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

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?

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