1. Fast Sweeping Direct Absorption (sub)Millimeter Spectroscopy Based on Chirped Pulse Technology Brian Hays 1, Steve Shipman 2, Susanna Widicus Weaver 1 1. Emory University 2. New College of Florida

2. Speeding up (sub)millimeter Spectroscopy Standard (sub)millimeter spectroscopy, lock-in detection, search problem Microwave spectroscopy 2-40 GHz (need R~2.5 x 10 6 ) (sub)Millimeter wave spectroscopy 50-1000 GHz (need R~1.5 x 10 8 ) We require a broadband but sensitive technique

3. Current Techniques cp-FTMW spectroscopy has been extended to the (sub)millimeter 1,2,3,4,5,6 Relies on expensive (sub)millimeter receivers FASSST technique uses time correlation to enable fast sweeping in the (sub)millimeter region 7,8,9,10 Used for full band scans in various experiments 1.Brown et al., Rev. Sci. Inst., 2008 2.Zaleski et al., J. Mol. Spec., 2012 3.Park at al., J. Chem. Phys., 2011 4.Steber et al., J. Mol. Spec., 2012 5.Gerecht at al., Opt. Express, 2011 6. Neill et al., Opt. Express, 2013 7. Petkie et al., Rev. Sci. Inst., 1997 8. Medvedev et al., J. Mol. Spec., 2004 9. Fortman et al., Ap. J., 2010 10. Medvedev et al., Opt. Lett., 2010

4. Fast Linear Frequency Sweeps Detect the time response of a frequency sweep Apply linear correction for the frequency calibration Detect with bolometer for high sensitivity Limited by the detector bandwidth (~500 kHz)

5. Experiment Microwave Synth 0 - 50 GHz AWG 0 – 5 GHz LPF 0 – 5 GHz DDG 10 MHz Rb Clock Mixer BPF 9.1 – 14.2 GHz Preamp x2 19 dB Attenuator AMC x3-27 Sample Cell Detector NI Digitizer Computer NI Digitizer Computer 50 GHz – 1 THz 600 Hz 1.5 ms sweeps

6. Time response of bolometer Frequency sweep from 144100 to 146100 MHz 1.5 ms duration at a rate of 600 Hz 1,000,000 averages taken over an hour Sweep rate of 1.333 THz/s (1.333 MHz/µs)

7. Time response of bolometer

9. Background subtracted response

10. Filter baseline Filter the Fourier transform using low pass, high pass, and notch filters Spline fit using masking

11. 2 nd derivative spectrum

12. Comparison with lock-in spectrum Compared to lock-in 2 nd derivative lineshape Lock-in gives higher SNR, but sweep is faster

13. Higher Frequency, Faster Sweep Frequency sweep from 336300 to 340300 MHz 1.5 ms duration at a rate of 600 Hz 10,000 averages taken over five minutes Background subtracted, then differentiated

14. Higher Frequency, Faster Sweep

15. Background subtraction at very high frequency

16. High Frequency Data Frequency sweep from 889500 to 891000 MHz 1.5 ms duration at a rate of 600 Hz 10,000 averages taken over five minutes Background subtracted, not differentiated

17. Broadband at high frequencies Frequency sweep from ~826000 to ~835000 MHz 1.5 ms duration at a rate of 600 Hz 10,000 averages taken over five minutes Background subtracted, not differentiated Very fast sweep rate, 6 THz/s

18. Fast Sweep Direct Absorption Fast sweeping without the AWG For use in pulsed experiments See Luyao Zou’s talk this afternoon Radicals TH03 See Luyao Zou’s talk this afternoon Radicals TH03

19. Conclusions Extension of broadband techniques into the (sub)millimeter for absorption spectroscopy The frequency agility using an AWG was combined with the sensitivity of a bolometer, to produce very fast and highly sensitive spectra Provides a solution for addressing the search problem in (sub)millimeter spectroscopy

20. Acknowledgements Widicus Weaver Lab NSF #CHE-1150492 NSF #CHE-1404341