Journal of Optoelectronical Nanostructures
Volume:8 Issue: 2, Spring 2023

In this paper, a tunable absorber structure based on a graphene hexagonal array in the terahertz range is investigated. The graphene hexagonal absorber is simulated by the finite element method. The effects of the geometry, graphene Fermi energy level and incident light angle, and light polarization on the absorptance of the structure are investigated. The results show that the absorptance spectrum of the proposed absorber is tuned from 6.1 THz to 9.1 THz when the Fermi energy increases from 0.4eV to 0.9eV. The absorptance peak shifts to lower and higher frequencies with increasing hexagonal side length and Fermi energy level, respectively. The absorption of the structure is over 90% in the incident light angle range from 0 to 80º for the TE polarization and in the range of 0-40º for the TM polarization. Also, results indicate that the absorption peaks shift to the lower energies with increasing the dielectric constant of the dielectric layer.

The electronic and transport properties of armchair α-graphyne nanoribbons (α-AGyNRs) are studied using density functional theory with non-equilibrium Green function formalism. The α-AGyNRs are considered with widths N = 6, 7 and 8 to represent three distinct families behavior  in presence of twisting. The band structure, current-voltage characteristic, transmission spectra, molecular energy spectrum, molecular projected self-consistent Hamiltonian (MPSH), and transmission pathways are studied for α-AGyNRs with θ= 0º, 30º, 60º and 90º. The results indicate that 6 and 7 α-AGyNRs devices are semiconductor, while 8 α-AGyNR device has metallic character. Moreover, these behaviors are preserved by applying the twist. Our theoretical study shows that the electronic  conduction of α-AGyNRs can be tuned by twisted deformation. The maximum modulation of conductance at 1.2 V is obtained 69.94% for 7 α-AGyNR device from θ=0º to θ=90º. The investigation of MPSH demonstrates that distribution of charge density get localized  on twisting sites which impact on the electron tunneling across the scattering region.

We purpose a hybrid energy harvester made of silicon solar cell, piezoelectric and thermoelectric. Our simulations are carried out using the COMSOL software. For this purpose, MEMS, heat transfer and electromagnetic modules were used. We connected nine piezoelectric, one thermoelectric and one solar cell modules in series to maximize the harvested energy and provide the appropriate voltage level. It is observed that the maximum electric current and voltage is about 200mA and 5V, respectively, which is equivalent to approximately 1W. The total obtained energy was amplified by two DC/DC converters and the voltage level increased to 5V.  Also, we theoretically proved that the use of an optical window (as top and bottom contact layers) based on photonic multilayer can control surface reflection. It is found that if we use two contact layers in the front and back of the solar cell, the transmittance increases from 33% (without contact layer) to 67% (with double contact layer).

NiO columnar nanostructure was prepared using the thermal evaporation technique with oblique angle deposition (OAD). The morphological, structural, and optical properties change with the creation of the substrate inclination. NiO columnar nanostructure was analyzed using X-ray diffraction (XRD) analysis and field emission scanning electron microscope (FESEM). The strain values ε obtained exhibit that the strain becomes tensile (ε>0) for (2 0 0) and (2 2 2) planes. Conversely, the strain returns to a compressive state (ε<0) for (1 1 1) and (2 2 0) planes. The average tensile strain in NiO columnar nanostructure is obtained at 1.4% while the average compressive strain is obtained at 2.04%. The value of the optical bandgap of the NiO columnar nanostructure is obtained at about 4.06 eV. The refractive index showed two absorption bands around the wavelengths of 520 nm and 700 nm with values of 2.19 and 2.22, respectively. Then, the refractive index increased from 2.22 at 700 nm to 2.35 at 920 nm and remained almost constant over 920 nm.

In this paper, we performed an experimental study of a LED-pumped Nd:YAG laser that works in QCW and Q-switch modes. We examined how the pump pulse duration affects the laser output. The laser rod was a Nd:YAG crystal with a diameter of 7 mm and a length of 95 mm, side-pumped by 30 LED arrays, each with 18 single dies at 810 nm. The maximum output energy at 1064 nm was 10.5 mJ in the QCW mode, with a pump energy of 81 mJ (230 µs pulses at 1 Hz). The optical conversion efficiency and the slope efficiency were 12.5% and 18%, respectively. In the PQS mode, the output energy was 250 µJ, with a pulse width of 190 ns (FWHM), corresponding to a peak power of 1.31 kW. The beam divergence was 0.3 mrad with TEM00 mode. This LED-pumped Q-switched Nd:YAG laser can be used for laser range finder applications.

This study employs a numerical model to analyze the non-radiative Auger current in c-plane InGaN/GaN multiple-quantum-well laser diodes (MQWLD) under hydrostatic pressure and temperature. Finite difference methods (FDMs) were used to acquire energy eigenvalues and their corresponding eigenfunctions of InGaN/GaN MQWLD. In addition, the hole eigenstates were calculated via a 6*6 k.p method under applied hydrostatic pressure and temperature. The calculations demonstrated that the hole-hole-electron (CHHS) and electron-electron-hole (CCCH) Auger coefficients had the largest contribution to the total Auger current (76% and 20%, respectively). Increasing the hydrostatic pressure could increase the amount of the carrier density and the electric field. On the other hand, this increase reduced the overlap integral of wave functions and the localized length of electrons, heavy, light and split of band holes. Also, for the hydrostatic pressure of about 10 GPa and the temperature ‎‎of 300 K, the non-radiative Auger current has an optimum value of 334 A/cm2. ‎The results reveal that the elevated hydrostatic pressure and temperature play a positive and negative role in the performance of laser diodes.