Demarcation Energy Research Articles

Lifespan prediction procedure of volume nanogratings imprinted by femtosecond laser in optical glasses

Volume nanogratings imprinted by infrared femtosecond laser in oxide glasses exhibit a characteristic birefringent signature, which translates into measurable retardance. Upon thermal annealing, such signature is progressively erased, typical of nanograting erasure. In this work, we propose a procedure to predict the lifespan of nanogratings by two main approaches: 1/ numerical modeling of optical retardance ageing using the Rayleigh-Plesset equation, and 2/exploiting VAREPA (VAriable REaction PAthways) framework fed by simulated ageing data. By considering experimental time – temperature annealing conditions, the modeled retardance is gathered as a function of demarcation energy to build a so-called Master Curve and then compared to accelerated ageing experiments. The erasure constant rate k0 can be determined for 8 commercial optical glasses. Based on a distributed Rayleigh-Plesset model, k0 and activation energy distribution are linked to glass viscosity and its temperature dependency. Finally, we discussed the restrictions on VAREPA application for an accurate lifetime prediction. This work provides guidelines for the future development of nanogratings based devices and applications, including optical data storage, birefringent devices, and optical sensors, through a judicious choice of glass composition and associated properties.

Analysis of thermally activated processes in bismuth-doped GeO<i/> 2-SiO<i/> 2 glass fibers using the demarcation energy concept

We report a study of the thermally activated processes occurring in Bi-doped GeO2-SiO2 fibers using the demarcation energy concept based on a distribution of the activation energies of a first order physico-chemical reaction. Our attention was paid to the processes of formation of the bismuth-related active centers (BACs) and Bi-centers responsible for the unsaturable loss, as well as to the restoration of the BACs in laser-bleached fibers. The activation energies of these processes were determined analyzing the thermal kinetics of these processes. The formation energies of new BACs and Bi-centers responsible for the unsaturable loss were determined to be around 1.1 eV and higher than 2 eV, respectively. The activation energy of thermal restoration process of the bleached BACs is ≈ 0.4 eV. The possible underlying mechanisms of these processes are discussed.

Demarcation energy properties of regenerated fiber Bragg grating sensors in few-mode fibers

Demarcation energy properties of regenerated fiber Bragg grating sensors in few-mode fibers

The Influence of Morphology and PbI2 on the Intrinsic Trap State Distribution in Perovskite Films Determined by Using Temperature-Dependent Fluorescence Spectroscopy.

Perovskite films with different particle sizes and PbI2 contents were prepared by using a controlled single or sequential method. By means of temperature-dependent fluorescence spectroscopy, the energetic distribution of intrinsic intragap trap states in perovskite was quantitatively determined, and the radiative charge recombinations through the band edge and via trap states were studied. Furthermore, a series of thermodynamic parameters, such as the demarcation energy between radiative and nonradiative recombination regions, detrapping activation energy, and characteristic temperature, were extracted based on which of the possible radiative and nonradiative recombination mechanisms were proposed. In addition, the correlation between the morphology of the perovskite films, the PbI2 content, and the energetic distribution of the trap states was investigated. Finally, we discuss the structure-function relationship of perovskite films prepared by different methods.

Thermal decay analysis of fiber Bragg gratings at different temperature annealing rates using demarcation energy approximation

Abstract In this study the thermal degradation of gratings inscribed in three types of fiber namely, PS 1250/1500, SM 1500 and zero water peak single mode fiber is demonstrated. A comparative investigation is carried out on the aging characteristics of the gratings at three different temperature ramping rates of 3 °C/min, 6 °C/min and 9 °C/min. During the thermal annealing treatment, a significant enhancement in the grating reflectivity is observed for PS 1250/1500 fiber from ∼1.2 eV until 1.4 eV which indicates a thermal induced reversible effect. Higher temperature ramping rates lead to a higher regeneration temperature. In addition, the investigation also reflects that regardless of the temperature ramping rate the thermal decay behavior of a specific fiber can be successfully characterized when represented in a demarcation energy domain. Moreover, this technique can be accommodated when predicting the thermal decay characteristics of a specific fiber.

Fabrication of thermal enduring FBG sensor based on thermal induced reversible effect

In this study, the thermal degradation of a fiber Bragg grating written on hydrogen loaded photosensitive Germanium/Boron co-doped fiber with 193nm ArF excimer laser is investigated. A noteworthy increase in the grating reflectivity during annealing at 425°C is observed indicating a thermal induced reversible effect deviating from the general thermal decay behavior. Both stepwise and continuous annealing procedures indicate a similar trend in the accelerated aging characteristics when analyzed with respect to demarcation energy (Ed). In the characterization of the lifetime of the treated FBG, a thermal sustainability of thousand years with less than 2% decay resulting in a grating reflectivity of 98% is observed at 300°C for a grating as short as 10mm. The ease of characterization and obtaining detailed information through the analysis of transmission spectra is also considered as an advantage in this study. These thermal enduring FBG sensors based on thermal annealing over a temperature range of 200–400°C lay the foundation for a new direction of research in the investigation of durable temperature sensors.

Effects of water absorption on surface charge and dielectric breakdown of polyimide/Al2O3nanocomposite films

The incorporation of inorganic nano particles (TiO2, Al2O3 and SiO2 etc.) with polyimide (PI) greatly improves its dielectric property. The dielectric properties of PI films are largely influenced by the absorbed water and humidity. What is more, the water absorption will be affected greatly by nano particles. In this paper, to study the effects of absorbed water on dielectric properties of moistened PI/Al2O3 composite film, PI film specimens were prepared by incorporating 12 wt% nano Al2O3 particles with PI. The original specimens were also prepared for contrast. Then the specimens were immersed in high purity water for 12, 24, 48, 72, 96, 120, 144 and 168 h. The water absorption and breakdown strength under different immersion time were measured. Meanwhile, given that the existence of surface charge had a critical influence on the breakdown characteristics, the corona charging tests were conducted at room temperature under pulse voltage. The pulse frequency was 300, 600 and 900 Hz, respectively. The pulse amplitude was +4 kV. The charge dissipation was investigated based on the demarcation energy theory. Obtained results show that the moisture condition affects more on the nano Al2O3 composite films than the original ones. Compared to original PI films, the water absorption is larger and the breakdown strength falls faster for the nano ones. The decay process of surface charge and the change of detrapping charges in nanocomposite films are affected greatly by the absorbed water.

A Panorama of Electrical Conduction Models in Dielectrics, With Application to Spacecraft Charging

Conduction models in disordered materials are described, with a special focus on the transient behavior appearing on a broad timescale as a consequence of disorder. Multiple trapping models, hopping models, or random walks coupled with waiting-time distributions are commonly used to describe charge transport in semiconductors. Important concepts have been introduced in this field, such as demarcation energy, percolation, or transport energy. Dispersive transport appears as a consequence of the disorder, together with a memory effect, that may be described using various mathematical tools. The interest of this research field concerning the charging behavior of materials used in spacecraft applications, especially polymers, is underlined and a practical model of the insulator is suggested.

Reliable Lifetime Prediction for Passivated Fiber Bragg Gratings for Telecommunication Applications

This paper is dedicated to the lifetime prediction of Type I Fiber Bragg gratings (FBG) and to problems that happen when stabilization (also called passivation) conditions or the industrial conditioning procedure depart from ageing ones (e.g., presence of hydrogen during the passivation process). For the first time, a reliable procedure to certify the predicted lifetime based on a “restricted” master curve built on real components (i.e., passivated FBG) is presented. It is worth noting that both procedures (master curve built on non-passivated or on passivated components) are based on the same model (demarcation energy approximation and the existence of a master curve) fed with ageing data (reflectivity decay vs. time and temperature). If the Master Curve (MC) build on passivated components can be derived from the original one, we can certify the lifetime prediction in a reliable manner.

Annealing of UV Ar^+ and ArF excimer laser fabricated Bragg gratings: SMF-28e fiber

Fiber Bragg gratings fabricated in pristine SMF-28e fibers using pulsed ArF-excimer and cw 244-nm Ar+ laser were annealed using tempering rates from 0.0038 to 0.25 K/s. Demarcation energy mapping allowed for the determination of the frequency factors and the master curves for the SMF-28e fiber under different irradiation conditions. A Gaussian decomposition of the underlying energy distribution revealed several individual activation energy distributions characteristic for the fiber with peak energies and widths that were independent of the laser used. From a fit of the integrated Gaussian distributions to the master curves the relative contributions of the individual energy distributions that appeared in both irradiation conditions were calculated. The difference in the activation energy spectra obtained from the two laser irradiations is explained by the relative contributions of the individual distributions that differ. Using the analytical description of the master curve, thermal stability maps were obtained.

Charge carriers' trapping states in pentacene films studied by modulated photocurrent

The modulated photocurrent (MPC) technique is employed to study the charge carriers' trapping states of pentacene films. The characteristics of the experimental MPC spectra were found to be compatible with trapping-detrapping process of holes in gap states in which their occupancy can be modified by the bias illumination. A demarcation energy level separating empty from partially occupied traps was deduced from the MPC spectra, which can be used to monitor bias-light induced changes in the quasi Fermi level. An exponential trap distribution from structural disorder and a deep metastable gaussian trap distribution from adsorbed environmental impurities were extracted by means of the MPC spectroscopy. An attempt to escape frequency of the order of 1010s−1 was deduced for the gap sates. The derived trap distributions agree with those found before by means of other techniques. The present results indicate that the MPC technique can be used as a valuable tool for pentacene films characterization since it can be also applied to field effect samples.

Theory of modified thermally stimulated current and direct determination of trap level distribution

In order to investigate the trap level distribution in polymer films, a new method is proposed based on modified thermally stimulated current (TSC) theory and numerical calculation of the TSC measurement. In this method, a new function is defined to weight the contribution of every trap level to the external current. The demarcation energy is used to study the trap emptying process. The modified TSC theory shows that only the electrons with trap levels very close to the demarcation energy can significantly contribute to the external circuit at any instant temperature. Based on this method, the trap level distribution of the DuPont original polyimide film 100HN and nanocomposite polyimide film 100CR are investigated as an application example. The effectiveness of the method is confirmed by the experiments. The experimental results show that the trap level density in the 100CR PI films is about six times larger than that in the 100HN PI films through the investigated trap level ranges 06–1.3 eV. The increased traps in 100CR should be introduced by nanofillers, probably come from the interfaces formed between nanofillers and the polymer matrix.

Modified isothermal discharge current theory and its application in the determination of trap level distribution in polyimide films

Modified isothermal discharge current theory (MIDC) was thoroughly investigated in order to obtain the trap level distribution in polymers. In this theory, a new function was defined to weight the contribution of any trap level to the depolarization current at an instant time. The demarcation energy was used to study the trap empty process. Analysis showed that only electrons with trap levels very close to the demarcation energy significantly contribute to the external current. As an application example, this theory was applied to determine the trap level distribution in Kapton 100HN and 100CR (nanodielectrics) PI films before and after corona ageing. Experiments showed that traps in 100CR PI films were about twice as much as that in the 100HN PI films through all trap levels. It's also found that the quantity of traps was increased evidently after corona ageing and lots of deeper traps were produced in 100HN film, whereas for 100CR, changes were not very significant. Since both physical and chemical defects intrinsic or extrinsic can change the trap level distribution and make a significant impact on the dielectric behavior. So modified isothermal current theory may be a useful tool for determining the defect characteristic and investigating the electrical properties of polymer dielectrics.

Thermally stimulated currents in amorphous solids due to complex heating regimes

The paper contains a theoretical description of thermally stimulated currents (TSCs) in amorphous solids. Earlier approximate formulas for TSCs are extended to the cases of delayed and step sample heating. TSC features characteristic for these heating schemes can be attributed to the non-linear dependence of the demarcation energy level on temperature. In particular, the initial TSC rise has a thermally activated character, with the activation energy equal to the demarcation energy at the onset of sample heating. The accuracy of the obtained formulas is verified by numerical calculations of TSCs for exponential trap distribution. The predicted TSC features are shown to be consistent with those established experimentally for amorphous hydrogenated silicon.

Precise life-time prediction using demarcation energy approximation for distributedactivation energy reaction

The requirement from the industry of precise lifetime estimation of Bragg gratingsin optical fibres and other optical devices motivated us to perform calculationsin a region of a master curve (describing the time evolution of a system at anytemperature) where an approximation, called demarcation energy, is no longervalid. We describe a correction procedure that leads to reintroducing afterwards atemperature dependence lost in this approximation while preserving most of theadvantages of the demarcation energy approach. With our procedure, a precisedetermination of burning-in parameter for achieving lifetime specification is possible.A Bragg grating written in H2 loaded Ge doped silica core optical fibres is used as application example.

Thermally stimulated carrier transport due to stepwise sample heating

AbstractThe article concerns non‐isothermal dispersive carrier transport in an insulating solid with traps. The approximate solutions of transport equations derived previously are extended to the case of stepwise sample heating. The specific features of thermally stimulated currents (TSCs) can be attributed to non‐linear dependence of the demarcation level on temperature. In particular, the initial TSC rise has a thermally activated character where activation energy equals to the demarcation energy at the end of previous heating cycle. The accuracy of the formulae describing TSCs is verified by Monte Carlo calculations for Gaussian trap distribution.

Electric-field-dependent dispersive transport of carriers in band tails of low-dimensional systems

A theory for the dispersive transport of carriers in the presence of an electric field for the quasi-one-dimensional systems has been developed. It is assumed that localized states are distributed randomly in both space and energy coordinates and that hopping of the carriers occurs in both coordinates. The exponential form of the density of states for band tails is considered. Expressions for the time-dependent demarcation energy and mobility are calculated. The theory predicts that the mobility has three power-law decay branches with exponents nm 1, mm 1 and mm m1. Here nand mare directly proportional to the temperature and electric field respectively. The first power law is associated with the multiple-trapping mechanism and is well known. The other two power laws are new findings of the present theory. The time-of-flight experiments for hydrogenated amorphous SiN x Si/hydrogenated amorphous multilayers can be understood by the present theory on the qualitative basis.

The Intuitive Thermalisation Model Compared to Explicit Multiple Trapping Calculations

ABSTRACTA simple model (the HTMT model) describing the interaction of excess mobile charge carriers with an exponential bandtail is presented. The new model differs from the commonly used intuitive thermalization (TROK) model only by the definition of the demarcation energy. By comparison of the results of the TROK and the HTMT model with numerical calculation of the interaction it is shown that the HTMT model yields an adequate description, whereas the TROK model does not reproduce the absolute values and also has a deviating time dependence at higher temperatures. The origin of these differences is discussed.

Transient behaviour of carriers in band tails of quasi-one dimensional systems

We have studied the effect of an electric field on the transient behaviour of localized carriers in quasi-one dimensional (QOD) systems. We assume that the localized states have an exponential form of the density of states in band tails. The localized states are distributed randomly in both space and energy coordinates and the hopping of the carriers occurs in both coordinates. The critical hopping range, the time dependent hopping mobility and the demarcation energy are calculated in the activated and the non activated regimes. Our expressions for the mobility and the demarcation energy are reduced to the expressions given in the literature. Numerical calculations are performed for the demarcation energy and the mobility as a function of an electric field and time. It is found that the electric field has dominant contribution at low temperatures.

Dynamics of Carrier Hopping in Exponential Band Tails of Quasi‐One‐Dimensional Systems in Electric Field

AbstractThe dynamics of carrier hopping in band tails of a quasi‐one‐dimensional (QOD) system is studied in the presence of temperature and electic field. A method is used in which it is assumed that localized states are distributed randomly in both space and energy coordinates and the hopping of carriers occurs in both coordinates. The exponential form of the density of states for band tails is considered. The hopping rate and the time dependent demarcation energy are calculated in the activated and nonactivated relaxation regimes. The hopping rate varies with the electric field as (1  β2)−1/γ and exp [(1  β2)] for the AR and the NAR regions, respectively. Here β and γ are directly proportional to the electric field and the temperature, respectively. Numerical calculations are performed for the hopping rate and the demarcation energy as a function of β and γ for both regions.