Terahertz Signature Science:

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Terahertz Signature Science The Second Gap in
the Electromagnetic Spectrum Frank C. De
Lucia Department of Physics Ohio State
University International Symposium on Spectral
Sensing Research Bar Harbor, Maine June 1, 2006
The New York Times - July 11, 2005 High-Tech
Antiterror Tools A Costly, Long-Range Goal
Millimeter wave machines . . .use trace amounts
of heat released by objects . . .to create images
that can identify hidden bombs . . . from about
30 feet away. Terahertz radiation devices can
create images of concealed objects as well as
identify the elemental components of a hidden
item. The terahertz devices may be more
promising since they could sound an alarm if
someone entering a subway or train station had
traces of elements used in bombs on them.
CONCEPTS ARE NICE BUT THE NUMBERS MATTER
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The Beginning Theses No Public THz
application has yet come to fruition - Why? What
do we need to do to remedy this? 1. We need
End-to-End systems analyses for the most widely
discussed applications. 2. We need to consider
appropriate figures of merit for the several
technical approaches and choose the technology
best suited for the application. 3. Often a lot
more is known about the signatures and
phenomenology than we consider as we propose
applications. (this leads to proposed
applications which threaten our credibility) 4.
But in other cases the phenomenology and
signature science is only poorly know. (this
leads to missed opportunities)
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Two SMM/THz Legacy Public Applications
-- Clear, but Challenging Paths to Success --
IMAGING
ANALYTICAL CHEMISTRY
Technical and Scientific R D Impact The
Breadth of Applicability
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Why is there a Clear Path to Imaging? Many
special purpose imagers have been built over the
last 40 years Detectors - scientifically
we understand - in single element
receivers we can approach well understood
fundamental limits Transmit power -
acceptable solid state sources for some
applications exist Propagation -overall
absorption generally known -impact of
fluctuations noise less clear Recognition/targets/
clutter -preliminary studies completed
-nature of active images complex, but large
contrast in images provides opportunities
-TIFT can start to do realistic end to end
calculations Where can we get to on
sensitivity-speed-size-cost tradeoff? Quantitative
ly, what are the target signatures, clutter, and
phenomenology for scenarios of interest? These
are not show stoppers, but the answers will
determine the Breath of Application
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Why is there a Clear Path to Analytical
Chemistry? A well understood spectroscopic
foundation is in place False alarm rates in
complex environments have been studied and can be
shown to be low because of the number of
resolution elements and complex redundancy of
molecular fingerprints Background
clutter/interference at trace levels have been
studied and can be shown to be low What we need
to know/develop Trade-offs among size, cost,
sensitivity, power consumption Speed of cost
reduction from mass wireless market?
Development of a signature science for larger
molecules These are not show stoppers, but the
answers will determine the Breath of Application
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Quantitative end-to-end designs
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Parallel and On Going MM/SMM Science Field
Technology and Systems Grew out of Lab Science
NASA JPL catalog HITRAN, KOLN, GEISA data
bases Ongoing Data Base Development
ALMA - Denmark HITRAN
NASA
The GEISA/IASI spectroscopic database
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Applications Matrix as introduction to Signature
Science
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Impact Order demonstrated demonstrated clea
r path Phenomena VLP (spent or potential) best
method To be demo Cancer/deep(spectra) X Ca
ncer/surface(spectra) X T-Ray (deep
medical) X Mutation(spectra) X Broadband
communications 100 GHz gt1 THz Explosives
remote with specificity X Classical
imaging X Point gas detection absolute
specificity X Astrophysics (gt2x109) X Atmosph
eric (gtn x 108) X Remote gas detection modest
specificity X specificity in mixtures at
1km X See
through walls 100 GHz gt1 THz Buried land
mines gt 6 100 GHz gt 1THz lt 6 gt1
THz Cancer/surface (water) X Incapacitate and
kill X Explosives/other solids close, sm
obstruct, mixtures X Explosives close,
sort, sm obstruct some materials Pharmaceuticals,
bio close, sort, sm obstruct some
materials
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Cost? Size? Speed? Breadth of Application?
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Impact Order demonstrated demonstrated clea
r path Phenomena VLP (spent or potential) best
method to be demo Cancer/deep(spectra) X Ca
ncer/surface(spectra) X T-Ray (deep
medical) X Mutation(spectra) X Broadband
communications 100 GHz gt1 THz Explosives
remote with specificity X Classical
imaging X Remote gas detection modest
specificity X Point gas detection absolute
specificity X Astrophysics (gt2x109) X Atmospheri
c (gtn x 108) X See through walls 100
GHz gt1 THz Buried land mines gt 6 100 GHz gt
1THz lt 6 gt1 THz Cancer/surface
(water) X Incapacitate and kill X Explosives
/other solids close, sm obstruct,
mixtures X Explosives close, sort, sm
obstruct some materials Pharmaceuticals, bio
close, sort, sm obstruct some materials
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Impact Order demonstrated demonstrated clea
r path Phenomena VLP (spent or potential) best
method To be demo Cancer/deep(spectra) X Ca
ncer/surface(spectra) X T-Ray (deep
medical) Mutation(spectra) X Broadband
communications 100 GHz gt1 THz Explosives
remote with specificity X Astrophysics
(gt2x109) X Atmospheric (gtn x 108) X Classical
imaging TS Remote gas detection modest
specificity TS See through walls 100
GHz gt1 THz Point gas detection absolute
specificity X Buried land mines gt 6 100
GHz gt 1THz lt 6 gt1 THz Cancer/surface
(water) X Incapacitate and kill X Explosives
close, sort, sm obstruct some
materials Pharmaceuticals, bio close, sort,
sm obstruct some materials
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Impact Order demonstrated demonstrated clea
r path Phenomena VLP (spent or potential) best
method To be demo Cancer/deep(spectra) X Ca
ncer/surface(spectra) X T-Ray (deep
medical) Mutation(spectra) X Broadband
communications 100 GHz gt1 THz Explosives
remote with specificity X Astrophysics
(gt2x109) X Atmospheric (gtn x 108) X Classical
imaging TS Remote gas detection modest
specificity TS See through walls 100
GHz gt1 THz Point gas detection absolute
specificity X Buried land mines gt 6 100
GHz gt 1THz lt 6 gt1 THz Cancer/surface
(water) X Incapacitate and kill X Explosives/
other solids close, sm obstruct,
mixtures X Explosives close, sort, sm
obstruct some materials Pharmaceuticals, bio
close, sort, sm obstruct some materials
?
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Signatures Explosives Spectra
Clearly spurious results in both gas and solids
have been reported
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From THz-Bridge
Are any of us willing to say that we are sure
that the sharp lines are spurious?
The solid line shows the reflectivity of the meat
part normalized of the reflectivity of the fat
part of Black Forrest ham averaged on three
points each.
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Clutter and Noise in the SMM/THz
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The THz is VERY Quiet even for CW Systems in
Harsh Environments
Experiment SiO vapor at 1700 K
Good News - CW systems are 1010 better than has
been widely claimed
All noise from 1.6 K detector system
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Impact of Atmospheric Transmission on Spectral
Fingerprints - Whats a THz?
Signature, with perfect atmo model
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Signatures vs Pictures Humans at 650 GHz
Active Image Skin
is close to specular - Hair really lights up At
least 40 db of dynamic range across this target A
high contrast target signature is very good for
recognition if you have system sensitivity to
observe
Thermal Image DT/T 0.1
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Clutter Limits in Imaging Temporal and Spatial
Scales
When do we reach clutter limits as a function of
frequency?
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Systems Remote Spectroscopic Sensing
Gas Phase Example 100 m, 1 ppm plume gt
10-2 absorption fraction, with 10 GHz linewidth
sharp lines 10-7 detectable (noise limits),
105 resolution elements broad lines 10-1
detectable (clutter limits), lt102 resolution
elements Solids What is the
concentration and absorption fraction (in
reflection)? What is the signature, the
linewidths, the clutter? Are their equivalent
double resonance schemes for solids?
3-D Specificity Matrix
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What are the Characteristics of Compact THz
Technologies? 1. CW multiplied or fundamental
oscillators 2. THz-TDS 3. FTFIR How do
these Relate to Signatures? Quantifiable Figures
of Merit
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• Spectroscopic Sensor Figures of Merit - I
• Sensitivity - Dynamic Range is widely abused
• 1. Only source power in the signature bandwidth
  (Brightness - W/Hz) is useful
• - the rest often causes additional noise (a
  fundamental limit for FTFIR)
• 2. Detectors
• -NEP (W/Hz1/2) vs NEP(W/Hz)
• 3. Noise and Dynamic Range Example
• - 1 mW in a 100 Hz bandwidth, 3000K noise
  temperature gtdynamic range of gt140 db
• This is good for the imager because the
  bandwidth of the reciever can be matched to the
  source and frame rate of the imager
• But people who build spectrometers never discuss
  dynamic range because the detection of a small
  amount of power in a narrow bandwidth is
  fundamentally different than the detection of a
  small change in a large amount of power.
• - in ideal noise limited spectrometer, the
  minimum detectable absorption is only - 90 db

A 50 db Difference
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• Spectroscopic Sensor Figures of Merit - II
• Specificity
• 1. Scenario Clutter must be understood -
  spectroscopic clutter is much more complex than
  radar clutter.
• 2. A vs B demonstrations relate to a
  relatively small fraction of the scenarios of
  interest
• 3. Calculation of scenario dependant PFA or ROC
  is useful

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How do we Move Beyond
Whispered Excitement about the THz Graham
Jordan Opening Plenary Presentation SPIE
Symposium Optics/Photonics in Security and
Defense Bruges, Belgium, 26 September, 2005
to A Field with many Public Applications?
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What Needs to be Done to Enable the SMM/THz
Spectral Region? 1. Classical penetrability,
scatter, and specular reflection as a function of
frequency and material. 2. What is the origin of
linewidths in solids? 3. What are the signatures
of solids and large molecules in the gas phase?
Distribution in frequency relative to
penetration? 4. What are the signatures of
clutter for scenarios of interest? 5. Develop
schemes for using time domain or other X
factors? 6. A closer connection between the
technology community and the applications and
science community.
A litmus test A reproducible, well founded
signature science catalogue