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THz Studies of Water Vapor Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic NIST/Optical Technology Division/Physics Lab Gaithersburg,

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Presentation on theme: "THz Studies of Water Vapor Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic NIST/Optical Technology Division/Physics Lab Gaithersburg,"— Presentation transcript:

1 THz Studies of Water Vapor Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic NIST/Optical Technology Division/Physics Lab Gaithersburg, MD 20899

2 Motivation THz studies are of importance to Climate modeling Radio Astromony Satellite-based remote sensing Acura/Aura/Far IR Space Telescope EM wave propagation over wide range of atmospheric conditions mm-wave have less sensitivity to cloud contamination vs infrared and UV Major importance for ozone chemistry and for the greenhouse effect Experimental advantages in the THz region for water vapor Discrete line shape is nearly pure Lorentzian for pressures > 1 Torr Doppler contributions are <5 MHz at room temperature Continuum aborption Insensitivity to far-wind line shape model

3 Challenges Two sources of absorption in this region Discrete line absorption Continuum absorption Self- and air-pressure broadened widths, shifts, and the temperature dependence of these parameters needed before estimates of continuum absorption

4 The Terahertz Gap Pure Rotational Spectroscopy for H 2 O ( 18 O), HDO and D 2 O 1 THz  33.3 cm -1 or  300  m Terahertz (THz) ν0.06 THz to 3 THz ν 2 cm -1 to 100 cm -1 λ5 mm to 100 μm Pure Rotational Lines Far-infrared MW

5 Photomixer chip – 5 x 5 mm + - V bias  15 V 0.2 μm wide fingers separated by 1 μm THz radiation is emitted The photomixers are epitaxial low-temperature-grown GaAs with a gold spiral antenna structure Two CW lasers, offset by THz, illuminate the fingers Conduction band Valence band e-e- ~850 nm + - Photoconductive Switches or Photomixers Photoexcitation produces an acceleration of charge at the beat note of the two lasers 8 x 8  m

6 Performance Limitations 2i o 2 R L  c (  )[  m P 1 P 2 /P o 2 ] [(1+  2  2 )(1+  2 R L 2 C 2 )] P rf (  ) = Conduction band Valence band e-e- ~850 nm   0.25 psec  1/  4 LT-GaAs poor conductor of heat time NIR Driving Fields Beat Note Amplitude on Mixer Surface

7 Bolometer sensitivity 1 pW/Hz 1/2

8 ErAs:GaAs LT GaAs ErAs:GaAs Photomixers New Photomixers deliver more than >5-fold power

9 Why is resolution important in the THz region? S/N Limit ~1% Repeatability minimizes spectral artifacts Current resolution is 2 parts in 10,000 Δν Laser ~ 0.2 cm -1 (0.006 THz) Δν Laser < 0.02 cm -1 ( THz)

10 THz Photomixer Spectrometer for Line Shape Studies

11 Polarization Stabilized HeNe Laser Stabilization Electronics PID Servo Intensity Stabilizer RF Driver Analog Sum Lock-in THz Frequency Calibration System CW Ring Ti:Sap Laser Programmable Ramp (12 Bits) Ti:Sap Laser Electronics AOM Computer 16 Bit Ramp Evacuated Reference Cavity Heater PZT Δν LOCK < 0.5 MHz PID Servo Lock-in Diode Laser Electronics Diode Laser/ Amplifier Δν LOCK < 150 kHz Δν LOCK < 0.5 MHz

12 THz Studies of Ions and Radicals in Etching Plasmas used to Validate plasma models and improve recipes to increase etch uniformity and feature fidelity Instrumental Linewidth < 3.0 MHz

13 AM methods optimal between 10% and 90 % fractional absorption L=53 cm for weak lines 0 THz 3.0 x Abs L=0.3 – 1 cm for strong lines

14 Pure Lorentzian 4 MHz Doppler limited Spike small contribution to line shape Shift <1/20 of line width

15 Self-Width vs H 2 O Pressure Residuals

16 x3 different Temperatures 263, 300, 340 K Self-Width vs H 2 O Pressure

17 Error bars are included

18 Self-Shift vs H 2 O Pressure Error bars are included

19 Temperature Dependence on Width Γ(T) / Γ(T 0 )=(T 0 / T) n where n found between 0.56 – 0.81 δ(T) = (2-5) x cm -1 /atm kHz/Torr is comparable to 100 kHz/Torr found for the line in the mm region At 1.5 Torr H 2 O, MHz changes

20 Parameter Summary for weak lines of H 2 O V. B. Podobedov, D. F. Plusquellic, G. T. Fraser, JQSRT, 87, 377 (2004) 1% on self-widths 5% on self-shifts 10-20% on temp dependence on widths >2-fold variation in shifts

21 THz Studies vs HITRAN for Pure H 2 O at 300 K

22 a W. S. Benedict, L. D. Kaplan, JQSRT, 4, 453 (1964) Open – Experiment, Solid – Theory a J init = J + K a - K c

23 FTIR Instrument Δν Range = 10–250 cm -1 Δν Inst = 0.07 cm -1 Time = 35 min Ti:Sapp Instrument Δν Range = cm -1 / 1 cm -1 Δν Inst = cm -1 Time = 10 min New Ti:Sapp Instrument (single knob tunable) Δν Range = cm -1 Δν Inst = <0.01 cm -1 Time = 30 min THz Instrumentation for H 2 O Foreign Gas Parameters Δν Inst ~ 0.07 cm -1 (2000 MHz) 15 Torr 0.2 cm -1 /atm 0.9 cm -1 /atm 975 Torr

24 M = grooves/mm M4M4 Stepper driven micrometer stage-mounted retro-reflector 10% M6M6 M 3 OC M5M5 M2M2 Ti:Sapp M1M1 M8M8 532 nm Pump 6:1 beam expander Single Knob Tunable Ti:Sapp Laser

25 2 parts in 10,000 High resolution Broadband THz Laser system Range >100 cm -1 at <0.02 cm -1 step resolution

26 Necessary for accurate retrievals of temperature and humidity profiles by EOS Water Vapor Continuum Absorption Water Vapor Continuum High Sensitivity Long Path Length THz Studies V. B. Podobedov, D. F. Plusquellic, G. T. Fraser, JQSRT, 91, 287 (2005)

27 THz White Cell Path Length = 24 m Temperature controlled to >70 C No optical saturation issues LHe cooled Bolometer Evacuated Sample Chamber M0 & M6 Parabolic Photomixer or FTFIR Spec M1 M2 M4 M3 M5 40 Pass White Cell M6 M0 Au Mirrors Vol 3 ft 3 60 mm beam aperature

28 FTFIR Instrument and Sensitivity Polarizing Michelson Interferometer w/ Hg Lamp Source Range = cm -1 Time = cm -1 resolution Drift less than ±1.5 % T Abs 10 = ±0.007 Minimum Values for Continuum Absorption T=297(1) K 2.5 Torr H 2 O 375 Torr N 2 A = A R + A NR A NR = C 1 P 2 H2O + C 2 P N2 P H2O + C 3 P 2 N2 THz Water Vapor Continuum

29 Pure H 2 O Line shape model important for local line absorption

30 Basic choices before application of far-wing absorption model Choice of lineshape function Lorentzian, Van Vleck Weisskopf How far to extend the lineshape Cutoff = 25 cm -1, 100 cm -1, infinite Typically 25 cm-1 used a or no cutoff b Number of water lines to consider Upper cutoff = cm -1 a T. Kuhn, A. Bauer, M. Godon, S. Buhler, K. Kunzi, JQSRT 74, 545 (2002) b J. R. Pardo, E. Serabyn, J. Cernicharo, JQSRT 68, 419 (2001) Models of Local & Far-Wing Line Absorption

31 Continuum Absorption of H 2 O Change is <10 % above 1 THz

32 Continuum Absorption of Pure H 2 O Windows where continuum absorbance largest relative to discrete line absorption and uncertainties in line intensities smallest HITRAN 01 Γ self = 4.8 Γ air Expected ν 2 dependence found P air = 1.11 P N2

33 Continuum Absorption of H 2 O / N 2 Mixtures

34 A H2O-N2 = A NR – A H2O Continuum Absorption of H 2 O / N 2 Mixtures A NR = A Total - A R A NR

35 Potential Sources of discrepancy Near-wing line shape model Number of lines included to model resonant absorption Self-broadening and foreign parameters used α(ν,T) = A * P H2O * P N2 * ν 2 * (300/T) B Continuum Absorption of H 2 O / N 2 Mixtures Q. Ma, R. H. Tipping, J. Chem. Phys. 117, (2002) T. Kuhn, A. Bauer, M. Godon, S. Buhler, K. Hunzi, JQSRT, 74, 545 (2002) From the perspective of atmospheric modeling, the total absorption is what is important!

36 Conclusions Current results on Self-width (1%), self-shift (%5) and temperature dependence of 6 weak lines from 12 cm cm -1 ( THz) Continuum absorption of H 2 O-H 2 O and H 2 O-N 2 Planned or in progress: Self-width, self-shift and temperature dependence for strong lines Foreign-width, shift and temperature dependence for strong lines Temperature dependence of the H 2 O-H 2 O and H 2 O-N 2 continuum

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38 Overlapping Scans to within +250 MHz FSR = MHz

39 Optically pumped THz photomixer Operational range0.1 – 4.5 THz Output power W Linewidth1 MHz Frequency drift<0.3 MHz/hour

40 Chamber Lens

41 Slow Wave Structure Electron Beam Backward Wave Oscillators B Waveguide Cathode Collector d < + Strong Interaction of e and electromagnetic waves v L Fast Feedback R k = k ph  = L 1 v ~ v e  ~ V L 2eV m e v e = - 1 to 20 mW 3 to 6 kV ~1 to 0.03 mm v e - ~ 6 to 10 kG f f ~ 30% 

42 Continuous-Wave Backward-Wave Oscillators Power: 1 mW to 50 mW Linewidth: ~ 10 kHz Frequency Range: to 1.2 THz Bandwidth: 30 GHz to 200 GHz, dependent on frequency Magnetic Field: 10 kG using permanent or electromagnetics. Sensitivity approximately % fractional absorption for 1 s integration. BWO’s used: 78 – 118 GHz (156 – 236 GHz with doubling) GHz GHz

43 Frequency Modulation Synthesizer GHz Mixer SRS Lock-In InSb Bolometer 4.2K BWO BWO-based Spectrometer GHz PC A/D & D/A Beam Splitter PLL Synchronizer BWO Control  F=100 MHz,  =2  s  IF=350 MHz R=100  Reference Clock f ref Low-noise Amp High Voltage Power Supply FuG Voltage Control From D/A Card GPIB

44 TT GG TG Agent precursor diethyl sulfide – CH 2 -CH 3 -S-CH 2 -CH 3 > 15% fractional absorption predicted Detection limit using AM methods demonstrated near 0.2% Potential of THz Methods for Detection of Chemical Agents 0.1 Torr in 100 Torr air sample Three conformers populated at room temperature Conformers intensities scaled according to MP2/ G(d,p) energies and dipole moments squared. Most vibrational sequence levels overlap within the pressure broadened linewidth ~1 GHz

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47 Continuum Absorption of H 2 O

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49 Grating-tuned Ti:Sap Laser Pump Laser 30 mW 850 nm Diode Laser Diode Amplifier Laser Cal. & Stabilization Isolator PhotoCurrent Photomixer and Si lens Computer Lock-in Bolometer Evacuated Sample Chamber waveplate THz Spectrometer BS Chopper Amplitude Modulation ~ 400Hz

50 Transmission Properties in the THz Region THz Scans Performed in Vacuum Plastic, Paper, Wood transparent

51 Multi-pass White Cell M1 M4 M2 M6 Bolometer Far IR Spectrometer or THz photomixer Source M5 M3 Size 3 ft 3 High-Resolution THz Laser Studies of H 2 O Path Length = 20 to 40 m No optical saturation issues Heatable to 100 C


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