Iranian Journal of Applied Physics
Year:2020 | Volume:10 | Issue:2 (21)|Papers Count:6

The optical coefficients and thickness of thin films are essential parameters in many branches of science and technology. Ellipsometry is a well-known and powerful technique for calculating the essential optical parameters of thin films, due to its accuracy and non-destructive capabilities. In this paper, a single wavelength (SW) ellipsometer which is relatively simple and inexpensive compared to other conventional ellipsometers have been designed. The light source used in this design was a He-Ne laser with wavelength of 633 nm. Rotating analyzer ellipsometry (RAE) technique at different incidence angles was used to measure the ellipsometry parameters (𝜓 , Δ ). The efficiency and accuracy of this method were examined by calculating the optical coefficients and thickness of TiO2 thin film (with known optical parameters). In order to increase accuracy of measurements, the ellipsometry parameters (𝜓 , Δ ) were acquired from different regions of the film surface. Refractive index obtained for the substrate was 1. 48 and refractive index, extinction coefficient and film thickness obtained for TiO2 thin film, was 2. 58, 0. 04 and 49. 60 nm, respectively. These results, extracted from the ellipsometry data curve fitting and employing the numerical inverse method, showed the efficiency and reliability of designed configuration.

When an intense laser beam propagates in plasma, due to relativistic electrons resulting from the laser electric field, the plasma refractive index alters. Subsequently, the plasma behaves initially similar to a positive lens that decreases the laser spot size. In this article, the propagation of the laser beam in collisional quantum plasmas is investigated with considering a ramped density profile. Using WKB and paraxial approximations through parabolic equation, a mathematical formulation for the beam width parameter is obtained from the wave equation. Acquired equations are numerically solved by employing the fourth-order Runge-Kutta method. In the collisionless plasma, by increasing the ramp slope, the beam width focuses with less oscillation amplitude, smaller laser spot size and more oscillations. In the collisional quantum plasma, due to energy absorption, the oscillation amplitude enhances by passing through the plasma and the laser beam defocuses at a few lengths. For the greater values of the slope and the collision frequency, the laser spot size oscillates with the higher amplitude and defocuses in a shallower plasma depth. Also, greater plasma density results in smaller laser spot size and bigger oscillation frequency. Then, this effect is compared in the strong and weak quantum and classical plasmas. Decreasing the plasma temperature improves the magnetization due to the ponderomotive force related to the time variation, the beam-width oscillates with greater frequency and deeper penetration in the strong quantum plasma in comparison with the weak quantum or the classical cases.

Controlling the orientational ordering of liquid crystal molecules by confined surfaces is one of the main reasons for the widespread use of liquid crystals in the industry. The anchoring condition is an essential feature of the liquid crystalline materials in vicinity of boundaries. As known, surface interaction energy in a confined liquid crystal can be studied in the framework of Rapini-Papoular model. However, for the uneven surfaces, other models, such as a model based on Fukuda theory should be used. The surface-like energy is not usually considered in energy calculations. In this work, we consider a nematic slab confined with two infinite surfaces and investigate the director field and anchoring effects in the total energy of the slab with a sinusoidal surface by taking into account this term of energy. We consider a surface with a grooved sinusoidal form and calculate the anchoring energy within the framework of Fukuda theory. We also obtain the dependence of energy on geometric parameters, such as grooves amplitude. By utilizing theoretical model proposed by Fukuda, we determine the director components and anchoring energy of the system under an external electric field. The variation of the director components and surface effects are discussed. It is shown that the anchoring energy has a maximum value in the absence of the field. Moreover, we find that the trend of anchoring energy variations is reduced due to the presence of an electric field.

In this paper, we have calculated the InAs structural parameters such as lattice constant, bulk modulus and its derivative, stability and density state in zincblende and wurtzite phases. The calculations have been performed using pseudopotential method in the framework of density functional theory by Espresso package. The calculated structural parameters are in good agreement with experimental results. The results showed that zincblende phase is more stable than wurtzite phase and InAs in zincblende and wurtzite phases is a narrow gap semiconductor. Also, the results of the contribution of different orbitals show that in the conduction band, there is a strong overlap between p orbitals of In and As atoms and the orbital s of In, while at the top of the valence band, the overlap of p orbitals of In and As atoms is predominant.

This paper is a review on Terahertz generation, based on a method called “ Two-Color Laser Pulses” , in which terahertz pulse is generated by focusing a pulse and its second harmonic into gas target at the same time, in the output. Terahertz generation set up, physical processes involved in terahertz generation in this method and the most acceptable reason for the amplification of terahertz pulse in this process is discussed. Also, the role of phase difference between the laser pulse and its second harmonic for generated pulse is reviewed. At last, generated THz amplitude based on different phases between the two amplitudes is surveyed. Nonlinear crystal is used in this generation method and the method of generation is a nonlinear process. Therefore, the role of nonlinear crystal in the process of Terahertz pulse generation, its position in optical set-up and the advantage of using it, are pointed out.

In this research, level structure of two-electron double quantum dots in terms of energy levels crossings a five-level system is modeled. This work is done for explaining reasons of happening multiphoton resonances and their different behavior for the odd and evenmultiples of photon energynear interdot charge transitions that shows the current detuning dependence on this asymmetry. First, interference phase and transition rates up to fourth order for zero and arbitrary detunings were calculated making use of non-adiabatic and strong-field approximations. Second, by numerical simulation and its analysis, the steady-state current is calculated in spin-blockaded situation that it perfectly agrees with the measured current in experiments done on InAs and GaAs double quantum dots. At last, it was shown that the obtained results in this model contain all main features of the experimental data and this behavioral difference of resonances for the frequency integer multiples is specific to multilevel systems in the presence of the large-amplitude applied fields. This study is closely related to the nano and micro scales, solid state and atomic or molecular systems.