Direct Comb Spectroscopy of Buffer-Gas Cooled Molecules Ben Spaun ISMS, 2015 JILA, NIST and University of Colorado at Boulder

The Optical Frequency Comb.... f rep f CEO N = f CEO + N x f rep

Frequency Comb Applications The Astro-Comb E. Hand, Nature 452, 514 (April 3, 2008). N. Newbury, Nature Photonics 5, 186–188 (2011) Schibli et al. Nature Phonics. (2008) Precision Metrology

N. Newbury, Nature Photonics 5, 186–188 (2011) Molecular absorption Comb modes Spectroscopy with an OFC : -Thousands of simultaneous absorption measurements. -Inherent high spectral resolution. -Broad bandwidth. -Long interaction length (when comb is coupled to an optical cavity) Frequency Comb Applications

N. Newbury, Nature Photonics 5, 186–188 (2011) From Mid-IR to XUV A Cingöz et al. Nature 482, (2012) Frequency Comb Applications

Mid-IR OPO Freq. Comb Tunable mid-IR frequency comb Yb:fiber Comb 10W, 1 μm Fan-out PPLN Crystal μm Tunable Comb Light Adler et al. Optics Letters (2009) Beat note Supercontinuum Fiber To f CEO control f rep lock: Cs clock

Mid-IR OPO Freq. Comb Cavity-Enhanced Direct Comb Spectroscopy High-refl. mirror Broadband, high-reflectivity cavity mirrors: Finesse > 6000 Over 100 nm of comb spectrum coupled through cavity f rep

Mid-IR OPO Freq. Comb Cavity-Enhanced Direct Comb Spectroscopy High-refl. mirror

Mid-IR OPO Freq. Comb Cavity-Enhanced Direct Comb Spectroscopy High-refl. mirror Scan comb f rep to ensure that no absorption features are skipped over. f rep Molecular absorption

Mid-IR OPO Freq. Comb Cavity-Enhanced Direct Comb Spectroscopy High-refl. mirror FTIR Spectrometer wavelength (  m) Fast scanning Michelson interferometer provides singe comb-mode resolution

Applications and Limitations Applications: Trace-gas detection (e.g. breath analysis) -- sensitivity down to ppb demonstrated for multiple molecules Isotopic analysis Tomography of super-sonic jet -- a complete spatial mapping of density, velocity, and temperature Methane Frequency (cm -1 )

Methane Frequency (cm -1 ) Applications and Limitations Applications: Trace-gas detection (e.g. breath analysis) Isotopic analysis Tomography of super-sonic jet Limitations: Spectral congestion makes more complex molecules unresolvable at room temp: Doppler width > line spacing Nitrous Oxide Isoprene

T = 300 K Cooling molecules below 10 K: Simplifies spectra since only lowest rotational levels are populated Reduces Doppler linewidth by at least factor of 5 Nitromethane (CH 3 NO 2 ) Frequency (cm -1 ) Absorption Simplify spectra by cooling molecules T = 150 K T = 50 K T = 8 K

Mid-IR OPO Freq. Comb Add buffer-gas cell inside cavity High-refl. mirror FTIR Spectrometer 300 K 35 K 8 K Molecules He Buffer Gas Edge-welded bellows D. Patterson & J. M. Doyle. Molecular Physics (2012)

Buffer Gas Cell (8K) Cavity Mirror and Piezo Cavity Mirror Molecule Inlet Mechanical Isolation (Bellows) Mid-IR Comb Light to FTIR Stainless Steal Cavity Structure Apparatus

3 + 6 (sym + asym NO stretch) 1 (sym CH stretch) (asym CH stretch) Fractional Absorption Frequency (cm -1 ) CH 3 NO 2 is a model system for large amplitude motion & internal rotation First high resolution (30 MHz Doppler-limited) spectra of the CH stretch region: >1000 transitions observed in three vibrational bands ( cm -1 ) Nitromethane spectrum near 3.3  m

Resolving DC Stark shifts High resolution allows unique Stark- shift signatures to be identified: Excited torsional states (|m|=1,2) split symmetrically in applied E-field Energy E-field m = +1 m = -1 m = 0 E-field = 0 V/cm E-field = 400 V/cm

Resolving DC Stark shifts High resolution allows unique Stark- shift signatures to be identified: Excited torsional states (|m|=1,2) split symmetrically in applied E-field Closely spaced opposite parity states mix to allow for new transitions Energy E-field E-field = 0 V/cm E-field = 400 V/cm E-field = 0 V/cm E-field = 400 V/cm

Resolving DC Stark shifts Grouping lines based on Stark-shift signatures simplifies line assignment process Successfully assigned most lines in ν 3 +ν 6, including many excited torsional transitions.

Observed perturbations in ν 3 +ν 6 Interactions with dark states are indicative of intramolecular (ro)vibrational coupling Mixed bright + dark state

Conclusion Buffer-gas cooling has been successfully integrated with cavity-enhanced direct frequency comb spectroscopy. Over 1000 nitromethane absorption lines spanning multiple vibrational bands in the mid-IR have been observed for the first time. Line assignment process simplified by resolution of small ( MHz) DC-stark shifts. Outlook: probe larger molecules and search for molecule clusters.

Acknowledgments Bryan Changala, Bryce Bjork, Oliver Heckl, Jun Ye JILA|NIST and CU David Patterson and John Doyle Harvard University Physics Dept.