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Purpose: Radiofrequency field inhomogeneity is a significant issue in imaging large fields of view in high‐ and ultrahigh‐field MRI. Passive shimming with coupled coils or dielectric pads is the most common approach at 3 T. We introduce and test light and compact metasurface, providing the same homogeneity improvement in clinical abdominal imaging at 3 T as a conventional dielectric pad. Methods: The metasurface comprising a periodic structure of copper strips and parallel‐plate capacitive elements printed on a flexible polyimide substrate supports propagation of slow electromagnetic waves similar to a high‐permittivity slab. We compare the metasurface operating inside a transmit body birdcage coil to the state‐of‐the‐art pad by numerical simulations and in vivo study on healthy volunteers. Results: Numerical simulations with different body models show that the local minimum of B1+ causing a dark void in the abdominal domain is removed by the metasurface with comparable resulting homogeneity as for the pad with decreasing maximum and whole‐body SAR values. In vivo results confirm similar homogeneity improvement and demonstrate the stability to body mass index. Conclusion: The light, flexible, and inexpensive metasurface can replace a relatively heavy and expensive pad based on the aqueous suspension of barium titanate in abdominal imaging at 3 T. [ABSTRACT FROM AUTHOR]

The existence of non‐radiating electromagnetic sources attracts much attention in photonic community and gives rise to extensive discussions of various applications in lasing, medical imaging, sensing, and nonlinear optics. In this article, the existence of magnetic anapole states (or magnetic‐type non‐radiating sources) characterized by a suppressed magnetic dipole radiation in a dielectric cylindrical particle is theoretically predicted and experimentally demonstrated. The specific features of the magnetic anapole state under ideal conditions are identified, followed by a demonstration of how their existence can be detected in practical structures. The concept is valid in various frequency bands from visible range for nanoparticles to microwave range for millimeter size objects. The experimental study is performed in microwave frequency range which allows not only to measure the far‐field (scattered field) characteristics, but also to probe the peculiar field profile directly inside the dielectric particle. The experimental results agree well with the analytical ones and pave the way to detect and identify nontrivial different‐type anapole states. [ABSTRACT FROM AUTHOR]

One of the most intriguing problems of light transport in solids is the localization that has been observed in various disordered photonic structures. The light localization in defect‐free icosahedral quasicrystals has recently been predicted theoretically without experimental verification. Herein, the fabrication of submicron‐size dielectric icosahedral quasicrystals is reported and the results of detailed studies of the photonic properties of these structures are demonstrated. The first direct experimental observation of intrinsic light localization in defect‐free quasicrystals is presented. This result is obtained in time‐resolved measurements at different laser wavelengths in the visible. The localization is linked with the aperiodicity of the icosahedral structure, which leads to uncompensated scattering of light from an individual structural element over the entire sphere, providing multiple scattering inside the sample and, as a result, the intrinsic localization of light. [ABSTRACT FROM AUTHOR]

In this paper we address the possibility to perform imaging of two samples within the same acquisition time using coupled ceramic resonators and one transmit/receive channel. We theoretically and experimentally compare the operation of our ceramic dual‐resonator probe with a wire‐wound solenoid probe, which is the standard probe used in ultrahigh‐field magnetic resonance microscopy. We show that due to the low‐loss ceramics used to fabricate the resonators, and a favorable distribution of the electric field within the conducting sample, a dual probe, which contains two samples, achieves an SNR enhancement by a factor close to the square root of 2 compared with a solenoid optimized for one sample. [ABSTRACT FROM AUTHOR]

Besides classical dipole moments, the metaparticles are characterized by toroidal dipole moments that are important for numerous applications ranging from strong field localizations to anapole modes and the dynamic Aharonov–Bohm effect. On the other hand, the toroidal and electric dipole radiation are undistinguished by external observers due to identical radiation patterns. Therefore, the importance of toroidal dipole moments in multipole expansion is questioned by many researchers. However, a long‐awaited unanswered question is—what is going on inside a toroidal metamolecule and is its near‐field distribution distinct from the electric dipole field? Herein, the toroidal dipole mode is experimentally confirmed in situ by proof‐of‐concept measurements of electric and magnetic fields inside properly fabricated water metamolecules with toroidal topology in the microwave frequency range. [ABSTRACT FROM AUTHOR]

Purpose: Design and characterization of a new inductively driven wireless coil (WLC) for wrist imaging at 1.5 T with high homogeneity operating due to focusing the B1 field of a birdcage body coil. Methods: The WLC design has been proposed based on a volumetric self‐resonant periodic structure of inductively coupled split‐loop resonators with structural capacitance. The WLC was optimized and studied regarding radiofrequency fields and interaction to the birdcage coil (BC) by electromagnetic simulations. The manufactured WLC was characterized by on‐bench measurements and in vivo and phantom study in comparison to a standard cable‐connected receive‐only coil. Results: The WLC placed into BC gave the measured B 1 + increase of the latter by 8.6 times for the same accepted power. The phantom and in vivo wrist imaging showed that the BC in receiving with the WLC inside reached equal or higher signal‐to‐noise ratio than the conventional clinical setup comprising the transmit‐only BC and a commercial receive‐only flex‐coil and created no artifacts. Simulations and on‐bench measurements proved safety in terms of specific absorption rate and reflected transmit power. Conclusions: The results showed that the proposed WLC could be an alternative to standard cable‐connected receive coils in clinical magnetic resonance imaging. As an example, with no cable connection, the WLC allowed wrist imaging on a 1.5 T clinical machine using a full‐body BC for transmitting and receive with the desired signal‐to‐noise ratio, image quality, and safety. [ABSTRACT FROM AUTHOR]

Abstract: Plasmonic nanoparticles coupled with metallic films forming nanometer scale cavities have recently emerged as a powerful tool for enhancement of light‐matter interaction. Despite high efficiency for sensing and light emission, such nanocavities exhibit harmful and uncontrolled optical heating which limits the ranges of light intensities and working temperature. In contrast to plasmonic nanoparticles, all‐dielectric counterparts possess low Ohmic losses, high temperature stability along with a strong temperature‐dependent Raman response. Here, we demonstrate that a silicon nanoparticle coupled with a thin gold film can serve as a multifunctional metal‐dielectric (hybrid) nanocavity operating up to 1200 K. Resonant interaction of light with such nanocavity enables molecular sensing, heat‐induced molecular events (protein unfolding), and their real‐time tracing with a nanoscale thermometry through the monitoring enhanced Raman scattering both from the nanoparticle and analyzed molecules. We model numerically the thermo‐optical properties of the hybrid nanocavity and reveal two alternative regimes of operation ‐ with and without strong optical heating while other functionalities are preserved. We believe that the concept of the multifunctional hybrid nanocavities holds great potential for diverse photochemical and photophysical applications. [ABSTRACT FROM AUTHOR]

Asymmetric metal-dielectric nanostructures are demonstrated superior optical properties arising from the combination of strong enhancement of near-fields and controllable scattering characteristics which originate from plasmonic and high-index dielectric components. Here, being inspired by the recent advances of the asymmetric hybrid nanoparticles fabrication [Dmitry Zuev, et al., Adv. Mater. 28, 3087 (2016)], we suggest and study numerically a novel type of hybrid dimer nanoantennas. The nanoantennas consist of asymmetric metal-dielectric (Au/Si) nanoparticles and allow tuning of the near- and far-field properties via laser induced reshaping of the metal part at the nanoscale. We demonstrate an ability to modification of the scattering properties, near-field distribution profilis, normalized local density of states, and radiation patterns of the nanoantenna upon the laser reshaping. The parameters used to investigate these effects correspond to experimentally demonstrated values. [ABSTRACT FROM AUTHOR]

Photoconductive antennas are promising sources of terahertz radiation that is widely used for spectroscopy, characterization, and imaging of biological objects, deep space studies, scanning of surfaces, and detection of potentially hazardous substances. These antennas are compact and allow for generation of both ultrabroadband pulses and tunable continuous wave terahertz signals at room temperatures, with no need for high-power optical sources. However, such antennas have relatively low energy conversion efficiency of femtosecond laser pulses or two close pump wavelengths (photomixers) into the pulsed and continuous terahertz radiation, correspondingly. Recently, an approach to solving this problem that involves known methods of nanophotonics applied to terahertz photoconductive antennas and photomixers has been proposed. This approach comprises the use of optical nanoantennas for enhancing the absorption of pump laser radiation in the antenna gap, reducing the lifetime of photoexcited carriers, and improving the antenna thermal efficiency. This Review is intended to systematize the main results obtained by researchers in this promising field of hybrid optical-to-terahertz photoconductive antennas and photomixers. We summarize the main results on hybrid THz antennas, compare the approaches to their implementation, and offer further perspectives of their development including an application of all-dielectric nanoantennas instead of plasmonic ones. [ABSTRACT FROM AUTHOR]

Employing nonlocal homogenization approach, we investigate the properties of a metamaterial consisting of parallel metallic wires with dielectric coating. We demonstrate that manipulation of dielectric contrast between wire dielectric shell and host material at fixed frequency results in dynamic switching of metamaterial dispersion regime from elliptic to the hyperbolic one, i.e. the topological transition takes place. It is proved that such transition can be induced by the variation of the metamaterial temperature. Our findings thus pave a way to the implementation of a tunable 'elliptic-hyperbolic' metamaterial. [ABSTRACT FROM AUTHOR]

All-optical ultrafast signal modulation and routing by low-loss nanodevices is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron-hole plasma with ultrafast relaxation rate. On the basis of this concept, we demonstrate the effect of beam steering by up to 20 degrees through simple variation of the intensity of incident light. The effect, which is suitable for ultrafast light routing in an optical chip, is demonstrated both in the visible and near-IR spectral regions for silicon- and germanium-based nanoantennas. We also reveal the effect of electron-hole plasma photoexcitation on the local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50%, whereas modification of the projected LDOS can be even more dramatic, almost five-fold for transverse dipole orientation. Moreover, our analytical model sheds light on the transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the suggested ultrafast nanodevice. The proposed concept paves the way to the creation of low-loss, ultrafast, and compact devices for optical signal modulation and routing. [ABSTRACT FROM AUTHOR]

Dewetting of thin metal films is one of the most widespread method for functional plasmonic nanostructures fabrication. However, simple thermal-induced dewetting does not allow to control degree of nanostructures order without additional lithographic process steps. Here we propose a novel method for lithography-free and large-scale fabrication of plasmonic nanostructures via controllable femtosecond laser-induced dewetting. The method is based on femtosecond laser surface pattering of a thin film followed by a nanoscale hydrodynamical instability, which is found to be very controllable under specific irradiation conditions. We achieve control over degree of nanostructures order by changing laser irradiation parametrs and film thickness. This allowed us to exploit the method for the broad range of applications: resonant light absorbtion and scattering, sensing, and potential improving of thin-film solar cells. [ABSTRACT FROM AUTHOR]

Background and Purpose: Aging of the healthy brain is characterized by focal or nonfocal white matter (WM) signal abnormality (SA) changes, which are typically detected as leukoaraiosis (LA). Hypertension is a risk factor for WM lesion formation. This study investigated whether LA might be associated with increased cerebrospinal fluid (CSF) pulsatility linked to arterial hypertension. Methods: A total of 101 individuals without neurologic diseases (53 females and 48 males) aged between 18 and 75 years underwent 3T brain MRI with cine phase contrast imaging for CSF flow estimation, after providing their informed consent. LA was defined as the presence of focal T2 WM SA changes and/or nonfocal uniform areas of signal increase termed dirty appearing white matter (DAWM). Relevant information relating to cardiovascular risk factors was also collected. Results: When controlled for age and hypertension, significant partial correlations were observed between: DAWM volume and: net negative flow (r = -.294, P = .014); net positive flow (NPF) (r = .406, P = .001); and peak positive velocity (r = .342, P = .004). Multiple linear regression analysis revealed DAWM volume to be significantly correlated with CSF NPF (P = .019) and hypertension (P = .007), whereas T2 WM SA volume was only significantly correlated with age (P = .002). Combined DAWM and T2 WM SA volumes were significantly related with age (P = .001) and CSF peak negative velocity (P = .041). Conclusions: Rarefaction of WM leading to LA is a multifactorial process, in which formation of DAWM induced by hypertension and increased aqueductal CSF pulsatility, may play a contributory role. These two factors appear to act independently of each other in a process that is independent of age. [ABSTRACT FROM AUTHOR]

A novel approach to facilitate excitation and readout processes of isolated negatively charged nitrogen-vacancy (NV) centers is proposed. The approach is based on the concept of all-dielectric nanoantennas. It is shown that the all-dielectric nanoantenna can significantly enhance both the emission rate and emission extraction efficiency of a photoluminescence signal from a single NV center in a diamond nanoparticle on a dielectric substrate. The proposed approach provides high directivity, large Purcell factor, and efficient beam steering, thus allowing an efficient far-field initialization and readout of several NV centers separated by subwavelength distances. [ABSTRACT FROM AUTHOR]

Following the suggestion of Rodriguez-Fortuno et al. [Phys. Rev. Lett. 112, 033902 (2014)], we study the repulsive force acting on a electric dipole placed over a surface of epsilon-near-zero (ENZ) metamaterial. The dependence of the repulsive force value on the dipole size has been studied. We show that the effect of finite size drastically affects the values of the repulsive force as compared to the point-dipole case. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Purpose: To investigate cerebrospinal fluid (CSF) dynamics in the aqueduct of Sylvius (AoS) in chronic cerebrospinal venous insufficiency (CCSVI)-positive and - negative healthy individuals using cine phase contrast imaging. Materials and Methods: Fifty-one healthy individuals (32 CCSVI-negative and 19 age-matched CCSVI-positive subjects) were examined using Doppler sonography (DS). Diagnosis of CCSVI was established if subjects fulfilled ⩾2 venous hemodynamic criteria on DS. CSF flow and velocity measures were quantified using a semiautomated method and compared with clinical and routine 3T MRI outcomes. Results: CCSVI was associated with increased CSF pulsatility in the AoS. Net positive CSF flow was 32% greater in the CCSVI-positive group compared with the CCSVInegative group (P = 0.008). This was accompanied by a 28% increase in the mean aqueductal characteristic signal (ie, the AoS cross-sectional area over the cardiac cycle) in the CCSVI-positive group compared with the CCSVI-negative group (P = 0.021). Conclusion: CSF dynamics are altered in CCSVI-positive healthy individuals, as demonstrated by increased pulsatility. This is accompanied by enlargement of the AoS, suggesting that structural changes may be occurring in the brain parenchyma of CCSVI-positive healthy individuals. [ABSTRACT FROM AUTHOR]

In order to achieve interaction between light beams, a mediating material object is required. Nonlinear materials are commonly used for this purpose. Here a new approach to control light with light, based on a nano-opto-mechanical system integrated in a plasmonic waveguide is proposed. Optomechanics of a free-floating resonant nanoparticle in a subwavelength plasmonic V-groove waveguide is studied. It is shown that nanoparticle auto-oscillations in the waveguide induced by a control light result in the periodic modulation of a transmitted plasmonic signal. The modulation depth of 10% per single nanoparticle of 25 nm diameter with the clock frequencies of tens of MHz and the record low energy-per-bit energies of 10−18 J is observed. The frequency of auto-oscillations depends on the intensity of the continuous control light. The efficient modulation and deep-subwavelength dimensions make this nano-optomechanical system of significant interest for opto-electronic and opto-fluidic technologies. [ABSTRACT FROM AUTHOR]

Purpose: To investigate cerebrospinal fluid (CSF) dynamics in the aqueduct of Sylvius in multiple sclerosis (MS) patients and healthy controls (HC) using cine phase contrast imaging. Materials and Methods: In all, 67 MS patients (48 relapsing-remitting [RR] and 19 secondary-progressive [SP]), nine patients with clinically isolated syndrome (CIS), and 35 age- and sex-matched HC were examined. CSF flow and velocity measures were quantified using a semiautomated method and compared with clinical and magnetic resonance imaging (MRI) disease outcomes. Results: Significantly decreased CSF net flow was detected in MS patients compared to HC (−3.7 vs. −7.1 μL/beat, P = 0.005). There was a trend for increased net positive flow between SP, RR, and CIS patients. Altered CSF flow and velocity measures were associated with more severe T1 and T2 lesion volumes, lateral and fourth ventricular volumes, and third ventricular width in MS and CIS patients ( P < 0.01 for all). In CIS patients, conversion to clinically definite MS in the following year was related to decreased CSF net flow ( P = 0.007). There was a trend between increased annual relapse rate and altered CSF flow/velocity measures in RRMS patients ( P < 0.05). Conclusion: CSF flow dynamics are altered in MS patients. More severe clinical and MRI outcomes in RRMS and CIS patients relate to altered CSF flow and velocity measures. J. Magn. Reson. Imaging 2012;36:825-834. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In this article, the authors discuss various articles on modern trends in metamaterial applications and photonic devices published in this issue of the periodical.

No abstract is available for this article. [ABSTRACT FROM AUTHOR]