Fast Sweeping Direct Absorption subMillimeter Spectroscopy Based on
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
Speeding up (sub)millimeter Spectroscopy • Standard (sub)millimeter spectroscopy, lock-in detection, search problem • Microwave spectroscopy 2 -40 GHz (need R~2. 5 x 106 ) • (sub)Millimeter wave spectroscopy 50 -1000 GHz (need R~1. 5 x 108 ) • We require a broadband but sensitive technique
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. 2. 3. 4. 5. 6. Brown et al. , Rev. Sci. Inst. , 2008 Zaleski et al. , J. Mol. Spec. , 2012 Park at al. , J. Chem. Phys. , 2011 Steber et al. , J. Mol. Spec. , 2012 Gerecht at al. , Opt. Express, 2011 Neill et al. , Opt. Express, 2013 7. Petkie et al. , Rev. Sci. Inst. , 8. Medvedev et al. , J. Mol. Spe 9. Fortman et al. , Ap. J. , 2010 10. Medvedev et al. , Opt. Lett.
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 k. Hz)
Experiment Mixer AWG 0 – 5 GHz BPF 9. 1 – 14. 2 GHz LPF 0 – 5 GHz Detector DDG 10 MHz Rb Clock Sample Cell NI Digitizer Computer x 2 19 d. B Attenuator Microwave Synth 0 - 50 GHz Preamp AMC x 3 -27 50 GHz – 1 THz 600 Hz 1. 5 ms sweeps
Time response of bolometer • Frequency sweep from 144100 to 146100 MHz • 1. 5 ms duration at a rate of 600 Hz • 1, 000 averages taken over an hour • Sweep rate of 1. 333 THz/s (1. 333 MHz/µs)
Time response of bolometer
Time response of bolometer
Background subtracted response
Filter baseline • Filter the Fourier transform using low pass, high pass, and notch filters • Spline fit using masking
2 nd derivative spectrum
Comparison with lock-in spectrum • Compared to lock-in 2 nd derivative lineshape • Lock-in gives higher SNR, but sweep is faster
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
Higher Frequency, Faster Sweep
Background subtraction at very high frequency
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
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
Fast Sweep Direct Absorption • Fast sweeping without the AWG • For use in pulsed experiments See Luyao Zou’s talk this afternoon Radicals TH 03
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
Acknowledgements • Widicus Weaver Lab • NSF #CHE-1150492 • NSF #CHE-1404341
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