Absorption (electromagnetic radiation)

In physics, absorption of electromagnetic radiation is how matter (typically electrons bound in atoms) takes up a photon's energy — and so transforms electromagnetic energy into internal energy of the absorber (for example, thermal energy). A notable effect is attenuation, or the gradual reduction of the intensity of light waves as they propagate through a medium. Although the absorption of waves does not usually depend on their intensity (linear absorption), in certain conditions (optics) the medium's transparency changes by a factor that varies as a function of wave intensity, and saturable absorption (or nonlinear absorption) occurs. Quantifying absorption Mathematical descriptions of opacity Many approaches can potentially quantify radiation absorption, with key examples following.

The absorption coefficient along with some closely related derived quantities
The attenuation coefficient (NB used infrequently with meaning synonymous with "absorption coefficient")

Accurate, Stable and Efficient Modal Calculations of Photoelectrically Useful Absorption in Lamellar Metallic and Semiconductor Diffraction Gratings

David Isaak Gablinger

Solar energy has seen tremendous advances in the past years. For thin film photovoltaics, which use less of the expensive semiconductor materials, insufficient light absorption can be a limiting factor. It is hoped that by using diffractive optics to improve the light absorption, the cost per Watt could sink. Correspondingly, the optics of such structures need to compensate for the low absorption by high (structural) resonance, which is challenging to calculate. To estimate optimal structures, a numerical method should be able to assess feasible structures with widely varying geometries quickly. Modal methods allow for an efficient analysis of structures with varying height through the separation of eigenvalue and boundary value problem. First, the thesis aspires to further develop the modal methods for the calculation of optical properties of layered structures containing weakly absorbing metals and semiconductors. Second, the thesis aims to calculate absorption enhancements in idealized, prototypical structures by applying the newly developed methods. The calculations should only depend on material parameters and not contain additional assumptions. These absorption enhancements are not tied to a priori assumptions such as mode couplings, but they solely follow the physics of the structure investigated. The first part of the thesis is concerned with the methodical improvements. A first emphasis is put on studying peculiar properties of the eigenvalue problem, and on new developments of methods to solve it within a layer. Furthermore, it shows several variants for the numerical implementation of the eigenvalue problem. This part includes a new method to calculate the eigenvalues that can be adapted to two dimensional grating problems of arbitrary shape. The new method integrates the eigenvalue problem by making use of a two point trapezoidal formula, and satisfies the boundary condition between different materials exactly. It is energy conserving and the rate of convergence depends on the approximation order. The eigenvalues show a monotonic convergence that allows for extrapolation. The second methodical emphasis is placed on variants of the implementation of the boundary value problem that connects the grating to the incoming and outgoing plane waves. This algorithm describes the propagation of the incident energy to the semiconductor layer and the substrate by solving a non-recursive and numerically stable system of linear equations. A novel variant reduces the bandwidth of the corresponding matrix by a third. The third part of the thesis concerns calculations using the improved methods. First, the improved calculations are verified by showing that the energy conservation of the modal method, as well as the well-behavedness of the condition number of the calculation. Next, numerical results for the new methods are compared to results from the literature for analytic modal methods, and a comparison with existing software is made. Thereafter, the interface plasmons occuring for H polarization are investigated. In the last part of the thesis, calculations are made for the material specific absorption of light in metallic gratings covered by semiconductors, with a special interest in the absorption in the semiconductor. Here, the spectra for rectangular, sinusoidal gratings, and asymmetric gratings are calculated, and the absorption improvement is investigated through an analysis of the involved modes.

EPFL2016

Preparation, characterization and photocatalytic activity of commercial TiO2 powders co-doped by N and S

Julian Andrés Rengifo Herrera

Heterogeneous photocatalysis over TiO2 using the sunlight seems to be a promising technology for waste-water treatment and drinking water production. However, the overall efficiency of TiO2 under natural sunlight is limited to the UV-driven activity (λ < 400 nm), accounting only to ~ 4% of the incoming solar energy on the Earth's surface. The increase of the TiO2 absorption towards wavelengths more abundant on the planet's surface has, thus become recently an interesting challenge. In this framework, the main objective of this work was to study the modification of TiO2 by non metallic doping (N and S) in order to increase its light absorption in the visible region (45% of the light hitting the terrestrial surface). Commercial TiO2 powders were doped by a simple preparation method consisting of manual grinding them with a N or S precursor (thiourea) and then annealing at 400 and 500 °C. At both temperatures N, S co-doped commercial TiO2 powders with visible response were obtained. However, the annealing step produced a reduction on the specific surface area and dehydroxylation causing a detrimental effect on the photocatalytic UV-driven activity principally on photocatalyst annealed at 500 °C. E. coli bacteria inactivation, phenol and di-chloroacetate (DCA) oxidation was achieved when the co-doped photocatalyst annealed at 400 °C was illuminated upon UV light the •OH radical being the main oxidative species, that was detected by Electronic Spin Resonance (ESR) spin-trapping with DMPO. In contrast, upon visible light irradiation, N, S co-doped TiO2 powders showed a diminished oxidation power since phenol and DCA were not oxidized. On the other hand, it was possible to inactivate E. coli cells demonstrating that under these conditions, the photocatalytic mechanism was different. In order to elucidate this mechanism, characterization by Diffuse Reflectance Time Resolved Spectroscopy (DRTRS), Low Temperature-ESR and ESR-spin trapping measurements were done. By DRTRS measurement, it was found that under visible light excitation, electrons would be promoted from N, S localized states within the band gap to the conduction band. These electrons were probably trapped on shallow traps such as oxygen vacancies (Vo) allowing them to reach lifetimes of ms. LT-ESR data revealed that N, S co-doping probably could benefit the formation of Vo. ESR spin trapping with TMP-OH as a singlet oxygen quencher, revealed the formation of singlet oxygen as the main oxidative species instead of the •OH radical. Thus, it was suggested that a photo-promoted electron, instead of the localized hole on N, S states, would be the carrier charge playing the main role in photocatalytic reactions on N, S co-doped commercial TiO2 powders. When the electron is trapped on Vo, it could react with molecular oxygen previously adsorbed producing superoxide radical (•O2-). Finally, the oxidation of •O2- by localized holes seems to be thermodynamically favored leading the singlet oxygen (1O2) production. It is well known that singlet oxygen is a Reactive Oxygen Species with a lower oxidative power than the hydroxyl radical. Singlet oxygen is not able to oxidize organic substances such as phenol and DCA but this species is very toxic to microorganism producing lipid peroxidation reaction on biological membranes. It was also concluded that N, S co-doped TKP 102 annealed at 400 °C did not present an enhancement on their photocatalytic activity towards phenol oxidation and E. coli inactivation when simulated solar light was used. Undoped Degussa P-25 was the commercial powder with highest photocatalytic activity. Evidences are reported about the classical photocatalytic process where •OH radicals are produced mainly by oxidation of water or hydroxyl ions with photo-induced valence band holes prevails upon simulated solar light exposition. Localized states induced by N or S-doping and responsible of visible light absorption did not play an important role on the photocatalytic activity of these novel materials under the experimental conditions used.

EPFL2009

Spectroscope method for determining the concentration of ethanol in beverages

Luc Thévenaz

A method and apparatus serve to determine spectroscopically a concentration of ethanol in a test sample of an alcoholic aqueous beverage. The determination is based upon providing calibration data which establish a relation between (A) a plurality of transmission values of an electromagnetic radiation in the near infrared region, measured at a unique wavelength at which water is relatively opaque to the radiation while ethanol is relatively transparent thereto, of a plurality of calibration samples of the beverage containing ethanol in differing known concentrations, and (B) the known concentrations of ethanol in the calibration samples. Then, at least one light transmission value of the test sample at the unique wavelength, at which said calibration data were established, is measured. The measured transmission value of the test sample is transformed, by way of the relation established by the calibration data, into an indication of the concentration of ethanol in the test sample.

United States Patent and Trademark Office1997