Heterogeneous THz quantum cascade lasers: Broadband operation

We demonstrate the operation of broadband heterogeneous terahertz quantum cascade lasers by carefully designing sub-stacks to align at the same field. Time domain spectroscopy measurements confirm that a flat gain spectrum is present and when incorporated into metal-metal waveguides we find broadband operation over 380 GHz when metal-metal ridges with non-vertical side-walls are used. In recent years there has been substantial growth in the research and development of terahertz (THz) sources. A THz source which has yet to reach its potential is the THz quantum cascade laser (QCL). These sources are based on intersubband transitions in quantum wells and are powerful, electrically driven and show operating temperatures of up to 187 K [1]. QCLs have intrinsically narrow gain spectrum, however for some applications a broad gain spectrum is very desirable. A THz QCL with a broad gain bandwidth could be used as an amplifier in THz TDS systems, as the gain medium in a tunable cavity [2] or in modelocking [3]. Broadband mid-IR QCLs make use of heterogeneous active regions [4], where two or more sub-stacks are combined into the same active region. We have recently shown two-colour lasing [5] from heterogeneous THz QCL designs and here we build on these results to produce devices which show broad band lasing. The heterogeneous two-colour design of reference [5] did not show the performance that was expected from the constituent sub-stacks; there was an elevated threshold for the higher frequency sub-stack and evidence from TDS measurements to suggest that a difference in alignment fields between the two sub-stacks was the cause. The solution to the issue of different electric fields is to use the flexibility of the QCL design to re-engineer the active regions to operate at the same electric field as well as the same current.