THz Studies of Water Vapor Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic...
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Transcript of THz Studies of Water Vapor Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic...
THz Studies of Water Vapor
Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic
NIST/Optical Technology Division/Physics LabGaithersburg, MD 20899
Motivation
THz studies are of importance toClimate modelingRadio AstromonySatellite-based remote sensingAcura/Aura/Far IR Space TelescopeEM wave propagation over wide range of atmospheric conditions
mm-wave have less sensitivity to cloud contamination vs infrared and UVMajor importance for ozone chemistry and for the greenhouse effect
Experimental advantages in the THz region for water vaporDiscrete 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
Challenges
Two sources of absorption in this regionDiscrete line absorption Continuum absorption
Self- and air-pressure broadened widths, shifts, and the temperature dependence of these parameters needed before estimates of continuum absorption
The Terahertz Gap
Pure Rotational Spectroscopy for H2O (18O), HDO and D2O
1 THz 33.3 cm-1 or 300 m
Terahertz (THz)
ν 0.06 THz to 3 THzν 2 cm-1 to 100 cm-1
λ 5 mm to 100 μm
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Pure Rotational Lines
Far-infraredMW
Photomixer chip – 5 x 5 mm
+ -
Vbias 15 V
0.2 μm wide fingers separated by 1 μmTHz 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-
~850 nm+
-
Photoconductive Switches or Photomixers
Photoexcitation produces an acceleration of charge at the beat note of the two lasers
8 x 8 m
Performance Limitations
2io2RLc()[mP1P2/Po
2][(1+ 22)(1+ 2RL
2C2)]Prf() =
Conduction band
Valence band
e-
~850 nm
0.25 psec
1/ 4
LT-GaAs poor conductor of heat
time
NIR Driving Fields
Beat NoteAmplitude on MixerSurface
-9
-8
-7
-6
0 1 2 3 4
THz Power
Lo
g10
( P
) /
W
/ THz
Antenna Theory
Measured on a 4.2 K Bolometer
Bolometer sensitivity 1 pW/Hz1/2
ErAs:GaAs
LT GaAs
ErAs:GaAs Photomixers
New Photomixers deliver more than >5-fold power
0.0 0.2 0.4 0.6 / THz
Rel
ati
ve
Tra
ns
mit
tan
ce
0
50
100
0.0 0.2 0.4 0.6 / THz
% T
ran
sm
itta
nc
e
0
50
100
0.285 0.290 0.295 0.300 / THz
% T
ran
sm
itta
nc
e
0.285 0.290 0.295 0.300
/ THz
Rel
ati
ve
Tra
ns
mit
tan
ce
Why is resolution important in the THz region?
S/N Limit ~1%
Repeatability minimizes spectral artifacts
Current resolution is2 parts in 10,000
ΔνLaser ~ 0.2 cm-1 (0.006 THz)
ΔνLaser < 0.02 cm-1 (0.0006 THz)
THz Photomixer Spectrometer for Line Shape Studies
Nd:YAG (x2)+
Ti:Sap RingLaser
Laser Cal. &Stabilization
Bolometer
Chopper
Evacuated Sample Chamber
DiodeLaser
DiodeAmplifier
BS
40 mWatts @ 850 nm
Photomixer
Single-mode Fiber
OpticalIsolator
1/2 Wave
Brass Cell
Fill/Pump Ports
Thermo-electric PID Controller ±0.2 C
T = 260 – 340 K
Polarization StabilizedHeNe Laser
StabilizationElectronics
PID Servo
IntensityStabilizer
RF Driver
Analog Sum
Lock-in
THz Frequency Calibration System
CW RingTi:Sap Laser
ProgrammableRamp (12 Bits)
Ti:Sap LaserElectronics
AOM
Computer
16 Bit Ramp
Evacuated Reference Cavity
HeaterPZT
ΔνLOCK< 0.5 MHz
PID Servo
Lock-in
Diode LaserElectronics
Diode Laser/Amplifier
ΔνLOCK< 150 kHz
ΔνLOCK< 0.5 MHz
THz Studies of Ions and Radicals in Etching Plasmas used toValidate plasma models and improve recipes to increase etch uniformity and feature fidelity
1.232460 1.232490
0
1
Ab
sorb
ance
(e)
(THz)
HF in Cell
Path = 0.39 m
J = 1 0T ~ 36 %
Gau
= 4.65(3)
T = 300 K
[HF] = 1.5x1013/cm3
19 sccm Ar
1 sccm HF
P=10 mTorr
Lor
= 0.53(4)
1.232460 1.232490
0
1
Ab
sorb
ance
(e)
(THz)
HF in CF3H Plasma
Path = 0.39 m
J = 1 0T ~ 37 %
Gau
= 4.70(3)
T = 300 K
[HF] = 1.4x1013/cm3
PRF
=300 W
P=10 mTorr
19 sccm Ar
1 sccm CF3H
Lor = 0.32(4)
Instrumental Linewidth < 3.0 MHz
AM methods optimal between 10% and 90 % fractional absorption
L=53 cm for weak lines
0 THz 3.0
x175
1
0
Ab
s 10
1
0
Ab
s 10
L=0.3 – 1 cmfor strong lines
1.541520 1.542375
Reproducibility
0
0.4
0.8A
bso
rpti
on
(B
ase
10)
/ THz
349(1) MHz
Lor
(P)
15.2(3) MHz
Trace Water
51.433 cm-1
Rep
= 0.3 MHz+_
300 K / 10 Torr
Pure Lorentzian
4 MHz Doppler limited Spike small contribution to line shape
Shift <1/20 of line width
Self-Width vs H2O Pressure
Residuals
Residuals
1.540600 1.543200
Water Line at 51.433 cm-1A
bso
rpti
on
(B
ase
10)
/ THz
10.02
4.02
0.5
0.0
1.51
0.51
349(1)
Pressure / Torr Lor
FWHM / MHz
19.3(1)
53.0(2)
143(1)
x3 different Temperatures263, 300, 340 K
Self-Width vs H2O Pressure
0
100
200
300
400
0.0 3.0 6.0 9.0 12.0
H2O Self-Pressure Broadening
FW
HM
/ M
Hz
Torr
51.54 cm-1 Line
263 K
340 K
300 K
Self-Width vs H2O Pressure
Error bars are included
Self-Shift vs H2O Pressure
0
5
10
15
0.0 3.0 6.0 9.0 12.0
H2O Self-Shift
F
WH
M /
MH
z
Torr
51.54 cm-1 Line
263 K
340 K
300 K
Error bars are included
Temperature Dependence on Width
60
36
48
Γ(T) / Γ(T0)=(T0 / T)n where n found between0.56 – 0.81
δ(T) = (2-5) x 10-3 cm-1/atm
80-200 kHz/Torr iscomparable to 100 kHz/Torrfound for the 643-550 linein the mm region
At 1.5 Torr H2O,10-12 MHz changes
EXPT 263 K 300 K 340 K
cm-1 Line ΓFWHM ΓFWHM δν ΓFWHM δν
12.68 414-321 1.07(1) 0.970(8) 0.870(5)
20.70 532-441 0.86(2) 0.800(3) 0.046(2) 0.710(4) 0.041(1)
30.56 422-331 0.95(2) 0.910(9) 0.035(1) 0.840(7) 0.033(2)
32.37 524-431 0.92(2) 0.870(5) 0.019(1) 0.790(3) 0.015(2)
42.64 743-652 0.85(2) 0.760(5) 0.018(2) 0.670(4) 0.013(1)
51.43 633-542 0.99(2) 0.890(5) 0.038(1) 0.810(4) 0.036(1)
cm-1/atm
Parameter Summary for weak lines of H2O
V. B. Podobedov, D. F. Plusquellic, G. T. Fraser, JQSRT, 87, 377 (2004)
1% on self-widths5% on self-shifts10-20% on temp dependence on widths
>2-fold variation in shifts
ΓFWHM
cm-1 Line THz HT OmC / %
12.68 414-321 0.970(8) 1.0809 +11
20.70 532-441 0.800(3) 0.8697 +9
30.56 422-331 0.910(9) 0.9231 +1
32.37 524-431 0.870(5) 0.8697 0
42.64 743-652 0.760(5) 0.8389 +10
51.43 633-542 0.890(5) 0.8508 -4
cm-1 / atm
THz Studies vs HITRAN for Pure H2O at 300 K
0
0.6
1.2
2 3 4 5 6 7
FW
HM
, c
m-1
atm
-1
Jinit
THz Studies vs HITRAN for Pure H2O at 300 K
aW. S. Benedict, L. D. Kaplan, JQSRT, 4, 453 (1964)
Open – Experiment, Solid – Theorya Jinit= J + Ka - Kc
FTIR InstrumentΔνRange = 10–250 cm-1
ΔνInst = 0.07 cm-1 Time = 35 min
Ti:Sapp InstrumentΔνRange = 2-100 cm-1 / 1 cm-1
ΔνInst = 0.0005 cm-1 Time = 10 min
New Ti:Sapp Instrument (single knob tunable)
ΔνRange = 2-100 cm-1
ΔνInst = <0.01 cm-1 Time = 30 min
THz Instrumentation for H2O Foreign Gas Parameters
ΔνInst ~ 0.07 cm-1 (2000 MHz)
15 Torr
0.2 cm-1/atm
0.9 cm-1/atm
975 Torr
M = 1
1800 grooves/mmM4
Stepper driven micrometer
stage-mountedretro-reflector
10%
M6
M3 OCM5
M2
Ti:SappM1
M8
532 nm Pump
6:1 beam expander
Single Knob Tunable Ti:Sapp Laser
0 1 2 3
Atmospheric Water Absorption
0
1
2
3
4
5
6
/ THz
Ab
sorb
an
ce
(Ba
se 1
0)
0.5 0.6 0.7 0.8
Atmospheric Water Absorption
0
1
/ THz
Ab
sorb
an
ce
(Ba
se 1
0)
FWHM
= 0.2 cm-1
Laser
= 0.02 cm-1
2 parts in 10,000
High resolution Broadband THz Laser system
Range >100 cm-1 at <0.02 cm-1 step resolution
Necessary for accurate retrievals of temperature and humidity profiles by EOS Water Vapor Continuum Absorption
Water Vapor ContinuumHigh Sensitivity Long Path Length THz Studies
V. B. Podobedov, D. F. Plusquellic, G. T. Fraser, JQSRT, 91, 287 (2005)
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 ft3
60 mm beam aperature
FTFIR Instrument and SensitivityPolarizing Michelson Interferometer w/ Hg Lamp SourceRange = 7-250 cm-1
Time = 35 min @ 0.07 cm-1 resolutionDrift less than ±1.5 % TAbs10 = ±0.007
Minimum Values for Continuum Absorption T=297(1) K2.5 Torr H2O375 Torr N2
A = AR + ANR
ANR = C1P2H2O + C2 PN2PH2O + C3 P2
N2
THz Water Vapor Continuum
0.0
0.5
1.0
10 20 30 40 50
Single vs Multi-pass Sensitivity
Ab
sorb
ance
(b
ase
10)
cm-1
10.5 Torr H2O297 K
Single Pass
Multi PassPure H2O
Line shape model important for local line absorption
Basic choices before application of far-wing absorption model
Choice of lineshape functionLorentzian, Van Vleck Weisskopf
How far to extend the lineshapeCutoff = 25 cm-1, 100 cm-1, infiniteTypically 25 cm-1 useda or no cutoffb
Number of water lines to consider Upper cutoff = 100 - 300 cm-1
aT. Kuhn, A. Bauer, M. Godon, S. Buhler, K. Kunzi, JQSRT 74, 545 (2002)bJ. R. Pardo, E. Serabyn, J. Cernicharo, JQSRT 68, 419 (2001)
Models of Local & Far-Wing Line Absorption
Continuum Absorption of H2O
0.5
1.0
1.5
2.0
10 20 30 40 50
Lineshape Functions and Cutoff RangesN
orm
aliz
ed A
bso
rban
ce (
bas
e 10
)
cm-1
Lorentzian - Width = 100 cm-1
Lorentzian - Width = 25 cm-1
Van Vleck - Weisskopf - Width = 25 cm-1
Van Vleck - Weisskopf - Width = 100 cm-1
Change is <10 % above 1 THz
Continuum Absorption of Pure H2O
Windows where continuum absorbance largest relative to discrete line absorption and uncertainties in line intensities smallest
HITRAN 01Γself = 4.8 Γair
Expected ν2 dependence found
Pair = 1.11 PN2
0.0
0.5
1.0
10 20 30 40 50
Continuum Absorption in H2O
Ab
sorb
ance
(b
ase
10)
cm-1
10.5 Torr H2O
10.5 Torr H2O + 608 Torr N
2
Continuum Absorption of H2O / N2 Mixtures
0
20
40
60
80
100
120
140
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
H2O Continuum Absorption
A. Bauerh2o-h2oQ. Matotalh2o-n2
Frequency, THz
TOTAL
(H2O - H
2O)
(H2O - N
2)
AH2O-N2 = ANR – AH2O
Continuum Absorption of H2O / N2 Mixtures
ANR = ATotal - AR
ANR
Parameter Q. Ma, et al. A. Bauer, et al. Our work
1.5 THz <0.35 THz 0.3-1.5 THz
(H2O – H2O) 9.55 E-8 4.22 E-8
(H2O – N2) 2.16 E-9 2.55 E-9 3.53 E-9
(dB/km) / (hPa GHz)2
Potential Sources of discrepancyNear-wing line shape modelNumber of lines included to model resonant absorptionSelf-broadening and foreign parameters used
α(ν,T) = A * PH2O * PN2 * ν2 * (300/T)B
Continuum Absorption of H2O / N2 Mixtures
Q. Ma, R. H. Tipping, J. Chem. Phys. 117, 10581 (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!
Conclusions
Current results onSelf-width (1%), self-shift (%5) and temperature dependence of 6 weak lines from 12 cm-1 - 55 cm-1 (0.4 - 1.7 THz)
Continuum absorption of H2O-H2O and H2O-N2
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 H2O-H2O and H2O-N2 continuum
0.00
0.10
20 30 40 50 60 70 80 90 100
O2 Absorption / 24 m Path
Ab
sorb
ance
(b
ase
10)
cm-1
608 Torr
Overlapping Scans to within +250 MHz
FSR = 249.058 MHz
Optically pumped THz photomixerOperational range 0.1 – 4.5 THzOutput power 10-6 - 10-8 WLinewidth 1 MHzFrequency drift <0.3 MHz/hour
10 20 30 40 500.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Approximate Spatial Resolution as aFunction of Chamber Size
= 1 mm or = 300 GHz
= 0.3 mm or = 1000 GHz
Ap
pro
xim
ate
Sp
atia
l Re
solu
tion
/ cm
Diameter of Chamber / cm
Chamber
Lens Lens
Slow Wave Structure
Electron Beam
Backward Wave Oscillators
B
Waveguide
Cathode
Collector
d <
+
Strong Interaction of e and electromagnetic waves
v
L
FastFeedbackR
k =k
ph
=L1
vph ~ ve ~ V
L
2eVme
ve=
-
1 to 20 mW
3 to 6 kV
~1 to 0.03 mm
ve-
~ 6 to 10 kG
ff
~ 30%
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 0.001 % fractional absorption for 1 s integration.
BWO’s used: • 78 – 118 GHz (156 – 236 GHz with doubling).• 220-380 GHz• 450-750 GHz
Frequency Modulation
Synthesizer54-118 GHz
Mixer
SRS Lock-In
InSb Bolometer
4.2K
BWO
BWO-based Spectrometer 50-850 GHz
PCA/D & D/A
BeamSplitter
PLL Synchronizer
BWO ControlF=100 MHz, =2 s
IF=350 MHz
R=100
Reference Clock
fref
Low-noiseAmp
High VoltagePower Supply
FuG
Voltage ControlFrom D/A Card
GPIB
TT
GG
TG
Agent precursor diethyl sulfide – CH2-CH3-S-CH2-CH3 • > 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 MP2/6311++G(d,p) energies and dipole moments squared.
Most vibrational sequence levels overlap within the pressure broadened linewidth ~1 GHz
Continuum Absorption of H2O
Grating-tunedTi:Sap Laser
Pump Laser
30 mW each @ 850 nm
DiodeLaser
DiodeAmplifier
Laser Cal. &Stabilization
Isolator
Isolator
PhotoCurrent
Photomixer and Si lens
Computer
Lock-in
Lock-in
Bolometer
Evacuated Sample Chamber
waveplate
THz SpectrometerTHz Spectrometer
BS
BS
BS BS
ChopperAmplitude Modulation
~ 400Hz
Transmission Properties in the THz Region
THz Scans Performed in Vacuum Plastic, Paper, Wood transparent
0 1 2 3
Atmospheric Transmission - 0.5 m Path
0
100
/ THz
% T
ran
sm
iss
ion
0 1 2 3
Polyethylene - 3 mm Path
0
100
% T
ran
sm
iss
ion
/ THz
4 K
Multi-pass White Cell
M1
M4M2 M6 Bolometer
Far IR Spectrometeror THz photomixer Source
M5M3
Size 3 ft3
High-Resolution THz Laser Studies of H2O
• Path Length = 20 to 40 m• No optical saturation issues• Heatable to 100 C