Biomedical subjects

Juan I Larruquert

Publications and source records attributed to Juan I Larruquert.

11 recordsLinked to original sources

PubMed2006

Reflectance optimization of inhomogeneous coatings with continuous variation of the complex refractive index.

A model is derived for the reflectance optimization of an inhomogeneous coating made of absorbing materials. The model is applicable mainly for spectral regions where no transparent materials are available, such as in the extreme ultraviolet. The complex refractive index is assumed to take values within a given continuous domain and in a given sequence. The coating design is generated through a series of layer elements with a small refractive-index contrast across interfaces; the thickness of the element is calculated in terms of the refractive-index increment at the interface. The coating is optimized element by element starting from the substrate. When the refractive index varies both continuously and smoothly, the thickness element is of first order in the refractive-index increment. Suggestions are given on how to optimize a more general coating that alternates continuous and smooth refractive-index domains along with discrete indices, which results in a succession of inhomogeneous coatings and finite layers. An example is given to illustrate the model. A new material selection rule is obtained to discriminate whether the addition of a material on top of a partly grown coating will increase or decrease the reflectance of the coating. As a consequence, the model, which is highlighted toward the maximization of reflectance, can be used analogously for reflectance minimization such as for anti-reflection coatings.

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PubMed2006

Can multilayers be optimized sequentially when radiation is partially polarized?

For radiation-absorbing materials and nonnormal incidence, multilayers with the largest reflectance at a desired wavelength are here demonstrated to require nonsequential optimization when radiation is partially polarized. The thicknesses of multilayers with the largest possible reflectance at a desired wavelength can be calculated sequentially either for s- or p-polarized radiation or for normal incidence and any polarization. The equations that impose the condition of maximum reflectance in the general case of partial polarization do not allow for the simplification that is performed when radiation is s or p polarized, which implies that multilayer optimization for partially polarized radiation is not sequential. An example is given in which the importance of nonsequential optimization is displayed. In contrast to the case of s- or p-polarized radiation, when radiation is partially polarized extra maxima may be found within an optical path difference of much less than half a wavelength.

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PubMed2006

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.

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PubMed2005

Inreflectance: a new function for the optimization of multilayers with absorbing materials.

A new magnitude is unveiled from the fact that a multilayer can be optimized for the largest reflectance in a layer-by-layer sequence: This magnitude has been named inreflectance. Inreflectance equals the modulus of the complex reflectance once elevated to the inverse of the complex refractive index of the next outer layer, including the inclination term. The maximization of inreflectance at every internal layer, proceeding sequentially starting with the innermost layer, results in a multilayer with the largest reflectance. At a given layer in the multilayer, inreflectance is different from reflectance when the next outer material absorbs radiation, whereas the two magnitudes are coincident when the next outer layer is transparent. The outermost layer of the multilayer is intrinsically different from the internal layers, and its optimization is performed through reflectance and not through inreflectance. Every maximum of inreflectance is found at a layer thickness that is larger than that for the reflectance maximum. The new magnitude is illustrated with some examples in the extreme ultraviolet, a spectral range in which all materials absorb radiation. Inreflectance can also be used to design multilayers in which some layers have fixed thicknesses and the rest of the layers have to be optimized taking into account the fixed layers.

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PubMed2004

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.

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PubMed2004

Layer-by-layer design method for multilayers with barrier layers: application to Si/Mo multilayers for extreme-ultraviolet lithography.

A previous layer-by-layer multilayer design method [J. Opt. Soc. Am. A 19, 385 (2002)] is completed by adding the possibility of alternating layers with fixed thicknesses along with layers whose thicknesses are optimized for the largest possible reflectance at a desired wavelength. The previous algorithm did not allow for layers with fixed thicknesses. The current formalism is particularly suited for a multilayer design in which barrier layers of given thicknesses are used to prevent diffusion and/or reaction between the multilayer constituents. The design method is also useful both when intermixing zones develop at multilayer interfaces and when capping layers are used. The algorithm allows the design of multilayers with complex barrier layers with any number of layers of any optical constants. The optimization can be performed either for normal incidence or for nonnormal incidence with either s- or p-polarized radiation. The completed method provides a fast and accurate procedure for multilayer optimization regardless of the number of different materials used in the multilayer. The optimum layer thickness is determined by means of functions suitable for implementation in a computer code. The performance of the current algorithm is exemplified through the design of Si/Mo multilayers with intermixing layers or with barrier layers that are optimized for the largest reflectance at 13.4 nm. The use of specific barrier layers on each multilayer interface is also discussed.

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PubMed2003

Optical properties of ytterbium films in the far and the extreme ultraviolet.

The optical properties of thin film of ytterbium in the 53.6-183.6-nm spectral range are described. Yb film were deposited in ultrahigh-vacuum conditians, and their transmittance and reflectance were measured in situ. Transmittance measurements showed that Yb has a certain window of lower absorption at approximately 54-100 nm, which makes Yb an interesting material for filters in this difficult spectral range. The optical constants were obtained from transmittance measurements and from multiangle reflectance measurements. These are what we believe are the first reported optical measurements of fresh Yb films at wavelengths shorter than 107.8 nm. Aging studies were performed both under vacuum and in a desiccator and showed that the optical properties of Yb are strongly modified on aging.

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PubMed2002

Sub-quarter-wave multilayer coatings with high reflectance in the extreme ultraviolet.

Multilayer coatings with a small number of layers were designed and prepared to provide an increase in normal-incidence reflectance in the extreme ultraviolet compared with the reflectance of available single-layer coatings, namely, SiC, B4C, and Ir. Multilayers were designed to produce coatings with the highest possible reflectance at 91.2 and at 58.4 nm. At these wavelengths all the materials absorb radiation strongly, but still a reflectance enhancement can be obtained by means of sub-quarter-wave multilayer coatings with two or more different materials. Sub-quarter-wave multilayer coatings based on Al, MgF2, diamondlike carbon, B4C, SiC, and Ir showed higher reflectance than single-layer coatings of SiC and B4C not only at the target wavelength but in a wide band ranging from 50 nm to the 121.6-nm H Lyman-alpha line. Multilayer coatings suffered some reflectance degradation over time. However, after approximately 80-100 days of aging in a desiccator, the reflectance for the multilayer coatings was greater than for the single-layer coatings.

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PubMed2002

Optical properties and quantum efficiency of thin-film alkali halides in the far ultraviolet.

The optical constants of thin films of CsI, KI, and KBr and the quantum efficiency (QE) of planar photocathodes made with these alkali halides in the 53.6-174.4-nm spectral range are presented. The optical constants were obtained from measurements of the reflectance as a function of incidence angle. The effect of film heating and exposure to UV irradiation on the optical properties and on the QE of the three alkali halides was investigated. KBr was found to be the most stable material for both heating and UV irradiation. KI appeared to be close to temperature stable, whereas UV exposure affected its optical constants. CsI optical constants changed after 420 K heating and after UV exposure. The changes in the optical constants were related to the QE changes, and a certain correlation between both variations was determined. However, it was also demonstrated that the QE changes cannot be explained solely by the changes in optical constants.

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PubMed2002

New layer-by-layer multilayer design method.

A new layer-by-layer multilayer design method is presented. The method is demonstrated mathematically and makes possible the optimization of the multilayer for the highest possible reflectance either at normal incidence or at nonnormal incidence for s- or p-polarized radiation. With the current method multilayers can be designed regardless of the number of different materials used. The optimum layer thickness is determined by means of functions suitable for implementation in a computer code. The new multilayer design method is fast and accurate.

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PubMed2002

Reflectance enhancement in the extreme ultraviolet and soft x rays by means of multilayers with more than two materials.

Sub-quarterwave multilayer coatings with more than two different materials are shown to provide a reflectance enhancement compared with the standard two-material multilayer coatings when reflectance is limited by material absorption. A remarkable reflectance enhancement is obtained when the materials in the multilayer are moderately absorbing. A simple rule based on the material optical constants is provided to select the most suitable materials for the multilayer and to arrange the materials in the correct sequence in order to obtain the highest possible reflectance. It is shown that sub-quarterwave multilayers generalize the concept of multilayers, of which the standard two-material multilayers are a particular case. Various examples illustrate the benefit of sub-quarter-wave multilayer coatings for highest reflectance in the extreme ultraviolet. Applications for sub-quarterwave multilayer coatings are envisaged for astronomy in the extreme ultraviolet (EUV) and soft x rays and also for future EUY lithography.

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