Extreme Ultraviolet Polarimetry
Utilizing Laser-Generated HighOrder Harmonics
N. Brimhall, M. Turner, N. Herrick, D. Allred,
R. S. Turley, M. Ware, J. Peatross
Department of Physics and Astronomy
Brigham Young University
1
Overview and Conclusions
 We have constructed an extreme ultraviolet (EUV) polarimeter that
employs laser-generated high-order harmonics as the light source.
 This instrument represents a potential ‘in-house’ instrument at
facilities developing EUV thin films.
 The source has high flux, a wavelength range from 8-62 nm, and
easily rotatable linear polarization.
 The instrument has a versatile positioning system and can
measure reflectance of multiple wavelengths of light
simultaneously.
 We have compared reflectance data with that taken at the
Advanced Light Source (ALS) and with calculated data. These
measurements agree well.
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Introduction: Extreme Ultraviolet Optics and
Optical Constants
 Optical constants in the EUV are typically
unknown, incomplete, or inaccurate.
 This is important for those designing EUV
optics for applications such as astronomy,
lithography, or microscopy.
Two examples


IMAGE satellite 2000 (above)
ThO2 optical constants (right)
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Optical Constants
Optical constants are determined by measuring reflectance as a
function of angle of a sample at a fixed wavelength and
polarization, then fitting this data to the Fresnel equations.
EUV light
sample
incident angle (Θ)
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Sources of EUV light
 Synchrotron Source
 High flux
 Wide, continuous wavelength range
 Not local, expensive to run, large
footprint
 Fixed polarization
 Plasma Source
 Low Flux
 Wide wavelength range, only a few
wavelengths in the range
 Local
 Unpolarized
 High Harmonics
 Fairly high flux
 Wide wavelength range, good spacing of
wavelengths throughout the range
 Local
 Easily rotatable linear polarization 5
High Harmonic Generation
EUV Grating
EUV Generation
EUV Light
800•nm,Wavelength range from 8-62 nm
30 fs, 10 mJ
• Flux of 6x108 photons/second
Laser Pulses
•
MCP
Detector
Easily rotatable linear polarization
λ = 800 nm / q
Orders 37 to 77
Wavelengths of 10-22 nm
Gas (He, Ne, Ar)
 Fairly high flux
 Wide wavelength range with good
spacing of wavelengths within the
range
 Easily rotatable linear polarization
 Small footprint, low cost of
operation
 Potential ‘in-house’ instrument at
facilities developing EUV thin films
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Instrument Overview
EUV
generation
f=100 cm
focusing lens
dual rotation
stages
turbo pumps secondary gas
cell
gas (He, Ne, Ar)
sample
800 nm, 30
fs, 10 mJ
laser pulses
rotatable
half-wave
plate
EUV grating
aperture
turbo pump
turbo pump
MCP
CCD
• Easily rotatable linear polarization
• Ability to measure reflectance of multiple wavelengths
simultaneously
• Extensive scanning ability
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Polarimeter Positioning System
Linear Translation
Grating
Sample
Secondary Vacuum
Chamber
MCP
CCD
camera
Grating
Rotation
Turbo
Pump
Sample
Rotation
Detector
Rotation
MCP
Rotation
Linear Translation
for Focusing
 The positioning system is made up of six motors, each
controlled by a single computer.
 The diffraction grating is placed after the sample, allowing
simultaneous reflectance measurements at multiple
wavelengths.
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Controlled Harmonic Attenuator
We increase the dynamic range of our detection system with a
secondary gas cell that acts as a controlled harmonic
attenuator.
90%
90%
secondary gas cell
0.01%
0.01%
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Laser Power Discriminator
Stability of our high harmonic source is important to the
accuracy of polarimetry measurements.
 Shot-to-shot variations in the
laser pulse energy lead to
about 37% variation in
harmonic signal.
 Averaging 100 shots decreases
variation to about 7%.
A sample of the incident laser beam is
imaged in real time simultaneously
with harmonics to provide per-shot
energy monitoring
 To further increase
repeatability, we implemented a
laser energy discriminator,
decreasing variations to about
2%.
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Reflectance Measurements
 Sample:
 thermally oxidized silicon, 27.4 nm SiO2 layer.
 High-harmonic generation parameters:
 100 torr helium gas
 Measurement parameters:
 all measurements averaged over 100 shots where the
variation in the laser power was +/-5%
 secondary gas cell pressures ranged in value from 0 to 2.8
torr (attenuation of about 3 orders of magnitude)
 dark signal taken simultaneously with measurements
 measurements taken on three separate days to examine
possible systematics in repeatability.
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Compare
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Conclusions
• We have constructed a new instrument that uses high-order
harmonics to measure optical properties of materials in the
EUV.
• Our source has a wide wavelength range, high flux, and easily
rotatable linear polarization.
• Our instrument has a sophisticated positioning system and is
efficient in that simultaneous reflectance measurements can
be made at multiple wavelengths.
• We have compared reflectance measurements with those
taken at the ALS and computed data. These measurements
agree.
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Future Work
 Investigate a new measurement technique
 In some regions where reflectance is very low, it may be difficult to
measure absolute reflectance accurately (at near-normal angles, absolute
reflectance is often on the order of 10-4).
 It may, however, be possible to measure a very accurate ratio of p- to spolarized reflectance. Our instrument has the capability to quickly toggle
between polarizations to measure a very accurate ratio.
 Variation in the laser source or harmonic generation parameters over time
scales longer than minutes will no longer be a concern. Also, dynamic
range issues will no longer be a problem.
 Measure optical properties of materials in this wavelength
range
 Optical constants
 Bonding effects on optical properties
 Oxidation rates
 Roughness effects
14
Thank you
We would like
to recognize
NSF grant
PHY0457316
and Brigham
Young
University for
supporting
this project.
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Spectral Resolution
•
Defocusing is the
limiting factor, giving
a spectral resolution
of about 184.
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Future Work
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