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 , Charlottesville, VA Kevin O. Douglass, David F. Plusquellic NIST, Optical Technology Division, Gaithersburg, MD Eyal Gerecht NIST, Electromagnetics Division, Boulder, CO 80305

Extending Chirped Pulse Spectroscopy into the Millimeter/Submillimeter More difficult due to lower power levels available Field group (MIT): GHz, 30 mW G.B. Park et al., 64 th ISMS, 2009, RH07 NIST: , 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

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

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

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

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

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

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

Segmented Chirped-Pulse Spectroscopy black: chirped pulses (Ch1) blue: local oscillator (Ch2) -detection bandwidth: 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)

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, GHz) GHz GHz GHz

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

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 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) S.L. Whittenburg, Spectrochim. Acta. A 52 (1996) 1169.

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

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

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

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

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

Compression of Chirped Pulse Frequency Combs Unfold comb cis-trans ethyl formate, averages GHz, detected in 923 MHz

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.

+ 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, /2 Expanded View Problem: Noise Folding

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,