Fluctuations in a diffusive medium with gain

We present a stochastic model for amplifying, diffusive media like, for instance, random lasers. Starting from a simple random-walk model, we derive a stochastic partial differential equation for the energy field with contains a multiplicative random-advection term yielding intermittency and power-law distributions of the field itself. Dimensional analysis indicate that such features are more likely to be observed for small enough samples and in lower spatial dimensions

ELECTRO-ACOUSTIC ANALOGIES BETWEEN THERMOELASTIC COMPONENT OF THE PHOTOACOUSTIC SIGNAL AND LOW-PASS RC FILTER

This paper presents a new approach to the thermal characterization of aluminum, based on the electro-acoustic analogy between the thermoelastic component of the photoacoustic signal and the passive RC low-pass filter. The analogies were used to calculate the characteristic thermoelastic cut-off frequencies of the photoacoustic component and obtain their relationship with the thickness of the aluminum samples. Detailed numerical analysis showed that the required relationship is linear in the log-log scale and can serve as a reference curve for the given material. The results of the numerical analysis were also confirmed experimentally

A study of random laser modes in disordered photonic crystals

We studied lasing modes in a disordered photonic crystal. The scaling of the lasing threshold with the system size depends on the strength of disorder. For sufficiently large size, the minimum of the lasing threshold occurs at some finite value of disorder strength. The highest random cavity quality factor was comparable to that of an intentionally introduced single defect. At the minimum, the lasing threshold showed a super-exponential decrease with the size of the system. We explain it through a migration of the lasing mode frequencies toward the photonic bandgap center, where the localization length takes the minimum value. Random lasers with exponentially low thresholds are predicted.Comment: 4 pages, 4 figure

EFFECT OF THE BIMODAL STRUCTURE PROCESSED BY ECAP AND SUBSEQUENT ROLLING ON STATIC STRENGTH AND SUPERPLASTICITY OF Al-Mg-Sc-Zr ALLOY

Microstructure and mechanical properties of the 1570C aluminum alloy were studied after equal channel angular pressing (ECAP) to the strain of 3 at 325°C and subsequent warm and cold rolling with near 80% reductions at 325°C and 20°C, respectively. Even containing a partially recrystallized bimodal structure with a volume fraction of ultrafine grains of 0.3 and their size not exceeding 2 mm, the alloy after ECAP demonstrated an excellent balance of room temperature static strength parameters (yield strength (YS) » 300 MPa, tensile strength (UTS) » 400 MPa and elongation (El) » 26%), and high strain rate superplasticity (with maximum elongation exceeding 2500% at 520°C and a strain rate of 1.4 × 10-2 s- 1). Subsequent warm and cold rolling resulted in an increase in YS to 340 and 430 MPa and UTS to 415 and 485 MPa amid El decreased to 24 and 11%, respectively. Despite the difference in the deformation structures formed in both rolling states, similar superplastic behavior was observed with maximum elongations of up to 3000% at temperatures of 500-520°C and strain rates of about 10-2 s-1. It was concluded that the initial processing of the alloy to relatively low ECAP strains before warm/cold rolling, leading to bimodal structure with a low fraction of ultrafine grains, is sufficient to ensure a favorable combination of both service and technological properties of the sheets obtained

Еvaluation of Thermophysical Properties of Semiconductors by Photoacoustic Phase Neural Network

The idea of this paper is to develop a method for determination thermal diffusivity, linear expansion coefficient and thickness of a semiconductor sample from photoacoustic phase measurement by using neural network. The neural network has been trained on photoacoustic phases obtained from a theoretical model of measured signal for Si n-type in the range of 20Hz to 20kHz. For the analysis of parameter determination from phases, we trained phase neural networks on a large database obtained from numerical experiments in the expected range of parameter changes. An analysis of a theoretical photoacoustic model with a phase neural network is demonstrated. The advantages of using a phase neural network with high accuracy and precision in prediction depending on the number of epochs are presented, as well as analyzes of the application of random Gaussian noise to the network in order to better predict the experimental photoacoustic signal.BPU11 : 11th International Conference of the Balkan Physical Union : Proceedings book; Aug 11 - Sep 1, 2022S07-OP Optics and Photonic

Photoacoustic characterization of TiO2 thin-films deposited on Silicon substrate using neural networks

In this paper, the possibility of determining the thermal, elastic and geometric characteristics of a thin TiO2 film deposited on a silicon substrate, thickness 30 mikrons, in the frequency range of 20 to 20 kHz with neural networks was analyzed. For this purpose, the substrate parameters remained the known and constant in the two-layer model and nano layer thin-film parameters were changed: thickness, expansion and thermal diffusivity. Prediction of these three parameters was analyzed separately with three neural networks and all of these together by fourth neural network. It was shown that neural network, which analyzed all three parameters at the same time, achieved the highest accuracy, so the use of networks that provide predictions for only one parameter is less reliable.Comment: 21 pages, 5 figure

Effects of Spatially Nonuniform Gain on Lasing Modes in Weakly Scattering Random Systems

A study on the effects of optical gain nonuniformly distributed in one-dimensional random systems is presented. It is demonstrated numerically that even without gain saturation and mode competition, the spatial nonuniformity of gain can cause dramatic and complicated changes to lasing modes. Lasing modes are decomposed in terms of the quasi modes of the passive system to monitor the changes. As the gain distribution changes gradually from uniform to nonuniform, the amount of mode mixing increases. Furthermore, we investigate new lasing modes created by nonuniform gain distributions. We find that new lasing modes may disappear together with existing lasing modes, thereby causing fluctuations in the local density of lasing states.Comment: 26 pages, 10 figures (quality reduced for arXiv

Influence of thin oxide layer to photoacoustic signal of nano-mechanical structures

Nano mechanical structures (NEMS) are being developed for application in pressure and temperature sensors. They consist of a very thin oxide layer on a silicon wafer. This layer is placed to improve their sensitivity to pressure changes. In this paper, it was investigated how much this layer affects the physical properties of such structures. In the research, the photoacoustic gas microphone experimental measurement method was applied and neural networks were developed for solving the inverse photoacoustic problem. The obtained results show that controlling the thickness of the oxide layer can be done by engineering the thermal elastic and optical properties of the NEMS.IX International School and Conference on Photonics : PHOTONICA2023 : book of abstracts; August 28 - September 1, 2023; Belgrad

THE SURFACE RECOMBINATION VELOCITY AND BULK LIFETIME INFLUENCES ON PHOTOGENERATED EXCESS CARRIER DENSITY AND TEMPERATURE DISTRIBUTIONS IN N-TYPE SILICON

The temperature distributions in the n-type silicon circular plate, excited by a frequency-modulated light source from one side, are investigated theoretically in the frequency domain. The influence of the photogenerated excess carrier density on the temperature distributions is considered with respect to the sample thickness, surface quality and carrier lifetime. The presence of the thermalization and non-radiative recombination processes are taken into account. The existence of the fast and slow heat sources in the sample is recognized. It is shown that the temperature distribution on sample surfaces is a sensitive function of an excess carrier density under a bulk and surface recombination. The most favorable values of surface velocities ratio and bulk lifetime are established, assigned for a simpler and more effective analysis of the carrier influence in semiconductors. The photothermal and photoacoustic transmission detection configuration is proposed as a most suitable experimental scheme for the investigation of the excess carrier influence on the silicon surface temperatures

THE EFFECTS OF DIFFERENT COLLABORATION TYPES AND LEADERSHIP STYLES UPON GROUP PRODUCTIVITY

The paper studies how different collaboration types affect group productivity. The study provides an experimental investigation of the effects the combinations of collaboration types (by L. Umansky, i.e. individual, consequent and interactive) and leadership styles (by K. Lewin, i.e. autocratic and democratic) upon the group productivity. The experiment described simulates groupwork under different collaboration types and leadership styles. The six experimental stages allowed to register several objective and subjective productivity indicators. The analysis revealed the regularities between different combinations of the collaboration types and the leadership styles on one hand, and the productivity indicators on the other