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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 3 — Jan. 30, 2012
  • pp: 3235–3240
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Continuous-wave interband cascade lasers operating above room temperature at λ = 4.7-5.6 μm

William W. Bewley, Chadwick L. Canedy, Chul Soo Kim, Mijin Kim, Charles D. Merritt, Joshua Abell, Igor Vurgaftman, and Jerry R. Meyer  »View Author Affiliations


Optics Express, Vol. 20, Issue 3, pp. 3235-3240 (2012)
http://dx.doi.org/10.1364/OE.20.003235


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Abstract

We have substantially improved the performance of interband cascade lasers emitting at λ = 4.7 and 5.6 μm, by applying the recently-pioneered approach of heavily doping the injector regions to rebalance the electron and hole concentrations in the active quantum wells. Ridges of ≈10 μm width, 4 mm length, and high-reflectivity back facets achieve maximum continuous wave operating temperatures of 60°C and 48°C, respectively. The threshold power density of ≈1 kW/cm2 at T = 25°C is over an order of magnitude lower than for state-of-the-art quantum cascade lasers emitting in this spectral range.

© 2012 OSA

1. Introduction

These initial successes in the 3.6-3.9 μm spectral range raised the question of whether similar improvements might now be possible at somewhat longer wavelengths beyond 4.5 μm. In that region the highest reported cw lasing temperatures have been 229 K for λ = 5.1 μm [8

8. C. L. Canedy, W. W. Bewley, J. R. Lindle, J. A. Nolde, D. C. Larrabee, C. S. Kim, M. Kim, I. Vurgaftman, and J. R. Meyer, “Interband cascade lasers with wavelenghths spanning 2.9 μm to 5.2 μm,” J. Electron. Mater. 37(12), 1780–1785 (2008). [CrossRef]

] and 184 K for λ = 5.9 μm [9

9. Z. Tian, C. Chen, R. Q. Yang, T. D. Mishima, M. B. Santos, J. C. Keay, M. B. Johnson, and J. F. Klem, “InAs-based plasmon-waveguide interband cascade lasers,” Proc. SPIE 7616, 76161B, 76161B-9 (2010). [CrossRef]

]. We report in this letter that the rebalancing scheme indeed dramatically improves the performance of ICLs emitting at λ = 4.7-5.6 μm, enabling RT cw operation. It will be seen that while the current- and power-density thresholds are somewhat higher than at shorter wavelengths, they are nonetheless substantially improved over any reported to date for state-of-the-art mid-IR quantum cascade lasers (QCLs) [10

10. 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]

,11

11. Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, “Room-temperature continuous-wave operation of quantum cascade lasers with 12.5% wall plug efficiency,” Appl. Phys. Lett. 93(2), 021103 (2008). [CrossRef]

].

2. Design

The two designs (Samples A and B) based on rebalancing of the hole/electron density ratio are similar to those reported previously for 3.6-3.9 μm devices [7

7. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nature Commun 2, 585 (2011). [CrossRef] [PubMed]

]. The main finding of that work is that while holes generated at the semimetallic interface in each stage of the ICL transfer efficiently to the active GaInSb hole QW via the very thin GaSb/AlSb hole injector, transfer from the InAs/AlSb electron injector is inhibited by its much greater thickness and is energetically unfavorable because of the need to generate electrons and holes at the semimetallic interface separating the electron and hole injectors. It was found that a slight excess of electrons over holes in the active region led to the most efficient production of gain when Auger recombination dominates and is split nearly equally between multi-electron and multi-hole processes.

3. Pulsed characterization

The two ICL wafers were grown on n-GaSb (100) substrates by molecular beam epitaxy [12

12. C. L. Canedy, C. S. Kim, M. Kim, D. C. Larrabee, J. A. Nolde, W. W. Bewley, I. Vurgaftman, and J. R. Meyer, “High-power, narrow-ridge, mid-infared interband cascade lasers,” J. Vac. Sci. Technol. 26(3), 1160–1162 (2008). [CrossRef]

]. Contact lithography and wet chemical etching were used to produce 150-μm-wide ridges with intentional lateral corrugation of the sidewalls to eliminate parasitic lasing modes. The uncoated cavities were cleaved to 2 mm length and tested at a pulse repetition rate of 3 kHz and width of 250 ns. At 300 K, the peak emission wavelengths are 4.5 and 5.5 μm for Samples A and B, respectively.

Sample A displays a threshold voltage (Vth) of 2.2 V and threshold current density (Jth) of 460 A/cm2 at 300 K, while the corresponding values for Sample B are Vth = 2.1 V and Jth = 650 A/cm2. The characteristic temperature T0 for both samples above room temperature is ≈39 K. The external differential quantum efficiencies from both facets above threshold are 13.8% and 15.1% for Samples A and B, respectively. Although these RT efficiencies are much lower than the 25-30% results exhibited by carrier-rebalanced 3.6-3.9 μm devices [7

7. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nature Commun 2, 585 (2011). [CrossRef] [PubMed]

], they are nonetheless higher than for any earlier ICLs emitting at λ > 4.5 μm. Assuming the internal efficiency to be independent of λ, the considerably lower slope efficiencies imply higher internal loss, possibly associated with larger free-career absorption cross sections in the SCL and active regions.

4. Continuous-wave performance

Narrow ridges were fabricated by photolithography and reactive-ion etching, using a Cl-based inductively coupled plasma (ICP) process, followed by cleaning with a phosphoric-acid-based wet etch. The etching stopped within the active core for Sample A and in the bottom SCL for Sample B. It should be noted that the threshold current densities reported below for Sample A may be slightly overestimated because there was some current spreading in the unetched part of the active region. The ridge widths were 10.9 μm for Sample A and 10.3 and 7.3 μm for Sample B. A 200-nm-thick Si3N4 layer was deposited by plasma-enhanced chemical vapor deposition, and a top contact window etched back using SF6-based ICP. Next, 100 nm of SiO2 was sputtered to block occasional pinholes in the Si3N4. The ridges were metallized and then electroplated with 4-5 μm of Au. Cavities of 4 mm length were cleaved, and an HR coating comprised of 46 nm Al2O3 topped by 100 nm Au was applied to each back facet. The micrograph in Fig. 1
Fig. 1 Micrograph of a 10.3-μm-wide ridge fabricated from Sample B and covered with 4 μm of electroplated gold. The positions of the top and bottom cladding layers, separate confinement layers, and active core are indicated.
shows a finished 10.3-μm-wide narrow ridge fabricated from Sample B, as seen from the front (output) facet. The devices were mounted epitaxial side up on a copper heat sink attached to a thermoelectric cooler.

The inset of Fig. 2
Fig. 2 Threshold current densities for the 10.9- and 10.3-μm-wide 4-mm-long ridges with one HR coating fabricated from Samples A and B, respectively, vs. of temperature in continuous-wave mode. The inset shows the cw emission spectra at 25°C.
shows the cw spectra at T = 25°C. The emission wavelength for Sample A increases from 4.75 μm at 25°C to 4.9 μm at 60°C, while that for Sample B varies from 5.6 μm at 25°C to 5.65 μm at 45°C. The main body of Fig. 2 illustrates the temperature-dependent cw threshold current densities for Samples A (10.9-μm-wide ridge, red points) and B (10.3-μm-wide ridge, blue points). For temperatures not too close to the maximum, both Jth(T) are described reasonably well by exponential fits that yield T0 = 31 K (A) and 35 K (B). The cw threshold of 480 A/cm2 for Sample A at T = 25°C is similar to the RT broad-area pulsed result reported above, while the value of 530 A/cm2 for Sample B is actually lower owing to lower reflection losses associated with the narrow ridge’s longer cavity and HR-coated back facet. Because the baseline loss in broad-area ICLs is higher at longer wavelengths, the additional loss arising from roughness scattering at the narrow-ridge sidewalls has less impact on the net efficiency.

Light-current-voltage characteristics for the 10.9-μm-wide ridge fabricated from Sample A and the 10.3-μm-wide ridge fabricated from Sample B are shown in Figs. 3
Fig. 3 Light-current-voltage characteristics at a series of temperatures for a 10.9 μm × 4 mm ridge with one HR coating fabricated from Sample A.
and 4
Fig. 4 Light-current-voltage characteristics at a series of temperatures for a 10.3 μm × 4 mm ridge with one HR coating fabricated from Sample B.
, respectively. Both structures generate more than 15 mW of cw output power at T = 25°C. The RT threshold voltages of 1.98 V for Sample A and 1.84 V for Sample B are again lower than for the corresponding broad-area devices, because current spreading from the narrow ridge into the much wider bottom cladding layer and substrate substantially reduces the differential series resistivity (0.2-0.3 mΩ-cm2 in both samples). The maximum cw operating temperatures are 60°C for Sample A and 48°C for Sample B.

Because single-mode cw output powers on the order of 1 mW will be sufficient for many chemical sensing systems, sources for that application can often operate very close to threshold. It is therefore noteworthy that at T = 25°C, the cw threshold power densities of 0.95 kW/cm2 for Sample A and 0.98 kW/cm2 for Sample B are more than an order of magnitude lower than the best values of ≈12 kW/cm2 ever reported for state-of-the-art QCLs with similar cavity dimensions [10

10. 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]

,11

11. Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, “Room-temperature continuous-wave operation of quantum cascade lasers with 12.5% wall plug efficiency,” Appl. Phys. Lett. 93(2), 021103 (2008). [CrossRef]

]. This is due in large part to the much lower operating voltage requirement of < 2.5 V for an ICL as compared to 10-15 V for most QCLs. Perhaps the most directly relevant figure of merit is threshold power, for which the two devices from Figs. 3 and 4 display RT values of 410 mW and 400 mW, respectively. The narrower ridge of width 7.3 μm from Sample B required a lower input of 320 mW at T = 25°C, due to its smaller active volume, and even lower values would have been possible were the cavity length shortened from 4 mm (down to the point where the accompanying additional mirror losses induce excessive threshold current density). Typical threshold powers for QCLs in this wavelength range from 2 to 5 W, although lower values have been demonstrated using a partially-transmissive HR coating on the front facet in addition to the HR-coated back facet [13

13. Y. Bai, S. R. Darvish, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Optimizing facet coating of quantum cascade lasers for low power consumption,” J. Appl. Phys. 109(5), 053103 (2011). [CrossRef]

]. The application of that approach to ICLs would also lower their required input powers from the values reported in this work. We conclude that if input power is the critical figure of merit, the ICL holds the edge even in the λ = 4-6 μm range where the QCL threshold current densities are relatively low. While the need for narrow spectral linewidth will further affect the power requirement, shorter-wavelength ICLs (processed from wafers grown before the carrier rebalancing strategy was introduced) have exhibited attractive single-mode performance [14

14. C. S. Kim, M. Kim, J. R. Lindle, W. W. Bewley, C. L. Canedy, J. Abell, I. Vurgaftman, and J. R. Meyer, “Corrugated-sidewall interband cascade lasers single-mode midwave-infrared emission at room temperature,” Appl. Phys. Lett. 95(23), 231103 (2009). [CrossRef]

].

5. Conclusion

To summarize, extension of the rebalanced carrier density approach to ICLs designed for longer wavelengths has led to much lower threshold current densities, as well as the first room-temperature cw operation of interband semiconductor lasers emitting beyond 4.5 μm. We find that even at wavelengths approaching 6 μm, the cw power densities needed to reach threshold are an order of magnitude lower than state-of-the-art QCL values. For fielded sensing systems, this implies enhanced battery lifetimes as well as a significant relaxation of packaging and size/weight constraints. On the other hand, owing to their lower internal loss and larger optimal number of stages, QCLs are expected to maintain their edge in longer-wavelength applications requiring high output powers.

References and links

1.

R. Q. Yang, “Infrared-laser based on intersubband transitions in quantum-wells,” Superlattices Microstruct. 17(1), 77–83 (1995). [CrossRef]

2.

J. R. Meyer, I. Vurgaftman, R. Q. Yang, and L. R. Ram-Mohan, “Type-II and type-I interband cascade lasers,” Electron. Lett. 32(1), 45–46 (1996). [CrossRef]

3.

C.-H. Lin, R. Q. Yang, D. Zhang, S. J. Murry, S. S. Pei, A. A. Allerman, and S. R. Kurtz, “Type-II interband quantum cascade laser at 3.8 μm,” Electron. Lett. 33(7), 598–599 (1997). [CrossRef]

4.

I. Vurgaftman, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, J. Abell, and J. R. Meyer, “Mid-infrared interband cascade lasers operating at ambient temperatures,” New J. Phys. 11(12), 125015 (2009). [CrossRef]

5.

W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, J. R. Lindle, J. Abell, I. Vurgaftman, and J. R. Meyer, “Ridge width dependence of mid-infrared interband cascade laser characteristics,” Opt. Eng. 49(11), 111116 (2010). [CrossRef]

6.

I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, J. R. Lindle, C. D. Merritt, J. Abell, and J. R. Meyer, “Mid-IR type-II interband cascade lasers,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1435–1444 (2011). [CrossRef]

7.

I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nature Commun 2, 585 (2011). [CrossRef] [PubMed]

8.

C. L. Canedy, W. W. Bewley, J. R. Lindle, J. A. Nolde, D. C. Larrabee, C. S. Kim, M. Kim, I. Vurgaftman, and J. R. Meyer, “Interband cascade lasers with wavelenghths spanning 2.9 μm to 5.2 μm,” J. Electron. Mater. 37(12), 1780–1785 (2008). [CrossRef]

9.

Z. Tian, C. Chen, R. Q. Yang, T. D. Mishima, M. B. Santos, J. C. Keay, M. B. Johnson, and J. F. Klem, “InAs-based plasmon-waveguide interband cascade lasers,” Proc. SPIE 7616, 76161B, 76161B-9 (2010). [CrossRef]

10.

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]

11.

Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, “Room-temperature continuous-wave operation of quantum cascade lasers with 12.5% wall plug efficiency,” Appl. Phys. Lett. 93(2), 021103 (2008). [CrossRef]

12.

C. L. Canedy, C. S. Kim, M. Kim, D. C. Larrabee, J. A. Nolde, W. W. Bewley, I. Vurgaftman, and J. R. Meyer, “High-power, narrow-ridge, mid-infared interband cascade lasers,” J. Vac. Sci. Technol. 26(3), 1160–1162 (2008). [CrossRef]

13.

Y. Bai, S. R. Darvish, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Optimizing facet coating of quantum cascade lasers for low power consumption,” J. Appl. Phys. 109(5), 053103 (2011). [CrossRef]

14.

C. S. Kim, M. Kim, J. R. Lindle, W. W. Bewley, C. L. Canedy, J. Abell, I. Vurgaftman, and J. R. Meyer, “Corrugated-sidewall interband cascade lasers single-mode midwave-infrared emission at room temperature,” Appl. Phys. Lett. 95(23), 231103 (2009). [CrossRef]

OCIS Codes
(140.2020) Lasers and laser optics : Diode lasers
(140.5960) Lasers and laser optics : Semiconductor lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 10, 2012
Revised Manuscript: January 23, 2012
Manuscript Accepted: January 23, 2012
Published: January 26, 2012

Virtual Issues
March 22, 2012 Spotlight on Optics

Citation
William W. Bewley, Chadwick L. Canedy, Chul Soo Kim, Mijin Kim, Charles D. Merritt, Joshua Abell, Igor Vurgaftman, and Jerry R. Meyer, "Continuous-wave interband cascade lasers operating above room temperature at λ = 4.7-5.6 μm," Opt. Express 20, 3235-3240 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-3-3235


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References

  1. R. Q. Yang, “Infrared-laser based on intersubband transitions in quantum-wells,” Superlattices Microstruct.17(1), 77–83 (1995). [CrossRef]
  2. J. R. Meyer, I. Vurgaftman, R. Q. Yang, and L. R. Ram-Mohan, “Type-II and type-I interband cascade lasers,” Electron. Lett.32(1), 45–46 (1996). [CrossRef]
  3. C.-H. Lin, R. Q. Yang, D. Zhang, S. J. Murry, S. S. Pei, A. A. Allerman, and S. R. Kurtz, “Type-II interband quantum cascade laser at 3.8 μm,” Electron. Lett.33(7), 598–599 (1997). [CrossRef]
  4. I. Vurgaftman, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, J. Abell, and J. R. Meyer, “Mid-infrared interband cascade lasers operating at ambient temperatures,” New J. Phys.11(12), 125015 (2009). [CrossRef]
  5. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, J. R. Lindle, J. Abell, I. Vurgaftman, and J. R. Meyer, “Ridge width dependence of mid-infrared interband cascade laser characteristics,” Opt. Eng.49(11), 111116 (2010). [CrossRef]
  6. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, J. R. Lindle, C. D. Merritt, J. Abell, and J. R. Meyer, “Mid-IR type-II interband cascade lasers,” IEEE J. Sel. Top. Quantum Electron.17(5), 1435–1444 (2011). [CrossRef]
  7. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nature Commun2, 585 (2011). [CrossRef] [PubMed]
  8. C. L. Canedy, W. W. Bewley, J. R. Lindle, J. A. Nolde, D. C. Larrabee, C. S. Kim, M. Kim, I. Vurgaftman, and J. R. Meyer, “Interband cascade lasers with wavelenghths spanning 2.9 μm to 5.2 μm,” J. Electron. Mater.37(12), 1780–1785 (2008). [CrossRef]
  9. Z. Tian, C. Chen, R. Q. Yang, T. D. Mishima, M. B. Santos, J. C. Keay, M. B. Johnson, and J. F. Klem, “InAs-based plasmon-waveguide interband cascade lasers,” Proc. SPIE7616, 76161B, 76161B-9 (2010). [CrossRef]
  10. 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]
  11. Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, “Room-temperature continuous-wave operation of quantum cascade lasers with 12.5% wall plug efficiency,” Appl. Phys. Lett.93(2), 021103 (2008). [CrossRef]
  12. C. L. Canedy, C. S. Kim, M. Kim, D. C. Larrabee, J. A. Nolde, W. W. Bewley, I. Vurgaftman, and J. R. Meyer, “High-power, narrow-ridge, mid-infared interband cascade lasers,” J. Vac. Sci. Technol.26(3), 1160–1162 (2008). [CrossRef]
  13. Y. Bai, S. R. Darvish, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Optimizing facet coating of quantum cascade lasers for low power consumption,” J. Appl. Phys.109(5), 053103 (2011). [CrossRef]
  14. C. S. Kim, M. Kim, J. R. Lindle, W. W. Bewley, C. L. Canedy, J. Abell, I. Vurgaftman, and J. R. Meyer, “Corrugated-sidewall interband cascade lasers single-mode midwave-infrared emission at room temperature,” Appl. Phys. Lett.95(23), 231103 (2009). [CrossRef]

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