1. Amanda L. Steber, Brent J. Harris, Justin L. Neill, Kevin K. Lehmann, Brooks H. Pate Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904.

2. 260-296 GHz Schematic 2-3.5GHz

3. Frequency Combs  Traditionally been used in IR and Optical spectroscopy 1,2,3  Good for high pressure systems: pressure broadened lines (as opposed to emission techniques)  Potential for broadband absorption data with fast collection rates  Split the power over many frequencies FASSST: µW 4 CPFC: mW 1 F Adler, M.J. Thorpe, K.C. Cossel, J. Ye, Annu. Rev. Anal. Chem. 3 (2010) 175-205. 2 P. Mddaloni, P. Cancio, P. De Natale, Meas. Sci. Technol. 20 (2009) 052001. 3 I. Coddington, W.C. Swann, N.R. Newbury, Phys. Rev. 82, 2010, 043817. 4 I.R. Medvedev, C.F. Neese, G.M. Plummer, F.C. De Lucia, Opt. Lett. 35 (2010) 1533 – 1535.

4. Laser Frequency Combs  Low duty cycle  Three independent paramenters: T, t rep, Δ Repeat n times T t rep ΔtΔt 1/t rep Bandwidth  1/Δt ν sp 1/t rep = ν sp 1/ T = Δ 1 F Adler, M.J. Thorpe, K.C. Cossel, J. Ye, Annu. Rev. Anal. Chem. 3 (2010) 175-205.

5. Chirped-Pulse Frequency Combs  100% duty cycle E(t) Repeat n times t T t chirps f1f1 f2f2 http://www.nanotron.com/EN/CO_techn-css.php ϕ1ϕ1 Φ 1 + Δ ϕ 1 1/t chirp = ν sp 1/ T = Δ Δf = f 2 – f 1 Bandwidth  Δf x 24 Δ ν / ν sp = 1/n

6. Demonstration of Frequency Combs Field Amplitude (V) Time Domain Spectrogram Fourier Transform t rep Field Amplitude (mV) 1/t rep Expanded View (roll-off due to digitizer) From Neill, J.L. et al. International Symposium on Molecular Spectroscopy, 66 th meeting, Talk RC06

7. Demonstration of Frequency Combs Bandwidth is extended; frequency comb spacing remains the same Before Multiplication After Multiplication 1/t rep From Neill, J.L. et al. International Symposium on Molecular Spectroscopy, 66 th meeting, Talk RC06

8. Frequency Comb Shape Micropulse: 20  s Macropulse: 1 ms Tooth Spacing: 50 kHz Tooth Width: 1 kHz

9.  Acrylonitrile  5 mTorr  400 MHz bandwidth for the frequency combs

10. Absorption vs Emission S/N: 7:1 Single Acquisition: 1 ms Pressure: 5 mTorr S/N: 45:1 Single Acquisition: 2 µs Pressure: 2 mTorr

11. High Pressure Spectrum  OCS 5 Torr spectrum 5 GHz bandwidth

12. Transients  ~10ns for this spectrum

13. Challenges  Phase Stability  Tooth to tooth fluctuations  Transients (reduce efficiency)  Time resolution is set by the tooth resolution  Large Data sets

14. Potential Solutions  Shape the pulse in the Arb  Can use filters and windows in post processing to remove transients  Use smaller bandwidths

15. Three Possible Techniques  Fullband Spectra 36GHz of spectrum in a single chirp Large variations in the overall profile  Comb Compression 3  Segmented Frequency Combs 3 I. Coddington, W.C. Swann, N.R. Newbury, Phys. Rev. 82, 2010, 043817

16. Advantages of Segmented Frequency Combs  Noise won’t fold over  LO purity not much of an issue  Make smaller data files that can be manipulated much faster  Sweeping across the bandwidth would be easy with an Arb  Due to device performances there seems to be: Reduced spurious signals (FC08) Better power response for smaller bandwidths More consistent response of device for smaller bandwidths

17. Summary  Pressure Broadened lines that would not be measurable by emission can be detected.  Allows for fast collection of broadband absorption spectra  Can potentially be collected by 3 methods, using one instrumental setup

18. Acknowledgements  Pate Lab  NSF CCI (Center for Chemistry of the Universe) CHE-0847919

20. + 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, 043817 /2 Expanded View Problem: Noise Folding

21. Using Chirped-Pulse in the mmw Regime  Technological improvements in power sources and digitizers allow chirped- pulse fourier transform mmw spectroscopy  These improvements are: Solid-state active multiplier chains (VDI) Large bandwidth oscilloscopes/faster acquisition oscilloscopes New data collection techniques