Why a Quantum Cascade Laser-Based Spectrometer?

1. High-Resolution Spectroscopy of the ν8 Band of Methylene Bromide Using a Quantum Cascade Laser-Based Cavity Ringdown Spectrometer Jacob T. Stewart and Brian E. Brumfield, Department of Chemistry, University of Illinois at Urbana-Champaign Matthew D. Escarra and Claire F. Gmachl, Department of Electrical Engineering, Princeton University Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

2. Why a Quantum Cascade Laser-Based Spectrometer? • C60 spectroscopy at ~8.5 μm in order to perform an astronomical search • Quantum cascade lasers (QCLs) offer performance necessary for high-resolution spectroscopy at this wavelength

3. How do QCLs work? • Semiconductor laser based on stacks of quantum wells • Lasing occurs through transitions within the conduction band • Different frequencies possible by changing thickness of quantum wells • Each QCL has limited tunability

4. Testing our QCL Spectrometer • Decided on methylene bromide as a test molecule • No previous high-resolution work in IR • Probe of temperature conditions in supersonic jet • ν8 band of CH2Br2 ~1197 cm-1 • Tuning range of our QCL ~1182-1200 cm-1

5. Initial Layout of QCL Spectrometer 800 μm pinhole PC-MCT

6. Experimental Spectra and Spectrometer Performance Reference (SO2) Spectrum Wavemeter CH2Br2 Spectrum Noise equivalent absorption = 1.4×10-8 cm-1 Linewidth = 40 - 60 MHz Step size = ~21 MHz (0.0007 cm-1) Sensitivity = 5×10-8 cm-1Hz-1/2

7. 79 81 79 79 81 81 Assigning the Spectrum • Two Br isotopes with almost equal abundance (79Br & 81Br) • Near-prolate top • Ground state rotational constants known from microwave • Fitting done using PGOPHER • Three isotopologues: Abundance • CH279Br2 1 • CH279Br81Br 2 • CH281Br2 1 PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of Bristol, http://pgopher.chm.bris.ac.uk

8. Room Temperature Spectra

9. Room Temperature Spectra

10. Room Temperature Spectra

11. Jet-Cooled Spectra Experimental Spectrum of Jet-Cooled Sample Combination Trot = 300 K Simulated spectrum Trot = 7 K B. E. Brumfield, J. T. Stewart, S. L. Widicus Weaver, M. D. Escarra, S. S. Howard, C. F. Gmachl, B. J. McCall, Rev. Sci. Instrum. (2010), 81, 063102.

12. Molecular Constants

13. PC-MCT PV-MCT Improvements to the Spectrometer • Faster detector (PV-MCT from Kolmar) • Using a 150 μm×12 mm slit instead of 800 μm pinhole • Ten times smaller current step size • New piezo driver to increase ringdowns per second • New mirror mounts with flexible bellows

14. Not Noise! New and Improved Spectra Noise equivalent absorption = ~4×10-9 cm-1 Linewidth = ~10 MHz Step size = ~2.4 MHz (0.00008 cm-1) Sensitivity = ~8×10-9 cm-1 Hz-1/2

15. Conclusions • Quantum cascade lasers are useful light sources for high-resolution infrared spectroscopy • We have constructed the first QCL-based cw-ringdown spectrometer coupled with a supersonic expansion source • We have obtained and assigned the previously unobserved rotational structure of the ν8 band of methylene bromide

16. What’s Next? • Collect high-resolution spectrum of pyrene (C16H10) • Collect high-resolution spectrum of C60

17. Ringdown Cavity Laser ZnSe Frsenel Rhomb Polarizer Dealing with Back-Reflection • Fresnel rhomb uses total internal reflections to act like a quarter wave plate

18. Acknowledgments • Brian Siller • Andrew Mills • Richard Saykally • Kevin Lehmann • Funding • NASA • Packard Foundation • Dreyfus Foundation • University of Illinois