1. Extreme Ultraviolet Polarimetry with Laser-Generated High-Order Harmonics
N. BRIMHALL, N. HEILMANN, N. HERRICK, D. D. ALLRED, R. S. TURLEY, M. WARE, J. PEATROSS
Brigham Young University, Provo, UT 84602

2. 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 (as opposed to synchrotron).
We have compared reflectance data with that taken at the Advanced Light Source (ALS) and with calculated data. These measurements agree well.
In addition to absolute reflectance, we can extract all desired information out of relative measurements of p- and s-polarized reflectance, reducing systematic errors.
The exciting thing about this instrument is not that the measurement is new, but that we can now do with a laser what was previously done with a synchrotron source.

3. 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)

4. Optical Constants
Optical constants are typically determined by measuring reflectance as a function of angle.
Reflectance is then fitted to the Fresnel equations to find the optical constants.
sample
incident angle (Θ)
EUV light

5. Sources of EUV light Synchrotron source Plasma source High Harmonics
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.
Easily rotatable linear polarization

6. High Harmonic Generation
800 nm, fs, 10 mJ Laser Pulses
Gas (He, Ne, Ar)
EUV Grating
MCP Detector
EUV Generation
EUV Light
Fairly high flux (6x108 photons/sec at a spectral resolution of 180)
Wide wavelength range with good spacing of wavelengths within the range (8-62 nm)
Easily rotatable linear polarization
Small footprint, low cost of operation
Potential ‘in-house’ instrument at facilities developing EUV thin films
λ = 800 nm / q
Orders 37 to 77
Wavelengths of 10 nm-22 nm

7. Instrument Overview Easily rotatable linear polarization
Ability to measure reflectance of multiple wavelengths simultaneously
Extensive scanning ability

8. Reflectance Measurements

9. Ratio Method
Noise (especially systematic noise) is a problem for retrieving accurate optical constants
A measurement of p- to s-polarized reflectance reduces systematic noise significantly

10. Can we extract the same information?
Yes!

11. Future Work One step away from an ellipsometer.
Can we measure phase information?
This is difficult in the EUV because there are no good polarizers
Diffraction pattern depends on the phase difference between the reflection from the two materials
“Unknown” material
“Known” material

12. Conclusions
We have constructed a new instrument that uses high-order harmonics to measure optical properties of materials in the EUV.
Our compact source has a wide wavelength range, high flux, and easily rotatable linear polarization.
We have compared reflectance measurements with those taken at the ALS and computed data. These measurements agree.
We can reduce systematic noise by measuring the ratio of p-polarized to s-polarized reflectance, and we can extract the same information from this as from absolute reflectance.

13. Acknowledgements
We would like to recognize NSF grant PHY and Brigham Young University for supporting this project.