Rozalia Lukacs

On photoinduced volume change in amorphous selenium: Quantum chemical calculation and Raman spectroscopy

Hartree–Fock ab initio Raman spectra calculations and Raman spectroscopic measurements were carried out on amorphous selenium in order to identify the characteristic vibrational mode due to sigma bonds. Variations in measured peak intensity were observed as a consequence of band gap light illumination. Based on our previous theoretical investigations we originate these intensity variations from photoinduced covalent bond breaking.

Structure and photoinduced volume changes of obliquely deposited amorphous selenium

Atomic scale computer simulations on structures and photoinduced volume changes of flatly and obliquely deposited amorphous selenium (a-Se) films have been carried out in order to understand how the properties of chalcogenide glasses are influenced by their preparation method. Obliquely deposited a-Se thin films contain more coordination defects and larger voids than the flatly deposited ones. To model the photoinduced volume changes the electron excitation and hole creation were treated independently within the framework of tight-binding formalism. Covalent and interchain bond breakings and formations were found. The obliquely deposited samples containing voids showed a wide spectrum of photoinduced structural changes in the microscopic level and volume changes in the macroscopic level.

Infrared Refractive Index Dispersion of Polymethyl Methacrylate Spheres from Mie Ripples in Fourier-Transform Infrared Microscopy Extinction Spectra

We performed high-resolution Fourier-transform infrared (FTIR) spectroscopy of a polymethyl methacrylate (PMMA) sphere of unknown size in the Mie scattering region. Apart from a slow, oscillatory structure (wiggles), which is due to an interference effect, the measured FTIR extinction spectrum exhibits a ripple structure, which is due to electromagnetic resonances. We fully characterize the underlying electromagnetic mode structure of the spectrum by assigning mode numbers to each of the ripples in the measured spectrum. We show that analyzing the ripple structure in the spectrum in the wavenumber region from about 3000  cm-1 to 8000  cm-1 allows us to determine both the unknown radius of the sphere and the PMMA index of refraction, which shows a strong frequency dependence in this infrared spectral region. While in this paper we focus on examining a PMMA sphere as an example, our method of determining the refractive index and its dispersion from infrared extinction spectra is generally applicable for the determination of the index of refraction of any transparent substance that can be shaped into micron-sized spheres.

To the kinetics of photoinduced volume changes in chalcogenide glasses

Recent experimental data on the dynamics of photoinduced volume changes in chalcogenide glasses are analyzed within a simple phenomenological model. Both kinetics of the volume expansion under continuous irradiation and that of the relaxation after the illumination is switched off are considered. Comparison between theoretical results and experimental data provides a quantitative estimate of the local conversion rate of structural units responsible for the effect into the expanded states. Furthermore, this comparison shows that the conversion rate in a-As2Se3 is several times smaller than that in a-Se.

Photoinduced Volume Changes of Obliquely and Flatly Deposited Amorphous AsSe Films Universal Description of the Kinetics

Recently, we have constructed structural models for flatly deposited amorphous Se (a-Se) and using these configurations we have developed atomic scale model of photoinduced volume changes in this type of films. In this paper the same work on obliquely deposited thin film is presented. Universal description for the kinetics of photoinduced volume change in chalcogenide glasses is proposed based on our results. Our model containing reversible (transient) and irreversible (metastable) parts for kinetics fits well the experimental results of both obliquely and flatly deposited a-AsSe thin films. Finally, we make a brief comment on stretched exponential function which could also provide a quite good fit to experimental data.