Fast Sweeping Direct Absorption (sub)Millimeter Spectroscopy

Based on Chirped Pulse Technology

Brian Hays1, Steve Shipman2, Susanna Widicus Weaver1

1. Emory University2. 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)millimeter1,2,3,4,5,6

• Relies on expensive (sub)millimeter receivers• FASSST technique uses time correlation to enable

fast sweeping in the (sub)millimeter region7,8,9,10

• Used for full band scans in various experiments

1. Brown et al., Rev. Sci. Inst., 20082. Zaleski et al., J. Mol. Spec., 20123. Park at al., J. Chem. Phys., 20114. Steber et al., J. Mol. Spec., 20125. Gerecht at al., Opt. Express, 20116. Neill et al., Opt. Express, 2013

7. Petkie et al., Rev. Sci. Inst., 19978. Medvedev et al., J. Mol. Spec., 20049. Fortman et al., Ap. J., 201010. 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 Synth0 - 50 GHz

AWG0 – 5 GHz

LPF 0 – 5 GHz

DDG

10 MHz Rb Clock

Mixer

BPF 9.1 – 14.2 GHz

Preampx2

19

dB

Atten

uato

r

AMC x3-27Sample Cell

Detector

NI DigitizerComputer

50 GHz – 1 THz

600 Hz1.5 ms sweeps

Time response of bolometer

• Frequency sweep from 144100 to 146100 MHz

• 1.5 ms duration at a rate of 600 Hz

• 1,000,000 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

2nd derivative spectrum

Comparison with lock-in spectrum

• Compared to lock-in 2nd 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 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-1150492• NSF #CHE-1404341