R. Barry Johnson, D.Sc.

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Terahertz Sensors and Applications

• R. Barry Johnson, D.Sc.
• Research ProfessorPhysics Department
  (A-145)Alabama AM University
• P.O. Box 1268
• Normal, Alabama 35762
• 256.372.8148ralph.johnson_at_aamu.edu

18 July 2008
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Terahertz Detectors

• Bolometers
• Conventional
• Electrostatic
• Golay
• Pyroelectric
• Diodes

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Terahertz Spectral Region0.06 10 THz 2
300 cm-1 5000-30 µm
Typical Range 300 GHz - 3 THz 1000-100 µm
X-rays
Radio wave
UV
microwaves
Infrared
NIR
visible
Molecular rotations (gas)
Low frequency bond vibrations
Crystalline phonon Vibrations (solid)
Hydrogen-bonding stretches and torsions (liquids)
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Bolometer

• Samuel Langley invented the bolometer in 1878.
• Any radiation absorbed by the bolometer raises
  its temperature above that of its heat sink.
  Temperature change causes a change in some
  parameter, such as device resistance, that can be
  measured directly or indirectly.
• Often used in a Wheatstone configuration with a
  hot and a cold detector.
• Can be made very sensitive, but have low
  frequency response.
• Used from mm-wave to beyond visible light.

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Principle of Bolometer
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Electrostatic Bolometer

• Broad spectral coverage.
• MEMS structure, which allows arrays to be easy
  fabricated.
• Cantilevered configuration electrostatically
  charged.
• Incident flux converted to heat which then
  discharges the electrostatic charge on the
  device.
• Good sensitivity and modest speed possible.

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Cryogenically Cooled Bolometer

• The cryogenically cooled silicon bolometers offer
  excellent signal to noise ratio and nearly flat
  response for THz wavelengths from15 µm to 2 mm.

RadiaBeam Technologies BLIS-03-BLM
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Silicon Bolometer
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Golay Cell Detector

• A Golay Cell is a room temperature bolometer,
  which is a convenient choice for the moderate to
  high intensity THz signal measurements.
• Cell is a metal cylinder having a blackened metal
  plate at one end and a flexible metalized
  diaphragm at the other. It is filled with an
  inert gas and then sealed. Radiation incident
  upon the blackened metal plate is absorbed and
  heats the gas which increases the pressure
  thereby deforming the deforms the diaphragm.
• Light is reflected by the diaphragm motion onto a
  detector to measure the incident flux.
• Wide spectral range of 0.02-20 THz

RadiaBeam Technologies BLIS-03-GYC and Microtech
Instruments, Inc.
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Pyroelectric Detector

• Convert the changes in incoming flux to electric
  signals.
• Pyroelectric materials are characterized by
  having spontaneous electric polarization, which
  is altered by temporal temperature changes(
  ) when irradiated by flux.
• High sensitivity,
• Room temperature operation
• Low cost
• Robust under severe environmental conditions
• Suffers from microphonics (minimal for SBN)

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LiTaO3 Pyroelectric Detector

• Large area to 9 mm diameter
• Broad spectral response 0.1 to 1000 µm
• Current and Voltage hybrid circuits
• NEP 3x10-10 W/(Hz)1/2
• High bandwidth to 20 MHz
• High voltage output, 50KV/W

Spectrum Detector Inc.
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Superconducting Hot Electron Bolometer

• Operates at superconducting transition region.
• Small temperature change yields large change in
  device resistance.

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B-field Tuned InSb Detectors

• Magnetic Resonance Enhanced Indium Antimonide
  (InSb) Hot Electron Bolometer Type QFI/XB
• Fast and sensitive detection from below 100 GHz
  to 3 THz
• In the type QFI/XB device, the detector is
  mounted within a quasi-uniform magnetic field
  geometry so that magnetic resonance effects can
  be used to enhance free carrier absorptivity at
  much higher frequencies.
• Speed Approx. 1MHz (-3dB) at 4.2K.
• Detector Optical N.E.P is below1 x 10-12 W
  Hz-1/2

QMC Instruments Ltd
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Zero-Bias GaAs SchottkyDiode Detectors
Responsivity and Noise Measurements of Zero-Bias
Schottky Diode Detectors http//www.virginiadiod
es.com/VDI/pdf/VDI20Detector20Char20ISSTT2007.p
df
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THz Source
J. Hesler, D. Porterfield, W. Bishop, T. Crowe,
A. Baryshev, R. Hesper and J. Baselmans,
"Development and Characterization of an
Easy-to-Use THz Source", Proc. 16th Intl.
Symposium on Space Terahertz Technology, May,
2005, Goteborg, Sweden. http//www.virginiadiodes.
com/VDI/pdf/Hesler20200520stt20thz20source20a
nd20measurements.pdf
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Applications of THz Sensors

• Pharmaceutical
• Medical
• Industrial
• Security

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Terahertz Spectral RegionMolecular Vibrations

• Terahertz Spectral Region
• Intermolecular bond vibrations
• Directly affected by crystal changes

• Infrared Spectral Region
• Intramolecular bond vibrations
• Indirectly affected by crystal changes

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THz Pulsed Imaging Basics

• THz Pulsed Imaging
• Time-of-flight analysis
• Production of spectral information
• Refractive index discontinuities reflect back a
  part of the incident pulse
• Imaging
• Depth profiling using multiple detected pulses
• 3D image created by raster scanning

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TeraView Ltd. Terahertz Pulsed Imaging and
Spectroscopy
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Photoconductive THz Generator
Zhang, J. Hong, Y. Braunstein, S.L. Shore,
K.A., Terahertz pulse generation and detection
with LT-GaAs photoconductive antenna,
Optoelectronics, IEE Proceedings, Vol. 151, Issue
2, 26 April 2004 (98 101). The characteristics
of optically induced teraherz (THz) radiation
from a biased low-temperature-grown GaAs
(LT-GaAs) photoconductive antenna were
investigated using a femtosecond Tisapphire
laser.
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Photoconductive THz Detection
J. Zhang et al., Terahertz pulse generation and
detection with LT-GaAs photoconductive antenna,
IEE Proceedings Optoelectronics, April 2004,
Vol.151, Issue 2, (98-101 ).
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Applications of Terahertz Sensorsto
Pharmaceutical Analysis
With Courtesy ofDr. Philip F Taday TeraView
Limited Cambridge, UK
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Pharmaceuticals

• Applications
• Process improvement
• Polymorph screening
• Tablet Inspection
• Early stage of application
• Commercial instrumentation available

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Consequences of Bad Coating Quality

• Unpredictable dosing rate
• Dose dumping life threatening
• Legal and commercial implications

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Coating Integrity Investigation using Terahertz
Pulsed Imaging
Reflected Thz pulses probe coating structures.
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Non-Destructive Mapping of Coating Thickness in
Tablets

• Terahertz pulses reflect from each coating layer.
• Mapping of coating layers accomplished by time of
  flight and x-y scanning.

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TPI - Coating Layer Thickness
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Enteric Coated Tablets
15 solids level
16 w.g. enteric coating
10 w.g. enteric coating
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Terahertz Pulsed ImagingPenetration through most
pharmaceutical excipients. Non-destructive
coating analysis.Fully automated process.
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Initial Setup for Measuring Water Ingress
10 ml water
HPMC tablet
900 microns
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K4M Change In Terahertz Image with Time After
the Addition of Water
30 minutes
30 minutes
40 minutes
50 minutes
60 minutes
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TPI Tablet Evaluation
TabletCoating Structure
Comparison ofX-ray CT TPI
Good vs. PoorTablet Coatings
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THz Medical Imaging

• Applications
• Skin cancer basal cell carcinoma
• Aid for surgeon in tissue typing
• Endoscopy prostrate other cancers
• In use for clinical trials

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Non-Destructive Testing3D THz Imaging of IC
Package
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Security Applications

• Checkpoint screening of people to locate hidden
  weapons and explosives.
• Stand-off detection of explosives.
• Baggage screening for explosives.
• Screening for biological and chemical agents.
• Drug detection.

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Issues RegardingTerahertz Technology for
Security

• Signatures
• Do threat materials have characteristic
  signatures?
• Are they distinct from non-threat materials?
• Shielding/Barriers
• Can terahertz flux penetrate clothing and other
  barriers?
• Mode
• Can signature be detected in reflection?
• Performance
• Can systems be used at distance up to 10 m?
• Source power and detector sensitivity
• Atmospheric absorption
• Practical Systems Achievable?

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Terahertz Spectra of Explosives

• Energetic compounds and explosives
• Most features above 500 GHz.
• Barrier material absorption limits upper
  frequency to lt 3 THz.

Kemp et al., Proc. SPIE 5070, 44 (2003)
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Water Windows Correspond to Spectral Features of
Explosives
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Possible Confusion Materials

• Large data base of materials has been collected.
• No significant confusion found between explosives
  and harmless materials.

Tribe et al., Proc SPIE 5354, 168 (2004)
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Clothing and Barrier Materials

• Clothing materials are partially transparent.
• Absorption increases with frequency.
• Useful frequency range limited tolt 2-3 THz.

Tribe et al., Proc SPIE 5354, 168 (2004)
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Detecting Materials Hidden Under Clothing
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Terahertz Image of Shoe with Hidden Ceramic Knife
Plastic Explosive
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Conclusions

• Terahertz technology has made significant
  progress in recent years and is being exploited
  for a variety of applications.
• Instrumentation is becoming available
  commercially.
• Components are available for various vendors.
• Research continues to improve performance and
  lower cost of terahertz components and systems.
• Terahertz pulsed imaging and spectroscopy has
  been shown to be of use in a number of key areas.
• Understanding of the thermodynamics of
  polymorphic systems
• Process understanding of complex coating
  structures
• Techniques are fast enough to be used in
  environments where tablets have fast random
  motions
• Advances in medical applications have been
  demonstrated and expected to be further
  exploited.
• Industrial applications for examining a variety
  of products is expanding.
• Terahertz systems have demonstrated definitive
  capability is addressing important security
  applications. Initial deployment of screening
  systems in airports around the world.