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
richard.dudley@npl.co.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 !
1
What are you terahertz experiences ?
Outline
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•
•
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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
2
The Electromagnetic Spectrum
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•
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•
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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.
3
How can we use terahertz waves ?
Just like any other EM wave !
How can we use terahertz waves ?
• Spectroscopy / Absorption Spectra
freq
THz IN
freq
K.Wayne
THz Out
Material Under Test
• 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.
4
Terahertz Spectrum
• Atmospheric Water Absorption
Raw THz
Absorption Spectra
2
1.166
1.113
Absorption (Arb.)
1.096
1.415
1.208
1.230
0.558
0.753
0.989
0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Frequency (THz)
How can we use terahertz waves ?
• Communications
• High data rate but line of sight
NTT
• 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.
5
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 µm (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 chemicalstructure 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
Source Examples
•
•
The selected source will
depend on your
application/measurement
and possible power
requirement.
•
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
Terahertz Sources
Two of the more difficult sources to operate.
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
ELRP, UK
superconducting
magnet
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Terahertz Sources
Two high power options but difficult to operate and tune.
CO2 Pumped Molecular Laser
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CO2 Difference Frequency Mixing
•
Narrow line width.
Tuneable !
•
•
Mix two stabilised CO2 with rf source.
• Appl. Phys. Lett. 44 576 (1984)
100 kHz resolution
Needs careful stabilisation circuits
FAR-IR Laser
100 W @
10um
CO2 Laser
Terahertz Sources
Semiconductor, electrical devices are attractive but not fully developed for
practical use fully as yet..
Quantum Cascade Laser (QCLs)
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•
•
•
•
Complex structures enable
population inversion
High output powers, c.w operation
Low temperature operation
From 2 THz up, narrow line
Not tuneable
Gunn & TUNNETT diodes
•
High power but limited frequencies
available and banded devices
•
Multiplier can help extend band
•
Heat and output power is a problem
R. Köhler et al., Nature417, 156 (2002)
TheEconomist,August 10th, 73 (2002)
H. Eisele, University of Leeds
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Terahertz Sources
Backward-wave oscillators
•
•
•
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AB Millimetre System
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.
Terahertz Sources
Difference frequency generation
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)
•
Use two lasers, c.w or pulsed.
Mix lasers optical beating in a nonlinear material/device such as
• Photoconductir, GaP, GaSe,
DAST
Adjusting laser properties allows
tuning of terahertz frequencies
generated
•
•
Basically required the illumination of
a crystal or semiconductor with a
very short optical laser pulse (subpicosecond)
Creates a terahertz pulse
corresponding to the optical pulse.
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Terahertz Optical Rectification
+++
++-++
+-+- - --
Laser
Ultrafast Laser
System
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+
Battery
Semiconductor
Terahertz Optical Rectification
• Example terahertz pulse created by optical
rectification
THz Pulse generated by
rectification
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
. 0
0
5
F
2
Z
0
0
. 0
0
0
0
5
0
. 0
0
0
0
4
0
. 0
0
0
0
3
0
. 0
0
0
0
2
0
. 0
0
0
0
1
2
0
4
0
6
0
t i m
8
e
,
p
N
e
o
v
2
0
0
4
m
t o L a s e r
m
i r r o r s y s t e m
n T e
1 / 1 m
m
, F
0
1
0
0
1
S
2
0
s
Fourier Transform
of pulse gives
frequency content
0
1
2
f r e
q
u
e
n
3
c y ,
T
H
4
z
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Ultra-broadband Optical Rectification
Terahertz Detectors
• Detector development lags source development !
Detectors
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•
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Thermal
• Golay Cell
• Bolometer
• Pyroelectric device
Electrical
• Photo-acoustic
• Diode
Optical / Laser Based
• Terahertz Pulse Methods
12
Terahertz Detectors
Bolometer
Golay Cell
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•
•
•
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Thermal device
Frequency insensitive
Operates at 4 Kelvin (close to -270
Centigrade)
Excellent sensitivity
Thermal device.
Room temperature operation
Vibration sensitive
Easily damaged
Ageing and linearity issues
Terahertz Detectors
Photo-acoustic
Pyroelectric
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•
•
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A closed air-cell and a pressure
transducer detects terahertz.
Calibration can be provided by
ohmic heating of a thin metal film
within the cell
Good sensitivity, robust and easy
to operate.
•
Thermal detection converted
into electrical signal.
Limited sensitivity but easy to
use.
13
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
•
•
EO- Detection
part
Appl. Phys. Lett. 67, 3523 (1995) & Appl. Phys. Lett. 71, 1285 (1997)
•
TDS 2
Detector Linearity
Frequency
(THz)
0.25
0.5
1.0
1.5
2.0
2.5
3.0
1E-3
Amplitude (a.u.)
TDS System Linearity
TDS 1
1E-4
1E-5
1
1E-6
Spectral amplitude (a.u.)
slope = 0.7
NPL
LinCal
Kit
0.1
0
2
3
4
5
6
7
8
9
0.2 THz
0.5 THz
1.0 THz
1.5 THz
2.0 THz
2.5 THz
3.0 THz
1E-3
slope = 0.7
1E-4
0
1
2
3
4
5
6
7
8
9
10
Number of plates
Golay
1
•
•
•
Slope = 0.78
Golay signal (V)
1
Number of plates
0.01
0.1
Not linear
Behaviour differs at high and low powers
Probable explanation: membrane has been
over-stretched and lost elasticity
Slope = 0.82
High power
Low power
Linear fit
0.01
-1
0
1
2
3
4
5
6
7
8
9
10
11
Number of plates
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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.
15
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
Metallic Waveguide
THz Wire 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
Planar Circuits
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•
•
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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.
Optical rectification and EO
detection
Poor dynamic range below 10 THz.
http://www.brucherseifer.com/html/dna_analysis.html
NPL DFTS
Broadband systems
Near-field systems
Time Domain Spectrometer
•
•
•
Use metal tips to concentrate
terahertz energy to nano-meter
regions.
Often uses optical rectification
and EO detection but limited
dynamic range
•
•
•
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
23
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.
•
•
•
2
1.166
1.113
Absorption (Arb.)
1.096
1.415
1.208
1.230
0.558
0.753
0.989
0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Frequency (THz)
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.
24
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.
25
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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