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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 18 — Aug. 29, 2011
  • pp: 17203–17211
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λ~7.1 μm quantum cascade lasers with 19% wall-plug efficiency at room temperature

Richard Maulini, Arkadiy Lyakh, Alexei Tsekoun, and C. Kumar N. Patel  »View Author Affiliations


Optics Express, Vol. 19, Issue 18, pp. 17203-17211 (2011)
http://dx.doi.org/10.1364/OE.19.017203


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Abstract

Strain-balanced In0.6Ga0.4As/Al0.56In0.44As 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

The performance of room temperature infrared quantum cascade lasers (QCLs) has improved significantly in the past few years, which saw the demonstration of multi-watt continuous wave (cw) output power [1

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

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]

], pulsed mode wall-plug efficiency (WPE) of the order of 25% [2

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

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]

], and watt-level uncooled operation [4

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

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]

]. So far, most of these improvements have been realized in the mid-wave infrared (MWIR) spectral range, between 4 and 5 μm. QCL performance in the long-wave infrared (LWIR) range (7-12 μm), where principal applications are chemical sensing, is still significantly lower. The maximum pulsed WPE demonstrated in this spectral region is 11.5%, at a wavelength of 8.5 μm [6

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]

]. At a wavelength of ~7 μm [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

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]

], the maximum cw output power and wallplug efficiency at room temperature reported in the literature are 0.7 W and 5%, respectively [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]

]. In this letter, we report 7.1 μm QCLs with a pulsed WPE of 19% and cw output power and WPE of 1.4 W and 10%, respectively. A maximum average power of 1.2 W and a WPE of 12.5% at 1 W power level are demonstrated in uncooled operation.

2. Laser design and fabrication

The active region of our QCL is based on a three-phonon-resonance design [12

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. 94(1), 011103 (2009). [CrossRef]

]. Each gain stage is composed of a five-quantum-well (QW) active region followed by a five-QW injector. The conduction band diagram of one gain stage under an applied electric field of 51 kV/cm is shown in Fig. 1
Fig. 1 Schematic conduction band diagram of one gain stage of the QCL heterostructure under an applied electric field of 51 kV/cm, corresponding to a voltage defect of ~100 meV. The moduli squared of the relevant wavefunctions are shown at the corresponding energies. The upper and lower laser levels are shown in red and the ground state of the injector is shown in black. The red arrow marks the radiative transition.
. Strain-balanced material compositions with ~0.5% strain were used in the active region to increase the conduction band offset to ~600 meV and hence improve carrier confinement. The use of similar strain has already been reported by several authors at this wavelength [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]

,8

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. 49(11), 111109 (2010). [CrossRef]

]. It is particularly important in our case because the upper laser level is higher in the quantum well, in our three-phonon-resonance design, than that in traditional two-phonon-resonance designs [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]

,8

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. 49(11), 111109 (2010). [CrossRef]

]. Our simulations show an energy difference ΔE C4 of ~250 meV between the upper laser level and the continuum (see Fig. 1). The energy spacing between the upper laser level and the first state above it is ΔE 54 = 60 meV.

The QCLs active region and InP waveguide were grown by molecular beam epitaxy (MBE) on an n-doped InP substrate (3 × 1018 cm−3) in a single growth step. The epitaxial layer sequence, starting from the substrate, was as follows: InP cladding layer (n = 1 × 1017 cm−3, thickness 2.5 μm), InP cladding layer (3 × 1016 cm−3, 2.5 μm), 45-stage strain-balanced In0.6Ga0.4As/Al0.56In0.44As active region (2.43 μm), InP cladding layer (3 × 1016 cm−3, 2.5 μm), InP cladding layer (1 × 1017 cm−3, 2.5 μm), InP plasmon layer (8 × 1018 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/cm2), assuming an electron mobility of 5000 cm2V−1s−1.

The epi-wafer was processed into buried heterostructure lasers with active region width varying between 8 and 12 μm and cleaved into chips of 2 to 5.8 mm length, which were then mounted epi-side down on aluminum nitride submounts with gold-tin hard solder [13

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

Low-duty-cycle pulsed testing was performed at chip-on-carrier level. Devices were pulsed at 10 kHz repetition rate with a pulse width of 500 ns and the output power was measured with a calibrated thermopile detector placed directly in front of the output facet. Peak output power (for two facets), voltage, and WPE as function of current of an uncoated 3 mm x 8 μm chip at a temperature of 293 K are shown in the main panel of Fig. 2
Fig. 2 Peak optical power, voltage, and wall-plug efficiency of a 3 mm x 8 μm quantum cascade laser emitting at 7.1 μm as function of current. Top: Measured voltage defect Δ per gain stage as a function of current.
. The threshold and roll-over current densities are 1.45 and 5.38 kA/cm2, 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]

]. To our knowledge, this is the first time that this limit, which, as stated in [11

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

], should not be interpreted as a fundamental limit, is experimentally exceeded at any wavelength. By performing similar measurements for temperatures between 293 K and 348 K, we deduced characteristic temperatures of 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

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]

], indicating that the temperature dependence of the performance of our QCLs is not affected by the low voltage defect, in this temperature range. The maximum WPE is still 13.4% at 348 K.

The top panel of Fig. 2 shows the measured voltage defect Δ = V/N phν, where V is the voltage drop across the entire structure, N p = 45 is the number of gain stages, and hν = 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/cm2) 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.

Waveguide losses were estimated by measuring slope efficiency as a function of mirror losses for cavity lengths between 2 and 5.8 mm and performing the same analysis as in [4

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]

], resulting in αw = 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

For thermoelectrically-cooled cw operation, thermistors were mounted ~0.5 mm away from the QCL chips on the AlN submounts [13

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]

]. Chips-on carriers were then mounted in hermetically-sealed butterfly-type packages containing a thermoelectric cooler (TEC) and collimating optics. The back facet was high-reflection (HR) coated to provide single-ended emission, and the front facet was partially anti-reflection (AR) coated to maximize WPE [4

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]

]. The partial AR coating was designed to produce the same mirror losses αm as for a 1.5 mm-long HR/uncoated chip, i.e. αm = 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
Fig. 3 Continuous-wave voltage, single-facet output power, and wall-plug efficiency of an 8 μm-wide, 4 mm-long, thermoelectrically-cooled quantum cascade laser as function of current at a temperature of 293 K. Inset: cw laser spectrum at a current of 1.12 A.
. Data were corrected for the lens collection efficiency which is equal to 0.9. A threshold current density of 2.12 kA/cm2, 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/cm2, which is ~30% lower than in pulsed mode. Cw performance will be improved by regrowing the structure with a lower active region doping.

A beam picture of the packaged QCL is shown in Fig. 4
Fig. 4 Beam picture of a continuous-wave packaged QCL at 1.06 W output power level. The picture was taken with a pyroelectric camera 2 m away from the laser.
. The picture was taken with a pyroelectric camera placed 2 m away from the laser, without additional optics. The laser was operated in cw mode at 293 K at a current of 1.12 A, corresponding to an output power level of 1.06 W (not corrected for the lens collection efficiency). A zeroth order beam profile is observed along both axes (TM00). 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

Chips were also tested in high-duty-cycle uncooled operation [4

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

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]

]. In this case, chips-on-carrier were mounted in hermetically-sealed packages containing a heat spreader and collimating optics, but no TEC. A thermistor was mounted close to the chip for temperature monitoring. Larger chip dimensions of 5.8 mm x 12 μm were chosen to improve heat dissipation [5

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]

]. Devices were HR/AR coated. An AR coating reflectivity of 0.7% was chosen, in order to obtain the same value of mirror losses as for cw operation. Measured average power as a function of duty cycle of such a device operated at a heatsink temperature of 293 K is shown in the top of Fig. 5
Fig. 5 Top: Single-facet average optical power as a function of duty cycle of an uncooled 7.1 μm QCL at a heatsink temperature of 293 K. Bottom: Wall-plug efficiency as a function of average optical power for the same laser.
. The laser was operated at 12.3 V and 2.3 A with 500 ns pulses. Data were corrected for the lens collection efficiency. The maximum average power is 1.18 W at a duty cycle of 42.5%. WPE as a function of output power is shown in the bottom of Fig. 5. The WPE is equal to 14.5% and 12.5% at 0.5 W and 1 W average power levels, respectively. Since these lasers do not contain a TEC, the efficiency of the overall laser system is roughly equal to that of the laser, making them appealing for battery-operated portable and handheld LWIR applications.

4. Backfilling model

As discussed in the introduction, one of the critical design parameters, which influence QCL WPE in the LWIR, is the voltage defect Δ. Faist [11

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

] and Howard [14

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]

] predict optimal voltage defects of 150 meV and 175 meV, respectively, for room temperature operation. Both of these models described backfilling of the lower laser level as 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.

We assume that the injector states are equally spaced by ΔE 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:
D(E)=D0i=0Ninjθ(EiΔEinj),
(1)
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 is
n(E)=nsZD(E)f(E),
(2)
where f(E) is the Fermi-Dirac distribution and Z=0D(E)f(E)dE is the partition function. Due to the low carrier density in QCLs, f(E) can be approximated by the Boltzmann distribution exp(-E/kT). The lower laser level backfilling is calculated as:
ntherm=1Ninj+1Δn(E)dE,
(3)
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:

ntherm=nseΔ2kTsinh[Δ2NinjkT]sinh[(Ninj+1)Δ2NinjkT].
(4)

In the limit Δ >> 2N inj kT, our formula tends towards the single-subband approximation: n thermn 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
Fig. 6 Calculated maximum wall-plug efficiency of a 7.1 μm quantum cascade laser as a function of voltage defect. The lower laser level backfilling was computed using the model presented in this article. Inset: backfilling of the lower laser level as a function of voltage defect calculated with the traditional single-subband model and with our model.
. 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.

We now apply our model to determine the voltage defect, which maximizes WPE at room temperature. The WPE as a function of Δ is plotted in Fig. 6 for N inj = 8, using the same numerical parameters as in [11

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

]. The maximum WPE is reached for Δ ~100 meV, which is significantly lower than the values given in [11

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

] and [14

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]

]. This is in good agreement with our experimentally observed maximum WPE of 19% for Δ ~95 meV. The maximum WPE of 27% at 4.9 μm reported by Bai et al. in [2

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]

] was also obtained at Δ ~100 meV. The single-subband model, on the other hand predicts that lasing is not possible for Δ < 65 meV, which is in contradiction with our experimental results presented in Fig. 2.

Our model reveals a dependence of backfilling on the number of subbands in the injector, with a noticeable decrease of 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

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

] in which the number of injector states only influences the WPE indirectly via the transport time. We believe that this is one of the reasons why the various attempts of increasing QCL WPE by shortening the injector, in order to decrease the transport time, reported in the literature (see for instance [15

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]

] and [16

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]

]) did not work as expected: shortening the injector resulted in an increased backfilling at room temperature.

5. Conclusion

In conclusion, we report 7.1 μm QCLs with strain-balanced active region and low voltage defect. The maximum WPE is 19% in pulsed mode at room temperature. This is the first report of a WPE higher than the Faist limit (for a dephasing of 70 fs). Output power of nearly 1.4 W and WPE of 10% were measured in cw mode, at 293 K. We believe that cw WPE can be further increased for the same active region design by reducing the doping level. Average output power of nearly 1.2 W and WPE in excess of 10% were measured from uncooled lasers. Finally, we have presented a model for backfilling of the lower laser level and have given a new design rule for the voltage defect, which according to our calculations is ~100 meV for maximum WPE at room temperature, in agreement with our experimental results. Based on the experimental and theoretical results presented here, we conclude that the WPE predictions for the LWIR spectral range [11

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

] have been underestimated thus far and should be revised.

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. 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]

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]

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. 49(11), 111109 (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]

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]

11.

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

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. 94(1), 011103 (2009). [CrossRef]

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]

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]

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]

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

  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]
  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]
  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. 49(11), 111109 (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]
  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]
  11. J. Faist, “Wallplug efficiency of quantum cascade lasers: critical parameters and fundamental limits,” Appl. Phys. Lett. 90(25), 253512 (2007). [CrossRef]
  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. 94(1), 011103 (2009). [CrossRef]
  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]
  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]
  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]

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