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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6

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Department: search.filters.department.Department of PhysicsSubject: search.filters.subject.Applied physics

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Now showing 1 - 10 of 26
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    PublicationOpen Access
    Collective excitations of a laser driven atomic condensate in an optical cavity
    (Maik Nauka/Interperiodica Publishing, 2013) Öztop, B.; Türeci, H. E.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674
    We theoretically examine collective excitations of an optically driven atomic Bose-Einstein condensate, coupled to a high-finesse optical cavity. This open system has been recently used for the experimental demonstration of the Dicke superradiance of cavity photons, which is simultaneously and mutually triggered by spontaneous breaking of translational symmetry of the condensate into a crystalline order. We first develop a Hartree-Fock mean field dynamical model of the physical system. Using this model, we compute the dynamics of the cavity photons, the condensate density profile and the Dicke phase transition diagram. Both the imaginary-time and real-time evolution methods are used in the calculations. Collective excitations are determined by the solving Bogoliubov-de Gennes equations. The spectrum, softening of the modes and energetic hierarchy of excitations are determined.
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    PublicationOpen Access
    Modeling and experimental observation of parametric processes in segmented KTiOPO4 channel waveguides
    (American Institute of Physics (AIP) Publishing, 1999) Aschieri, P.; Baldi, P.; Bierlein, J.; Department of Physics; Sundheimer, Michael; Faculty Member; Department of Physics; College of Sciences
    This letter presents a modeling technique for quasiphase-matched parametric processes in segmented channel waveguides and the results of measurements of infrared parametric fluorescence in segmented KTiOPO4 channel waveguides. We demonstrate both theoretically and experimentally a strong dependence of optical parametric oscillator tuning curves and parametric conversion efficiency (parametric gain) on the segmentation duty cycle. We show that the optimum conversion efficiency for parametric interactions in segmented waveguides is obtained for duty cycles greater than 50%. Our results validate the use of an effective continuous waveguide model for segmented channel waveguide calculations over a wide range of wavelengths and duty cycles. (C) 1999 American Institute of Physics. [S0003-6951(99)02812-0].
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    PublicationOpen Access
    Optimization of room-temperature continuous-wave cr (4+)-Doped solid-state lasers: experiment and modeling
    (Institute of Electrical and Electronics Engineers (IEEE), 2001) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851
    A novel study to determine the optimum crystal and resonator parameters for Cr4+ doped lasers subjected to lifetime thermal loading was permormed. The comparison of the results of lasing threshold, pump absorption saturation, and power efficiency measurements was done with the predictions of a theoretical model to determine the laser cross sections. Using the best-fit cross-section values, numerical optimization studies were carried out to determine the optimum crystal absorption, resonator reflectivity, and crystal length which maximized the output power.
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    PublicationOpen Access
    Tunable integrated optical filters based on sapphire microspheres and liquid crystals
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2010) Gilardi, Giovanni; Yılmaz, Hasan; Sharif, Murib Mohammed; Asquini, Rita; D'Alessandro, Antonio; Beccherelli, Romeo; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851
    We present an integrated optical narrowband electrically tunable filter based on the whispering gallery modes of sapphire microspheres and double ion-exchanged channel BK7 glass waveguides. Tuning is provided by a liquid crystal infiltrated between the spheres and the glass substrate. By suitably choosing the radii of the spheres and of the circular apertures, upon which the spheres are positioned, arrays of different filters can be realized on the same substrate with a low cost industrial process. We evaluate the performance in terms of quality factor, mode spacing, and tuning range by comparing the numerical results obtained by the numerical finite element modeling approach and with the analytical approach of the Generalized Lorenz-Mie Theory for various design parameters. By reorienting the LC in an external electrical field, we demonstrate the tuning of the spectral response of the sapphire microsphere based filter. We find that the value of the mode spacing remains nearly unchanged for the different values of the applied electric field. An increase of the applied electric field strength, changes the refractive index of the liquid crystal, so that for a fixed geometry the mode spacing remains unchanged.
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    PublicationOpen Access
    Controlled photoluminescence in amorphous-silicon-nitride microcavities
    (American Institute of Physics (AIP) Publishing, 2001) Tanrıseven, S.; Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855
    Narrow-band and enhanced photoluminescence have been observed in hydrogenated amorphous-silicon-nitride microcavities. The distributed Bragg reflectors were fabricated using alternating layers of hydrogenated amorphous-silicon nitride and hydrogenated amorphous-silicon oxide. The microcavity resonance wavelength was designed to be at the maximum of the bulk hydrogenated amorphous-silicon-nitride luminescence spectrum. At the microcavity resonance, the photoluminescence amplitude is enhanced, while the photoluminescence linewidth is reduced with respect to the bulk hydrogenated amorphous-silicon nitride. (C) 2001 American Institute of Physics.
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    PublicationOpen Access
    Self-Q-Switched Cr:LiCAF laser near 800 nm
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2013) Department of Physics; Beyatlı, Ersen; Sennaroğlu, Alphan; Demirbaş, Ümit; PhD Student; Faculty Member; Master Student; Department of Physics; College of Sciences; N/A; 23851; N/A
    Self-Q-switching (SQS) of lasers enable the generation of Q-switched pulses from simple laser cavities without using any additional saturable absorbers or active modulators. Earlier studies have reported SQS in ruby, Nd:YAG, and Cr:LiSAF lasers. However, these systems were mostly flashlamp pumped and required cooling of the crystal and/or misalignment of the laser cavity for the observation of SQS. In this presentation, for the first time to our knowledge, we report SQS operation of a Cr:LiCAF laser. SQS was achieved in an astigmatically compensated x-cavity containing only a Cr:LiCAF crystal that was end-pumped with a 130-mW continuous-wave (cw) diode at 660 nm. During regular cw operation, the laser produced a diffraction-limited beam with 50 mW of output power and had a spectral width of 0.5 nm near 795 nm. SQS operation of the Cr:LiCAF laser could be initiated by fine adjustment of the separation between the curved mirrors of the cavity and was observed at several mirror separations within the stability range of the resonator. During SQS operation, the laser generated pulses with about 5 microsecond duration in the 780-800 nm wavelength range, at repetition rates between 10 and 30 kHz. SQS operation was further accompanied with a decrease in the output power to the 30-45 mW range. In this regime, the output beam became multimode and spectral broadening up to 12.5 nm (FWHM) was observed.
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    PublicationOpen Access
    Dielectric resonators based on amorphous silicon
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2001) Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855
    In this paper, we report on the design, modeling, fabrication, and characterization of dielectric microresonators based on hydrogenated amorphous silicon nitride and hydrogenated amorphous silicon oxide. The microresonators were modelled using the transfer matrix method (TMM). Quarter wavelength thick stacks of hydrogenated amorphous silicon nitride and hydrogenated amorphous silicon oxide were consecutively deposited using low temperature plasma enhanced chemical vapor deposition (PECVD). For the characterization of the dielectric microresonators the intrinsic photoluminescence of the amorphous silicon nitride is used. The photoluminescence is enhanced by at least an order of magnitude at the resonance wavelength of 710 nm. The minimum resonance linewidth is 6 nm, corresponding to a quality factor of 118. The maximum rejection bandwidth of the distributed Bragg reflector (DBR) is 150 nm. The enhancement and inhibition of the photoluminescence is understood by the modified photon density of states of the dielectric microresonator. The linewidth of the photoluminescence is also narrowed with respect to the linewidth of the bulk amorphous silicon nitride, again due to the presence of the electromagnetic modes of the microresonator.
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    PublicationOpen Access
    Droplet resonator based optofluidic microlasers
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2014) Brzobohaty, Oto; Jezek, Jan; Pilat, Zdenek; Zemanek, Pavel; Anand, Suman; McGloin, David; Department of Physics; Kiraz, Alper; Aas, Mehdi; Karadağ, Yasin; Jonas, Alexandr; Faculty Member; PhD Student; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; 22542; N/A; N/A; N/A
    An SU-8 polymer microdisk resonator coated with a palladium (Pd) layer and coupled to a single-mode optical waveguide is used to as a hydrogen (H-2) gas sensor. In the presence of H2 a red shift is observed in the spectral positions of the microdisk whispering gallery modes (WGMs) due to the expansion in the Pd lattice. H-2 concentrations below the flammable limit (4%) down to 0.3% could be detected in nitrogen atmosphere at room temperature. For H-2 concentrations between 0.3 1%, WGM spectral positions shifted linearly with H-2 concentration at a rate of 32 pm/%H-2. Average response time of the devices was measured to be 50 s for 1% H-2. The proposed device concept can also be used to detect different chemical gases by using appropriate sensing layers.
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    PublicationOpen Access
    Spectral tuning of liquid microdroplets standing on a superhydrophobic surface using electrowetting
    (American Institute of Physics (AIP) Publishing, 2008) Department of Physics; Kiraz, Alper; Karadağ, Yasin; Coşkun, Ahmet Faruk; Faculty Member; PhD Student; Department of Physics; College of Sciences; 22542; N/A; N/A
    Using electrowetting, we demonstrate reversible spectral tuning of the whispering gallery modes of glycerol/water microdroplets standing on a superhydrophobic surface by up to 4.7 nm at 400 V. Our results can inspire electrically tunable optical switches and filters based on microdroplets on a superhydrophobic surface. The sensitivity of the observed spectral drift to the contact angle can also be used to measure the contact angles of microdroplets on a superhydrophobic surface. (C) 2008 American Institute of Physics.
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    PublicationOpen Access
    Tunable WGM resonators from optically trapped dye doped liquid crystal emulsion droplets
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2014) Sery, Mojmir; Pilat, Zdenek; Jezek, Jan; Zemanek, Pavel; Department of Physics; Department of Electrical and Electronics Engineering; Aas, Mehdi; Kiraz, Alper; Faculty Member; Department of Physics; Department of Electrical and Electronics Engineering; College of Sciences; N/A; 22542
    We have built a complex apparatus for optical trapping, stretching, heating and concurrent whispering gallery mode (WGM) lasing excitation of liquid crystal (LC) emulsion micro-droplets doped with various fluorescent dyes. We have explored the changes of WGM lasing wavelength when the LC droplets were optically stretched or electrically heated beyond the transition to the isotropic phase. We have found that the range of lasing wavelengths was in some cases considerably higher than when we optically stretched ordinary fluorescent oil droplets in our previous experiments.