Development of an External
Cavity Quantum Cascade Laser
for High-Resolution
Spectroscopy of Molecular
Ions
JACOB T. STEWART, BRADLEY M. GIBSON, BENJAMIN J.
MCCALL
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS
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).
2
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
from http://www.qoe.ethz.ch/research/t-bbmirqcl
Curl et al., Chem. Phys. Lett., 487, 1 (2010).
Controlling wavelength with
an external cavity
First order
diffraction is
coupled back into
the QCL, forming
the external cavity
Broad gain QCL
chip with
thermoelectric
cooler
Three ways wavelength can be controlled: laser current,
diffraction grating angle, and EC length
Wysocki et al., Appl. Phys. B: Lasers Opt. (2008), 92, 305.
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 together
output
mirrors
laser
mount
diffraction
grating
Can be used with other
broadband QCLs from
7-14 µm
EC-QCL performance
• Tuning range
increased to
~85 cm-1
• Power
comparable to
previous lasers
(Lack of) Mode-hop free
tuning
QCL chip
mode hop
EC
mode hop
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?
H5+
CH5+
Our usual target near 1184 cm-1
Band near 1180 cm-1
Broad peak centered at 1250 cm-1
in IRMPD spectrum
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