THz Studies of Water Vapor

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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, MD 20899
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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 gt 1
Torr Doppler contributions are lt5 MHz at room
temperature Continuum aborption Insensitivity
to far-wind line shape model
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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
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The Terahertz Gap
Pure Rotational Spectroscopy for H2O (18O), HDO
and D2O
Terahertz (THz)
1 THz ? 33.3 cm-1 or ? ? 300 ?m
? 0.06 THz to 3 THz ? 2 cm-1 to
100 cm-1 ? 5 mm to 100 µm
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Photoconductive Switches or Photomixers
The photomixers are epitaxial low-temperature-grow
n GaAs with a gold spiral antenna structure
Photomixer chip 5 x 5 mm
Two CW lasers, offset by THz, illuminate the
fingers
Conduction band
-
e-
850 nm

Vbias ? 15 V
Valence band

8 x 8 ?m
-
Photoexcitation produces an acceleration of
charge at the beat note of the two lasers
0.2 µm wide fingers separated by 1 µm
THz radiation is emitted
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Performance Limitations
Beat Note Amplitude on Mixer Surface
Conduction band
e-
850 nm
NIR Driving Fields
Valence band
? ? 0.25 psec
time
2io2RL?c(?)?mP1P2/Po2 (1 ?2?2)(1 ?2RL2C2)
Prf(?)
? 1/ ?4
LT-GaAs poor conductor of heat
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Bolometer sensitivity 1 pW/Hz1/2
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ErAsGaAs Photomixers
New Photomixers deliver more than gt5-fold power
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Why is resolution important in the THz region?
??Laser 0.2 cm-1 (0.006 THz)
??Laser lt 0.02 cm-1 (0.0006 THz)
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THz Photomixer Spectrometer for Line Shape Studies
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THz Frequency Calibration System
??LOCKlt 0.5 MHz
??LOCKlt 150 kHz
Stabilization
Polarization Stabilized
Lock-in
HeNe Laser
Electronics
PID Servo
PZT
Heater
Evacuated Reference Cavity
Intensity
??LOCKlt 0.5 MHz
Stabilizer
Lock-in
AOM
PID Servo
Programmable
RF Driver
Ramp (12 Bits)
Analog Sum
16 Bit Ramp
Computer
CW Ring
TiSap Laser
Electronics
TiSap Laser
Diode Laser
Diode Laser/
Electronics
Amplifier
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Instrumental Linewidth lt 3.0 MHz
THz Studies of Ions and Radicals in Etching
Plasmas used to Validate plasma models and
improve recipes to increase etch uniformity and
feature fidelity
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AM methods optimal between 10 and 90
fractional absorption
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Pure Lorentzian 4 MHz Doppler limited Spike
small contribution to line shape Shift lt1/20 of
line width
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Self-Width vs H2O Pressure
Residuals
Residuals
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Self-Width vs H2O Pressure
x3 different Temperatures 263, 300, 340 K
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Self-Width vs H2O Pressure
Error bars are included
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Self-Shift vs H2O Pressure
Error bars are included
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Temperature Dependence on Width
At 1.5 Torr H2O, 10-12 MHz changes
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G(T) / G(T0)(T0 / T)n where n found
between 0.56 0.81
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d(T) (2-5) x 10-3 cm-1/atm
80-200 kHz/Torr is comparable to 100
kHz/Torr found for the 643-550 line in the mm
region
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Parameter Summary for weak lines of H2O
gt2-fold variation in shifts
1 on self-widths 5 on self-shifts 10-20 on
temp dependence on widths
V. B. Podobedov, D. F. Plusquellic, G. T. Fraser,
JQSRT, 87, 377 (2004)
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THz Studies vs HITRAN for Pure H2O at 300 K
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THz Studies vs HITRAN for Pure H2O at 300 K
Jinit J Ka - Kc
Open Experiment, Solid Theorya
aW. S. Benedict, L. D. Kaplan, JQSRT, 4, 453
(1964)
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THz Instrumentation for H2O Foreign Gas Parameters
FTIR Instrument
975 Torr
??Range 10250 cm-1 ??Inst 0.07 cm-1 Time
35 min
0.9 cm-1/atm
TiSapp Instrument
??Range 2-100 cm-1 / 1 cm-1 ??Inst 0.0005
cm-1 Time 10 min
0.2 cm-1/atm
New TiSapp Instrument (single knob tunable)
15 Torr
??Inst 0.07 cm-1 (2000 MHz)
??Range 2-100 cm-1 ??Inst lt0.01 cm-1 Time
30 min
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Single Knob Tunable TiSapp Laser
stage-mounted retro-reflector
M8
TiSapp
M1
532 nm Pump
M2
10
M5
M3 OC
61 beam expander
M 1
M4
1800 grooves/mm
M6
Stepper driven micrometer
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High resolution Broadband THz Laser system
Range gt100 cm-1 at lt0.02 cm-1 step resolution
2 parts in 10,000
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Water Vapor Continuum High Sensitivity Long Path
Length THz Studies
Necessary for accurate retrievals of temperature
and humidity profiles by EOS
Water Vapor Continuum Absorption
V. B. Podobedov, D. F. Plusquellic, G. T. Fraser,
JQSRT, 91, 287 (2005)
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THz White Cell
Photomixer or FTFIR Spec
Evacuated Sample Chamber
60 mm beam aperature
M1
M0
Vol 3 ft3
M2
M3
M4
40 Pass White Cell
M5
M6
Au Mirrors
M0 M6 Parabolic
LHe cooled Bolometer

• Path Length 24 m
• Temperature controlled to gt70 C
• No optical saturation issues

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THz Water Vapor Continuum
FTFIR Instrument and Sensitivity Polarizing
Michelson Interferometer w/ Hg Lamp Source Range
7-250 cm-1 Time 35 min _at_ 0.07 cm-1
resolution Drift less than 1.5 T Abs10
0.007
Minimum Values for Continuum Absorption T297(1)
K 2.5 Torr H2O 375 Torr N2
A AR ANR ANR C1P2H2O C2 PN2PH2O C3 P2N2
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Pure H2O
Line shape model important for local line
absorption
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Models of Local Far-Wing Line Absorption
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 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)
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Continuum Absorption of H2O
Change is lt10 above 1 THz
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Continuum Absorption of Pure H2O
HITRAN 01 Gself 4.8 Gair
Expected ?2 dependence found
Pair 1.11 PN2
Windows where continuum absorbance largest
relative to discrete line absorption and
uncertainties in line intensities smallest
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Continuum Absorption of H2O / N2 Mixtures
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Continuum Absorption of H2O / N2 Mixtures
ANR ATotal - AR
ANR
AH2O-N2 ANR AH2O
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Continuum Absorption of H2O / N2 Mixtures
Potential Sources of discrepancy Near-wing line
shape model Number of lines included to model
resonant absorption Self-broadening and foreign
parameters used
From the perspective of atmospheric modeling, the
total absorption is what is important!
a(?,T) A PH2O PN2 ?2 (300/T)B
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)
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Conclusions
Current results on Self-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
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Overlapping Scans to within 250 MHz
FSR 249.058 MHz
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Optically pumped THz photomixer Operational
range 0.1 4.5 THz Output power 10-6 - 10-8
W Linewidth 1 MHz Frequency drift lt0.3
MHz/hour
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Lens
Lens
Chamber
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Backward Wave Oscillators
B
6 to 10 kG
l
Electron Beam
1 to 0.03 mm
d lt
Cathode
v
-
e
Fast
R
3 to 6 kV
Feedback
Collector

L
Slow Wave Structure
Waveguide
1 to 20 mW
-
Strong Interaction of e and electromagnetic waves
w
1
2eV

v
v
k

ph
e
L
m
k
e
f
D
V
w

v
v
30


ph
e
L
f
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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.

• BWOs used
• 78 118 GHz (156 236 GHz with doubling).
• 220-380 GHz
• 450-750 GHz

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BWO-based Spectrometer 50-850 GHz
GPIB
PLL Synchronizer
f
Frequency
Synthesizer

PC
Reference
F
ref
Modulation
54-118 GHz
A/D D/A
Clock
SRS Lock-In
IF350 MHz
Mixer
Low-noise
BWO Control
Amp
BWO
DF100 MHz, t 2 ms
InSb
Bolometer
4.2K

Beam
W
R100
Splitter
High Voltage
Voltage Control
Power Supply
From D/A Card
FuG
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Potential of THz Methods for Detection of
Chemical Agents

• Agent precursor diethyl sulfide
  CH2-CH3-S-CH2-CH3
• gt 15 fractional absorption predicted
• Detection limit using AM methods demonstrated
  near 0.2

0.1 Torr in 100 Torr air sample Three conformers
populated at room temperature Conformers
intensities scaled according to MP2/ 6311G(d,p)
energies and dipole moments squared. Most
vibrational sequence levels overlap within the
pressure broadened linewidth 1 GHz
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Continuum Absorption of H2O
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THz Spectrometer
BS
BS
waveplate
Isolator
Diode Laser
Diode Amplifier
BS
BS
Isolator
30 mW each _at_ 850 nm
Photomixer and Si lens
Bolometer

Evacuated Sample Chamber
Chopper Amplitude Modulation 400Hz
PhotoCurrent
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Transmission Properties in the THz Region
THz Scans Performed in Vacuum
Plastic, Paper, Wood transparent
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High-Resolution THz Laser Studies of H2O
Multi-pass White Cell
Size 3 ft3

• Path Length 20 to 40 m
• No optical saturation issues
• Heatable to 100 C