R. Barry Johnson, D.Sc.Research Professor

Physics Department (A-145)Alabama A&M University

P.O. Box 1268Normal, Alabama 35762

256.372.8148ralph.johnson@aamu.edu

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.