R. Barry Johnson, D.Sc. Research Professor Physics Department (A-145) Alabama A&M University P.O. Box 1268 Normal, Alabama July 2008

Terahertz Detectors Bolometers –Conventional –Electrostatic –Golay Pyroelectric Diodes

visible Radio wave X-rays UVmicrowavesInfrared 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 Region 0.06 – 10 THz ; 2 – 300 cm -1 ; μm Typical Range: 300 GHz - 3 THz; μm

Bolometer Samuel Langley invented the bolometer in 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 from 15 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 THz RadiaBeam Technologies BLIS-03-GYC and 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)

LiTaO 3 Pyroelectric Detector Large area to 9 mm diameter Broad spectral response 0.1 to 1000 µm Current and Voltage hybrid circuits NEP 3x 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 below 1 x W Hz -1/2 QMC Instruments Ltd

Zero-Bias GaAs Schottky Diode Detectors “Responsivity and Noise Measurements of Zero-Bias Schottky Diode Detectors”

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.

Applications of THz Sensors Pharmaceutical Medical Industrial Security

Terahertz Spectral Region Molecular 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, ( ).

Applications of Terahertz Sensors to Pharmaceutical Analysis With Courtesy of Dr. Philip F Taday TeraView Limited Cambridge, 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 incident THz pulse multiple return pulses Coated tablet Terahertz Pulsed Imaging Penetration through most pharmaceutical excipients. Non-destructive coating analysis. Fully automated process.

Initial Setup for Measuring Water Ingress HPMC tablet 10 l water ~900 microns

30 minutes 40 minutes50 minutes60 minutes 30 minutes K4M – Change In Terahertz Image with Time After the Addition of Water

TPI Tablet Evaluation Tablet Coating Structure Comparison of X-ray CT & TPI Good 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 Testing 3D 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 Regarding Terahertz 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.