Research Outputs
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Publication Metadata only 10-NJ multipass-cavity femtosecond CR3+: LiCAF laser pumped by low-power single-mode diodes(Optical Society of America, 2009) Kärtner, Franz X.; Fujimoto, James G.; Demirbaş, Ümit; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851We report on the generation of 9.9-nJ, 95-fs pulses at a repetition rate of 9.58 MHz from a multipass-cavity Cr3+:LiCAF laser pumped by single-mode diodes with a total absorbed pump power of only 540 mW.Publication Metadata only 80-NJ multipass-cavity chirped-pulse Cr4+: forsterite laser(Optical Society of America, 2010) Fujimoto, James G.; Department of Physics; Sennaroğlu, Alphan; Çankaya, Hüseyin; Faculty Member; Researcher; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 23851; N/ABy using 8.5 W of incident pump power, we obtained 80-nJ, 5.5-ps pulses at 1260 nm with a spectral width of 17 nm from a multipass-cavity, chirped-pulse Cr4+:forsterite laser operated at 4.9-MHz repetition rate. © 2010 Optical Society of America.Publication Open Access A broken gauge approach to gravitational mass and charge(Springer, 2002) Tucker, R. W.; Department of Physics; Dereli, Tekin; Faculty Member; Department of Physics; College of Sciences; 201358We argue that a spontaneous breakdown of local Weyl invariance offers a mechanism in which gravitational interactions contribute to the generation of particle masses and their electric charge. The theory is formulated in terms of a spacetime geometry whose natural connection has both dynamic torsion and non-metricity. Its structure illuminates the role of dynamic scales used to determine measurable aspects of particle interactions and it predicts an additional neutral vector boson with electroweak properties. © SISSA/ISAS 2002.Publication Open Access A critical approach to the biocompatibility testing of NiTi orthodontic archwires(Vibgyor Online Publishers, 2016) Şahbazoğlu, D.; Toker, S. M.; Saher, D.; Department of Mechanical Engineering; Canadinç, Demircan; Gümüş, Berkay; Uzer, Benay; Yıldırım, Cansu; Polat-Altıntaş, Sevgi; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433; N/A; N/A; N/A; N/AThe biocompatibility of Nickel-Titanium (NiTi) archwires was investigated by simulating actual contact state of archwires around brackets, which enabled incorporation of realistic mechanical conditions into ex situ experiments. Specifically, archwires (undeformed, and bound to brackets on acrylic dental molds) were statically immersed in artificial saliva (AS) for 31 days. Following the immersion, the archwires and the immersion solutions were analyzed with the aid of variouselectron-optical techniques, and it was observed that carbon-rich corrosion products formed on both archwire sets upon immersion. The corrosion products preferentially formed at the archwire–bracket contact zones, which is promoted by the high energy of these regions and the micro-cracks brought about by stress assisted corrosion. Moreover, it is suggested that these corrosion products prevented significant Ni or Ti ion release by blocking the micro-cracks, which, otherwise, would have led to enhanced ion release during immersion. The current findings demonstrate the need for incorporating both realistic chemical and mechanical conditions into the ex situ biocompatibility experiments of orthodontic archwires, including the archwire-bracket contact.Publication Metadata only Acousto-optic mode-locking of a Cr2+: ZnSe laser(Optica Publishing Group, 2000) Carrig, Timothy J.; Wagner, Gregory J.; Jeong, Jay Y.; Pollock, Clifford R.; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851Acousto-optic mode-locking of a Cr2+:ZnSe laser that produces 4.4 psec duration, transform-limited, Gaussian shaped pulses is described. The laser outputs 82 mW of output power at an 81 MHz pulse repetition frequency.Publication Open Access Addendum to 'Unidirectionally invisible potentials as local building blocks of all scattering potentials'(American Physical Society (APS), 2014) Department of Mathematics; Department of Physics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; Department of Physics; College of Sciences; 4231In [Phys. Rev. A 90, 023833 (2014)], we offer a solution to the problem of constructing a scattering potential v(x) which possesses scattering properties of one's choice at an arbitrarily prescribed wave number. This solution involves expressing v(x) as the sum of n <= 6 finite-range unidirectionally invisible potentials. We improve this result by reducing the upper bound on n from 6 to 4. In particular, we show that we can construct v(x) as the sum of up to n = 3 finite-range unidirectionally invisible potentials, unless if it is required to be bidirectionally reflectionless.Publication Open Access An optofluidic FRET laser using aqueous quantum dots as donors(Society of Photo-optical Instrumentation Engineers (SPIE), 2016) Chen, Qiushu; Fan, Xudong; Department of Physics; Kiraz, Alper; Faculty Member; Department of Physics; College of Sciences; 22542The integration of optofluidic laser and FRET mechanism provides novel research frontiers, including sensitive biochemical analysis and novel photonic devices, such as on-chip coherent light sources and bio-tunable lasers. Here we investigated an optofluidic FRET laser using quantum dots (QDs) as FRET donors. We achieved lasing from Cy5 as the acceptor in the QD-Cy5 pair with excitation at 450 nm where Cy5 has negligible absorption by itself. The threshold was approximately 14 mu J/mm(2). The demonstrated capability of QDs as the donor in a FRET laser greatly improves the versatility of optofluidic laser operation due to the broad and large absorption cross section of QDs in the blue and UV spectral region. The excitation efficiency of the acceptor molecules through FRET channel was also analyzed, showing that the energy transfer rate and the non-radiative Auger recombination rate of QDs plays a significant role in FRET laser performance.Publication Open Access Artificial gauge fields for the Bose-Hubbard model on a checkerboard superlattice and extended Bose-Hubbard model(Springer, 2012) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659We study the effects of an artificial gauge field on the ground-state phases of the Bose-Hubbard model on a checkerboard superlattice in two dimensions, including the superfluid phase and the Mott and alternating Mott insulators. First, we discuss the single-particle Hofstadter problem, and show that the presence of a checkerboard superlattice gives rise to a magnetic flux-independent energy gap in the excitation spectrum. Then, we consider the many-particle problem, and derive an analytical mean-field expression for the superfluid-Mott and superfluid-alternating-Mott insulator phase transition boundaries. Finally, since the phase diagram of the Bose-Hubbard model on a checkerboard superlattice is in many ways similar to that of the extended Bose-Hubbard model, we comment on the effects of magnetic field on the latter model, and derive an analytical mean-field expression for the superfluid-insulator phase transition boundaries as well.Publication Metadata only Atmospheric pressure plasma jet treatment of human hair fibers(2015) N/A; N/A; Department of Molecular Biology and Genetics; Department of Chemistry; Birer, Özgür; Acar, Erhan; Keleş, Merve; Öngel, Cansu; Researcher; Master Student; Undergraduate Student; Undergraduate Student; Department of Molecular Biology and Genetics; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; Graduate School of Sciences and Engineering; College of Science; College of Science; N/A; N/A; N/A; N/AHuman hair fibers in virgin and dyed forms were treated with atmospheric pressure helium, helium/oxygen, argon, and argon/oxygen plasma jets at 20 W of power. The effects of 10-min plasma treatment on surface morphology and chemistry were studied by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The plasma treatment was quite effective for removing the organic residues from the surface and creating oxidized functional groups. Helium plasma had a mild cleaning effect on the surfaces while argon/oxygen plasma had the strongest corrosive effect. Mild hydrogen peroxide treatment for the same duration had neither the cleaning nor the oxidizing power of the plasma jets. These types of plasma jets have the potential to replace peroxide treatment. The corrosive jets can be used to restore dyed hair fibers. In addition, the jets can be used to clean the surfaces of hair fibers to prepare samples for analytical investigations where the organic residues may induce problems. © 2015, Springer International Publishing AG.Publication Metadata only Attosecond timing jitter pulse trains from semiconductor saturable absorber mode-locked Cr:LiSAF lasers(Optical Society of America, 2012) Kärtner, Franz X.; Fujimoto, James G.; Demirbaş, Ümit; Benedick, Andrew; Li, Duo; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851An integrated timing jitter of 43 as [10 kHz-50 MHz] is measured from a semiconductor saturable absorber modelocked Cr:LiSAF laser. AM-to-PM transfer functions' characterization shows self-steepening is the dominant effect of excess timing jitter. © OSA 2012.