Optical Properties Of New Materials Research Articles

Relationships among optical and structural characteristics of ABSO4 crystals

We systematize the data for refractive indices and optical birefringences of ABSO4 crystals (A, B = Li, Na, K, NH4, Rb, Cs) and analyze their interrelations with structural and energy-band parameters. Besides of a number of empirical relationships that reveal low correlation coefficients and cannot be used for predictions, we find nearly functional links of the average refractive index n with the average lattice-cell parameter and the ionic radius of alkali cation for most of the ABSO4 crystals. The refractive index n correlates perfectly with the combinations of crystal density and average atomic mass that follow from Gladstone–Dale, Lorentz and Lorentz–Lorenz formulae. Moreover, we obtain a fair correlation of n with the unit-cell volume per one sulphur atom, which is associated with chemical-bond ionicity. We discuss also the links of the refractive index and the birefringence with the bandgap and its anisotropy, which follow from the Moss and Ravindra–Gupta formulae. Resources for ascertaining the birefringence size and availability of a zero-birefringence point in the ABSO4 crystals are poorer, when compared with the case of refractive indices. Our results suggest some physical and practical means for predicting basic optical properties of new materials.

Use of x-ray scattering functions in Kramers-Kronig analysis of reflectance

Kramers-Kronig analysis is commonly used to estimate the optical properties of new materials. The analysis typically uses data from far infrared through near ultraviolet (say, 40--40,000 cm$^{-1}$ or 5 mev--5 eV) and uses extrapolations outside the measured range. Most high-frequency extrapolations use a power law, 1/$\omega^n$, transitioning to $1/\omega^{4}$ at a considerably higher frequency and continuing this free-carrier extension to infinity. The mid-range power law is adjusted to match the slope of the data and to give pleasing curves, but the choice of power (usually between 0.5 and 3) is arbitrary. Instead of an arbitrary power law, it is is better to use X-ray atomic scattering functions such as those presented by Henke and co-workers. These basically treat the solid as a linear combinations of its atomic constituents and, knowing the chemical formula and the density, allow the computation of dielectric function, reflectivity, and other optical functions. The "Henke reflectivity" can be used over photon energies of 10 eV--34 keV, after which a $1/\omega^{4}$ continuation is perfectly fine. The bridge between experimental data and the Henke reflectivity as well as two corrections that needed to be made to the latter are discussed.

Quantum dots as biophotonics tools.

This chapter provides a short review of quantum dots (QDs) physics, applications, and perspectives. The main advantage of QDs over bulk semiconductors is the fact that the size became a control parameter to tailor the optical properties of new materials. Size changes the confinement energy which alters the optical properties of the material, such as absorption, refractive index, and emission bands. Therefore, by using QDs one can make several kinds of optical devices. One of these devices transforms electrons into photons to apply them as active optical components in illumination and displays. Other devices enable the transformation of photons into electrons to produce QDs solar cells or photodetectors. At the biomedical interface, the application of QDs, which is the most important aspect in this book, is based on fluorescence, which essentially transforms photons into photons of different wavelengths. This chapter introduces important parameters for QDs' biophotonic applications such as photostability, excitation and emission profiles, and quantum efficiency. We also present the perspectives for the use of QDs in fluorescence lifetime imaging (FLIM) and Förster resonance energy transfer (FRET), so useful in modern microscopy, and how to take advantage of the usually unwanted blinking effect to perform super-resolution microscopy.

Effective Manipulation of the Electronic Effects and Its Influence on the Emission of 5‐Substituted Tris(8‐quinolinolate) Aluminum(III) Complexes

The unique electron-transport and emissive properties of tris(8-quinolinolate) aluminum(III) (Alq(3)) have resulted in extensive use of this material for small molecular organic light-emitting diode (OLED) fabrication. So far, efforts to prepare stable and easy-to-process red/green/blue (RGB)-emitting Alq(3) derivatives have met with only a limited success. In this paper, we describe how the electronic nature of various substituents, projected via an arylethynyl or aryl spacer to the position of the highest HOMO density (C5), may be used for effective emission tuning to obtain blue-, green-, and red-emitting materials. The synthetic strategy consists of four different pathways for the attachment of electron-donating and electron-withdrawing aryl or arylethynyl substituents to the 5-position of the quinolinolate ring. Successful tuning of the emission color covering the whole visible spectrum (lambda=450-800 nm) was achieved. In addition, the photophysical properties of the luminophores were found to correlate with the Hammett constant of the respective substituents, providing a powerful strategy with which to predict the optical properties of new materials. We also demonstrate that the electronic nature of the substituent affects the emission properties of the resulting complex through effective modification of the HOMO levels of the quinolinolate ligand.