Researcher: Morova, Yağız
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Morova, Yağız
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Publication Metadata only A novel machine learning method for the design optimization of diamond waveguides fabricated by femtosecond laser writing(Elsevier, 2024) Ince, Faik Derya; Ozel, Tugrul; Department of Physics; Department of Physics; Morova, Yağız; Sennaroğlu, Alphan; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences;We report on a novel machine learning method for the design optimization of femtosecond (fs) laser written dielectric waveguides. Experimental results previously obtained from the optical characterization of fs laser written depressed cladding diamond waveguides have been used to form statistically generated regression models. Design variables such as core diameter and number of written tracks were varied to both minimize the propagation loss as well as to establish a full-factorial experimental design. The regression models were used to conduct a multi-objective optimization study to optimize the competing objectives such as maximizing the refractive index contrast while minimizing the propagation loss and V-number by using a genetic algorithm. Optimization was subject to a nonlinear Rayleigh range constraint to ensure that the structure was in the waveguiding regime. Results from the optimization revealed the optimum variables to achieve low-loss and nearly single-mode guiding for a fs laser written diamond waveguide. Using the solution sets of design parameters resulting from the optimization study and their corresponding objective function values, important correlations between the design parameters and the objective functions have been revealed. With this regard, it has been shown that the number of written tracks is a much more dominant parameter, when compared to core diameter, during the design of a fs laser written circular depressed cladding diamond waveguide. The proposed method should be applicable not only to diamond waveguides but also to a wide range of dielectric waveguides fabricated by fs laser writing.Publication Metadata only Er3+:YLiF4 channeled waveguide laser near 2.7-2.8 μm fabricated by femtosecond laser inscription(Optica Publishing Group, 2024) Tonelli, Mauro; Department of Physics; Department of Physics; Ayevi, Berke; Morova, Yağız; Sennaroğlu, Alphan; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences;We report, for the first time to our knowledge, a demonstration of robust waveguide lasing near 2.7-2.8 mu m in an erbium -doped fluoride host. Femtosecond laser inscription was employed to fabricate 50- and 70-mu m diameter channeled waveguides inside an Er3+:YLiF4 crystal. The best power performance was obtained with the 70-mu m diameter waveguide and 16% transmitting output coupler. The propagation loss and refractive index contrast were measured as 0.23 dB/cm and 7.1 x 10-4, respectively, for the 70-mu m diameter waveguide. Both self-Q-switched (SQS) and continuous-wave (CW) operations could be obtained. During the SQS operation, as short as 240-ns pulses with average power of 51 mW, repetition rate of 368 kHz, and power slope efficiency of 15.2% were generated at the wavelength of 2717 nm with 465 mW of the pump power. During the CW operation, as high as 66 mW of output power was achieved at 2808 nm by using 460 mW of pump power at 798 nm, with a power slope efficiency of 19.6%. (c) 2024 Optica Publishing GroupPublication Metadata only Near-infrared triggered degradation for transient electronics(American Chemical Society, 2024) İstif, Emin; Department of Physics;Department of Mechanical Engineering; Ali, Mohsin; Özüaçıksöz, Elif Yaren; Morova, Yağız; Beker, Levent; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; College of EngineeringElectronics that disintegrate after stable operation present exciting opportunities for niche medical implant and consumer electronics applications. The disintegration of these devices can be initiated due to their medium conditions or triggered by external stimuli, which enables on-demand transition. An external stimulation method that can penetrate deep inside the body could revolutionize the use of transient electronics as implantable medical devices (IMDs), eliminating the need for secondary surgery to remove the IMDs. We report near-infrared (NIR) light-triggered transition of metastable cyclic poly-(phthalaldehyde) (cPPA) polymers. The transition of the encapsulation layer is achieved through the conversion of NIR light to heat, facilitated by bioresorbable metals, such as molybdenum (Mo). We reported a rapid degradation of cPPA encapsulation layer about 1 min, and the rate of degradation can be controlled by laser power and exposure time. This study offers a new approach for light triggerable transient electronics for IMDs due to the deep penetration depth of NIR light through to organs and tissues.Publication Metadata only Femtosecond laser written Tm3+:BaY2F8 waveguide laser operating near 1.9 μm(Optica Publishing Group (formerly OSA), 2021) Tonelli, Mauro; Department of Physics; Department of Physics; Department of Physics; Sennaroğlu, Alphan; Morova, Yağız; Faculty Member; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of Sciences; College of Sciences; 23851; N/AWe report the first experimental demonstration of continuous-wave laser action near 1.9 μm in a femtosecond laser written Tm3+ ion-doped BaY2F8 crystal waveguide laser with incident lasing threshold pump power as low as 23 mW.Publication Metadata only Femtosecond laser-mediated preparation of HfNbTaTiZr refractory high-entropy alloy nanoparticles for photothermal therapy applications: influence of solvent and fluence(Elsevier, 2023) Guo, Sheng; Department of Physics; Department of Physics; N/A; Department of Chemistry; Department of Chemistry; N/A; N/A; Department of Physics; Department of Chemistry; Sennaroğlu, Alphan; Morova, Yağız; Alamdari, Armin Asghari; Eroğlu, Zafer; Metin, Önder; Motallebzadeh, Amir; Jahangiri, Hadi; Faculty Member; Researcher; PhD Student; Researcher; Faculty Member; Researcher; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 23851; N/A; N/A; N/A; 46962; N/A; N/ANanoparticles (NPs) have become popular photothermal therapy (PTT) photosensitizers because they can be targeted to cancer tissues and deliver a chemotherapeutic medication. Pulsed laser ablation in liquid media (PLAL) can produce stable colloidal NPs without any stabilizing agents. The stability, size distribution, and morphology of the produced colloids are influenced by the nature of NPs, laser parameters such as laser power, laser frequency repetition, ablation rate, and the specific ionic effects from the solvent. In this work, HfNbTaTiZr refractory high entropy alloy (RHEA) NPs were prepared by PLAL method in different solvents, including distilled water, ethanol, and n-hexane. The experiments were performed by irradiating a HfNbTaTiZr target with a femtosecond laser outputting 120-fs pulse at varying levels of ablation fluence (0.1, 0.16, 0.23 mJ/cm2). The elemental and structural characteristics of the prepared HfNbTaTiZr NPs were elucidated by using several advanced analytical techniques. The XRD pattern of NPs revealed that the liquid medium significantly affects the type of crystallized phases and the surface composition. The surface composition was studied by XPS, indicating that the fabricated NPs were oxidized. SEM and TEM analysis have evidenced the generation of NPs with an average diameter of less than 50 nm. The results revealed that the higher ablation fluence resulted in the formation of NPs with a larger average diameter. The highest intrinsic photothermal conversion efficiency and 12 °C time-dependent solvent heating produced in ethanol at 640 nm irradiation after 20 min were demonstrated.Publication Metadata only 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; Department of Physics; Morova, Yağız; Morova, Berna; Jahangiri, Hadi; Toker, Işınsu Baylam; Sennaroğlu, Alphan; Researcher; Researcher; Researcher; PhD Student; Faculty Member; 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; 23851We 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.Publication Metadata only Laser-micromachined zebra-patterned graphene as a mode locker with adjustable loss(Optical Soc Amer, 2020) Bae, Ji Eun; Rotermund, Fabian; Department of Physics; Department of Physics; Department of Physics; Morova, Yağız; Sennaroğlu, Alphan; Researcher; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of Sciences; College of Sciences; N/A; 23851In this Letter, we describe a novel, to the best of our knowledge, device based on micro-structured graphene, referred to as zebra-patterned graphene saturable absorber (ZeGSA), which can be used as a saturable absorber with adjustable loss to initiate femtosecond pulse generation. Femtosecond laser micro-machining was employed to ablate monolayer graphene on an infrasil substrate in the form of stripes with a different duty cycle, resulting in the formation of regions with variable insertion loss in the 0.21%-3.12% range. The mode-locking performance of the device was successfully tested using a Cr4+: forsterite laser, operating near 1250 nm. In comparison with mode locking using non-ablated graphene, the ZeGSA device with regions of decreasing graphene, enabled improved power performance where the mode-locked output power increased from 68 mW to 114 mW, and the corresponding pulse duration decreased from 62 to 48 fs at the same incident pump power of 6.3 W. These experiments indicate that ZeGSA shows great potential as a laser mode locker with adjustable loss and that it should find applications in the development of femtosecond lasers over a broad spectral range.Publication Metadata only Tunable laser operation of Tm3+:KY3F10 near 19 µm and 23 µm via upconversion pumping at 1064 nm(Optical Soc Amer, 2021) Tonelli, Mauro; Department of Physics; N/A; N/A; Department of Physics; Morova, Yağız; Kamun, Eylül Nihan; Sennaroğlu, Alphan; Researcher; Master Student; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; Graduate School of Sciences and Engineering; School of Medicine; N/A; N/A; 205925We experimentally demonstrate, for the first time to our knowledge, that upconversion pumping can be employed with a 1064-nm fiber laser to achieve tunable laser operation with 8 at. % Tm3+:KY3F10 crystal near 1.9 mu m, in addition to the previously explored upconversion pumping of the 2.3-mu m laser transition. Two different sets of cavity optics were used to investigate lasing at 1.9 and 2.3 mu m, corresponding to the laser transitions F-3(4)-H-3(6) and H-3(4)-H-3(5), respectively. In the case of the F-3(4)-H-3(6) laser transition, an x-cavity laser oscillator was constructed with a 2.3% output coupler and as high as 142 mW of output power was obtained at an incident pump power of 1.9 W at 1064 nm. Tunable laser operation could be obtained between 1849 and 1994 nm by using CaF2 and suprasil prisms. The highest slope efficiency of 29% with respect to average absorbed pump power was obtained with a 5.5% output coupler. For the case of 1.9 mu m lasing, it was further demonstrated that the nonlinear absorption of the crystal at 1064 nm depends on the intracavity laser intensity as well as the pump intensity. The H-3(4)-H-3(5) laser transition was also investigated with the same crystal in a z-cavity configuration under upconversion pumping, giving as high as 130 mW of output power at the central wavelength of 2344 nm with 1.6 W of incident pump power.Publication Metadata only Upconversion pumping o a 2.3 μm Tm3+:KY3F10 laser with a 1064 nm ytterbium fiber laser(Optical Society of America (OSA), 2020) Tonelli, Mauro; Petrov, Valentin; Department of Physics; Department of Physics; Department of Physics; Morova, Yağız; Sennaroğlu, Alphan; Researcher; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; College of Sciences; N/A; 23851We report efficient Iasi ng ofthe isotropic Tm3+:KY3F10 crystal near 2.3 mu m via upconversion pumping with a 1064 nm ytterbium fiber laser as the pump source. When pumped at 1064 nm, an x-cavity Tm3+:KY3F10 laser operated at the free-running wavelength of 2344 nm. Lasing was obtained with output couplers having transmissions in the range of 1-3%, and as high as 124 mW of continuous-wave (cw) output power was generated with 604 mW of absorbed pump power by using the 3% output coupler. Broadly tunable cw lasing could be obtained in the 2268-2373 nm wavelength range. An analysis of the experimental power efficiency data shows that nearly all of the absorbed pump photons were converted to 2.3 mu m laser output after accounting for the quantum defect of the laser transition and resonator losses. We expect that higher lasing efficiency should be possible by using longer crystals to increase the pump absorption.Publication Metadata only Tunable continuous-wave laser operation of Tm3+ ion doped tellurite glass near 2 μm(Elsevier, 2022) Denker, Boris; Galagan, Boris; Sverchkov, Sergei; Department of Physics; N/A; Department of Physics; Department of Physics; Morova, Yağız; Khan, Minahil; Sennaroğlu, Alphan; Researcher; PhD Student; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 23851We provide an experimental investigation of the absorption saturation and 1.9-mu m lasing characteristics of a bulk Tm3+ :TZLN glass with the host composition of 74TeO(2)-12ZnO-4La(2)O(3)-10Na(2)O. By using a z-scan saturation measurement, the absorption cross section at 794 nm was determined to be 5 x 10(-21) cm(2) , in reasonable agreement with the value of 6 x 10(-21) cm(2) obtained from small-signal transmission measurement. In lasing experiments, double pumping was employed at 794 nm and the bulk Tm3+ :TZLN glass laser operated at the free running wavelength of 1926 nm. As high as 15.4 mW of continuous-wave output power was obtained with a 2.15% transmitting output coupler at the absorbed pump power of 284 mW. The slope efficiency with respect to the absorbed pump power was 7.1%. By using the threshold and power efficiency data, the stimulated emission cross section was further determined to be 6.8 x 10(-21) cm(2) at 1926 nm. Finally, continuous tuning of the laser output was demonstrated between 1845 and 1973 nm by using a quartz birefringent plate.