مجله فیزیک کاربردی
سال یازدهم شماره 1 (پیاپی 24، بهار 1400)

In the process of crystal growth by Czochralski technique, lower part and core of the crystal are warmer than other parts of crystal and its environment, which leads to expansion in different parts of the crystal. The result of this thermal gradient is strain, which eventually causes thermo-elastic stress in the crystal. Increasing this stress leads to transition of the material from elastic limit and entering plastic area. To show thermo-elastic stress in crystals, a criterion called Von Misses stress is used. Using the solid mechanics approach, the mechanical response of crystal to the stresses can be determined through appropriate structural equations. In this paper, using appropriate structural equations, a set of numerical simulations of temperature field, thermal stress and dislocation density for a Czochralski setup used to grow Ge single crystal have been done for different heights of crystal. In order to investigate dislocation density, using a simple first-order approximation, in which the dislocation density is proportional to radial gradient of temperature is used. A two-dimensional steady state finite element method has been applied for all calculations. The numerical results reveal that the thermal field and thermal stress are mainly dependent on the crystal height, heat radiation and gas flow in the growth system. As the height of the crystal increases and the shape of the crystal-melt interface changes, we see an increase in thermo-elastic stress and dislocation density.

All-optical comparators can play crucial roles in the next generation of optical integrated circuits. In this paper, a novel structure was proposed for designing and implementing an all-optical comparator. Two nonlinear ring resonators were used for designing the proposed structure. These ring resonators can drop optical waves with central wavelength and optical intensities equal to 1550 nm and 0.1 W/m2, respectively. The simulation results show that the proposed structure can perform optical cooperation. At the proposed structure when X is ON and Y is OFF (i.e. X > Y) O1 will be ON and O2 will be OFF. When X is OFF and Y is ON (i.e. X < Y) O1 will be OFF and O2 will be ON. Finally, when X and Y are in similar states (i.e. X=Y) both output ports will be OFF. By Combining these output states one can implement optical comparison. The maximum rise time is about 1/5 ps.

Intensity Modulated Radiation Therapy (IMRT) is one of the modern technologies of external radiotherapy, which underwent widespread clinical adoption in medical centers. Modern radiotherapy makes use of new technologies in design, treatment, and delivery systems. However, despite the advantages of IMRT compared to previous methods, radiotherapy errors are still an obstacle to achieving the desired dose distribution. In this paper, effective errors of the IMRT technique, together with the sensitivity of different quality assurance (QA) procedures in diagnosis are investigated and classified. According to these studies, in addition to the importance of human errors in delivery and patient positioning, beam correction device errors are other most effective sources of delivery errors, responsible for 35% to 50% of radiotherapy uncertainties. Thus, IMRT QA methods such as diode detectors, films, electronic portal images, log files, and artificial intelligence methods have been used extensively to investigate the MLC leaf positioning errors. Moreover, uncertainties of treatment couch design and MLC modeling in TPS should not be underestimated, since numerous studies have demonstrated that various couch tops include non-negligible beam attenuation, ranging from 4% to 9% for a gantry angle of 0. Whereas posterior oblique beams are often used in the IMRT process. This article aims to highlight the importance of recognition and correction of radiotherapy uncertainties and reduce possible accidents during an IMRT process by precisely knowing various IMRT QA procedures.

The entanglement of the helium and helium-like atoms have been studied. The calculations are performed considering the radial wave function as a linear combination of exponential functions and making use of Von Neumann entropy, Linear entropy and reduced density matrix. The coefficients and powers of the wave function were calculated using variational method and the energy and entanglement are obtained considering the optimized wave function. The results show that the entanglement tends to increase with increasing energy. The results also show that with increasing atomic number (in helium-like atoms), the electron-electron interaction becomes weaker and as a result the entanglement decreases.

In this study, we consider a photonic crystal slab with a triangular lattice, which consists of air holes with a circular shape in a tellurium background on top of a Teflon substrate. In this structure, by enlarging the size of an air hole and infiltrating it with nonlinear polystyrene material we introduce a nonlinear cavity in the mentioned structures. Then, we optimize the geometrical parameters, using the finite-difference time-domain method, to obtain optimal parameters. The results reveal that the triangular lattice represents a nonlinear cavity with a large quality factor (). The mentioned value is much greater than the reported values in similarly designed structures. Then, the designed high-quality cavity is placed between two waveguides symmetrically, and thus a coupled cavity-waveguide structure is created. These waveguides are used to couple light in and out of the cavity. Our investigation shows that by changing the structural parameters such as distance between the cavity and waveguides, the strong coupling between the cavity and waveguides is obtained. In the end, the optical bistability diagram of the structure corresponding to optimum parameters is presented. It is observed that the threshold power is significantly low in the designed structure. In the optical switching phenomenon, the threshold intensity and the response time of the nonlinear materials are very important. The response time of polymers is significantly shorter than that of semiconductors and due to the use of polymers instead of semiconductors in the current study, the obtained results represent some advantages compared with the previously published results.

Regarding the rapid development of optical communication, finding and preparing new materials with very fast and large optical responses are essential for making optical switches and processing equipment. Recently the use of materials smaller than micrometers and nanometers has been studied by researchers for a wide range of applications. The most important of these materials are nanofibers. Also, titanium dioxide is used as one of the most important materials in the field of optical communication. In this research, titanium dioxide nanoparticles with anatase phase were prepared using the sol-gel method and transformed into nanofibers using the electrospinning technique. To improve the performance of these nanofibers, they were also doped with silver nanoparticles. Raman spectroscopy was performed to ensure the purity of the particles of this material. It was found that the samples are all pure titanium dioxide (TiO2) powder in nanoscale and there are no impurities in their structure. The particle distribution of these nanomaterials showed that their size is in the range of 10-20 nm. Conditions for nanofiber electrospinning are: 10 cm distance between the needle and the collector, the feed rate of 2 mL/h, and the maximum voltage used was 15.5 kV. Also, silver colloidal nanoparticles were prepared by Li and Masil experimental method. The structure of fibers was studied in terms of material type using Raman spectroscopy and morphology with an optical microscope and fiber conductivity by FT-IR spectroscopy by determining optical coefficients. Optical microscopy showed that the surface of the nanofibers was smooth and flat and without beads. The diameter distribution of nanofibers was also very narrow (155±5 nm). FT-IR spectroscopy showed that on the surface of nanofibers, there were hydroxyl functional groups that were formed during the preparation of nanofibers. Finally, Raman spectroscopy showed the presence of impurities of silver nanoparticles (15±2 nm).