High-Power, High-Efficiency Mid-Infrared Quantum Cascade Lasers D. Botez1, J. D. Kirch1, C. Boyle1, K. Oresick1, C. Sigler1, D. Lindberg2, T. Earles2 and L. J. Mawst1 1
ECE Department, Univ. of Wisconsin-Madison, 1415 Engineering Dr., Madison WI 53706 2 Intraband, LLC, 200 N. Prospect Ave., Madison WI 53726
[email protected]
Abstract: Carrier-leakage suppression and fast carrier extraction allow quantum cascade lasers to reach internal efficiencies close to fundamental limits (~ 95 %). Then, CW wallplug efficiencies ≥ 40 % and powers ≥ 10 W become possible. OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (140.3070) Infrared and far-infrared lasers
High-CW-power (i.e., watt-range), efficient mid-infrared (IR) (λ= 3-15 µm) quantum cascade lasers (QCLs) are needed for a wide range of applications, from remote sensing to infrared countermeasures. A key parameter is the internal efficiency (per QCL period) ηi , a factor in the external-differential-efficiency, ηd , expression: !! ! 𝜂! = 𝜂! 𝑁! ; 𝜂! = 𝜂!"# !" (1) !! !!!
!!" !!!!
where αm and αw are the mirror and waveguide losses; Np is the number of periods; ηinj is the injection efficiency; and τul and τll are the effective upper-level and lower-level lifetimes, respectively [1]. For high output power and high wallplug efficiency (WPE) the ηd value needs to be maximized. While αw was reduced to values as low as ~ 0.5 cm-1 and Np of 40-45 has been used for state-of-the-art devices, for conventional QCLs the ηi values have been found to be rather low: 50-60 % in the 4.5-6.0 µm wavelength range [1] and 57-67 % in the 7-11 µm range [2] with, until recently, no clear explanation why that was the case. First of all, to increase ηinj , the existing carrier leakage was strongly suppressed by using conduction-band engineering, such as step-tapering the active-region (AR) barrier heights [1], which resulted in ηinj values in the 80-85 % range [3]. In parallel, the τll value was reduced to ~ 0.19 ps, and miniband-like carrier extraction from the ARs was employed [2,3]. In turn, the τul/(τul + τll) ratio, so-called lasertransition efficiency, ηtr , increased from ~ 75 % to ~ 85, %, when considering only LO-phonon scattering. Thus, by combining carrier-leakage suppression with fast extraction, ηi values have increased and are approaching the upper limit of ~ 90 % for mid-IR QCLs [3]. In contrast, for diode lasers the ηi upper limit is 100 %, since ηtr is 100 %. Fig. 1 shows a comparison of obtained ηi values, for various QCL types, over the 4-11 µm wavelength range. Devices with both carrier-leakage suppression and miniband-like extraction: the step-taper AR – resonant-extraction (STA-RE) QCL [2,3] and the TA-RE (also called shallow-well) QCL [4] have significantly higher ηi values than conventional QCLs. For example, STA-RE QCLs have ηi values 30-50 % higher than conventional QCLs: ~ 77 % at λ = 5 µm, over the 4-6 µm range [3]; and ~ 86 % at both λ = 8.4 µm and λ = 8.8 µm, over the 7-11 µm range [2].
Fig. 1. Internal efficiency as a function of emitting wavelength for various mid-IR QCL types. The references for the data points are given in [3].
One result of a ~ 90% upper ηi limit is that the WPE ultimate limits increase by ~ 35 % vs. previous estimates that considered ηi ≈ 67 % [2] (Fig. 2). The data points are referred to in [2] except for a 14 % single-facet value for STARE QCLs, due to unoptimized, high αw values; and a 12 % both-facets point at λ = 8 µm [5]. For λ = 4.6 µm it is clear that WPE values of ~ 40 % can be reached. For STA-RE-type QCLs, due to much stronger impact of elastic
Fig. 2. Fundamental limit for the WPE of mid-IR QCLs (i.e., the red, ηi = 90 % curve) vs. emission wavelength, and WPE data points [2,5]
scattering on τll than on τul [3], the ηi upper limit becomes ~ 95 %, which leads to pulsed WPE upper limits as high as 44 % [3]. As a consequence, ~ 40 % CW WPE becomes possible for 4.6 µm-emitting QCLs. In the 4.6-5.0 µm range, single-facet CW power as high as 5.1 W has been achieved from MBE-grown TA-REtype QCLs [4]. MOCVD-grown QCLs have also achieved multiwatt CW powers, as seen from a 5.0 µm-emitting STA-RE QCL (Fig. 3 a): 2.6 W single-facet CW power at 15 oC temperature. The maximum, single-facet CW WPE is 12 %. For 8.0 µm-emitting STA-RE QCLs a maximum CW single-facet power of 1 W is obtained (Fig. 3 b); same value reported for other MOCVD- and MBE-grown QCLs emitting in the 8-10 µm range [5]. The maximum CW WPE value is 6 %, which represents the highest achieved single-facet CW value for ~ 8 µm-emitting QCLs.
(a)
(b) Fig. 3. CW power and CW WPE curves for STA-RE QCLs at: (a) ~ 5.0 µm emission wavelength (b) ~ 8.0 µm emission wavelength
In conclusion, by increasing the internal efficiency close to ultimate values: ~ 95 % (i.e., including the effect of elastic scattering), in low-waveguide-loss STA-RE-type QCLs, single-facet CW wallplug efficiencies as high as 40 % and CW output powers in excess of 10 W become possible to obtain from 4.6 µm-emitting QCLs. This work was supported in part by the Air Force Research Laboratory under Grant FA8650-13-2-1616, the Army Contract W911NF-12-C-0033, and the Navy Contract N68335-15-C-0073. [1] Dan Botez, Chun-Chieh Chang and Luke J. Mawst, J. Phys. D: Appl. Phys. 49, 043001 (2016). [2] J. D. Kirch, C.-C. Chang, C. Boyle, L. J. Mawst, D. Lindberg, T. Earles and D. Botez, Opt. Express 24, 24483-24494 (2016). [3] D. Botez, J. Kirch, C. Boyle, K. Oresick, C. Sigler, H. Kim, B. Knipfer, JH Ryu, D. Lindberg et al., Opt. Mater. Express 8, 1378-1398 (2018). [4] Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, Appl. Phys. Lett. 98, 181102 (2011). [5] B. Schwarz, C. A. Wang, L. Missaggia, T. S. Mansuripur, P. Chevalier et al., ACS Photonics 4, 1225–1231 (2017).
