Browsing by Subject "Laser"
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Item Open Access Characteristic equations for the lasing Modes of infinite periodic chain of quantum wires(IEEE, 2008-06) Byelobrov, V. O.; Benson, T. M.; Altıntaş, Ayhan; Nosich, A.I.In this paper, we study the lasing modes of a periodic open optical resonator. The resonator is an infinite chain of active circular cylindrical quantum wires standing in tree space. Characteristic equations for the frequencies and associated linear thresholds of lasing are derived. These quantities are considered as eigenvalues of specific electromagnetic-field problem with "active" imaginary part of the cylinder material's refractive index - Lasing Eigenvalue Problem (LEP). ©2008 IEEE.Item Open Access Coreless fiber‐based whispering‐gallery‐mode assisted lasing from colloidal quantum well solids(Wiley-VCH Verlag, 2020-01) Sak, Mustafa; Taghipour, Nima; Delikanlı, Savaş; Shendre, S.; Tanrıöver, İbrahim; Gao, Y.; Yu, J.; Yanyan, Z.; Yoo, S.; Dang, C.; Demir, Hilmi Volkan; Foroutan, SinaWhispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm−2 and ≈1.39 mJ cm−2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm−1 and a high net modal gain coefficient of ≈485 cm−1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.Item Open Access Defect structures in metallic photonic crystals(A I P Publishing LLC, 1996-12-16) Özbay, Ekmel; Temelkuran, B.; Sigalas, M.; Tuttle, G.; Soukoulis, C. M.; Ho, K. M.We have investigated metallic photonic crystals built around a layer‐by‐layer geometry. Two different crystal structures (face‐centered‐tetragonal and tetragonal) were built and their properties were compared. We obtained rejection rates of 7–8 dB per layer from both metallic crystals. Defect modes created by removing rods resulted in high peak transmission (80%), and high quality factors (1740). Our measurements were in good agreement with theoretical simulations.Item Open Access Demonstration of a cavity-enhanced optical parametric chirped-pulse amplification system(Optical Society of America, 2011-03-28) Siddiqui, A.; Hong, K. H.; Moses, J.; Chen, J.; Ilday, F. O.; Kartner, F. X.The use of a low finesse enhancement cavity resonant with a low average power (< 1W) and a high repetition rate (78MHz) pump source is shown to achieve 55% conversion efficiency into signal and idler from the coupled pump in an optical parametric process, whereas an equivalent amount of pump power in a single-pass configuration leads to negligible conversion. Careful comparison of the intracavity conversion process to the single-pass case is performed to assess the underlying impedance matching that yields the high conversion results.Item Open Access Effects of laser ablated silver nanoparticles on Lemna minor(Elsevier, 2014) Üçüncü, E.; Özkan, A. D.; Kurşungöz, C.; Ülger, Z. E.; Ölmez, T. T.; Tekinay, T.; Ortaç, B.; Tunca E.Item Open Access Excitonics of colloidal nanocrystals for next-generation optoelectronics(Bilkent University, 2016-05) Güzeltürk, BurakItem Open Access Experimental realization of a high-contrast grating based broadband quarter-wave plate(Optical Society of America, 2012-12-03) Mutlu, M.; Akosman, A. E.; Kurt, G.; Gokkavas, M.; Özbay, EkmelFabrication and experimental characterization of a broadband quarter-wave plate, which is based on two-dimensional and binary silicon high-contrast gratings, are reported. The quarter-wave plate feature is achieved by the utilization of a regime, in which the proposed grating structure exhibits nearly total and approximately equal transmission of transverse electric and transverse magnetic waves with a phase difference of approximately pi/2. The numerical and experimental results suggest a percent bandwidth of 42% and 33%, respectively, if the operation regime is defined as the range for which the conversion efficiency is higher than 0.9. A compact circular polarizer can be implemented by combining the grating with a linear polarizer. (C) 2012 Optical Society of AmericaItem Open Access Femtosecond pulse generation with voltage-controlled graphene saturable absorber(Optical Society of America, 2014) Baylam, M. N.; Cizmeciyan, S.; Ozharar, S.; Polat, E. O.; Kocabas, C.; Sennaroglu, A.We report, for the first time to our knowledge, the demonstration of a graphene supercapacitor as a voltage-controlled saturable absorber for femtosecond pulse generation from a solid-state laser. By applying only a few volts of bias, the Fermi level of the device could be shifted to vary the insertion loss, while maintaining a sufficient level of saturable absorption to initiate mode-locked operation. The graphene supercapacitor was operated at bias voltages of 0.5-1V to generate sub-100 fs pulses at a pulse repetition rate of 4.51 MHz from a multipass-cavity Cr4+:forsterite laser operating at 1255 nm. The nonlinear optical response of the graphene supercapacitor was further investigated by using pump-probe spectroscopy. (C) 2014 Optical Society of AmericaItem Open Access Generation of InN nanocrystals in organic solution through laser ablation of high pressure chemical vapor deposition-grown InN thin film(Springer, 2012-07-27) Alkis, S.; Alevli, M.; Burzhuev, S.; Vural, H. A.; Okyay, Ali Kemal; Ortaç, B.We report the synthesis of colloidal InN nanocrystals (InN-NCs) in organic solution through nanosecond pulsed laser ablation of high pressure chemical vapor deposition-grown InN thin film on GaN/sapphire template substrate. The size, the structural, the optical, and the chemical characteristics of InN-NCs demonstrate that the colloidal InN crystalline nanostructures in ethanol are synthesized with spherical shape within 5.9-25.3, 5.45-34.8, 3.24-36 nm particle-size distributions, increasing the pulse energy value. The colloidal InN-NCs solutions present strong absorption edge tailoring from NIR region to UV region.Item Open Access Generation of picosecond pulses directly from a 100 W, burst-mode, doping-managed Yb-doped fiber amplifier(Optical Society of America, 2014) Elahi, P.; Yilmaz, S.; Eldeniz, Y. B.; Ilday, F. O.Burst-mode laser systems offer increased effectiveness in material processing while requiring lower individual pulse energies. Fiber amplifiers operating in this regime generate low powers in the order of 1 W. We present a Yb-doped fiber amplifier, utilizing doping management, that scales the average power up to 100 W. The laser system produces bursts at 1 MHz, where each burst comprises 10 pulses with 10 mu J energy per pulse and is separated in time by 10 ns. The high-burst repetition rate allows substantial simplification of the setup over previous demonstrations of burst-mode operation in fiber lasers. The total energy in each burst is 100 mu J and the average power achieved within the burst is 1 kW. The pulse evolution in the final stage of amplification is initiated as self-similar amplification, which is quickly altered as the pulse spectrum exceeds the gain bandwidth. By prechirping the pulses launched into the amplifier, 17 ps long pulses are generated without using external pulse compression. The peak power of the pulses is similar to 0.6 MW. (C) 2014 Optical Society of AmericaItem Open Access Highly directional enhanced radiation from sources embedded inside three-dimensional photonic crystals(Optical Society of America, 2005) Caglayan, H.; Bulu, I.; Özbay, EkmelWe have experimentally studied emission of microwave radiation from a monopole source embedded in a three-dimensional photonic crystal. We have demonstrated enhancement of microwave radiation at the band edge and cavity mode frequencies. Furthermore, we have shown that it is possible to obtain highly directive microwave radiation sources operating at the band edge of the three-dimensional photonic crystal. We have measured half power beam widths of 13 degrees for both E and H planes, corresponding to a maximum directivity of 245. (c) 2005 Optical Society of America.Item Open Access Lasing action in single subwavelength particles supporting supercavity modes(American Chemical Society, 2020-05) Mylnikov, V.; Ha, S. T.; Pan, Z.; Valuckas, V.; Paniagua-Domínguez, R.; Demir, Hilmi Volkan; Kuznetsov, A. I.On-chip light sources are critical for the realization of fully integrated photonic circuitry. So far, semiconductor miniaturized lasers have been mainly limited to sizes on the order of a few microns. Further reduction of sizes is challenging fundamentally due to the associated radiative losses. While using plasmonic metals helps to reduce radiative losses and sizes, they also introduce Ohmic losses hindering real improvements. In this work, we show that, making use of quasibound states in the continuum, or supercavity modes, we circumvent these fundamental issues and realize one of the smallest purely semiconductor nanolasers thus far. Here, the nanolaser structure is based on a single semiconductor nanocylinder that intentionally takes advantage of the destructive interference between two supported optical modes, namely Fabry–Perot and Mie modes, to obtain a significant enhancement in the quality factor of the cavity. We experimentally demonstrate the concept and obtain optically pumped lasing action using GaAs at cryogenic temperatures. The optimal nanocylinder size is as small as 500 nm in diameter and only 330 nm in height with a lasing wavelength around 825 nm, corresponding to a size-to-wavelength ratio as low as 0.6.Item Open Access Lasing modes of infinite periodic chain of quantum wires(IEEE, 2009-06-07) Byelobrov, V. O.; Benson, T. M.; Sewell, P.; Altıntaş, Ayhan; Nosich, A. I.In this paper, we study the scattering and eigenvalue problems for a periodic open optical resonator that is an infinite chain of active circular cylindrical quantum wires standing in free space. The scattering problem is solved by the method of partial separation of variables. The eigenvalue problem differs from the first one by the absence of the incident field and presence of "active properties" of cylinders and yields the frequencies and thresholds of lasing. ©2009 IEEE.Item Open Access Micro tool design and fabrication: a review(Elsevier, 2018) Oliaei, S. N. B.; Karpat, Yiğit; Davim, J. P.; Perveen, A.Mechanical micromachining is considered as a cost-effective and efficient fabrication technique to produce three dimensional features and free-form surfaces from various engineering materials. Micro cutting tools are an essential part of mechanical micromachining and they are exposed to harsh conditions which reduces tool life and adversely affect the economics of the process. The challenge is therefore to maintain the tool rigidity and cutting edge sharpness for extended period of time. Thus, the design, fabrication and durability of micro cutting tools are of significant importance for successful micromachining operations. This review paper aims to provide a comprehensive understanding about the capabilities, characteristics, and limitations of different fabrication techniques used in the manufacturing of micro cutting tools. State-of-the-art micro cutting tool design and coating technology has been presented for various micromachining applications. Possible future research direction and development in the field of micro tool design and fabrication has also been discussed.Item Open Access Mid-infrared quantum cascade laser(Bilkent University, 2017-05) Bozok, BerkayMid-infrared quantum cascade laser technology has been developed rapidly since it was demonstrated in 1994. Since quantum cascade laser technology is small, robust and efficient, they have become commonly used mid-infrared laser source for various applications. The particular application fields of quantum cascade lasers are free space communication, chemical spectroscopy, environmental monitoring and infrared countermeasure. This work presents a comprehensive study over mid-infrared quantum cascade laser including review of theoretical background, theoretical analyses, active layer design, quantum mechanical, thermal and optical simulations, device fabrication, optimization and experimental characterization. As a result of this work mid-infrared quantum cascade laser have been successfully demonstrated.Item Open Access Miniaturized fiber optic transmission system for magnetic resonance imaging signals(Bilkent University, 2005) Memiş, Ömer GökalpIn conventional Magnetic Resonance Imaging (MRI) instruments, after echo signals are received by an MRI coil, they are transmitted through an ultra low noise transmission system consisting of electrical cables. Although this design proved to be effective over the years, there are recent developments in the MRI technology which require a better, more sophisticated design. One of these emerging technologies is “parallel imaging”, where total size of interconnections is the primary problem, and the other is “interventional MRI”, where safety needs to be improved. The Miniature Fiber Optic Transmission System was developed to serve these needs. The system consists of a receiver MRI coil with passive detuning, a two-stage low-noise preamplifier and a low-noise laser diode connected to a photodetector with fiber optic cable in between. The overall noise figure of the system is measured to be lower than 1 dB which guarantees that total signal-to-noise ratio (SNR) reduction in the images due to optical MRI signal transmission system is less than 15%. Total power consumption is 50mW and the device is switchable by another fiber optic line, which can also control active detuning circuit if it is present. A prototype device was tested in a GE 1.5 Tesla MRI instrument and several images were acquired with a slight SNR drop, due to problems with passive detuning. We believe that this design will significantly reduce the size of parallel imaging arrays and enable placement of internal coils into body cavities without providing any safety hazard to the patient, such as electrical shock or burns.Item Open Access Nano patterning of AISI 316L stainless steel with nonlinear laser lithography: sliding under dry and oil-lubricated conditions(Elsevier Ltd, 2016) Gnilitskyi, I.; Rotundo, F.; Martini, C.; Pavlov, I.; Ilday, S.; Vovk, E.; Ilday, F. Ö.; Orazi, L.Femtosecond laser-based Nonlinear Laser Lithography (NLL) was applied to AISI 316L stainless steel, which requires surface modification to achieve satisfactory tribological behaviour. NLL advances over the well-known Laser Induced Periodic Surface Structures (LIPSS) in terms of uniformity and long-range order of high speeds, over large areas. A galvanometric scanner head was used for an high production rate. Dry and lubricated sliding tests, considering different orientations of the nanotexture showed that COF values after NLL treatment are significantly lower. In lubricated tests, COF values of NLL-treated surfaces are nearly half the values of untreated surfaces, whereas the difference further increases when measured in dry conditions, where the orientation of the surface texturing influences the results.Item Open Access Performance evaluation of fiber-based ballistic composites against laser threats(Elsevier, 2019) Candan, C.; Seymen, A. Aytaç; Karatutlu, Ali; Tiken, M.; Midilli, Yakup; Orhan, E.; Berberoğlu, H.; Ortaç, BülendThe interaction between a laser and a composite material has been an intense subject within the past decade and become an emerging field for the defense and manufacturing industry since high-power lasers were initiated to be utilized for the directed-beam applications. In this study, a specially developed composite material for the ballistic applications was shined to a continuous wave (CW) laser beam at 915 nm. The ballistic material was composed of 77 layers of the single sheet of the SR-3136 by Spectra Shield® from Honeywell consisting of ultra-high molecular weight polyethylene (UHMW-PE) fibers reinforced with low-density PE (LD-PE) fibers and a polyurethane-based thermoplastic resin. At the instant of the exposure, the region of interest was completely evaporated and punctured with a slight swelling around the hole where the temperature was over 450 °C. The composite material was drilled completely upon exceeding 20 kJ of laser energy. The chemical and physical changes on the composite material after the laser exposure were extensively studied by a combination of techniques including High-Resolution Scanning Electron Microscopy (HR-SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). The physical properties of a single layer of the SR-3136 were also studied using HR-SEM, UV-VIS-NIR Absorption Spectroscopy, Thermogravimetric analysis (TGA), Differential Scanning Calorimetry (DSC) and XPS. The research presented here reveals the first study on the effects of the high-power laser beam irradiance on the fiber-reinforced composite materials utilized for the ballistic protection.Item Open Access Physics and applications of photonic nanocrystals(Inderscience Publishers, 2004) Özbay, Ekmel; Güven, Kaan; Aydın, Koray; Bayındır, MehmetPhotonic nanocrystals are periodic dielectric or metallic structures having photonic bands in analogy to electronic bands of semiconductors. The presence of photonic band-gaps, where the propagation of photons of certain frequencies is prohibited, and the variety of photon dispersions give rise to novel and unusual optical phenomena. Consequently, photonic crystals are now envisaged as an essential building block of future photonic devices. This paper aims to provide a review of contemporary developments on the physics and applications of photonic crystals with an emphasis on optical properties of coupled microcavity waveguides and on the negative refraction phenomenon. The enhancement of spontaneous emission in a silicon nitride photonic nanocrystal is investigated in detail. Both the negative refraction of a Gaussian beam and the focusing of a microwave point source through a photonic crystal slab with subwavelength resolution are studied experimentally.Item Open Access Three dimensional processing of silicon with pulsed lasers for optical applications(Bilkent University, 2015-05) Turnalı, AhmetMicromachining of silicon with lasers is being investigated for the last three decades. Until now, interior silicon modification without inscribing the surface has not resulted in success. Such an ability could enable disruptive technologies in nanophotonics by paving the way for producing monolithic optoelectronic chips. Here, we report a maskless, one step photo-induced method to generate subsurface modifications in silicon with pulsed infrared lasers for indefinitely large areas. We demonstrate continuous, highly controllable structures buried in the bulk of silicon wafers and investigate the underlying mechanism. Further, we utilize the method for spatial information encoding and fabrication of optical components in the infrared regime in silicon. This silicon processing technology can be useful in various applications, including multilayer silicon chips, solar cells and opto uidics.