6 research outputs found
Optical nonlinearities of small polarons in lithium niobate
An overview of optical nonlinearities of small bound polarons is given, which
can occur in the congruently melting composition of LiNbO3. Such polarons
decisively influence the linear and nonlinear optical performance of this
material that is important for the field of optics and photonics. On the basis
of an elementary phenomenological approach, the localization of carriers in a
periodic lattice with intrinsic defects is introduced. It is applied to
describe the binding energies of four electron and hole small polarons in
LiNbO3: small free Nb4+Nb polarons, small bound Nb4+Li polarons, small bound
Nb4+Li:Nb4+Nb bipolarons, and small bound Oâ hole polarons. For the
understanding of their linear interaction with light, an optically induced
transfer between nearest-neighboring polaronic sites is assumed. It reveals
spectrally well separated optical absorption features in the visible and near-
infrared spectral range, their small polaron peak energies and lineshapes.
Nonlinear interaction of light is assigned to the optical formation of short-
lived small polarons as a result of carrier excitation by means of band-to-
band transitions. It is accompanied by the appearance of a transient
absorption being spectrally constituted by the individual fingerprints of the
small polarons involved. The relaxation dynamics of the transients is
thermally activated and characterized phenomenologically by a stretched
exponential behavior, according to incoherent 3D small polaron hopping between
regular and defect sites of the crystal lattice. It is shown that the analysis
of the dynamics is a useful tool for revealing the recombination processes
between small polarons of different charge. Nonlinear interaction of small
polarons with light furthermore results in changes of the index of refraction.
Besides its causal relation to the transients via Kramers-Kronig relation,
pronounced index changes may occur due to optically generated electric fields
modulating the index of refraction via the linear electro-optic effect, also.
Based on a microscopic picture and by considering the local structural
environment of bound polarons, the appearance of photovoltaic currents is
explained straightforwardly as a result of the optically induced carrier
transfer. Both transient absorption and index changes are spatially confined
to the intensity profile of the interacting light allowing for the recording
of efficient mixed absorption and phase volume holograms. By means of
holographic spectroscopy, these small-polaron based optical nonlinearities are
verified either without or with the action of the linear electro-optic effect;
their prominent features are highlighted by appropriate experimental studies
wherin the ultrafast response on the picosecond time scale is the most
recognized one. Based on these findings, the consequences for applications of
LiNbO3 in the field of nonlinear optics and photonics are presented. Besides
visionary examples like real-time, 3D holographic displays, the impact of
optical nonlinearities of small polarons for present applications are
discussed with frequency conversion and respective limiting effects, such as
green-induced infrared absorption and optical damage, as important example
Impact of deposition conditions on nanostructured anisotropic silica thin films in multilayer interference coatings
Recent developments of nanostructured coatings have reached a point where extensive investigations within multi-layer systems are necessary for further implementation in novel photonic systems. Although sculptured thin films are explored for decades, no optical and structural measurements have been performed for anisotropic nanostructured multi-layer coatings with different deposition conditions of the dense layer. In this paper, we present extensive morphological analysis on silica nanostructured anisotropic films. Changing the deposition angle from 66° to 84°, indicate the changes in surface filling from 84% to 57%, respectively, while phase retardance has a maximal value of 0.032°/nm at 70° and 72° angles. We also present the investigation of covering such structures with the dense layer at different conditions. As a result, the technology for maintaining initial anisotropic properties is developed for extending spectral difference 1.6 times and phase retardation by 5% in anisotropic multi-layer coatings. Furthermore, we present simulations of growing silica layer using experimental conditions in the Virtual Coater framework resulting in virtual anisotropic films for comparison with measurements. The minimal impact on the anisotropy of porous layer is reached with the deposition of dense layer at 30° angle during constant substrate rotation. © 202
Correlation of structural and optical properties using virtual materials analysis
Thin film growth of TiO2 by physical vapor deposition processes is simulated in the Virtual Coater framework resulting in virtual thin films. The simulations are carried out for artificial, simplified deposition conditions as well as for conditions representing a real coating process. The study focuses on porous films which exhibit a significant anisotropy regarding the atomistic structure and consequently, to the index of refraction. A method how to determine the effective anisotropic index of refraction of virtual thin films by the effective medium theory is developed. The simulation applies both, classical molecular dynamics as well as kinetic Monte Carlo calculations, and finally the properties of the virtual films are compared to experimentally grown films especially analyzing the birefringence in the evaluation
Nichtlineare Optik mit ultrakurzen Laserpulsen: SuszeptibilitÀt dritter Ordnung und kleine Polaronen sowie Interferenz und Holographie verschiedenfarbiger Laserpulse
In der vorliegenden Arbeit werden die nichtlinearen optischen Eigenschaften der Materialien Lithiumniobat und Di-Zinn-Hexathiohypodiphosphat aufgrund der SuszeptibilitÀt 3. Ordnung und kleiner Polaronen untersucht. Zudem wird gezeigt, dass die Interferenz verschiedenfarbiger Laserpulse die Aufzeichnung von statischen und dynamischen holographischen Gittern ermöglicht. Ein Teil dieser Arbeit ist in den im Anhang angegebenen 6 Publikationen bereits veröffentlicht.
Lithiumniobat wird mit einer Erweiterung des Z-Scan Experiments untersucht, welches die PulslĂ€ngenabhĂ€ngige Messung der nichtlinearen Absorption und der nichtlinearen BrechungsindexĂ€nderung ermöglicht. Dabei konnte festgestellt werden, dass bei sehr kurzen PulslĂ€ngen von 70 fs ein Effekt der Polaronen auf die nichtlineare Absorption vernachlĂ€ssigbar ist und die Zwei-Photonen-Absorption die nichtlineare Absorption dominiert. Mit gröĂerer PulslĂ€nge gibt es allerdings Abweichungen zwischen der Theorie der Zwei-Photonen-Absorption und den Messergebnissen. Mit der Entwicklung eines Polaronen-Anregungs-Modells, welches eine polaronische Absorption aufgrund wiederholtem optisch induziertem Hopping annimmt, konnte dieser Effekt konsistent erklĂ€rt werden. Die Messungen der nichtlinearen BrechungsindexĂ€nderung lassen darauf schlieĂen, dass sowohl freie LadungstrĂ€ger als auch kleine Polaronen neben der SuszeptibilitĂ€t 3. Ordnung einen Einfluss auf die BrechungsindexĂ€nderung haben, da eine nichtlineare AbhĂ€ngigkeit von der IntensitĂ€t auch bei PulslĂ€ngen von 70 fs festgestellt werden konnte.
Analog dazu konnte in Di-Zinn-Hexathiohypodiphosphat ein groĂer Zwei-Photonen-Absorptionskoeffizient festgestellt werden, welcher fĂŒr Photonenenergien nahe der Bandkante Werte zeigt, die gröĂer sind als theoretischen Ăberlegungen zeigen. Eine transiente Absorption nach optischer Anregung, gemessen durch ein Anreg-Abtast-Experiment, sowie Literatur legen nahe, dass in Di-Zinn-Hexathiohypodiphosphat gebundene Lochpolaronen durch optische Anregung entstehen können.
Durch den hohen Zwei-Photonen-Absorptionskoeffizienten konnte das Aufzeichnen eines kontrastreichen, dynamischen Amplitudengitters mittels Femtosekundenpulsen gezeigt und nachgewiesen werden.
Die KĂŒrze der Femtosekundenpulse ermöglicht aber nicht nur das Aufzeichnen eines Zwei-Photonen-Absorptionsgitters aufgrund der hohen IntensitĂ€ten, sondern erlaubt zudem die Beobachtung von Interferenz zwischen verschiedenfarbigen Pulsen. In der Zeitspanne der PulslĂ€nge betrĂ€gt die Bewegung der Interferenzstreifen, welche in der GröĂenordnung der Lichtgeschwindigkeit liegt, nur ein Bruchteil der Streifendistanz, sodass das Interferenzmuster eingefroren und beobachtbar erscheint. Somit lassen sich statische Hologramme in holographischen Filmen, wie auch dynamische Hologramme aufzeichnen. Ăber ein dynamisches holographisches Gitter mittels Zwei-Photonen-Absorption konnte so eine Frequenzkonversion durch Dopplerverschiebung in Lithiumniobat gezeigt werden
Statistical Analysis on the Structural Size of Simulated Thin Film Growth with Molecular Dynamics for Glancing Angle Incidence Deposition
For the purpose of a deeper understanding of thin film growth, in the last two decades several groups developed models for simulation on the atomistic scale. Models using molecular dynamics as their simulation method already give results comparable to experiments, however statistical analysis of the simulations themselves are lacking so far, reasoned by the limits imposed by the computational power and parallelization that can only be used in lateral dimensions. With advancements of software and hardware, an increase in simulation speed by a factor of up to 10 can be reached. This allows either larger structures and/or more throughput of the simulations. The paper analyses the significance of increasing the structure size in lateral dimensions and also the repetition of simulations to gain more insights into the statistical fluctuation contained in the simulations and how well the coincidence with the experiment is. For that, glancing angle incidence deposition (GLAD) coatings are taken as an example. The results give important insights regarding the used interaction potential, the structure size and resulting important differences for the density, surface morphology, roughness and anisotropy. While larger structures naturally can reproduce the real world in more detail, the results show which structure sizes are needed for these aspects without wasting computational resources
Statistical Analysis on the Structural Size of Simulated Thin Film Growth with Molecular Dynamics for Glancing Angle Incidence Deposition
For the purpose of a deeper understanding of thin film growth, in the last two decades several groups developed models for simulation on the atomistic scale. Models using molecular dynamics as their simulation method already give results comparable to experiments, however statistical analysis of the simulations themselves are lacking so far, reasoned by the limits imposed by the computational power and parallelization that can only be used in lateral dimensions. With advancements of software and hardware, an increase in simulation speed by a factor of up to 10 can be reached. This allows either larger structures and/or more throughput of the simulations. The paper analyses the significance of increasing the structure size in lateral dimensions and also the repetition of simulations to gain more insights into the statistical fluctuation contained in the simulations and how well the coincidence with the experiment is. For that, glancing angle incidence deposition (GLAD) coatings are taken as an example. The results give important insights regarding the used interaction potential, the structure size and resulting important differences for the density, surface morphology, roughness and anisotropy. While larger structures naturally can reproduce the real world in more detail, the results show which structure sizes are needed for these aspects without wasting computational resources