Ultrafast Terahertz Radiation

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Ultrafast Terahertz Radiation
The THz spectral region Natural sources of
THz Ultrafast THz Measuring THz
pulses Spectrometry with THz light Imaging with
THZ light
Most slides courtesy of Daniel
Mittleman Electrical Computer Engineering Rice
University
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Ultrashort pulses are the best way to make
Terahertz radiation (T-rays).
n 1 THz
l 300 mm
hn 33 cm-1
T 48 K
4.1 meV
THz light has very unusual absorption properties.
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THz light is heavily absorbed by the atmosphere,
and especially by water.
Atmospheric absorption spectrum
Note that the vertical axis is a log of a log
scale!
Long distance atmospheric transmission is very
challenging (but possible for 100 m or so).
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Blackbody spectra roll off very rapidly in the
THz spectral range.
Natural sources of THz are very weak.
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Traditional electronic sources of THz light
Peter Siegel, JPL
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THz and Optical Parametric Generators
THz picks up where OPGs leave off.
Gavin D. Reid, University of Leeds, and Klaas
Wynne, University of Strathclyde
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Mixing of optical sources yields some THz.
LTG-GaAs Epitaxial Layer
Photomixer Electrodes
Silicon Hyper-Hemispherical Lens
on One-Inch Semi-Insulating
GaAs Substrate
Pump Beams
Spiral Antenna
10 µm
Photomixer
Fiber
Terahertz Signal
Diode Lasers
S. Duffy K. McIntosh, MIT Lincoln Labs
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Power of THz by optical mixing
Tunability is good, but power is poor.
S. Duffy K. McIntosh, MIT Lincoln Labs
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Generation of free-space THz pulses using
ultrashort pulses
A fs pulse induces conductivity in a biased
photoconductive switch.
When the pulse is on, current flows.
Accelerating charges emit light.
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A typical THz Photoconductive Emitter
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Generation of free-space THz pulses
Emission is proportional to the second time
derivative of the induced polarization
J is the current density
Ultrashort-pulse-induced THz light is typically a
single-cycle pulse!
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Measuring THz pulses using photoconductive
sampling
Detecting or measuring THz usually uses the same
fs optical pulse that generates it. The fs
optical pulse induces conductivity in a
photoconductive switch, which then yields more
current when the THz pulse is present.
This yields a cross-correlation between the fs
optical pulse and the THz pulse.
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Measuring THz pulses using photoconductive
sampling
Scanning the delay is performed as in
cross-correlation.
Antenna impedance vs. time
Because the fs optical pulse is shorter than the
THz pulse, this works.
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Typical THz wave-forms
Typical THz bandwidth is 2.5 - 5
THz. Photoconductive sampling is a coherent
detection of electric field. So it can be Fourier
transformed to yield the spectrum.
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THz yields a time-domain spectrometer
Simply measuring the THz spectrum before and
after a sample tells us its absorption spectrum
in the THz range.
THz has been used to measure species in a flame
(Grischkowski and coworkers).
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THz free-space electro-optic sensing
THz can measure absorption over large distances.
THz emitter
Up to 100 THz of bandwidth!
X.-C. Zhang, Rensselaer Polytechnic Institute
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THz image of a semiconductor integrated circuit
Visible image
THz image
THz sees the metal leads through the plastic
packaging.
0.25 millimeter spatial resolution
Useful for fault detection, delamination
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THz image of an automobile dashboard
Visible Images
THz Image
Front View
Gap in foam filling hidden beneath opaque plastic
Easily visible in THz image
Rear View
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Water content in a living leaf
After
Before
Proof of principle experiment
Plant is allowed to dry somewhat, and then
watered As the leaf rehydrates, THz transmission
decreases
Changes smaller than 1 are detectable
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THz imaging for tumor detection
Tumors appear to have different THz absorption
properties from normal tissue.
metastasis
paraffin
healthy tissue
50 mm
Optical image of a liver sample containing tumors
THz image 0.2 - 0.5 THz
M. Koch, TU Braunschweig
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THz imaging of tooth decay
M. Pepper, Teraview Ltd.
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Attainable resolution is only 100 mm.
THz image of object
Actual object
Dave Zimdars, Picometrix, Inc.
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Near-field imaging (using an aperture at the
sample) can enhance the spatial resolution.
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THz imaging in a reflection geometry
Reflection at nearly normal incidence. Time-of-fli
ght imaging for 3D information.
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Stand-off imaging and sensing
target
THz transceiver
Distance 15 meters
X.-C. Zhang, RPI
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THz time-of-flight imaging
40
20
0
Amplitude (nanoamps)
-20
-40
-60
0
10
20
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Delay (picoseconds)
Internal dielectric interfaces can be
distinguished Depth resolution 1/Dw (approx.
100 mm)
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THz Time-of-Flight images of a 3.5" floppy disk
"Normal" THz image (total reflected energy)
Time-of-flight THz image
This image is 3D.
D. Mittleman, et al., Opt. Lett., 22, 904 (1997).
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Improving the depth resolution by making
interferometric THz measurements
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Security screening
Because clothes are transparent to THz and the
human body (its mostly water) and metals are
not, THz can see weapons through
clothes. Unfortunately, I couldnt get
permission to show the movies of THz images of
people wearing clothesbut looking very naked!
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THz Conclusions

• THz imaging and sensing has many applications!
• Unique possibilities with single-cycle optical
  pulses

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