José A. Méndez

Determination of optical constants of scandium films in the 20-1000 eV range

The transmittance of thin films of Sc deposited by evaporation in ultrahigh vacuum conditions has been investigated in the 20-1000 eV spectral range. Transmittance measurements were performed in situ on Sc layers that were deposited over grids coated with a C support film. Transmittance measurements were used to obtain the extinction coefficient of Sc films at each individual photon energy investigated. These data, along with the data available in the literature for the rest of the spectrum, were used to obtain the refractive index of Sc by means of the Kramers-Krönig analysis. Sum-rule tests indicated an acceptable consistency of the data.

Nonlinear absorption of InN/InGaN multiple-quantum-well structures at optical telecommunication wavelengths

We report on the nonlinear optical absorption of InN / InxGa1−xN (x=0.8, 0.9) multiple-quantum-well structures characterized at 1.55 µm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2~−9x103 cm /GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption.

Optical constants of electron-beam evaporated boron films in the 6.8-900 eV photon energy range

The optical constants of electron-beam evaporated boron from 6.8 to 900 eV were calculated through transmittance measurements of boron thin films deposited onto carbon-coated microgrids or LiF substrates in ultrahigh-vacuum conditions. In the low-energy part of the spectrum the measurements were performed in situ on freshly deposited samples, whereas in the high-energy range the samples were exposed to the atmosphere before the measurements. The extinction coefficient was calculated directly from the transmittance data, and a Kramers-Kronig analysis that combined the current data with data from the literature was performed to determine the dispersive part of the index of refraction. Finally, two different sum-rule tests were performed that indicated the good consistency of the data.

Triple-wavelength, narrowband Mg/SiC multilayers with corrosion barriers and high peak reflectance in the 25-80 nm wavelength region

We have developed new, Mg/SiC multilayer coatings with corrosion barriers which can be used to efficiently and simultaneously reflect extreme ultraviolet (EUV) radiation in single or multiple narrow bands centered at wavelengths in the spectral region from 25 to 80 nm. Corrosion mitigation was attempted through the use of Al-Mg or Al thin layers. Three different multilayer design concepts were developed and deposited by magnetron sputtering and the reflectance was measured at near-normal incidence in a broad spectral range. Standard Mg/SiC multilayers were also deposited and measured for comparison. They were shown to efficiently reflect radiation at a wavelength of 76.9 nm with a peak reflectance of 40.6% at near-normal incidence, the highest experimental reflectance reported at this wavelength for a narrowband coating. The demonstration of multilayer coatings with corrosion resistance and multiple-wavelength EUV performance is of great interest in the development of mirrors for space-borne solar physics telescopes and other applications requiring long-lasting coatings with narrowband response in multiple emission lines across the EUV range.

FUV reflectometer for in situ characterization of thin films deposited under UHV

A far UV (FUV) reflectometer was developed at the Metal Optics Laboratory (Instituto de Fisica Aplicada, CIC, Madrid) for in situ reflectance as well as transmittance measurements of ultra high vacuum (UHV) deposited thin films. The spectral region covered by the reflectometer is 50 - 200 nm. The angle of incidence can be continuously changed from 3 degrees to 87 degrees. The sample holder is provided with two perpendicular rotation axes to perform reflectance measurements in two perpendicular planes of incidence. Thin films of the materials to be investigated can be deposited by evaporation in an adjacent chamber that is connected to the reflectometer through a gate valve and a long linear/rotary feedthrough. In this way, thin films are deposited and their reflectance is measured in UHV conditions without breaking vacuum. Two different deposition systems, including an electron gun and resistive evaporation sources, can be used for multilayer deposition. The instrument is furnished with a substrate heating system for deposition on a heated substrate, and/or for post-deposition sample annealing. A gas entrance system allows exposing the sample to controlled doses of different gases to analyze their effect over the sample reflectance. An atomic oxygen source is also installed in the reflectometer for aging simulations of in orbit operating optical instruments. The instrument is particularly useful to investigate the effect on the sample FUV reflectance of exposure to controlled atmospheres and other in situ treatments.

Transmittance and optical constants of Ce films in the 6–1200eV spectral range

The optical constants of Ce films were obtained in the 6–1200eV range from transmittance measurements obtained at room temperature. Thin films of Ce were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Ce films were deposited onto grid-supported, thin C films. Transmittance measurements of various film thicknesses were used to obtain the extinction coefficient k of Ce films at each individual photon energy investigated. The refractive index n of Ce was calculated with the Kramers–Kronig analysis using the current k data, which were extended toward smaller and larger energies with available data from literature and extrapolations. Ce has a low-absorption band right below O2,3 edge, with lowest absorption at 16.1eV. This makes Ce a promising material for the development of new filters and multilayer coatings below Ce O2,3 edge, in which few developments have been performed due to the lack of low-absorption materials. A good consistency of the data was ev...

Optical constants of Yb films in the 23-1700 eV range

The optical constants of Yb films have been determined in the 23-1700 eV spectral range from transmittance measurements performed in situ on Yb films deposited by evaporation in ultrahigh vacuum conditions. Yb films were deposited over grids coated with a thin carbon film. Transmittance measurements were used to obtain the extinction coefficient of Yb films at each individual photon energy investigated. The energy range investigated encompasses Yb edges from M(4,5) to O(2,3). The current results, along with data in the literature, show that Yb has an interesting low-absorption band in the approximately 12-24 eV range, which may be useful for the development of transmittance filters and multilayer coatings. The current data along with literature data and extrapolations were used to obtain n, the real part of the complex refractive index, using a Kramers-Krönig analysis. The application of the sum rules showed a good consistency of the results.

Optical properties of scandium films in the far and the extreme ultraviolet

The optical properties of thin Sc films deposited in ultrahigh-vacuum conditions have been investigated in the 6.7-174.4-nm spectral range. We measured transmittance and multiangle reflectance in situ in the 53.6-174.4 nm spectral range and used these measurements to obtain the complex refractive index of a Sc film at every individual wavelength investigated. Transmittance measurements were made of Sc samples that were deposited over grids coated with a support C film. The transmittance and the extinction coefficient of Sc films at wavelengths shorter than 30 nm were measured ex situ. The ex situ samples were protected with an additional top C film before their removal from vacuum. To our knowledge, these are the first optical measurements of Sc films reported in the spectral ranges cited.

Retrieval of the zero-dispersion wavelength map of an optical fiber from measurement of its continuous-wave four-wave mixing efficiency

We present a novel algorithm for reconstruction of the longitudinal distribution of zero-dispersion wavelength in an optical fiber from low power, end-to-end measurements of four-wave mixing efficiency. It is based on a modified version of the error-reduction approach used in classic phase-retrieval problems. Experimental results demonstrate the validity of this algorithm.

Transmittance and optical constants of Pr films in the 4–1600eV spectral range

The optical constants of Pr films were obtained in the 4–1600eV range from transmittance measurements performed at room temperature. Thin films of polycrystalline Pr were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Pr films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the optical extinction coefficient k of Pr films in the 4–1600 investigated photon energy range. The refractive index n of Pr was calculated using the Kramers–Kronig analysis. Data were extrapolated both on the high and low-energy sides by using experimental and calculated extinction coefficient data available from the literature. Pr, similar to other lanthanides, has a low-absorption band right below the O2,3 edge onset; the lowest absorption was measured at about 17eV. Therefore, Pr is a promising material for filters and multilayer coatings in the energy range below O2,3 edge in which most materials have a strong absor...