Can Koral - Academia.edu (original) (raw)

Papers by Can Koral

Journal of the Optical Society of America A

Micromachines

We report the design, fabrication and experimental validation of a THz all-dielectric multi-mode ... more We report the design, fabrication and experimental validation of a THz all-dielectric multi-mode q-plate having a fixed rate of change of the optical axis. The device consists of space-variant birefringent slabs manufactured by 3D printing using melt-extruded Acrylonitrile Butadiene Styrene (ABS). The desired form birefringence is analytically evaluated and experimentally measured by the THz time domain spectroscopy technique. The manufactured q-plate design is characterized using a polarization-sensitive imaging setup. The full electric field spatial maps are acquired from the beam propagating through the q-plate. The device enables the realization of both radial and azimuthal vector beams at discrete frequency intervals by controlling the space-dependent orientation of the ordinary and extraordinary axes in the transverse plane with a multi-mode sequence.

Optics Letters

A novel, to the best of our knowledge, methodology based on the combination of experimental measu... more A novel, to the best of our knowledge, methodology based on the combination of experimental measurements and simulations of the wave transmission through a metasurface at different angles is presented, enabling us to identify the fundamental and first high-order mode of spoof surface plasmon polaritons (SSPPs) excited in the terahertz regime. The approach offers a new way, an alternative to standard near field imaging, to trace out the presence of SSPPs on a metal-dielectric interface.

$Research paper thumbnail of Tuning of the silicon nitride refractive index by RF sputtering power$

The fabrication of thin-film multilayer structures by sputtering technique usually requires multi... more The fabrication of thin-film multilayer structures by sputtering technique usually requires multi-cathode deposition machine. This study proposes a simpler approach based on the RF power modulation: silicon nitride (SiNx) thin films were prepared by RF reactive sputtering in (Ar + N2) atmosphere at room temperature. The samples were analyzed to highlight the effects of different deposition conditions on the morphological and optical properties of the films in the visible and near/mid-IR regions. The refractive index of the films was changed in the range of 1.5 2.5 (at λ=800 nm) by tuning the sputtering power. IR reflectance measurements revealed the absence of spurious (oxygenor hydrogen-based) phases, while atomic force and scanning electron microscopies confirmed the presence of flat and defect-free samples surfaces.

Frontiers in Physics, 2022

Free-Electron Lasers are a rapidly growing field for advanced science and applications, and world... more Free-Electron Lasers are a rapidly growing field for advanced science and applications, and worldwide facilities for intense field generation, characterization and usage are becoming increasingly popular due to their peculiarities, including extremely bright, coherent, wide band tunable ultra-short pulses which are not achievable with other techniques up to now. In this review we give a thorough survey of the latest advances in the Free-Electron Laser-based field generation and detection methodologies and then present the main characteristics of a future THz/IR source, named TerRa@BriXSinO, based on a superconducting linear accelerator. The foreseen source is strongly monochromatic, with a bandwidth of 1% or smaller, highly coherent both transversally and longitudinally, with extreme versatility and high frequency tunability. After introducing the most recent and novel FEL-assisted scientific investigations, including fundamental explorations into complex systems and time-dependent ...

2019 Thirteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), 2019

We study the sensitivity of two different metagrids to the presence of biological fluids on the s... more We study the sensitivity of two different metagrids to the presence of biological fluids on the surface. When a THz beam impinges on each metagrid capped with a dielectric substrate, peaks in transmission related to the onset of high order surface plasmon polaritons appear and allow to estimate the properties of a guest fluid filling the partition volume with a very high sensitivity, comparable with record literature values.

In the new generation of particle accelerators and storage rings, collective effects have to be c... more In the new generation of particle accelerators and storage rings, collective effects have to be carefully analyzed. In particular, the finite conductivity of the beam pipe walls is a major source of impedance and instabilities. A reliable electromagnetic (EM) characterisation of different coating materials is required up to hundreds of GHz due to very short bunches. We propose two different measurement techniques for an extended frequency characterization: (i) a THz time domain setup based on the signal transmission response of a tailored waveguide to infer the coating EM properties from 100 to 300 GHz or even further*.**. This technique has been tested both on NEG and amorphous Carbon films. (ii) a resonant method, based on dielectric cavities, to evaluate the surface resistance Rs of thin conducting samples at low (GHz) frequencies***. Due to its high sensitivity, Rs values can be obtained for very thin (nanometric) coatings or for copper samples with a laser treated surface, sinc...

2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), 2018

Resistive-wall impedance constitutes a significant percentage of the total beam-coupling impedanc... more Resistive-wall impedance constitutes a significant percentage of the total beam-coupling impedance budget of an accelerator. Under extreme conditions, like large power handling and ultrahigh vacuum, metamaterial-based absorbers can represent a valid alternative to other approaches for impedance mitigation in specific accelerator components, like resonant (parasitic) cavities or collimators operating along the beam line. We design sub-wavelength 2D metallic resonant structures based on split rings or on 3D hyperbolic tapered waveguide arrays that can be employed as mode dampers in accelerating structures. A number of prototypes are fabricated and then measured in a "test model" pillbox cavity.

Physical Review Accelerators and Beams, 2018

This paper describes a reliable, handy, and inexpensive measurement system to characterize, in th... more This paper describes a reliable, handy, and inexpensive measurement system to characterize, in the sub-THz frequency range, the coating materials used in the beam pipes of accelerators. The method is based on time domain measurements of an electromagnetic wave passing through a waveguide having a thin central slab, where the coating material is deposited on both sides. Two horn antennas are integrated on both sides of the device to optimize the signal collection and detection. This novel technique is tested on slabs coated with a nonevaporable getter (NEG) and allows to evaluate the surface impedance in the frequency range from 0.1 to 0.3 THz.

Terahertz (THz), Mid Infrared (MIR) and Near Infrared (NIR) Technologies for Protection of Critical Infrastructures Against Explosives and CBRN, 2021

Materials research is an essential component of modern terahertz systems: novel, broadband, low l... more Materials research is an essential component of modern terahertz systems: novel, broadband, low loss and tunable devices all heavily rely on new materials, ranging from graphene to transition metal dichalcogenides and doped semiconductors. At the same time, terahertz spectroscopy provide a powerful tool for the characterisation of a broad range of materials, including composite polymers and biomolecules. In this review, we will present our recent progress on the use of time domain spectroscopy to extract microscopic information in an otherwise inaccessible portion of the electromagnetic spectrum.

2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2019

The electromagnetic interference (EMI) shielding mechanisms of graphene/PMMA multilayered structu... more The electromagnetic interference (EMI) shielding mechanisms of graphene/PMMA multilayered structures are experimentally investigated by using time domain spectroscopy (TDS) in the THz range. Stacked plates of similar thickness (~ 5μm), starting from a single layer up to 100 layers, were produced by a novel approach combining ultra-thin polymer casting and wet deposition techniques. These nano laminates show enhanced electrical conductivity (~100 S/cm) and superior specific shielding effectiveness (~2.104 dB cm2 g-1).

We report on the realisation of a customized THz Time Domain Spectroscopic Ellipsometer (THz-TDSE... more We report on the realisation of a customized THz Time Domain Spectroscopic Ellipsometer (THz-TDSE) based on fiber-coupled photoconductive antennas, operating in a wide range of incident angles and allowing also standard transmission spectroscopy without any optical realignment. To ensure accurate parameter extraction for a broad range of materials, we developed a fast and effective algorithm-assisted method to calibrate the setup and compensate for the nonideality in the response of the THz system. The procedure allows to minimise errors induced by imperfect response of the antennas and polarizers, imprecise setting of the impinging and receiving angles in the goniometric mechanical arms, and unavoidable mismatches in the THz beam optics. Differently from other calibration methods applied in the literature, our approach compares in time domain the ellipsometric derived electric field s- and p-polarised components at a given angle of incidence with the reconstructed ones, attained by...

The electromagnetic characterization of coating materials is fundamental to build a reliable impe... more The electromagnetic characterization of coating materials is fundamental to build a reliable impedance model of any accelerator. In particular, since the CLIC (Compact LInear Collider) necessitates bunches with short longitudinal length, a full electromagnetic characterization of the coating material surface impedance is needed at high frequencies in the millimetric wave length and beyond. The goal of this paper is to develop a measurement method to characterize the coating materials in the sub-THz frequency range. The electromagnetic characterization of the material is performed using a time domain coherent THz spectrometer. The method is based on the attenuation measurement of the signal passing through a waveguide specifically designed, having a very thin central layer where the coating material is deposited on both sides. The guide is equipped with two "horn" antennas integrated on both sides of the device to enhance the electromagnetic signal collection. This novel te...

Nature Communications, 2021

The use of graphene in a form of discontinuous flakes in polymer composites limits the full explo... more The use of graphene in a form of discontinuous flakes in polymer composites limits the full exploitation of the unique properties of graphene, thus requiring high filler loadings for achieving- for example- satisfactory electrical and mechanical properties. Herein centimetre-scale CVD graphene/polymer nanolaminates have been produced by using an iterative ‘lift-off/float-on’ process and have been found to outperform, for the same graphene content, state-of-the-art flake-based graphene polymer composites in terms of mechanical reinforcement and electrical properties. Most importantly these thin laminate materials show a high electromagnetic interference (EMI) shielding effectiveness, reaching 60 dB for a small thickness of 33 μm, and an absolute EMI shielding effectiveness close to 3·105 dB cm2 g−1 which is amongst the highest values for synthetic, non-metallic materials produced to date.

$Research paper thumbnail of Tuning silicon nitride refractive index through radio-frequency sputtering power$

Thin Solid Films, 2021

Fabrication of thin-film multilayer structures by sputtering typically requires a multi-cathode d... more Fabrication of thin-film multilayer structures by sputtering typically requires a multi-cathode deposition machine. This study proposes a simpler approach based on the radio-frequency power modulation; thin silicon nitride (SiN x) thin films were prepared by radio-frequency reactive sputtering in an (Ar + N 2) atmosphere at room temperature. The samples were analyzed to highlight the effects of different deposition conditions on the morphological and optical properties of the films in the visible and near/mid-infrared (IR) regions. The refractive index of the films was varied from 1.5-2.5 (at λ=800 nm) by tuning the sputtering power. IR reflectance measurements indicated the absence of spurious (oxygen-or hydrogen-based) phases; atomic force microscopy and scanning electron microscopy indicated flat and homogeneous sample surfaces.

20th Italian National Conference on Photonic Technologies (Fotonica 2018), 2018

Graphene Technology, 2020

The incorporation of graphene-related materials as nanofiller can produce multifunctional foams w... more The incorporation of graphene-related materials as nanofiller can produce multifunctional foams with enhanced specific properties and density reduction. Herein we report on the preparation of microcellular thermoplastic polyurethane/graphene foams by batch foaming. Solution blending was first adopted to disperse graphene nanoplatelets (GNP) in the elastomeric matrix. Then a foaming process based on the use of supercritical CO 2 was adopted to produce the microcellular TPU/GNP composite foams with graphene content up to 1 wt%. The EMI shielding behaviour of the TPU/GNP foams has been assessed in the THz range, and has revealed their potential in comparison with other graphene-filled foams presented in literature, that exhibit similar specific shielding effectiveness but at much higher content of graphene-related materials (10-30 wt%).

Optics & Laser Technology, 2020

• Metasurfaces can be realized employing laser ablation as fabrication technique. • Transmission ... more • Metasurfaces can be realized employing laser ablation as fabrication technique. • Transmission of metasurfaces present resonances with high quality factor. • Full wave simulations show a very high control of transmission characteristics.

Carbon, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

RSC Advances, 2020

Defective TiO2/C bulk heterostructures exhibit visible light photoresponsivity and remarkable H2 ... more Defective TiO2/C bulk heterostructures exhibit visible light photoresponsivity and remarkable H2 evolution rates under both UV and visible light irradiation.