Researcher:
Morova, Berna

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Berna

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Morova

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Morova, Berna

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2018 - 201942020 - 202217

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    Femtosecond laser written continuous-wave Nd3+:BaY2F8 waveguide laser at 1.3 mu m
    (Elsevier, 2022) Di Lieto, Alberto; Cittadino, Giovanni; Damiano, Eugenio; Tonelli, Mauro; Department of Physics; N/A; N/A; N/A; Department of Physics; Morova, Yağız; Morova, Berna; Jahangiri, Hadi; Toker, Işınsu Baylam; Sennaroğlu, Alphan; Researcher; Researcher; Researcher; PhD Student; Faculty Member; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; N/A; N/A; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; N/A; 23851
    We experimentally demonstrate, for the first time to our knowledge, robust operation of a Nd3(+:)BaY(2)F(8) waveguide laser at 1.3 mu m without any parasitic lasing at any other wavelength. The best power performance was obtained with a depressed cladding waveguide, having a diameter of 70 mu m, which was fabricated by employing femtosecond laser writing. The propagation loss of the waveguide was measured as 0.33 dB/cm at the wavelength of 761 nm. The power performance of the waveguide laser was investigated for E//z and E//y pumping polarizations by using butt-coupled flat resonator mirrors. Higher power performance was obtained for E//z pumping, where the resonator with the 6.2% transmitting output coupler produced 157 mW of continuous-wave output power at 1318 nm with 882 mW of pump power at 800 nm. The measured power slope efficiency was 19% with respect to the incident pump power. By using a different set of cavity optics, 1050-nm lasing performance was also studied, where E//z pumping at 800 nm with the 12% transmitting output coupler yielded 307 mW of output power with 31% slope efficiency. Use of a host medium with relatively low refractive index around 1.5 was instrumental in reducing the facet reflectivity and eliminating the possibility of parasitic lasing at 1.05 mu m during 1.3-mu m lasing.
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    Passive sorting of emulsion droplets with different interfacial properties using laser-patterned surfaces
    (Springer Heidelberg, 2019) Erten, Ahmet; Jonas, Alexandr; N/A; Department of Physics; Department of Mechanical Engineering; Department of Physics; Rashid, Muhammed Zeeshan; Morova, Berna; Muradoğlu, Metin; Kiraz, Alper; PhD Student; Researcher; Faculty Member; Faculty Member; Department of Mechanical Engineering; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; College of Sciences; N/A; 152935; 22542
    We demonstrate passive sorting of emulsion microdroplets based on differences in their interfacial tension and contact angle. The sorted droplets are flowing inside a microfluidic channel featuring a shallow guiding track (depth similar to 0.6 mu m) defined by femtosecond laser micromachining in polydimethylsiloxane coating deposited on glass. Under these flow conditions, the droplets experience a confinement force that pulls them into the track; this force depends on the interfacial tension and the difference between the contact angles inside and outside the ablated track. The interplay between the confinement force, fluid drag, and wall friction then determines the trajectory of the droplet along the guiding track. We investigate experimentally the droplet trajectory as a function of droplet velocity and angle between the track and the channel axis and demonstrate precise control of droplet direction by adjusting the track angle. Moreover, we show that droplets of liquids with different interfacial tensions and contact angles travel different distances along the guiding track at a constant flow rate, which can be used for droplet sorting. We develop a theoretical model that incorporates the droplet position with respect to the ablated track, interfacial tension, and contact angles to predict the droplet trajectory under given experimental conditions. Thus, the dynamic behavior of the droplets leading to different guiding scenarios can be studied without the need of computationally expensive fluid dynamics simulations. The presented study paves the way for designing and optimizing new systems for advanced manipulation of droplets of different content using potentially reconfigurable guiding tracks.
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    Femtosecond laser ablated tracks on smart surfaces for droplet manipulation applications
    (Institute of Electrical and Electronics Engineers (IEEE), 2018) Coskun, Umut Can; Morova, Yaǧız; Bozkurt, Asuman Asikoglu; Erten, Ahmet Can; Jonas, Alexandr; Aktürk, Selcuk; N/A; Department of Physics; Department of Physics; Rashid, Muhammed Zeeshan; Morova, Berna; Kiraz, Alper; PhD Student; Researcher; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 22542
    In droplet based microfluidics, ability of controlling the motion of the droplets insidechips have great importance for the applications such guiding, sorting, mixing and dividing of liquid droplets. In this study, we fabricated hydrophilic microchannels inside the chips by femtosecond laser ablation technique in order to control droplet motion. The laser beam focused on PDMS converts the irradiated regions to hydrophilic by ablating the coating entirely from the surface. Droplets follow the tracks because of the difference in wettability of ablated region and PDMS surfaces together with the topographical profile of the ablated tracks. Within this study, we investigated the manipulation of three kind of liquid droplets (water/surfactant mixture, pure water and pure etylene glycol) along microchannels with depths of 1μm, 1.5μm and 2μm. We verified the experimental results with simulations and analyzed the trajectories of the moving droplets with the different wettability along the tracks of different depths. Our results showed that, while topographical effects play role in droplet guiding for 1.5μm and 2μm tracks, for the case of 1μm depth of channel, surface energy modifications cause the guiding rather than the topographical step on the surface. These results will pioneer for sorting applicaitons of different microdroplets mixed in the same microfluidic chip. / Damla temelli mikroakışkan sistemlerinde, damlaların yonga içerisindeki hareketlerinin kontrolü, yönlendirme, ayrıştırma, birleştirme ve bölme gibi uygulamalarda oldukça önemlidir. Bu çalışmada, damlaların hareketinin kontrolünü sağlamak için mikroakışkan yonga içerisinde hidrofilik mikrokanalları femtosaniye lazer ablasyon tekniği ile ürettik. Polydimethylsiloxane (PDMS) üzerine odakladığımız fs lazer hüzmesi uyarılan bölgeyi tamamen aşındırarak hidrofilik hale getirmektedir. Damlalar, aşındırılan hidrofilik bölge ile PDMS yüzeyi arasındaki ıslanabilirlik ve topografik farklılık sebebiyle bu kanallara eşlenerek ilerlemektedir. Bu çalışma kapsamında, üç farklı polar akışkandan (su/yüzey aktif madde karışımı, saf su ve saf etilen glikol) yağ içerisinde ürettiğimiz damlaların, 1μm, 1.5μm ve 2μm derinliklerine sahip işlenmiş mikrokanallar aracılığıyla yönlendirilmesini araştırdık. Deneysel olarak gerçekleştiridiğimiz çalışmaların sonuçlarını simülasyon değerleri ile karşılaştırdık ve farklı yapıdaki akışkan damlaların farklı derinliklerdeki kanallara sahip yongalardaki hareket yörüngelerini inceledik. Elde ettiğimiz sonuçlarla 1.5μm ve 2μm derinliğindeki kanallarda damlaların yönlendirilmesi hususunda kanal topografisinin etkili olduğunu, 1μm derinliğindeki kanallar için ise kanalın yüzeyde sahip olduğu derinlik yerine yüzey enerjisindeki değişimin etkili olduğunu gösterdik. Elde ettiğimiz bu sonuçlar, aynı yonga içerisinde bulunan farklı ıslanabilirlik özeliklerine sahip akışkanların ayrıştırılması gibi uygulamalara imkan sağlayacaktır.
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    Image reconstruction in frequency space using sinusoidal illumination patterns
    (IEEE, 2020) Aydın, Musa; Department of Physics; N/A; N/A; Department of Physics; Kiraz, Alper; Uysallı, Yiğit; Özgönül, Ekin; Morova, Berna; Faculty Member; PhD Student; PhD Student; Researcher; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; 22542; N/A; N/A; N/A
    Structured Illumination Patterns is an imaging technique used in microscopic imaging to achieve super-resolution image by exceeding the diffraction limit. In microscopic imaging, the light projected onto the sample to be imaged is modulated into two dimensional sinusoidal illumination patterns and the raw image is obtained. By using this technique, the image reconstruction algorithm applied to the raw images in the frequency space is provided to increase the resolution of the final image up to two times. In this study, to obtain the high resolution target image, convolution multiplication of the structured illumination patterns with a test image is applied and a moire fringe pattern is formed as a result of this product. Next, the steps of the structured illumination microscopy technique algorithm are described. Finally, the algorithm for image reconstruction in frequency space has been developed and the results are shown.
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    Use of an elastic buffer layer for improved performance of a polymer microcylinder ring resonator hydrogen sensor
    (Elsevier Science Sa, 2022) N/A; N/A; Department of Physics; Department of Mechanical Engineering; Department of Physics; Bavili, Nima; Ali, Basit; Morova, Berna; Alaca, Burhanettin Erdem; Kiraz, Alper; PhD Student; PhD Student; Researcher; Faculty Member; Faculty Member; Department of Mechanical Engineering; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; College of Sciences; N/A; N/A; 152935; 115108; 22542
    The impact of substrate on Pd nanofilm expansion in a Pd-H-2 system is investigated using polymer microcylinder ring resonator (PMRR) platform. Being a highly sensitive platform for H-2 gas detection, PMRR comprises of an inner sensitive Pd nanofilm and an outer PDMS layer coated on a standard optical fiber. Optical whispering gallery modes (WGMs) are excited in the rim of the outermost PDMS layer through evanescent field of a tapered fiber. H-2 molecules penetrating the H-2-sensitive Pd nanofilm through the PDMS layer cause reversible expansion in the PMRR. This translates into shifts in spectral positions of the WGMs that are observed with tapered fiber transmission spectroscopy. Two types of PMRRs were fabricated. In the first type, Pd nanofilm was directly deposited on the silica surface of an optical fiber. In the other one, a PDMS buffer layer was precoated between Pd nanofilm and the silica surface, with different thicknesses. It is demonstrated that, the use of a PDMS buffer layer yields higher radial expansion of the nanofilm during the interaction with H-2 gas. A 180-nm-thick Pd nanofilm coated on similar to 2.5-mu m-thick PDMS buffer layer showed at least 18% higher radial expansion compared to the case without buffer layer. Identical thickness of Pd nanofilm on a similar to 3.5-mu m-thick PDMS buffer layer showed 30% higher radial expansion. Numerical and analytical calculations were also performed confirming the experimental results. Among mechanical properties of the PDMS buffer layer, Poisson's ratio was found to be the most significant parameter affecting the expansion of the nanofilm.
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    Optical sensors based on palladium and polymer-coated optical fiber resonators
    (Spie-Int Soc Optical Engineering, 2019) Eryürek, Mustafa; N/A; Department of Physics; Department of Chemistry; Department of Chemistry; Department of Physics; Bavili, Nima; Morova, Berna; Yılgör, İskender; Yılgör, Emel; Kiraz, Alper; PhD Student; Researcher; Faculty Member; Researcher; Faculty Member; Department of Chemistry; Department of Physics; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); N/A; College of Science; College of Sciences; College of Sciences; College of Sciences; N/A; 152935; 24181; N/A; 22542
    In this work, we present robust and easy-to-fabricate optical gas and vapor sensors based on optical fiber resonators (OFR) coated with palladium (Pd) thin films, Pd micro-particles and polymer brushes (PB). Pd based sensors are used for hydrogen (H-2) gas detection in concentration range of 0% to 1% and polymer brush-coated OFR are used for detection of vapor in concentration range of 0 to 25%. Sensing mechanism of these sensors is based on spectral shift of resonance wavelength which are called whispering gallery modes (WGMs). This spectral shift is due to volume expansion of the sensing material. Tapered fiber is used in order to excite WGMs in coated OFRs. Good sensitivity and repeatability results are obtained for all three types of sensors.
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    Three-dimensional imaging of cleared human liver tissues reveals extensive fibrosis heterogeneity in non-alcoholic fatty liver disease
    (Elsevier, 2022) Ulukan, Burge; Tas, Yagmur Cetin; Morova, Berna; Aydin, Musa; Uysalli, Erkan, Mert; Karahuseyinoglu, Sercin; Yurdaydin, Cihan; Akyildiz, Murat; Sheth, Abhishek; Kirimlioglu, Hale; Dayangac, Murat; Ferhanoglu, Onur; Kiraz, Alper; Zeybel, Mujdat; N/A; N/A; N/A; N/A; N/A; N/A; Department of Physics; N/A; N/A; N/A; N/A; Özdemir, Yasemin Gürsoy; Yiğit Alpdoğan, Buket; Özgönül, Ekin; Yaman, Ömer; Morova, Berna; Uysallı, Yiğit; Kiraz, Alper; Taş, Yağmur Çetin; Demirtaş, Elif; Karahüseyinoğlu, Serçin; Zeybel, Müjdat; Faculty Member; Researcher; PhD Student; PhD Student; Researcher; PhD Student; Faculty Member; Researcher; Master Student; Faculty Member; Faculty Member; Department of Physics; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); N/A; School of Medicine; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; Graduate School of Sciences and Engineering; College of Sciences; School of Medicine; Graduate School of Health Sciences; School of Medicine; School of Medicine; 170592; N/A; N/A; N/A; N/A; N/A; 22542; N/A; N/A; 110772; 214694
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    Amplified phase shift - fiber cavity ring down spectroscopy for biosensing applications at 1550nm
    (Spie-Int Soc Optical Engineering, 2020) Cheema, Muhammed Imran; Ullah, Ubaid; Ghauri, M. Daniyal; N/A; N/A; N/A; Department of Physics; Ayaz, Rana Muhammed Armaghan; Uysallı, Yiğit; Morova, Berna; Kiraz, Alper; PhD Student; PhD Student; Researcher; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; College of Sciences; N/A; N/A; N/A; 22542
    We present a novel active fiber cavity platform for biosensing applications at 1550nm. We employed the phase shift-cavity ring down spectroscopy to the amplified fiber cavity and demonstrate sensing of sugar solutions with sensitivity and detection limit of 2659 degrees/RIU and 1.11 x 10(-5) RIU, respectively.
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    Linear cavity tapered fiber sensor using amplified phase-shift cavity ring-down spectroscopy
    (Optical Society of America (OSA), 2021) N/A; N/A; N/A; Department of Physics; Ayaz, Rana Muhammed Armaghan; Uysallı, Yiğit; Morova, Berna; Kiraz, Alper; PhD Student; PhD Student; Researcher; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; College of Sciences; N/A; N/A; N/A; 22542
    Phase-shift cavity ring-down spectroscopy (PS-CRDS) enables the measurement of minute fluctuations in optical loss encountered by an intensity modulated laser beam via lock-in demodulation of the accumulated signal phase as the laser beam propagates through the cavity. A linear fiber cavity containing two highly reflective fiber Bragg gratings (FBGs) and a tapered fiber is a favorable cavity geometry for sensor applications due to its compact nature and wide availability of its components at telecom wavelengths. However, due to the optical absorption of water, usage of telecom wavelengths for sensing of aqueous solutions degrades the sensitivity. This problem can be worked around via an optical amplifier placed inside the fiber cavity, compensating for intrinsic cavity losses. In this work, we demonstrate amplified PS-CRDS using such a linear cavity tapered fiber sensor utilizing an optical amplifier that enables the use of thinner tapered fibers, thus achieving higher sensitivities. We also employ continuous laser wavelength sweeps and analyze peak-to-peak PSs of individual cavity modes instead of using laser-cavity mode locking with the Pound-Drever-Hall technique. This enables further simplification of the experimental arrangement without compromising sensor performance. To test the performance of the reported sensor, solutions of varying sucrose concentrations in deionized water were measured systematically by tracking the average peak-to-peak PS of the cavity modes. Tapered fibers with waist diameters (D-taper) around 2.2 mu m were used during the experiment and limit of detection (LOD) values were measured down to 2.7 mu M, corresponding to 1.01 x 10(-7) RIU. The reported LOD values can be further improved by mechanical stabilization and thermal control of the device by the use of FBGs with higher reflectivity, by applying automatic optical gain control, and by spectral filtering to remove errors caused by amplified spontaneous emission. The presented concentration sensing device can be suitable for developing compact and highly sensitive, label-free optofluidic sensors.
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    Refractive index sensing by phase shift cavity ringdown spectroscopy
    (Springer Science and Business Media B.V., 2022) Cheema, M. Imran; N/A; N/A; N/A; N/A; Department of Physics; Ayaz, Rana Muhammed Armaghan; Uysallı, Yiğit; Bavili, Nima; Morova, Berna; Kiraz, Alper; PhD Student; PhD Student; Researcher; Researcher; Faculty Member; Department of Physics; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; N/A; College of Sciences; N/A; N/A; N/A; N/A; 22542
    Cavity ring down spectroscopy is a sensitive optical detection technique but it makes use of expensive electronics and data fitting algorithm. Both of these factors put a limitation on its performance for sensing applications. In this study cost effective and easy to use phase shift cavity ring down spectroscopy (PS-CRDS) technique, has been demonstrated for refractive index sensing. A refractive index sensor consisting of a tapered single mode optical fiber and two fiber Bragg gratings (FBGs) is proposed. In this sensing methodology, an intensity modulated laser beam centered at 1550 nm from a DFB laser is scanned at a narrow wavelength range and cavity modes are excited. Later the phase shift corresponding to these cavity modes is measured using a lock-in amplifier. Sucrose solutions of various concentrations are used for performance analysis of the refractive index sensing device. The resultant limit of detection (LOD) came out to be ~6.4 × 10−6 refractive index units (RIUs), which can be improved further by using thinner fiber tapers or fiber Bragg gratings with higher reflectivity. © 2022, Springer Nature B.V.