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