Increase of refractive index induced by absorbing centres

An analytical expression of the complex permittivity is derived for absorbing centres featuring inhomogeneous absorption-line broadening. Such an expression gives the dispersion law of the real part of the permittivity when the imaginary part has a Gaussian lineshape. Our mathematical approach starts from an overlap integral of Lorentzian-type dielectric susceptibilities weighted by a Gaussian probability distribution of the resonance absorption energies. The analytical solution found is consistent with the Kramers–Kronig relation. We demonstrate that, like in the case of homogeneous absorption-line broadening, the refractive index increases at photon energies lower than the resonance absorption energy also for inhomogeneous absorption-line broadening; if the absorbing centres emit Stokes-shifted radiation, such an increase can be exploited for passive and active waveguiding applications. An example is reported regarding active waveguides based on colour centres in a lithium fluoride crystal.     

Miniaturized structures based on color centers in lithium fluoride: Optical properties and applications

Research activities concerned with color centers in alkali halide films started recently. The use of versatile, well-assessed, and low-cost fabrication techniques consisting of physical vapor deposition of Lithium Fluoride (LiF) films combined with direct writing lithographic processes allows the realization of miniaturized structures, like broad-band emitters, channel waveguides, optical microcavities and point-light sources emitting in the visible spectral range. Promising results have been obtained in the generation, amplification and waveguiding of visible light in LiF treated by low energy electron beams, where the efficient formation of stable primary and aggregate color centers also induces a local modification of the refractive index. A brief overview of the investigated optical properties is presented together with a short discussion about their perspectives of applications in optoelectronics.     

Color centres in crystalline Lif films

Abstract

Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F₂ and F₃ ⁺ aggregate color centres produced by electron irradiation were performed.     

Optical reflectance and transmittance of a multilayer coating affected by refractive-index inhomogeneity,interface roughness,and thickness wedge

A theoretical model to evaluate normal-incidence specular, direct, hemispherical, and diffuse reflectance and transmittance of a single layer affected by refractive-index inhomogeneity along the growth axis, roughness, and thickness wedge was introduced in recent papers. In the present work, the model is extended to the case of a multilayer optical coating. To that purpose, suitable transfer matrices that account for layer-index inhomogeneity and random deviations of the interfaces from smooth planes are introduced, and statistical averages are applied to the resulting amplitude and intensity coefficients. Eventually, thickness wedge is included as a deterministic average on the intensity coefficients.     

Infrared near-field microscopy with the Vanderbilt free electron laser: overview and perspectives

Scanning near-field optical microscopy (SNOM) makes it routinely possible to overcome the fundamental diffraction limit of standard (far-field) microscopy. Recently, aperture-based infrared SNOM performed in the spectroscopic mode, using the Vanderbilt University free electron laser, started delivering spatially-resolved information on the distribution of chemical species and on other laterally-fluctuating properties. The practical examples presented here show the great potential of this new technique both in materials science and in life sciences.     

A preliminary study on electrochemical biosensors for the determination of total cholinesterase inhibitors in strawberries

Organophosphorous (OP) insecticides reveal acute toxicity because of their capability to affect the nervous system through the inhibition of acetyl cholinesterase function in regulating the neurotransmitter acetylcholine. The present work shows an example of an easy to be handled inhibition electrochemical biosensor, based on thick film technology for low cost production of screen printed electrodes. Anti-cholinesterase activity in specific fruits was determined measuring the inhibition of acetyl cholinesterase enzyme owing to the presence of OP pesticides. Paraoxon was taken as reference pesticide for each measurement. The main fluidic critical parameters were investigated under flow injection analysis, through the comparison of different enzymatic immobilisation methods. Analytical features were evaluated as a function of experimental parameters. The analytical detection was developed in a three step procedure and the pesticides content was measured in strawberries samples taken from the local market. The separation between the acetyl cholinesterase inhibition and the electrochemical detection with the choline oxidase biosensor decreases the total analysis time, allowing improvements in reproducibility and stability of the system. A comparison with reference materials and standard analytical procedures for pesticides will be required in the future for evaluating the reliability of the method.     

Optical Investigation of Metallic Colloids in Ion-Irradiated Lithium Fluoride (Lif) Crystals

The measured room-temperature absorption spectra of LiF crystals implanted with 1.5 MeV He^? ions at different ion doses are presented. The application of a theoretical model allowed us to give an estimation of the band intensities associated with various kinds of electronic defects and to study their dependence on the ion dose. The model also considers the contribution due to nanometric lithium colloids, whose formation starts in the dose range at which the production of colour centres saturates.     

Energy transfer among color centers in LiF crystals

In colored LiF crystals there are many absorption and emission bands which cover a wide region of the spectrum from 200 up to 1 300 nm without breaking the continuity. In the frame of these favourable conditions we have performed some experiments of energy transfer among various color centers. The results indicate the existence of efficient exchanges of radiative energy among several bands by using only one exciting wavelength. In particular, the emissions of the F₃ ^- and F₂ ^- color centers centered at 900 and 1 100 nm, respectively, have been observed by pumping at 672 nm completely outside their absorption bands. Received 25 June 2001 and Received in final form 16 July 2001     

Spectroscopy of F 3 + -color centers in lithium fluoride crystals (review)

Results of investigations of radiative F ₃ ⁺ -color centers in lithium fluoride obtained by the methods of one- and two-photon absorption, polarization spectroscopy, and luminescence in singlet, triplet and triplet-singlet channels are reported. The scheme of energy levels of the F ₃ ⁺ -center is revised. The probabilities of singlet-triplet conversion and the rates of depletion of the lower triplet state in the range of 80–350 K are presented. Recommendations are given on the technology of radiative LiF coloration that provide an increase in the F ₃ ⁺ -center concentration and a decrease in the concentration of some other centers that prevent lasing on the LiF:F ₃ ⁺ active medium from being obtained. The phototransformations of color centers in LiF are analyzed. The parameters of lasers with LiF:F ₃ ⁺ active media that provide generation of radiation tunable in the green region of the spectrum are considered. Based on spectroscopic data the maximum attainable time characteristics of such lasers are discussed. The prospects for further investigations of LiF:F ₃ ⁺ lasers are outlined in brief. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 745–760, September–October, 1998.