1 Extreme Ultraviolet Polarimetry Utilizing Laser-Generated High- Order Harmonics N. Brimhall, M. Turner, N. Herrick, D. Allred, R. S. Turley, M. Ware, J. Peatross Department of Physics and Astronomy Brigham Young University

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.  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.

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)  ThO 2 optical constants (right)

4 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. sample incident angle (Θ) EUV light

5 Sources of EUV light  High Harmonics  Fairly high flux  Wide wavelength range, good spacing of wavelengths throughout the range  Local  Easily rotatable linear polarization  Plasma Source  Low Flux  Wide wavelength range, only a few wavelengths in the range  Local  Unpolarized  Synchrotron Source  High flux  Wide, continuous wavelength range  Not local, expensive to run, large footprint  Fixed polarization

6 High Harmonic Generation Wavelength range from 8-62 nm Flux of 6x10 8 photons/second Easily rotatable linear polarization  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 800 nm, 30 fs, 10 mJ Laser Pulses Gas (He, Ne, Ar) EUV Grating MCP Detector EUV Generation EUV Light λ = 800 nm / q Orders 37 to 77 Wavelengths of nm

7 Instrument Overview Easily rotatable linear polarization Ability to measure reflectance of multiple wavelengths simultaneously Extensive scanning ability CCD turbo pumps rotatable half-wave plate gas (He, Ne, Ar) turbo pump MCP sample EUV grating dual rotation stages EUV generation turbo pump 800 nm, 30 fs, 10 mJ laser pulses secondary gas cell f=100 cm focusing lens aperture

8 Polarimeter Positioning System  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.

9 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% 0.01% 90% secondary gas cell 0.01%

10 Laser Power Discriminator  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%.  To further increase repeatability, we implemented a laser energy discriminator, decreasing variations to about 2%. Stability of our high harmonic source is important to the accuracy of polarimetry measurements. A sample of the incident laser beam is imaged in real time simultaneously with harmonics to provide per-shot energy monitoring

11 Reflectance Measurements  Sample:  thermally oxidized silicon, 27.4 nm SiO 2 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.

12Compare

13Conclusions 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.

14 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 ).  It may, however, be possible to measure a very accurate ratio of p- to s- polarized 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

15 Thank you We would like to recognize NSF grant PHY and Brigham Young University for supporting this project.

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18 Spectral Resolution Defocusing is the limiting factor, giving a spectral resolution of about 184.

19 Future Work incident angle (from grazing) R p / R s n k

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