JP2010238711A - Quantum-cascade laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum-cascade laser having a short wavelength, a narrow beam width, and less restriction of material, and capable of performing ultrafast modulation. <P>SOLUTION: The quantum-cascade laser includes an active layer in which a multiquantum well 12 and a semiconductor intermediate layer 14 are alternately laminated. A carrier passing through a miniband included in a conductive band of a first multiquantum well comes into collision with a minigap 13 having an energy level higher than that of a barrier layer of a second multiquantum well arranged at the side lower than the first multiquantum well to transit to the minigap included in the conductive band of the second multiquantum well for light emission. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a quantum cascade laser, and more particularly to a quantum cascade laser having a quantum well structure and utilizing transition of carriers between minibands.

  Conventionally, a semiconductor laser element in an infrared region having an oscillation wavelength exceeding 3 μm did not oscillate unless cooled below room temperature. In 1994, as a semiconductor laser in the infrared region, a quantum cascade laser element having an oscillation wavelength of 4 μm or more and using intersubband transition in the conduction band of a quantum well was announced (Non-patent Document 1). Thereafter, quantum cascade lasers having an oscillation wavelength of 4 μm or more and 130 μm (2.31 terahertz (THz)) have been announced.

  In a normal semiconductor laser, electrons in the conduction band transition within the forbidden band and combine with holes in the valence band, so that light is emitted and laser oscillation occurs due to resonance. In the quantum cascade laser, light is emitted when only electrons in the conduction band or only holes in the valence band transition between subbands in the conduction band, and laser oscillation occurs due to resonance. Quantum cascade lasers are classified as unipolar devices for electron or hole only device operation.

  Patent Document 1 discloses a quantum cascade laser that uses transitions between minibands. FIG. 6 shows a band profile in the quantum cascade laser proposed in Patent Document 1. The quantum cascade laser has an active layer having a structure in which a plurality of multiple quantum wells (superlattice structure) are stacked with a semiconductor intermediate layer interposed therebetween. Within the conduction band of each multiple quantum well, two minibands where electrons can exist and a minigap that cannot separate electrons between them are formed depending on the materials that make up the quantum well layer and barrier layer. Is done.

  Each miniband is formed as a set of subbands of close energy levels that are formed in the conduction band. For example, when electrons are injected from the upper quantum well layer into the lower quantum well layer in the quantum well, a part of the electrons injected into the miniband is one or more barriers due to the tunnel effect. The layers pass through and are injected into a lower quantum well layer disposed within the same quantum well. If the quantum well layer has a minigap at the same level as the injected electrons, the electrons transition from the injected miniband to the lower miniband in the quantum well layer, and the next barrier. It penetrates the layer and is injected into the adjacent lower quantum well.

  As described above, when electrons injected into the quantum well from the upper side collide with the minigap in the quantum well, the electrons transit from the upper miniband to the lower miniband in the quantum well. The electrons emit light having a wavelength that depends on the amount of energy reduction during the band transition.

  In each miniband, paying attention to the relationship between the energy of the subband and the momentum (Ek space), the energy curvature is the same in the upper subband and the lower subband. For this reason, all the electrons on the subbands in the miniband transition with the same transition energy. That is, the coupled state density becomes a delta function, and not only the optical gain increases greatly, but also the wavelength fluctuation of light emission decreases. For this reason, there is an advantage that the spectral line width of laser oscillation becomes extremely small.

  A normal semiconductor laser element is an interband transition laser element in which a transition between bands occurs between a conduction band and a valence band. In the transition between the bands, the emission recombination lifetime is about several nanoseconds, whereas the transition between the minibands in the quantum cascade laser is extremely fast, about several picoseconds. For this reason, Auger recombination adversely affects the laser characteristics in the interband transition laser element, whereas Auger recombination is hardly a problem in the quantum cascade laser.

  Further, in the interband transition laser element, there is a problem that the interband transition probability of electrons is lowered when a level caused by crystal defects or impurities is formed in the forbidden band. However, in the quantum cascade laser, since the carriers do not transition within the forbidden band, the deterioration of the crystal quality is not a problem. Further, the quantum cascade laser has an advantage that the fluctuation of the oscillation wavelength due to temperature is extremely small because the wavelength of light is determined not between the forbidden bandwidth but between subbands.

JP 10-144959A

J. Faist et al., “Quantum Cascade Laser,” Science 264 (1994), pp.563

  In the quantum cascade lasers reported so far, such as the quantum cascade laser described in Patent Document 1, the oscillation wavelength is limited by the band offset of the quantum well. For this reason, in order to realize laser oscillation with a short wavelength such as an interband transition laser, it is essential to select a material having a larger band offset.

  For example, in order to realize a 1.55 μm band wavelength used in optical communication with a quantum cascade laser, a material having a band offset of at least 0.8 eV or more must be selected. For this reason, there is almost no applicable material except nitride semiconductors such as AlN / GaN and GaN / InN. Also, in semiconductor lasers, defect levels due to interface irregularities and dangling bonds have a large effect on laser characteristics, so a lattice-matching system in which the lattice constants of the quantum well and quantum barrier layer match must be used. There are material restrictions. Furthermore, when a semiconductor laser is composed of an Al-based compound, there is a restriction that Al is not used because oxidation of Al greatly deteriorates the reliability of the laser.

  In view of the above, an object of the present invention is to provide a quantum cascade laser capable of short wavelength, narrow line width, high-speed modulation, and small material constraints.

In order to achieve the above object, the present invention is a quantum cascade laser comprising an active layer in which multiple quantum wells and semiconductor intermediate layers are alternately stacked, and emitting light by transition between minibands of carriers,
The energy of the barrier layer of the second multiple quantum well in which the carriers transmitted through the miniband included in the conduction band of the first multiple quantum well are disposed on the lower level side than the first multiple quantum well. Provided is a quantum cascade laser that emits light by colliding with a mini-gap at an energy level higher than the level and transitioning to a mini-gap included in the conduction band of the second multiple quantum well.

  In the quantum cascade laser according to the present invention, the carrier transmitted through the first multi-quantum well mini-band transitions to the second multi-quantum well mini-band on the lower side of the first multi-quantum well and emits light. It was adopted. For this reason, a large band gap can be obtained compared with the conventional quantum cascade laser using the transition between minibands in which carriers transition in one multi-quantum well. Wavelength light oscillation is possible. Further, since the transition between minibands is used, the advantages of narrow line width and high-speed modulation can be obtained.

  In the quantum cascade laser of the present invention, it is possible to adopt a configuration in which the multiple quantum well includes a quantum well layer containing InGaAs and a barrier layer containing InP. Further, an oscillation wavelength of 1.55 μm or less is possible as the oscillation wavelength.

  In the quantum cascade laser of the present invention, the active layer can be composed of a III-V group compound semiconductor. In this case, the active layer may include a plurality of elements selected from the group consisting of In, Ga, As, P, Sb, and N.

  It is possible to adopt a configuration in which the difference in energy level between the mini-gap of the first multiple quantum well and the mini-gap of the second multiple quantum well is 10 meV or more and 1 eV or less. In this case, oscillation at an optical wavelength used in optical communication is possible.

  It is possible to adopt a configuration in which the active layer forms a slab type waveguide that is easy to manufacture. It is also possible to adopt a configuration in which multiple quantum wells and semiconductor intermediate layers are repeatedly formed in 15 pairs or more. Further, a configuration in which the active layer forms a single plasmon type waveguide or a double plasmon type waveguide is also possible.

The band profile in the active layer which shows the principle of the quantum cascade laser of this invention. The band profile which shows the result of the simulation which demonstrates the feasibility of this invention. Sectional drawing of the quantum cascade laser of the 1st Embodiment of this invention. Sectional drawing of the quantum cascade laser of the 2nd Embodiment of this invention. Sectional drawing of the quantum cascade laser of the 3rd Embodiment of this invention. The band profile in the quantum cascade laser described in Patent Document 1.

  FIG. 1 illustrates a band profile in the active layer, which illustrates the principle of the quantum cascade laser of the present invention. In the quantum cascade laser 10 of the present invention, the laser active layer includes a plurality of stacked multiple quantum wells 12, and a semiconductor intermediate layer 14 is disposed between two adjacent multiple quantum wells 12. In each multiple quantum well 12, a plurality of quantum well layers and a plurality of barrier layers are disposed in pairs. In the upper part of the band profile of each multiple quantum well 12, a mini-gap 13 is formed adjacent to and above the energy level of the barrier layer profile.

  In the above configuration, the electrons 16 injected into the miniband 15 of the multiple quantum well 12 from the high potential side pass through the semiconductor intermediate layer 14 without making a transition in the quantum well 12. The electrons 16 transmitted through the semiconductor intermediate layer 14 collide with the minigap 13 formed at a level higher than the band profile level of the barrier layer of the next multiple quantum well 12, and a level lower than that. Transition to the mini-band 15. By adopting this structure, the transition energy of the electrons 16 becomes larger than the band offset in a normal quantum cascade laser, so that the wavelength of oscillation can be shortened. Therefore, it is possible to realize a quantum cascade laser that oscillates in the 1.55 μm band by using an InGaAs quantum well layer, which has been difficult with conventional quantum cascade lasers.

  FIG. 2 is a band profile showing the results of simulation performed to realize the quantum cascade laser of the present invention. In the figure, two adjacent multiple quantum wells 12A and 12B in the active layer and a semiconductor intermediate layer 14 disposed therebetween are shown. Each wavy line 11 represents a subband, and a set of subbands 11A and 11B constitutes minibands 15A and 15B. The adjacent mini bands 15A and 15B are separated by a mini gap 13. By adjusting the composition of the quantum well layers and the barrier layers constituting the multiple quantum wells 12A and 12B, a mini-gap 13 is formed at the upper level of the barrier layer profile.

  As a result of the simulation, a material having a specific composition can be selected, and the minigap 13 can be arranged at a desired level higher than the energy level of the barrier layer. That is, by appropriately adjusting the material composition of the quantum well layer and the barrier layer, it is possible to form the minigap 13 having the energy level immediately above the energy level of the barrier layer. Further, it is possible to configure so that one miniband 15A, 15B is formed in one multiple quantum well 12A, 12B.

As an example, when a quantum well is formed by combining an InGaAs quantum well layer and an InP barrier layer, the band gap of InP is 1.35 eV, the In composition is 0.48, and the Ga composition is 0.52 (hereinafter referred to as InGaAs). _0.48 Ga _0.52 As)) is 0.75 eV. At this time, as a result of the simulation, as shown in FIG. 2, it has been found that a mini-gap 13 having an energy level at a high level immediately adjacent to the energy level of the barrier layer of the multiple quantum well 12B is formed. For this reason, the electrons 16 injected from the upper multiple quantum well 12A collide with the minigap 13 and transition to the miniband 15B at the next energy level of the multiple quantum well 12B. From this simulation, the feasibility of the quantum cascade laser of the present invention was shown.

Other combinations of quantum wells include a barrier layer made of Al _0.3 Ga _0.7 As with a band gap of 1.63 eV and a quantum well layer made of GaAs with a band gap of 1.42 eV (hereinafter referred to as Al _0.3 Ga _0.7 As (1.63 eV) / GaAs (1.42 eV).), In _0.53 Al _0.47 As (1.42 eV) / Ga _0.46 In _0.54 As (0.86 eV), GaN (3.4 eV) / In _0.2 Ga _0.8 N (1.24 eV)
Etc.

First Embodiment FIG. 3 shows a cross-sectional view of a semiconductor laser as a quantum cascade laser according to a first embodiment of the present invention. The semiconductor laser 10A includes an InP substrate (100) 21 constituting a lower cladding layer, an active layer 22, an InP upper cladding layer 23, and an upper electrode (negative electrode) sequentially stacked on the surface of the InP substrate 21. 24 and a lower electrode (positive electrode) 25 formed on the back surface of the InP substrate 21. The active layer 22 includes a plurality of multiple quantum wells 26 and a semiconductor intermediate layer 27 sandwiched between two adjacent multiple quantum wells 26. Each multiple quantum well 26 includes a plurality of pair layers each including an InGaAs quantum well layer and an InP barrier layer.

  The quantum cascade laser 10A is configured as a ridge type laser having a cavity length of 1 mm and having a mesa stripe, and its ridge width is 10 μm. The lower electrode 25 is made of a Ti / Pt / Au layer, and the upper electrode 24 is made of a Pd / Pt / Au layer.

The band gap of both InP clad layers 21 and 23 is 1.35 eV. The InGaAs quantum well layer has an In composition of 0.48 and a Ga composition of 0.52 in order to lattice match this InP. A band gap of In _0.48 Ga _0.52 As having this composition is 0.75 eV. The thickness of the InGaAs quantum well layer is 2 nm, and the thickness of the InP barrier layer is 1 nm. The number of pair layers of the quantum well layer and the barrier layer is, for example, 4 pairs. A plurality of the multiple quantum wells 26 are formed in the active layer 22, and an In _0.48 Ga _0.52 As layer having a thickness of 40 nm is formed as the semiconductor intermediate layer 27 therebetween.

  The total thickness of the active layer 22 is about 800 nm in order to increase the optical confinement factor as much as possible. For this purpose, the number of pairs of each multiple quantum well 26 and the semiconductor intermediate layer 27 is, for example, 15.5 pairs, and the lowermost layer and the uppermost layer are the multiple quantum wells 26. Furthermore, the thickness of the InP upper cladding layer 23 is, for example, 3 μm. The InP lower cladding layer 21 is thinned by lapping after crystal growth of the laser structure. For example, the thickness after lapping is set to about 50 μm in consideration of ease of handling.

Hereinafter, a manufacturing method of the quantum cascade laser 10A of the first embodiment will be described. First, on the lower clad layer 21 constituting the InP substrate, by MOCVD (metal organic chemical vapor deposition), the growth temperature is 600 ° C., and 2 nm-thick InGaAs and 1 nm-thick InP are paired to form four pairs. A multiple quantum well 26 is grown. Next, an In _0.48 Ga _0.52 As layer constituting the semiconductor intermediate layer 27 is grown to a thickness of 40 nm. The growth of the multiple quantum well 26 and the growth of the semiconductor intermediate layer 27 are alternately repeated 15 times.

Next, the InP upper cladding layer 23 is grown to a thickness of 3 μm. As described above, the semiconductor multilayer structure constituting the semiconductor laser 10A is completed. Next, a stripe layer patterned with a SiN _x layer or the like is formed on the surface of the semiconductor multilayer structure, and the semiconductor multilayer structure is patterned using this as a mask. For patterning, a dry etching apparatus is used to form a mesa stripe having a ridge width of 10 μm.

Thereafter, a SiN _x film (not shown) is deposited to a thickness of 120 nm as a passivation film on the surface of the semiconductor multilayer structure patterned as described above by PECVD (plasma chemical vapor deposition). Next, an opening matching the shape of the upper electrode 24 is formed on the SiN _x film by etching.

  Next, the upper electrode 24 made of Pd / Pt / Au is formed on the surface of the upper cladding layer exposed from the opening by using the EB vapor deposition method. Note that vapor deposition may be performed by resistance heating vapor deposition, sputtering, or the like.

  Next, using a lapping apparatus, polishing is performed from the back surface of the InP substrate 21 so that the thickness of the InP substrate 21 becomes 50 μm. Thereafter, in order to form a resonator structure, the substrate including the semiconductor stack is cleaved in a direction perpendicular to the extension direction of the stripe mesa. By this cleavage, a mesa having a resonator length of 1 mm is obtained. Further, the substrate is diced in a direction parallel to the mesa in units where the mesa is sandwiched, and the substrate is separated into individual semiconductor laser elements. If necessary, a dielectric reflection film is formed on the cleavage plane to complete the semiconductor laser 10A of the present embodiment.

Second Embodiment FIG. 4 is a sectional view showing a semiconductor laser comprising a quantum cascade laser according to the second embodiment of the present invention. The semiconductor laser 10B of this embodiment includes a multiple quantum well 26 formed by alternately laminating InGaAs quantum well layers and InP quantum barrier layers on an InP cladding layer 21 constituting a substrate, an InGaAs semiconductor intermediate layer 27, and the like. Are alternately stacked to form the active layer 22. An upper electrode 24 made of Au is deposited on the active layer 22 to form a semiconductor multilayer structure. Such a laminated structure having no cladding layer is called a single plasmon structure and has an advantage that light leakage to the upper part of the laminated layer is extremely small because of a large extinction coefficient in the Au electrode. For this reason, the generated light is efficiently confined in the active layer 22. The semiconductor laser 10B is a ridge type laser having a mesa stripe with a resonator length of 1 mm, for example, and the ridge width is, for example, 10 μm.

  Here, the configuration of the active layer 22 is the same as that of the first embodiment.

  The total thickness of the active layer 22 is about 200 nm in order to increase the optical confinement factor as much as possible. For this reason, the number of pairs of the multiple quantum well 26 and the semiconductor intermediate layer 27 is 3.5 pairs, and the uppermost layer and the lowermost layer are the multiple quantum wells 26. The thickness of the Au upper electrode 24 is 10 nm. The InP cladding layer 21 is thinned by lapping after crystal growth of the laser laminated structure. Considering the ease of handling, the final thickness is about 50 μm.

  Hereinafter, a method for manufacturing the semiconductor laser 10B constituting the quantum cascade laser according to the second embodiment will be described. First, a quantum well structure comprising an InGaAs layer having a thickness of 2 nm and an InP barrier layer having a thickness of 1 nm on an InP substrate constituting the InP clad layer 21 at a growth temperature of 600 ° C. by MOCVD (metal organic chemical vapor deposition). 4 pairs are grown to form a multiple quantum well 26. Next, an InGaAs layer 27 as a semiconductor intermediate layer is grown to a thickness of 40 nm. The growth of the multiple quantum well 26 and the semiconductor intermediate layer 27 is repeated 15 times alternately. In this way, a semiconductor multilayer structure constituting the semiconductor laser is completed.

A striped SiN _x layer is formed on the surface of the semiconductor multilayer structure, and the semiconductor multilayer structure is patterned using this as a mask. For patterning, a mesa stripe having a ridge width of 10 μm is formed using a dry etching apparatus. Thereafter, a SiN _x film is deposited as a passivation film to a thickness of 120 nm on the surface of the semiconductor multilayer structure by using PECVD (plasma chemical vapor deposition). Next, an opening matching the shape of the upper electrode is formed on the SiN _x film.

An Au upper electrode is deposited on the surface of the semiconductor stack exposed in the opening of the SiN _x film by using the EB vapor deposition method. As a vapor deposition method, a resistance heating vapor deposition method, a sputtering method, or the like may be used. Next, the lapping apparatus is used to polish from the back surface of the InP substrate 21 so that the final thickness of the InP substrate becomes 50 μm.

  Thereafter, the substrate is cleaved in a direction perpendicular to the mesa stripe to form a semiconductor laser resonator structure. By this cleavage, a laser having a resonator length of 1 mm is obtained. Further, the semiconductor substrate is separated into individual semiconductor laser elements by performing dicing in a direction parallel to the mesa in units of positions where the mesa is sandwiched. If necessary, a semiconductor laser is completed by forming a dielectric reflection film on the cleavage plane. Through the above steps, the semiconductor laser of the second embodiment can be manufactured.

Third Embodiment FIG. 5 is a cross-sectional view of a semiconductor laser constituting a quantum cascade laser according to a third embodiment of the present invention. In the semiconductor laser 10C, a plurality of multiple quantum wells 26 each including an InGaAs quantum well layer and an InP quantum barrier layer are stacked, and the semiconductor intermediate layer 27 is sandwiched between adjacent multiple quantum wells 26 to form an active layer 22. An Au upper electrode 24 is formed on the active layer 22. An Au lower electrode 25 is formed below the active layer 22. Such a structure having no cladding layer above and below the active layer is called a double plasmon structure. Since the double plasmon structure has a large extinction coefficient in the Au electrodes 24 and 25 sandwiching the upper and lower layers, light leakage to the upper and lower portions is extremely small, and almost all light is confined in the active layer 22. The cavity length of this semiconductor laser is, for example, 1 mm, and is configured as a ridge type laser having a mesa, and the ridge width is, for example, 10 μm.

  Here, the configuration of the active layer 22 is the same as that of the first embodiment.

  The total thickness of the active layer 22 is about 200 nm in order to increase the optical confinement factor as much as possible. For this purpose, the number of pairs of the multiple quantum well 26 and the semiconductor intermediate layer 27 is 3.5. The thickness of the upper and lower Au electrodes is, for example, 50 nm.

  Hereinafter, a method for manufacturing the semiconductor laser constituting the quantum cascade laser of FIG. 5 will be described. First, a multiquantum well structure including four pairs of a pair of InGaAs quantum well layers having a thickness of 1 nm and an InP barrier layer having a thickness of 2 nm is formed on an InP substrate by MOCVD (metal organic chemical vapor deposition) at a growth temperature of 600 ° C. 4 pairs grow. Next, an InGaAs semiconductor intermediate layer is grown to a thickness of 40 nm. The growth of these multiple quantum wells and the semiconductor intermediate layer is repeated 15 times alternately. As described above, the semiconductor multilayer structure constituting the semiconductor laser is completed.

Next, a mixed solution containing hydrochloric acid and phosphoric acid is used as an etchant to remove the InP substrate. In this case, since the InP etching rate is sufficiently faster than the InGaAs rate, when InP is removed, the etching is stopped at the InGaAs layer. A patterned SiN _x stripe layer is formed on the surface of the semiconductor multilayer structure, and patterning is performed using this as a mask to etch the semiconductor multilayer structure. Etching is performed by a dry etching apparatus to form a mesa stripe of a semiconductor laser having a ridge width of 10 μm.

Thereafter, a SiN _x film is deposited as a passivation film to a thickness of 120 nm on the surface of the semiconductor multilayer structure by PECVD (plasma chemical vapor deposition). Next, an opening matching the shape of the upper electrode is formed on the SiN _x film. An Au electrode is formed on the surface of the semiconductor stack exposed in the opening by EB vapor deposition. Note that vapor deposition may be performed by resistance heating vapor deposition, sputtering, or the like.

  Thereafter, the substrate is cleaved in a direction perpendicular to the mesa stripe to form a resonator structure. The cleavage is performed so that the resonator length is 1 mm. Further, the laminated structure is separated into individual semiconductor laser elements by performing dicing in the direction parallel to the mesa in units where the mesa is sandwiched. If necessary, a semiconductor laser is completed by forming a dielectric reflection film on the cleavage plane. Through the above steps, the semiconductor laser according to the third embodiment can be manufactured.

  The present invention provides a quantum cascade laser capable of realizing a semiconductor laser for optical communication with a short wavelength, a narrow line width, and ultra-high speed modulation. The quantum cascade laser of the present invention can be used to realize an increase in transmission capacity and speed in information communication.

  Although the invention has been particularly shown and described with reference to illustrative embodiments, the invention is not limited to these embodiments and variations thereof. It will be apparent to those skilled in the art that various modifications can be made to the present invention without departing from the spirit and scope of the invention as defined in the appended claims.

10: Quantum cascade laser 11: Subband 12: Multiple quantum well 13: Mini gap 14: Semiconductor intermediate layer 15: Mini band 16: Electron 21: Lower cladding layer (InP substrate)
22: Active layer 23: Upper clad layer 24: Upper electrode 25: Lower electrode 26: Multiple quantum well 27: Semiconductor intermediate layer

Claims (9)

A quantum cascade laser comprising an active layer in which multiple quantum wells and semiconductor intermediate layers are alternately stacked, and emitting light by transition of carriers between minibands,
The energy of the barrier layer of the second multiple quantum well in which the carriers transmitted through the miniband included in the conduction band of the first multiple quantum well are disposed on the lower level side than the first multiple quantum well. A quantum cascade laser that emits light by colliding with a mini-gap at an energy level higher than the level and transitioning to a mini-gap included in a conduction band of the second multiple quantum well.   The quantum cascade laser according to claim 1, wherein the multiple quantum well has a quantum well layer containing InGaAs and a barrier layer containing InP.   The quantum cascade laser according to claim 2, having an oscillation wavelength of 1.55 μm or less.   The quantum cascade laser according to any one of claims 1 to 3, wherein the active layer is made of a group III-V compound semiconductor.   The quantum cascade laser according to claim 4, wherein the active layer includes a plurality of elements selected from the group consisting of In, Ga, As, P, Sb, and N.   The energy level difference between the mini-gap of the first multiple quantum well and the mini-gap of the second multiple quantum well is 10 meV or more and 1 eV or less. The quantum cascade laser described.   The quantum cascade laser according to claim 1, wherein the active layer forms a slab waveguide.   The quantum cascade laser according to claim 1, wherein the multiple quantum well and the semiconductor intermediate layer are repeatedly formed by 15 pairs or more.   The quantum cascade laser according to claim 1, wherein the active layer forms a single plasmon waveguide or a double plasmon waveguide.

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