Nowadays, due to the specific properties of lasers (especially from the high power point of view) and the need for their utilization in industry and medicine, the generation of high power and high efficiency laser has been of interested. As the generation of coherent beams from the linear crystal is not possible in every desired spectral region, the generation of laser beam in the spectral regions which are non-access, is possible though the use of nonlinear crystals. In the recent years, the optical parametric oscillator has been acknowledged as a method to generate new frequencies, especially in mid infrared region.
With respect to this fact that the tunability and stability of pulsed optical parametric oscillators are influenced under the various thermal effects, one has to be aware of thermal effects, which are happened in a nonlinear crystal under pumping with high powers. The nonlinear crystal of KTP with unique properties such as large acceptance angle, high damage threshold, high effective nonlinear coefficient, and small walk-off, it is a suitable nonlinear crystal for pumping with multimode lasers and running in mid infrared region. However, heat generation in the crystal is one of the limitations of achieving t high efficiency.
In this thesis, a theoretical model is presented to investigate the thermal effects in an optical parametric oscillator laser. To this end, we first solve the heat and then the thermally induced phase mismatching equations. In the second stage, coupled wave equations with and without considering optical absorption coefficients for pump, idler, and signal were solved. Finally, thermally induced phase mismatching for several temperatures were calculated and were inserted in the wave equations. With this, we could investigate the effects of heat in nonlinear interactions. Due to the mathematical complications, differential equations were solved through a numerical analysis of finite difference method. The equations were solved via codding in FORTRAN.
 

Abstract: Nowadays, the use of second harmonic of neodymium doped solid-state lasers such as Nd: YAG is one of the most useful methods to generate coherent green light. Second harmonic of 1064 nm emitting from Nd:YAG laser cab be generated by nonlinear crystals such as KTP, LBO, BBO, etc. Despite the relatively high efficiency of solid-state lasers compared to green gas lasers such as argon, thermal effects caused by light absorption in the solid-state crystals is the main problem. The heat causes effects such as thermal dispersion, terminal bulging, and thermal stressed. These effects ultimately reduce the efficiency and the quality of laser output beams. Effects such as thermal lensing, thermally induced phase mismatching, crystal fracture, and swelling occur in the high power solid-state lasers. In this thesis, the thermal effects happened in the KTP crystal pumped by 1064 nm near-infrared laser were investigated by analytical and approximated models. The efficiency of second harmonic generation was investigated under the thermal effects. To this end, at first the Fourier heat equation with considering the crystal heating by fundamental and second harmonic waves was solved analytically and then the temperature distribution throughout the crystal was calculated. The thermal induced phase mismatching was then calculated. Using a two-wave nonlinear interaction model, the nonlinear coupled wave equations were solved ideally, and the fundamental and second harmonic fields changes in along the length and the radius of crystal were calculated. The thermal induced phase mismatching term as a function of position was finally inserted in the the field equations and the new solutions are extracted. The results show that the efficiency of second harmonic generation under the thermal effects are significantly reduced compared to the ideal one. In particular, with increased the power, efficiency is reduced. This model can be used in experimental designs of nonlinear solid-state lasers to evaluate and mitigate the effects of heat.

Graphene, as the first two-dimensional material, has attracted wide range of researches due to its amazing applications. In this thesis, few layer graphene was synthesized on copper substrate by chemical vapor deposition (CVD) method using a mixture of methane, hydrogen and argon gases at the atmospheric pressure. After synthesis of the graphene, it was transferred to SiO₂/Si substrates using poly methyl methacrylate (PMMA). The graphene sheets were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), optical microscope and Raman spectroscopy methods. At the next stage, the I-V characterization of graphene sheets on the FTO substrate was carried out. The response of photodetector, rise time, fall time, and external quantum efficiency of the graphene at the xenon lamp exposure using blue, green, and red filters were measured. The results show that the rise and fall time of the graphene photodetector was about several tens of seconds. Finally, the graphene were exposed to 0.02% volume of CH₄, CO₂, H₂ and O₂ gases. The sensivity, response time and recovery time of the graphene at the gases exposure were measured. The response and recovery time of the graphene was about few hundred of seconds with a sensivity of 19%, 23%, 17% and 18% for CH₄, CO₂, H₂ and O₂ gases, respectively.c

 Fano effect is known as a result of a two-mode interference. In the quantum physics context, the coupling between two transitions from ground state, one towards a continuum (ionization) and another towards discrete state (autoionization) is called Fano effect. The pronounced characteristic of such a resonance is an asymmetric profile at the energy (frequency) spectrum which is a result of destructive and constructive interferences at the short frequency interval between two modes with a specific phase difference. The Fano effect appears in various fields of physics such as nuclear physics atomic physics, etc. In the recent decade, this effect has showed interesting properties in plasmonics (the science of metallic nanoparticle optics). The plasmonic Fano effect is a coupling between a single broadband bright mode and a narrowband dark mode with a phase difference leading to an asymmetric Fano extinction profile. One of the outstanding properties of plasmonic Fano effect is a strong enhancement in electromagnetic field at the frequency interval of the maximum and the minimum of asymmetric extinction profile. Dark modes are modes which are not brightened by incident light at all, and therefore they should be exposed to a strong near field of bright modes (dipole mode) or in mixed modes. As the frequency position and the field distribution of plasmonic modes depend totally on the geometry and the material of the nanostructures, one can control the plasmonic Fano effect with adjusting these two parameters. A satisfied Fano effect happens when it can enhance a noticeable field, makes a deep valley at the asymmetric profile, and possesses a small frequency interval between the maximum and the minimum of the extinction profile. In this thesis crescent-shaped metallic nanoparticles in triple (dolmen-shaped) asymmetric systems were used to show the plasmonic Fano effect in the optical (IR to UV) range. The problem has been handled through the solving of Maxwell’s electromagnetic wave equation with a numerical method of finite element and then calculating the extinction cross section for single-, mutual dual-, and triple-crescent systems. Due to the inherent asymmetric property of crescent particles, they can display Fano resonance in their extinction cross section profile. Dolmen-shaped systems can also show Fano effect through the brightening their own dark modes in the near field of others system elements. These triple systems can also show the Fano effect through interference of mixed and near field modes leading to multiple Fano resonances. Since the plasmonic modes excitation depend on the incident light polarization, on the contrary to common (rectangular) dolmen-shaped systems, these systems can display the multiple Fano resonances for two polarizations seperately. Also in this thesis, the effect of narrowing the crescents in generating the high-order modes leading to sharp profiles has been investigated, too. Eventually, as an applied example, the effect of Fano resonance on the solar cells has been inspected.

Argon ion laser with output strong wavelengths at 488 and 514 nm in green-blue area is considered one of the most important green ion gas lasers. The most important feature of this laser is the ability to produce a power about ten continuous wavelength in the visible region with few milli watts to tens of watts. In this thesis, a Argon ion laser beam with a rated power of 40 W was analyzed, repaired and rebuilt . All the pieces and parts include laser tube, vacuum systems and gas distribution, power supply, cooling system, trigger system and control panel were rebuilt and repaired. The power supply of laser that is current source and ability to produce 120 amps at 550 volts, with switching capacitors and thyristors and Repair of other circuits, power electronics, was launched. The laser tube which contains 74 hard anodized aluminum and corrosion due to the leak, repair and launched. Also the cooling system using a closed circuit was built deionized water. Gas supply system tube that is filled with argon gas at a pressure m bar, was repair and rebuilt. System optical cavity consists of two mirrors with high reflectivity (in back cavity) and concave mirror with transmittance (in front cavity) is set up and configured. While this system should be set up, to prevent gas leakage and away from the hot plasma. The triggering system that starting up the laser is in work, repair and reconstruction and well able to create a high-voltage pulses and to do initial discharge. finally The laser was switched on and It was detected output coherent green light. Then, a titanium sapphire laser cavity solid-state, argon ion laser is blown by the green output, and the redesign was made and it was confirmed stability. This can be in the range of 660 to 1180 nm laser output is tunable to produce research and spectroscopy particularly very practical.

In this thesis, the argon and krypton arc and flash lamps along with their manual switching driving circuit were design and constructed. At the first, for optimization of important flash lamp parameters such as threshold voltage, working voltage and current, light intensity, and emission spectrum, T-shaped lamps were constructed. After optimization, linear lamps were constructed. Essentially, each lamp device is comprised of a lamp tube and a deriving circuit. In this work, for tube construction, generally borosilicate glassy tube with wall thickness of 1.5 mm, bore of 5 mm, and various lengths were used. Also for electrodes, we used alloyed argon welding electrodes (which are found in market) such as tungsten-cesium oxide alloy with a diameter of 3 mm as cathodes and tungsten-thorium oxides with wt 2% with a diameter of 3 mm as anodes. To seal the tube with electrodes, we have introduced an alloy having a minimum thermal linear expansion difference with that of borosilicate glass to minimize the thermal stresses. Both electrodes which are comprised of two parts of emissive (the top part) and connecting (the back part) bars, have been attached to the glass tube through a 50%-50% Ni-Fe alloy which had been welded to the emissive part through argon welding. With the use of such alloy, we were successful to seal the tube with a strong welding and without leakage. To remove the residual stresses, the annealing treatment has been made in a thermal oven. We could design lamps with tungsten-cesium oxide as cathode and tungsten-thorium oxide wt 2% as anode with the minimum threshold and working voltages compared to other used electrodes. Also, we constructed a manual switching deriver circuit. This circuit is comprised of four parts of pulse maker, voltage booster, discharge and external triggering. The main discharge circuit was designed with a 700 DCV, 62 kHz frequency, 5.75 J discharge energy, and 375 kW power. This circuit was applied for an argon flash lamp with an arc length of 13 mm and gas pressure of 62.03 and 87.9 Torr, which was constructed with a borosilicate glassy tube with the wall thickness of 1.5 mm and bore of 5 mm with a satisfying performance.

Among the wide variety of gas lasers, the far-IR CO2 laser with a highest possible power and a strong absorption in organic and mineral materials has wide applications in industry and medicine. In this thesis, the report of the design and construction of a continuous wave carbon dioxide laser with the emission wavelength of 10.6 µm is presented. In this laser, various gases such as helium (as a CO2 depleting and a cooling gas) and nitrogen (as an assistance in excitation of CO2) with the various portions were used. The main laser tube was pumped longitudinally with high voltages at the pressures lower than that of atmosphere. A concave high reflective mirror with a radius of curvature of 5 m with the silicon coating as the back mirror, and a plane mirror with zinc selenide coating with some percentages of transmission as output coupler were used. Due to the dissociation of CO2 when lasing occurs which leads to a reduction in output power, a flowing gas system was used, such that fresh gas is injected into the tube. In this gas laser, new designs for cathode and anode were made, such that besides the capability of replacing the damaged elements, such as mirrors, electrodes, gas iterance, and high voltage connections, easily, the laser efficiency is also increased. Total laser elements were designed in our laboratory and then machined. For constructing the double-wall tube, two thick pyrex pipes for gas (inner pipe) and water (outer pipe) were used. A polyamide piece, an O-ring, and silicon past were used for coaxial connection of two pipes. The laser cathode and anode, with a new design, as four-piece element was constructed, such that the gas and water connecting element (from polyamide), gas entrance and high voltage connection, mirror mount, and movable electrodes (from 304 steel) by screw and O-ring were fixed. Three gas mole mixtures of 1:1:3, 1:12.5:6.5, and 1:1:19, for carbon dioxide, nitrogen, and helium for optimization of laser power were used, respectively. Eventually, a gas mixture of 1:3:3 and pressure of 200 Torr at the gas tube and a voltage of 27 kV as optimized state were selected.

In this thesis, the single- to four-layered organic light emitting diodes (OLEDs) with an active layer of MEH-PPV were fabricated and their threshold voltages were decreased as possible. To fabricate the diodes, the spin coating for polymers and thermal evaporation in vacuum for small molecules and aluminum were used. At first, a single-layered diode of ITO/MEH:PPV/Al(165 nm) was fabricated. Then, to improve the charge carrier injection and to investigate the influence of the hole transport and buffer layers on the threshold voltage, a hole injection layer of PEDOT:PSS and an electron transport layer of Alq3 were added. Finally, the buffer and injection layer of LiF was added. To optimize the diode performance, various thicknesses were examined. The reduction of operation voltage, enhancement of efficiency and the color changing were the final aim of this thesis. For each diode structure, four different thicknesses for MEH-PPV were investigated. The selected round per minute (rpm) for this layer were 3000, 4000, 5000 and 6000 rpm. For single-layered diodes, the minimum lightening voltage was 12.98 V and belongs to the ITO/MEH-PPV(6000 rpm)/Al(165 nm). For two-layered diodes, the minimum lightening voltage was 7.1 V which belongs to ITO/PEDOT:PSS(2000 rpm)/MEH-PPV(6000 rpm)/Al(165 nm). For three-layered diodes, the minimum lightening voltage was 7.09 and belongs to ITO/PEDOT:PSS(2000 rpm)/MEH-PPV(4000 rpm)/Alq3(50 nm)Al(165 nm). The presence of Alq3 increased the current noticeably. For four-layered diode of ITO/PEDOT:PSS(2000 rpm)/MEH-PPV(4000 rpm)/Alq3(50 nm)/LiF(5 nm)/Al(165 nm) the lightening voltage reduced to 3.97 V. Also, we fabricated a two-layered OLED of ITO/TPD/Alq3/Al which is a green light emitting diode with various thicknesses. For ITO/TPD(61 nm)/Alq3(40 nm)/Al(180 nm), the minimum lightening voltage was 6.1 V. In this thesis, also the optical and surface properties of diodes with methods such as absorption (UV-Visible) spectroscopy, emittion spectroscopy, CIE, photoluminescence, AFM, and DEKTAKT were investigated

The vast majority of emitted light by quantum dots, which is determined by the size and the materials constituting them, is due to the spontaneous emission. Nowadays, due to the wide utilization of quantum dots in various fields, such as single-photon sources, photo-detectors, lasers, light emitting diodes, solar cells, and quantum information and communications systems, controlling the spontaneous emission which can enhance the systems’ efficiencies, has been of critical importance. With respect to the formulation of cavity quantum electrodynamics, the rate of spontaneous emission has been found to be directly related with the density of localized photon states. An optical cavity provides an environment at which density of localized photon states can intensively be increased. This issue leads to an enhancement in spontaneous emission rate of the emitters. High quality factor and low effective mode volume are of crucial factors to enhance the spontaneous emission rate. In this thesis, aimed to controlling the spontaneous emission of InGaN/GaN quantum dots, three types of cavities are introduced and investigated: (i) two-dimensional and slab photonic crystal micro-cavities which are created by a defect at the center of them; (ii) plasmonic bowtie nano-antennas which are formed by opposing high aspect ratio metallic nano-metals; (iii) mixed plasmonic cavities which are form by adjusting the plasmonic nano-antennas in the center of a dielectric photonic crystal. With respect to the emitting spectrum of InGaN/GaN quantum dots, which are lied in the green band, the cavity parameters are optimized such that the cavity wavelength is pulled in this region. Through the solving electromagnetic wave equations in various cavities, quality factor and effective mode volume of cavities were calculated. Calculations were carried out by finite element method in three-dimensional space coordinates. High performance computing machines with processors and GBytes RAM were used to accomplish this work.

Combination of plasmonic and photovoltaic is one of the novel approaches to enhance the solar cells’
efficiencies. Utilization of metallic nano-structures in third generation solar cells, along with
excitation of various plasmonic modes such as localized surface plasmons and surface plosmon
polaritons, provides the possibility of light trapping absorption layers and thus leads to noticeable
enhancements in these third generation solar cells. In this thesis, the influence of metallic nano-strips
of various metals such as Ag, Al, Cu, and Au on the top of the ultra-thin solar cells has been
investigated. To this end, numerical method of finite element and the Poynting theory are used to,
respectively, calculate the electromagnetic field within the solar cells and Si thin film optical
absorption. In this thesis, for Au, Al, Cu and Ag nano-strips the optical absorption enhancement,
electron-hole generation rate enhancement and short-circuit current density as a function of strips
cross section and grating periodic were calculated. Three phenomena of localized surface plasmons,
surface plasmon polaritons and effective scattering for TM polarized mode and waveguide modes for
TE polarized mode were studied. In this thesis, for the first time, the effect of nano-strips cross section
on the absorption enhancement, electron-hole generation rate and short-circuit-current density have
been investigated. The result showed that although the semiconductor-on-insulator (SOI) solar cells,
because of their industrialization, has become feasible and are currently used in photovoltaic designs,
however metal-dielectric-metal (MIM) solar cells results in higher short-circuit current density than
SOI in similar conditions. In TM mode, the SOI structures are only capable to exit the waveguide and
localized surface plasmonic modes around the nano-strips, while MIM structures, in addition to these
two phenomena, are capable of exciting surface plasmon polaritons modes, too. Eventually, by a
comparison between two SOI and MIM structures and various nano-strips with various cross sections
and materials, the best structure for absorption of un-polarized light (such as sun-light), the Ag nanostrips in MIM structure was introduced

In this thesis, the effect of size on the laser properties of cone-shaped quantum dots (QDs) coupled to wetting layer has been investigated. Using numerical method of finite element and single-band Schrodinger equation in effective mass approximation, the energy eigenvalues and wave functions of ground state, first excited state, and second excited state of two groups of QDs have been solved. Then, using electron wave functions, the electric dipole moments and transition lifetimes have been calculated as a function of QD size. The effects of wetting layer thickness on the wave functions and energy eigenvalues have also been investigated in detail. Until now, we have ignored the effects of Coulomb potential due to the electron concentration in the dot. In order to reach to more accurate solutions, the Schrodinger and Poisson equations have been coupled and solved via an iteration method. In continue, the effect of mole fraction of x, in InxGa1-xAs/GaAs compound has been investigated. The results show with increasing the QD size, the energy eigenvalues decrease and go towards the bulk system. For small sizes, the wave functions are mainly localized in the wetting layer, relaxing then towards the dot regions by increasing the dot size further. Eventually, we investigated the laser properties of QDs through solving the laser rate equation for a three-level system. The results showed that for small sizes, the laser output decreases and reaches to its minimum at the cone radius of 9 nm. It then increases with increasing the radius. The reason of such behavior is attributed to the entrance of wave functions contributing in lasing process, from wetting layer towards the dot region