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Introduction to Terahertz
The ins and outs of terahertz technologies, what it
does and how it works
Richard Dudley, Mira Naftaly
National Physical Laboratory
Teddington, UK
Aim:
• Introduce the history of terahertz science
• Understand what terahertz waves are, how they
interact with matter, how they are generated, detected and manipulated.
• Look at common measurement systems and discuss what can and can’t be measurement.
Please do ask questions and interrupt !
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What are you terahertz experiences ?
Outline
• What is terahertz (THz)
• Recent History
• How can we use terahertz waves ?
• Terahertz Components• Sources
• Detectors
• Other
• Measurement systems• Spectrometers
• Solid, liquid & gas measurement.
• Terahertz Measurements
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The Electromagnetic Spectrum
• Terahertz EM RADIATION / Rays / Waves ?
• 1012 Hz = 300 µm = 33 cm-1 = 4 meV = 50 Kelvin
• Room temperature objects = 6 THz
• Half the Cosmic background from Big Bang is THz
• Terahertz Gap ?
Terahertz Gap ?
• Terahertz has always been here !• “Heat Rays of Great Wavelength”, H. Rubens & E. F.
Nichols, Phys. Rev. 4, 314 (1897)
• Generating terahertz has just been difficult.• “Short Electric Waves”, E. F. Nichols and J. D. Tear,
Physical Review21, 587 (1923) – spark gaps
• Townes† attributes some of his motivation for
creating a maser (leading to the laser) was to
create a ‘molecular generator’ for the terahertz
band. (1957)
† Townes, How the laser happened, Oxford Uni Press, 1999.
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How can we use terahertz waves ?
Just like any other EM wave !
How can we use terahertz waves ?
• Spectroscopy / Absorption Spectra
• Organic molecules exhibit strong absorption from GHz to THz
through rotational and vibration transitions providing fingerprints
in the THz band.
• THz material ‘signatures’
• The crystalline or physical structure of materials provides further
terahertz absorptions.
K.Wayne THz OutTHz INMaterial Under Test
freq freq
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Terahertz Spectrum
• Atmospheric Water Absorption
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
0
2
1.415
1.230
1.208
1.166
1.096
1.113
0.989
0.558
Absorp
tion (
Arb
.)
Frequency (THz)
0.753
Raw THz Absorption Spectra
How can we use terahertz waves ?
• Communications
• High data rate but line of sight
• Current wireless communication systems utilise carrier waves
less than 5 GHz which restrict their maximum data rate, 100’s
Mbit/s typically.
• Higher frequency carriers enable high data rates and therefore
1000’s Gbit/s dates rates are on offer with terahertz carrier
waves.
NTT
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How can we use terahertz waves ?
• To see through objects or layers.• Terahertz waves can penetrate through materials
opaque to other parts of the EM spectrum.
• Many non-metallic or non-polar materials are transparent to terahertz to some degree
• Many plastics, glasses, woods, cardboards, earth/soil,
fabric, ceramics are transparent to some degree.
• Hidden or buried layers can be observed in structures containing layers, packaging or clothing.
• Use in non-destructive testing (NDT)
A few terahertz limitations.
• Penetration depth into material• Penetration is limited in high-water content or metal objects.
• Most materials have terahertz attenuation and thus a finite thickness before they become opaque.
• Terahertz cannot travel large distances in earth atmosphere free space• Greater distances are possible in space.
• Terahertz cannot be focused to spot sizes below 100 µµµµm (diffraction limit) unlike optical waves• Near field techniques can get round the limit.
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A few terahertz limitations.
• Spectral Features
• Terahertz is often sold as a mass spectrometer type
instrument, able to identify complex chemical substances
just by spectral absorption feature.
• Often terahertz reveals more information about the chemical-
structure rather than the chemical make up of a test sample.
• The quality of any features deteriorates as you move from
gas to liquid to solid.
• Solutions and mixtures can create very difficult to interpret
absorption spectra.
Terahertz Components
Sources, detectors and everything else…
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Terahertz Sources
• I like to categories sources into • Single frequency (c.w)
• Single frequency but tuneable
• Broadband or pulsed • Thermal
• Lamps/Black Bodies
• Electrical
• Gunn Diodes/Mixers
• Backward Wave Oscillators
• Optical / Laser Based
• CO2 Pumped Gas Laser
• Optical Parametric Oscillator
• Hetrodyne C.W Photo-mixing
• Terahertz Pulse Methods
Source Examples
The selected source will depend on your
application/measurement and possible power
requirement.
Terahertz Sources
Free Electron Laser (FEL)
• An ideal THz source: high power, large bandwidth, coherent
• But not portable!
p-Germanium laser
• 10-100 µW power.
• 1.5-5 THz tuning range
• A large cryogenic installation requiring a superconducting
magnet
ELRP, UK
Two of the more difficult sources to operate.
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Terahertz Sources
CO2 Pumped Molecular Laser
• Narrow line width.
• Tuneable !
CO2 Difference Frequency Mixing
• Mix two stabilised CO2 with rf source.
• Appl. Phys. Lett. 44 576 (1984)
• 100 kHz resolution
• Needs careful stabilisation circuits
100 W @ 10um
CO2 Laser
FAR-IR Laser
Two high power options but difficult to operate and tune.
Terahertz Sources
Quantum Cascade Laser (QCLs)• Complex structures enable
population inversion
• High output powers, c.w operation
• Low temperature operation
• From 2 THz up, narrow line
• Not tuneable
R. Köhler et al., Nature417, 156 (2002)
TheEconomist,August 10th, 73 (2002)
Gunn & TUNNETT diodes• High power but limited frequencies
available and banded devices
• Multiplier can help extend band
• Heat and output power is a problem
Semiconductor, electrical devices are attractive but not fully developed for practical use fully as yet..
H. Eisele, University of Leeds
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Terahertz Sources
Backward-wave oscillators• A vacuum tube that is used to
generate microwaves up to the
terahertz range.
• Some tuning, but limited in range
• High power output possible
• High voltages required and some operational difficulties.
AB Millimetre System
Terahertz Sources
Difference frequency generation
• Use two lasers, c.w or pulsed.
• Mix lasers optical beating in a non-linear material/device such as
• Photoconductir, GaP, GaSe, DAST
• Adjusting laser properties allows
tuning of terahertz frequencies generated
Optical Rectification
Probably, most commonly used method to generate 0.1 to 5 THz
• First observed by Austen et al, with
femtosecond optical pulses in an electro-optic material.
• Phys. Rev. Lett. Vol.53 p.1555 (1984)
• Basically required the illumination of a crystal or semiconductor with a very short optical laser pulse (sub-picosecond)
• Creates a terahertz pulse corresponding to the optical pulse.
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Terahertz Optical Rectification
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+
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+
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+Laser
SemiconductorBattery
- +Ultrafast Laser System
Terahertz Optical Rectification
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0
- 0 . 0 0 5
- 0 . 0 0 4
- 0 . 0 0 3
- 0 . 0 0 2
- 0 . 0 0 1
0 . 0 0 0
0 . 0 0 1
0 . 0 0 2
0 . 0 0 3
0 . 0 0 4
0 1 2 3 4
0 . 0 0 0 0 1
0 . 0 0 0 0 2
0 . 0 0 0 0 3
0 . 0 0 0 0 4
0 . 0 0 0 0 5
f r e q u e n c y , T H z
t i m e , p s
F e m t o L a s e r
2 m i r r o r s y s t e m
Z n T e 1 / 1 m m , F S
5 N o v 2 0 0 4
• Example terahertz pulse created by optical
rectificationTHz Pulse generated by rectification
Fourier Transform of pulse gives frequency content
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Ultra-broadband Optical Rectification
Terahertz Detectors
• Thermal
• Golay Cell
• Bolometer
• Pyroelectric device
• Electrical
• Photo-acoustic
• Diode
• Optical / Laser Based
• Terahertz Pulse Methods
Detectors
• Detector development lags source development !
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Terahertz Detectors
Bolometer• Thermal device
• Frequency insensitive
• Operates at 4 Kelvin (close to -270 Centigrade)
• Excellent sensitivity
Golay Cell• Thermal device.
• Room temperature operation
• Vibration sensitive
• Easily damaged
• Ageing and linearity issues
Terahertz Detectors
Photo-acoustic
• A closed air-cell and a pressure transducer detects terahertz.
• Calibration can be provided byohmic heating of a thin metal film within the cell
• Good sensitivity, robust and easy to operate.
Pyroelectric
• Thermal detection converted into electrical signal.
• Limited sensitivity but easy to use.
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Electro-Optic (EO) Detection
• Normally a pump-probe method and requires the laser pulse used in optical rectification generation to sample the detect the signal.
• Terahertz co-propagates with optical beam through a non-linear crystal and encodes a polarisation change on the optical beam proportional to the terahertz field strength
• Excelent bandwidth and sensitivity
• Appl. Phys. Lett. 67, 3523 (1995) & Appl. Phys. Lett. 71, 1285 (1997)
EO- Detection part
Detector Linearity
0 1 2 3 4 5 6 7 8 9 10
1E-4
1E-3
0.01
0.1
1
slope = 0.7
Spe
ctr
al a
mplit
ud
e (
a.u
.)
Number of plates
0.2 THz
0.5 THz
1.0 THz
1.5 THz
2.0 THz
2.5 THz
3.0 THz
slope = 0.7
0 1 2 3 4 5 6 7 8 9
1E-6
1E-5
1E-4
1E-3
Am
plitu
de (
a.u
.)
Number of plates
Frequency
(THz)
0.25
0.5
1.0
1.5
2.0
2.5
3.0
-1 0 1 2 3 4 5 6 7 8 9 10 11
0.01
0.1
1
Slope = 0.78
High power
Low power
Linear fit
Gola
y s
ign
al (V
)
Number of plates
Slope = 0.82
Golay • Not linear
• Behaviour differs at high and low powers
• Probable explanation: membrane has been over-stretched and lost elasticity
TDS System Linearity
TDS 1
TDS 2
NPL
LinCal
Kit
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Other terahertz components
Optics, waveguides and materials….
Some Important Materials
• Silicon & the plastics PTFE, HDPE and TPX are key terahertz materials
• They exhibit low loss and dispersion over the 0.1 to 5 THz band.
• If you need to make
• Lenses
• beam splitters
• Windows
• sample holders etc…..
• You will probably need to use these materials to design it.
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Free Space Optics
• Mirrors• Metal plane and curved can be used.
• Parabolic mirrors most commonly used to enable terahertz beam manipulation (focus, steering, collimating..)
• Lenses• Designs as with optics but silicon and plastic being
most common materials rather than glass.
• Polarising• Wire grids
• Filters• Metal grids
• Modulators• Phase modulation with wire grids
• Chopper type low frequency modulation.
Terahertz Waveguides
• Coax / Copper wire becomes too small and very high loss
above a few 100 GHz.
• Metallic waveguides can be used to many THz but become small, expensive and narrow band (typically only covering 100 GHz per physical structure).
• Metal and Polymer Wires have shown some promise
• Nature 432, 376-379 (18 November 2004)
• Surface modes so not tolerant of bending or being enclosed
THz Wire Waveguide
Metallic Waveguide
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Terahertz Waveguides
• Dielectric Waveguide and Photonic Crystal Fibres are possible future directions• Fibres like optical containing mainly vacuum have shown promise
by having guiding abilities but with low loss.
• Coupling terahertz into the devices can be challenging
• Has potential to be used as a sensor too for long interaction lengths or flows.
• Appl. Phys. Lett. 80, 2634 (2002)
Terahertz Planar Waveguides
• Small metal lines are placed on a low loss material
called the substrate
• Terahertz waves can propagate over tens of millimetres.
• Coupling is very challenging and is generally done on/in
the substrate.
• Can also be used as an on chip test module.
GaAs
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Terahertz Measurement Systems
Source, detectors and optics selection for some measurement examples
Single Frequency Systems
• Imaging
• For x-ray type operation to reveal hidden structures
• CO2, Diodes or QCL good options as sources.
• Bolometer or Golay cell easy options but not ideal due to robustness or operational difficulties.
• Communications
• Industry demands electrical, easy to operate devices.
• Diodes based devices are current favourites
• Detectors are not ideal and are either the same diodes or thermal devices which have limited bandwidth.
• Modulation is a problem.
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Broadband systems
FT Spectrometer
• Fourier transform spectrometer is a Michelson interferometer with a movable mirror and sample under test placed in one arm.
• Thermal source and golay or bolometer detection.
• Poor dynamic range below 10 THz.
Planar Circuits
• Optical rectification and EO detection
http://www.brucherseifer.com/html/dna_analysis.html
NPL DFTS
Broadband systems
Near-field systems
• Use metal tips to concentrate terahertz energy to nano-meter regions.
• Often uses optical rectification and EO detection but limited dynamic range
Time Domain Spectrometer• Has become the instrument/technique
of choice for the majority of terahertz spectroscopy and NDT applications.
• NPL Instrument
• Optical rectification and EO detection
• 10,000 dynamic range possible many sample measurement configurations.
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Time Domain Spectrometer (TDS)
TDS - System
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TDS – Delay Line
TDS Emitter
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TDS EO Detector
TDS Measurement Space
Emitter Detector
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Time Domain Spectrometer
• Parabolic metal mirrors, TPX lenses, lock-in techniques.
• Measurement systems built using the new method of
generation and detection to create systems with better
dynamic range than previously possible.
• Resolution can be issue.
Making Terahertz Spectroscopic Measurements
Higher frequency resolution requires longer acquisition time – few minutes to an hour is possible
for a single measurement
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Measurement of Gas Samples
• Typically requires long interaction lengths to get good absorption, a
parallel beam is best here.
• Gas cell needed with transparent windows.
• Produces narrow line spectral features with few distortions or
errors, thus long delay lines scans for high resolution required.
• Knowledge of gas pressure and cell length for absorption
information.
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
0
2
1.415
1.2301.208
1.166
1.096
1.113
0.989
0.558
Ab
so
rptio
n (
Arb
.)
Frequency (THz)
0.753
Measurement of Solid/Liquid Samples
Solid
• Spectra may show some features but not sharp as with gases.
• Ideally placement in collimated beam, but often done in focused beam region.
• Care must be taken to avoid internal pulse reflections within the sample.
• Pellets made from sample and PTFE powder can be made to aid measurement.
Liquid
• Spectra are often lacking in features and just show a general attenuation over
the band.
• Losses in water rich samples generally high so a thin cell is required.
• Techniques such as Attenuated Total Reflection can be used to measure
water samples but analysis is difficult.
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Non-Destructive Testing
• Send terahertz pulse into a
multilayer structure and reflections will occur from each boundary.
• Timing the delay of each
reflection can revel its thickness / refractive index.
• TDS systems are universally used for this type of application.
• See Teraview presentation
Dynamic Range
• A key parameter for any measurement system which will
define if you can undertake a measurement or not.
M Naftaly, R A Dudley, Linearity calibration of amplitude and power measurements in terahertz systems and detectors, Opt. Lett., Vol. 34 (5), 2009, pp. 674-676.
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Quantitative Terahertz Measurements
• Most terahertz measurements are made using
arbitrary units for the vertical scale.
• Absorption measurements are rarely quantitatively
and only comparable with the measured reference.
• Calibration of terahertz measurement systems is
rare even for basic parameters such as power,
frequency, linearity, spatial resolution ….
• Care must therefore be taken when comparing
data between measurement systems.
A few commercial measurement system examples
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Teraview System
• Time-domain spectrometers
• Many configurations and
options available for
spectrometers, imagers and
testing.
Picometrix T-Ray
• Time-domain spectrometers
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Thruvision
• Passive security imaging camera
Applications
• Historically Terahertz applications were dominated by,
• Astronomy
• Remote Sensing
• High energy experiments
• Terahertz Instrumentation has created applications in,
• Material Testing
• Medical Imaging
• Dentistry
• Drug Detection
• Security Scanning (People and Baggage)
• Non-destructive testing
• Communications
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