1. Techniques for High-Bandwidth (> 30 GHz) Chirped-Pulse Millimeter/Submillimeter Spectroscopy Justin L. Neill, Amanda L. Steber, Brent J. Harris, Brooks H. Pate University of Virginia, Department of Chemistry, University of Virginia, McCormick Rd, PO Box 400319, Charlottesville, VA 22904 Kevin O. Douglass, David F. Plusquellic NIST, Optical Technology Division, Gaithersburg, MD 20899 Eyal Gerecht NIST, Electromagnetics Division, Boulder, CO 80305

2. Extending Chirped Pulse Spectroscopy into the Millimeter/Submillimeter More difficult due to lower power levels available Field group (MIT): 70-100 GHz, 30 mW G.B. Park et al., 64 th ISMS, 2009, RH07 NIST: 540-620, 810-870 GHz, ~1-10 mW (trace gas sensing) K. Douglass, 65 th ISMS, 2010, WH09 E. Gerecht, K.O. Douglass, D.F. Plusquellic, Optics Express, 19, 8973 (2011) Pulse GenerationDetection

3. Extending Chirped Pulse Spectroscopy into the Millimeter/Submillimeter Power increasing in solid-state active multiplier chains System bandwidth exceeds that of currently available oscilloscopes Fast digitization rates limit data throughput speed Goal: Techniques that can measure broadband (>30 GHz) spectra in < 1 ms Virginia Diodes 840 GHz Active Multiplier Chain http://vadiodes.com/images/stories/systems/788-860-S155-wr12.pdf

4. AWG Can Be the Local Oscillator Two-channel AWGs are available with up to 6 GHz of bandwidth per channel (more than enough for mmw/sub-mmw spectroscopy) Single system can measure both absorption and emission spectra: Absorption: chirped-pulse frequency combs Emission: segmented chirped-pulse Fourier transform spectroscopy

5. Field Amplitude (V) Chirped Pulse Frequency Combs (13.5-18.0 GHz) Time Domain Spectrogram Fourier Transform t rep Field Amplitude (mV) 1/t rep Expanded View (roll-off due to digitizer)

6. Multiplication of Frequency Comb (27-36 GHz) Bandwidth is extended; frequency comb spacing remains the same Before Multiplication After Multiplication 1/t rep

7. Measuring Absorption Spectra with Chirped Pulse Frequency Combs 3,3,3-trifluoropropyne 1,000 signal averages (pulsed jet) 1 ms freq. combs, 9 GHz bandwidth 50 kHz comb spacing J=5-4 J=6-5 single pulse noise level

8. Measuring Absorption Spectra with Chirped Pulse Frequency Combs Methyl cyanide, 10 mTorr (J=30-29, K=3, 2, 1, 0)

9. Segmented Chirped-Pulse Spectroscopy black: chirped pulses (Ch1) blue: local oscillator (Ch2) -detection bandwidth: 100-350 MHz -in weak pulse limit, equal sensitivity per measurement time segmenting -many fewer data points collected because of low digitization rate -issue: spectral purity of arb (need good pulse filtering)

10. Segmented Chirped-Pulse Spectroscopy -65 GHz spectrum, 60  s data collection (“1,000,000 GHz/s” scan rate) FASSST: ~10 GHz/s (Inset: 1.5  s, 1.728 GHz) 804.063-804.351 GHz 804.351-804.639 GHz 804.639-804.927GHz

11. Segmented Chirped-Pulse Spectroscopy -room temperature -25 ns polarization, 125 ns detection (elevated pressure) -S/N ratio ~200:1 on strongest transition

12. Segmented Chirped-Pulse Spectroscopy From time-domain FID can fit Lorentzian and Gaussian components in time domain (need near-zero dead time!) Fit of 8 unblended methanol lines from 510-580 GHz system (4 parameters: amplitude, Doppler width, frequency, phase) 9 mTorr pressure; ~250 ns dead time (due to echo); fixed Lorentzian component  G = 2.375(12) MHz (~5% spread) mass = 29.4(29) amu (actual: 32 amu) Could offer at least “heavy atom” (~10%) mass resolution for each molecule in a complex mixture Sample single-frame fit (FID filtered in frequency domain) R. Coerdt and H. Gronig, Appl. Opt. 28 (1989) 3021. S.L. Whittenburg, Spectrochim. Acta. A 52 (1996) 1169.

13. Conclusions and Future Applications -single instrument design for absorption and emission spectroscopy -low digitizer demands -bandwidth limited only by the device bandwidth -“video frame rate” monitoring of molecular concentrations -coherent detection: signal averaging to improve sensitivity can be performed -Applications: -characterization of complex mixtures (with mass resolution: library-free detection) -broadband detection of transient species -kinetics monitoring

14. Acknowledgements Funding: NSF CCI (Center for Chemistry of the Universe) CHE-0847919 NSF MRI-R 2 CHE-0960074 -Kevin Lehmann -Brian Drouin

16. Amplitude Stability of Chirped Pulse Frequency Comb (frequencies shifted to show amplitude variation) Two single CPFC pulses Single shot fluctuations are ~2%.

17. Upconversion of Chirped-Pulse Frequency Comb: Filtration of Micropulses with Tukey (Tapered Cosine) Filter Suppresses Comb Wings

18. Upconversion of Chirped-Pulse Frequency Comb: Filtration of Macropulse with Kaiser-Bessel Window Improves Baseline Resolution

19. Compression of Chirped Pulse Frequency Combs Unfold comb cis-trans ethyl formate, 6 06 -5 05 4100 averages 27-36 GHz, detected in 923 MHz

20. Frequency Shifting of Chirped-Pulse Frequency Comb The overall frequency of the comb can be shifted by adding in a variable phase shift to the chirped pulses, which can be accomplished because of the complete phase control of the arbitrary waveform generator.

21. + Because there are 5 teeth in the mix comb, the resulting comb is compressed by 10. Compression of Chirped Pulse Frequency Combs Coddington, I., Swann, W.C., Newbury, N.R. Phys. Rev. 82, 2010, 043817 /2 Expanded View Problem: Noise Folding

22. Bandwidth Compression Mixer Compresses 2.4 GHz of spectral coverage (160 combs) into 52.6 MHz Coddington, I., Swann, W.C., Newbury, N.R. Phys. Rev. A 82, 2010, 043817.