Improved Back-Projection Cortical Potential Imaging by Multi-resolution Optimization Technique.

Electroencephalogram (EEG) has evolved to be a well-established tool for imaging brain activity. This progress is mainly due to the development of high-resolution (HR) EEG methods. One class of HR-EEG is the cortical potential imaging (CPI), which aims to estimate the potential distribution on the cortical surface, which is much more informative than EEG. Even though these methods exhibit good performance, most of them have inherent inaccuracies that originate from their operating principles that constrain the solution or require a complex calculation process. The back-projection CPI (BP-CPI) method is relatively new and has the advantage of being constraint-free and computation inexpensive. The method has shown relatively good accuracy, which is necessary to become a clinical tool. However, better performance must be achieved. In the present study, two improvements are proposed. Both are embedded as adjacent stages to the BP-CPI and are based on the multi-resolution optimization approach (MR-CPI). A series of Monte-Carlo simulations were performed to examine the characteristics of the proposed improvements. Additional tests were done, including different EEG noise levels and variation in electrode-numbers. The results showed highly accurate cortical potential estimations, with a reduction in estimation error by a factor of 3.75 relative to the simple BP-CPI estimation error. We also validated these results with true EEG data. Analyzing these EEGs, we have demonstrated the MR-CPI competence to correctly localize cortical activations in a real environment. The MR-CPI methods were shown to be reliable for estimating cortical potentials, enabling researchers to obtain fast and robust high-resolution EEGs.

Wireless Communication with Nanoplasmonic Data Carriers: Macroscale Propagation of Nanophotonic Plasmon Polaritons Probed by Near-Field Nanoimaging.

The ability to control the energy flow of light at the nanoscale is fundamental to modern communication and big-data technologies, as well as quantum information processing schemes. However, since photons are diffraction-limited, efforts of confining them to dimensions of integrated electronics have so far proven elusive. A promising way to facilitate nanoscale manipulation of light is through plasmon polaritons-coupled excitations of photons and charge carriers. These tightly confined hybrid waves can facilitate compression of optical functionalities to the nanoscale but suffer from huge propagation losses that limit their use to mostly subwavelength scale applications. With only weak evidence of macroscale plasmon polaritons, propagation has recently been reported theoretically and indirectly, no experiments so far have directly resolved long-range propagating optical plasmons in real space. Here, we launch and detect nanoscale optical signals, for record distances in a wireless link based on novel plasmonic nanotransceivers. We use a combination of scanning probe microscopies to provide high resolution real space images of the optical near fields and investigate their long-range propagation principles. We design our nanotransceivers based on a high-performance nanoantenna, Plantenna, hybridized with channel plasmon waveguides with a cross-section of 20 nm × 20 nm, and observe propagation for distances up to 1000 times greater than the plasmon wavelength. We experimentally show that our approach hugely outperforms both waveguide and wireless nanophotonic links. This successful alliance between Plantenna and plasmon waveguides paves the way for new generations of optical interconnects and expedites long-range interaction between quantum emitters and photomolecular devices.

Back-Projection Cortical Potential Imaging: Theory and Results.

Electroencephalography (EEG) is the single brain monitoring technique that is non-invasive, portable, passive, exhibits high-temporal resolution, and gives a directmeasurement of the scalp electrical potential. Amajor disadvantage of the EEG is its low-spatial resolution, which is the result of the low-conductive skull that "smears" the currents coming from within the brain. Recording brain activity with both high temporal and spatial resolution is crucial for the localization of confined brain activations and the study of brainmechanismfunctionality, whichis then followed by diagnosis of brain-related diseases. In this paper, a new cortical potential imaging (CPI) method is presented. The new method gives an estimation of the electrical activity on the cortex surface and thus removes the "smearing effect" caused by the skull. The scalp potentials are back-projected CPI (BP-CPI) onto the cortex surface by building a well-posed problem to the Laplace equation that is solved by means of the finite elements method on a realistic head model. A unique solution to the CPI problem is obtained by introducing a cortical normal current estimation technique. The technique is based on the same mechanism used in the well-known surface Laplacian calculation, followed by a scalp-cortex back-projection routine. The BP-CPI passed four stages of validation, including validation on spherical and realistic head models, probabilistic analysis (Monte Carlo simulation), and noise sensitivity tests. In addition, the BP-CPI was compared with the minimum norm estimate CPI approach and found superior for multi-source cortical potential distributions with very good estimation results (CC >0.97) on a realistic head model in the regions of interest, for two representative cases. The BP-CPI can be easily incorporated in different monitoring tools and help researchers by maintaining an accurate estimation for the cortical potential of ongoing or event-related potentials in order to have better neurological inferences from the EEG.

Secondary Electron Imaging of Light at the Nanoscale.

The interaction of fast electrons with metal atoms may lead to optical excitations. This exciting phenomenon forms the basis for the most powerful inspection methods in nanotechnology, such as cathodoluminescence and electron-energy loss spectroscopy. However, direct nanoimaging of light based on electrons is yet to be introduced. Here, we experimentally demonstrate simultaneous excitation and nanoimaging of optical signals using unmodified scanning electron microscope. We use high-energy electron beam for plasmon excitation and rapidly image the optical near fields using the emitted secondary electrons. We analyze dipole nanoantennas coupled with channel nanoplasmonic waveguides and observe both surface plasmons and surface plasmon polaritons with spatial resolution of 25 nm. Our experimental results are confirmed by rigorous numerical calculations based on full-wave solution of Maxwell's equations, showing high correlation between optical near fields and secondary electrons images. This demonstration of optical near-field mapping using direct electron imaging provides essential insights to the exciting relations between electrons plasmons and photons, paving the way toward secondary electron-based plasmon analysis at the nanoscale.

Enabling High Efficiency Nanoplasmonics with Novel Nanoantenna Architectures.

Surface plasmon polaritons (SPPs) are propagating excitations that arise from coupling of light with collective electron oscillations. Characterized by high field intensity and nanometric dimensions, SPPs fashion rapid expansion of interest from fundamental and applicative perspectives. However, high metallic losses at optical frequencies still make nanoplasmonics impractical when high absolute efficiency is paramount, with major challenge is efficient plasmon generation in deep nanoscale. Here we introduce the Plantenna, the first reported nanodevice with the potential of addressing these limitations utilizing novel plasmonic architecture. The Plantenna has simple 2D structure, ultracompact dimensions and is fabricated on Silicon chip for future CMOS integration. We design the Plantenna to feed channel (20 nm × 20 nm) nanoplasmonic waveguides, achieving 52% coupling efficiency with Plantenna dimensions of λ(3)/17,000. We theoretically and experimentally show that the Plantenna enormously outperforms dipole couplers, achieving 28 dB higher efficiency with broad polarization diversity and huge local field enhancement. Our findings confirm the Plantenna as enabling device for high efficiency plasmonic technologies such as quantum nanoplasmonics, molecular strong coupling and plasmon nanolasers.

Observing optical plasmons on a single nanometer scale.

The exceptional capability of plasmonic structures to confine light into deep subwavelength volumes has fashioned rapid expansion of interest from both fundamental and applicative perspectives. Surface plasmon nanophotonics enables to investigate light-matter interaction in deep nanoscale and harness electromagnetic and quantum properties of materials, thus opening pathways for tremendous potential applications. However, imaging optical plasmonic waves on a single nanometer scale is yet a substantial challenge mainly due to size and energy considerations. Here, for the first time, we use Kelvin Probe Force Microscopy (KPFM) under optical illumination to image and characterize plasmonic modes. We experimentally demonstrate unprecedented spatial resolution and measurement sensitivity both on the order of a single nanometer. By comparing experimentally obtained images with theoretical calculation results, we show that KPFM maps may provide valuable information on the phase of the optical near field. Additionally, we propose a theoretical model for the relation between surface plasmons and the material workfunction measured by KPFM. Our findings provide the path for using KPFM for high resolution measurements of optical plasmons, prompting the scientific frontier towards quantum plasmonic imaging on submolecular scales.

Towards integrated nanoplasmonic logic circuitry.

Surface plasmon polaritons (SPPs) may serve as ultimate data processing expedients in future nanophotonic applications. SPPs combine the high localization of electrons with the bandwidth, frequency and propagation properties of photons, thus supplying nature with the best of two worlds. However, although plasmonics have recently gained constantly growing scientific attention, logic devices that operate on SPPs on a deep nanometer scale are yet to be demonstrated. Here, we design, fabricate and experimentally verify the smallest, first ever reported all optical nanoplasmonic XOR logic gate. The introduced XOR device is based on a novel engineerable interferometry scheme with extremely compact dimensions of λ(3)/15,500, which can be used to realize a variety of plasmonic logic functionalities. We use frequency modulated Kelvin probe microscopy to provide evidence of binary XOR functionality performed directly on SPPs with λ(3)/80,000 mode volumes. An extinction ratio of 10 dB is achieved for a device length of 150 nm, increasing up to 30 dB for a device length of 280 nm. Our findings confirm plasmonics as the favorite data carriers in integrated all optical logic devices operating on the deep nanoscale, and pave the way to the development of future ultrafast information processing technologies based on SPPs.

Microwave whirlpools in a rectangular waveguide cavity with a thin ferrite disk.

We study a three-dimensional system of a rectangular waveguide resonator with an inserted thin ferrite disk. The interplay of reflection and transmission at the disk interfaces together with a material gyrotropy effect, gives rise to a rich variety of wave phenomena. We analyze the wave propagation based on full Maxwell-equation numerical solutions of the problem. We show that the power-flow lines of the microwave-cavity field interacting with a ferrite disk, in the proximity of its ferromagnetic resonance, form whirlpool-like electromagnetic vortices. Such vortices are characterized by the dynamical symmetry breaking. The role of ohmic losses in waveguide walls and dielectric and magnetic losses in a disk are the subjects of our investigations.