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

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.

Introduction: Extreme Ultraviolet Introduction: Extreme Ultraviolet Optics and Optical ConstantsOptics and Optical Constants

Two examples IMAGE satellite 2000

(above) ThO2 optical constants

(right)

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.

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

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

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

800 nm, 35 fs, 10 mJ Laser Pulses

Gas (He, Ne, Ar)

EUV Grating

MCP Detector

EUV GenerationEUV Light

λ = 800 nm / q

Orders 37 to 77

Wavelengths of 10 nm-22 nm

High Harmonic Generation

Instrument Overview

Easily rotatable linear polarization Ability to measure reflectance of multiple wavelengths simultaneously Extensive scanning ability

Reflectance Measurements

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

Can we extract the same information?

Yes!

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

“Known” material

“Unknown” material

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.

Acknowledgements

We would like to recognize NSF grant PHY-0457316 and Brigham Young University for supporting this project.