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 , Charlottesville, VA

GHz Schematic 2-3.5GHz

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) P. Mddaloni, P. Cancio, P. De Natale, Meas. Sci. Technol. 20 (2009) I. Coddington, W.C. Swann, N.R. Newbury, Phys. Rev. 82, 2010, I.R. Medvedev, C.F. Neese, G.M. Plummer, F.C. De Lucia, Opt. Lett. 35 (2010) 1533 – 1535.

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)

Chirped-Pulse Frequency Combs  100% duty cycle E(t) Repeat n times t T t chirps f1f1 f2f2 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

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

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

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

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

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

High Pressure Spectrum  OCS 5 Torr spectrum 5 GHz bandwidth

Transients  ~10ns for this spectrum

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

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

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,

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

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

Acknowledgements  Pate Lab  NSF CCI (Center for Chemistry of the Universe) CHE

+ 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

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