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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Item Metadata only Dissipative phase transition in an open Tavis-Cummings model with a two-photon drive(IOP Publishing Ltd, 2023) 0000-0002-9134-3951; Adhikary, Kingshuk; Deb, Bimalendu; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; College of Sciences; 1674We investigate the steady-state quantum phases and associated collective phenomena of an open Tavis-Cummings (TC) model driven by a two-photon source. The standard (non-dissipative) TC model describes the quantum phases of an ensemble of N ( q ) two-level quantum emitters interacting with a single-mode electromagnetic field. The interplay among coherent driving, dissipation, and dipole interactions of the open TC model results in emergent collective phenomena, leading to a dissipative or non-equilibrium phase transition from the normal to the superradiant phase. We solve the Liouvillian equation analytically using the semi-classical mean-field approximation. We carry out stability analysis of the steady-state phases and determine the phase boundary. The use of Holstein-Primakoff transformation in the thermodynamic limit Nq & RARR;& INFIN;Item Metadata only Continuous-variable polarization mode entanglement in a V-type micromaser(Elsevier, 2023) 0000-0002-9134-3951; Mousavitaha, Kowsar Al-Sadat; Faizi, Esfandyar; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; College of Sciences; 1674The micromaser is an archetype experimental setting where a beam of excited two-level atoms is injected into a high-finesse cavity. It has played a pivotal role as a testbed for predictions of quantum optics. We consider a generalized micromaser setting consisting of a high-quality cavity pumped by a beam of three -level atoms. The atoms are assumed to be prepared to carry quantum coherence between their excited state doublet. Our objective is to produce quantum entanglement between the right-handed circular (RHC) and left-handed circular (LHC) polarized photons in the cavity, exploiting the quantum coherence in the pump atoms. For that aim, we derive the generalized micromaser master equation for our system. We find that the dynamics of the micromaser field driven by the pump beam is equivalent to two non-interacting RHC and LHC photonic systems sharing a common non-equilibrium environment. The effect of the shared bath is to mediate an incoherent interaction between the otherwise non-interacting cavity photons, which emerges only if the atoms carry quantum coherence. We take into account cavity losses as a source of quantum decoherence and characterize the quantum entanglement between the LHC and RHC polarized photons in terms of logarithmic negativity, Hillery-Zubairy and spin squeezing criterion, calculated using the dynamical solution of the master equation. We show that, in the same parameter regime, one of the criteria shows entanglement while for the other never detects entanglement. Our results reveal that LHC and RHC polarized photons can be entangled in the transient regime according to the logarithmic negativity criterion.Item Metadata only Broadly tunable continuous wave 2.3-μm Tm3+:tellurite bulk glass laser(Optica Publishing Group, 2023) 0000-0003-4391-0189; 0000-0003-3446-2062; 0009-0009-7375-3090; Denker, Boris; Galagan, Boris; Sverchkov, Sergei; Department of Physics; Department of Physics; N/A; Sennaroğlu, Alphan; Morova, Yağız; Ardalı, Emir; Faculty Member; Researcher; Master Student; 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; 23851; N/A; N/AWe report, for the first time to the best of our knowledge, a continuous wave trivalent thulium ion (Tm3+)-doped bulk glass near 2.3 mu m. In the experiments, a bulk Tm3+- doped tellurite glass with the stoichiometric composition of 74TeO2-12ZnO-4La2O3-10Na2O (Tm3+:TZLN) was used. Lasing operation was achieved by using an x-fold cavity at the free-running wavelength of 2303 nm. The maximum slope efficiency of 6.2% was obtained with respect to the absorbed pump power with a 1% transmitting output coupler. In this case, as high as 100-mW output power was generated with 2.2 W of absorbed pump power. Continuous, broad tuning was achieved from 2233 nm to 2400 nm. The excitation spectrum of the laser was also investigated and 2.3-mu m lasing was obtained by varying the pump wavelength over the 773-809-nm range. The absorption cross section was determined to be 4.4 x 10-21 cm2, based on open-aperture z-scan measurement. By using the laser efficiency data, the emission cross section of the Tm3+:TZLN glass was further determined to be 1.3 x 10-20 cm2 at 2.3 mu m.(c) 2023 Optica Publishing GroupItem Metadata only Self-Q-switched and widely tunable continuous-wave operation of a Tm3+: Lu2O3 ceramic laser near 2 μm(Elsevier, 2023) 0000-0003-4391-0189; 0000-0003-1164-1973; 0000-0003-3446-2062; N/A; Department of Physics; Department of Chemistry; Department of Physics; N/A; Sennaroğlu, Alphan; Aydemir, Umut; Morova, Yağız; içli, Suat; Faculty Member; Faculty Member; Researcher; PhD Student; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); College of Sciences; College of Sciences; College of Sciences; Graduate School of Sciences and Engineering; 23851; 58403; N/A; N/AWe report, for the first time to our knowledge, self-Q-switched (SQS) operation of a Tm3+:Lu2O3 ceramic laser near 2 mu m based on the 3F4 -> 3H6 optical transition. By careful optimization of cavity focusing, SQS operation could be achieved at several curved mirror separations without any external modulator and the laser delivered 9.7-mu s pulses at the wavelength of 2091 nm with a pulse repetition rate of 26 kHz. In the experiments, a tight-focusing resonator design was further employed to achieve low-threshold continuous-wave (CW) operation of the laser with lasing threshold pump powers as low as 33 mW. With an incident pump power of 590 mW at 776 nm, a maximum output power of 164 mW was obtained at 2097 nm with a slope efficiency of 30% with respect to the incident pump power. The tuning range of the CW Tm3+:Lu2O3 ceramic laser could be extended up to the wavelength of 2235 nm.Item Metadata only Terahertz wavefront engineering using a hard-coded metasurface(Springer, 2023) 0000-0002-1097-5106; Noori, Aileen; Akyurek, Bora; Demirhan, Yasemin; Ozyuzer, Lutfi; Altan, Hakan; Aygun, Gulnur; Department of Physics; Güven, Kaan; Faculty Member; College of Sciences; 52290During the past few years, coding metamaterials (MM) drew significant attention, where the far-field scattering/transmission pattern of the electromagnetic wave (particularly in the THz regime) can be encoded into a single or few-bit digitized phase-response of the metasurface, thereby enabling a full digital control. Single-bit MMs contain two types of unit cells where the phase becomes 0 and 1 (in units of pi ), respectively. By arranging these unit cells into a 2D surface pattern, the THz wavefront can be shaped. In this work, a novel hard-coded metasurface was designed, fabricated, and experimentally investigated for multi-beam reflection of incident THz beam. The design employs stripe and checkerboard patterns of bilayer MM unit cells consisting of square gold patches with a polymer spacing layer from a gold backplane. Experimental and simulation results show that the incident wave in the 0.500-0.750 THz range can be reflected with > 95% efficiency in uniform amplitude and 1-bit coded phase. For the checkerboard metasurface pattern, the measured and analytically calculated reflection angle shows good agreement. The metasurface design is suitable for large-scale fabrication and can potentially be used as a template in the development of actively coded metasurfaces.Item Metadata only Tapered tip optical fibers for measuring ultra-small refractive index changes with record high sensitivity(Optica Publishing Group, 2022) 0000-0003-1727-8239; Lu, Chunyu; Nikbakht, Hamed Yusuf; van Someren, Bob; Akca, B. Imran; Department of Physics; Erdolu, Mert Yusuf; Undergraduate Student; College of SciencesHere we demonstrate an inexpensive, simple, and ultra-sensitive refractive index sensor based on a tapered tip optical fiber combined with a straightforward image analysis method. The output profile of this fiber exhibits circular fringe patterns whose intensity distribution dramatically changes even with ultra-small refractive index variations in the surrounding medium. The sensitivity of the fiber sensor is measured using different concentrations of saline solutions with a transmission setup consisting of a single wavelength light source, a cuvette, an objective lens, and a camera. By analyzing the areal changes in the center of the fringe patterns for each saline solution, we obtain an unprecedented sensitivity value of 24,160 dB/RIU (refractive index unit), which is the highest value reported so far among intensity-modulated fiber refractometers. The resolution of the sensor is calculated to be 6.9 x10(-9). Moreover, we measure the sensitivity of the fiber tip in the backreflection mode using salt-water solutions and obtained a sensitivity value of 620 dB/RIU. This sensor is ultra-sensitive, simple, easy to fabricate, and low-cost, which makes it a promising tool for on-site measurements and point-of-care applications.Publication Metadata only Tackling the focal shift effect for metalenses(IOP Publishing Ltd, 2024) Department of Physics; Department of Physics; Ali, Farhan; Yazdaanpanah, Ramin; Ramazanoğlu, Serap Aksu; Graduate School of Sciences and Engineering; College of SciencesWe present a theoretical analysis aimed at comprehending and mitigating the focal shift phenomenon in planar dielectric metalenses. To conduct this analysis, we introduce metalens designs consisting of silicon and germanium nanoblocks on a calcium fluoride substrate, operating in the mid-IR frequency range. The lensing performance of these metalenses is investigated using the finite-difference time-domain method, and they operate at wavelengths of 3 and 4 mu m with a polarization conversion efficiency close to unity. Our findings indicate a strong correlation between the focal shift phenomena on dielectric metalenses and the numerical aperture (NA), revealing that increasing the Fresnel number is not always an effective approach to minimizing the focal shift. In contrast to previous studies, we define a critical NA, independent of the lens size, where the focal shift reaches a minimum, resulting in a symmetric focal intensity distribution and ultimately yielding a better-performing metalens. We demonstrate that for NAs greater than the determined critical value, a positive focal shift is observed on planar metalenses, diverging from the conventional negative shift predicted by existing models. Additionally, we show that by selecting a metalens within a specific NA range and with smaller diameters, high focusing efficiencies can be achieved. The focusing efficiency of the studied metalenses is measured as high as 70%, marking one of the best values reported for the IR range to date. These results serve as a guide for improving the agreement between experimental and designed metalens features, enhancing their practical applications.Publication Metadata only Localized thermal emission from topological interfaces(American Association for the Advancement of Science, 2024) Ergöktaş, M. Said; Keçebaş, Ali; Despotelis, Konstantinos; Soleymani, Sina; Bakan, Gökhan; Principi, Alessandro; Rotter, Stefan; Özdemir, Şahin K.; Kocabaş, Coşkun; Department of Physics; Department of Physics; Kocabaş, Aşkın; College of SciencesThe control of thermal radiation by shaping its spatial and spectral emission characteristics plays a key role in many areas of science and engineering. Conventional approaches to tailoring thermal emission using metamaterials are hampered both by the limited spatial resolution of the required subwavelength material structures and by the materials' strong absorption in the infrared. In this work, we demonstrate an approach based on the concept of topology. By changing a single parameter of a multilayer coating, we were able to control the reflection topology of a surface, with the critical point of zero reflection being topologically protected. The boundaries between subcritical and supercritical spatial domains host topological interface states with near-unity thermal emissivity. These topological concepts enable unconventional manipulation of thermal light for applications in thermal management and thermal camouflage.Publication Metadata only Geometrical optimization of spin clusters for the preservation of quantum coherence(American Physical Society, 2024) ; Department of Physics; Department of Physics; Gassab, Lea; Pusuluk, Orhan; Müstecaplıoğlu, Özgür Esat; ; Graduate School of Sciences and Engineering; College of Sciences;We investigate the influence of geometry on the preservation of quantum coherence in spin clusters subjected a thermal environment. Assuming weak interspin coupling, we explore the various buffer network configura yons that can be embedded in a plane. Our findings reveal that the connectivity of the buffer network is crucial indetermining the preservation duration of quantum coherence in an individual central spin. Specifically, we observe that the maximal planar graph yields the longest preservation time for a given number of buffer spins. Interestingly, our results demonstrate that the preservation time does not consistently increase with an increasing #umber of buffer spins. Employing a quantum master equation in our simulations, we further demonstrate that a Cetrahedral geometry comprising a four-spin buffer network provides optimal protection against environmental Tects.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.