Post on 15-Dec-2015
Development of an External Cavity Quantum Cascade Laser for High-Resolution Spectroscopy of Molecular IonsJACOB T. STEWART, BRADLEY M. GIBSON, BENJAMIN J . MCCALL
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS
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Quantum cascade lasers (QCLs)
•Made from multiple stacks of quantum wells
•Thickness of wells determines laser frequency
•Frequency is adjusted through temperature and current
Curl et al., Chem. Phys. Lett., 487, 1 (2010).
QCLs in spectroscopy• Usage has flourished since
introduction in 1994• Available throughout the
mid-IR (~4-10 µm) – cw and pulsed
• Many commercial vendors sell QCLs
• Good performance for spectroscopy
Our QCL spectrometer• Goal to observe C60 near
8.5 µm• Based on a Fabry-Perot
quantum cascade laser (QCL)
• Uses cavity ringdown spectroscopy
• Has been used to observe CH2Br2, C16H10, Ar-D2O, and (D2O)2
Talk TJ14
Advantages and disadvantages• Good sensitivity• High resolution
(linewidths as narrow as ~12 MHz)
• Ability to observe fundamental bands
• Liquid nitrogen cooling for laser
• Limited frequency tuning (1180-1200 cm-1)
Disadvantages can be overcome with new QCL technology
Broadband gain QCLs•Have several active region designs on a single chip
•Bound-to-continuum active region design
•Combination of the two approaches
Curl et al., Chem. Phys. Lett., 487, 1 (2010).
from http://www.qoe.ethz.ch/research/t-bbmirqcl
Controlling wavelength with an external cavity
Wysocki et al., Appl. Phys. B: Lasers Opt. (2008), 92, 305.
Broad gain QCL chip with thermoelectric cooler
First order diffraction is coupled back into the QCL, forming the external cavity
Three ways wavelength can be controlled: laser current, diffraction grating angle, and EC length
Building the external cavity
• Need to be able to control diffraction grating angle and cavity length
• Entire assembly on optics breadboard for mobility
Putting it all togetherlaser
mount
diffractiongrating
outputmirrors
Can be used with other broadband QCLs from 7-14 µm
Mode-hop free tuning• Mode-hops
can be avoided by controlling all tuning elements
• >0.6 cm-1 of tuning achieved
Frequency instability• Frequency instability has been observed by wavemeter
and aligning ringdown cavity• Jitter of about 225 MHz as measured by wavemeter• Most likely sources: mechanical vibrations coupling into
the external cavity• Have put rubber under laser and cavity to try and damp
vibrations• May need to use active feedback and lock laser to
ringdown cavity
What do we want to do with the EC-QCL?
Band near 1180 cm-1
Our usual target near 1184 cm-1
Broad peak centered at 1250 cm-1 in IRMPD spectrum
H5+
CH5+
Future work• Improve
frequency stability
• Initial testing of EC-QCL with neutral molecules
• Integrate EC-QCL system with ion sources
Conclusions• We have built a EC-QCL capable of tuning over 85 cm-1 • The external cavity system can also be used for other
QCLs throughout the 7-14 µm region• We have achieved mode-hop free tuning over a range
of >0.6 cm-1
• The EC-QCL is capable of observing H5+ and CH5
+, as well as other molecules and molecular ions
Acknowledgments• McCall Group• Tracy Tsai• Gerard Wysocki
Springborn Endowment
http://bjm.scs.illinois.edu