repost- Dave in Ohio "Terahertz -Quantum Cascade Laser update from SPIE D&S 2009

This post comes from Dave in Ohio, on the companion group page.

Thanks Dave!

"Federico Capasso who is working on QCL in the terahertz range at
Harvard University presented a paper at the recent SPIE Defense &
Security Symposium entitled "Terahertz quantum cascade lasers;
physics, design and applications."
I quote the session summary on the SPIE web page: (http://spie.org/
x6768.xml)
"Capasso noted that terahertz quantum cascade lasers are still in the
research phase. Their main limitation, at this time, is operation at
only very low temperatures, below the range that can be reached by the
use of thermoelectric coolers (TECs). The root cause of this appears
to be the depopulation of the upper excited states thru nonradiative
transitions. This has prompted research in new approaches to designing
terahertz sources. One approach is a nonlinear QCL design utilizing
difference frequency generation (DFG). In this approach, a dual
wavelength (short wavelengths) laser is designed and the difference
frequency light is generated (at terahertz frequency) in the nonlinear
gain medium.
"The issue with this approach is that the coherence length for the
nonlinear region is very short. To address this, current designs
incorporate a surface emission scheme that allows multiple regions to
contribute to the output power. With this approach, fractions of a
microwatt have been achieved at room temperature. Other design
enhancements hope to improve on these results. Terahertz laser sources
will find applications in medical imaging, imaging for security
applications and local oscillators for coherent astronomical
measurements."