Jacob T. Stewart , Bradley M. Gibson, Benjamin J. McCall

1. 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

2. 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).

3. 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

4. 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

5. 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

6. 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).

7. 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.

8. Building the external cavity • Need to be able to control diffraction grating angle and cavity length • Entire assembly on optics breadboard for mobility

9. Putting it all together laser mount output mirrors diffraction grating Can be used with other broadband QCLs from 7-14 µm

10. EC-QCL performance • Tuning range increased to ~85 cm-1 • Power comparable to previous lasers

11. (Lack of) Mode-hop free tuning QCL chip mode hop EC mode hop

12. Mode-hop free tuning • Mode-hops can be avoided by controlling all tuning elements • >0.6 cm-1 of tuning achieved

13. 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

14. What do we want to do with the EC-QCL? H5+ CH5+ Band near 1180 cm-1 Our usual target near 1184 cm-1 Broad peak centered at 1250 cm-1 in IRMPD spectrum

15. Future work • Improve frequency stability • Initial testing of EC-QCL with neutral molecules • Integrate EC-QCL system with ion sources

16. 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

17. Acknowledgments • McCall Group • Tracy Tsai • Gerard Wysocki Springborn Endowment http://bjm.scs.illinois.edu