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 10 6 ) (sub)Millimeter wave spectroscopy GHz (need R~1.5 x 10 8 ) 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.Brown et al., Rev. Sci. Inst., Zaleski et al., J. Mol. Spec., Park at al., J. Chem. Phys., Steber et al., J. Mol. Spec., Gerecht at al., Opt. Express, Neill et al., Opt. Express, Petkie et al., Rev. Sci. Inst., Medvedev et al., J. Mol. Spec., Fortman et al., Ap. J., Medvedev et al., Opt. Lett., 2010

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)

Experiment Microwave Synth 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

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

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 to 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 to 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 ~ to ~ 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 TH03 See Luyao Zou’s talk this afternoon Radicals TH03

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 NSF #CHE