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We propose a new class of oscillators by engineering the dispersion of two-coupled periodic waveguides to exhibit a degenerate band edge (DBE). The DBE is an exceptional point of degeneracy (EPD) of order four, i.e., representing the coalescence of four eigenmodes of a waveguide system without loss and gain. We present a distributed DBE oscillator realized in periodic coupled transmission lines with a unique mode selection scheme that leads to a stable single-frequency oscillation, even in the presence of load variation. The DBE oscillator potentially leads to a boost of the efficiency and performance of radio frequency (RF) sources, due to the unique features associated with the EPD concept. This class of oscillators is promising for improving discrete-distributed coherent sources and can be extended to radiating structures to achieve a new class of active integrated antenna arrays.

We present a novel paradigm for dispersion engineering in coupled transmission lines (CTLs) based on exceptional points of degeneracy (EPDs). We also develop a theory for fourth-order EPDs consisting of four Floquet-Bloch eigenmodes coalescing into one degenerate eigenmode; present unique wave propagation properties associated with the EPD; develop a figure of merit (FOM) to assess the practical occurrence of the fourth-order EPDs in CTLs with tolerances and losses; and experimentally verify for the first time the existence of a fourth EPD (the degenerate band edge), through dispersion and transmission measurements in microstrip-based CTLs at microwave frequencies. In addition, we report that based on experimental observation and the developed FOM, the EPD features are still observable in structures that radiate (leak energy away), even in the presence of fabrication tolerances and dissipative losses. We investigate the 'gain and loss balance' regime in CTLs as a means of recovering an EPD in the presence of radiation and/or dissipative losses, without necessarily resorting to parity-time (PT)-symmetry regimes. The versatile EPD concept is promising in applications such as high-intensity and power-efficiency oscillators, spatial power combiners, or low-threshold oscillators and opens new frontiers for boosting the performance of large coherent sources.

We explore the use of cylindrical metasurfaces in providing several illusion mechanisms including scattering cancellation and creating fictitious line sources. We present the general synthesis approach that leads to such phenomena by modeling the metasurface with effective polarizability tensors and by applying boundary conditions to connect the tangential components of the desired fields to the required surface polarization current densities that generate such fields. We then use these required surface polarizations to obtain the effective polarizabilities for the synthesis of the metasurface. We demonstrate the use of this general method for the synthesis of metasurfaces that lead to scattering cancellation and illusion effects and discuss practical scenarios by using loaded dipole antennas to realize the discretized polarization current densities. This study is the first fundamental step that may lead to interesting electromagnetic applications, like stealth technology, antenna synthesis, wireless power transfer, sensors, cylindrical absorbers, etc.

We propose a two-scale architecture by simultaneously exploiting both the Rayleigh anomaly and localized surface plasmon resonance in colloidal directly assembled oligomers to achieve giant field enhancement, overcoming limitations by losses and the intrinsic nonlocality of the dielectric response of metal nanoparticles. We show that the total field enhancement in the nanogap hotspots of this two-scale structure is the product of the enhancements of each individual geometrical scale. Under this scheme, chemically assembled metallic oligomers enable strong enhancement of the near-field in resultant few-nanometer gaps at their plasmonic resonance. An extra enhancement factor results from combining the plasmonic resonance enhancement of these oligomers (nanometer scale) with a Rayleigh anomaly caused by a one-dimensional (1D) periodic set of nanorods (micrometer scale) fabricated using a lithographic method. This extra field enhancement cannot be obtained by further reducing the nanoantennas' gaps, since they are already in the few-nm ranges thanks to direct chemical assembly of oligomers. Experimental results verify that surface-enhanced Raman spectroscopy (SERS) signal is enhanced by more than one order of magnitude due to the Rayleigh anomaly as compared to the signal taken by using only metallic oligomers without periodic nanorods. The proposed two-scale structure could serve as a substrate for different spectroscopy techniques including SERS and can improve the sensitivity of spectroscopy for ultrasensitive biomolecule detection and single molecule spectroscopy studies.

We provide a new angle to investigate exceptional points of degeneracy (EPD) relating the current linear-algebra point of view to bifurcation theory. We apply these concepts to EPDs related to propagation in waveguides supporting two modes (in each direction), described as a coupled transmission line. We show that EPDs are singular points of the dispersion function associated with the fold bifurcation connecting multiple branches of dispersion spectra. This provides an important connection between 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 algebraic EPD formalism. Since bifurcation theory involves only eigenvalues, we also establish the connection to the linear-algebra point of view by casting the system eigenvectors in terms of eigenvalues, analytically showing that the coalescence of two eigenvalues results automatically in the coalescence of the two respective eigenvectors. Therefore, for the studied two-coupled transmission-line problem, the eigenvalue degeneracy explicitly implies an EPD. Furthermore, we discuss in some detail the fact that EPDs define branch points in the complex frequency plane, we provide simple formulas for these points, and we show that parity-time (PT) symmetry leads to real-valued EPDs occurring on the real-frequency axis.

We present a general theory of exceptional points of degeneracy (EPD) in periodically time-variant systems. We show that even a single resonator with a time-periodic component is able to develop EPDs, contrary to parity-time- (PT) symmetric systems that require two coupled resonators. 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 when they are directly induced by time-periodic variation of a system without loss and gain elements. We also show that a single resonator system with zero time-average loss-gain exhibits EPDs with purely real resonance frequencies, yet the resonator energy grows algebraically in time since energy is injected into the system from the time-variation mechanism. Although the introduced concept and formalism are general for any time-periodic system, here, we focus on the occurrence of EPDs in a single LC resonator with time-periodic modulation. These findings have significant importance in various electromagnetic and photonic systems and pave the way for many applications, such as sensors, amplifiers, and modulators. We show a potential application of this time-varying EPD as a highly sensitive sensor.

We show that the gradient force generated by the near field of a chiral nanoparticle carries information about its chirality. On the basis of this physical phenomenon we propose a new microscopy technique that enables the prediction of spatial features of chirality of nanoscale samples by exploiting the photoinduced 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's 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 and probe the longitudinal chirality, 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. The proposed technique may benefit branches of science such as stereochemistry, biomedicine, physical and material science, and pharmaceutics.

We propose a novel scheme for enhancing the quality factor (Q factor) of coupled resonators optical waveguides (CROWs) side coupled to a straight waveguide when operating near a degenerate band edge (DBE). The DBE is an exceptional point of degeneracy occurring without the need of loss or gain when four Bloch eigenwaves coalesce, providing a resonance condition with field enhancement and other unique properties associated with such frozen mode. We derive an analytic expression that describes the flatness of the dispersion diagram at the DBE and show how this is related to engineering the quality factor. We report an anomalous scaling law of the quality factor for a cavity made of the DBE-CROW with the fifth power of the number of rings when operating at the resonance frequency closest to the DBE frequency, even in the presence of losses and structural perturbations. Remarkably, the Q factor of the proposed DBE-CROW can be engineered to exceed that of a single ring resonator having a diameter equal to the total DBE-CROW length; hence, the DBE leads to an overall huge area reduction. We also demonstrate the robustness of the DBE-CROW's Q factor scaling near the DBE against perturbations and disorders that could arise from tolerances in fabrication. The findings reported in this paper are critical for enhancing field's 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.

We propose a 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 light-matter interaction mechanism that leads also to a unique 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.

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 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. The SIP structure when synchronized with an electron beam has potential benefits for amplification which include: 1) gain enhancement; 2) gain-bandwidth product improvement; and 3) 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.

A conventional periodic LC ladder circuit forms a transmission line that has a regular band edge between a passband and a stopband. 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 a 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 a 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.

We propose a high-resolution microscopy technique for enantiospecific detection of chiral samples down to sub-100-nm size based on force measurement. We delve into the differential photoinduced optical force ΔF 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 ΔF is entangled with the enantiomer type of the sample enabling enantiospecific detection of chiral inclusions. Moreover, we demonstrate that ΔF 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 enantiospecify nanoscale chiral samples with chirality parameter on the order of 0.01 and discuss how the sensitivity of our proposed technique can be further improved.

We present an approach and a theoretical framework for generating high-order exceptional points of degeneracy (EPDs) in photonic structures based on periodic coupled resonator optical waveguides (CROWs). Such EPDs involve the coalescence of Floquet-Bloch eigenwaves in CROWs, without the presence of gain and loss, which contrasts with the parity-time symmetry required 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 periodic coupling 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 factors) 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 orders, including the degenerate band edge (DBE) and the stationary inflection point. The proposed CROW of finite length shows an enhanced quality factor when operating near the DBE, and the Q factor exhibits an unconventional 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.

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.

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 the fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. The DBE is associated with four Bloch eigenmodes representing wave propagation in the periodic structure that coalesce in both wavenumbers and eigenvectors (i.e., polarizations), at a single frequency. It leads to a very flat wavenumber-frequency dispersion relation, and the finite-length structure's quality factor scales as N5 , 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. 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, and scaling of the quality factor and group delay with length. 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-power microwave generation in amplifiers and oscillators based on an electron beam interaction or solid-state devices, pulse compressors, and microwave sensors.

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 show that coupled transmission lines (CTLs) at radio frequencies having gain (active devices) and loss (e.g., material, radiation) balance exhibit EPDs. We demonstrate the concept of parity-time (PT)-symmetry in uniform CTLs that involve symmetric gain and loss and how this condition is associated with a second-order EPD. Furthermore, we also demonstrate that $\mathcal {PT}$-symmetry is not a necessary condition for realizing EPDs, and indeed, we show that EPD is also obtained with asymmetric distributions of gain and loss in uniform CTLs. We further propose potential applications of the EPDs in designing leaky-wave antennas with the capability of beam and directivity control as well as enhanced sensitivity. Operating near such special degeneracy conditions leads to potential performance enhancement in a variety of microwave and optical resonators, antennas, and devices such as distributed oscillators, including lasers, amplifiers, radiating oscillators, pulse compressors, and Q-switching sensors.

Colloidal self-assembly combined with templated surfaces holds the promise of fabricating large area devices in a low cost facile manner. This directed assembly approach improves the complexity of assemblies that can be achieved with self-assembly while maintaining advantages of molecular scale control. In this work, electrokinetic driving forces, i.e., electrohydrodynamic flow, are paired with chemical crosslinking between colloidal particles to form close-packed plasmonic metamolecules. This method addresses challenges of obtaining uniformity in nanostructure geometry and nanometer scale gap spacings in structures. Electrohydrodynamic flows yield robust driving forces between the template and nanoparticles as well as between nanoparticles on the surface promoting the assembly of close-packed metamolecules. Here, electron beam lithography defined Au pillars are used as seed structures that generate electrohydrodynamic flows. Chemical crosslinking between Au surfaces enables molecular control over gap spacings between nanoparticles and Au pillars. An as-fabricated structure is analyzed via full wave electromagnetic simulations and shown to produce large magnetic field enhancements on the order of 3.5 at optical frequencies. This novel method for directed self-assembly demonstrates the synergy between colloidal driving forces and chemical crosslinking for the fabrication of plasmonic metamolecules with unique electromagnetic properties.

We report that a longitudinal epsilon-near-zero (LENZ) film leads to giant field enhancement and strong radiation emission of sources in it and these features are superior to what was 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, whereas 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's field enhancement not only is remarkably larger, but 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 what happens in 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 ten times larger than that obtained with realistic ENZ materials.

We demonstrate the existence of exceptional points of degeneracy (EPDs) 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 the emergence of a second-order modal EPD associated with the parity-time (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 a 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 PT symmetry is not necessary to attain EPDs. Moreover, we investigate the degree of detuning of the EPD when the geometrical symmetry or balanced condition is broken. Furthermore, we demonstrate a realistic implementation of the EPD in a coupled chain made of pairs of plasmonic nanospheres and active core-shell nanospheres at optical frequencies. These findings open avenues toward superior light localization and transport with application to high-Q resonators utilized in sensors, filters, low-threshold switching and lasing.

The integration of a leaky-wave antenna with a ring resonator is presented based on the leaky-wave theory and integrated waveguide models. The device consists of a ring resonator fed by a directional coupler, where the ring resonator path includes a segment that is turned into a leaky-wave antenna. We devise an analytical model of the guided wave propagation along a directional coupler and the ring resonator path, including the antenna and non-radiating segments. The resonator integration provides two main advantages: 1) the high-quality factor resonance ensures effective control of radiation intensity by controlling the resonance conditions and 2) the efficient radiation from a leaky-wave antenna even when its length is much smaller than the propagation length of the leaky wave. The tradeoffs regarding the quality factor of resonance and the antenna efficiency of such a design are reported in terms of the coupler parameters, leaky-wave constant, and radiation length. Finally, a CMOS-compatible OLWA suitable for the ring resonator integration is designed where silicon is utilized as the waveguide material for a possible electronic control of radiation intensity. The simulation results together with the analytical model show that slight variations in the leaky wave's propagation constant, realized through excitation of excess carriers in the Si domain, is sufficient to control the far-field radiation modulation with high extinction ratio.

The concept of magnetic nanoprobes (or magnetic nanoantennas), their excitation, and the capability of providing a magnetic near-field enhancement and vanishing electric field is presented and investigated. It is established that a particular type of cylindrical vector beam called an azimuthally electric polarized vector beam yields a strong longitudinal magnetic field on the beam axis where the electric field is ideally null. These beams, with an electric polarization vortex and cylindrical symmetry, are important in generating high magnetic to electric field contrast, i.e., large local field admittance, and in allowing selective excitation of magnetic transitions in matter located on the beam axis. We demonstrate that azimuthally polarized vector beam excitation of a photoinduced magnetic nanoprobe made of a magnetically polarizable nanocluster leads to an enhanced magnetic near field with resolution beyond the diffraction limit. We introduce two figures of merit, magnetic field enhancement and local field admittance normalized to that of a plane wave, that are useful to classify magnetic nanoprobes and their excitation. The performance of magnetic nanoprobes and azimuthal polarized beams is quantified in comparison to other illumination schemes and with several defect scenarios. The proposed probes may be useful in spectroscopy and scanning probe microscopy applications.

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 photoin duced 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 photo induced force microscopy and spectroscopy systems based on the excitation of photoinduced magnetic dipolar transitions.

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.

We present the theory of a new amplification regime in traveling wave tubes (TWTs) composed of a slow wave periodic structure that supports multiple electromagnetic modes that can all be synchronized with the electron beam. The interaction between the multimodal slow wave structure and the electron beam is quantified using a multi-transmission line (MTL) approach based on a generalized Pierce model and transfer matrix analysis, leading to the identification of modes with complex Bloch wavenumber. In particular, we address a new possible operation condition for TWTs based on the supersynchronism between an electron beam and four modes exhibiting a degeneracy condition near a band edge of the periodic slow wave MTL. We show a phenomenological change in the band structure of a periodic MTL, where we observe at least two growing modal cooperating solutions as opposed to a uniform MTL, interacting with an electron beam where there is rigorously only one growing modal solution. We discuss the advantage of using such a degeneracy condition in TWTs that leads to larger gain conditions in amplifier regimes and also to very low starting beam current in high-power oscillators.

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.

We propose a new amplification regime based on a 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-Pérot cavity 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 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.

We propose a new approach leading to giant gain enhancement. It is based on unconventional slow-wave resonance associated with a degenerate band edge (DBE) in the dispersion diagram for a special class of photonic crystals supporting four modes at each frequency. We show that the gain enhancement in a Fabry-Pérot cavity (FPC) when operating at the DBE is several orders of magnitude stronger when compared to a cavity of the same length made of a standard photonic crystal with a regular band edge (RBE). The giant gain condition is explained by a significant increase in the photon lifetime and in the local density of states. We have demonstrated the existence of DBE operated special cavities that provide for superior gain conditions for solid-state lasers, quantum cascade lasers, traveling wave tubes, and distributed solid-state amplifiers. We also report the possibility to achieve low-threshold lasing in FPC with DBE compared to RBE-based lasers.

Photoinduced magnetic force between nanostructures, at optical frequencies, is investigated theoretically. Till now optical magnetic effects were not used in scanning probe microscopy because of the vanishing natural magnetism with increasing frequency. On the other hand, artificial magnetism in engineered nanostructures led to the development of measurable optical magnetism. Here two examples of nanoprobes that are able to generate strong magnetic dipolar fields at optical frequency are investigated: first, an ideal magnetically polarizable nanosphere and then a circular cluster of silver nanospheres that has a looplike collective plasmonic resonance equivalent to a magnetic dipole. Magnetic forces are evaluated based on nanostructure polarizabilities, i.e., induced magnetic dipoles, and magnetic-near field evaluations. As an initial assessment on the possibility of a magnetic nanoprobe to detect magnetic forces, we consider two identical magnetically polarizable nanoprobes and observe magnetic forces on the order of piconewtons, thereby bringing it within detection limits of conventional atomic force microscopes at ambient pressure and temperature. The detection of magnetic force is a promising method in studying optical magnetic transitions that can be the basis of innovative spectroscopy applications.

We demonstrate the existence of a special degeneracy condition, called degenerate band edge (DBE), between two Bloch modes in periodically-loaded circular all-metallic waveguides at microwave frequencies. The DBE condition has been associated with a dramatic reduction in group velocity and with some unique resonance properties, but it has not been shown in hollow waveguide structures yet. Hence, we show here its existence in two periodic waveguide examples. The unit cell of the first structure is composed of a circular waveguide loaded with two inner cylinders with elliptical irises with misaligned angles. The second structure is composed by loading the waveguide with elliptical rings. The demonstration of DBE in those waveguide is explained through a simple multi-transmission line approach where the conditions to obtain DBE are clarified, and suggests that the DBE can occur in several other analogous periodic waveguides. These structures can be potentially used to investigate unconventional gain schemes in traveling wave tubes or other kinds of distributed amplifiers, oscillators and novel pulse compressors.

Plasmonic nano antennas like dimers, have been investigated for their capability to provide a strong near-field enhancement when illuminated by external light. Traditionally these nano antennas, isolated or arrayed, are placed on a substrate and used in spectroscopy techniques. Surfaces made of such plasmonic nano antennas have been very useful for applications like surface enhanced Raman scattering in which it provides various orders of magnitude of enhanced sensitivity. These instruments however are not economic and are often not mobile since surfaces require an external beam illumination and the Raman scattering is detected by a large aperture microscope. The goal of this paper is to combine nano antennas made of gold dimers with integrated waveguide to make a spectrometer which has low cost and volume in comparison with the structure mentioned above. A technique in which optical plasmonic nano antennas are located in proximity of silicon nitride waveguide is proposed that is useful both for illumination and detection channels. The waveguide evanescent field, which is decaying outside of the waveguide, excites the dimer and causes it to resonate which results in a very strong electric field enhancement of approximately 25 times in the antenna gap. Also the coupling effect of dimer resonance on waveguide modes is investigated. To show the efficiency of the proposed structure, full wave analysis has been done and its results are compared with the multilayer structure case. The simulation results demonstrate that this structure can be designed and fabricated for the purpose of spectroscopy application.

Generalized Lorentz-Lorenz formulas are developed for the effective parameters of binary lattice metamaterials composed of a periodic arrangement of electric and/or magnetic inclusions. The proposed homogenization approach is based on a dual dipole approximation for the induced currents. The obtained formulas for the metamaterial effective electric and magnetic characteristics duly consider both electric and magnetic polarizabilities of the inclusions and completely describe the effects of frequency and spatial dispersion. Several numerical examples are provided to demonstrate the general applicability of the proposed formulas to different types of binary lattices and inclusions. It is shown that the proposed effective parameters have the capability of providing a physically sound and accurate description of wave propagation in the metamaterials in an extended range of frequencies in contrast to the equivalent parameters that can be defined in the absence of impressed sources and assuming a local anisotropic constitutive model, which hides inherent spatial dispersion effects and nonphysical features. To gain further insight into the metamaterial response and the physical meaningfulness of calculated effective parameters, the power flow of metamaterial supported modes is analyzed and its homogenized representation is compared to the complete description. A correspondence between the power flow due to the microscopic field and the effect of spatial dispersion in the homogenized parameters is established.

The possibility of integration of two important categories of optical components, i.e., circular polarizer and lens, into a thin plasmonic metasurface is examined, for the realistic case when metal losses cannot be neglected, for example when operating in the visible spectrum, or at infrared when non-noble metals are used. We introduce a metasurface made of nanoantennas that offer enough degrees of freedom to control both amplitude and phase of the reflection coefficient. First a theoretical formulation is developed and then an optimal design for a polarizing lens at mid-infrared wavelengths near 4.3 μmis presented.Atwo-dimensional array of Y-shaped nanoantennas with polarization-dependent and spatially varying phase response offers the possibility of balancing the losses experienced by the x- and y-polarized incident fields and is the basis for such a compact polarizing lens.

Plasmonic nano antennas like dimers, bowties, and others have been investigated for their capability to provide a strong near-field enhancement when illuminated by external light. Traditionally these nano antennas, isolated or arrayed, are placed on a substrate and used in spectroscopy techniques. Surfaces made of such plasmonic nano antennas have been very useful, for example for surface enhanced Raman scattering, providing various orders of magnitude of enhanced sensitivity. These instruments however are not economic and are often not mobile since surfaces require an external beam illumination and the Raman scattering is detected by a large aperture microscope. We propose a technique where optical plasmonic nano antennas are located in proximity of silicon based waveguides that provide both for illumination and detection channels. The evanescent field surrounding the waveguide is capable to excite the proximity resonant nano antenna and generate the very strong field enhancement in the antenna gaps. Intensity enhancement reaches three or four orders of magnitude. Such nano antennas are used also for detection schemes and they provide very high sensitivity to molecular scattering, for example, when molecules are placed in the antenna 'hot spot'. A couple of different silicon based waveguide will be investigated with coupling to various possible plasmonic nano antennas. First we investigate the field enhancement and then the coupling to waveguide modes.

Directive optical leaky wave antennas (OLWAs) with tunable radiation pattern are promising integrated optical modulation and scanning devices. OLWAs fabricated using CMOS-compatible semiconductor planar waveguide technology have the potential of providing high directivity with electrical tunability for modulation and switching capabilities. We experimentally demonstrate directive radiation from a silicon nitride (Si3N4) waveguide-based OLWA. The OLWA design comprises 50 crystalline Si perturbations buried inside the waveguide, with a period of 1 μm, each with a length of 260 nm and a height of 150 nm, leading to a directive radiation pattern at telecom wavelengths. The measured far-field radiation pattern at the wavelength of 1540 nm is very directive, with the maximum intensity at the angle of 84.4° relative to the waveguide axis and a half-power beam width around 6.2°, which is consistent with our theoretical predictions. The use of semiconductor perturbations facilitates electronic radiation control thanks to the refractive index variation induced by a carrier density change in the perturbations. To assess the electrical modulation capability, we study carrier injection and depletion in Si perturbations, and investigate the Franz-Keldysh effect in germanium as an alternative way. We theoretically show that the silicon wire modulator has a -3 dB modulation bandwidth of 75 GHz with refractive index change of 3×10-4 in depletion mode, and 350 MHz bandwidth with refractive index change of 1.5×10-2 in injection mode. The Franz-Keldysh effect has the potential to generate very fast modulation in radiation control at telecom wavelengths.

Fano resonances are features in transmissivity/reflectivity/absorption that owe their origin to the interaction between a broad bright resonance and a dark (i.e., sub-radiant) narrower one. They may emerge in the optical properties of planar two-dimensional (2D) periodic arrays (metasurfaces) of plasmonic nanoparticles. In this letter, we provide a thorough assessment of their nature for the general case of normal and oblique plane wave incidence, highlighting when a Fano resonance is affected by the mutual coupling in an array and its capability to support free modal solutions. We analyze the representative case of a metasurface of plasmonic nanoshells at ultraviolet frequencies and compute its absorption under TE- and TM-polarized, oblique plane-wave incidence. In particular, we find that plasmonic metasurfaces display two distinct types of resonances observable as absorption peaks: one is related to the Mie electric dipolar resonance of each nanoparticle and the other is due to the forced excitation of free modes with small attenuation constant, usually found at oblique incidence. The latter is thus an array-induced collective Fano resonance. This realization opens up to manifold flexible designs at optical frequencies mixing individual and collective resonances. We explain the physical origin of such Fano resonances using the modal analysis through which we calculate the free modes with complex wavenumber supported by the metasurface. We define equivalent array dipolar polarizabilities that are directly related to the absorption physics at oblique incidence and show a direct dependence between array modal phase and attenuation constants and Fano resonances. We thus provide a more complete picture of Fano resonances that may lead to the design of filters, energy-harvesting devices, photodetectors, and sensors at ultraviolet frequencies. Similar resonances may be also extended to the visible range with an appropriate choice of geometries and materials.

A hyperbolic metamaterial (HM) resonator is analyzed as a nano-antenna for enhancing the radiative emission of quantum emitters in its vicinity. It has been shown that the spontaneous emission rate by an emitter near a hyperbolic metamaterial substrate is enhanced dramatically due to very large density of states. However, enhanced coupling to the free-space, which is central to applications such as solid-state lighting, has not been investigated significantly. Here, we numerically demonstrate approximately 100 times enhancement of the free-space radiative emission at 660nm wavelength by utilizing a cylindrical HM resonator with a radius of 54nm and a height of 80nm on top of an opaque silver-cladded substrate. We also show how the free-space radiation enhancement factor depends on the dipole orientation and the location of the emitter near the subwavelength resonator. Furthermore, we calculate that an array of HM resonators with subwavelength spacings can maintain most of the enhancement effect of a single 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.

We develop an electrodynamic model based on dyadic Green's functions for analyzing the near-field interactions between a dipolar scatterer (DS) and a plasmonic nanosphere (PN) under external excitation, accounting for multipolar contributions in the evaluation of the scattered fields. In particular, we include all the radiative and nonradiative field interactions between the DS and the PN, particularly the physical mechanism of DS's self-coupling through the PN, which is either neglected or approximated in previous work. Our objective is to show under which conditions self-coupling is important for strong excitation-rate enhancement of the DS and provide a description of the system's properties. We analytically investigate the conditions under which the excitation rate of a DS, such as an organic dye or a quantum dot, is enhanced when located in close proximity to a PN. We show the existence of critical conditions in terms of polarizabilities and distances that lead to large enhancement based on self-coupling and how to predict it.

A novel fully on-chip antenna based on a metasurface fabricated in a 180-nm BiCMOS process is presented. Inspired by the concept of high impedance surface (HIS), this metasurface is not used as a reflector below an antenna as commonly done. Instead, it is used as a radiator by itself. The extremely thin metasurface is composed of a patterned top two metal layers and the ground plane placed in the lowest metal layer in the process. The ground plane on the lowest metal layer of the process provides a solid shielding from the substrate and other possible circuitries. The fundamental of the antenna radiation and design are described. The measured antenna shows 2.5 dBi peak broadside gain with 8-GHz 3-dB gain bandwidth and an impedance bandwidth larger than 10 GHz. In its class of broadside radiating fully on-chip antennas, with a ground plane on the lower metal layer of the process, and without additional fabrication processing, this structure achieves the widest impedance bandwidth at W-band and one of the highest gain and gain bandwidth. It is noteworthy that this is achieved with an extremely thin antenna substrate thickness and a shielding ground plane.

We provide for the first time the detailed study of the radiation performance of an optical leaky wave antenna (OLWA) integrated into a Fabry-Pérot resonator. We show that the radiation pattern can be expressed as the one generated by the interference of two leaky waves counter-propagating in the resonator leading to a design procedure for achieving optimized broadside radiation, i.e., normal to the waveguide axis. We thus report a realizable implementation of the OLWA made of semiconductor and dielectric regions. The theoretical modeling is supported by full-wave simulation results, which are found to be in good agreement. We aim to control the radiation intensity in the broadside direction via excess carrier generation in the semiconductor regions. We show that the presence of the resonator can provide an effective way of enhancing the radiation level modulation, which reaches values as high as 13.5 dB, paving the way for novel promising radiation control capabilities that might allow the generation of very fast optical switches, as an example. © 2014 IEEE.

We employ the generalized Lorentz-Lorenz method to investigate how both magnetoelectric coupling and spatial dispersion influence the artificial magnetic capabilities at terahertz frequencies of the representative case of a metamaterial consisting of a three-dimensional (3D) lattice of TiO2 microspheres. The complex wavenumber dispersion relations pertaining to modes supported by the array, traveling along one of the principal axes of the array with electric or magnetic field polarized transversely and longitudinally (with respect to the mode traveling direction), are studied and thoroughly characterized. One mode with transverse polarization is dominant at any given frequency for the analyzed dimensions, proving that the 3D lattice can be treated as a homogeneous medium with defined electromagnetic material parameters. We show, however, that bianisotropy is a direct consequence of magnetoelectric coupling, and the dyadic expressions of both effective and equivalent material parameters are derived. In particular, we analyze the effect of spatial dispersion on the effective parameters relative to a composite material made by a 3D lattice of TiO2 microspheres with filling fraction around 30% and near the first Mie magnetic dipolar resonance. Finally, we homogenize the metamaterial in terms of equivalent index and impedance, and by comparison with full-wave simulations, we explain the presence of the unphysical antiresonance permittivity behavior observed in previous work. © 2014 Optical Society of America.

Strong light-matter coupling has recently been demonstrated in subwavelength volumes by coupling engineered optical transitions in semiconductor heterostructures (e.g., quantum wells) to metasurface resonances via near fields. It has also been shown that different resonator shapes may lead to different Rabi splittings, though this has not yet been well explained. In this paper, our aim is to understand the correlation between resonator shape and Rabi splitting, in particular to determine and quantify the physical parameters that affect strong coupling by developing an equivalent circuit network model whose elements describe energy and dissipation. Because of the subwavelength dimension of each metasurface element, we resort to the quasistatic (electrostatic) description of the near field and hence define an equivalent capacitance associated to each dipolar element of a flat metasurface. We show that this is also able to accurately model the phenomenology involved in strong coupling between the metasurface and the intersubband transitions in quantum wells. We show that the spectral properties and stored energy of a metasurface/quantum-well system obtained using our model are in good agreement with both full-wave simulation and experimental results. We then analyze metasurfaces made of three resonator geometries and observe that the magnitude of the Rabi splitting increases with the resonator capacitance in agreement with our theory, providing a phenomenological explanation for the resonator shape dependence of the strong coupling process. © 2014 American Physical Society.

We investigate the capabilities of hyperbolic metamaterials (HMs) to couple near-fields (i.e., evanescent waves) emitted by a two-dimensional periodic array of electric dipoles to propagating waves. In particular, large-order Floquet harmonics with transverse-magnetic polarization that would be evanescent in free space, and therefore confined near the array surface, are transformed into a propagating spectrum inside the HM and thus carry power away. Because of this property, independent of the finite or infinite extent of the HM, the power generated by an array of elementary electric dipoles is strongly enhanced and is mostly directed into the HM when the array is located near a HM surface. In particular, the power coupled to the HM exhibits narrow frequency features that can be employed in detection applications. The results shown in this paper provide a clear signature on wave dynamics in HMs. A link between the results pertaining to the case of an isolated dipole on top of HM and the planar array is found to be convenient in explaining both wave dynamics and spectral power distribution. The narrow frequency emission features appear in the array case only; they depend on its spatial periodicity and remarkably on the HM thickness. © 2014 American Physical Society.

We study the formation of second-harmonic double-resonance cones in hyperbolic metamaterials. An electric dipole on the surface of the structure induces second-harmonic light to propagate into two distinct volume plasmon-polariton channels: a signal that propagates within its own peculiar resonance cone and a phase-locked signal that is trapped under the pump's resonance cone. Metamaterial dispersion and birefringence induce a large angular divergence between the two volume plasmon polaritons, making these structures suitable for subwavelength second- and higher-harmonic imaging microscopy. © 2014 American Physical Society.

A possible route toward achieving high-power microwave (HPM) devices is through the use of novel slow-wave structures, represented by multiple coupled transmission lines (MTLs), and whose behavior when coupled to electron beams has not been sufficiently explored. We present the extension of the 1-D linearized Pierce theory to MTLs coupled to a single electron beam. We develop multiple formalisms to calculate the k-ω dispersion relation of the system and find that the existence of a growing wave solution is always guaranteed if the electron propagation constant is larger than or equal to the largest propagation constant of the MTL system. We verify our findings with illustrative examples that bring to light unique properties of the system in which growing waves were found to exist within finite bands of the electron propagation constant and also present possible means to improve the gain. By treating the beam-MTL interaction as distributed dependent current generators in the MTL, we derive relations characterizing the power flux and energy exchange between the beam and the MTLs. For the growing wave, we show that the beam always behaves as an energy source causing power flux along the transmission lines. The theory developed in this paper is the basis for the possible use of degenerate band edges in periodic MTL systems for HPM amplifiers. © 2014 IEEE.

We predict enhanced second-harmonic generation in low-damping epsilon-near-zero slabs made of plasmonic nanoshells. We further discuss the contribution of nonlocal effects induced by free-electron gas pressure in metals to the nonlinear response of the slab. © OSA 2013.

A simple solution for a 63 GHz planar single-feed Fabry-Pérot cavity (FPC) antenna covered by a thick metallic frequency selective surface (FFS) is proposed. The antenna is fed through a planar feed consisting of a coplanar-waveguide-fed slot on the ground plane of the cavity. The FSS is made of periodic circular slots in a thick Brass plate. Moreover, the sides of the metallic FSS are terminated with vertical metallic walls leading to a self-supported low-profile FPC filled with air. Good agreement is observed between measured and simulated results. © 2013 IEEE.

We investigate the radiation of an optical leaky-wave antenna embedded in a Fabry-Pérot resonator (FPR), resonating in the direction of the guided leaky wave. Leaky-wave radiation is achieved via the generation of a -1 Floquet harmonic in a dielectric waveguide due to the inclusion of a periodic set of perturbations. The FPR is formed by confining the dielectric waveguide and the leaky-wave section between two partially reflective mirrors. We report the design principles that are required to achieve a pencil beam at broadside as a superposition of leaky waves that travel inside the FPR in two opposite directions. We also investigate the tunability of the radiation by using semiconductor regions in the waveguide and thereby exciting and controlling excess carriers. We report simulation results related to a realistic design as a proof of the concept. © 2013 IEEE.

We introduce a graphene-based composite multilayer structure that exhibits hyperbolic-like wavevector dispersion at terahertz and mid-infrared frequencies. The multilayer structure comprises graphene sheets separated by dielectric layers. We formulate the effective permittivity tensor of the multilayer using a simple homogenization scheme. In addition, we employ Bloch theory for evaluating the wavevector dispersion for a propagation mode inside the HM, and show that the homogenization scheme is in good agreement with Bloch theory in a very wide spatial spectrum. We report the tunability of transition from elliptic to hyperbolic iso-frequency wavevector dispersion by varying the chemical potential of graphene sheets. © 2013 IEEE.