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CN-113471815-B - Quantum cascade laser

CN113471815BCN 113471815 BCN113471815 BCN 113471815BCN-113471815-B

Abstract

The QCL includes a semiconductor substrate and an active layer provided on the semiconductor substrate. The active layer has a cascade structure in which a unit stack including a light-emitting layer that generates light and an injection layer that transports electrons from the light-emitting layer is stacked in multiple stages. The light-emitting layer and the injection layer each have a quantum well structure in which quantum well layers and barrier layers are alternately stacked. A separation layer including a separated quantum well layer having a layer thickness smaller than the average layer thickness of the quantum well layers contained in the light-emitting layer and smaller than the average layer thickness of the quantum well layers contained in the injection layer is provided between the light-emitting layer and the injection layer in the unit stack.

Inventors

  • FUJITA KAZUUE
  • HIDAKA MASAHIRO

Assignees

  • 浜松光子学株式会社

Dates

Publication Date
20260508
Application Date
20210329
Priority Date
20200330

Claims (20)

  1. 1. A quantum cascade laser, comprising: a substrate; an active layer disposed on the substrate, The active layer has a cascade structure in which a plurality of stages of unit laminated bodies including a light-emitting layer that generates light and an injection layer that transports electrons from the light-emitting layer are laminated, The light-emitting layer and the injection layer are respectively provided with a quantum well structure formed by alternately stacking quantum well layers and barrier layers, A separation layer including a separation quantum well layer as the quantum well layer having a layer thickness smaller than an average layer thickness of the quantum well layer contained in the light-emitting layer and smaller than an average layer thickness of the quantum well layer contained in the injection layer is provided between the light-emitting layer and the injection layer in the unit stack, The unit stack has an upper emission level, a lower emission level, and a nonlinear level caused by a base level of the separation quantum well layer in a sub-band level structure according to the quantum well structure, An anticrossing energy gap between a low energy level, which is the lowest energy level in a first unit stack, which is the unit stack, and the light-emitting upper energy level in a second unit stack, which is the unit stack, arranged at a later stage of the first unit stack, is set to be larger than an anticrossing energy gap between the low energy level and the nonlinear energy level in the second unit stack.
  2. 2. The quantum cascade laser of claim 1, wherein, The layer thickness of the separated quantum well layer is smaller than the layer thickness of a first quantum well layer adjacent to the separated quantum well layer in the quantum well layer contained in the light emitting layer, and smaller than the layer thickness of a second quantum well layer adjacent to the separated quantum well layer in the quantum well layer contained in the injection layer.
  3. 3. The quantum cascade laser of claim 2, wherein, The layer thickness of the separated quantum well layer is 1/2 or less of the layer thickness of the first quantum well layer and 1/2 or less of the layer thickness of the second quantum well layer.
  4. 4. The quantum cascade laser of claim 1, wherein, The energy interval between the luminescence upper level and the nonlinear level is set smaller than the energy E LO of the longitudinal optical phonon.
  5. 5. The quantum cascade laser of claim 2, wherein, The energy interval between the luminescence upper level and the nonlinear level is set smaller than the energy E LO of the longitudinal optical phonon.
  6. 6. The quantum cascade laser of claim 3, wherein, The energy interval between the luminescence upper level and the nonlinear level is set smaller than the energy E LO of the longitudinal optical phonon.
  7. 7. The quantum cascade laser according to any of claims 1-6, wherein, In the unit laminate, the separation quantum well layer is formed of any one of the 4 th to 6 th quantum well layers from the foremost quantum well layer.
  8. 8. The quantum cascade laser according to any of claims 1-6, wherein, The unit laminate is configured to generate terahertz waves of a first frequency ω 1 and a second frequency ω 2 , which are mid-infrared light, and a difference frequency ω THz of the first frequency ω 1 and the second frequency ω 2 by a double resonance process in which the light emission upper level, the light emission lower level, and the nonlinear level resonate.
  9. 9. The quantum cascade laser of claim 7, wherein, The unit laminate is configured to generate terahertz waves of a first frequency ω 1 and a second frequency ω 2 , which are mid-infrared light, and a difference frequency ω THz of the first frequency ω 1 and the second frequency ω 2 by a double resonance process in which the light emission upper level, the light emission lower level, and the nonlinear level resonate.
  10. 10. The quantum cascade laser according to any of claims 1-6, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the average layer thickness of the barrier layer contained in the light-emitting layer and smaller than the average layer thickness of the barrier layer contained in the injection layer.
  11. 11. The quantum cascade laser of claim 7, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the average layer thickness of the barrier layer contained in the light-emitting layer and smaller than the average layer thickness of the barrier layer contained in the injection layer.
  12. 12. The quantum cascade laser of claim 8, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the average layer thickness of the barrier layer contained in the light-emitting layer and smaller than the average layer thickness of the barrier layer contained in the injection layer.
  13. 13. The quantum cascade laser of claim 9, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the average layer thickness of the barrier layer contained in the light-emitting layer and smaller than the average layer thickness of the barrier layer contained in the injection layer.
  14. 14. The quantum cascade laser according to any of claims 1-6, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.
  15. 15. The quantum cascade laser of claim 7, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.
  16. 16. The quantum cascade laser of claim 8, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.
  17. 17. The quantum cascade laser of claim 9, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.
  18. 18. The quantum cascade laser of claim 10, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.
  19. 19. The quantum cascade laser of claim 11, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.
  20. 20. The quantum cascade laser of claim 12, wherein, The separation layer includes separation barrier layers as the barrier layers arranged on both sides of the separation quantum well layers in a stacking direction of the unit stack, The layer thickness of the separation barrier layer is smaller than the layer thickness of a first barrier layer adjacent to the separation barrier layer among the barrier layers included in the light-emitting layer and the layer thickness of a second barrier layer adjacent to the separation barrier layer among the barrier layers included in the injection layer.

Description

Quantum cascade laser Technical Field The present disclosure relates to a quantum cascade laser. Background A quantum cascade laser (hereinafter, referred to as "QCL") is known in which a terahertz wave of a third frequency ω3 (= |ω 1－ω2 |) which is a frequency Difference between a first frequency ω 1 and a second frequency ω 2 is generated by Difference frequency generation (DFG: difference-Frequency Generation) (for example, refer to patent document 1 (japanese patent No. 2010-521815)). Disclosure of Invention In the QCL described above, pump light components of two wavelengths (ω 1、ω2) as mid-infrared light are generated, and terahertz waves are generated by nonlinear optical effects (NL: nonlinear mixing) and Difference Frequency Generation (DFG) inside the QCL. As a method of using such a technique and having a high mid-infrared-terahertz conversion efficiency (high second-order nonlinear susceptibility χ (2)), for example, a combined double-upper-state (DAU) structure designed to sufficiently inject carriers (electrons) into both of the two light-emitting upper-level stages is known. However, the conventional DAU structure has a problem that the continuous operation at room temperature is difficult because the threshold current density required for laser oscillation is high. It is therefore an object of an aspect of the present disclosure to provide a quantum cascade laser capable of reducing a threshold current density. An aspect of the present disclosure provides a quantum cascade laser including a substrate, and an active layer provided on the substrate, the active layer having a cascade structure in which a unit stack including a light-emitting layer that generates light and an injection layer that transports electrons from the light-emitting layer is stacked in multiple stages, the light-emitting layer and the injection layer each having a quantum well structure in which a quantum well layer and a barrier layer are alternately stacked, and a separation layer provided between the light-emitting layer and the injection layer in the unit stack, the separation layer including a separation quantum well layer as a quantum well layer having a layer thickness smaller than an average layer thickness of the quantum well layer included in the light-emitting layer and smaller than an average layer thickness of the quantum well layer included in the injection layer. In the quantum cascade laser, a separation layer is provided between the light-emitting layer and the injection layer in each unit stack constituting the active layer. The separation layer includes a separation quantum well layer having a layer thickness smaller than an average layer thickness of the quantum well layer contained in the light-emitting layer and an average layer thickness of the quantum well layer contained in the injection layer. According to such a split quantum well layer, a nonlinear energy level contributing to nonlinear optical effects can be formed in the subband energy level structure of the unit stack based on the quantum well structure. In addition, since the amount of injection of carriers (electrons) from the injection layer of the unit stack of the preceding stage to the nonlinear energy stage is small, the number of carriers of the nonlinear energy stage is suppressed to be low. As a result, compared with the conventional DAU structure in which electrons are positively injected into both of the two emission upper potential levels, the threshold current density required for laser oscillation in the active layer can be reduced. The layer thickness of the separated quantum well layer may be smaller than the layer thickness of a first quantum well layer adjacent to the separated quantum well layer among the quantum well layers included in the light-emitting layer and smaller than the layer thickness of a second quantum well layer adjacent to the separated quantum well layer among the quantum well layers included in the injection layer. The layer thickness of the split quantum well layer may be 1/2 or less of the layer thickness of the first quantum well layer, and 1/2 or less of the layer thickness of the second quantum well layer. According to this structure, the efficiency of transporting electrons from the light-emitting layer to the injection layer via the separation layer can be improved. As a result, the efficiency of laser oscillation can be improved. The unit stack may have a light emission upper level, a light emission lower level, and a nonlinear level due to a base level of the separation quantum well layer in a subband level structure based on the quantum well structure. According to the above structure, the threshold current density can be reduced and the second-order nonlinear susceptibility χ (2) can be improved by separating the nonlinear energy level formed by the base energy level of the quantum well layer from the light-emitting upper energy level and the light-emitting lower energy level. The energy int