TW. When the thickness TG of the light guiding layer 25 is set to 100nm or more, the light confinement effect can be improved as will be described later by simulation. On the other hand, if the active layers 24a and 24b are too separated, the light recovery action is weakened. Therefore, the thickness TG of the light guiding layer 25 is preferably 500nm or less.
Further, it is preferable to provide the light guide layer 25 so as to divide the active layer 24 into substantially equal parts, because the light intensity distribution can be made nearly symmetrical. For example, if the number of the cascade-connected unit stacks 100 is an even number, the light guide layer 25 may be provided so that the number of the unit stacks 100 is divided into 2 equal parts. The number of layers of the unit laminated body 100 is only an odd number, and the number of layers on either side is slightly increased. However, since the number of stacked unit stacks 100 is as large as 30 to 200, the unit stacks may not be divided into two parts strictly.
The 1 st clad layer 23 and the 2 nd clad layer 26 are provided so as to sandwich the active layer 24 from both sides. The refractive index of the 1 st cladding 23 and the 2 nd cladding 26 is lower than the effective refractive index of each light emitting quantum well region.
In embodiment 1, the light guiding layer 25 is disposed near the center of the active layer 24, whereby the light confinement effect can be enhanced. Therefore, the threshold current can be reduced, and the quantum efficiency and the light output can be improved.
Fig. 2(a) is a graph illustrating the effect of light confinement in the vertical direction of the active layer of the quantum cascade laser according to comparative example 1, and fig. 2(b) is a graph illustrating the effect of light confinement in the vertical direction of the active layer of the quantum cascade laser according to comparative example 2.
In FIGS. 2(a) and (b), the vertical axes represent the refractive index n and the light intensity ILAnd the horizontal axis is the vertical position Z. In the quantum cascade laser of comparative example 1 shown in fig. 2(a), the active layer 124 is sandwiched between the 1 st cladding layer 123 and the 2 nd cladding layer 126, and no optical guiding layer having an effective refractive index higher than the refractive index of the injection quantum well region and the refractive index of the light-emitting quantum well region is provided. Therefore, the light confinement effect is weak, and the light intensity ILExpands into the active layer 124. Therefore, the threshold current cannot be reduced, and it is difficult to increase the output.
In the quantum cascade laser according to comparative example 2 shown in fig. 2(b), the light guiding layer 125 having a refractive index higher than the effective refractive index of each light-emitting quantum well region is provided between the active layer 124 and the 1 st cladding layer 123 and between the active layer 124 and the 2 nd cladding layer 126. Since the light guiding layer 125 is provided, the light confinement effect can be enhanced as compared with comparative example 1, but the light confinement effect is weaker as compared with embodiment 1 in which the light guiding layer 125 is disposed at the center portion. Further, with respect to the light confinement effect, a detailed description is given later based on the simulation result.
Fig. 3(a) is a schematic perspective view of the quantum cascade laser according to embodiment 1, and fig. 3(b) is a schematic cross-sectional view taken along line a-a.
The quantum cascade laser may further include a substrate 10, and a stacked body 20 in which crystals are grown by MOCVD (Metal Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy) may be further provided on the substrate 10. The laminate 20 is processed into a stripe-shaped ridge waveguide RG. Dielectric layers 40 and 42 are provided so as to cover the substrate 10 and the laminate 20. The dielectric layers 40 and 42 are opened so that the upper surface of the laminate 20 is exposed, and the 1 st electrode 50 is provided. In addition, a 2 nd electrode 52 is provided on the back surface of the substrate 10.
The width WA of the ridge waveguide RG is set to 10 μm to 50 μm, for example. In fig. 3(b), the cross section of the ridge waveguide RG is shown as a rectangle having side surfaces 20a and 20 b. However, the shape of the ridge waveguide RG is not limited to this, and may be a mesa shape.
The laminate 20 includes, in the following order from the substrate 10 side: a base layer 21, a contact layer 22, a 1 st clad layer 23, an active layer 24a, a light guiding layer 25, an active layer 24b, a 2 nd clad layer 26, and a contact layer 27.
As shown in fig. 3(a), laser light 60 having a wavelength of infrared rays to terahertz waves is emitted from an end surface of the active layer 24 along an optical axis 62.
Next, a light intensity distribution and a threshold current obtained by simulation will be described with respect to a specific example. The substrate 10 contains InP or GaAs, and the light-emitting quantum well region 80 contains InxGa1-xWell layer of As (0 < x < 1) and containing InyAl1-yAnd barrier layer of As (0 < y < 1). In addition, the optical guiding layer 25 contains InzGa1-zAs(0＜z＜1)。
Fig. 4(a) is a graph showing a simulation result of a relative light intensity distribution of specific example 1 of embodiment 1, and fig. 4(b) is a graph showing a simulation result of a dependency of a threshold current on a thickness of an optical guiding layer.
In fig. 4(a), the vertical axis represents the refractive index (solid line) n or the light intensity (broken line) ILAnd the horizontal axis represents the vertical position Z.
Table 1 shows a unit laminate 100 of example 1.
[ TABLE 1 ]
The laminate 20 includes, from the substrate 10 side, in the following order: a base layer 21 (thickness: 3 μm, refractive index: 2.97) containing n-type InP; a contact layer 22 comprising n-shaped InGaAs (thickness: 1 μm, refractive index: 3.34); a 1 st cladding layer 23 (thickness: 2.5 μm, refractive index: 3.09) containing n-type InP; an active layer 24a (thickness 1.19 μm, refractive index: 3.26); a light guiding layer 25 containing InGaAs (thickness: 0.3 μm, refractive index: 3.4); an active layer 24b (thickness 1.19 μm, refractive index: 3.26); a 2 nd cladding layer 26 (thickness: 2.5 μm, refractive index 3.09) containing n-shaped InP; and a contact layer 27 (thickness: 1 μm, refractive index: 3.34) comprising n-shaped InGaAs. The base layer 21 may be a part of the substrate 10. In addition, In is used for the contact layers 22 and 27, the optical guiding layer 25, the active layers 24a and 24b, and the likexGa1-xIn the case of As (0 < x < 1), when the molar ratio x is set to about 0.532, lattice matching with InP becomes easy and crystallinity becomes good.
As shown in fig. 1(b), the light-emitting quantum well region 80 can have two or more well layers. The thicker well layer thickness TW is set to 4.5nm or the like, for example. The thickness TG of the optical guiding layer 25 is 0.3 μm, which is sufficiently thicker than the well layer thickness TW. Therefore, the optical transition between the sub-bands does not occur in the optical guiding layer 25. That is, electrons as the carriers 102 are confined to the well layer of the light emitting quantum well region 80. On the other hand, light is confined to the vicinity of the light guiding layer 25 having a refractive index higher than the effective refractive index of the light emitting quantum well layer 80. When the well layer thickness TW is 4.5nm, the peak of the gain is in the infrared band of 3.7 μm or 4.3 μm or so.
As shown in FIG. 4(a), light intensity ILThe width (stacking direction) of the laser beam, which is one-half of the peak value, can be narrowed to approximately 1.8 μm. Therefore, as shown in point A of FIG. 4(b), the threshold current can be as low as approximately 0.392kA/cm2. Therefore, quantum efficiency and light output can be improved. Further, strain may be introduced into the luminescence quantum well region 80.For example, the well layer can be In0.669Ga0.331As (lattice constant larger than that of InP), and In is used for the barrier layer0.362Al0.638As (lattice constant is smaller than that of InP), and the like. In this way, since the strain is compensated for between the well layer and the barrier layer, for example, the threshold current can be further reduced while maintaining the crystallinity well.
Fig. 5 is a graph showing the simulation result of the relative light intensity distribution of comparative example 2.
In fig. 5, the vertical axis represents the refractive index (solid line) n or the light intensity (broken line) ILAnd the horizontal axis represents the position Z in the vertical direction.
Table 2 shows a unit laminate of comparative example 2.
[ TABLE 2 ]
The laminate includes, in the following order from the substrate side: a base layer 121 (thickness: 3 μm, refractive index: 2.97) containing n-shaped InP; a contact layer 122 composed of n-shaped InGaAs (thickness: 1 μm, refractive index: 3.34); a 1 st cladding layer 123 (thickness: 2.5 μm, refractive index: 3.09) comprising n-shaped InP; a light guiding layer 125a (thickness: 0.3 μm, refractive index: 3.40); an active layer 124 (thickness 2.38 μm, refractive index: 3.26); a light guiding layer 125b (thickness: 0.3 μm, refractive index: 3.40); a 2 nd cladding 126 (thickness: 2.5 μm, refractive index 3.09) comprising n-shaped InP; and a contact layer 127 (thickness: 1.0 μm, refractive index: 3.34) comprising n-shaped InGaAs.
Light intensity ILThe width (stacking direction) of the laser beam, which is one-half of the peak value, is increased to approximately 2.29 μm. Thus, the threshold current is as high as approximately 0.498kA/cm2The quantum efficiency and the light output are reduced as compared with example 1.
Fig. 6(a) is a graph showing a simulation result of a relative light intensity distribution of specific example 2 of embodiment 1, and fig. 6(b) is a graph showing a simulation result of a dependency of a threshold current on a thickness of an optical guiding layer.
In fig. 6(a), the vertical axis represents the refractive index (solid line) n or the light intensity (broken line) ILAnd the horizontal axis represents the vertical position Z.
Table 3 shows a unit laminate 100 of example 2.
[ TABLE 3 ]
The laminate 20 is arranged from the substrate 10 side: a base layer 21 (thickness: 3 μm, refractive index: 2.97) comprising n-type InP; a 1 st cladding layer 23 (thickness: 2.5 μm, refractive index: 3.09) containing n-type InP; an active layer 24a (thickness 1.19 μm, refractive index: 3.26); a light guide layer 25 (thickness: 0.3 μm, refractive index: 3.4); an active layer 24b (thickness 1.19 μm, refractive index: 3.26); a 2 nd cladding layer 26 (thickness: 2.5 μm, refractive index 3.09) containing n-shaped InP; a contact layer 27 comprising n-shaped InGaAs (thickness: 0.1 μm, refractive index: 3.34).
As shown in FIG. 6(a), light intensity ILThe width (stacking direction) of the laser beam, which is one-half of the peak value, can be narrowed to approximately 1.8 μm. Therefore, as shown by point B in fig. 6 (B). The threshold current can be as low as approximately 0.384kA/cm2. Therefore, quantum efficiency and light output can be improved.
Fig. 7(a) is a graph showing a simulation result of a relative light intensity distribution of specific example 3 of embodiment 1, and fig. 7(b) is a graph showing a simulation result of a dependency of a threshold current on a thickness of an optical guiding layer.
In fig. 7(a), the vertical axis represents the refractive index (solid line) n or the light intensity (broken line) ILAnd the horizontal axis represents the vertical position Z.
Table 4 shows a unit laminate of example 3.
[ TABLE 4 ]
The laminate 20 includes, from the substrate 10 side, in the following order: a base layer 21 (thickness: 3 μm, refractive index: 2.97) containing n-type InP; a 1 st cladding layer 23 (thickness: 2.5 μm, refractive index: 3.09) containing n-type InP; an active layer 24a (thickness 0.69 μm, refractive index: 3.29); a light guiding layer 25 (thickness: 0.3 μm, refractive index: 3.42); an active layer 24b (thickness 0.69 μm, refractive index: 3.29); a 2 nd cladding layer 26 (thickness: 2.5 μm, refractive index 3.09) containing n-shaped InP; and a contact layer 27 (thickness: 0.1 μm, refractive index: 3.37) comprising n-shaped InGaAs.
As shown in FIG. 7(a), light intensity ILThe width (stacking direction) of the laser beam, which is one-half of the peak value, can be narrowed to approximately 1.27 μm. Therefore, as shown by point C in FIG. 7(b), the threshold current can be reduced to approximately 0.206kA/cm2. Therefore, quantum efficiency and light output can be improved.
A quantum cascade laser with a reduced threshold current is provided by the present embodiment. By setting the threshold current to a low value, the quantum efficiency and the optical output of the quantum cascade laser can be improved. These quantum cascade lasers can be widely used for gas analysis, environmental measurement, hazardous substance detection, and the like.
While several embodiments of the present invention have been described, these embodiments have been presented by way of example, and are not intended to limit the scope of the invention. These new embodiments can be implemented in other various ways, and various omissions, substitutions, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalent scope thereof.
Claims (9)
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