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.

260-296 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 frequenciesFASSST: µW4 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.

Laser Frequency Combs Low duty cycle Three independent paramenters: T, trep, Δ

Repeat n times

T

trep

Δt

1/trep

Bandwidth 1/Δtνsp

1/trep = νsp

1/ T = Δ

1 F Adler, M.J. Thorpe, K.C. Cossel, J. Ye, Annu. Rev. Anal. Chem. 3 (2010) 175-205.

Chirped-Pulse Frequency Combs

100% duty cycle

E(t)

Repeat n times

t

T

tchirps

f1f2

http://www.nanotron.com/EN/CO_techn-css.php

ϕ1Φ1 + Δϕ11/tchirp = νsp

1/ T = Δ

Δf = f2 – f1

Bandwidth Δf x 24Δν/ νsp = 1/n

Demonstration of Frequency Combs

Fie

ld A

mp

litu

de

(V)

Time Domain Spectrogram

Fourier Transform

trep

Fie

ld A

mp

litu

de

(mV

)

Fie

ld A

mp

litu

de

(mV

)

1/trep

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/trep

1/trep

From Neill, J.L. et al. International Symposium on Molecular Spectroscopy, 66 th meeting, Talk RC06

Frequency Comb Shape

Micropulse: 20 sMacropulse: 1 msTooth Spacing: 50 kHzTooth Width: 1 kHz

Acrylonitrile 5 mTorr 400 MHz

bandwidth for the frequency combs

Absorption vs Emission

S/N: 7:1Single Acquisition: 1 msPressure: 5 mTorr

S/N: 45:1Single Acquisition: 2 µsPressure: 2 mTorr

High Pressure Spectrum

OCS5 Torr

spectrum5 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 chirpLarge variations in the overall profile

Comb Compression3

Segmented Frequency Combs

3 I. Coddington, W.C. Swann, N.R. Newbury, Phys. Rev. 82, 2010, 043817

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-0847919

+

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