Mehdi Veysi

Giant amplification in degenerate band edge slow-wave structures interacting with an electron beam

We propose a new amplification regime based on synchronous operation of four degenerate electromagnetic (EM) modes in a slow-wave structure and the electron beam, referred to as super synchronization. These four EM modes arise in a Fabry-Perot cavity (FPC) when degenerate band edge (DBE) condition is satisfied. The modes interact constructively with the electron beam resulting in superior amplification. In particular, much larger gains are achieved for smaller beam currents compared to conventional structures based on synchronization with only a single EM mode. We demonstrate giant gain scaling with respect to the length of the slow-wave structure compared to conventional Pierce type single mode traveling wave tube amplifiers. We construct a coupled transmission line (CTL) model for a loaded waveguide slow-wave structure exhibiting a DBE, and investigate the phenomenon of giant gain via super synchronization using the Pierce model generalized to multimode interaction.

Vortex beams with strong longitudinally polarized magnetic field and their generation by using metasurfaces

A novel method of generation and synthesis of azimuthally E-polarized vortex beams is presented. Along the axis of propagation such beams have a strong longitudinally polarized magnetic field where ideally there is no electric field. We show how these beams can be constructed through the interference of Laguerre-Gaussian beams carrying orbital angular momentum. As an example, we present a metasurface made of double-split ring slot pairs and report a good agreement between simulated and analytical results. Both a high magnetic-to-electric-field contrast ratio and a magnetic field enhancement are achieved. We also investigate the metasurface physical constraints to convert a linearly polarized beam into an azimuthally Epolarized beam and characterize the performance of magnetic field enhancement and electric field suppression of a realistic metasurface. These findings are potentially useful for novel optical spectroscopy related to magnetic dipolar transitions and for optical manipulation of particles with spin and orbital angular momentum.A novel method of generation and synthesis of azimuthally E-polarized vortex beams is presented. Along the beam axis such beams have a strong longitudinally polarized magnetic field where ideally there is no electric field. We show how these beams can be constructed through the interference of Laguerre–Gaussian beams carrying orbital angular momentum (OAM), and then quantify the longitudinal magnetic field of such beams. As an example, we present a metasurface made of double-split ring slot pairs and report a good agreement between simulated and analytical results. Both a high magnetic-to-electric-field contrast ratio and a magnetic field enhancement are achieved. We also investigate the metasurface physical constraints to convert a linearly polarized beam into an azimuthally E-polarized beam and characterize the performance of magnetic field enhancement and electric field suppression of a realistic metasurface. These findings are potentially useful for novel optical spectroscopy related to magnetic dipolar transitions and for optical manipulation of particles with spin and OAM.

Low Starting Electron Beam Current in Degenerate Band Edge Oscillators

We propose a new principle of operation in vacuum electron-beam-based oscillators that leads to a low beam current for starting oscillations. The principle is based on supersynchronous operation of an electron beam interacting with four degenerate electromagnetic modes in a slow-wave structure (SWS). The four-mode supersynchronous regime is associated with a very special degeneracy condition in the dispersion diagram of a cold periodic SWS called degenerate band edge (DBE). This regime features a giant group delay in the finite-length SWS and low starting-oscillation beam current. The starting beam current is at least an order of magnitude smaller compared with a conventional backward-wave oscillator of the same length. As a representative example, we consider an SWS conceived by a periodically loaded metallic waveguide supporting a DBE and investigate starting-oscillation conditions using the Pierce theory generalized to coupled transmission lines. The proposed supersynchronism regime can be straightforwardly adapted to waveguide geometries others than the periodically loaded waveguide considered here since DBE is a general property that can be realized in a variety of structures.

Focused azimuthally polarized vector beam and spatial magnetic resolution below the diffraction limit

An azimuthally electric-polarized vector beam (APB), with a polarization vortex, has a salient feature that it contains a magnetic-dominant region within which the electric field is ideally null while the longitudinal magnetic field is maximum. Fresnel diffraction theory and plane-wave spectral calculations are applied to quantify field features of such a beam upon focusing through a lens. The diffraction-limited full width at half-maximum (FWHM) of the beam’s longitudinal magnetic field intensity profile and complementary FWHM of the beam’s annular-shaped total electric field intensity profile are examined at the lens’s focal plane as a function of the lens’s paraxial focal distance. Then, we place a subwavelength dense dielectric Mie scatterer in the minimum-waist plane of a self-standing converging APB and demonstrate for the first time, to the best of our knowledge, that a very-high-resolution magnetic near-field at optical frequency is achieved with total magnetic near-field FWHM of 0.23λ (i.e., magnetic near-field spot area of 0.04λ2) within a magnetic-dominant region located one radius (0.12λ) away from the scatterer. In particular, the utilization of the nanosphere as a magnetic nanoantenna (so-called magnetic nanoprobe) illuminated by a tightly focused APB is instrumental in boosting the photoinduced magnetic response and suppressing the electric response of a sample matter. The access to the weak photoinduced magnetic response in sample matter would add extra degrees of freedom to future optical photoinduced force microscopy and spectroscopy systems based on the excitation of photoinduced magnetic dipolar transitions.

Tunable optical response of bowtie nanoantenna arrays on thermoplastic substrates

Thermally responsive polymers present an interesting avenue for tuning the optical properties of nanomaterials on their surfaces by varying their periodicity and shape using facile processing methods. Gold bowtie nanoantenna arrays are fabricated using nanosphere lithography on prestressed polyolefin (PO), a thermoplastic polymer, and optical properties are investigated via a combination of spectroscopy and electromagnetic simulations to correlate shape evolution with optical response. Geometric features of bowtie nanoantennas evolve by annealing at temperatures between 105 °C and 135 °C by releasing the degree of prestress in PO. Due to the higher modulus of Au versus PO, compressive stress occurs on Au bowtie regions on PO, which leads to surface buckling at the two highest annealing temperatures; regions with a 5 nm gap between bowtie nanoantennas are observed and the average reduction is 75%. Reflectance spectroscopy and full-wave electromagnetic simulations both demonstrate the ability to tune the plasmon resonance wavelength with a window of approximately 90 nm in the range of annealing temperatures investigated. Surface-enhanced Raman scattering measurements demonstrate that maximum enhancement is observed as the excitation wavelength approaches the plasmon resonance of Au bowtie nanoantennas. Both the size and morphology tunability offered by PO allows for customizing optical response.

Large magnetic to electric field contrast in azimuthally polarized vortex beams generated by a metasurface (Presentation Recording)

We investigate azimuthally E-polarized vortex beams with enhanced longitudinal magnetic field. Ideally, such beams possess strong longitudinal magnetic field on the beam axis where there is no electric field. First we formulate the electric field vector and the longitudinal magnetic field of an azimuthally E-polarized beam as an interference of right- and left-hand circularly polarized Laguerre Gaussian (LG) beams carrying the orbital angular momentum (OAM) states of -1 and +1, respectively. Then we propose a metasurface design that is capable of converting a linearly polarized Gaussian beam into an azimuthally E-polarized vortex beam with longitudinal magnetic field. The metasurface is composed of a rectangular array of double-layer double split-ring slot elements, though other geometries could be adopted as well. The element is specifically designed to have nearly a 180° transmission phase difference between the two polarization components along two orthogonal axes, similar to the optical axes of a half-wave plate. By locally rotating the optical axes of each metasurface element, the transmission phase profile of the circularly polarized waves over the metasurface can be tailored. Upon focusing of the generated vortex beam through a lens with a numerical aperture of 0.7, a 41-fold enhancement of the magnetic to electric field ratio is achieved on the beam axis with respect to that of a plane wave. Generation of beams with large magnetic field to electric field contrast can find applications in future spectroscopy systems based on magnetic dipole transitions, which are usually much weaker than electric dipole transitions.

Enantiospecific Detection of Chiral Nanosamples Using Photoinduced Force

Author(s): Kamandi, M; Albooyeh, M; Guclu, C; Veysi, M; Zeng, J; Wickramasinghe, K; Capolino, F | Abstract:

Optical leaky-wave antenna integrated in ring resonator

A leaky-wave antenna at optical frequencies is designed and integrated with a ring resonator at 1550 nm wavelength. The leaky wave is generated by using periodic perturbations in the integrated dielectric waveguide that excite the -1 spatial harmonic. The antenna consists of a dielectric waveguides with semiconductor corrugations, and it is compatible with CMOS fabrication technology. We show that integrating the leaky wave antenna in an optical ring resonator that is fed by directional couplers, we can improve the electronic control of the radiation through carrier injection into the semiconductor corrugations.

Cylindrical metasurfaces for exotic electromagnetic wave manipulations

We present closed-boundary cylindrical metasurfaces for the manipulation of electromagnetic waves. We suggest a systematic analytical approach for the analysis and synthesis of such metasurfaces. Moreover, we demonstrate a conceptual synthesis example i.e., an electromagnetic illusion device and suggest other applications using the proposed approach. The suggested approach paves the way for more complex and interesting wave manipulations.

Reflective metasurface lens with an elongated needle-shaped focus

Novel multifocal flat metasurface (MS) lenses are developed using two techniques: (i) polarization diversity, and (ii) annular segmentation of the lens aperture. The polarization-diversity technique enables overall lens aperture reuse, thus doubling the number of foci through the simultaneous focusing of two orthogonal linearly polarized incident beams at two distinct foci using the lens aperture. The annular-segmentation technique, on the other hand, is independent of incident beam polarization and is only based on dividing the lens aperture into concentric annular segments that converge different portions of the illuminating beam at different foci. The total number of foci can be further increased by combining the polarization-diversity and the annular-segmentation techniques. Subsequently, the concept of multifocality is further extended to design a novel flat lens with an overall single needle-like focal region with elongated depth of focus (DOF) without loss of lateral resolution. To this goal, we design a multifocal lens with overlapping profiles of foci superposed into a single elongated needle-shaped focal region. Using the combination of polarization-diversity and annular-segmentation techniques, we develop a novel MS flat lens made of Y-shaped nanoantennas, whose polarization-dependent reflection phase and amplitude can be controlled independently via their geometrical parameters. Via numerical calculations, we demonstrate that the proposed MS lens has an overall single focal region with an extremely long DOF of about 74.1 λ, a lateral full width at half maximum varying in the range of 1.37 λ to 2.8 λ, and a numerical aperture of about 0.26 (considering the center of the focal region as the effective focal point). Here, the MS lens’s capability to synthesize extremely long DOF is conceptually demonstrated without resorting to time-consuming and complicated wavefront synthesis methods. The engineering of focal intensity profiles with flat MSs may introduce significant advancement in nanoimaging, many areas of microscopy, and ophthalmic applications.