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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. Motivation. THz studies are of importance to Climate modeling Radio Astromony Satellite-based remote sensing

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slide1

THz Studies of Water Vapor

Vyacheslav B. Podobedov, Gerald T. Fraser and

David. F. Plusquellic

NIST/Optical Technology Division/Physics Lab

Gaithersburg, MD 20899

slide2

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

slide3

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

slide4

Far-infrared

MW

Pure Rotational Lines

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

The Terahertz Gap

Pure Rotational Spectroscopy for H2O (18O), HDO and D2O

Terahertz (THz)

1 THz  33.3 cm-1 or   300 m

ν 0.06 THz to 3 THz

ν2 cm-1 to 100 cm-1

λ 5 mm to 100 μm

slide5

Photoconductive Switches or Photomixers

The photomixers are epitaxial low-temperature-grown 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

slide6

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

slide8

ErAs:GaAs

LT GaAs

ErAs:GaAs Photomixers

New Photomixers deliver more than >5-fold power

slide9

Why is resolution important in the THz region?

S/N Limit ~1%

Repeatability minimizes spectral artifactsCurrent resolution is2 parts in 10,000

ΔνLaser ~ 0.2 cm-1 (0.006 THz)

ΔνLaser < 0.02 cm-1 (0.0006 THz)

slide10

Diode

Amplifier

Diode

Laser

+

Nd:YAG (x2)

Optical

Isolator

Ti:Sap Ring

Laser

BS

1/2

Wave

Chopper

Laser Cal. &

Stabilization

Single-mode

Fiber

Evacuated Sample Chamber

40 mWatts @ 850 nm

T = 260 – 340 K

Photomixer

Bolometer

Brass Cell

Thermo-electric PID Controller ±0.2 C

Fill/Pump Ports

THz Photomixer Spectrometer for Line Shape Studies

slide11

THz Frequency Calibration System

ΔνLOCK< 150 kHz

ΔνLOCK< 0.5 MHz

Stabilization

Polarization Stabilized

Lock-in

HeNe Laser

Electronics

PID Servo

PZT

Heater

Evacuated Reference Cavity

Intensity

ΔνLOCK< 0.5 MHz

Stabilizer

AOM

Lock-in

PID Servo

Programmable

RF Driver

Ramp (12 Bits)

Analog Sum

16 Bit Ramp

Computer

Ti:Sap Laser

CW Ring

Electronics

Ti:Sap Laser

Diode Laser

Diode Laser/

Electronics

Amplifier

slide12

Instrumental Linewidth < 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

slide13

1

1

L=0.3 – 1 cm

for strong lines

Abs10

Abs10

0

0

L=53 cm

for weak lines

x175

0 THz 3.0

AM methods optimal between 10% and 90 % fractional absorption

slide14

Pure Lorentzian

4 MHz Doppler limited

Spike small contribution

to line shape

Shift <1/20 of line width

slide15

Self-Width vs H2O Pressure

Residuals

Residuals

slide16

Self-Width vs H2O Pressure

x3 different

Temperatures

263, 300, 340 K

slide17

Self-Width vs H2O Pressure

Error bars are included

slide18

Self-Shift vs H2O Pressure

Error bars are included

slide19

Temperature Dependence on Width

At 1.5 Torr H2O,

10-12 MHz changes

60

Γ(T) / Γ(T0)=(T0 / T)n

wheren found between

0.56 – 0.81

48

36

δ(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

slide20

Parameter Summary for weak lines of H2O

>2-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)

slide22

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)

slide23

THz Instrumentation for H2O Foreign Gas Parameters

FTIR Instrument

975 Torr

ΔνRange = 10–250 cm-1

ΔνInst = 0.07 cm-1

Time = 35 min

0.9 cm-1/atm

Ti:Sapp Instrument

ΔνRange = 2-100 cm-1 / 1 cm-1

ΔνInst = 0.0005 cm-1

Time = 10 min

0.2 cm-1/atm

New Ti:Sapp Instrument

(single knob tunable)

15 Torr

ΔνInst ~ 0.07 cm-1 (2000 MHz)

ΔνRange = 2-100 cm-1

ΔνInst = <0.01 cm-1

Time = 30 min

slide24

Single Knob Tunable Ti:Sapp Laser

stage-mounted

retro-reflector

M8

Ti:Sapp

M1

532 nm Pump

M2

10%

M5

M3 OC

6:1 beam

expander

M = 1

M4

1800 grooves/mm

M6

Stepper driven

micrometer

slide25

High resolution Broadband THz Laser system

Range >100 cm-1 at <0.02 cm-1 step resolution

2 parts in 10,000

slide26

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)

slide27

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 >70 C
  • No optical saturation issues
slide28

THz Water Vapor Continuum

FTFIR Instrument and Sensitivity

Polarizing Michelson Interferometer w/ Hg Lamp Source

Range = 7-250 cm-1

Time = 35 min @ 0.07 cm-1 resolution

Drift less than ±1.5 % T

Abs10 = ±0.007

Minimum Values for Continuum Absorption T=297(1) K

2.5 Torr H2O

375 Torr N2

A = AR + ANR

ANR = C1P2H2O + C2 PN2PH2O+ C3 P2N2

slide29

Pure H2O

Line shape model important for local line absorption

slide30

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)

slide31

Continuum Absorption of H2O

Change is <10 %

above 1 THz

slide32

Continuum Absorption of Pure H2O

HITRAN 01

Γself = 4.8 Γair

Expected ν2 dependence found

Pair = 1.11 PN2

Windows where continuum absorbance largest relative to discrete line absorption and uncertainties in line intensities smallest

slide34

Continuum Absorption of H2O / N2 Mixtures

ANR = ATotal - AR

ANR

AH2O-N2 = ANR – AH2O

slide35

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!

α(ν,T) = A * PH2O * PN2 * ν2 * (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)

slide36

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

slide39

Optically pumped THz photomixer

Operational range 0.1 – 4.5 THz

Output power 10-6 - 10-8 W

Linewidth 1 MHz

Frequency drift <0.3 MHz/hour

slide40

Lens

Lens

Chamber

slide41

Backward Wave Oscillators

B

~ 6 to 10 kG

l

Electron Beam

~1 to 0.03 mm

d <

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

slide42

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
slide43

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

IF=350 MHz

Mixer

Low-noise

BWO Control

Amp

BWO

DF=100 MHz, t =2 ms

InSb

Bolometer

4.2K

Beam

W

R=100

Splitter

High Voltage

Voltage Control

Power Supply

From D/A Card

FuG

slide44

TT

TG

GG

Potential of THz Methods for Detection of Chemical Agents

  • Agent precursor diethyl sulfide – CH2-CH3-S-CH2-CH3
  • > 15% fractional absorption predicted
  • Detection limit using AM methodsdemonstrated near 0.2%

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

slide49

Grating-tuned

Ti:Sap Laser

Pump Laser

Laser Cal. &

Stabilization

Computer

Lock-in

Lock-in

THz Spectrometer

BS

BS

waveplate

Isolator

Diode

Amplifier

Diode

Laser

BS

BS

Isolator

30 mW each

@ 850 nm

Photomixer

and Si lens

Bolometer

Evacuated

Sample Chamber

Chopper

Amplitude

Modulation

~ 400Hz

PhotoCurrent

slide50

Transmission Properties in the THz Region

THz Scans Performed in Vacuum

Plastic, Paper, Wood transparent

slide51

M3

M5

M1

Bolometer

M4

M2

M6

Far IR Spectrometer

or THz photomixer Source

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
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