## λ~7.1 μm quantum cascade lasers with 19% wall-plug efficiency at room temperature |

Optics Express, Vol. 19, Issue 18, pp. 17203-17211 (2011)

http://dx.doi.org/10.1364/OE.19.017203

Acrobat PDF (2793 KB)

### Abstract

Strain-balanced In_{0.6}Ga_{0.4}As/Al_{0.56}In_{0.44}As quantum cascade lasers emitting at a wavelength of 7.1 μm are reported. The active region is based on a three-phonon-resonance quantum design with a low voltage defect of 120 meV at injection resonance. A maximum wall-plug efficiency of 19% is demonstrated in pulsed mode at 293 K. Continuous-wave output power of 1.4 W and wall-plug efficiency of 10% are measured at the same temperature, as well as 1.2 W of average power in uncooled operation. A model for backfilling of the lower laser level which takes into account the number of subbands in the injector is presented and applied to determine the optimum value of the voltage defect to maximize wall-plug efficiency at room temperature, which is found to be ~100 meV, in good agreement with experimental results.

© 2011 OSA

## 1. Introduction

1. A. Lyakh, R. Maulini, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, Q. J. Wang, F. Capasso, and C. K. N. Patel, “3 W continuous-wave room temperature single-facet emission from quantum cascade lasers based on nonresonant extraction design approach,” Appl. Phys. Lett. **95**(14), 141113 (2009). [CrossRef]

2. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. **98**(18), 181102 (2011). [CrossRef]

2. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. **98**(18), 181102 (2011). [CrossRef]

3. Y. Yao, X. Wang, J.-Y. Fan, and C. F. Gmachl, “High performance ‘continuum-to-continuum’ quantum cascade lasers with a broad gain bandwidth of over 400 cm^{−1},” Appl. Phys. Lett. **97**(8), 081115 (2010). [CrossRef]

4. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. **95**(15), 151112 (2009). [CrossRef]

5. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, and C. K. N. Patel, “High average power uncooled mid-wave infrared quantum cascade lasers,” Electron. Lett. **47**(6), 395 (2011). [CrossRef]

6. A. Bismuto, R. Terazzi, M. Beck, and J. Faist, “Electrically tunable, high performance quantum cascade laser,” Appl. Phys. Lett. **96**(14), 141105 (2010). [CrossRef]

7. M. Troccoli, X. Wang, and J. Fan, “Quantum cascade lasers: high-power emission and single-mode operation in the long-wave infrared (λ*>*6 μm),” Opt. Eng. **49**(11), 111106 (2010). [CrossRef]

9. J. S. Yu, S. Slivken, and M. Razeghi, “Injector doping level-dependent continuous-wave operation of InP-based QCLs at λ ~ 7.3 μm above room temperature,” Semicond. Sci. Technol. **25**(12), 125015 (2010). [CrossRef]

7. M. Troccoli, X. Wang, and J. Fan, “Quantum cascade lasers: high-power emission and single-mode operation in the long-wave infrared (λ*>*6 μm),” Opt. Eng. **49**(11), 111106 (2010). [CrossRef]

4. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. **95**(15), 151112 (2009). [CrossRef]

10. A. Lyakh, R. Maulini, A. Tsekoun, R. Go, S. Von der Porten, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High-performance continuous-wave room temperature 4.0-μm quantum cascade lasers with single-facet optical emission exceeding 2 W,” Proc. Natl. Acad. Sci. U.S.A. **107**(44), 18799–18802 (2010). [CrossRef]

_{w}, and the voltage defect Δ, defined as the energy difference between the lower laser level of one gain stage and the upper laser level of the next one, which, in this spectral range, is comparable to the photon energy. We address waveguide losses by growing a thick, low-doped dielectric waveguide. To optimize the voltage defect, we introduce a model for the backfilling of the lower laser level which, unlike the conventional model [11

11. J. Faist, “Wallplug efficiency of quantum cascade lasers: critical parameters and fundamental limits,” Appl. Phys. Lett. **90**(25), 253512 (2007). [CrossRef]

## 2. Laser design and fabrication

**×**10

^{18}cm

^{−3}) in a single growth step. The epitaxial layer sequence, starting from the substrate, was as follows: InP cladding layer (

*n*= 1

**×**10

^{17}cm

^{−3}, thickness 2.5 μm), InP cladding layer (3

**×**10

^{16}cm

^{−3}, 2.5 μm), 45-stage strain-balanced In

_{0.6}Ga

_{0.4}As/Al

_{0.56}In

_{0.44}As active region (2.43 μm), InP cladding layer (3

**×**10

^{16}cm

^{−3}, 2.5 μm), InP cladding layer (1

**×**10

^{17}cm

^{−3}, 2.5 μm), InP plasmon layer (8

**×**10

^{18}cm

^{−3}, 1.5 μm), and a heavily-doped InGaAs contact layer (0.2 μm). Waveguide simulations resulted in an optical confinement factor of 0.76 and waveguide losses of 0.65 cm

^{−1}(not taking into account intersubband absorption in the active region) at a wavelength of 7.1 μm for 10 μm-wide lasers processed in buried heterostructure geometry. The voltage drop across the low doped InP cladding layers is estimated to be 28 mV/(kA/cm

^{2}), assuming an electron mobility of 5000 cm

^{2}V

^{−1}s

^{−1}.

13. A. Tsekoun, R. Go, M. Pushkarsky, M. Razeghi, and C. K. N. Patel, “Improved performance of quantum cascade lasers through a scalable, manufacturable epitaxial-side-down mounting process,” Proc. Natl. Acad. Sci. U.S.A. **103**(13), 4831–4835 (2006). [CrossRef] [PubMed]

## 3. Laser characterization

### 3.1 Low-duty-cycle pulsed operation

^{2}, respectively. The slope efficiency is 3.59 W/A, and the maximum WPE 18.9%. This is in excess of the room temperature wall-plug efficiency limit of 18% predicted by Faist at this wavelength, for a dephasing time of 70 fs [11

11. J. Faist, “Wallplug efficiency of quantum cascade lasers: critical parameters and fundamental limits,” Appl. Phys. Lett. **90**(25), 253512 (2007). [CrossRef]

11. J. Faist, “Wallplug efficiency of quantum cascade lasers: critical parameters and fundamental limits,” Appl. Phys. Lett. **90**(25), 253512 (2007). [CrossRef]

*T*

_{0}= 158 K and

*T*

_{1}= 441 K for threshold current and slope efficiency, respectively. These values are comparable to those previously reported in the literature for high performance QCLs operating at similar wavelengths [7

7. M. Troccoli, X. Wang, and J. Fan, “Quantum cascade lasers: high-power emission and single-mode operation in the long-wave infrared (λ*>*6 μm),” Opt. Eng. **49**(11), 111106 (2010). [CrossRef]

9. J. S. Yu, S. Slivken, and M. Razeghi, “Injector doping level-dependent continuous-wave operation of InP-based QCLs at λ ~ 7.3 μm above room temperature,” Semicond. Sci. Technol. **25**(12), 125015 (2010). [CrossRef]

*N*

_{p}–

*h*ν, where V is the voltage drop across the entire structure,

*N*

_{p}= 45 is the number of gain stages, and

*h*ν = 175 meV is the photon energy, as a function of current. The voltage drop across the InP cladding layers (~0.1 V at

*J*= 4 kA/cm

^{2}) represents only ~1% of the total voltage and, therefore, was neglected in the voltage defect calculation. The voltage defect at threshold, maximum WPE, and roll-over is measured to be ~45, 95, and 120 meV, respectively.

4. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. **95**(15), 151112 (2009). [CrossRef]

_{w}= 1.7 ± 0.2 cm

^{−1}. The difference between the experimental value and waveguide simulations is attributed to sidewall roughness and non-resonant intersubband absorption in the active region, which were not taken into account in our simulations.

### 3.2 Continuous-wave operation

13. A. Tsekoun, R. Go, M. Pushkarsky, M. Razeghi, and C. K. N. Patel, “Improved performance of quantum cascade lasers through a scalable, manufacturable epitaxial-side-down mounting process,” Proc. Natl. Acad. Sci. U.S.A. **103**(13), 4831–4835 (2006). [CrossRef] [PubMed]

**95**(15), 151112 (2009). [CrossRef]

_{m}as for a 1.5 mm-long HR/uncoated chip, i.e. α

_{m}= 4.3 cm

^{−1}, which for a 4 mm-long device requires a reflectivity of ~3%. Cw output power, voltage, and WPE as function of current of a 4 mm

**×**8 μm laser at a temperature of 293 K are shown in Fig. 3 . Data were corrected for the lens collection efficiency which is equal to 0.9. A threshold current density of 2.12 kA/cm

^{2}, slope efficiency of 2.81 W/A, maximum power of 1.38 W and maximum WPE of 10.0% are measured at this temperature. The inset in Fig. 3 shows the laser spectrum in cw mode at 293 K at a current of 1.12 A. The spectrum is centered at 7.14 μm under these operating conditions. The significant increase in threshold current and decrease in slope efficiency and WPE observed between pulsed and cw modes indicates that the doping level is too high for optimum cw operation. This is also confirmed by the fact that the power roll-over in cw operation occurs at a current density of ~3.9 kA/cm

^{2}, which is ~30% lower than in pulsed mode. Cw performance will be improved by regrowing the structure with a lower active region doping.

_{00}). The measured divergence half-angles at 1/

*e*

^{2}of the collimated beam are 1.6 mrad and 1.5 mrad along the horizontal (x) and vertical (y) axes, respectively.

### 3.3 High-duty-cycle uncooled operation

**95**(15), 151112 (2009). [CrossRef]

5. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, and C. K. N. Patel, “High average power uncooled mid-wave infrared quantum cascade lasers,” Electron. Lett. **47**(6), 395 (2011). [CrossRef]

5. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, and C. K. N. Patel, “High average power uncooled mid-wave infrared quantum cascade lasers,” Electron. Lett. **47**(6), 395 (2011). [CrossRef]

## 4. Backfilling model

**90**(25), 253512 (2007). [CrossRef]

14. S. S. Howard, Z. Liu, D. Wasserman, A. J. Hoffman, T. S. Ko, and C. F. Gmachl, “High-performance quantum cascade lasers: optimized design through waveguide and thermal modeling,” IEEE J. Sel. Top. Quantum Electron. **13**(5), 1054–1064 (2007). [CrossRef]

*n*

_{therm}=

*n*

_{s}exp(-Δ/

*kT*), where

*n*

_{s}is the sheet carrier density per gain stage. This implicitly assumes a constant density of states in the injector, i.e. an injector consisting of a single subband. Here we introduce a more refined model, which takes into account the number of subbands in the injector.

*E*

_{inj}= Δ /

*N*

_{inj}, where

*N*

_{inj}is the number of injector subbands ( = numbers of subbands below the lower laser level, per gain stage). This is a good approximation of a typical energy level distribution in a QCL injector (see Fig. 1) and it has the advantages that it does not require to know the exact positions of the levels and that it introduces only one additional parameter (

*N*

_{inj}) compared to the traditional model. Neglecting non-parabolicity, the two-dimensional density of states can be written as:where

*D*

_{0}is the density of states of one subband and

*θ*is the Heaviside step function. Assuming, as in the conventional model, a thermal distribution in the injector, the carrier density per unit energy and area iswhere

*f*(

*E*) is the Fermi-Dirac distribution and

*f*(

*E*) can be approximated by the Boltzmann distribution exp(-

*E*/

*kT*). The lower laser level backfilling is calculated as:where the 1/(

*N*

_{inj}+ 1) factor accounts for the degeneracy of the energy states due to the presence of multiple subbands. Calculations can be performed analytically in the case of the Boltzmann distribution, resulting in the following formula for backfilling:

*N*

_{inj}

*kT*, our formula tends towards the single-subband approximation:

*n*

_{therm}≈

*n*

_{s}exp(-Δ/

*kT*), because only the lowest injector subband is significantly populated. However, this limit does not correspond to a practical case at room temperature. At

*T*= 300 K, for instance, it corresponds to Δ >> 400 meV.

*n*

_{therm}/

*n*

_{s}as a function of Δ at

*T*= 300 K obtained with this model and with the usual approximation are plotted in the inset in Fig. 6 . Comparing the two, we find that, for the design presented in this paper in which

*N*

_{inj}= 8, the single-subband approximation overestimates the backfilling by factors of ~2, 2.5, and 4 for Δ = 150, 100, and 50 meV, respectively.

*N*

_{inj}= 8, using the same numerical parameters as in [11

**90**(25), 253512 (2007). [CrossRef]

**90**(25), 253512 (2007). [CrossRef]

14. S. S. Howard, Z. Liu, D. Wasserman, A. J. Hoffman, T. S. Ko, and C. F. Gmachl, “High-performance quantum cascade lasers: optimized design through waveguide and thermal modeling,” IEEE J. Sel. Top. Quantum Electron. **13**(5), 1054–1064 (2007). [CrossRef]

2. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. **98**(18), 181102 (2011). [CrossRef]

*n*

_{therm}with increasing

*N*

_{inj}. For a typical

*N*

_{inj}of 8, this translates into a significant reduction of the optimum value of Δ, which results in an increased WPE, especially in the LWIR, where the photon energy is comparable to the voltage defect, and in the very long-wave infrared (VLWIR, λ > 12 μm), where the photon energy is smaller than the voltage defect. Quantitatively, a voltage defect reduction from 150 meV to 100 meV corresponds to a relative increase in voltage efficiency of 18% at 7 μm, 22% at 10 μm, and 25% at 12 μm. This is in contrast to the model of [11

**90**(25), 253512 (2007). [CrossRef]

15. S. Katz, A. Vizbaras, G. Boehm, and M.-C. Amann, “Injectorless quantum cascade laser operating in continuous wave above room temperature,” Semicond. Sci. Technol. **24**(12), 122001 (2009). [CrossRef]

16. K. J. Franz, P. Q. Liu, J. Raftery, M. D. Escarra, A. J. Hoffman, S. S. Howard, Y. Yao, Y. Dikmelik, X. Wang, J. Fan, J. B. Khurgin, and C. Gmachl, “Short injector quantum cascade lasers,” IEEE J. Quantum Electron. **46**(5), 591–600 (2010). [CrossRef]

## 5. Conclusion

**90**(25), 253512 (2007). [CrossRef]

## References and links

1. | A. Lyakh, R. Maulini, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, Q. J. Wang, F. Capasso, and C. K. N. Patel, “3 W continuous-wave room temperature single-facet emission from quantum cascade lasers based on nonresonant extraction design approach,” Appl. Phys. Lett. |

2. | Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. |

3. | Y. Yao, X. Wang, J.-Y. Fan, and C. F. Gmachl, “High performance ‘continuum-to-continuum’ quantum cascade lasers with a broad gain bandwidth of over 400 cm |

4. | R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. |

5. | R. Maulini, A. Lyakh, A. Tsekoun, R. Go, and C. K. N. Patel, “High average power uncooled mid-wave infrared quantum cascade lasers,” Electron. Lett. |

6. | A. Bismuto, R. Terazzi, M. Beck, and J. Faist, “Electrically tunable, high performance quantum cascade laser,” Appl. Phys. Lett. |

7. | M. Troccoli, X. Wang, and J. Fan, “Quantum cascade lasers: high-power emission and single-mode operation in the long-wave infrared (λ |

8. | R. P. Leavitt, J. L. Bradshaw, K. M. Lascola, G. P. Meissner, F. Micalizzi, F. J. Towner, and J. T. Pham, “High-performance quantum cascade lasers in the 7.3- to 7.8-μm wavelength band using strained active regions,” Opt. Eng. |

9. | J. S. Yu, S. Slivken, and M. Razeghi, “Injector doping level-dependent continuous-wave operation of InP-based QCLs at λ ~ 7.3 μm above room temperature,” Semicond. Sci. Technol. |

10. | A. Lyakh, R. Maulini, A. Tsekoun, R. Go, S. Von der Porten, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High-performance continuous-wave room temperature 4.0-μm quantum cascade lasers with single-facet optical emission exceeding 2 W,” Proc. Natl. Acad. Sci. U.S.A. |

11. | J. Faist, “Wallplug efficiency of quantum cascade lasers: critical parameters and fundamental limits,” Appl. Phys. Lett. |

12. | Q. J. Wang, C. Pflügl, L. Diehl, F. Capasso, T. Edamura, S. Furuta, M. Yamanishi, and H. Kan, “High performance quantum cascade lasers based on three-phonon-resonance design,” Appl. Phys. Lett. |

13. | A. Tsekoun, R. Go, M. Pushkarsky, M. Razeghi, and C. K. N. Patel, “Improved performance of quantum cascade lasers through a scalable, manufacturable epitaxial-side-down mounting process,” Proc. Natl. Acad. Sci. U.S.A. |

14. | S. S. Howard, Z. Liu, D. Wasserman, A. J. Hoffman, T. S. Ko, and C. F. Gmachl, “High-performance quantum cascade lasers: optimized design through waveguide and thermal modeling,” IEEE J. Sel. Top. Quantum Electron. |

15. | S. Katz, A. Vizbaras, G. Boehm, and M.-C. Amann, “Injectorless quantum cascade laser operating in continuous wave above room temperature,” Semicond. Sci. Technol. |

16. | K. J. Franz, P. Q. Liu, J. Raftery, M. D. Escarra, A. J. Hoffman, S. S. Howard, Y. Yao, Y. Dikmelik, X. Wang, J. Fan, J. B. Khurgin, and C. Gmachl, “Short injector quantum cascade lasers,” IEEE J. Quantum Electron. |

**OCIS Codes**

(140.3070) Lasers and laser optics : Infrared and far-infrared lasers

(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

(250.5590) Optoelectronics : Quantum-well, -wire and -dot devices

**ToC Category:**

Lasers and Laser Optics

**History**

Original Manuscript: July 13, 2011

Revised Manuscript: August 9, 2011

Manuscript Accepted: August 13, 2011

Published: August 17, 2011

**Citation**

Richard Maulini, Arkadiy Lyakh, Alexei Tsekoun, and C. Kumar N. Patel, "λ~7.1 μm quantum cascade lasers with 19% wall-plug efficiency at room temperature," Opt. Express **19**, 17203-17211 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-18-17203

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### References

- A. Lyakh, R. Maulini, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, Q. J. Wang, F. Capasso, and C. K. N. Patel, “3 W continuous-wave room temperature single-facet emission from quantum cascade lasers based on nonresonant extraction design approach,” Appl. Phys. Lett. 95(14), 141113 (2009). [CrossRef]
- Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98(18), 181102 (2011). [CrossRef]
- Y. Yao, X. Wang, J.-Y. Fan, and C. F. Gmachl, “High performance ‘continuum-to-continuum’ quantum cascade lasers with a broad gain bandwidth of over 400 cm−1,” Appl. Phys. Lett. 97(8), 081115 (2010). [CrossRef]
- R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. 95(15), 151112 (2009). [CrossRef]
- R. Maulini, A. Lyakh, A. Tsekoun, R. Go, and C. K. N. Patel, “High average power uncooled mid-wave infrared quantum cascade lasers,” Electron. Lett. 47(6), 395 (2011). [CrossRef]
- A. Bismuto, R. Terazzi, M. Beck, and J. Faist, “Electrically tunable, high performance quantum cascade laser,” Appl. Phys. Lett. 96(14), 141105 (2010). [CrossRef]
- M. Troccoli, X. Wang, and J. Fan, “Quantum cascade lasers: high-power emission and single-mode operation in the long-wave infrared (λ>6 μm),” Opt. Eng. 49(11), 111106 (2010). [CrossRef]
- R. P. Leavitt, J. L. Bradshaw, K. M. Lascola, G. P. Meissner, F. Micalizzi, F. J. Towner, and J. T. Pham, “High-performance quantum cascade lasers in the 7.3- to 7.8-μm wavelength band using strained active regions,” Opt. Eng. 49(11), 111109 (2010). [CrossRef]
- J. S. Yu, S. Slivken, and M. Razeghi, “Injector doping level-dependent continuous-wave operation of InP-based QCLs at λ ~ 7.3 μm above room temperature,” Semicond. Sci. Technol. 25(12), 125015 (2010). [CrossRef]
- A. Lyakh, R. Maulini, A. Tsekoun, R. Go, S. Von der Porten, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, “High-performance continuous-wave room temperature 4.0-μm quantum cascade lasers with single-facet optical emission exceeding 2 W,” Proc. Natl. Acad. Sci. U.S.A. 107(44), 18799–18802 (2010). [CrossRef]
- J. Faist, “Wallplug efficiency of quantum cascade lasers: critical parameters and fundamental limits,” Appl. Phys. Lett. 90(25), 253512 (2007). [CrossRef]
- Q. J. Wang, C. Pflügl, L. Diehl, F. Capasso, T. Edamura, S. Furuta, M. Yamanishi, and H. Kan, “High performance quantum cascade lasers based on three-phonon-resonance design,” Appl. Phys. Lett. 94(1), 011103 (2009). [CrossRef]
- A. Tsekoun, R. Go, M. Pushkarsky, M. Razeghi, and C. K. N. Patel, “Improved performance of quantum cascade lasers through a scalable, manufacturable epitaxial-side-down mounting process,” Proc. Natl. Acad. Sci. U.S.A. 103(13), 4831–4835 (2006). [CrossRef] [PubMed]
- S. S. Howard, Z. Liu, D. Wasserman, A. J. Hoffman, T. S. Ko, and C. F. Gmachl, “High-performance quantum cascade lasers: optimized design through waveguide and thermal modeling,” IEEE J. Sel. Top. Quantum Electron. 13(5), 1054–1064 (2007). [CrossRef]
- S. Katz, A. Vizbaras, G. Boehm, and M.-C. Amann, “Injectorless quantum cascade laser operating in continuous wave above room temperature,” Semicond. Sci. Technol. 24(12), 122001 (2009). [CrossRef]
- K. J. Franz, P. Q. Liu, J. Raftery, M. D. Escarra, A. J. Hoffman, S. S. Howard, Y. Yao, Y. Dikmelik, X. Wang, J. Fan, J. B. Khurgin, and C. Gmachl, “Short injector quantum cascade lasers,” IEEE J. Quantum Electron. 46(5), 591–600 (2010). [CrossRef]

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