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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only A computational study of drop formation in an axisymmetric flow-focusing device(Amer Soc Mechanical Engineers, 2006) Department of Mechanical Engineering; Department of Mechanical Engineering; Filiz, İsmail; Muradoğlu, Metin; N/A; Faculty Member; Department of Mechanical Engineering; College of Engineering; College of Engineering; N/A; 46561We investigate the formation and dynamics of drops computationally in an axisymetric geometry using a Front-Tracking/Finite-Difference (FT/FD) method. The effects of viscosity ratio between inner and outer liquids on the drop creation process and drop size distribution are examined. It is found that the viscosity ratio critically influences the drop formation process and the final drop distribution. We found that, for small viscosity ratios, i.e., 0.1 < lambda < 0.5 drop size is about the size of the orifice and drop distribution is highly monodisperse. When viscosity ratio is increased, i.e., 0.5 < lambda < I a smaller drop is created just after the main drop. For even higher viscosity ratios, the drop distribution is usually monodisperse but a satellite drop is created in some cases. The effect of the flow rates in the inner jet and the co flowing annulus are also studied. It is found that the drop size gets smaller as Q(in) / Q(out) is reduced while keeping the outer flow rate constant.Publication Metadata only Optical modulation with silicon microspheres(IEEE-Inst Electrical Electronics Engineers Inc, 2009) Gürlü, Oğuzhan; N/A; Department of Physics; Yüce, Emre; Serpengüzel, Ali; Master Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; 245435; 27855In this letter, a silicon microsphere coupled to a silica optical fiber half coupler has been characterized for electrooptical modulation in the L-band at 1.55 mu m. Electrooptical modulation of the transmitted and the 90 degrees elastic scattered signals for both the TE and the TM polarizations of the microsphere resonances has been observed.Publication Metadata only Ruby microsphere and liquid cyrstal based tunable optical filter(Ieee, 2009) Beccherelli, Romeo; Department of Physics; N/A; N/A; Serpengüzel, Ali; Murib, Mohammed Sharif; Hüseyinoğlu, Ersin; Faculty Member; PhD Student; Master Student; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 27855; N/AA microsphere placed in a liquid crystal is used as an optical filter. By changing the refractive index of the liquid cyrstal, the resonance frequency of the sphere is controlled.Publication Metadata only High-resolution beam steering using microlens arrays(Optical Soc Amer, 2006) N/A; N/A; Department of Electrical and Electronics Engineering; Akatay, Ata; Ataman, Çağlar; Ürey, Hakan; Master Student; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 8579Imaging or beam-steering systems employing a periodic array of microlenses or micromirrors suffer from diffraction problems resulting from the destructive interference of the beam segments produced by the array. Simple formulas are derived for beam steering with segmented apertures that do not suffer from diffraction problems because of the introduction of a moving linear phase shifter such as a prescan lens before the periodic structure. The technique substantially increases the resolution of imaging systems that employ microlens arrays or micromirror arrays. Theoretical, numerical, and experimental results demonstrating the high-resolution imaging concept using microlens arrays are presented.Publication Metadata only Efficient continuous-wave radiatively cooled Cr4+: forsterite lasers at room temperature(Optical Soc Amer, 1998) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851Results of a detailed experimental investigation aimed at reducing the thermal loading problem in a cw Cr4+:forsterite laser at elevated temperatures are presented. From a Cr4+:forsterite crystal with a differential absorption coefficient of 0.57 cm(-1), as much as 900 mW of cw output power has been obtained at 1.26 mu m and at a crystal boundary temperature of 15 degrees C with an absorbed pump power of only 4.5 W at 1.06 mu m. No chopping of the the pump beam was necessary. An efficient radiative cooling technique was further employed to cool the laser and no subsequent power fading was observed. To the author's knowledge, the measured absorbed power slope efficiency of 29.5% represents the highest cw power performance reported to date: from a Cr4+:forsterite laser pumped by a Nd:YAG laser around room temperature. The role of the low differential absorption coefficient in the reduction of thermal loading is further elucidated by presenting comparative cw power performance data with a second Cr4+:forsterite crystal having a differential absorption coefficient of 1.78 cm(-1) in the temperature range between 12 and 35 degrees C. Finally, some interesting multipulse effects of the laser observed in the millisecond regime during quasi-cw operation at 50% duty cycle are described.Publication Metadata only Broadly tunable continuous-wave orange-red source based on intracavity-doubled Cr4+: forsterite laser(Optical Soc Amer, 2002) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851The operation of a room-temperature, continuous-wave, intracavity frequency-doubled Cr4+:forsterite laser capable of producing broadly tunable output in the orange-red region of the electromagnetic spectrum is described. Intracavity doubling was achieved in a periodically poled lithium niobate crystal that had gratings with different periods. Tunable second-harmonic output could be obtained between 613 and 655 am. At a wavelength of 630 nm, intracavity doubling yielded as much as 45 mW of continuous-wave output. To the author's knowledge, this represents the highest second-harmonic-power generation obtained to date with a continuous-wave Cr4+:forsterite laser.Publication Metadata only Amorphous silicon nitride microcavities(Optical Soc Amer, 2001) Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Amorphous silicon nitride microcavities were fabricated by use of distributed Bragg reflectors. The distributed Bragg reflectors were fabricated with alternating layers of quarter-wavelength-thick hydrogenated amorphous silicon nitride and amorphous silicon oxide. The spectral peak of the bulk amorphous silicon nitride photoluminescence spectrum was chosen as the microcavity resonance wavelength. The amorphous silicon nitride microcavity enhances the photoluminescence amplitude and reduces the photoluminescence linewidth with respect to the bulk amorphous silicon nitride. This narrowing and enhancement of the photoluminescence can he understood by the redistribution of the density of optical modes owing to the presence of the microcavity. The microcavity narrowing and enhancement of luminescence in hydrogenated amorphous silicon nitride opens up a variety of possibilities for optoelectronic applications such as resonant-cavity-enhanced light-emitting diodes and color flat-panel displays.Publication Metadata only Determination of the stimulated emission cross section from laser-induced pump saturation data in Cr4+-doped solid-state lasers(Optical Soc america, 2001) Department of Physics; Department of Physics; Sennaroğlu, Alphan; Yaman, Fatih; Faculty Member; Undergraduate Student; Department of Physics; College of Sciences; College of Sciences; 23851; N/AWe describe a novel method for the determination of the stimulated emission cross section in Cr4+-doped solid-state lasers from laser-induced pump saturation data. the technique uses the fact that increasing intra-cavity laser intensity stimulates a faster decay of excited atoms and reduces the saturation of pump absorption. a numerical model was developed to analyze the experimental data collected from Cr4+:YaG and Cr4+:forsterite lasers. the best-fit values of the stimulated emission cross section agree with previous results. To demonstrate the applicability of the method to other solid-state lasers, the same technique was also used to determine the emission cross section in a Cr2+:ZnSe laser.Publication Metadata only Closed-form representations of field components of fluorescent emitters in layered media(Optical Soc Amer, 2009) Doğan, Mehmet; Swan, Anna K.; Goldberg, Bennett B.; Ünlü, M. Selim; Department of Electrical and Electronics Engineering; Aksun, M. İrşadi; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 28358Dipole radiation in and near planar stratified dielectric media is studied theoretically within the context of fluorescence microscopy, as fluorescent emitters are generally modeled by electric dipoles. Although the main emphasis of this study is placed on the closed-form representations of the field components of fluorescent emitters in layered environments in near- and far-field regions, the underlying motive is to understand the limits of spectral self-interference fluorescence microscopy in studying the dipole orientation of fluorophores. Since accurate calculations of the field components of arbitrarily polarized electric dipoles in layered environments are computationally very time-consuming, a method for finding their closed-form representations is proposed using the closed-form potential Green's functions previously developed for microwave applications. The method is verified on typical geometries used in spectral self-interference microscopy experiments, where a dipole emitter is positioned over a slab of SiO2 on top of a Si substrate. In addition to facilitating efficient calculation of near and intermediate fields of fluorescent emitters, closed-form Green's functions for fields would also play a crucial role in developing efficient and rigorous computational analysis and design tools for optical passive devices such as optical antennas by significantly improving the computational cost of the numerical solution of the integral equation.Publication Metadata only Efficient continuous-wave operation of a diode-pumped Nd:YVO4 laser at 1342 nm(Elsevier Science Bv, 1999) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851The experimental study described in this paper investigates the continuous-wave power performance of an efficient diode-pumped Nd:YVO4 laser at 1342 nm by employing four different end pumping configurations. When pumped by a high-power fiber-coupled diode array at 806 nm, the compact resonator consisting of a 9.25-mm long Nd:YVO4 crystal and a 3.6% transmitting output coupler produced as high as 3550 mW of output power. The absorbed power slope efficiency was measured to be 28.1%. By using the experimentally measured threshold data, the stimulated emission cross-section of the gain medium was determined to be 13 X 10(-19) cm(2) at 1342 nm. Above absorbed pump powers of 11 W, strong thermal loading caused saturation of the output power and the focal length of the induced thermal lens was measured as a function of the pump power. Results further showed that the laser output was insensitive to variations in the crystal boundary temperature between 20 degrees C and 40 degrees C.