Abdalla M. Darwish

Investigations of charge transfer and photosensitivity in single- and double-doped LiNbO 3 single crystals: an optical-electron paramagnetic resonance study: I

Electron paramagnetic resonance (EPR) for specially doped LiNbO3 crystals was determined. Study of the charge transfer and the photosensitivity in these crystals determined the ions, which participate on the charge transfer processes. In doubly doped crystals with Fe:Cr and Fe:Ce the decay of the EPR signal was less than the decay time in LiNbO3:Fe:Mn. some inherited impurities in these crystals like Mn and Fe were detected. The effect of the inherited impurities on the decay and growth of the EPR signal was estimated by comparing the results with homegrown LiNbO3 crystals.

Lasing and intermode correlation of whispering gallery mode in dye-doped polystyrene microspheres

The advantage of organic materials like polystyrene is their plasticity so that they can be made into any shape. Polystyrene has a rather large refractive index of 1.58 and can be easily doped with the required laser dye and also can be made into spheres of the required size. Fabrication of microspheres was basically carried out by J. Vanderhoff and his assistants at Lehigh University. The host material in all of these samples is polystyrene. Microspheres doped with Bis-MSB, PPO and alpha-NPO dyes were studied with suitable lasers like UV, N2 and XeCl excimer. We have observed enhanced emission and lasing action at certain wavelengths which correspond to the whispering gallery modes of polystyrene spheres. Characterization of the lasing process in microspheres with respect to their sizes will be presented. The 2D crystalline structure on a periodic array thin films of polystyrene microspheres has been studied by both diffraction and microscopic techniques and will be presented as well.

Oscillations in coherent-beam-pumped mutual phase-conjugate emissions

Oscillations are observed in mutual pumped phase conjugate emission when two coherent laser beams are incident on the positive C face of a BaTiO3 crystal in a symmetric configuration. The angle between the laser beams is varied from 30 degrees to 120 degrees. Self-pumped emission and oscillation are also studied with one of the laser beams making angels between 15 degrees and 36 degrees with the crystal. Growth and decay of the mutual pumped phase conjugate signals when either of the laser beams is cut off are also studied. A plausible explanation of the observed features is given in terms of formation and competition between different types of gratings formed in the crystal by the laser beams.

Photoinduced charge transfer in BSO:Cr3+ homegrown single crystal at room temperature during grating formation under Ar+ laser illumination in EPR experiment

With a view to understand the microscopic origin of photoinduced charge transfer in the cubic BSO crystal, we have studied the EPR spectrum of BSO:Fe3+ and the effect of laser illumination on the EPR signal during photorefractive grating formation at liquid nitrogen and room temperatures. The BSO:Cr crystal was grown at Alabama A&M University using Czochralski technique and the EPR spectrum was recorded at room temperature. Three EPR signals were observed. The EPR signals are due to the main dopant Cr3+ and the weak presence of Fe3+ which is inherent in BSO crystal. The effect of Ar+ laser illumination revealed new facts about the behavior of Cr3+ under laser illumination which is opposite to the behavior of Fe3+ in BSO:Fe3+ under laser illumination. Forming a grid on the face of the crystal increases the light-induced charge transfer Cr2+/Cr4+ is equivalent to Cr3+. The use of laser illumination helped to shed more understanding on the assignment of the EPR signals as well as the dynamic behavior of the charge transfer processes.

Transient multiple diffraction rings induced by Ar+ laser from poly (methyl methacrylate)(PMMA) polymer doped with organic dyes

The transient multiple diffraction rings associated with self-phase modulation is observed from PMMA doped with different dyes. The phenomenon is shown to be the result of laser-induced refractive-index change. The experiment is done on three different films fo PMMA doped with sulfrodamine, DCM, and Accriden yellow. The films were fabricated using the spin coating technique. The Ar+ laser at 514 nm was used as the exciting beam to the thin film. The transmitted laser formed interference fringes on a projection screen. It is observed that by increasing the thickness of the film, the number of rings increased. The relation between the thickness of the thin film and the number of rings was found to be different for the three films. This could be explained as the induced thermal refractive index coefficient in each of the samples depending on the strength of absorption of the 541 nm line by each material. It is also found that the third order nonlinear susceptibility enhanced with increasing the thickness of PMMA doped with Accriden yellow. The results also indicate the sensitivity of the transport phenomena in PMMA by changing the guest/host combination and the polymer doped with dye could be optimized for certain applications.

Polymer nanocomposite luminescent films for solar energy harvesting made by concurrent multi-beam multi-target pulsed laser deposition

We report on the polymer nanocomposite films doped with the nanoparticles of rare-earth (RE)-doped fluoride phosphor NaYF4:Yb3+,Er3+ (molar proportion: a = 3% of Er3+, b = 1 to 5% of Er3+, and [100% - (a + b)] of Y3+) for efficient luminescent solar concentrators (LSCs). The films are deposited using the concurrent multi-beam multi-target pulsed laser deposition of the inorganic target material and matrix assisted pulsed laser evaporation of the polymer (MBMTPLD/MAPLE). Polymer poly(methyl methacrylate) known as PMMA was evaporated and deposited on a glass plate from its solution in chlorobenzene frozen in liquid nitrogen with the fundamental harmonic (1064 nm) of a Q-switched Nd:YAG laser concurrently with the inorganic phosphor target ablated with the 2-nd harmonic (532 nm) of the same laser. The sun light is absorbed by the phosphor nanoparticles embedded in the 250-nm thick polymer film and converted in near-infrared (NIR) radiation via the mechanism of downconversion (quantum cutting). The NIR radiation propagates via the glass plate as a light guide and is converted in electric power with photovoltaic cells attached to the edges of the plate. The advantage of the proposed polymer nanocomposite LSCs is a broad absorption spectrum covering a significant portion of the solar radiation spectrum, high spectral conversion efficiency, and low reabsorption due to minimal overlap between the absorption and emission spectra (large Stokes shift). The power concentration factor of the polymer nanocomposite LSC is expected to be of the order of 10.

Inorganic nanocomposite films with polymer nanofillers made by the concurrent multi-beam multi-target pulsed laser deposition

We report on the new class of inorganic nanocomposite films with the inorganic phase hosting the polymer nanofillers made by the concurrent multi-beam multi-target pulsed laser deposition of the inorganic target material and matrix assisted pulsed laser evaporation of the polymer (MBMT-PLD/MAPLE). We used the exemplary nanocomposite thermoelectric films of aluminum-doped ZnO known as AZO with the nanofillers made of poly(methyl methacrylate) known as PMMA on various substrates such as SrTiO3, sapphire, fused silica, and polyimide. The AZO target was ablated with the second harmonic (532 nm) of the Nd:YAG Q-switched laser while PMMA was evaporated from its solution in chlorobenzene frozen in liquid nitrogen with the fundamental harmonic (1064 nm) of the same laser (50 Hz pulse repetition rate). The introduction of the polymer nanofillers increased the electrical conductivity of the nanocomposite films (possibly due to the carbonization of PMMA and the creation of additional channels of electric current) three times and reduced the thermal conductivity by 1.25 times as compared to the pure AZO films. Accordingly, the increase of the thermoelectric figure-of merit ZT would be ~ 4 times. The best performance was observed for the sapphire substrates where the films were the most uniform. The results point to a huge potential of the optimization of a broad variety of optical, opto-electronic, and solar-power nanocomposite inorganic films by the controllable introduction of the polymer nanofillers using the MBMT-PLD/MAPLE method.

Fabrication of MEMS using liquid jet dispensing technique

We describe a simple yet efficient technique of delineation of various polymer MEMS structures using computer driven liquid jet dispenser. A syringe-type dispenser with a replaceable needle is filled with a polymer solution and mounted on a vertical translation stage. The substrate is mounted on two-coordinate horizontal translation stage. Additional vertical translation arm is connected to the plunger of the syringe. Vertical translation provides positioning of the needle at a certain height over the substrate. Delineation is accomplished when the syringe plunger, being pushed down by the vertical translation arm, dispenses the liquid on the substrate at a programmable flow rate simultaneously with horizontal motion of the substrate with respect to the needle. Horizontal motion is done along a programmable trajectory at a programmable sped. Comparing to conventional methods, the proposed device consumes three order of magnitude less amount of material for the fabrication of similar MEMS structures. In addition, we describe a supplement to the liquid delineation system in the form of a microscope objective head connected to a noncoherent UV source with a multimode optical fiber. The head can serve as a tool for UV curing of the polymer material deposited with the jet dispenser as well as a separate UV writing tool. In conclusion, we describe the calculation technique for the distribution of the electric field in electrically or optically controlled ferroelectric polymer MEMS fabricated by the proposed technique.

Charge transfer and the photosensitivity in single- and double-doped LiNbO3 single crystals; an optical-electron paramagnetic resonance study: II

Transition metals in particular increase the photorefractive sensitivity. We have reported in part I of the observed enhancement for the nonlinearity of these crystals due to the single and doubly dopants. In this part, we tried to answer some of the questions about the enhancement of the photorefractive sensitivity due to gamma irradiation. The crystals were irradiated by gamma ray and the EPR spectra were obtained. A new EPR center was observed at 3420G in Mn:Fe:LiNbO3 crystals. The Degenerate four wave mixing experiment was performed for these crystals and we have observed that the intensity of the PC signal from Ni:LiNbO3 crystal was increased by almost 30 percent. By applying the Ar+ laser in situ the EPR signal was decreased and the center was bleached after almost nine minutes in the case of LiNbO3:Ni and four minutes in the case of LiNbO3:Fe:Mn crystal. An interpretation for this phenomenon will be given on the light of Kuktarev model.

Optical characterization of organic nonlinear optical crystals irradiated with MeV protons: an electron paramagnetic resonance study

In processes of obtaining a new NLO materials, modifying and enhancing the existing materials, various aromatic compounds were synthesized with the intent of comparing their nonlinear optical properties. Different techniques were used to characterize the proton-irradiated N-isobutyl-4-methyl-6- nitro-2-quinolinamine sample. From the electron paramagnetic resonance measurements; the spin concentration seemed to increase significantly when the proton beam increased from 0 to 2.2 Mev and then the spin concentration starts decreasing up to 3 Mev. The EPR signal for N-isobutyl was singlet up to 1.8 Mev then split into duplet around 2 Mev. Then EPR signal for N-isobutyl was singlet up to 1.8 Mev-irradiated-sample. FTIR analysis also shows a dramatic increase in transmission in different bands of the spectra. Due to higher energy proton irradiation, a high significant improvement in the nonlinear characteristics of the sample was observed. A theoretical interpretation for the effect of the proton irradiation enhancement of the nonlinearity of these will be presented as well.