Recent Progress in Chirped-Pulse Fourier Transform THz Spectroscopy Eyal Gerecht1, Ron Goldfarb1, David Plusquellic2 and Kevin Douglass2 National Institute of Standards and Technology 1. Electromagnetics Division, Boulder, CO 2. Optical Technology Division, Gaithersburg, MD

Introduction : Quantifying Greenhouse Gases There is an initiative at NIST to develop measurement technologies to quantify greenhouse gas emissions over a range of length scales for studying Climate change or to develop carbon mitigation strategies As well as provide a supportive role such as providing spectroscopic parameters for HITRAN The technique that I will be talking about could be used as a point source monitor May be possible to develop an open path version

Multi-Component Gas Monitor GHGs, VOCs, or breath analysis N2O NO Formaldehyde Acetone Methanol CO2 (18O) 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 Ethanol 0.805 0.875 THz

Room Temperature D2O 4000 avgs 10 mTorr 2µs pulse 60 MHz bandwidth Low Q cavity 4000 avgs 10 mTorr 120:1 RMS 40:1 pp BW 60 MHz power 1 watt 2 us pulse Maybe Q 100

Room Temperature D2O Simulation using JPL database

Predicting Sensitivity in the THz S/N = 120:1 for D2O at 10 mTorr total pressure at 10 GHz Signal loss due to limited power - 660 times stronger line intensity for N2O at 853.3 GHz Operate at Doppler Limited Pressure ~ 250 mTorr ~ 20 – 200 Net Gain Does not take into account Q or coupling efficiency (which was likely small in the semi-confocal cavity) not more than 16 Really just tells us we think this should work, this is just for emission measurement For absorption if we can get the light through the cell and pick a strongly absorbing line we are guaranteed to see signal 4000 Expect 36000 : 1 S/N Low ppm sensitivity if scaling from 20:1 signal 1%D2O in N2

Chirped THz Pulse Experimental Setup: Gaithersburg Lab 10 MHz Ru Standard ARB 480 MHz bandwidth Chirp at 540 GHz Synthesizer 10-18 GHz x48 Digital Oscilloscope - LeCroy 8zi the next step is actually making chirped pulses – you don’t need much bandwidth in MW so a lower cost ARB is fine. You also need to make a really clean chirp at MW freqs (with either filtering or special mixers Explain diagram One of the great things about the chirp is that the bandwidth gets multiplied up which so you can tell immediately if you have generated a chirp at the final output frequency IF amp KVARZ Synthesizer 78 - 118 GHz THz Harmonic Mixer G.G.Brown, B.C.Dian, K.O.Douglass, S.M.Geyer, S.Shipman and B.H.Pate, Rev.Sci.Instrum. 79 (2008) 053103.

Chirped Pulse before Multiplication

Chirped THz Pulse Experimental Setup: Gaithersburg Lab - Planned 10 MHz Ru Standard DG ARB Synthesizer 10-18 GHz x48 Digital Oscilloscope - LeCroy 8zi 40 GS/s Receiver is on order as well as an upgrade to existing multiplier chain Capability will be from 100 GHz to 900 GHz various gaps Long path absorption cell IF amp HP Synthesizer 10 - 18 GHz

White Cell Proof of Principle D2O Room Temperature 10 mTorr Room Temperature measurements of D2O At Microwave Frequencies 220:1 RMS or 80:1 pp Likely a factor of 10 due to poor coupling

FT-THz Setup: Boulder Lab DPO70604 6 GHz Bandwidth 10 MHz Ru Standard DG Synthesizer 10-18 GHz nx72 = 849.6 GHz x72 1 Meter Cell Room temperature detector!! x72 Synthesizer 2 10-18 GHz IF amp nx72 = 850.6 GHz mixer Room Temp. nbeat = 1GHz

THz Chirp Pulse dBm

N2O: Absorption and Emission For the Measurement we chose N2O in the range of our multiplier 853 GHz in a 1 meter path length cell which is predicted to absorb nearly 90% of the light Emission and absorption are square of mag FT ~ 3 MHz FWHM

Absorption 40,000 avgs HITRAN Measured 490:1 RMS 175:1 pp Widths about 2.6 MHz and 3 MHz 10% Absorbance 1.92 and 1.5 ~20% Step size ~380 kHz measured slightly asymetric

Free Induction Decay Emission Room temp. N2O 250 mTorr RMS 100:1 JPL center Freq. Δ 40 kHz 160,000 avgs – 20 minutes

N2O and Methanol: 300 MHz Bandwidth 250 mTorr total 40000 avgs

FT-THz Setup – Enhanced Detection DPO70604 6 GHz Bandwidth 10 MHz Ru Standard DG ARB Synthesizer 10-18 GHz nx72 = 849.6 GHz x72 Long path absorption cell 4K system nbeat = 1GHz x72 Synthesizer 2 10-18 GHz nx72 = 850.6 GHz IF amp 8 GHz Bandwidth HOT Electron Bolometer A passive heterodyne hot electron bolometer imager operating at 850 gigahertz”, Gerecht, E., Gu, D., You, L., and Yngvesson, K. S. IEEE Trans.Microw.Theory Tech. 2008, 56(5), 1083-1091

Conclusions Demonstrated x72 and x48 multiplication of a chirped pulse Phase locked Chirped Pulse measurements at 850 GHz of both absorption and emission signals of N2O and Methanol with a bandwidth of 300 MHz Next Generation system expect a factor ~400 improvement in sensitivity

Acknowledgements Virginia L. Perkey – SURF student Gerald Fraser