Condensed Matter Seminar Series
THz Quantum-Cascade Lasers
Holger T. Grahn
Paul-Drude-Institut for Solid State Electronics, Berlin, Germany
Thursday
May 6, 12:50 pm,
Room 298, Physics Building
Quantum-cascade lasers
(QCLs) are unipolar semiconductor lasers, which were originally
developed for the mid-infrared spectral region. The laser emission is
based on intersubband transitions either in the conduction or in the
valence band within coupled quantum wells. The active regions,
typically consisting of a few quantum wells, are connected by injector
structures containing again a certain number of quantum wells. The
carrier injection into the active regions occurs via resonant
tunneling. A single period of a cascade structure typically consists of
6 to 20 ultrathin layers. The period is repeated between 20 and 200
times so that the total structure typically contains on the order of
1,000 layers. The complete structures are usually grown by
molecular-beam epitaxy. QCLs for the THz spectral range were
originally developed in 2002. They are very compact THz laser sources
with a significant output power. They are almost exclusively based on
the GaAs/(Al,Ga)As materials system with a typical Al content in the
barriers of 10 to 15%. The total thickness of a complete laser
structure including the waveguides is about 10 µm. In recent
years, THz QCLs with typical output powers in the range of 10 mW have
been realized. The frequency range currently covers 1.2 to 5 THz. The
maximum operating temperatures for pulsed operation are generally below
200 K and for continuous-wave operation below 120 K so that QCLs can
only be operated with cooling. The goal in our design studies is
to achieve a low operating voltage and, at the same time, a high output
power. The THz QCLs are designed and simulated using the
self-consistent solution of the Schrödinger and Poisson equations
in the framework of a one-dimensional scattering-rate approach. Output
powers of up to 10 mW and threshold voltages below 5 V have been
achieved. The maximum operating temperature is 78 K for continuous-wave
operation and 116 K for pulsed operation. For this type of QCL, the
frequency can be electrically tuned over a range of up to 10% of the
central frequency.
Host: Stephen Teitsworth