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

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    PublicationOpen Access
    Perfect broadband invisibility in isotropic media with gain and loss
    (The Optical Society (OSA) Publishing, 2017) Department of Mathematics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; College of Sciences; 4231
    We offer a simple route to perfect omnidirectional invisibility in a spectral band of desired width. Our approach is based on the observation that in two dimensions a complex potential v(x; y) is invisible for incident plane waves with a wavenumber not exceeding a pre-assigned value a, provided that its Fourier transform with respect to y, which we denote by v (x; R-y), vanishes for R-y <= 2a. We can fulfill this condition for potentials modeling the permittivity profile of an optical slab. Such a slab is perfectly invisible for any transverse electric wave whose wavenumber is in the range [0; a]. Our results also apply to transverse magnetic waves propagating in a medium with a relative permittivity epsilon (x; y) that is a smooth bounded function with a positive real part.
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    PublicationOpen Access
    Multi-view autostereoscopic projection display using rotating screen
    (Optical Society of America (OSA), 2013) Department of Electrical and Electronics Engineering; Eldeş, Osman; Akşit, Kaan; Ürey, Hakan; Master Student; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 8579
    A new technique for multi-view autostereoscopic projection display is proposed, and demonstrated. The technique uses two mobile projectors, a rotating retro-reflective diffuser screen, and a head-tracking camera. As two dynamic viewing slits are created at the viewer's position, the slits can track the position of the eyes by rotating the screen. The display allows a viewer to move approximately 700 mm along the horizontal axis, and 500 mm along the vertical axis with an average crosstalk below 5 %. Two screen prototypes with different diffusers have been tried, and they provide luminance levels of 60 Cd/m(2), and 160 Cd/m(2) within the viewing field.
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    PublicationOpen Access
    Low-signal limit of X-ray single particle diffractive imaging
    (Optical Society of America (OSA), 2019) Ayyer, Kartik; Morgan, Andrew J.; Aquila, Andrew; Hogue, Brenda G.; Kirian, Richard A.; Xavier, P. Lourdu; Yoon, Chun Hong; Chapman, Henry N.; Barty, Anton; Department of Molecular Biology and Genetics; Demirci, Hasan; Faculty Member; Department of Molecular Biology and Genetics; College of Sciences; 307350
    An outstanding question in X-ray single particle imaging experiments has been the feasibility of imaging sub 10-nm-sized biomolecules under realistic experimental conditions where very few photons are expected to be measured in a single snapshot and instrument background may be significant relative to particle scattering. While analyses of simulated data have shown that the determination of an average image should be feasible using Bayesian methods such as the EMC algorithm, this has yet to be demonstrated using experimental data containing realistic non-isotropic instrument background, sample variability and other experimental factors. In this work, we show that the orientation and phase retrieval steps work at photon counts diluted to the signal levels one expects from smaller molecules or with weaker pulses, using data from experimental measurements of 60-nm PR772 viruses. Even when the signal is reduced to a fraction as little as 1/256, the virus electron density determined using ab initio phasing is of almost the same quality as the high-signal data. However, we are still limited by the total number of patterns collected, which may soon be mitigated by the advent of high repetition-rate sources like the European XFEL and LCLS-II.
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    PublicationOpen Access
    Graphene mode-locked femtosecond Cr:ZnSe laser at 2500 nm
    (Optical Society of America (OSA), 2013) Kim, J. W.; Bae, S.; Hong, B. H.; Rotermund, F.; N/A; Department of Physics; Çizmeciyan, Melisa Natali; Sennaroğlu, Alphan; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 23851
    We report, for the first time to our knowledge, femtosecond pulse generation from a graphene mode-locked Cr:ZnSe laser at 2500 nm. To minimize the insertion losses at the lasing wavelength, high-quality monolayer graphene transferred on a CaF2 substrate was used in the experiments. Once mode-locking was initiated, the laser generated a stable train of 226 fs pulses with a time-bandwidth product of 0.39. The mode-locked laser operated at a pulse repetition rate of 77 MHz and produced 80 mW output power with an incident pump power of 1.6 W. To our knowledge, this is the longest laser wavelength at which graphene-based passive mode-locking has been demonstrated to date.
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    PublicationOpen Access
    Light-efficient augmented reality display with steerable eyebox
    (Optical Society of America (OSA), 2019) Department of Electrical and Electronics Engineering; Hedili, M. Kıvanç; Soner, Burak; Ulusoy, Erdem; Ürey, Hakan; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; N/A; N/A; 111927; 8579
    We present a novel head-mounted display setup that uses the pinhole imaging principle coupled with a low-latency dynamic pupil follower. A transmissive LCD is illuminated by a single LED backlight. LED illumination is focused onto the viewer's pupil to form an eyebox smaller than the average human pupil, thereby creating a pinhole display effect where objects at all distances appear in focus. Since nearly all the light is directed to the viewer's pupil, a single low-power LED for each primary color with 0.42 lumens total output is sufficient to create a bright and full-color display of 360 cd/m(2) luminance. In order to follow the viewer's pupil, the eyebox needs to be steerable. We achieved a dynamic eyebox using an array of LEDs that is coupled with a real-time pupil tracker. The entire system is operated at 11 msec motion-to-photon latency, which meets the demanding requirements of the real-time pupil follower system. Experimental results effectively demonstrated our head-mounted pinhole display with 37 degrees FOV and very high light efficiency, equipped with a pupil follower with low motion-to-photon latency.
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    PublicationOpen Access
    Integrated CMOS-compatible Q-switched mode-locked lasers at 1900nm with an on-chip artificial saturable absorber
    (Optical Society of America (OSA), 2019) Shtyrkova, Katia; Callahan, Patrick T.; Li, Nanxi; Ruocco, Alfonso; Vermeulen, Diedrik; Kaertner, Franz X.; Watts, Michael R.; Ippen, Erich P.; Department of Electrical and Electronics Engineering; Mağden, Emir Salih; Department of Electrical and Electronics Engineering; College of Engineering; 276368
    We present a CMOS-compatible. Q-switched mode-locked integrated laser operating at 1.9 mu m with a compact footprint of 23.6 x 0.6 x 0.78mm. The Q-switching rate is 720 kHz, the mode-locking rate is 1.2 GHz, and the optical bandwidth is 17nm, which is sufficient to support pulses as short as 215 fs. The laser is fabricated using a silicon nitride on silicon dioxide 300-mm wafer platform, with thulium-doped Al2O3 glass as a gain material deposited over the silicon photonics chip. An integrated Kerr-nonlinearity-based artificial saturable absorber is implemented in silicon nitride. A broadband (over 100 nm) dispersion-compensating grating in silicon nitride provides sufficient anomalous dispersion to compensate for the normal dispersion of the other laser components, enabling femtosecond-level pulses. The laser has no off-chip components with the exception of the optical pump, allowing for easy co-integration of numerous other photonic devices such as supercontinuum generation and frequency doublers which together potentially enable fully on-chip frequency comb generation.
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    PublicationOpen Access
    Pupil steering holographic display for pre-operative vision screening of cataracts
    (Optica Publishing Group, 2021) Ulusoy, Erdem; Department of Electrical and Electronics Engineering; Kavaklı, Koray; Aydındoğan, Güneş; Kesim, Cem; Hasanreisoğlu, Murat; Şahin, Afsun; Ürey, Hakan; Teaching Faculty; Faculty Member; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; School of Medicine; College of Engineering; Koç University Hospital; N/A; N/A; N/A; 182001; 171267; 8579
    Cataract is the most common cause of preventable blindness and vision loss where the only treatment is surgical replacement of the natural lens with an intraocular lens. Computergenerated holography (CGH) enables to control phase, size, and shape of the light beam entering through the eye-pupil. We developed a holographic vision simulator to assess visual acuity for patients to experience the postoperative corrected vision before going through surgery. A holographically shaped light beam is directed onto the retina using small non-cataractous regions of the lens with the help of a pupil tracker. A Snellen chart hologram is shown to subjects at desired depth with myopia and hyperopia correction. Tests with 13 patients demonstrated substantial improvements in visual acuity and the simulator results are consistent with the post-operative vision tests. Holographic simulator overperforms the existing vision simulators, which are limited to static pinhole exit pupils and incapable of correcting aberrations.
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    PublicationOpen Access
    Temperature dependence of Alexandrite effective emission cross section and small signal gain over the 25-450 degrees C range
    (Optical Society of America (OSA), 2019) Demirbaş, Ümit; Kaertner, Franz X.; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851
    We present detailed measurements of effective emission cross section spectra of the Alexandrite gain medium in the 25-450 degrees C temperature range and provide analytic formulas that can be used to match the measured spectra. The measurement results have been used to investigate the wavelength and temperature dependence of small signal gain, as well as gain bandwidth relevant for ultrafast pulse generation/amplification. We show that the estimated laser performance based on the measured spectroscopic data provides a good fit to the results in the literature. We further discuss the need for a detailed measurement of excited-state absorption cross section in future studies.
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    PublicationOpen Access
    Bidirectional optical neuromodulation using capacitive charge-transfer
    (The Optical Society (OSA) Publishing, 2020) Department of Electrical and Electronics Engineering; N/A; Department of Chemical and Biological Engineering; Department of Molecular Biology and Genetics; Melikov, Rustamzhon; Srivastava, Shashi Bhushan; Karatüm, Onuralp; Nizamoğlu, Sedat; Doğru-Yüksel, Itır Bakış; Dikbaş, Uğur Meriç; Kavaklı, İbrahim Halil; PhD Student; Researcher; PhD Student; Faculty Member; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Department of Chemical and Biological Engineering; Department of Molecular Biology and Genetics; Graduate School of Sciences and Engineering; College of Engineering; College of Sciences; N/A; N/A; N/A; 130295; N/A; N/A; 40319
    Artificial control of neural activity allows for understanding complex neural networks and improving therapy of neurological disorders. Here, we demonstrate that utilization of photovoltaic biointerfaces combined with light waveform shaping can generate safe capacitive currents for bidirectional modulation of neurons. The differential photoresponse of the biointerface due to double layer capacitance facilitates the direction control of capacitive currents depending on the slope of light intensity. Moreover, the strength of capacitive currents is controlled by changing the rise and fall time slope of light intensity. This approach allows for high-level control of the hyperpolarization and depolarization of membrane potential at single-cell level. Our results pave the way toward advanced bioelectronic functionalities for wireless and safe control of neural activity.
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    PublicationOpen Access
    Energy scaling of a carbon nanotube saturable absorber mode-locked femtosecond bulk laser
    (Optical Society of America (OSA), 2012) Ozharar, S.; Choi, S. Y.; Kim, K.; Rotermund, F.; Griebner, U.; Petrov, V.; Department of Physics; N/A; Toker, Işınsu Baylam; Çankaya, Hüseyin; Sennaroğlu, Alphan; Researcher; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 23851
    We report successful energy scaling of a room-temperature femtosecond Cr4+: forsterite laser by using a single-walled carbon nanotube saturable absorber (SWCNT-SA). By incorporating a q-preserving multipass cavity, a repetition rate of 4.51 MHz was realized, and the oscillator produced 121 fs, 10 nJ pulses at 1247 nm, with an average output power of 46 mW. To the best of our knowledge, the peak power of 84 kW is the highest generated to date from a SWCNT-SA mode-locked oscillator. Furthermore, energy scaling of a femtosecond multipass-cavity laser, mode-locked using a SWCNT-SA, is demonstrated for the first time.