arXiv subjects

Filippo Capolino

Publications and source records attributed to Filippo Capolino.

At least 91 records · Page 5Linked to original sources

arXiv2018

Exceptional Points of Degeneracy Induced by Linear Time-Periodic Variation Hamidreza

We present a general theory of exceptional points of degeneracy (EPD) in periodically time-variant systems that do not necessarily require the presence of loss or gain, and we show that even a single resonator with a time-periodic component may develop EPDs. An EPD is a special point in a system parameter space at which two or more eigenmodes coalesce in both their eigenvalues and eigenvectors into a single degenerate eigenmode. We demonstrate the conditions for EPDs to exist in time-periodic systems that are either lossless/gainless or with loss and/or gain and we show that a system with zero time-average loss/gain exhibits EPDs with purely real resonance frequencies, yet the resonator energy grows algebraically in time. We show the occurrence of EPDs in a single LC resonator while the introduced concept is general for any time-periodic system. These findings have significant importance in various electromagnetic/photonic systems and pave the way of applications in areas of sensors, amplifiers and modulators. A potential application of this time varying EPD is highlighted as a highly-sensitive sensor.

physics.app-ph↗

arXiv2018

Unscrambling Structured Chirality with Structured Light at Nanoscale Using Photo-induced Force

We introduce a microscopy technique that facilitates the prediction of spatial features of chirality of nanoscale samples by exploiting photo-induced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip-sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy, we propose a judiciously engineered combination of radially and azimuthally polarized beams, as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively, hence probing longitudinal chirality. The proposed technique may benefit branches of science like stereochemistry, biomedicine, physical and material science, and pharmaceutics

physics.optics↗

arXiv2018

Exceptional Points of Degeneracy and Branch Points for Transmission-Line Problems - Linear Algebra and Bifurcation Theory Perspectives

We demonstrate several new aspects of exceptional points of degeneracy (EPD) pertaining to propagation in two uniform coupled transmission-line structures. We describe an EPD using two different approaches - by solving an eigenvalue problem based on the system matrix, and as a singular point from bifurcation theory, and the link between these two disparate viewpoints. Cast as an eigenvalue problem, we show that eigenvalue degeneracies are always coincident with eigenvector degeneracies, so that all eigenvalue degeneracies are implicitly EPDs in two uniform coupled transmission lines. Furthermore, we discuss in some detail the fact that EPDs define branch points (BPs) in the complex-frequency plane; we provide simple formulas for these points, and show that parity-time (PT) symmetry leads to real-valued EPDs occurring on the real-frequency axis. We discuss the connection of the linear algebra approach to previous waveguide analysis based on singular points from bifurcation theory, which provides a complementary viewpoint of EPD phenomena, showing that EPDs are singular points of the dispersion function associated with the fold bifurcation. This provides an important connection of various modal interaction phenomena known in guided-wave structures with recent interesting effects observed in quantum mechanics, photonics, and metamaterials systems described in terms of the EPD formalism.

physics.app-ph↗

arXiv2018

Two-scale structure for giant field enhancement: combination of Rayleigh anomaly and colloidal plasmonic resonance

We demonstrate theoretically and experimentally a two-scale architecture able to achieve giant field enhancement by simultaneously exploiting both the Rayleigh anomaly and localized surface plasmon resonance. Metallic oligomers composed of colloidal nanospheres are well-known for the ability to strongly enhance the near-field at their plasmonic resonance. However, due to intrinsic nonlocality of the dielectric response of the metals along with their inherent loss, the achievable field enhancement has an ultimate constraint. In this paper we demonstrate that combining plasmonic resonance enhancements from oligomers, with feature size of tens of nanometers, with a Rayleigh anomaly caused by a 1-D set of periodic nanorods, having a period on the order of the excitation wavelength, provides a mean to produce enhancement beyond that constrained by losses in near field resonances. Metallic oligomers are chemically assembled in between the periodic set of nanorods that are fabricated using lithographic methods. The nanorod periodicity is investigated to induce the Rayleigh anomaly at the oligomers plasmonic resonance wavelength to further enhance the field in the oligomers hot spots. A thorough study of this structure is carried out by using an effective analytical-numerical model which is also compared to full-wave simulation results. Experimental results comparing enhancements in surface enhanced Raman scattering measurements with and without nanorods demonstrate the effectiveness of a Rayleigh anomaly in boosting the field enhancement. The proposed structure is expected to be beneficial for many applications ranging from medical diagnostics to sensors and solar cells.

physics.optics↗

arXiv2018

The degenerate band edge laser: a new paradigm for coherent light-matter interaction

We propose a novel class of lasers based on a fourth order exceptional point of degeneracy (EPD) referred to as the degenerate band edge (DBE). EPDs have been found in Parity-Time-symmetric photonic structures that require loss and/or gain, here we show that the DBE is a different kind of EPD since it occurs in periodic structures that are lossless and gainless. Because of this property, a small level of gain is sufficient to induce single-frequency lasing based on a synchronous operation of four degenerate Floquet-Bloch eigenwaves. This lasing scheme constitutes a new paradigm in the light-matter interaction mechanism that leads also to the unprecedented scaling law of the laser threshold with the inverse of the fifth power of the laser-cavity length. The DBE laser has the lowest lasing threshold in comparison to a regular band edge laser and to a conventional laser in cavities with the same loaded quality (Q) factor and length. In particular, even without mirror reflectors the DBE laser exhibits a lasing threshold which is an order of magnitude lower than that of a uniform cavity laser of the same length and with very high mirror reflectivity. Importantly, this novel DBE lasing regime enforces mode selectivity and coherent single-frequency operation even for pumping rates well-beyond the lasing threshold, in contrast to the multifrequency nature of conventional uniform cavity lasers.

physics.optics↗

arXiv2018

Functional metasurfaces: Do we need normal polarizations?

We consider reciprocal metasurfaces with engineered reflection and transmission coefficients and study the role of normal (with respect to the metasurface plane) electric and magnetic polarizations on the possibilities to shape the reflection and transmission responses. We demonstrate in general and on a representative example that the presence of normal components of the polarization vectors does not add extra degrees of freedom in engineering the reflection and transmission characteristics of metasurfaces. Furthermore, we discuss advantages and disadvantages of equivalent volumetric and fully planar realizations of the same properties of functional metasurfaces.

physics.app-ph↗

arXiv2017

A New Amplification Regime for Traveling Wave Tubes with Third Order Modal Degeneracy

Engineering of the eigenmode dispersion of slow-wave structures (SWSs) to achieve desired modal characteristics, is an effective approach to enhance the performance of high power traveling wave tube (TWT) amplifiers or oscillators. We investigate here for the first time a new synchronization regime in TWTs based on SWSs operating near a third order degeneracy condition in their dispersion. This special three-eigenmode synchronization is associated with a stationary inflection point (SIP) that is manifested by the coalescence of three Floquet-Bloch eigenmodes in the SWS. We demonstrate the special features of "cold" (without electron beam) periodic SWSs with SIP modeled as coupled transmission lines (CTLs) and investigate resonances of SWSs of finite length. We also show that by tuning parameters of a periodic SWS one can achieve an SIP with nearly ideal flat dispersion relationship with zero group velocity or a slightly slanted one with a very small (positive or negative) group velocity leading to different operating schemes. When the SIP structure is synchronized with the electron beam potential benefits for amplification include (i) gain enhancement, (ii) gain-bandwidth product improvement, and (iii) higher power efficiency, when compared to conventional Pierce-like TWTs. The proposed theory paves the way for a new approach for potential improvements in gain, power efficiency and gain-bandwidth product in high power microwave amplifiers.

physics.app-ph↗

arXiv2017

Enantio-specific Detection of Chiral Nano-Samples Using Photo-induced Force

We propose a novel high resolution microscopy technique for enantio-specific detection of chiral samples down to sub-100 nm size, based on force measurement. We delve into the differential photo-induced optical force exerted on an achiral probe in the vicinity of a chiral sample, when left and right circularly polarized beams separately excite the sample-probe interactive system. We analytically prove that the differential force is entangled with the enantiomer type of the sample enabling enantio-specific detection of chiral inclusions. Moreover, we demonstrate that the differential force is linearly dependent on both the chiral response of the sample and the electric response of the tip and is inversely related to the quartic power of probe-sample distance. We provide physical insight into the transfer of optical activity from the chiral sample to the achiral tip based on a rigorous analytical approach. We support our theoretical achievements by several numerical examples, highlighting the potential application of the derived analytic properties. Lastly, we demonstrate the sensitivity of our method to enantio-specify nanoscale chiral samples with chirality parameter in the order of 0.01, and discuss how this could be further improved.

physics.optics↗

arXiv2017

Theory of Coupled Resonator Optical Waveguides (CROW) Exhibiting High Order Exceptional Points of Degeneracy

We present a novel approach and a theoretical framework for generating high order exceptional points of degeneracy (EPD) in photonic structures based on periodic coupled resonators optical waveguides (CROWs). Such EPDs involve the coalescence of Floquet-Bloch eigenwaves in CROWs, without the presence of gain and loss, which is in contrast to the requirement of Parity-Time (PT) symmetry to develop exceptional points based on gain and loss balance. The EPDs arise here by introducing symmetry breaking in a conventional chain of coupled resonators through coupling the chain of resonators to an adjacent uniform optical waveguide, which leads to unique modal characteristics that cannot be realized in conventional CROWs. Such remarkable characteristics include high quality factors (Q-factor) and strong field enhancement, even without any mirrors at the two ends of a cavity. We show for the first time the capability of CROWs to exhibit EPDs of various order; including the degenerate band edge (DBE) and the stationary inflection point (SIP). The proposed CROW of finite length shows enhanced quality factor when operating near the DBE, and the Q-factor exhibits an anomalous scaling with the CROW's length. We develop the theory of EPDs in such unconventional CROW using coupled-wave equations, and we derive an analytical expression for the dispersion relation. The proposed unconventional CROW concepts have various potential applications including Q-switching, nonlinear devices, lasers, and extremely sensitive sensors.

physics.optics↗

arXiv2017

Giant resonance and anomalous quality factor scaling in coupled resonator optical waveguides at the degenerate band edge

We propose a novel scheme for enhancing the quality factor of coupled resonators optical waveguides (CROWs) when operating near a degenerate band edge (DBE). A DBE is a four-mode exceptional point of degeneracy (EPD) occurring when four Bloch eigenmodes coalesce providing a resonance condition with a giant enhancement in fields. We report an unprecedented scaling law of quality factor of CROWs when operating at the DBE, even in the presence of losses and structural perturbations. Remarkably, the Q factor of the proposed CROW can be engineered to exceed that of a single ring resonator having a diameter equal to the CROW length, hence having an overall strong area reduction. The findings reported in this letter are critical for enhancing fields amplitudes to giant levels and the Q factor of ring resonators and are very beneficial for various applications including four wave mixing, Q switching, lasers, and highly sensitive sensors.

physics.optics↗

arXiv2017

Oscillator Based on Lumped Double Ladder Circuit with Band Edge Degeneracy

An oscillator design based on a periodic, double ladder resonant circuit is proposed. The circuit exhibits a degenerate band edge (DBE) in the dispersion diagram of its phase-frequency eigenstates, and possesses unique resonance features associated with a high Q-factor resonance, compared to a single ladder or a conventional LC tank circuit. This oscillator is shown to have an oscillation threshold that is half that of a single LC ladder circuit having the same total quality factor, and thus is more robust than an LC oscillator in the presence of losses. It is also shown that the output and amplitude of the double-ladder oscillator is much less sensitive to the output loading compared to single-ladder oscillators. We show the analysis and design of such oscillators that potentially lead to enhancing the efficiency of RF components and sources.

physics.class-ph↗

arXiv2017

Theory of Double Ladder Lumped Circuits With Degenerate Band Edge

A conventional periodic LC ladder circuit forms a transmission line that has a regular band edge between a pass and a stop band. Here for the first time we develop the theory of simple yet unconventional double ladder circuit that exhibits a special degeneracy condition referred to as degenerate band edge (DBE). The degeneracy occurs when four independent eigenstates coalesce into a single eigenstate at the DBE frequency. In addition to possible practical applications, this circuit may provide insight into DBE behavior that is not clear in more complex systems. We show that double ladder resonators exhibit unusual behavior of the loaded quality factor near the DBE leading to a stable resonance frequency against load variations. These two properties in the proposed circuit are superior to the analogous properties in single ladder circuits. Our proposed analysis leads to analytic expressions for all circuit quantities thus providing insight into the very complex behavior near degeneracy points in periodic circuits. Interestingly, here we show for the first time that DBE is obtained with unit cells that are symmetric along the propagation direction. The proposed theory of double ladders presented here has potential applications in filters, couplers, oscillators, and pulse shaping networks.

physics.class-ph↗

arXiv2017

Giant Field Enhancement in Longitudinal Epsilon Near Zero Films

We report that a longitudinal epsilon-near-zero (LENZ) film leads to giant field enhancement and strong radiation emission of sources in it and that these features are superior to what found in previous studies related to isotropic ENZ. LENZ films are uniaxially anisotropic films where relative permittivity along the normal direction to the film is much smaller than unity, while the permittivity in the transverse plane of the film is not vanishing. It has been shown previously that realistic isotropic ENZ films do not provide large field enhancement due to material losses, however, we show the loss effects can be overcome using LENZ films. We also prove that in comparison to the (isotropic) ENZ case, the LENZ film field enhancement is not only remarkably larger but it also occurs for a wider range of angles of incidence. Importantly, the field enhancement near the interface of the LENZ film is almost independent of the thickness unlike for the isotropic ENZ case where extremely small thickness is required. We show that for a LENZ structure consisting of a multilayer of dysprosium-doped cadmium oxide and silicon accounting for realistic losses, field intensity enhancement of 30 is obtained which is almost 10 times larger than that obtained with realistic ENZ materials

physics.optics↗

arXiv2016

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically-Loaded Circular Waveguide

We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely the degeneracy of four Bloch modes, in loaded circular metallic waveguides. The four modes forming the DBE represent a degeneracy of fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. It leads to a very flat wavenumber-frequency dispersion relation, and the finite length structure's quality factor scales as $N^5$ where $N$ is the number of unit cells. The proposed waveguide in which DBE is observed here is designed by periodically loading a circular waveguide with misaligned elliptical metallic rings, supported by a low-index dielectric. We validate the existence of the DBE in such structure using measurements and we report good agreement between full-wave simulation and the measured response of the waveguide near the DBE frequency; taking into account metallic losses. We correlate our finding to theoretical and simulation results utilizing various techniques including dispersion synthesis, as well as observing how quality factor and group delay scale as the structure length increases. Moreover, the reported geometry is only an example of metallic waveguide with DBE: DBE and its characteristics can also be designed in many other kinds of waveguides and various applications can be contemplated as high microwave generation in amplifiers and oscillators based on an electron beam interaction or solid state devices, pulse compressors and microwave sensors.

physics.class-ph↗

arXiv2016

Exceptional points of degeneracy and $\cal{PT}$-symmetry in photonic coupled chains of scatterers

We demonstrate the existence of exceptional points of degeneracy (EPD) of periodic eigenstates in non-Hermitian coupled chains of dipolar scatterers. Guided modes supported by these structures can exhibit an EPD in their dispersion diagram at which two or more Bloch eigenstates coalesce, in both their eigenvectors and eigenvalues. We show a second-order modal EPD associated with the parity-time ($\cal{PT}$) symmetry condition, at which each particle pair in the double chain exhibits balanced gain and loss. Furthermore, we also demonstrate a fourth-order EPD occurring at the band edge. Such degeneracy condition was previously referred to as a degenerate band edge in lossless anisotropic photonic crystals. Here, we rigorously show it under the occurrence of gain and loss balance for a discrete guiding system. We identify a more general regime of gain and loss balance showing that $\cal{PT}$-symmetry is not necessary to realize EPDs. Furthermore, we investigate the degree of detuning of the EPD when the geometrical symmetry or balanced condition is broken. These findings open unprecedented avenues toward superior light localization and transport with application to high-Q resonators utilized in sensors, filters, low-threshold switching and lasing.

physics.optics↗

arXiv2016

Theory of Exceptional Points of Degeneracy in Uniform Coupled-Waveguides and Balance of Gain and Loss

We present a transmission line theory of exceptional points of degeneracy (EPD) in coupled-mode guiding structures, i.e., a theory that illustrates the characteristics of coupled electromagnetic modes under a special dispersion degeneracy condition, yet unexplored in the contest of gain and loss. We demonstrate the concept of Parity-Time ($\cal{PT}$)-symmetry in coupled uniform waveguides with balanced and symmetric gain and loss and how this condition is associated with a second order EPD. We show that by introducing gain into naturally lossy structures provides for the conditions whereby exceptional points of non-Hermitian degeneracies can be manifested, such as in $\cal{PT}$- symmetric structures. Furthermore, we also demonstrate that $\cal{PT}$- symmetry, despite being the method often suggested for obtaining non-Hermitian degeneracies at optical frequencies, is not a necessary condition and indeed we show that EPD can be obtained with broken topological symmetry in uniform TLs. Operating near such special degeneracy conditions leads to potential performance enhancement in a variety of microwave and optical resonators, and devices such as distributed oscillators, including lasers, amplifiers, radiating arrays, pulse compressors, and Qswitching sensors.

physics.optics↗

arXiv2016

Bridging the Gap between Crosslinking Chemistry and Directed Assembly of Metasurfaces Using Electrohydrodynamic Flow

Advances in understanding chemical and physical driving forces in self-assembly allow the fabrication of unique nanoarchitectures with subwavelength building blocks as the basis for plasmonic and metamaterial devices. Chemical crosslinking of colloidal nanospheres has produced among the smallest gap spacings, necessary to obtain regions of greatly enhanced electric field, hotspots, which are critical to tailor light-matter interactions. However, obtaining uniform electromagnetic response of dense nanoantennas over large area for use in devices remains challenging. In this work, electrohydrodynamic (EHD) flow and chemical crosslinking is combined to form dense, yet discrete, Au nanosphere clusters (oligomers) on a working electrode. EHD provides a long range driving force to bring nanospheres together and anhydride crosslinking yields 0.9 nm gap spacings. Using selective chemistry, nanospheres are simultaneously crosslinked onto a block copolymer template, producing oligomers with a narrower wavelength band width and higher hotspot intensity than monolayer structures produced without a template. We investigate nanoantenna response via full wave simulations, ultraviolet-visible spectroscopy, and surface enhanced Raman scattering (SERS). Nanoantennas exhibit uniform hotspot intensity and gap spacing. Simulations show field enhancements of 600, correlating well with measured average SERS enhancement of 1.4x10^9. Nanoantenna substrates produce a SERS signal with a relative standard deviation of 10% measured over a 1 mm2 area, crucial for nano-optical devices such as optical sensors, among other applications. Understanding long range (EHD flow) and short range (chemical crosslinking) driving forces provides the control for assembling colloidal nanoparticles in architectures for large area plasmonic and metasurface device fabrication.

cond-mat.soft↗

arXiv2016

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 electric field ideally has a null while longitudinal magnetic field is maximum. Fresnel diffraction theory and plane-wave spectral (PWS) 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 (CFWHM) 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 that a very high resolution magnetic field at optical frequency is achieved with total magnetic field FWHM of 0.23λ (i.e., magnetic field spot area of 0.04λ^2) within a magnetic-dominant region. The theory shown here is valuable for development of optical microscopy and spectroscopy systems based on magnetic dipolar transitions which are in general much weaker than their electric counterparts.

physics.optics↗