Pyroelectric Electron Beam Source for SmithPurcell Terahertz Generation

1
Pyroelectric Electron Beam Source for
Smith-Purcell Terahertz Generation

• Shayla M.L. Sawyer

Team Members Mike Mendez Ken Moy PI Bill
Quam Stephan Weeks
Walter Lincoln Hawkins 32 Graduate Research
Conference
November 17, 2004
2
Agenda

• Summer Research Experience
• Terahertz Technology Background
• Introduction to Terahertz
• Applications Defense and Security
• STL Concept
• 3-Part Breakdown
• Goals and Objectives
• Essential Parameters
• Experimental Setup
• Data Analysis
• Conclusions

3
The STL Experience
Bill Quam PI

• Department of Homeland Security Fellowship
  program
• 11-week internship at Bechtel Nevada Special
  Technologies Laboratory
• Government facility in Santa Barbara, CA within
  walking distance from UCSB
• Site Directed Research and Development (SDRD)
  project
• Team Members 3 Physicists, 1 Chemist, 1
  Electrical Engineer

4
Introduction to Terahertz
Portion of the submillimeter wavelength
electromagnetic spectrum between 1mm (300 GHz)
and 30 µm (10 THz)
1 Center for Material Sensing and Detection
2 S.J. Allen, Terahertz Transport in
Semiconductor Quantum Structures
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Introduction to Terahertz

• Important Properties
• Polar liquids absorb strongly (water)
• Metals are opaque
• Non-metals like paper or plastic are transparent
• Dielectrics have unique absorption spectra for
• each material
• Lightweight molecules
• have differentiable
• signatures

M. Pospiech, Teraherz Imaging 3
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Applications Defense and Security

• Screening (Spectroscopy)
• Harmful biological agents
• Anthrax strong attenuation in THz range 4
• Biological aerosol detection
• Illicit drugs
• Explosives (airborne residue)
• Imaging (2D-3D)
• Metal and non-metal weapons
• Reduced health risk Ionization (x-ray like image
  without harmful effects)

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Introduction to Terahertz

• Terahertz generation from free electron
  accelerators
• -Advantage high output power
• -Disadvantage difficult to scale down

Room Size
Table Top
Handheld?
STL
Concept
4 http//sbfel3.ucsb.edu/
5 http//www.biotech.iastate.edu/ facilities/BMF
/sem.html
J.E. Walsh Dartmouth College SEM and
Smith-Purcell Effect
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STL Concept

• 3-Part Breakdown
• 1) Electron Beam Generation
• Pyroelectric crystal accelerate focused, highly
  energetic electrons
• 2) Radiation Generation
• Electron beam passed over periodic structure
  Smith-Purcell effect
• 3) Radiation Detection
• Highly sensitive cryogenic bolometer

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Electron Beam Generation

• Pyroelectric crystal intrinsic properties
• Polarization coefficient
• High potential for electron acceleration
• LiTaO3 190 ?C m-2 K-1
• Curie temperature (point at which crystal loses
  polarization)
• High Tc wont lose effect while heating
• LiTaO3 665oC
• Electron energies of over 100 KeV
• X-ray radiation from e-beam impact on tungsten
  (images created)

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Electron Beam Generation
Room temperature 20 C Spontaneously
polarized Immediately compensated E-field is
zero No movement of electrons
gas
-z
z
Heating Temperature 60 C Polarization
decreases E-field is slightly positive
(E) Electrons accelerate toward the crystal (Ta
X-rays low count)
Heating Temperature 100 C Polarization
decreases E-field is strongly positive
(E) Electrons accelerate toward the crystal (Ta
X-rays high count)
ion
e-
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Electron Beam Generation
Cooling Temperature 90C Polarization
increases E-field is slightly negative
(E-) Electrons accelerate away from crystal (W
X-rays low count)
ion
e-
-z
z
Cooling Temperature 50C Polarization
increases E-field is strongly negative
(E--) Electrons accelerate away from crystal (W
X-rays high count)
ion
e-
Cooling Room Temperature 20C Polarization
increases E-field is 0 Time equilibrium occurs
depends upon pressure
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Radiation Emission

• Smith Purcell Effect radiation produced by the
  interaction of the electron with a periodic
  structure, such as a metallic grating.

• Radiation wavelength depends on electron beam
  energy and grating spacing.

Dgrating spacing
spectral order of radiation
Frequency
4 K.J Woods and J.E.Walsh, et. al., Forward
Directed Smith Purcell Radiation from
Relativistic Electrons
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Radiation Detection

• Cryogenic Si Bolometer
• Incident radiation causes a temperature change of
  absorber material which alters its resistivity
• Silicon and Germanium are often used for their
  quadratic dependence upon cryogenic temperatures
• High sensitivity but does not determine
  wavelength
• Borrowed from UCSB

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Goals and Objectives

• Characterize optimization of the electron beam
• Choose best metallic grating
• Detect radiation
• Create a literature review for potential field
  deployable THz sources and detectors
• Make contacts at UCSB

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Experimental Setup
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Essential Parameters

• Electron beam optimization
• Pressure
• Peak temperature
• Heating rate
• Cooling rate
• Focal length
• Crystal shape (concavity)
• Target selection
• Groove spacing
• Groove geometry

Gold Coated Diffraction Grating Groove Spacing
and Radiation Frequency at 60 KeV
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Data Analysis Electron Beam
All samples heated for 1 minute and 30 seconds at
pressures from 1-9 mTorr Varying input voltage
across the resistor (12V and 15V) results in
different peak temperatures Higher peak
temperatures give higher average electron current
during cooling
12V
15V
12V
15V
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Data Analysis Electron Beam
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Data Analysis Electron Beam
All samples were heated to 200oC at heating rates
varied by input current Measurements were taken
within time of illumination on the phosphor High
temperatures at the beginning of illumination
result in high electron current A slower heating
rate to a specific temperature and faster cooling
rate (LN) increased electron current output
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Qualitative Analysis
High temperature High electron current Somewhat
unstable
Low temperature Low electron current Stable
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Radiation Detection

• Poly IR II emission window material 85
  transmission
• Parabolic mirror
• Helium and nitrogen cooled bolometer from UCSB
• Two collimators (aluminum foil/cardboard)
• Optical chopper
• Lock-in amplifier
• Aluminum baffle on emission window side of the
  crystal (eliminate IR (heat) detection)

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Conclusions

• Experimental Results
• Low pressures (0.3-1 mTorr) give higher electron
  current
• Higher crystal temperatures yield less focused
  electrons
• Slow heating rate increases electron current
• LN cooling increases current (cumbersome)
• Target placement must be precise for optimal
  output
• Did we detect generated THz radiation?
• Signal detected on stable portion of cooling
  cycle for lower values of groove/mm spacings
  (1-10 THz)
• Further Research
• Must use filters and/or spectrometer to pinpoint
  emission frequency
• Crystal arrays and/or larger area crystals to
  increase output power and electron current

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References

• 1 Center for Materials Sensing and Detection,
  Johns Hopkins University, http//www.wse.jhu.edu/
  cmsd/Thz/, 2003.
• 2 S. J. Allen, Terahertz Transport in
  Semiconductor Quantum Structures, International
  Journal of High Speed Electronics and Systems,
  Vol 13, 4, 1129-1148, 2003.
• 3 B. Ferguson, S. Wang, H Zhong, D. Abbott, and
  X.-C Zhang, Powder Retection with T-ray
  Imaging, Terahertz for Military and Security
  Applications, Proceedings of SPIE, Vol. 5070,
  7-16.
• 4 K.J Woods and J.E.Walsh, et. al., Forward
  Directed Smith Purcell Radiation from
  Relativistic Electrons, Physical Review Letters,
  Vol. 74, 19, 1995.

Acknowledgments
THz Team Bill Quam, Mike Mendez, Stephan Weeks,
Ken Moy UCSB Contacts Prof. James Allen, Jerry
Ramian SUNY Binghampton Prof. James
Brownridge RPI contacts Jeff Geuther and Michael
Shur