CHIRPED-PULSE TERAHERTZ SPECTROSCOPY FOR BROADBAND TRACE GAS SENSING Eyal Gerecht, Kevin O. Douglass, David F. Plusquellic National Institute of Standards and Technology Optical Technology Division, Gaithersburg, MD Recently developed solid state sources and heterodyne detectors for the terahertz frequency range have made it possible to generate and detect precise digitally synthesized waveforms at THz frequencies with ultra-low phase noise. The sample gas is polarized using sub-?s chirped THz pulses and both the absorption and the free inductive decay (FID) signals are detected using a mixer amplifier multiplier chain. This approach allows for a rapid broadband multi-component detection with low parts-per-billion sensitivities and high frequency accuracy. Current acquisition time is 30 seconds for 10.6 GHz of bandwidth. Such a system can be configured into a portable, robust, and easy to use sensing platform. A full description of broadband trace gas sensor operating at 540 GHz to 620 GHz will be presented.

Multi-Component Gas Monitor GHGs, VOCs, or breath analysis Formaldehyde CO Methanol Acetone Ethanol CO2 (18O) N2O NO 0.805 0.875 THz in addition to making measurements to support the hitran database and doing fundamental spectroscopy our Goal is to develop rapid multi chemical gas sensor Variety of applications the common theme is small molecule with a dipole moment With high ppb sensitivity L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005).

Current Multiplier Chain Coverage 255 – 315 GHz 4 mW 520 – 630 GHz 0.8 mW 850 – 945 GHz 0.1 mW

Chirped-Pulse THz Spectrometer AWG 12 GS/s LO YIG 9 GHz Source x48 YIG Mix AMC x48 White Cell E. Gerecht, K.O. Douglass, D.F. Plusquellic, Optics Express, April 22, 2011, Vol. 19, Issue 9, pp. 8973-8984 (2011) Field group (MIT): 70-100 GHz,G.B. Park et al., 64th ISMS, 2009, RH07

High Purity MW Chirped Pulse

100 ns - 10 GHz Chirped THz pulse 12 Frequency (GHz) 140 MHz in the MW 100 Time (ns)

High Speed Data Transfer Remote Data Transfer Wired x4 PCI Express 360 MB/s  26.66 MS/s Record and Transfer  80 MS in 3 s At 40 GS/s: 4000 records in 3 s 80,000 averages in 60 sec. Factor of 4 over just using the scope This method improves the phase stability 1 trigger for every 80 M points – 4000 averages or 8000 avgs

White Cell Power Transmission ~20% Currently aligned for 25 meter path length M3 M5 M1 M2 M4 M6 0.5 m V. B. Podobedov, D. F. Plusquellic, and G. T. Fraser, J. Quant. Spectrosc. Radiat. Transf. 91(3), 287–295 (2005). V. B. Podobedov, D. F. Plusquellic, K. E. Siegrist, G. T. Fraser, Q. Ma and R. H. Tipping, J. Quant. Spectrosc. Radiat. Transf. 109(3), 458-467 (2008).

Artifact of White Cell Alignment 50 Meter path pulse Signal Background Subtracted

Artifact of White Cell Alignment 50 Meter path pulse Signal Background Subtracted

Artifact of White Cell Alignment 50 Meter path pulse Signal Background Subtracted

25 ns - 10 GHz Chirped THz pulse 12 Frequency (GHz) 100 Time (ns)

Direct Absorption of a 5 Component Gas Mix L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005). H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, 883-890 (1998).

Absorption - N2O at 853.3553 GHz Intensity Pressure (mTorr)

Absorption Data Results The Voigt profile is in good agreement for all of the observed lines. However, the residuals (shown below the line in the insert) indicate the observed line is slightly broader. The best fit line shape gives a Gaussian component of ≈1.49 MHz compared to the HITRAN value of 1.20 MHz. The additional 0.9 MHz Gaussian width contribution is very close to that expected for the transform limit of the 500 ns interval.

Correcting Simulated Intensities for FID Response

FID Detection of a 5 Component Gas Mix 10.6 GHz in 500 nsec – 80K averages in 60 sec 100,000:1 H2O 2 ppb x500 x500 OCS 280 ppb FID Signal (a.u.) Acetone 8000 ppb EtOH 1600 ppb MeOH 100 ppb N2O 170 ppb 0.54600 0.54866 0.55133 0.55399 0.55666 THz L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005). H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, 883-890 (1998).

FID Detection of MeOH 10 GHz Bandwidth Field Magnitude 2.5 mTorr Pure * * Signal Scaling? In between limits Intensity CPT66_06-17-11_AgilentTest – SG32_FID_Final.JBW * * 541060 ν / GHz 552505

Extending to Higher Bandwidths Please See Justin Neil RC06 CPT59_05-23-11 - SGBG00_FID_Final.JBW - Not Squared 90 GHz FID near 850 GHz MeOH -1.2 mTorr Pure 2ms acquisition time 779.760 ν / GHz 869.760

Measuring lineshapes from the FID See RB08

Conclusions Demonstrated Chirped pulse THz spectrometer operating in 530 – 630 GHz and 780 – 870 GHz regions Demonstrated phase stable operation Demonstrated high sensitivity and rapid throughput for both absorption and FID emission measurements We continue to make progress on the quantification of FID signals and sensitivities

Acknowledgements Virginia L. Perkey – SURF student Eric M. Vess - SURF student Tektronix – equipment loan NIST National Research Council Program Post Doctoral Research Opportunities http://www.nist.gov/pml/div685/grp08/biophysics-group-research-opportunities.cfm

5 Component Gas Mix – 3 GHz Section Acetone N2O MeOH EtOH