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