Scholarly Publications - NANOTAM
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Item Open Access Design and simulation of SWIR nBn-InGaAs photodetector with AlGaAs barrier(EDP Sciences, 2025-09-22) Tok, Çaǧrı; Satılmış, Mert; Keleş, Habibe; Oǧuz, Fikri; Sarı, Hüseyin; Özbay, EkmelThe article discusses the simulation of an nBn-InGaAs photodetector using Silvaco TCAD at 300 K, focusing on optimizing the barrier performance and reducing the valence band offset. It explores the device’s electrical and optical behaviors, including dark current, photocurrent, and capacitance. The results show a peak responsivity of 0.91 A/W at 1.55 µm and a junction capacitance of 9 pF at −5 V. Adding an anti-reflection coating notably improved the optical performance, highlighting the structure’s potential for efficient infrared photodetection.Item Open Access The origin of gate degradation under HTRB operation: Buffer engineering to suppress impact ionization in GaN HEMT(Institute of Electrical and Electronics Engineers Inc., 2025-12-15) Soydan, Mahmut Can; Joya, Amir Ali; Ghobadi, Amir; Özbay, EkmelThis study investigates the origin of gate degradation in AlGaN/GaN HEMTs under high-temperature reverse bias (HTRB) conditions and proposes a buffer engineering strategy to mitigate this degradation. Transmission electron microscopy (TEM) analysis reveals the presence of a thin oxide layer between the gate and AlGaN contact. It is found that the holes generated through impact ionization (under high electric fields of HTRB operation), are directed toward the gate due to the intense local electric field under the gate and the field plate overhang, and are accumulated under this energetic barrier. Consequently, these trapped holes cause gate degradation and increased gate leakage. To address this issue, impact ionization, as the initial forcing mechanism of the degradation, is suppressed via thinning the channel GaN (C-GaN) layer. This improvement is attributed to the suppression of the local electric field near the gate region in thin C-GaN HEMTs. Additionally, the impact of C-GaN thinning on breakdown characteristics and RF performance is discussed. Overall, the findings provide insights into the root cause of gate degradation and offer a buffer engineering strategy to minimize gate degradation under deep off-state stress. This approach enhances the reliability of future high-power and high-frequency GaN HEMTs, contributing to their long-term performance in demanding applications.Item Open Access GaN-on-SiC broadband driver amplifier for C- and X-band applications(John Wiley and Sons Inc, 2025-06-12) Hannan, Abdullah; Aras, Erdem; Gürdal, Armağan; Urfalı, Emirhan; Zafar, Salahuddin; Nawaz, Muhammad Imran; Özbay, EkmelA GaN-on-SiC-based broadband driver amplifier operating in the C- and X-bands from 5 to 12 GHz has been demonstrated. The MMIC has a typical small signal gain of 29.7 dB with a ±1.4 dB gain ripple. The input and output return losses are better than 10.5 and 8.8 dB, respectively. The average Psat is approximately 2.65 W with an OIP3 of 37.7 dBm, while the large signal gain is 22 dB. This design is distinguished by its low output power ripple and the low large signal gain fluctuations observed across the full frequency range of interest. Consistent load impedance matching at the output stage for the whole frequency range enabled an output power ripple less than ±1.1 dB and a large signal gain ripple less than ±0.6 dB at 10 dBm input power. This allowed for an output power density of at least 2.68 W/mm across the broad frequency range of 5–12 GHz. The typical power-added efficiency is 26.4%. To the best of the authors' knowledge, this DA design exhibits the best combination of gain, output power density, gain ripple, output power ripple and output return loss in this frequency band.Item Open Access A heterodox approach for designing iron photosensitizers: pentacyanoferrate(II) complexes with monodentate bipyridinium/pyrazinium-based acceptor ligands(American Chemical Society, 2025-04-01) Schmidt, Heiner; Oglou, Ramadan Chalil; Tunçer, Hüseyin Orhun; Ulusoy Ghobadi, Türkan Gamze; Tekir, Şafak; Özvural Sertçelik, Kübra Nur; Ibrahim, Abdelrahman; Döhler, Lotta; Özçubukçu, Salih; Kupfer, Stephan; Dietzek-Ivanšić, Benjamin; Karadaş, FerdiThe main obstacle in replacing well-established precious ruthenium photosensitizers with earth-abundant iron analogs is the short excited state lifetimes of metal-to-ligand charge transfer (MLCT) states due to relatively weak octahedral field splitting and relaxation via metal-centered (MC) states. In this study, we address the issue of short lifetime by using pentacyanoferrate(II) complexes and combat facile photodissociation by utilizing positively charged pyrazinium or bipyridinium ligands. We utilize femtosecond transient absorption spectroscopy alongside quantum chemical calculations to probe the excited states of three 4,4′-bipyridinium- or pyrazinium-based pentacyanoferrate(II) complexes. The 4,4′-bipyridinium-based complexes exhibit 3MLCT lifetimes of about 20 ps, while the pyrazinium-based complex exhibits a lifetime of 61 ps in an aqueous solution, setting a benchmark for cyanoferrate complexes. These results mark the foundation for a new group of easy-to-prepare iron photosensitizers that can be used for harvesting visible light.Item Open Access Improvement in ASM and GaN HEMT model for RF applications(Institute of Electrical and Electronics Engineers Inc., 2026-11-26) Hannan, Abdullah; Gürbüz, Abdulkadir; Nawaz, Muhammad Imran; Darwish, Haitham; Aras, Erdem; Özbay, EkmelGallium nitride (GaN) high electron mobility transistors (HEMTs) are widely used in high-frequency and high-power applications because of their exceptional electrical properties. Accurate modeling of HEMT is crucial for reliable circuit design, particularly as the device dimension scales. The measurement-based models are reliable but dependent on the periphery and do not offer flexibility to the designer. These can be used to design impedance-matching networks and perform small signal simulations only of the complete MMIC design. In this work, we have proposed an improvement in advanced Spice model for HEMT (ASM-HEMT) for better S22 performance at low frequencies by implementing an additional series RC network to address the parasitic path between the drain and the source through the buffer. Also, an electromagnetic (EM)-based modeling approach for obtaining scalable extrinsic parasitics is described. Equations are developed as a function of the number of gate fingers and gate widths from extrinsic parasitics of the gate, the drain, and the source. The proposed model was validated by fabricating six different peripheries of HEMTs. The S-parameters were verified from 400 MHz to 26.5 GHz for the quiescent bias of 28 V and 100 mA/mm. Large-signal validation for output power and power-added efficiency (PAE) was also performed on devices with the sizes of 6×100 μ m, 8×100 μ m, and 10×100 μ m, confirming the accuracy of the model under large-signal excitation. This work provides a comprehensive and scalable modeling solution for GaN HEMTs, suitable for advanced RF and microwave circuit design.Item Open Access Visible light-driven acetaldehyde production from CO₂ and H₂O via synergistic vacancies and atomically dispersed Cu sites(John Wiley and Sons Inc, 2025-05-12) Lei, Jian; Wang, Zhongliao; Huo, Jinquan; Sang, Shuaikang; Zhang, Chao; Zhu, Enquan; Kong, Tingting; Karadaş, Ferdi; Low, Jingxiang; Xiong, YujieAcetaldehyde (CH$_3$CHO) is of great industrial importance and serves as a key intermediate in various organic transformations. Photocatalytic production of acetaldehyde from CO2 represents a sustainable route compared to conventional oxidation processes. However, current photocatalytic systems often face challenges, including limited product selectivity and dependence on sacrificial reagents. Here, we present a Cd$_{0.6}$Zn$_{0.4}$S (CZS) photocatalyst co-modified with sulfur vacancies and atomically dispersed Cu (Cu/CZS−Vs) for the efficient conversion of CO₂ to acetaldehyde. Charge density analysis reveals that sulfur vacancies induce charge accumulation around the adjacent metal atoms, creating active sites that strongly anchor CO₂ and H$^+$, thereby promoting CO₂ conversion while suppressing the competing hydrogen evolution reaction. The atomically dispersed Cu sites facilitate the conversion of key intermediates (i.e., *CHO and *CO) to the crucial C₂ intermediate *OCCHO, which can subsequently be converted to acetaldehyde. As a result, this catalyst achieves an acetaldehyde yield of 121.5 μmol g$^{−1}$ h$^{−1}$ with a selectivity of ca. 80 % via photocatalytic CO₂ conversion in the absence of sacrificial agents, along with a quantum efficiency of ca. 0.53 % at 400 nm, underscoring its potential for practical CO₂ conversion applications. These results are expected to pave the way for future developments in green chemical processes.Item Open Access Thermal management to break the heat trap: scalable lithography-free multilayer films for radiative cooling(IEEE, 2025-07-02) Khalichi, Bahram; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Özbay, EkmelThis work presents a high-performance, lithography-free multilayer absorber/emitter for passive radiative cooling. The proposed structure exhibits a strong absorption resonance around 9 $μm$, aligning with the atmospheric transparency window to maximize thermal emission. The structure achieves a temperature reduction of −10.93°C at 20°C ambient temperature under purely radiative cooling conditions $(h_c = 0 W/m²K)$. As convective heat transfer increases, the cooling effect remains strong, reaching 6.68°C for $h_c = 3 W/m²K$ and 11.30°C for $h_c = 6 W/m²K$. The proposed scalable, cost-effective, and fabrication-friendly approach represents a significant advancement in energy-efficient cooling technologies for industrial and thermal management applications.Item Open Access Split-bridged disk-like terahertz metasurface: enabling ultra-broadband asymmetric transmissive co-to-cross polarization conversion(Institute of Electrical and Electronics Engineers Inc., 2025) Khalichi, Bahram; Elshurafa, Reham I.M.; Eishorafa, Mohammed; Ghobadi, Amir; Özbay, EkmelThis paper proposes an ultra-broadband transmissive polarization converter with an asymmetric transmission characteristic operating in the terahertz (THz) frequency range. The proposed design comprises a tri-layer metasurface structure, where a split-bridged disk-like resonator is sandwiched between two perpendicularly oriented grating layers separated by dielectric spacers. This configuration enables the efficient conversion of linearly polarized waves into their cross-polarized counterparts under normal incidence, achieving a high polarization conversion ratio exceeding 90% across the frequency range of 1.19–9.0 THz.Item Open Access Expanding the toolbox of carbohydrate recognizing moieties: demonstrating the interaction of carbohydrates with surface-immobilized RB221 and translating it into a tapered optical fiber cavity ring-down sensor(American Chemical Society, 2026-01-14) Malik, Bakhtawar; Ullah, Ubaid; Azeem, Iqra; Nayab, Sana; Talpur, Zakir H.; Masood, Rushba Saman; Duran, Hatice; Mirza, Shaper; Cheema, M. Imran; Yameen, BasitThe diversity of functions associated with carbohydrates in physiology and pathophysiological conditions has sparked significant scientific interest in understanding their molecular interactions for enabling the development of therapeutic and diagnostic interventions. Capitalizing on the affinity of reactive blue 221 (RB221) dye toward carbohydrates, herein, we demonstrate the potential of RB221 as a new carbohydrate-recognizing moiety. For this purpose, RB221 is immobilized on the surface of polyethylenimine (PEI)-functionalized iron oxide nanoparticles (IONPs-PEI) to produce an IONPs-PEI/RB221 construct. The RB221–carbohydrate interaction is determined by exposing the IONPs-PEI/RB221 to aqueous solutions of nine different carbohydrates. The interaction with d-maltose, d-sucrose, d-lactose, d-fructose, d-glucose, l-fucose, d-arabinose, d-mannose, and d-galactose suggests that the number of molecules of these carbohydrates that adsorbed on IONPs-PEI/RB221 per RB221 molecule is approximately 2, 2, 3, 3, 3, 4, 4, and 5, respectively. Furthermore, RB221 immobilized on a tapered optical fiber surface is demonstrated for glucose sensing using phase shift fiber cavity ring-down spectroscopy (PS-CRDS). Integrating RB221-functionalized tapered optical fiber in the PS-CRDS-based sensor results in a 47-fold increase in sensitivity over bare fiber with a minimum detection limit of 2 pg/mL. This study highlights the potential of RB221–carbohydrate interactions for developing affordable and reliable sensors.Item Open Access The Brettanomyces bruxellensis contamination of wines: a case study of Moldovan micro-winery(MDPI AG, 2024-12-25) Mitina, Irina; Grajdieru, Cristina; Sturza, Rodica; Mitin, Valentin; Rubtov, Silvia; Balanuta, Anatol; Behta, Emilia; İnci, Fatih; Hacıosmanoğlu, Nedim; Zgardan, DanBrettanomyces bruxellensis yeasts cause wine spoilage by producing volatile phenol compounds with specific off-odors. Assessing the propagation of this species is challenging, especially for micro-wineries. In this study, wines produced in a micro-winery from the grapes of different varieties collected from three PGI regions of Moldova over three years were studied for the presence and infection level of Brettanomyces spoilage yeasts, using traditional microbiological and molecular methods. The results of Brettanomyces infection monitoring in mature wines might speak in favor of the hypothesis that grape berries can be a potential source of B. bruxellensis in wine. The contamination levels of mature wines with respective species fluctuated in accordance with the year of grape cultivation, being the highest during the 2023 vintage. This study shows the potential of applying sequencing analysis for tracking the source of Brettanomyces contamination in wineries.Item Embargo Colloidal quantum wells enable phototransistors with enhanced visible light detection(John Wiley and Sons Inc, 2025-10-31) Liu, Ziqing; Durmusoglu, Emek Goksu; Ren, Yunfei; Xiao, Peng; Fang, Wenhui; Tan, Haixing; Zhang, Haojun; Lin, Xiaoguang; Dai, Yi; Wang, Sui-Dong; Liu, Chuan; Hernandez-Martinez, Pedro Ludwig; Dabard, Corentin; Zhao, Dongxu; Demir, Hilmi Volkan; Liu, BaiquanInGaZnO (IGZO) phototransistors have largely attracted research attention due to their excellent electrical properties. However, the wide bandgap (>3.0 eV) of IGZO phototransistors intrinsically limits their spectral response to UV wavelengths, hindering their applicability in visible-light optoelectronics. We report integrating colloidal quantum wells (CQWs, spin-coated at room temperature in ambient air) to extend the spectral sensitivity of IGZO phototransistors into the visible range. The resulting CQW/IGZO heterojunction phototransistor exhibits a six-order-of-magnitude enhancement in specific detectivity (D* = 1 × 10¹¹ Jones at 660 nm) across the visible spectrum (405–660 nm), while retaining excellent electrical performance, including a maximum mobility of 13.38 cm² V⁻¹ s⁻¹ and an on/off current ratio of 1.53 × 10⁹. Energy-band engineering at the CQW/IGZO interface, with a conduction band offset (ΔEc = 0.15 eV), facilitates efficient charge separation and promotes electron injection into the IGZO channel. Gate-modulation further enables the device to drive quantum-dot light-emitting diodes, underscoring its potential in adaptive sensing and integrated optoelectronics. This work demonstrates the integration of CQWs in thin-film transistors as a scalable and cost-effective strategy to broaden the optoelectronic functionality of wide-bandgap semiconductors. © 2025 Wiley-VCH GmbH.Item Open Access Hybridized plasmonic modes in semiconductor-based metasurfaces: a gateway to dual Fano resonances(IEEE, 2025-07-02) Khalichi, Bahram; Elshurafa, Reham I.M.; Ghobadi, Amir; Özbay, EkmelA metagrating structure comprising aluminum (Al) and gallium phosphide (GaP), along with a planar indium tin oxide (ITO) spacer on top of an Al substrate, is proposed to achieve dual Fano resonances in the absorption spectrum. Without GaP grating, the Fano resonance arises from the coupling between localized surface plasmon resonance (LSPR) mode in the Al grating and gap-surface plasmon resonance mode in the ITO layer. Introducing the GaP semiconductor grating between the Al grating and the ITO spacer excites a hybrid plasmonic waveguide resonance mode, which couples with the LSPR mode to produce another Fano resonance and enhance the absorption bandwidth within the shortwave infrared range.Item Open Access Development of magnetically driven contactless piezoelectric nanogenerators utilizing ZnO nanowires with different shaped top electrodes(Institute of Physics, 2024-12-13) Ucar, E.F.; Ilbay, A.; Simsek, T.; Kaynar, M.B.; Özbay, Ekmel; Ozcan, S.In this research, ZnO nanorod (NR)-based contactless piezoelectric nanogenerators (C-PENGs) with pyramidal and zigzag-shaped top electrodes were fabricated. ZnO NRs with different aspect ratios were grown on a Si (100) wafer, which acts as the bottom electrode. Two different top electrode architectures, zigzag-trenched and pyramidal cavity, were used. Additionally, different work function metallic coatings, platinum and nickel (as a cheaper alternative to noble metals) were applied to the architecturally shaped surfaces to achieve a Schottky contact between the top and bottom electrodes. A magnetic field ranging from 3.5 Oe to 55 Oe with a frequency range between 0.5 and 2 Hz was applied to drive the nanogenerator. The results showed that a maximum output voltage of 75 mV and a maximum power density of 20.7 pW cm$^{−2}$ were successfully generated. The measurement of output power densities revealed that C-PENGs are a promising candidate for future nanorobotics applications.Item Open Access Gamma radiation shielding efficiency of cross-linked polystyrene-b-polyethyleneglycol block copolymer nanocomposites doped arsenic (III) oxide and boron nitride nanoparticles(Multidisciplinary Digital Publishing Institute (MDPI), 2025-12-17) Ortaç, Bülend; Baskan, Taylan; Mutlu, Saliha; Yılmaz, Sevil Savaşkan; Yılmaz, Ahmet HakanIn recent years, polymer-based hybrid nanocomposites have emerged as promising alternatives to traditional heavy metal shields due to their low density, flexibility, and environmental safety. In this study, the synthesis of PS-PEG copolymers and the gamma radiation-shielding properties of PS-PEG/As₂O₃, PS-PEG/BN, and PS-PEG/As₂O₃/BN nanocomposites with different compositions are investigated. The goal is to find the optimal nanocomposite composition for gamma radiation shielding and dosimetry. Therefore, the mass attenuation coefficient (MAC), linear attenuation coefficient (LAC), half-value layer (HVL), tenth-value layer (TVL), effective atomic number, mean free path (MFP), radiation shielding efficiency (RPE), electron density, and specific gamma-ray constant were presented. Gamma rays emitted by the Eu source were detected by a high-purity germanium (HPGe) detector device. GammaVision was used to analyze the given data. Photon energy was in the vicinity of 121.8–1408.0 keV. The MAC values in XCOM simulation tools were used to compute. Gamma-shielding efficiency was increased by an increased number of NPs at a smaller photon energy. At 121.8 keV, the HVL of a composite with 70 wt% As₂O₃ NPs is 2.00 cm, which is comparable to the HVL of lead (0.56 cm) at the same energy level. Due to the increasing need for lightweight, flexible, and lead-free shielding materials, PS-b-PEG copolymer-based nanocomposites reinforced with arsenic oxide and BN NPs will be materials of significant interest for next-generation radiation protection applications.© 2025 by the authors.Item Open Access Ti₃C₂Tₓ/MoO₃ composite as saturable absorber for dissipative soliton 1.0 μm mode-locked fiber laser(Institute of Physics Publishing Ltd., 2025-05-15) Ahmad, H. Bin; Lutfi, Maisarah A.M.; Zaini, Muhammad Khairol Annuar; Samion, Muhamad Zharif; Suresh, Sagadevan; Ortaç, Bülend; Thambiratnam, Kavintheran A.L.This work demonstrated a passive mode-locked ytterbium-doped fiber laser operating at a 1-micron region. The saturable absorber incorporated was a Ti₃C₂Tₓ/MoO₃ deposited on a D-shaped fiber, generating stable dissipative soliton in all normal dispersion regimes. The all-fiber ring cavity laser configuration generates mode-locked pulses with a repetition rate of 24.2 MHz, which was further verified on the radio frequency spectrum. The dissipative soliton spectrum was centered explicitly at 1031 nm with a spectral edge-to-edge bandwidth of ∼3.7 nm. The Ti₃C₂Tₓ/MoO₃ saturable absorber provides mode-locked stability with ∼59 dB of signal-to-noise ratio. The Ti₃C₂Tₓ/MoO₃ mode-locked laser was amplified up to 101 mW, corresponding to 4.1 nJ of single pulse energy.Item Open Access Tunable multiresonant microcavity exciton-polaritons in colloidal quantum wells(American Chemical Society, 2025-03-13) Cam, Nhung Vu; Rahman, Md Abdur; Akhil, Syed; Durmusoglu, Emek Goksu; Do, Thi Thu Ha; Hernandez-Martinez, Pedro Ludwig; Dabard, Corentin; Arora, Deepshikha; Uddin, Siam; Zamiri, Golnoush; Wang, Hao; Ha, Son Tung; Mortensen, N. Asger; Demir, Hilmi Volkan; Yang, Joel K.W.Optical microcavities are widely used to confine photons for exciton-polariton formation. However, their compact design often imposes limitations on spatial freedom, particularly in controlling the cavity length with the nanometer precision required for effective coupling with excitons. Existing methods for tuning resonances by integrating cavities with dynamic structures often lack sufficient resolution or a complex operation. Here, we introduce a multiresonant microcavity array that provides a full spectral selection of cavity resonances with a sub-5 nm cavity length variation. We employed this platform to investigate room-temperature polariton formation using gradient core-crown colloidal quantum wells that host highly stable excitons. The strong coupling system exhibits longevity of Rabi oscillations with a quality factor of QR = 3.3 and a large Rabi splitting exceeding twice the thermal losses. Notably, we achieved control of the polariton mixed properties across the cavity arrays on a single substrate. This platform is promising for the development of on-chip polaritonic devices.Item Embargo Sintering temperature controlled giant dielectric, Ge-doped CaCu₃Ti₄O₁₂ ceramics for electrical energy storage, optoelectronics and supercapacitor applications(Elsevier Ltd, 2025-10-15) Satyarthi, Satyendra Kumar; Verma, Harish; Singh, Vishwa Pratap; Kumar, Vinod; Puri, Nitin Kumar; Rai, Rama Nand; Ramam, Koduri; Upadhyaya, Shail; Singh, Akhilesh KumarGe-doped CaCu₃Ti₄O₁₂ ceramics with composition CaCu₃Ti₃.₉Ge₀.₁O₁₂ were synthesized using a wet chemical method and sintered at different temperatures to study their properties. X-ray diffraction confirmed single-phase cubic structure for all sintering temperatures, with improved crystallinity observed at higher sintering temperatures. Morphological analysis showed 1050 °C as the optimal sintering temperature, balancing grain growth without excessive liquid phase. Optical studies revealed the lowest band gap of 2.16 eV for sample sintered at 1050 °C, along with strong photoluminescence dominated by cyan emission at 513.5 nm. Ge doping allowed tunable emissions from 470 to 620 nm with an effective half lifetime of 1.30 ns, which can be very useful for optoelectronics. Dielectric measurements peaked for sample sintered at 1050 °C, showing enhanced polarization and charge mobility, along with increased grain boundary resistance and non-Debye relaxation, indicating improved insulation and internal barrier layer capacitor behavior. Electrochemical tests demonstrated excellent pseudocapacitive performance with over 90 % capacitance retention after many cycles. Overall, CaCu₃Ti₃.₉Ge₀.₁O₁₂ is a promising multifunctional material for hybrid (electrical and electrochemical) energy storage and optoelectronic applications.Item Open Access GaN-based 6–18 GHz power amplifier MMIC(Institute of Electrical and Electronics Engineers, 2025-05-20) Zafar, S.; Aras, Erdem; Urfalı, Emirhan; Nawaz, Muhammad Imran; Hannan, Abdullah; Mohyuddin, N.; Baloch, S.; Özbay, EkmelA reactively matched wideband amplifier MMIC covering C-, X-, and Ku-band is presented. The matching networks are designed in such a way as to reduce the gain ripple without compromising large-signal parameters. The amplifier is fabricated using NANOTAM’s 0.25 µm GaN-on-SiC HEMT technology. The MMIC obtained a minimum small signal gain of 20 dB with a gain ripple of only ±2 dB in the entire frequency range from 6 GHz to 18 GHz. The output power (Pout) ranges from 34.8–39.4 dBm with an associated power-added efficiency (PAE) of 11.8–28.8 %. The variation of Pout and PAE for corresponding gain compression and input power levels is also reported. The overall chip size is 4.3 mm x 3.4 mm.Item Open Access A compliant mandrel-based fiber-optic hydrophone for underwater acoustic sensing(Institute of Electrical and Electronics Engineers Inc., 2025-09-15) Uyar, Faruk; Aldemir, Abdullah Sefa; Kartaloğlu, Tolga; Özbay, Ekmel; Özdür, İbrahimThis article presents the design, numerical modeling, and experimental validation of a mandrel-based fiber-optic hydrophone (FOH) tailored for underwater acoustic sensing. The hydrophone leverages a compliant mandrel structure to transduce radial pressure-induced strain into axial strain along an optical fiber wound around the mandrel, with acoustic signals retrieved via interferometric techniques. A comprehensive 3-D finite element model is developed to evaluate the mechanical response and sensitivity characteristics of the hydrophone. Experimental validation is conducted in a water tank using a calibrated piezoelectric transducer, confirming the simulated predictions. The designed FOH exhibits a mean acoustic pressure sensitivity of -135.28 dB re rad/µPa over the frequency range of 250 Hz–8 kHz, which is competitive with state-of-the-art FOH designs reported in recent literature. The pressure noise floor characterization yielded a value of 43.28 dB re rad2/Hz at 1 kHz, demonstrating the hydrophone’s capability to detect weak acoustic signals below deep-sea state 0 (DSS0) up to frequencies above 1 kHz. The results suggest that this mandrel-based FOH design offers a robust, scalable, and cost-effective solution for large-scale underwater acoustic monitoring systems, with strong potential for integration into fiber-optic sensor arrays used in oceanographic, industrial, and defense applications.Item Embargo Terahertz waveguiding through 3D-printed bar-nested hollow-core optical fibers(Academic Press Inc., 2025-12-21) Siddiqui, Muhammad Zain; Chiba, Kohei; Kanamori, Yoshiaki; Ordu, MustafaTerahertz (THz) frequencies, ranging from 0.1 to 10 THz, offer unique sensing, imaging, and high-speed communications applications. However, these electromagnetic radiations experience high attenuation in free-space transmission, emphasizing the need for a low-loss waveguiding solution, often limited by material absorption. Negative curvature hollow-core fibers offer a promising solution by confining the light within an air core region, further reducing material absorption losses. This study presents the design, fabrication, and characterization of a simple bar-nested tubular hollow core fiber tailored for operation in the THz regime. The bar nested four-tube fiber design parameters are numerically optimized using finite element simulations. The cyclic olefin copolymer-based fiber is fabricated through fused deposition modeling in a 3D printer, enabling rapid prototyping and intricate fiber designs. The experimental results demonstrated a measured transmission loss of 7.6 dB/m for the fabricated fiber at 750 GHz with a bandwidth of 100 GHz. The differences observed between numerical and experimental losses are attributed to the fabrication imperfections, surface scattering losses, and coupling losses, highlighting the challenges and potential of 3D printing for THz waveguide development. © 2025 Elsevier Inc.