Simulation of surface acoustic wave modulation of quantum cascade lasers

Frequency tunable quantum cascade lasers (QCLs) with a broad wavelength tuning range
are highly desirable in chemical sensor/spectroscopic applications owing to the wide range
of frequencies, from terahertz up to mid-infrared, which must be scanned. QCLs represent
the only convenient, low-cost radiation source over this range of frequencies, however little
progress has been made in achieving broadband tunability. Surface acoustic waves (SAWs)
present an opportunity for achieving broadband modulation by passing a SAW through the
gain medium of the QCL. The electric �eld generated by the SAW via the piezoelectric e�ect
will modulate the carrier concentration within the gain medium causing distributed feedback
(DFB) in the QCL. Unlike conventional DFB mechanisms such as etched gratings on the
QCL surface, the wavelength of the SAW, and therefore the pitch of the DFB, can be altered
allowing tunability in the QCL frequency.
In this work, a theoretical investigation into the interaction between a SAW and the
free-carriers within the QCL active region is presented, with particular focus on whether
this interaction is strong enough for DFB to occur. Numerical models of both QCL active
regions and SAW propagation through semiconductor materials are developed and used in
conjunction to simulate the modulating e�ect of the SAW on the carrier concentration in
the QCL. It is shown that the magnitude of this modulation is large enough for DFB occur,
giving a DFB coupling constant comparable to, if not larger than, many experimentally
demonstrated DFB QCLs. Finally, device design recommendations are presented which aim
to maximise this DFB coupling constant in order to give the widest possible tuning range of
the QCL emission frequency.