Self-heating in mid-infrared QCLs leads to beam instabilities and facet related failures. Single-element 4.6 μm-emitting BH QCLs were fabricated, where a tapered region scales the output power and, ahead of the emitting aperture, a narrow section provides mode filtering for suppressing high-order spatial modes. Beam-stability measurements indicate a small degree of collimated-beam centroid motion (< 0.25 mrad) can be achieved at >1.5W QCW output powers. Comparisons between short-pulse current and QCW operation reveal the impact of thermal lensing on the beam properties, while full 3D modeling provides insights into influence of device geometry on mode selection.
Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and fast, miniband-like lower-laser (ll) level depopulation. That has led to an internal-efficiency value of ~ 77 % for ~ 5 μmemitting devices, a record-high value for 4.5-6.0 μm-emitting QCLs. We have recently shown that that value can be basically accounted for when considering both LO-phonon- and interface-roughness (IFR)-triggered carrier leakage from the upper-laser (ul) level and injector states. The same carrier-leakage analysis is applied to MOCVD-grown ~ 8.0 μmemitting, STA-type QCLs, and we find that the internal efficiency reaches a high value of ~ 73.6 %, due to a record-high injection-efficiency value (86.7 %) and to an IFR-enhanced laser-transition efficiency value of ~ 85 %. In contrast, for a conventional MOCVD-grown, ~ 8.0 μm-emitting QCLs the internal efficiency is found to be only ~ 55 %, typical of values extracted from experimental data of mid-infrared-emitting conventional QCLs. The ul-level lifetime is found to be controlled by LO-phonon and alloy-disorder (AD) scattering for typical MOCVD-grown QCLs, just like for 4.5-5.0 μm-emitting QCLs. However, for typical MBE-grown QCLs the ul-level lifetime is controlled by LO-phonon, AD and IFR scattering. The ll-level lifetime is found to controlled by both LO-phonon and IFR scattering. Besides the high internal-efficiency value, the use of excited-state injection and a low voltage defect result in the STA QCL reaching a single-facet wall-plug efficiency value of 10.6 %, a record-high single-facet value for 8-11 μm-emitting QCLs grown by MOCVD and holding potential for CW operation.
Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and ultrafast, miniband-like carrier extraction. In turn, that has led to very high internal-efficiency ni values: ~ 77 % and 80-86 % from ~ 5.0 μm- and 8-9 μm-emitting QCLs, respectively. Based on extracted parameters that characterize the interfaceroughness (IFR) scattering, a study has been performed of the effects of elastic scattering, both IFR and alloy-disorder (AD) scattering, on 5.0 μm-emitting STA-QCLs. We find that the laser-transition efficiency ntr is enhanced by ~15 % (i.e., from 83 % to ~ 95 %) due to the much stronger effect of elastic scattering on the lower-laser-level lifetime than on the effective upper-laser-level lifetime. In turn, the injection efficiency: ninj = ni /ntr , reaches ~ 81 %; that is, the highest injection-efficiency value obtained to date from QCLs. Furthermore, we find that the projected upper limit for the pulsed wall-plug efficiency can reach values as high as 44.4 % for 4.6 μm-emitting devices; thus, raising the possibility of CW operation of 4.5-5.0 μm-emitting QCLs with wallplug-efficiency values as high as 40 %.
Implementation of the step-taper active-region (STA) design to 8-9 μm-emitting quantum cascade lasers (QCLs) has resulted in both high T0 and T1 values: 220 K and 665 K, and short lower-level lifetimes: 0.12 ps. In turn, the internal differential efficiency ηid, which is the product of the injection efficiency and the differential laser-transition efficiency, reaches values as high as 86 % for both 8.4 μm- and 8.8 μm-emitting QCLs. Such ηid values are 30-50% higher than those obtained from conventional QCLs emitting in the 7-11 μm wavelength range. Achieving both carrier-leakage suppression and miniband-like carrier extraction in mid-infrared (IR) QCLs leads to ηid values close to the fundamental limit of ~ 90 %. In turn, the currently employed fundamental wallplug-efficiency limits over the mid-IR wavelength range have to be increased by ~ 34 % (e.g., the wallplug-efficiency limit at λ= 4.6 μm increases from 29 % to 39 %). Preliminary results from STA-type 4.8-5.0 μm-emitting QCLs include 1.5 W CW operation, and 77 % internal differential efficiency; that is, 30-50% higher than the ηid values obtained from conventional 4.0-6.5μm-emitting QCLs.
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