Signature Science in the Terahertz

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Signature Science in the Terahertz The Second Gap
in the Electromagnetic Spectrum Frank C. De
Lucia Department of Physics Ohio State
University Optics East Boston October 2, 2006
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What is the Physics of the SMM/THz? The
Energetics hn kT The Classical Size Scale
1 mm Noise Interactions Gases, Liquids, and
Solids Atmospheric Absorption Classical
Scattering and Penetration
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Where is the New THz Excitement?
New Physical Regimes
Analytical Applications
Medical
Active and Passive Imaging
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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 good 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 good specificity in mixtures
at 1km X See through walls 100 GHz gt1
THz Buried land mines gt 6 100 GHz gt 1THz lt
6 100 GHz gt1 THz Cancer/surface
(water) X Incapacitate and kill X Explosives
/other solids close range, mixtures, sm
obstruct X Explosives close range, A vs.
B, sm obstruct some materials Pharmaceuticals
close range, A vs. B, sm obstruct some
materials
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Convolution of RDX Spectrum with Atmospheric
Transmission
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Signature of Explosive through 1 m of Atmosphere
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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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Quantitative end-to-end designs based on known
signatures
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Image Comparisons Angular Diversity
Cold Sky Illumination at 94 GHz
Uniform Passive Illumination at 94 GHz
MODES Co-propagated coherent illumination
(Active) Diffuse thermal illumination (Passive
?) Diffuse coherent illumination (Active
?) Thermal emission (Passive)
THz Passive Thermal Emission
Thermal Emission on Warm at 650 GHz Background
15 Degree Thermal Illumination at 650 GHz
Shadow gram of metallic object reflecting diffuse
colder room
Contrast of metal within angular diversity of
illuminator
Thermal Radiation from Object
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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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Log Scale
Linear Scale
The combination of strong spectral reflectivity
at 650 GHz and the lack of angular diversity in
simple active systems has a negative impact on
the information content of images and makes them
difficult to identify. However, with log signal
processing in the analog IF the large dynamic
range of the signal the information content of
the images can be restored.
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Quantitative Logarithmic Image The blue line
indicates the position of the cross section
shown below. The metal surface of the toy gun is
reflective but not perfect. A mirror has a
maximum signal power of 5 dBm. The Eccosorb
background varies between -45 and -55 dBm A
theme of the next few pictures is that an active
system can have very large dynamic range, but
that we need it to deal with specular
reflections In 100 Hz, 1 mW corresponds to 1018
K
0 degrees
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Logarithmic Image as a Function of Angle
10 degrees
40 degrees
20 degrees
Issue is not when you loose detectable signal,
but rather when does the signal become lost in
clutter background?
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Materials Measurements
Normal
10 degrees
With large dynamic range seeming small effects
are significant. Secondary illumination and
reflections can dominate off-axis specular
reflections which can be very black
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Modes and Angles Active and Passive Imaging in
the THz For a single mode, 100 Hz bandwidth, 300
K, the thermal power/noise is 4 x 10-19 W 1 mW
in 100 Hz corresponds to a noise temperature of
1018 K A reasonable receiver noise temperature
is 3000 K For diffuse target, the number of
return modes is NAD (spot
size/wavelength)2 100 (our system in portrait
mode) For a specular target, the number of
return modes is 1 Floodlight limit If an
illuminator of power PI is used to flood light
(i.e. fill all modes) of an object whose scale is
l, in a 100 Hz bandwidth the temperature/mode
is With l 1 m, l 0.5 mm TI 2 x 1011
K Random illumination limit A practical way to
get spotlight illumination would be to illuminate
the whole room or urban canyon' assume a 10
reflection, and let the target come into
equilibrium with the room. If we let l 100 m,
then TI 2.5 x 106 K.
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Point Chemical Sensors
Identification in a mixture of 20 gases
Spectra as a function of molecular size
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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. Quantitative
development of useful technological figures of
merit. 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
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