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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Tackling the focal shift effect for metalenses(IOP Publishing Ltd, 2024) Department of Physics; Ali, Farhan; Yazdaanpanah, Ramin; Ramazanoğlu, Serap Aksu; Department of Physics; 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 Optical modulation with silicon microspheres(IEEE-Inst Electrical Electronics Engineers Inc, 2009) Gürlü, Oğuzhan; N/A; Department of Physics; Yüce, Emre; Serpengüzel, Ali; Master Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; 245435; 27855In this letter, a silicon microsphere coupled to a silica optical fiber half coupler has been characterized for electrooptical modulation in the L-band at 1.55 mu m. Electrooptical modulation of the transmitted and the 90 degrees elastic scattered signals for both the TE and the TM polarizations of the microsphere resonances has been observed.Publication Metadata only Broadly tunable continuous-wave orange-red source based on intracavity-doubled Cr4+: forsterite laser(Optical Soc Amer, 2002) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851The operation of a room-temperature, continuous-wave, intracavity frequency-doubled Cr4+:forsterite laser capable of producing broadly tunable output in the orange-red region of the electromagnetic spectrum is described. Intracavity doubling was achieved in a periodically poled lithium niobate crystal that had gratings with different periods. Tunable second-harmonic output could be obtained between 613 and 655 am. At a wavelength of 630 nm, intracavity doubling yielded as much as 45 mW of continuous-wave output. To the author's knowledge, this represents the highest second-harmonic-power generation obtained to date with a continuous-wave Cr4+:forsterite laser.Publication Metadata only Amorphous silicon nitride microcavities(Optical Soc Amer, 2001) Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Amorphous silicon nitride microcavities were fabricated by use of distributed Bragg reflectors. The distributed Bragg reflectors were fabricated with alternating layers of quarter-wavelength-thick hydrogenated amorphous silicon nitride and amorphous silicon oxide. The spectral peak of the bulk amorphous silicon nitride photoluminescence spectrum was chosen as the microcavity resonance wavelength. The amorphous silicon nitride microcavity enhances the photoluminescence amplitude and reduces the photoluminescence linewidth with respect to the bulk amorphous silicon nitride. This narrowing and enhancement of the photoluminescence can he understood by the redistribution of the density of optical modes owing to the presence of the microcavity. The microcavity narrowing and enhancement of luminescence in hydrogenated amorphous silicon nitride opens up a variety of possibilities for optoelectronic applications such as resonant-cavity-enhanced light-emitting diodes and color flat-panel displays.Publication Metadata only Rectification of SEMG as a tool to demonstrate synchronous motor unit activity during vibration(Elsevier Sci Ltd, 2013) Karacan, İlhan; Çidem, Muharrem; Department of Physics; N/A; Sebik, Oğuz; Türker, Kemal Sıtkı; Researcher; Faculty Member; Department of Physics; College of Sciences; School of Medicine; N/A; 6741The use of surface electromyography (SEMG) in vibration studies is problematic since motion artifacts occupy the same frequency band with the SEMG signal containing information on synchronous motor unit activity. We hypothesize that using a harsher, 80-500 Hz band-pass filter and using rectification can help eliminate motion artifacts and provide a way to observe synchronous motor unit activity that is phase locked to vibration using SEMG recordings only. Multi Motor Unit (MMU) action potentials using intramuscular electrodes along with SEMG were recorded from the gastrocnemius medialis (GM) of six healthy male volunteers. Data were collected during whole body vibration, using vibration frequencies of 30 Hz, 35 Hz, 40 Hz or 50 Hz. A computer simulation was used to investigate the efficacy of filtering under different scenarios: with or without artifacts and/or motor unit synchronization. Our findings indicate that motor unit synchronization took place during WBV as verified by MMU recordings. A harsh filtering regimen along with rectification proved successful in demonstrating motor unit synchronization in SEMG recordings. Our findings were further supported by the results from the computer simulation, which indicated that filtering and rectification was efficient in discriminating motion artifacts from motor unit synchronization. We suggest that the proposed signal processing technique may provide a new methodology to evaluate the effects of vibration treatments using only SEMG. This is a major advantage, as this non-intrusive method is able to overcome movement artifacts and also indicate the synchronization of underlying motor units.Publication Metadata only Design principles of q-preserving multipass-cavity femtosecond lasers(The Optical Society (OSA) Publishing, 2006) Kowalevicz, A.M.; Zare, A.T.; Fujimoto, J.G.; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851We describe a new class of femtosecond laser cavity designs that are based on a Herriott-type multipass cavity (MPC) to effectively increase the length of a standard laser resonator. MPC laser designs can be used to increase the output pulse energies or to make more compact resonator configurations. A general theory for MPC lasers is developed by analyzing a periodic optical system, and the conditions are established for the case in which the q parameter of a Gaussian beam is left invariant after a single transit through the system. On the basis of this analysis, we determine the design criteria for two-mirror q-preserving MPCs. Practical laser cavity choices are presented and their trade-offs are examined. We also discuss various experimental setups that use these novel MPC designs to increase pulse energies while maintaining compact cavities.Publication Metadata only Experimental determination of fractional thermal loading in an operating diode-pumped Nd: YVO4 minilaser at 1064 nm(Optical Soc Amer, 1999) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851A practical in, situ method is described and used for determination of the fractional thermal-loading parameter eta(h) in an operating diode-pumped Nd:YVO4 minilaser at 1064 nm. Readily applicable to the thermal characterization of other solid-state media, the method is based on the fact that thermally induced lensing will cause the laser oscillation to be quenched at a critical pump power whose magnitude depends on the cavity configuration, thermo-optical properties of the gain medium, and, in particular, on the value of eta(h). In the experiments described here, a 0.5-mm-long coated Nd:YVO4 crystal with 3-at. % Nd concentration was used to construct the diode-pumped laser with a flat highly reflecting end mirror and an intracavity lens. For the method to be effective, the resonator was set up close to the edge of the stability range. Above the oscillation threshold, the pump power at which lasing was quenched because of the onset of the thermally induced resonator instability was measured as a function of the intracavity lens position. A numerical model that accounted for absorption saturation and pump-induced thermal lensing was then used to analyze the experimentally measured data with eta(h) as an adjustable parameter. The average best-fit value of eta(h) was determined to be 0.40 with an estimated statistical variation of 8%.Publication Metadata only Optimum crystal parameters for room-temperature Cr4+: forsterite lasers: experiment and theory(Elsevier, 2000) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851Room-temperature Cr4+:forsterite laser experience deterioration in continuous-wave power performance due to thermal loading caused by the temperature-dependent fluorescence lifetime and the low,heat conductivity of the host. The study presented in this paper uses a numerical model to analyze the experimental threshold and efficiency data of Cr4+:forsterite lasers by accounting for pump-induced thermal gradients, the temperature: dependence of the fluorescence lifetime, absorption saturation at the pump wavelength, and excited-state absorption at-the lasing wavelength. Very good agreement was obtained between theoretically predicted trends and the experimental data. The best-fit values of the stimulated emission cross-section and the excited-state absorption cross-section were determined to be 1.16 X 10(-19) cm(2) and 0.18 X 10(-19) cm(2), respectively. Optimization studies further predict that at an incident pump power of 8 W, a 2-cm-long Cr4+:forsterite crystal with a differential absorption coefficient of 0.31 cm(-1) should produce the highest continuous-wave output around room temperature.Publication Metadata only Silicon microsphere whispering gallery modes excited by femtosecond-laser-inscribed glass waveguides(Optical Soc Amer, 2018) Sotillo, Belen; Bharadwaj, Vibhav; Ramponi, Roberta; Eaton, Shane Michael; N/A; N/A; Department of Physics; Çirkinoğlu, Hüseyin Ozan; Bayer, Mustafa Mert; Gökay, Ulaş Sabahattin; Serpengüzel, Ali; Master Student; Master Student; PhD Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 27855We report on the coupling of whispering gallery modes in a 500-mu m-radius silicon microsphere to a femtosecond-laser-inscribed glass optical waveguide. The shallow glass waveguide with a large mode field diameter in the near-infrared is written at a depth of 25 mu m below the glass surface, resulting in a high excitation impact parameter of 525 mu m for the microsphere. The excited whispering gallery modes of the silicon microsphere have quality factors of approximately 10(5) in the 90 degrees elastic scattering and 0 degrees transmission. Integration of such spherical silicon microresonators on femtosecond-laser-inscribed glass waveguides is promising for photonic communication, computation, and sensing applications. (C) 2018 Optical Society of America.Publication Metadata only Passive sorting of emulsion droplets with different interfacial properties using laser-patterned surfaces(Springer Heidelberg, 2019) Erten, Ahmet; Jonas, Alexandr; N/A; Department of Physics; Department of Mechanical Engineering; Department of Physics; Rashid, Muhammed Zeeshan; Morova, Berna; Muradoğlu, Metin; Kiraz, Alper; PhD Student; Researcher; Faculty Member; Faculty Member; Department of Mechanical Engineering; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; College of Sciences; N/A; 152935; 22542We demonstrate passive sorting of emulsion microdroplets based on differences in their interfacial tension and contact angle. The sorted droplets are flowing inside a microfluidic channel featuring a shallow guiding track (depth similar to 0.6 mu m) defined by femtosecond laser micromachining in polydimethylsiloxane coating deposited on glass. Under these flow conditions, the droplets experience a confinement force that pulls them into the track; this force depends on the interfacial tension and the difference between the contact angles inside and outside the ablated track. The interplay between the confinement force, fluid drag, and wall friction then determines the trajectory of the droplet along the guiding track. We investigate experimentally the droplet trajectory as a function of droplet velocity and angle between the track and the channel axis and demonstrate precise control of droplet direction by adjusting the track angle. Moreover, we show that droplets of liquids with different interfacial tensions and contact angles travel different distances along the guiding track at a constant flow rate, which can be used for droplet sorting. We develop a theoretical model that incorporates the droplet position with respect to the ablated track, interfacial tension, and contact angles to predict the droplet trajectory under given experimental conditions. Thus, the dynamic behavior of the droplets leading to different guiding scenarios can be studied without the need of computationally expensive fluid dynamics simulations. The presented study paves the way for designing and optimizing new systems for advanced manipulation of droplets of different content using potentially reconfigurable guiding tracks.