Kinetics Of Photoinduced Changes Research Articles

Insight into the photoinduced phenomena in ternary Ge-Sb-Se sputtered thin films

The kinetics of photoinduced changes, namely, photobleaching and photodarkening in sputtered ternary Ge 29 Sb 8 Se 63 thin films, was studied. The study of time evolution of the absorption coefficient Δ α ( t ) upon room-temperature near-bandgap irradiation revealed several types of photoinduced effects. The as-deposited films exhibited a fast photodarkening followed by a dominative photobleaching process. Annealed thin films were found to undergo photodarkening only. The local structure studied by Raman scattering spectroscopy showed significant structural changes upon thermal annealing, which are presumably responsible for a transition from the photobleaching observed in as-deposited and reversible photodarkening in annealed thin films. Moreover, a transient photodarkening process was observed in both as-deposited and annealed thin films. The influence of the initial film thickness and laser optical intensity on the kinetics of photoinduced changes is discussed.

Spectral dependence of photoinduced optical effects in As40S60-xSex thin films

Spectral dependence of photoinduced structural transformation and related changes of Egopt, α and n of thermally evaporated ternary As40S60-xSex thin films have been studied under the exposure to bandgap and super-/sub-bandgap light of LED. The irradiation by bandgap and super-bandgap light with intensity 0.1W/cm2 leads to partial photoinduced polymerization of molecular structural units into continuous network that is characteristic for the bulk glasses with appropriate stoichiometric composition. The efficiency of structural transformation depends on penetration depth of excitation beam. The maximum photosensitivity (i.e. changes of Egopt, α and n) were achieved at the wavelengths ~100nm above the bandgap wavelength for each appropriate composition. Beyond the region of maximal photosensitivity the values of optical parameters and kinetics of photoinduced changes depend on relation between exposed and unexposed sublayer for super-bandgap beams or on absorption coefficient for sub-bandgap light.

PHOTOINDUCED CHANGES IN MICROHARDNES OF AMORPHOUS THIN FILMS OF THE Ge-As-Se SYSTEM

The results of experimental studies of the kinetics of photoinduced changes in microhardness of amorphous films Ge х As у Se 100-х-у under the irradiation with a laser of 50 mW power and 655 nm wavelength are presented. It is shown that the intense (10-27%) photoinduced changes of hardness of the films are observed for 25-30 minutes exposure: when the irradiation is enabled the microhardness decreases exponentially, and when it's disabled - increases exponentially. The value of photoinduced changes of microhardness depends on the average coordination number Z and has maximum at Z = 2.8. Minimum of the photoinduced microhardness changes in the topological structural 2D-3D transition at Z = 2.67 was observed. Giant growth of photoinduced changes of microhardness at Z > 2.67 is explained within the intramolecular structural model of photoinduced plasticity in chalcogenide glasses.

Millisecond kinetics of photoinduced changes in the optical parameters ofa−As2S3films

We report the kinetics of photoinduced changes in absorption coefficient, optical band gap, and refractive index of $\mathrm{a}\text{\ensuremath{-}}{\mathrm{As}}_{2}{\mathrm{S}}_{3}$ films by measuring the time evolution of complete transmission spectrum between 400 and $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at every $10\phantom{\rule{0.3em}{0ex}}\mathrm{milliseconds}$. The wavelength dependence of the time constants invalidates previously assumed parallel shift of the absorption edge, and provides a better description of photodarkening and other photoinduced effects. The time evolution of the absorption coefficient and optical band gap is significantly faster than that of the refractive index. The time constants of the reversible, transient part of the changes are also wavelength dependent. We use the photon-assisted site switching model and defects around $S$ atoms to explain the observed changes.

Kinetics of photoinduced changes in Ag nanoparticles deposited on an indium tin oxide surface

An enhancement of the UV-visible optical absorption spectra of Ag nanoparticles (NPs) deposited on indium tin oxide (ITO) surfaces of different resistance by a seed-mediated growth technique is presented. A bicolour coherent beam, obtained from a pulsed Nd:YAG laser providing a 1060-nm fundamental wavelength and a double-frequency one at 530 nm, was used. A significant change in the optical density is observed for Ag NPs on low-resistance ITO surfaces (≈4 Ω/square), while almost nothing occurs with high-resistance (≈50 Ω/square surfaces). The spectral position of the maximal absorption at 440 nm is almost preserved in both cases. This result is explained in terms of a local photoinduced static field and surface-plasmon polaritons interacting with photoexcited phonons. A grating effect, arising from the coherent bicolour light forming a superstructure as for quantum crystals, could be another contribution.

Optical gap and Urbach edge slope in a-Se

Good correspondence between the temperature shift of the optical gap and the Urbach edge slope has been found for a-Se. Such coherence, however, does not exist for photoinduced changes of the gap and Urbach edge slope measured at 77 K. The kinetics of photoinduced changes of the gap takes the form of first-order process, while the kinetics of photoinduced changes of Urbach edge obeys stretched exponential form. It is supposed that overheating due to illumination is most probably not responsible for photoinduced changes. Rather an increase in density of localized states at band edges, together with formation of self-trapped exciton-like states could explain the photoinduced changes observed in the gap and Urbach edge slope.

Relaxation kinetics of photogenerated charged excitations in conjugated polymer crystals

The recombination kinetics of photoinjected charge carriers in quasi-one-dimensional conducting polymers is studied. The dynamics of the correlated pair of photoexcitations is considered within the model of one-dimensional diffusion of quasiparticles in segments of a polymer chain confined by defects (traps). The main characteristics of the dynamics of a recombining pair (distribution function and survival probability in a confined segment, and the averaged survival probability in a polymer chain with random trap distribution) are calculated. The results obtained are associated with the observed characteristics, such as transient photocurrent decay kinetics of photoinduced changes in optical absorption (or reflectivity) and delayed fluorescence caused by charge carrier recombination. It is shown that the influence of the defects leads to acceleration of the photocurrent decay, namely, the usual power-law dependence varies as t-1/2 describing the geminate recombination kinetics in an infinite chain, at times approximately=(10 pi 2Wc2)-1 (W is the diffusion rate, c is the trap concentration), is changed to a faster exponential-like dependence varies as t1/6 exp(-t1/3). In contrast it is shown that the photoinduced absorption decay is slowed down due to trapping of photoexcitations by defects. The delayed fluorescence decay caused by recombination of photoexcitations also obeys an exponential-like dependence, varies as t-1/2 exp(-t1/3), unlike the case of an infinite chain, when delayed fluorescence decay follows a power-law dependence varies as t-3/2. The theoretical results are compared with the experimental data on relaxation kinetics of photoexcitations in trans-polyacetylene and restrictions on the model are considered.