THz-Imaging

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1. Physikalisches Institut Universität Stuttgart

• THz-Imaging
• with coherent cw-radiation
• Michael Gerull

19.04.05
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1. Physikalisches Institut Universität Stuttgart
Outline

• Introduction
• Motivation
• THz-Radiation
• Nearfield-optics
• Setup
• Results
• Summary
• Outlook

Michael Gerull
19.04.05
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1. Physikalisches Institut Universität Stuttgart
Introduction and Motivation
Michael Gerull
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1. Physikalisches Institut Universität Stuttgart

• Electromagnetic spectrum

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart

• Properties of THz-Radiation
• Not absorbed by
• Cardboard and paper
• Plastic
• Textiles
• Potential Applications - Security
  application
• - Medical Diagnostic
• - Quality control

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
Farfield optics

• Resolution is limited by diffraction
• normally known as Abbe's diffraction limit of
  ?/2
• The diffraction limit is a serious problem in
  the THz range where the wavelength is between
  0.1 and 3 mm.
• Diffraction-limited spotsize of radiation

f focal length D lens diameter/ beam
diameter

• Short focal length and large diameter needed
• gt - hyperbolic lenses
• - no spherical aberrations

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
Hyperbolic lenses

• made of polyethylene
• self made in the work-
• shop on a computer-
• controlled lathe

Michael Gerull
19.04.05
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1. Physikalisches Institut Universität Stuttgart
Nearfield optics

• Alternative approach
• Solid immersion lens (SIL) in the visible n_max
  2, in the
• THz-range materials with n up to 10
• higher resolution without loss of intensity

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
Setup
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1. Physikalisches Institut Universität Stuttgart
Transmission Geometry
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1. Physikalisches Institut Universität Stuttgart
General setup
Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
BWO

• Cathode current
• 1.5 A
• High voltage
• 2 5 kV
• water-cooled

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
BWO spectra
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1. Physikalisches Institut Universität Stuttgart
Detector
Golay-cell
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1. Physikalisches Institut Universität Stuttgart
Linear Measuring Tables

• Scanning Unit
• Movable in all 3 directions
• DC motor with position
• correction
• Speed up to 40 mm/s
• Position accuracy 0.5 µm
• Positioning range
• 55 mm in x y
• 10 mm in z (beam direction)
• Added analog position detector

LabVIEW-controlled
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1. Physikalisches Institut Universität Stuttgart
Nearfield probes

• thin metal pinhole for low distance probe -
  sample
• holes from 100 to 300 µm

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
Improved Nearfield-probe
solid immersion lens silicon n 3.5 factor in
resolution of ?/n
scheme of holder
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Results
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Resolution farfield 1.27 mm, at 470 GHz
10-90 criteria
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1. Physikalisches Institut Universität Stuttgart
Resolution with SIL 310 µm
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1. Physikalisches Institut Universität Stuttgart
Test-sample
test-sample, made of steel, lines and characters,
different thickness
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Corrected, a little bit strechted, nonlinear A/D
raw data
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nearfield
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1. Physikalisches Institut Universität Stuttgart
Anti-theft protection
example for resolution in far- and
nearfield printed metal lines/wires on
cardboard, covered with plastic
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Relay
Photo THz open cover
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Ivy leaf
Photo farfield nearfield, 4
days later
water content is decreasing see leaf veins
Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
Difference spectroscopy
k absorption coefficent
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Cancer

• Oesophagus (gullet) carcinoma of a horse
• embedded in paraffin

Photo farfield nearfield
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Difference spectroscopy
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Thin cut of rat brain
60 µm thick measured with offset- correction
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Summary Outlook
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Summary

• THz images with 0,25 MegaPixel (data points)
  resolution in less than 60 min for
  farfield-measurements
• Near-field measurements in transmission working
  and at higher resolution than in the far field,
  duration of 2-4 hrs
• Images of biological pathological samples show
  interesting results

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart
Outlook

• Application of a better A/D-converter with 12or
  16 bit for a better resolution
• Implementation of bidirectional scanning gt
  reducing measurement time
• Improvement of nearfield-probe

Michael Gerull
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1. Physikalisches Institut Universität Stuttgart

• Thank you for your attention

Michael Gerull
19.04.05