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R. Barry Johnson, D.Sc.Research Professor
Physics Department (A-145)Alabama A&M University
P.O. Box 1268Normal, Alabama 35762
18 July 2008
Terahertz Detectors
• Bolometers– Conventional– Electrostatic– Golay
• Pyroelectric
• Diodes
vis
ibleRadio wave
X-raysUVmicrowaves Infrared
0.01 0.1 1 10 100
1 10 100 1000
frequency / THz
wavenumber / cm -1
Low frequency bond vibrations
Hydrogen-bonding stretches and torsions (liquids)
Crystalline phonon Vibrations (solid)
Molecular rotations (gas)
NIR
Terahertz Spectral Region0.06 – 10 THz ; 2 – 300 cm-1; 5000-30 μm
Typical Range: 300 GHz - 3 THz; 1000-100 μm
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.
Principle of Bolometer
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.
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
Silicon Bolometer
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-GYCand
Microtech Instruments, Inc.
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)
/T t
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.
Superconducting Hot Electron Bolometer
• Operates at superconducting transition region.
• Small temperature change yields large change in device resistance.
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
Zero-Bias GaAs SchottkyDiode Detectors
“Responsivity and Noise Measurements of Zero-Bias Schottky Diode Detectors” http://www.virginiadiodes.com/VDI/pdf/VDI%20Detector%20Char%20ISSTT2007.pdf
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/Hesler%202005%20stt%20thz%20source%20and%20measurements.pdf
Applications of THz Sensors
• Pharmaceutical
• Medical
• Industrial
• Security
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
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
TeraView Ltd. Terahertz Pulsed Imaging and Spectroscopy
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 Ti:sapphire laser.
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 ).
Applications of Terahertz Sensorsto Pharmaceutical Analysis
With Courtesy ofDr. Philip F TadayTeraView LimitedCambridge, UK
Pharmaceuticals
• Applications– Process improvement– Polymorph screening– Tablet Inspection
• Early stage of application
• Commercial instrumentation available
Consequences of Bad Coating Quality
• Unpredictable dosing rate• Dose dumping – life threatening• Legal and commercial implications
Coating Integrity Investigation using Terahertz Pulsed Imaging
Reflected Thz pulses probe coating structures.
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.
TPI - Coating Layer Thickness
16% w.g. enteric coating10% w.g. enteric coating
15% solids level
Enteric Coated Tablets
Single incidentTHz pulse
multiple return pulses Coated
tablet
Terahertz Pulsed ImagingPenetration through most pharmaceutical excipients.
Non-destructive coating analysis.Fully automated process.
Initial Setup for Measuring Water Ingress
HPMC tablet
10 l water
~900 microns
Time Delay (mm)
B-scan
y-d
ire
cti
on
(m
m)
-1 0 1 2 3
-6
-4
-2
0
2
4
6
-0.1
-0.05
0
0.05
30 minutes
40 minutes 50 minutes 60 minutes
30 minutes
K4M – Change In Terahertz Image with Time After the Addition of Water
TPI Tablet EvaluationTablet
Coating Structure
Comparison ofX-ray CT & TPIGood vs. Poor
Tablet Coatings
THz Medical Imaging
• Applications– Skin cancer: basal cell
carcinoma– Aid for surgeon in
tissue typing– Endoscopy: prostrate
& other cancers
• In use for clinical trials
Non-Destructive Testing3D THz Imaging of IC Package
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.
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?
Terahertz Spectra of Explosives
• Energetic compounds and explosives
• Most features above 500 GHz.
• Barrier material absorption limits upper frequency to < 3 THz.
Kemp et al., Proc. SPIE 5070, 44 (2003)
Water Windows Correspond to Spectral Features of Explosives
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)
Clothing and Barrier Materials
• Clothing materials are partially transparent.
• Absorption increases with frequency.
• Useful frequency range limited to< 2-3 THz.
Tribe et al., Proc SPIE 5354, 168 (2004)
Detecting Materials Hidden Under Clothing
Terahertz Image of Shoe with Hidden Ceramic Knife & Plastic Explosive
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.