Scholarly Publications - NANOTAM

Permanent URI for this collectionhttps://hdl.handle.net/11693/115670

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Open Access
Robustness of GaN on SiC low-noise amplifiers in common source and cascode configurations for X-band applications
(John Wiley & Sons Ltd., 2024-08) Nawaz, Muhammad Imran; Zafar, Salahuddin; Gürdal, Armağan; Akoğlu, Büşra Çankaya
Cascode HEMTs exhibit high gain and broadband performance. Promising reverse transmission makes matching networks simpler and insensitive to impedance on either side of the HEMT. On the other hand, common source (CS) HEMTs with intentional small inductance at the source provide simultaneous match for optimum noise and input impedance. This paper provides a performance comparison of 4 x 50 mu m cascode HEMTs-based low -noise amplifier and 4 x 50 mu m CS HEMTs-based low -noise amplifiers with specific emphasis on robustness, including survivability and reverse recovery time (RRT). Cascode LNA survives an input power of 33 dBm while CS LNA handles 30 dBm power, each having a 1 k Omega passive limiting resistor on the gate bias line. RRT of cascode LNA is also better. Better survivability and RRT for cascode LNA are attributed to its HEMT's stacked configuration. The designs of LNAs are described, along with their small -signal, noise, and large -signal characteristics in the X -band. Cascode LNA has a better input match, while CS LNA has a better output match. Gains are comparable, while CS LNA has better P1dB at higher band edge frequency. The noise figure for both LNAs is less than 1.9 dB, with CS LNA having a slight edge over cascode. This study benefits RF designers in choosing appropriate HEMT topology as per application for designing robust low -noise amplifiers.
Open Access
Solid-mechanics analysis and modeling of the alloyed ohmic contact proximity in GaN HEMTs using μRaman spectroscopy
(Institute of Physics Publishing Ltd., 2024-07-17) Güneş, Burak; Bütün, Bayram; Özbay, Ekmel
This study explores the impact of alloyed ohmic contact separation on ungated GaN high electron mobility transistors (HEMTs) lattice stress by employing Raman spectroscopy and solid mechanics simulations for comprehensive analysis. Focusing on the substantial stresses exerted by ohmic contacts, our research introduces a novel mechanical calibration procedure. The proposed procedure demonstrates that the stress in the GaN buffer can be precisely modelled using Raman measurements taken from patterns of varying length, which in return reveals the impact of ohmic contacts on stress. We show that this technique shows a good alignment to the Raman measurement results. Moreover, we identify ohmic contact edges as potential sites for defect generation due to the accumulation of substantial elastic energy, a finding supported by experimental observations of crack formations in related studies. Our calibrated mechanical model not only enhances the understanding of stress distributions within GaN HEMTs but also lays the groundwork for future improvements in electro-thermo-mechanical simulations.
Open Access
Reprogrammable metasurface design for NIR beam steering and active filtering
(Institute of Physics Publishing Ltd., 2024-07-24) Hajian, Hodjat; Proffit, Matthieu; Özbay, Ekmel; Landais, Pascal; Bradley, A. Louise
Reprogrammable metasurfaces enable active modulation of light at subwavelength scales. Operating in the microwave, terahertz, and mid-infrared ranges, these metasurfaces find applications in communications, sensing, and imaging. Electrically tunable metasurfaces operating in the near-infrared (NIR) range are crucial for light detection and ranging (LiDAR) applications. Achieving a NIR reprogrammable metasurface requires individual gating of nano-antennas, emphasizing effective heat management to preserve device performance. To this end, here we propose an electrically tunable Au-vanadium dioxide (VO2) metasurface design on top of a one-dimensional Si-Al2O3 photonic crystal (PC), positioned on a SiC substrate. Each individual Au-VO2 nano-antenna is switched from an Off to ON state via Joule heating, enabling the programming of the metasurface using 1-bit (binary) control. While operating as a nearly perfect reflector at lambda(0)=1.55 mu m, the materials, thickness, and number of the layers in the PC are carefully chosen to ensure it acts as a thermal metamaterial. Moreover, with high optical efficiency (R similar to 40% at lambda(0)), appropriate thermal performance, and feasibility, the metasurface also enables broadband programmable beam steering in the 1.4-1.7 mu m range for a wide steering angle range. This metasurface design also offers active control over NIR light transmittance, reflectance and absorptance in the wavelength range of 0.75-3 mu m. These characteristics render the device practical for LiDAR and active filtering.
Open Access
Reduction in temperature-dependent fiber-optic gyroscope bias drift by using multifunctional integrated optical chip fabricated on pre-annealed LiNbO₃
(MDPI AG, 2024-12-11) Karagöz, Ercan; Aşık, Fatma Yasemin; Gökkavas, Mutlu; Akbaş, Erkut Emin; Yertutanol, Aylin; Özbay, Ekmel; Özcan, Sadan
The refractive index change obtained after annealed proton exchange (APE) in lithium niobate (LiNbO₃) crystals depends on both the proton exchange process carried out in hot acid and the structure of the crystals. In devices produced by the APE method, dislocations and lattice defects within the crystal structure are considered to be primary contributors to refractive index discontinuities and waveguide instability. In this study, the effects of pre-annealing LiNbO₃ crystals at 500 °C on multifunctional integrated optical chips (MIOCs) were investigated through interferometric fiber-optic gyroscope (IFOG) system-level tests. It was observed that the pre-annealing process resulted in an improvement in the optical throughput of MIOCs (from 34% to 51%) and the temperature-dependent bias drift stability of the IFOG (from 0.031–0.038°/h to 0.012–0.019°/h). The angle random walk (ARW) was measured as 0.0056 deg/√h.
Open Access
Laser nanofabrication inside silicon with spatial beam modulation and anisotropic seeding
(NATURE PORTFOLIO, 2024-07-16) Sabet, Rana Asgari; Ishraq, Aqiq; Saltık, Alperen; Bütün, Mehmet; Tokel, Onur
Nanofabrication in silicon, arguably the most important material for modern technology, has been limited exclusively to its surface. Existing lithography methods cannot penetrate the wafer surface without altering it, whereas emerging laser-based subsurface or in-chip fabrication remains at greater than 1 mu m resolution. In addition, available methods do not allow positioning or modulation with sub-micron precision deep inside the wafer. The fundamental difficulty of breaking these dimensional barriers is two-fold, i.e., complex nonlinear effects inside the wafer and the inherent diffraction limit for laser light. Here, we overcome these challenges by exploiting spatially-modulated laser beams and anisotropic feedback from preformed subsurface structures, to establish controlled nanofabrication capability inside silicon. We demonstrate buried nanostructures of feature sizes down to 100 +/- 20 nm, with subwavelength and multi-dimensional control; thereby improving the state-of-the-art by an order-of-magnitude. In order to showcase the emerging capabilities, we fabricate nanophotonics elements deep inside Si, exemplified by nanogratings with record diffraction efficiency and spectral control. The reported advance is an important step towards 3D nanophotonics systems, micro/nanofluidics, and 3D electronic-photonic integrated systems. The authors report controlled laser nanofabrication inside silicon. The dimensional barrier is overcome by spatially modulated lasers and anisotropic feedback from preformed structures. Features down to 100 nm is achieved, improving the state-of-the-art by an order-of-magnitude.
Open Access
Strongly polarized color conversion of isotropic colloidal quantum dots coupled to fano resonances
(Royal Society of Chemistry, 2024-07-24) Güngör, Kıvanç; Erdem, Onur; Güzeltürk, Burak; Ünal, Emre; Jun, Shinae; Jang, Eunjoo; Demir, Hilmi Volkan
Colloidal quantum dots (QDs) offer high color purity essential to high-quality liquid crystal displays (LCDs), which enables unprecedented levels of color enrichment in LCD-TVs today. However, for LCDs requiring polarized backplane illumination in operation, highly polarized light generation using inherently isotropic QDs remains a fundamental challenge. Here, we show strongly polarized color conversion of isotropic QDs coupled to Fano resonances of v-grooved surfaces compatible with surface-normal LED illumination for next-generation QD-TVs. This architecture overcomes the critically oblique excitation of surface plasmon coupled emission by using v-shapes imprinted on the backlight unit (BLU). With isotropic QDs coated on the proposed v-BLU surface, we experimentally measured a far-field polarization contrast ratio of similar to 10. Full electromagnetic solution shows Fano line-shape transmission in transverse magnetic polarization allowing for high transmission as an indication for forward-scattering configuration. Of these QDs coupled to the surface plasmon-polariton modes, we observed strong modifications in their emission kinetics revealed by time-resolved photoluminescence spectroscopy and via dipole orientations identified by back focal plane imaging. This collection of findings indicates conclusively that these isotropic QDs are forced to radiate in a linearly polarized state from the patterned planar surface under surface-normal excitation. For next-generation QD-TVs, the proposed polarized color-converting isotropic QDs on such v-BLUs can be deployed in bendable electronic displays. We demonstrate a novel backlight unit utilizing the plasmonic interaction of quantum dots with v-shaped metallic grooves, which are capable of enhancing the emission in the desired polarization while suppressing the unwanted polarization component.
Open Access
Improving the temperature stability of mems gyroscope bias with on-chip stress sensors
(IEEE, 2024-04-23) Erkan, Derin; Tatar, Erdinç
Temperature calibration is commonly used to suppress the bias drift of MEMS inertial sensors. Temperature compensation reduces the bias drift but cannot eliminate it. We report a compensation technique for temperature- induced drifts by incorporating temperature and on-chip stress, for the first time. Adding on-chip stress to the temperature captures the offset behavior with hysteresis more accurately. Our open and closed-loop sense mode temperature sweep results demonstrate almost three-fold offset stability improvement over only temperature calibration for a wide (65 degrees C) temperature range. Temperature and stress sensors provide data about thermal stress and stress mismatches in the sensor stack, respectively. We validate the calibration concept with a MEMS ring gyroscope integrated with eight capacitive stress sensors. We perform the temperature tests with an on-PCB heater that only heats the MEMS die and front-end amplifiers.
Open Access
On temperature effects in a mems ring gyroscope
(2024-04-23) Hosseini Pishrobat, Mehran; Erkan, Derin; Tatar, Erdinç
We report on experimental and analytical investigation of temperature effects in a 3.2mm-diameter, 57kHz ring gyroscope equipped with 16 capacitive stress sensors. According to the well-known ~-60ppm/°C temperature dependency of Young’s modulus of silicon, the temperature coefficient of frequency (TCF) is expected to be ~-30ppm/°C. Our experimentally observed TCFs, however, tend to be ~-14ppm/°C, pointing to thermal stresses as the countering factor. To find the root cause of the measured TCFs, we develop an analytical framework that enables us to calculate the temperature-induced stiffness variations, considering both thermal and mechanical strains. The model successfully predicts changes and hysteretic behavior of frequency over temperature using the measured stress and temperature data.
Open Access
Exploring localized ENZ resonances and their role in superscattering, wideband invisibility, and tunable scattering
(Nature Publishing Group, 2024-01-18) Serebryannikov, Andriy E.; Özbay, Ekmel
While the role and manifestations of the localized surface plasmon resonances (LSPRs) in anomalous scattering, like superscattering and invisibility, are quite well explored, the existence, appearance, and possible contribution of localized epsilon-near-zero (ENZ) resonances still invoke careful exploration. In this paper, that is done along with a comparison of the resonances of two types in the case of thin-wall cylinders made of lossy and loss-compensated dispersive materials. It is shown that the localized ENZ resonances exist and appear very close to the zero-permittivity regime, i.e., at near-zero but yet negative permittivity that is similar to the ENZ modes in thin planar films. Near- and far-field characteristics of the superscattering modes are investigated. The results indicate that the scattering regimes arising due to LSPRs and localized ENZ resonances are distinguishable in terms of the basic field features inside and around the scatterer and differ in their contribution to the resulting scattering mechanism, e.g., in terms of the occupied frequency and permittivity ranges as well as the sensitivity to the wall thickness variations. When the losses are either weak or tend to zero due to the doping with gain enabling impurities, the sharp peaks of the scattering cross-section that are yielded by the resonances can be said to be embedded into the otherwise wide invisibility range. In the case of lossy material, a wide and continuous invisibility range is shown to appear not only due to a small total volume of the scatterer in the nonresonant regime, but also because high-Q superscattering modes are suppressed by the losses. For numerical demonstration, indium antimonide, a natural lossy material, and a hypothetical, properly doped material with the same real part of the permittivity but lower or zero losses are considered. In the latter case, variations of permittivity with a control parameter can be adjusted in such a way that transitions from one superscattering mode to another can be achieved. In turn, transition from the strong-scattering to the invisibility regime is possible even for the original lossy material. The basic properties of the studied superscattering modes may be replicable in artificial structures comprising natural low-loss materials.
Open Access
A novel method to eliminate the symmetry dependence of fiber coils for shupe mitigation
(Nature Publishing Group, 2024-04-20) Şenol, Tuğba Andaç; Akçaalan, Önder; Yertutanol, Aylin; Özbay, Ekmel
It is a well-known fact that interferometric fiber optic gyroscopes (IFOGs) are easily distorted by thermal effects and distortion results in the degradation of the performance of these sensors. Changing the fiber coil geometry, increasing the winding symmetry, adding fiber buffer layers around the fiber coil, using different modulation methods for multifunctional integrated optic chips, and using special types of fibers, such as photonic crystal fibers, are some alternative solutions for preventing this degradation. This paper, theoretically and experimentally, investigates not only how different types of fiber coil winding methods behave under different rates of temperature change but also presents a novel method, to the best of our knowledge, to eliminate the Shupe effect, without violating the simplest IFOG scheme. This method rules out the importance of the winding symmetry epochally and the need of any extra treatment for the fiber coil to increase the thermal performance of the system. Regardless of the symmetry of the fiber coil winding, the rate error due to the Shupe effect can be reduced to about ±0.05◦/ h for any rate of temperature change with this new method according to the experimental results.
Open Access
Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale
(Nature Publishing Group, 2024-06-13) Gökkavas, Mutlu; Gökkavas, Tamara Funda Gündoğdu; Özbay, Ekmel; Serebryannikov, Andriy E.
We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ/7 and free of diffractions. Usually, the structure’s symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.
Open Access
Distributed strain sensing by frequency-selective fading in phase-OTDR
(IEEE, 2024-10-29) Yıldız, Muhammed Kaan; Uyar, Faruk; Kartaloğlu, Tolga; Özbay, Ekmel; Özdür, İbrahim
We demonstrate a novel approach using frequency-selective fading in phase-OTDR systems to measure dynamic strain on a fiber optic cable. We present the measurements of 200 Hz, 0.03 με strain at 2 kHz interrogation frequency. CLEO 2024 © Optica Publishing Group 2024 © 2024 The Author(s)
Open Access
Lithography-free tunable Fano resonator
(Institute of Electrical and Electronics Engineers, 2024-09-30) Omam, Zahra Rahimian; Khalichi, Bahram; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Khalily, Mohsen; Özbay, Ekmel
This paper presents a lithography-free thermally tunable Fano resonator obtained by strong coupling between the narrowband and broadband resonators. The unique optical phonon response of the Dolomite (DLM) thin film and the phase-change material (PCM) of vanadium dioxide (VO2) are utilized to design such a Fano resonator. The findings of this study can have multiple applications, ranging from biosensing and gas-sensing to the modulation of thermal emission.
Open Access
A polarization-dependent reflective color filter with grating-based Sb2S3 phase-change material
(Institute of Electrical and Electronics Engineers, 2024-09-30) Omam, Zahra Rahimian; Khalichi, Bahram; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Khalily, Mohsen; Özbay, Ekmel
This paper presents a reflective tunable color filter based on employing an antimony trisulfide (Sb2S3) grating structure. The design generates two different colors when illuminated by either one of two orthogonally polarized incident lights. Moreover, by adjusting the phase of the material, the resonances can be tuned, enabling the generation of different colors. This design offers a simple platform capable of generating four distinct colors across the visible spectrum.
Open Access
Active control of Fano resonance with phase-change-based metasurface
(Institute of Electrical and Electronics Engineers, 2024-09-30) Omam, Zahra Rahimian; Khalichi, Bahram; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Khalily, Mohsen; Özbay, Ekmel
This paper presents a phase-change-based metasurface absorber that can control the excitation of Fano resonance with temperature variations. The proposed thermally tunable metasurface-based configuration is designed based on strong coupling between the narrowband (discrete state) and broadband (continuum state) absorbers, leading to an excitation of Fano resonance within the infrared region.
Open Access
A highly survivable X-band low noise amplifier based on GaN HEMT technology and impact of pulse width on recovery time
(John Wiley and Sons Inc, 2024-09-20) Nawaz, Muhammad Imran; Zafar, Salahuddin; Akoğlu, Büşra Çankaya; Çağlar, Gizem Tendürüs; Hannan, Abdullah; Urfalı, Emirhan; Aras, Erdem; Özbay,Ekmel
GaN high electron mobility transistor (HEMT)-based low noise amplifiers (LNAs) are an integral part of microwave receiver systems to enhance signal-to-noise ratio (SNR). The noise of LNA itself becomes critical for systems requiring high SNR, such as imaging and satellite communication systems. This paper discusses the design of a three-stage LNA operating at the X-band in the frequency range of 8.0-12.0 GHz. The amplifier's design and small signal, noise, and linearity characterizations are discussed. Stagewise analysis for gain, noise figure (NF), and matching network losses at the design stage results in achieving promising results. The proposed LNA provides a gain of 23.2 dB with +/- 1.0 $\pm \ 1.0$ dB gain ripple. Its NF is below 1.5 dB, output power at 1 dB gain compression is 16.4 dBm, and third-order intercept point is 24.7 dBm at 10 GHz. LNA's survivability is validated to input stress as high as 42 dBm. This LNA is the best reported NF and survivability combination in the 8.0-12.0-GHz frequency range. The reverse recovery time of LNA is measured under two different pulse conditions, and it has been shown that LNA has better recovery times for lower pulse width signals. This LNA finds its applications in radars and satellite communication systems.
Open Access
Investigation of SiNx: H surface passivation impact on InAsP/InGaAs e-SWIR photodiodes
(IEEE, 2024-01-01) Satılmış, Mert; Gezgin, İrfan Alp; Tok, Çağrı; Keleş, Habibe; Kolkowski, Walery; Pasternak, Iwona; Oğuz, Fikri; Strupinski, Wlodek; Özbay, Ekmel
In this study, we investigate the deposition of SiNx:H thin films using the inductively coupled plasma chemical vapor deposition (ICPCVD) method with NH3/SiH4 ratios ranging from 1.3 to 25. The impacts of these variations on extended short-wave infrared region (e-SWIR) photodiodes (PDs) were characterized through stress, reflectance, photoresponse, and dark current modeling techniques. It was observed that an increase in H content leads to a decrease in both the refractive index and stress. To reveal the effects of H content on the bond numbers of N-H, Si-H, and Si-N within SiNx:H, FTIR measurements were conducted. This allowed us to investigate the K − trap centers depending on the change in NH3/SiH4 ratios. The influence of H content on the passivation layer was examined through dark current modeling and PD performance evaluations. The optimized passivation layer underscores that the dark current of the PD is G-R limited, with diffusion current emerging as the dominant component at high temperatures. Additionally, the PD performance was enhanced with optimized passivation, achieving a higher photo-to-dark current ratio, a responsivity of 0.97 A/W at $2~\mu$ m, an external quantum efficiency (E.Q.E.) peak of 70.8% at $1.6~\mu$ m, and a dark current density of $120~\mu$ A/cm2 at 300 K under a 100 mV reverse bias voltage.
Open Access
The structural, mechanical, electronic, and optical properties of multiferroic LiCu2O2 under different pressures
(Indian Academy of Sciences, 2024-12-18) Koç, Hüsnü; Şimşek, Şevket; Mamedov, Amirullah M.; Özbay, Ekmel
In this work, the structural, mechanical, and electronic properties of LiCu2O2 compound under different pressures were studied using the density functional theory. The spin-polarized generalized-gradient approximation has been used for modelling the exchange-correlation effects. In particular, the electronic structure under zero pressure was analysed using both conventional GGA-PBE and meta-GGA (mBJLDA) functional. The structural optimization was performed by using VASP-code, and the lattice parameters and magnetic moments were calculated. Bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, anisotropy factors, sound velocities, and Debye temperature were obtained from the calculated elastic constants for LiCu2O2 compound. While the electronic band structures obtained from both functionals for spin up under zero pressure are semiconductor in nature, the electronic band structures obtained from PBE and mBJLDA functionals for spin down are narrow semiconductor and semiconductor, respectively. For the spin-up state, the Eg value decreases linearly after 5 GPa, while the Eg value increases linearly for the spin-down state. The real and imaginary parts of the dielectric function along the x, y, and z axes and the optical constants, such as the energy loss function, refractive index, reflectivity coefficient, and extinction coefficient are also calculated and presented.
Embargo
The structural, elastic, electronic, and optical properties of the rhombohedral La₂NiMnO₆ double perovskite compound at different temperatures
(Elsevier BV, 2024-11-26) Kaya, F.; Koç, H.; Mamedov, Amirullah M.; Özbay, Ekmel
This study investigates the structural, mechanical, electronic, and optical properties of the rhombohedral $La_2NiMnO_6$ (LNMO) perovskite compound at different temperatures using density functional theory. The calculation employed the spin-polarized generalized gradient approximation (GGA). The calculated lattice parameter values agree (less than 1.2 % difference) with experimental values. We determined the mechanical properties using elastic constants calculated from the stress-strain relationship. The elastic constants obtained satisfy the Born stability criteria for the rhombohedral structure. Electronic band structure calculations revealed that LNMO exhibits the suitable narrow band gap semiconductor character in the spin-polarized state. Finally, the real and imaginary parts of the dielectric function and other optical functions along the x and z directions are calculated and interpreted.
Open Access
Modeling the synapse and neuromuscular junction using organ-on-a-chip technology
(Elsevier Inc., 2024-01-01) Yılmaz, Eylül Gülşen; Hacıosmanoğlu, Nedim; İnci, Fatih; Ashammakhi, Nureddin; Willerth, Stephanie