Abstract
Terahertz quantum cascade lasers (QCLs) with a broadband gain medium could play an important role for sensing and spectroscopy since then distributed-feedback schemes could be utilized to produce laser arrays on a single semiconductor chip with wide spectral coverage. QCLs can be designed to emit at two different frequencies when biased with opposing electrical polarities. Here, terahertz QCLs with bidirectional operation are developed to achieve broadband lasing from the same semiconductor chip. A three-well design scheme with shallow-well GaAs/Al0.10Ga0.90As superlattices is developed to achieve high-temperature operation for bidirectional QCLs. It is shown that shallow-well heterostructures lead to optimal quantum-transport in the superlattice for bidirectional operation compared to the prevalent GaAs/Al0.15Ga0.85As material system. Broadband lasing in the frequency range of 3.1–3.7 THz is demonstrated for one QCL design, which achieves maximum operating temperatures of 147 K and 128 K respectively in opposing polarities. Dual-color lasing with large frequency separation is demonstrated for a second QCL, that emits at ~3.7 THz and operates up to 121 K in one polarity, and at ~2.7 THz up to 105 K in the opposing polarity. These are the highest operating temperatures achieved for broadband terahertz QCLs at the respective emission frequencies, and could lead to commercial development of broadband terahertz laser arrays.
Highlights
We show that improved performance could be realized for bidirectional quantum cascade lasers (QCLs) when they are implemented with the more robust three-well design scheme; in the following, it is argued that shallow-wells need be employed to achieve the design flexibility that is needed for achieving gain at separate frequencies in opposing polarities
We have demonstrated high-temperature operation of broadband/dual-color terahertz QCLs based on the bidirectional three-well resonant-phonon superlattices
Design flexibility is demonstrated by developing a QCL with wide spectral coverage across a bandwidth of 0.7 THz centered around 3.5 THz for one QCL, whereas dual-color operation is demonstrated for a second bidirectional QCL with ~1 THz frequency separation when the QCL is operated in opposing polarities
Summary
Jth 79 A/cm[2] (4 K) 209 A/cm[2] (4 K) 490 A/cm[2] (forward) (4 K) 330 A/cm[2] (reverse) (4 K) 260 A/cm[2] (10 K) 950 A/cm[2] (10 K) 865 A/cm[2] (10 K) 325 A/cm[2] (10 K) 190 A/cm[2] (10 K) 380 A/cm[2] (forward) (46 K) 420 A/cm[2] (reverse) (46 K) 360 A/cm[2] (forward) (46 K) 700 A/cm[2] (reverse) (46 K). We show that improved performance could be realized for bidirectional QCLs when they are implemented with the more robust three-well design scheme; in the following, it is argued that shallow-wells (with 10%–Al barriers) need be employed to achieve the design flexibility that is needed for achieving gain at separate frequencies in opposing polarities. For such designs, the maximum operating temperature is better or at-par with that of any other previously developed broadband terahertz QCLs. Shallow-wells lead to lower threshold current-densities compared to the threshold current-densities in three-well terahertz QCLs that are predominantly based on 15%–Al barriers[17], which is an additional benefit in the presented designs
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