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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 7 — Apr. 7, 2014
  • pp: 7702–7710
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Pulsed and CW performance of 7-stage interband cascade lasers

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


Optics Express, Vol. 22, Issue 7, pp. 7702-7710 (2014)
http://dx.doi.org/10.1364/OE.22.007702


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Abstract

We report a narrow-ridge interband cascade laser emitting at λ ≈3.5 μm that produces up to 592 mW of cw power with a wallplug efficiency of 10.1% and beam quality factor of M2 = 3.7 at T = 25 °C. A pulsed cavity length study of broad-area lasers from the same wafer confirms that the 7-stage structure with thicker separate confinement layers has a reduced internal loss of ≈3 cm−1. More generally, devices from a large number of wafers with similar 7-stage designs and wavelengths spanning 2.95-4.7 μm exhibit consistently higher pulsed external differential quantum efficiencies than earlier state-of-the-art ICLs.

© 2014 Optical Society of America

1. Introduction

Some of the earlier studies also introduced corrugations to the ridge sidewalls as a means of improving the beam quality along the lateral (slow) axis [11

11. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]

]. This occurs because higher-order modes having greater optical intensity near the ridge boundaries are scattered preferentially over the fundamental mode. The accompanying penalty is that the corrugations typically lower the slope efficiency by ≈10-20% due to additional scattering losses. Nonetheless, the studies to date indicate that corrugated ridges typically display higher maximum brightness figures of merit (defined below) than conventional straight-sidewall ridges [11

11. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]

].

In this work, we report a significant further enhancement of the maximum cw output power, wallplug efficiency, and brightness attainable from ICL narrow ridges with corrugated sidewalls. This resulted primarily from the introduction of new designs that employ both more active stages and thicker low-loss separate confinement layers (SCLs) than earlier state-of-the-art structures.

2. Design considerations

On the other hand, the same simulations find a larger stage multiplicity to be preferable if high cw output power and brightness, rather than minimized drive power, are the primary objectives. Nominally, the slope efficiency scales with Nact, although this is accompanied by a decrease of the current at which heat accumulation in the active region causes the slope to roll over. It will be seen below that the optimal number of stages depends to a large degree on how Nact influences the internal loss.

The net internal loss combines contributions from up to four primary sources: (1) the active gain region comprising Nact stages layered in series, (2) the two n--GaSb SCLs that surround the active region, (3) the top and bottom optical cladding layers that surround the SCLs, and (4) absorption (e.g., in the dielectric coatings) and scattering (e.g., by sidewall corrugations or unintentional non-uniformities) in the narrow ridge waveguide. Since the fourth is nominally insensitive to the epitaxial layering design, it will be taken as fixed in this discussion. We also note that the various transition superlattices separating the cladding, SCL, and active regions may add a non-negligible contribution to the loss.

3. MBE growth and pulsed characterization

A series of 5-stage and 7-stage ICL wafers with carrier-rebalanced designs [10

10. 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,” Nat. Commun. 2, 585 (2011). [CrossRef] [PubMed]

], and having both baseline (500 nm) and thicker (700-800 nm) SCLs, were grown by molecular beam epitaxy (MBE) at NRL on n-GaSb (100) substrates using procedures similar to those reported previously [14

14. 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-infrared interband cascade lasers,” J. Vac. Sci. Technol. B 26(3), 1160–1162 (2008). [CrossRef]

]. The observed performance characteristics are generally consistent for wafers grown on either a Riber Compact 21T or a Gen-II MBE system. All pulsed characterization was carried out on standard broad-area test devices with 150 μm ridge width, 2 mm cavity length, and uncoated facets.

Fig. 1 Pulsed single-facet differential slope efficiencies above threshold for 5-stage (red) and 7-stage (blue) ICLs with both the baseline SCL thickness (500 nm, open points) and thicker SCLs (700-800 nm, filled points). All characterizations with pulse length 175 ns and repetition frequency 5 kHz were performed on standard broad-area test structures with 150 μm ridge width, 2 mm cavity length, and uncoated facets. With other design parameters held nearly fixed, results were nominally independent of emission wavelength between 3.2 μm and 3.9 μm.
Figure 1 plots the pulsed slope efficiencies vs. temperature for broad-area lasers processed from various 5-stage and 7-stage ICL wafers. At T = 300 K, the emission wavelengths ranged from 3.2 μm to 3.8 μm, the threshold current densities were 140-250 A/cm2 (similar for 5 and 7 stages) and the threshold voltages were 2.3-2.9 V for 5 stages and 3.5-3.7 V for 7 stages.

On the other hand, increasing the stage multiplicity to 7 (at fixed SCL thickness) provides additional gain. Our simulations show that even though the 7-stage design with thick SCLs has 6% less gain than the 5-stage structure with thin SCLs, it benefits by reducing the mode fraction in the cladding layers by 40%, with most of the difference being transferred into the SCLs. The observed effect is that the 7-stage devices with thicker SCLs are competitive at high T while remaining generally advantageous near ambient. This may be seen by comparing the open (500 nm SCLs) and filled (700-800 nm SCLs) blue points in the figure.

4. Narrow ridge processing and CW operation

The higher pulsed efficiencies observed for broad-area ICLs employing the new 7-stage designs with thicker SCLs appear promising for increasing the cw output power and wallplug efficiency. To confirm this, we fabricated narrow ridges from one of the 7-stage wafers, specifically that corresponding to the filled blue circles in Fig. 1, using photolithography and reactive ion etching. Although both Cl-based and BCl3–based inductively coupled plasma (ICP) procedures were employed to fabricate ridges of several widths, we focus here on the performance of a particular ridge that employed the Cl-based ICP. A more comprehensive comparison of the two etch processes will be reported elsewhere.

The dry etching was designed to stop just below the active core of the device, in the bottom GaSb SCL, and was followed by cleaning with a phosphoric-acid-based wet etch. The ridge of width 32.4 μm had sidewall corrugations with a peak-to-valley amplitude of ≈1.0 μm and period of 2.0 μm. A 250-nm-thick Si3N4 layer was deposited by plasma-enhanced chemical vapor deposition, after which a top contact window was etched back using SF6-based ICP. Approximately 100 nm of SiO2 was also deposited by sputtering to block occasional pinholes in the Si3N4.

The ridge was metallized with the top contact comprised of Ag/Ti/Pt/Au (5nm/20nm/150nm/200 nm for 5 stages and 1000 nm for 7 stages) and the bottom contact consisting of Ag/Cr/Sn/Pt/Au (5nm/30nm/40nm/150nm/100nm) and then electro-plated with ≈5 μm of Au. The electro-plating was patterned so as to leave non-plated gaps of ≈50 μm to allow cleaving into individual laser cavities. For 5-stage ICLs, the standard metallization thickness before electro-plating was 200 nm. However, when 7-stage structures were processed by the earlier protocol and tested in CW mode, damage inevitably occurred due to excessive heating of the non-plated portion of the ridge near each facet. This was most likely exacerbated by a current runaway process that occurs in ICLs (and QCLs) with laterally non-uniform temperature distributions, because a substantial decrease of the effective series resistance with increasing T causes most of the current to flow through those regions where the temperature is highest (causing them to heat up further). Damage to the 7-stage ridges was mitigated by depositing a much thicker Au layer (≈1 μm) prior to the patterned Au electro-plating. The thicker metallization did not degrade the cleave quality, and subsequent testing confirmed that the enhanced thermal conduction prevented thermal damage to the laser (unless operated beyond the L-I rollover at much higher currents).

The device was cleaved to a cavity length of 3 mm and mounted epitaxial-side-down, using a high-yield proprietary process [11

11. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]

], on a C-mount attached to a thermoelectric cooler. A high-reflection (HR) coating comprised of 200 nm Al2O3 topped by 100 nm Au was deposited on the device’s back facet, while an anti-reflection (AR) coating consisting of a λ/4 layer of Al2O3 (estimated reflection ≈2%) was deposited on the front facet.

Fig. 3 Emission spectra at T = 25°C for three CW injection currents. Using the centroid wavelength and the 2.3 nm/°C wavelength shift for the same device operated in pulsed mode, the temperature rise in the active region was estimated to be ≈36°C for a current of 1.4 A.
Figure 3 shows the ridge’s emission spectra at T = 25 °C for several cw injection currents. While the centroid wavelength at the lowest current is relatively close to its value for pulsed operation (λ = 3.45 μm), device heating induces a red shift of up to 70 nm with increasing current.

Fig. 4 CW L-I characteristics for a series of temperatures in the range T = 15-70 °C.
Figure 4 plots the ridge’s cw L-I characteristics at a series of temperatures between 15 °C and 70 °C. The maximum cw output powers of 696 mW at 15 °C and 592 mW at 25 °C are much higher than any reported previously for ICLs. Even at T = 65 °C, the power of 117 mW exceeds any attainable at room temperature just three years ago. The cw threshold bias voltage for the narrow ridge is 3.49 V at 25 °C, while its threshold current density of 219 A/cm2 is only slightly higher than the pulsed value of 188 A/cm2 measured for a broad-area device fabricated from the same wafer. The maximum cw slope efficiency of 815 mW/A exceeds the pulsed value of 601 mW/A, although that comparison is not very meaningful because the AR-coated mirror loss of the 3-mm-long cavity exceeds that of the uncoated facet in the 2-mm-long cavity.

As was the case for the pulsed characteristics described in Section 3, the observed improvement goes beyond what may be associated with adding more stages to increase the slope efficiency. Equally important is the higher wallplug efficiency, e.g., 15.2% at T = 15 °C, that results from the redesigned structure’s lower internal loss.
Fig. 5 Wallplug efficiency at T = 25 °C as a function of CW injection current for the 7-stage narrow ridge (blue curve), along with the corresponding dependence for an earlier 5-stage ICL (red dashed curve) [10].
Figure 5 compares the CW wallplug efficiencies vs. current at T = 25 °C for the 7-stage ridge of this work (Gen3B) with the corresponding result for the best earlier 5-stage device with similar ridge width and sidewall corrugations (Gen3) [11

11. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]

]. While it is unfortunately not possible to compare devices with the same dimensions, the 7-stage ICL’s WPE advantage clearly goes well beyond that attributable to its slightly shorter cavity (3 mm vs. 4 mm). The values of 13.2% at the WPE maximum and 10.1% at the highest current compare with 8.7% and 6.5% for the 5-stage design. As discussed briefly in the introduction, the 5-stage data from 2012 in turn represented a substantial improvement over all earlier CW WPEs for ICLs with cavity lengths ≥ 2 mm. A pulsed cavity length study of broad-area devices fabricated from the wafer used to process the 7-stage narrow ridge yielded an internal loss of 2.9 cm−1 and internal efficiency of 80% assuming an uncoated mirror reflectivity of 41%. Assuming the same internal efficiency, the 5-stage structure’s pulsed slope efficiency implies a somewhat higher internal loss of 4.1 cm−1.

In order to determine the “effective M2” [12

12. W. W. Bewley, C. S. Kim, C. L. Canedy, C. D. Merritt, I. Vurgaftman, J. Abell, J. R. Meyer, and M. Kim, “High-power, high-brightness continuous-wave interband cascade lasers with tapered ridges,” Appl. Phys. Lett. 103(11), 111111 (2013). [CrossRef]

,15

15. C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, C. L. Canedy, J. A. Nolde, D. C. Larrabee, I. Vurgaftman, and J. R. Meyer, “Broad-stripe, single-mode, mid-IR interband cascade laser with photonic-crystal distributed-feedback grating,” Appl. Phys. Lett. 92(7), 071110 (2008). [CrossRef]

,16

16. I. Vurgaftman, W. W. Bewley, R. E. Bartolo, C. L. Felix, M. J. Jurkovic, J. R. Meyer, M. J. Yang, H. Lee, and R. U. Martinelli, “Far-field characteristics of mid-IR angled-grating distributed feedback lasers,” J. Appl. Phys. 88(12), 6997–7005 (2000). [CrossRef]

] for the present 7-stage narrow ridge, we measured far-field intensity profiles along the slow axis for several injection currents at T = 25 °C, as shown in Fig. 6.
Fig. 6 Far-field emission profiles at T = 25 °C for the 7-stage narrow ridge at three cw injection currents. The extracted M2 values are discussed in the text.
The M2 values derived from the profiles range from 2.5 at I = 460 mA to 3.7 at 1400 mA. It is unclear why these values are somewhat higher than those observed previously for corrugated ICL ridges of similar width [11

11. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]

], although one possibility is that processing differences affected the strength of the corrugation-induced scattering of higher-order lasing modes.

4. Conclusion

To confirm that enhanced cw performance could also be achieved, a 32.4-μm-wide narrow ridge with corrugated sidewalls was fabricated from one of the 7-stage wafers. In cw mode, it emitted up to 592 mW of cw power at λ ≈3.5 μm and T = 25 °C, with a beam quality factor of M2 = 3.7. The maximum cw wallplug efficiency was 13.2%, its value at the highest output power was 10.1%, and the maximum brightness figure of merit was 160 mW. All of these characteristics represent substantial advances over earlier ICL performance results.

Unfortunately, the interpretation of these findings remains largely empirical, since the available data do not allow the loss distribution, e.g., between the active and cladding layers, to be determined, or the dominant loss processes to be identified. Future studies will attempt to better understand the relevant physical processes, as a step toward identifying the most promising pathways toward further loss mitigation. At this point, there is no reason to believe that the current generation of ICLs has already encountered a fundamental limit to loss minimization or enhancement of the lasing efficiency.

Acknowledgment

References and links

1.

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]

2.

R. Weih, A. Bauer, M. Kamp, and S. Höfling, “Interband cascade lasers with AlGaAsSb bulk cladding layers,” Opt. Mater. Express 3(10), 1624–1631 (2013). [CrossRef]

3.

R. P. Leavitt, J. D. Bruno, J. L. Bradshaw, K. M. Lascola, J. T. Pham, F. J. Towner, S. Suchalkin, G. Belenky, I. Vurgaftman, C. L. Canedy, W. W. Bewley, C. S. Kim, M. Kim, C. D. Merritt, and J. R. Meyer, “High performance interband cascade lasers at 3.8 microns,” Proc. SPIE 8277, 82771E (2012). [CrossRef]

4.

M. von Edlinger, J. Scheuermann, R. Weih, L. Naehle, C. Zimmermann, L. Hildebrandt, M. Fischer, M. Kamp, S. Höfling, and J. Koeth, “DFB interband cascade lasers for tunable laser absorption spectroscopy from 3 to 6μm”, SPIE Photonics West (San Francisco, 2–6 February 2014).

5.

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

6.

I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, and J. R. Meyer, “Interband cascade lasers with low threshold powers and high output powers,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1200210 (2013). [CrossRef]

7.

F. Capasso, “High-performance midinfrared quantum cascade lasers,” Opt. Eng. 49(11), 111102 (2010). [CrossRef]

8.

M. Kim, C. L. Canedy, W. W. Bewley, C. S. Kim, J. R. Lindle, J. Abell, I. Vurgaftman, and J. R. Meyer, “Interband cascade laser emitting at λ = 3.75 μm in continuous wave above room temperature,” Appl. Phys. Lett. 92(19), 191110 (2008). [CrossRef]

9.

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]

10.

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,” Nat. Commun. 2, 585 (2011). [CrossRef] [PubMed]

11.

W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]

12.

W. W. Bewley, C. S. Kim, C. L. Canedy, C. D. Merritt, I. Vurgaftman, J. Abell, J. R. Meyer, and M. Kim, “High-power, high-brightness continuous-wave interband cascade lasers with tapered ridges,” Appl. Phys. Lett. 103(11), 111111 (2013). [CrossRef]

13.

R. Weih, M. Kamp, and S. Höfling, “Interband cascade lasers with room-temperature threshold current densities below 100 A/cm2,” Appl. Phys. Lett. 102(23), 231123 (2013). [CrossRef]

14.

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-infrared interband cascade lasers,” J. Vac. Sci. Technol. B 26(3), 1160–1162 (2008). [CrossRef]

15.

C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, C. L. Canedy, J. A. Nolde, D. C. Larrabee, I. Vurgaftman, and J. R. Meyer, “Broad-stripe, single-mode, mid-IR interband cascade laser with photonic-crystal distributed-feedback grating,” Appl. Phys. Lett. 92(7), 071110 (2008). [CrossRef]

16.

I. Vurgaftman, W. W. Bewley, R. E. Bartolo, C. L. Felix, M. J. Jurkovic, J. R. Meyer, M. J. Yang, H. Lee, and R. U. Martinelli, “Far-field characteristics of mid-IR angled-grating distributed feedback lasers,” J. Appl. Phys. 88(12), 6997–7005 (2000). [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 16, 2014
Revised Manuscript: February 26, 2014
Manuscript Accepted: February 27, 2014
Published: March 26, 2014

Citation
Chadwick L. Canedy, Joshua Abell, Charles D. Merritt, William W. Bewley, Chul Soo Kim, Mijin Kim, Igor Vurgaftman, and Jerry R. Meyer, "Pulsed and CW performance of 7-stage interband cascade lasers," Opt. Express 22, 7702-7710 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-7-7702


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References

  1. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, J. R. Lindle, C. D. Merritt, J. Abell, J. R. Meyer, “Mid-IR type-II interband cascade lasers,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1435–1444 (2011). [CrossRef]
  2. R. Weih, A. Bauer, M. Kamp, S. Höfling, “Interband cascade lasers with AlGaAsSb bulk cladding layers,” Opt. Mater. Express 3(10), 1624–1631 (2013). [CrossRef]
  3. R. P. Leavitt, J. D. Bruno, J. L. Bradshaw, K. M. Lascola, J. T. Pham, F. J. Towner, S. Suchalkin, G. Belenky, I. Vurgaftman, C. L. Canedy, W. W. Bewley, C. S. Kim, M. Kim, C. D. Merritt, J. R. Meyer, “High performance interband cascade lasers at 3.8 microns,” Proc. SPIE 8277, 82771E (2012). [CrossRef]
  4. M. von Edlinger, J. Scheuermann, R. Weih, L. Naehle, C. Zimmermann, L. Hildebrandt, M. Fischer, M. Kamp, S. Höfling, and J. Koeth, “DFB interband cascade lasers for tunable laser absorption spectroscopy from 3 to 6μm”, SPIE Photonics West (San Francisco, 2–6 February 2014).
  5. R. Q. Yang, “Infrared-laser based on intersubband transitions in quantum-wells,” Superlattices Microstruct. 17(1), 77–83 (1995). [CrossRef]
  6. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Meyer, “Interband cascade lasers with low threshold powers and high output powers,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1200210 (2013). [CrossRef]
  7. F. Capasso, “High-performance midinfrared quantum cascade lasers,” Opt. Eng. 49(11), 111102 (2010). [CrossRef]
  8. M. Kim, C. L. Canedy, W. W. Bewley, C. S. Kim, J. R. Lindle, J. Abell, I. Vurgaftman, J. R. Meyer, “Interband cascade laser emitting at λ = 3.75 μm in continuous wave above room temperature,” Appl. Phys. Lett. 92(19), 191110 (2008). [CrossRef]
  9. I. Vurgaftman, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, J. Abell, J. R. Meyer, “Mid-infrared interband cascade lasers operating at ambient temperatures,” New J. Phys. 11(12), 125015 (2009). [CrossRef]
  10. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nat. Commun. 2, 585 (2011). [CrossRef] [PubMed]
  11. W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, J. R. Meyer, “High-power room-temperature continuous-wave mid-infrared interband cascade lasers,” Opt. Express 20(19), 20894–20901 (2012). [CrossRef] [PubMed]
  12. W. W. Bewley, C. S. Kim, C. L. Canedy, C. D. Merritt, I. Vurgaftman, J. Abell, J. R. Meyer, M. Kim, “High-power, high-brightness continuous-wave interband cascade lasers with tapered ridges,” Appl. Phys. Lett. 103(11), 111111 (2013). [CrossRef]
  13. R. Weih, M. Kamp, S. Höfling, “Interband cascade lasers with room-temperature threshold current densities below 100 A/cm2,” Appl. Phys. Lett. 102(23), 231123 (2013). [CrossRef]
  14. C. L. Canedy, C. S. Kim, M. Kim, D. C. Larrabee, J. A. Nolde, W. W. Bewley, I. Vurgaftman, J. R. Meyer, “High-power, narrow-ridge, mid-infrared interband cascade lasers,” J. Vac. Sci. Technol. B 26(3), 1160–1162 (2008). [CrossRef]
  15. C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, C. L. Canedy, J. A. Nolde, D. C. Larrabee, I. Vurgaftman, J. R. Meyer, “Broad-stripe, single-mode, mid-IR interband cascade laser with photonic-crystal distributed-feedback grating,” Appl. Phys. Lett. 92(7), 071110 (2008). [CrossRef]
  16. I. Vurgaftman, W. W. Bewley, R. E. Bartolo, C. L. Felix, M. J. Jurkovic, J. R. Meyer, M. J. Yang, H. Lee, R. U. Martinelli, “Far-field characteristics of mid-IR angled-grating distributed feedback lasers,” J. Appl. Phys. 88(12), 6997–7005 (2000). [CrossRef]

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