Imperial College London

Summary

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Visiting Researcher
 
 
 
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Contact

 

+44 (0)20 7594 6242m.navarro

 
 
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Assistant

 

Ms Susan Brace +44 (0)20 7594 6215

 
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Location

 

706Electrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

347 results found

Wu R, Nekovic E, Collins J, Storey CJ, Canham LT, Navarro-Cía M, Kaplan Aet al., 2022, Taming non-radiative recombination in Si nanocrystals interlinked in a porous network., Phys Chem Chem Phys

A range of the distinctive physical properties, comprising high surface-to-volume ratio, possibility to achieve mechanical and chemical stability after a tailored treatment, controlled quantum confinement and the room-temperature photoluminescence, combined with mass production capabilities offer porous silicon unmatched capabilities required for the development of electro-optical devices. Yet, the mechanism of the charge carrier dynamics remains poorly controlled and understood. In particular, non-radiative recombination, often the main process of the excited carrier's decay, has not been adequately comprehended to this day. Here we show, that the recombination mechanism critically depends on the composition of surface passivation. That is, hydrogen passivated material exhibits Shockley-Read-Hall type of decay, while for oxidised surfaces, it proceeds by two orders of magnitude faster and exclusively through the Auger process. Moreover, it is possible to control the source of recombination in the same sample by applying a cyclic sequence of hydrogenation-oxidation-hydrogenation processes, and, consequently switching on-demand between Shockley-Read-Hall and Auger recombinations. Remarkably, irregardless of the recombination mechanism, the rate constant scales inversely with the average volume of individual silicon nanocrystals contained in the material. Thus, the type of the non-radiative recombination is established by the composition of the passivation, while its rate depends on the degree of the charge carriers' quantum confinement.

Journal article

Du L, Liu Y, Zhou X, Tao L, Li M, Ren H, Ji R, Song K, Zhao X, Navarro-Cia Met al., 2022, Dual-band all-dielectric chiral photonic crystal, Journal of Physics D: Applied Physics, Vol: 55, ISSN: 0022-3727

We present an all-dielectric chiral photonic crystal that guides the propagation of electromagnetic waves without backscattering for dual bands. The chiral photonic crystal unit cell is composed of four dielectric cylinders with increasing inner diameter clockwise or anticlockwise, which leads to chirality. It is demonstrated that the proposed chiral photonic crystal can generate dual band gaps in the gigahertz frequency range and has two types of edge states, which is similar to topologically protected edge states. Hence, the interface formed by the proposed 2D chiral photonic crystal can guide the propagation of electromagnetic waves without backscattering, and this complete propagation is immune to defects (position disorder or frequency disorder). To illustrate the applicability of the findings in communication systems, we report a duplexer and a power divider based on the presented all-dielectric chiral photonic crystal.

Journal article

DeglInnocenti R, Lin H, Navarro-Cía M, 2022, Recent progress in terahertz metamaterial modulators, Nanophotonics, Vol: 11, Pages: 1485-1514, ISSN: 2192-8606

The terahertz (0.1–10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which have particularly benefitted from these developments.

Journal article

Nourinovin S, Navarro-Cia M, Rahman MM, Philpott MP, Abbasi QH, Alomainy Aet al., 2022, Terahertz Metastructures for Noninvasive Biomedical Sensing and Characterization in Future Health Care [Bioelectromagnetics], IEEE Antennas and Propagation Magazine, Vol: 64, Pages: 60-70, ISSN: 1045-9243

Journal article

Freer S, Sui C, Grover LM, Hanham SM, Navarro-Cía Met al., 2022, Temperature Dependent Hyperspectral Terahertz Imaging of Human Bone for Disease Diagnosis, ISSN: 1605-7422

Water is a fundamental component of many biological systems. The ability to detect water therefore provides great insight into system functionality, particularly in the development of disease. In this work, the high interaction of terahertz radiation with water, paired with the dependence of the dynamics of water molecules with varying temperature, is utilised to monitor changes in the composition of bone tissue. Heterotopic ossification (HO) bone samples and deionised free water are measured using terahertz time-domain spectroscopy for varying environmental temperatures, for prospective use in disease diagnosis.

Conference paper

Adams E, Attwood T, Freer S, Hanham SM, Constantinou C, Hu F, Azpilicueta L, Navarro-Cia Met al., 2021, Broadband characterisation of interior materials and surface scattering using terahertz time-domain spectroscopy, 2021 14th UK-Europe-China Workshop on Millimetre-Waves and Terahertz Technologies (UCMMT), Publisher: IEEE, Pages: 1-3

Indoor wireless communications need to move towards Terahertz (THz) frequencies in order to keep up with society's demand for data transmission, but this change is currently hindered by limited knowledge of material properties and propagation and scattering models at these frequencies. The dielectric properties of common household materials are investigated here with a twofold objective: (1) to extend the library of material properties at THz, and (2) to estimate and disentangle losses in scattering measurements in order to facilitate propagation, scattering and, ultimately, channel models.

Conference paper

Ivanov AV, Tatarenko AY, Gorodetsky AA, Makarevich ON, Navarro-Cía M, Makarevich AM, Kaul AR, Eliseev AA, Boytsova OVet al., 2021, Fabrication of epitaxial W-Doped VO2 nanostructured films for terahertz modulation using the solvothermal process, ACS Applied Nano Materials, Vol: 4, Pages: 10592-10600, ISSN: 2574-0970

We report a feasible and high-throughput method for high-quality W-doped VO2 nanostructured epitaxial films on r-sapphire substrate fabrication. Single-phase, smooth vanadium dioxide thin films with uniform distribution of tungsten (up to 2.3%) are formed using the solvothermal process from ethylene glycol/water V4+ and W6+ solutions. Compositional analysis by X-ray photoelectron and energy-dispersive X-ray spectroscopy (XPS and EDX, respectively); structural analysis (X-ray diffraction, Raman spectroscopy, selected area electron diffraction (SAED)); and detailed analysis of the surface morphology and substrate–film interface using scanning electron microscopy, atomic force microscopy, and high-resolution transmission electron microscopy (SEM, AFM, HRTEM, respectively) confirm the formation of nanoscale (50–60 nm) epitaxial W:VO2 (M1) on r-sapphire with epitaxial relationships (100)VO2∥(101̅2)Al2O3 and [010]VO2∥[011̅0]Al2O3. The nanostructured films demonstrate excellent terahertz (THz) transmission properties: a phase transition temperature of 31 °C, a huge THz modulation depth of over 60%, and broad bandwidth (≥2 THz) operation. Hence, they can be efficiently used as active material for tunable THz manipulation devices.

Journal article

Liu S, Ma S, Shao R, Zhang L, Yang B, Navarro-Cia M, Cui TJ, Zhang Set al., 2021, Edge state mimicking topological behavior in a one-dimensional electrical circuit, New Journal of Physics, Vol: 23, ISSN: 1367-2630

For one-dimensional (1D) topological insulators, the edge states always reside in the bulk bandgaps as isolated modes. The emergence and vanishing of these topological edge states are always associated with the closing/reopening of the bulk bandgap and changes in topological invariants. In this work, we discover a special kind of edge state in a 1D electrical circuit, which can appear not only inside the bandgap but also outside the bulk bands with the changing of bulk circuit parameters, resembling Tamm states or Shockley states. We prove analytically that the emergence/vanishing of this edge state and its position relative to the bulk bands depends on the intersections of certain critical frequencies. Specifically, the edge mode in the proposed circuit can be mathematically described by polynomials with roots equal to some critical frequencies in the bulk circuit. From this point of view, the transition of the edge state is uniquely determined by the order of the critical frequencies in the bulk circuit. Such topological behaviors shown by the edge state in the proposed electrical circuit may indicate, in a broader sense, the presence of certain type of topology.

Journal article

Cojocari M, Ospanova AK, Chichkov V, Navarro-Cia M, Gorodetsky A, Basharin AAet al., 2021, Pseudo-anapole regime in terahertz metasurfaces, Physical Review B: Condensed Matter and Materials Physics, Vol: 104, ISSN: 1098-0121

We present the numerical, theoretical, and experimental study of a terahertz metasurface supporting a pseudo-anapole. Pseudo-anapole effect arises when electric and toroidal dipole moments both tend to a minimum, instead of destructive interference between electric and toroidal dipole moments in conventional anapole mode. Such overlap allows resonance suppression of electric type radiation. Thus it becomes possible to study the multipoles of other families and higher order excitations. We estimate multipole contribution to the metasurface response via the multipole expansion method. The series is extended with such terms as mean-square radii and multipole interference. We also study the metasurface geometrical tunability. Via scaling, we demonstrate that it is possible to control the metasurface toroidal and electric responses independently. This in turn proves the fact that these multipoles have different physical origin. Moreover, we demonstrate that the proposed metasurface allows excitation of coherent magnetic dipole and electric quadrupole modes, which is crucial for planar cavities and lasing spasers in nanophotonics.

Journal article

Cojocari MV, Ospanova AK, Chichkov VI, Navarro-Cia M, Gorodetsky A, Basharin AAet al., 2021, Pseudo-Anapole Mode Establishment in Planar THz Metamaterial

In this paper, we propose new kind nonradiating state appearing in high Q planar toroidal THz metamaterial. So-called pseudo-anapole regime arises when the trivial solution to the non-radiating state condition is met. Here, both toroidal and electric dipole intensities are suppressed at resonance frequency while their far-field zone intensities tend to zero. The proposed effect is quite different from well-known anapole regime that is established by the condition p =- ikT that leads to nonradiating state [1]. The fundamental difference of pseudo-anapole state is that suppression of both electric and toroidal multipoles providing an opportunity for study of higher order multipoles from different families. This effect has been confirmed both numerically and experimentally in terahertz frequency range.

Conference paper

Freer S, Sui C, Hanham SM, Grover LM, Navarro-Cia Met al., 2021, Hybrid reflection retrieval method for terahertz dielectric imaging of human bone, Biomedical Optics Express, Vol: 12, Pages: 4807-4820, ISSN: 2156-7085

Terahertz imaging is becoming a biological imaging modality in its own right, alongside the more mature infrared and X-ray techniques. Nevertheless, extraction of hyperspectral, biometric information of samples is limited by experimental challenges. Terahertz time domain spectroscopy reflection measurements demand highly precise alignment and suffer from limitations of the sample thickness. In this work, a novel hybrid Kramers-Kronig and Fabry-Pérot based algorithm has been developed to overcome these challenges. While its application is demonstrated through dielectric retrieval of glass-backed human bone slices for prospective characterisation of metastatic defects or osteoporosis, the generality of the algorithm offers itself to wider application towards biological materials.

Journal article

Freer S, Sui C, Penchev P, Dimov S, Gorodetsky A, Hanham SM, Grover LM, Navarro-Cía Met al., 2021, Hyperspectral terahertz imaging for human bone biometrics, Terahertz Emitters, Receivers, and Applications XII, Publisher: SPIE, Pages: 1-6

The realisation of hyperspectral terahertz imaging is a significant step towards understanding of the life sciences on all scales. A key to this understanding is the retrieval of dielectric properties from such images, a task which is plagued by experimental limitations, challenging the terahertz community for more than two decades. In this contribution, we propose a new combined retrieval methodology to overcome misalignments and Fabry-Pérot effects on the extraction of the dielectric properties of human bone samples through the combination of the Kramers-Kronig relations and Fabry-Pérot reflection modelling. Results extracted from ∼100 µm human bone slices composed largely of collagen are consistent with those measured for pristine collagen samples. This represents another stepping-stone towards the adoption of terahertz imaging into pre- and clinical practice.

Conference paper

Sabery SM, Bystrov A, Navarro-Cia M, Gardner P, Gashinova Met al., 2021, Study of low terahertz radar signal backscattering for surface identification, Sensors, Vol: 21, Pages: 1-17, ISSN: 1424-8220

This study explores the scattering of signals within the mm and low Terahertz frequency range, represented by frequencies 79 GHz, 150 GHz, 300 GHz, and 670 GHz, from surfaces with different roughness, to demonstrate advantages of low THz radar for surface discrimination for automotive sensing. The responses of four test surfaces of different roughness were measured and their normalized radar cross sections were estimated as a function of grazing angle and polarization. The Fraunhofer criterion was used as a guideline for determining the type of backscattering (specular and diffuse). The proposed experimental technique provides high accuracy of backscattering coefficient measurement depending on the frequency of the signal, polarization, and grazing angle. An empirical scattering model was used to provide a reference. To compare theoretical and experimental results of the signal scattering on test surfaces, the permittivity of sandpaper has been measured using time-domain spectroscopy. It was shown that the empirical methods for diffuse radar signal scattering developed for lower radar frequencies can be extended for the low THz range with sufficient accuracy. The results obtained will provide reference information for creating remote surface identification systems for automotive use, which will be of particular advantage in surface classification, object classification, and path determination in autonomous automotive vehicle operation.

Journal article

Nekovic A, Camacho M, Freer S, Penchev P, Boix RR, Dimov S, Navarro-Cia Met al., 2021, Taming extraordinary THz transmission through sub-λ slot arrays via array truncation, slot rotation, polarization and angle of incidence, 2020 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-2

Accurate and time-effective simulation and design optimization of quasi-optical (QO) systems is an extremely challenging electromagnetic problem given the multi-scale dimension of QO components and the need to consider the finite size of such components to account for effects like diffraction. To show that the Method of Moments (MoM) provides an elegant solution for these problems, truncated rectangular arrays of tilted slots are measured in a QO Terahertz (THz) time-domain setup and comparison with MoM is carried out. The extraordinary transmission peaks are modulated by the size of the array and the orientation of the slots with respect to the incident electric field.

Conference paper

Gorodetsky A, Freer S, Navarro-Cia M, 2021, Continuous wave sub-Terahertz lensless holographic reflective imaging, 2020 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-1

We propose a simple setup involving standard commercially available sub-terahertz electronic source and camera in reflection layout. The setup is designed for imaging amplitude and phase objects. The construction allows for quick installation and potentially almost-realtime operation. Initial reconstruction results demonstrate resolution of about three wavelengths.

Conference paper

Freer S, Martinez R, Perez-Quintana D, Beruete M, Hanham SM, Attallah MM, Navarro-Cia Met al., 2021, Metal 3D printed D-band waveguide to surface wave transition, 2020 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-2

The coupling efficiency between free space waves and surface waves is low, narrowband, or both. Highly efficient broadband (better than 20% fractional bandwidth) coupling from waveguide modes can be achieved through sophisticated transitions whose fabrication can be enabled through additive manufacturing (e.g. selective laser melting). Here, we present alternative metallic transitions designed to couple the fundamental mode of a D-band waveguide to the fundamental transverse-magnetic surface mode supported by a periodic metal corrugated grating. Simulations of the coupling process and initial measurements have been undertaken.

Conference paper

Freer S, Gorodetsky A, Navarro-Cia M, 2021, Beam profiling of a commercial lens-assisted terahertz time domain spectrometer, IEEE Transactions on Terahertz Science and Technology, Vol: 11, Pages: 90-100, ISSN: 2156-342X

To undertake THz spectroscopy and imaging, and accurately design and predict the performance of quasi-optical components, knowledge of the parameters of the beam (ideally Gaussian) emitted from a THz source is paramount. Despite its proliferation, relatively little work has been done on this in the frame of broadband THz photoconductive antennas. Using primarily pinhole scanning methods, along with stepwise angular spectrum simulations, we investigate the profile and polarization characteristics of the beam emitted by a commercial silicon-lensintegrated THz photoconductive antenna and collimated by a TPX (polymethylpentene) lens. Our study flags the limitations of the different beam profiling methods and their impact on the beam Gaussianity estimation. A non-Gaussian asymmetric beam is observed, with main lobe beam waists along x and y varying from 8.4 ± 0.7 mm and 7.7±0.7 mm at 0.25THz,to1.4±0.7 mm and 1.4 ± 0.7 mm at 1 THz, respectively. Additionally, we report a maximum cross-polar component relative to the ON-axis co-polar component of -11.6 dB and -21.2 dB, at 0.25 THz and 1 THz, respectively.

Journal article

Pacheco-Pena V, Alves RA, Navarro-Cia M, 2020, Plasmonic nanoantennas and nanocavities: a transformation electromagnetics perspective, 2020 XXXIIIrd General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS), Publisher: IEEE, Pages: 1-4

Here, we apply the transformation electromagnetics technique to study the performance of plasmonic bowtie and diabolo nanoantennas as well as nanocavities. The nanoparticles are illuminated by a nanoemitter and their response is evaluated in terms of the non-radiative Purcell enhancement. The influence of the polarization of the emitter, metals and aperture of the arms is analyzed. Moreover, hidden symmetries between diabolo nanoantennas and nanocavities are unveiled and explained by means of the transformation electromagnetics approach.

Conference paper

Gorodetsky AA, Freer S, Navarro-Cía M, 2020, Assessment of cameras for continuous wave sub-terahertz imaging, Terahertz Emitters, Receivers, and Applications XI, Publisher: SPIE, Pages: 1-9

In this contribution, we present the direct comparison between Ophir Pyrocam IV and Terasense Tera-1024 cameras used for imaging of terahertz (THz) and sub-THz signals. We compare general properties, such as frequency dependent and polarisation dependent sensitivity, angle dependent sensitivity essential for holographic and noncollinear interferometric measurements, and draw a conclusion about the most suitable camera for the discussed imaging approaches. Both cameras show acceptable performance and sensitivity at imaging both 0.14 THz and 0.3 THz signals. The Terasense camera, expectedly, shows stronger polarisation dependent properties, however, is significantly more angle independent, showing an acceptable performance at all tested incident angles up to 50 degrees. At the same time, although the angle dependence is stronger for the Ophir camera, it has smaller pixel pitch and more extended post-processing features, thus making it somewhat better suited for noncollinear interferometric and holographic sub THz imaging.

Conference paper

Camacho M, Nekovic A, Freer S, Penchev P, Boix RR, Dimov S, Navarro-Cia Met al., 2020, Symmetry and finite-size effects in quasi-optical extraordinarily THz transmitting arrays of tilted slots, IEEE Transactions on Antennas and Propagation, Vol: 68, Pages: 6109-6117, ISSN: 0018-926X

Extraordinarily transmitting arrays are promising candidates for quasi-optical (QO) components due to their high frequency selectivity and beam scanning capabilities owing to the leaky-wave mechanism involved. We show here how by breaking certain unit cell and lattice symmetries, one can achieve a rich family of transmission resonances associated with the leaky-wave dispersion along the surface of the array. By combining 2-D and 1-D periodic method of moments (MoM) calculations with QO terahertz (THz) time-domain measurements, we provide physical insights, numerical, and experimental demonstration of the different mechanisms involved in the resonances associated with the extraordinary transmission peaks and how these evolve with the number of slots. Thanks to the THz instrument used, we are also able to explore the time-dependent emission of the different frequency components involved.

Journal article

Alves RA, Guerreiro A, Navarro-Cia M, 2020, Bridging the hydrodynamic Drude model and local transformation optics theory, PHYSICAL REVIEW B, Vol: 101, Pages: 1-9, ISSN: 2469-9950

The recent ability of plasmonic nanostructures to probe subnanometer and even atomic scales demands theories that can account for the nonlocal dynamics of the electron gas. The hydrodynamic Drude model (HDM) captures much of the microscopic dynamics of the quantum mechanical effects when additional boundary conditions are considered. Here, we revisit the HDM under the Madelung formalism to reexpress its coupled system of equations as a single nonlinear Schrödinger equation in order to have a natural quantum mechanical description of plasmonics. Specifically, we study the response of two overlapping nanowires with this formalism. We ensure that an proposed frame concurs with classical electrodynamics when the local response approximation holds in the plasmonic system by finding the correction needed.

Journal article

Razavizadeh SM, Kashani ZG, Sadeghzadeh R, Navarro-Cía Met al., 2020, Tunable compression of THz chirped pulses using a helical graphene ribbon-loaded hollow-core waveguide., Applied Optics, Vol: 59, Pages: 4247-4253, ISSN: 1559-128X

Pulse shaping is important for communications, spectroscopy, and other applications that require high peak power and pulsed operation, such as radar systems. Unfortunately, pulse shaping remains largely elusive for terahertz (THz) frequencies. To address this void, a comprehensive study on the dispersion tunability properties of THz chirped pulses traveling through a dielectric-lined hollow-core waveguide loaded with a helical graphene ribbon is presented. It is demonstrated that there is an optimal compression waveguide length over which THz chirped pulses reach the maximum compression. The optimal length is dependent on the chirp pulse duration. It is shown that by applying an electrostatic controlling gate voltage (Vg) of 0 and 30 V on the helical graphene ribbon, the temporal input pulses of width 8 and 12 ps, propagating through two different lengths, can be tuned by 5.9% and 8%, respectively, in the frequency range of 2.15-2.28 THz.

Journal article

Freer S, Camacho M, Kuznetsov SA, Boix RR, Beruete M, Navarro-Cia Met al., 2020, Revealing the underlying mechanisms behind TE extraordinary THz transmission, Photonics Research, Vol: 8, Pages: 430-439, ISSN: 2327-9125

Transmission through seemingly opaque surfaces, so-called extraordinary transmission, provides an exciting platform for strong light–matter interaction, spectroscopy, optical trapping, and color filtering. Much of the effort has been devoted to understanding and exploiting TM extraordinary transmission, while TE anomalous extraordinary transmission has been largely omitted in the literature. This is regrettable from a practical point of view since the stronger dependence of the TE anomalous extraordinary transmission on the array’s substrate provides additional design parameters for exploitation. To provide high-performance and cost-effective applications based on TE anomalous extraordinary transmission, a complete physical insight about the underlying mechanisms of the phenomenon must be first laid down. To this end, resorting to a combined methodology including quasi-optical terahertz (THz) time-domain measurements, full-wave simulations, and method of moments analysis, subwavelength slit arrays under s-polarized illumination are studied here, filling the void in the current literature. We believe this work unequivocally reveals the leaky-wave role of the grounded-dielectric slab mode mediating in TE anomalous extraordinary transmission and provides the necessary framework to design practical high-performance THz components and systems.

Journal article

Liu Y, Li M, Song K, Hu D, Liu H, Zhao X, Zhang S, Navarro-Cia Met al., 2020, Leaky-wave antenna with switchable omnidirectional conical radiation via polarization handedness, IEEE Transactions on Antennas and Propagation, Vol: 68, Pages: 1282-1288, ISSN: 0018-926X

Reconfigurable antennas capable of beam-steering offer an efficient solution to optimize the use of the crowded wireless medium and can serve as a multifunction antenna. Beam-steering is often achieved by antenna geometry switching at the expense of hardware complexity. Here, polarization is used to realize beam-steering without the need of antenna geometry modification. Depending on the handedness of the feed, backward or forward conical radiation is demonstrated in a ∼13λ0-long short-circuited helically slotted waveguide antenna. Tapering the slit width with a Taylor distribution reduces the measured sidelobe levels by ∼3 dB in average and results in a realized gain of 10-13 dB and 11-13 dB for right-handed (backward radiation) and left-handed circularly polarized (forward radiation) feeding, respectively, in the bandwidth from 8.5 to 9.5 GHz.

Journal article

Freer S, Gape J, Shalom E, Navarro-Cia M, Kuznetsov SA, Beruete Met al., 2020, Study of leaky waves responsible for terahertz TE extroardinary transmission, 2019 12th UK-Europe-China Workshop on Millimeter Waves and Terahertz Technologies (UCMMT), Publisher: IEEE

Extraordinary transmission (ET) via transverse electric (TE) modes through dielectric-backed periodic subwavelength slit arrays is largely owed to the leakage of energy from waves guided by the structure. This phenomenon is investigated in order to shed light on the leaky wave mechanism. Both angular transmission and temporal measurements are undertaken, revealing the origin of both the modes developed in the structures and the behaviour of the energy coupled to these modes.

Conference paper

Makarevich A, Makarevich O, Ivanov A, Sharovarov D, Eliseev A, Amelichev V, Boytsova O, Gorodetsky A, Navarro-Cía M, Kaul Aet al., 2020, Hydrothermal epitaxy growth of self-organized vanadium dioxide 3D structures with metal–insulator transition and THz transmission switch properties, CrystEngComm, Vol: 22, Pages: 2612-2620, ISSN: 1466-8033

The hydrothermal method is the most effective approach for the synthesis of VO2 metastable polymorphs with unique powder crystallite morphology. In this work, we expanded the capabilities of this method, directing it to the growth of oriented crystallites in self-organized systems on single crystal substrates. According to our investigations, a large variety of 3D structures of vanadium dioxide can be obtained using one single crystal substrate r-sapphire by fine tuning of synthesis parameters. The orientation growth of six-pointed vanadium dioxide crystallites fits into an epitaxial growth model describing unit cell relations between the VO2(M1) film and r-sapphire substrate. We describe the process of VO2(M1) phase stabilization in the films and the changes of resistivity and terahertz transparency of the films based on the metal–insulator transition (MIT).

Journal article

Pacheco-Peña V, Alves RA, Navarro-Cía M, 2020, From symmetric to asymmetric bowtie nanoantennas: electrostatic conformal mapping perspective, Nanophotonics, Vol: 0, ISSN: 2192-8606

Plasmonic nanoantennas have revolutionized the way we study and modulate light–matter interaction. Due to nanofabrication limitations, dimer-type nanoantennas always exhibit some degree of asymmetry, which is desirable in some cases. For instance, in sensing applications, asymmetry is sometimes induced by design in plasmonic nanoantennas to favor higher order nonradiative modes with sharp Fano line shapes. Regardless of the actual origin of the asymmetry, unintentional or intentional, an analytical frame that can deal with it in a seamless manner would be beneficial. We resort to conformal mapping for this task and we track the influence of the degree of asymmetry of the circular sectors composing gold bowtie nanoantennas on the nonradiative Purcell enhancement of a nearby nanoemitter. This manuscript reviews the contributions of conformal mapping to plasmonic nanoantennas and illustrates the advantages of the elegant analytical solution provided by conformal mapping to grasp physical insights, which can serve as a springboard for new plasmonic asymmetric nanoantenna designs.

Journal article

Pacheco-Pena V, Alves R, Navarro-Cia M, 2019, Conformal transformation in bowtie nanoantennas and nanocavities: unveiling hidden symmetries, 2019 Thirteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), Publisher: IEEE

In this work, bowtie nanoantennas and nanocavities are analyzed using the conformal transformation technique. Their performance is studied in terms of the non-radiative Purcell enhancement and self-induced optical forces experienced by quantum emitters. It is demonstrated how these two geometrically different plasmonic nanoparticles can share the same non-radiative Purcell spectra. This hidden symmetric response is unveiled by properly applying the conformal transformation technique, demonstrating that both nanoparticles share the same transformed geometry.

Conference paper

Ma M, Wang Y, Navarro-Cía M, Liu F, Zhang F, Liu Z, Li Y, Hanham SM, Hao Zet al., 2019, The dielectric properties of some ceramic substrate materials at terahertz frequencies, Journal of the European Ceramic Society, Vol: 39, Pages: 4424-4428, ISSN: 0955-2219

The terahertz (THz) dielectric constant (εr') and dielectric loss tangent (tanδ) of the commercial LTCC materials (Ferro A6M and DuPont 951), Al2O3 (ceramic and single crystal), AlN and β-Si3N4 ceramics were measured using a vector network analyzer (VNA) over the frequency range of 140–220 GHz and a time-domain spectrometer (TDS) from 0.2 to 1.0 THz. The results from the two instruments are compared with the literature and show good agreement and consistency. For Ferro A6M, εr' = 6.06, tanδ = 0.012 at 1.0 THz. For DuPont 951, εr' = 7.67, tanδ = 0.097 at 1.0 THz. For Al2O3 ceramic and single crystal, the measured THz dielectric properties are consistent with the reported works. The dielectric constant of AlN (εr' = 8.85) and β-Si3N4 (εr' = 8.41) ceramics in the THz region is a little lower than those reported for the MHz to GHz region. These results provide valuable and much needed reference information for device designers and material scientists.

Journal article

Freer S, Camacho M, Kuznetsov SA, Shalom E, Gape J, Boix RR, Beruete M, Navarro-Cia Met al., 2019, Modes and pseudo-modes in TE extraordinary THz transmission, 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Publisher: IEEE, ISSN: 2162-2027

Transverse electric (TE) transmission through dielectric-baked subwavelength slit arrays is inextricably linked to the grounded-dielectric slab TE1 mode, and thus, it depends highly on the electrical thickness of the substrate. We track the influence of the TE1 mode using both TDS measurements and the Method of Moments. For electrically thick dielectric samples, the TE1 mode is in propagation, and hence total transmission occurs (in excess of 0 dB, attributed to collimation), whereas in electrically thin samples, the TE1 mode is in cutoff. In this regime, the mode still contributes to transmission, and hence is referred to as a pseudo-mode.

Conference paper

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