arXiv subjects

Filippo Capolino

Publications and source records attributed to Filippo Capolino.

At least 19 recordsLinked to original sources

arXiv2026

Exceptional Points in Photonics: From Non-Hermitian Physics to Applications

Open photonic systems provide a versatile platform for non-Hermitian physics, enabling control over complex spectra, transport, and light-matter interactions. Exceptional points (EPs), at which eigenvalues and eigenvectors coalesce and the governing operator becomes defective, play a central role because they combine branch-point spectral topology, nonanalytic perturbative response, and controllable eigenstate conversion. This Review provides a unified framework for EP photonics by systematically distinguishing exceptional degeneracies according to the underlying operator, spectral variable, boundary conditions, and experimentally accessible observables. We discuss Hamiltonian EPs, absorbing EPs associated with scattering zeros, real-frequency scattering-matrix and Jones-matrix EPs, Bloch and Floquet EPs, and Liouvillian EPs in open quantum systems. We review their spectral topology, static and dynamical encircling, higher-order exceptional structures, and coexistence with bound states in the continuum, together with applications in sensing, lasing, coherent absorption, directional scattering, polarization and wavefront control, nonlinear optics, optical storage, nonreciprocal photonics, and quantum photonics. We also critically assess the current limitations, practical challenges, and future perspectives of EP-based photonic technologies, with particular attention to robustness, noise, scalability, and experimentally measurable performance.

physics.optics↗

arXiv2026

Second-Harmonic Generation at a Fourth-Order Exceptional Point Degeneracy

An anomalous flat-band dispersion provided by a degenerate band edge (DBE) of longitudinal optical modes in a double-grating waveguide is used to enhance second-harmonic generation (SHG). The DBE is a fourth-order exceptional point degeneracy (EPD) in a lossless and gainless waveguide, characterized by the coalescence of four eigenmodes that establish a frozen mode in a cavity. At a DBE resonance, the cavity quality factor scales $Q\propto N^5$, where $N$ is the number of unit cells of the grating waveguide. In our numerical experiments, we observe the peak intensity of the fundamental field in the edge-excited cavity scaling as $I_1\propto N^{3.6}$. This leads to a highly efficient SHG process that is radiated vertically from the cavity (i.e., normal to the grating) without requiring collinear phase matching, with a conversion efficiency scaling as $\eta\propto N^{8.27}$. These results establish DBE-based waveguides as promising platforms for miniaturized efficient nonlinear photonic devices.

physics.optics↗

arXiv2026

Frozen mode in coupled single-mode waveguides with gratings

We present a systematic methodology for designing slow-light photonic integrated circuits with a frozen mode based on a special kind of exceptional point of degeneracy (EPD) of order three named stationary inflection points (SIPs). This is realized through three-way coupled waveguides with lateral gratings operating at telecommunication wavelengths. We provide two designs and analyze sensitivity to geometric perturbations. We have fabricated a periodic waveguide with integrated taper loads and demonstrate reasonable agreement with full-wave simulations. These findings confirm the feasibility of integrating SIP-based delay functionalities in standard silicon photonic platforms.

physics.optics↗

arXiv2026

Chiral Terahertz Amplification and Lasing using Two-Dimensional Materials with Berry Curvature Dipole

Compact, electrically driven sources of coherent terahertz (THz) radiation remain a challenge due to the lack of efficient gain media and scalable device platforms. Here, we propose and theoretically investigate a cavity-based THz gain mechanism enabled by Berry curvature dipole (BCD) in a DC-biased, low-symmetry two-dimensional (2D) material. Placing the biased 2D layer at the center of a Fabry-Perot cavity enhances light-matter interactions, enabling direct conversion of DC electrical power into coherent THz radiation. We analyze the conditions for amplification and lasing, and identify the parameter regimes that support self-oscillatory coherent emission. Rather than introducing a specific device implementation, our work establishes the physical principles and operating conditions for BCD-enabled THz gain and lasing and provides the theoretical foundation for future realizations. The chiral nature of BCD-induced response enables bias-tunable chiral optical gain, selective polarization eigenstate amplification, and electrically controlled handedness of the emitted radiation. Importantly, substantial amplification and lasing are achieved using only a single 2D material, significantly simplifying device design while preserving scalability across the THz band via cavity-length tuning. This platform is broadly applicable to low-symmetry 2D materials with finite BCD, offering a general route toward compact, frequency-tunable, and polarization-selective THz sources.

physics.optics↗

arXiv2026

Steady-State Exceptional Point Degeneracy and Sensitivity of Nonlinear Saturable Coupled Oscillators

A coupled oscillator system displays enhanced sensitivity of its saturated steady-state (SS) oscillation frequency to small parameter perturbations near an exceptional point degeneracy (EPD), a property that can be used to realize EPD-based sensors. Linear $\mathcal{PT}$-symmetric systems, consisting of two coupled resonators, exhibit EPDs around which square-root sensitivity is observed. However, linear models are insufficient for realistic systems that rely on nonlinear, saturable gain elements, particularly when $\mathcal{PT}$-symmetry is broken. Thus, we study the SS of a general system of two coupled oscillators featuring EPDs and saturable nonlinear gain, using coupled-mode theory. We do this by synthesizing and extending prior SS analyses of the system's stability, and its square-root and cubic-root oscillation frequency sensitivity at a unique third-order SS-EPD. We include an SS analysis of the saturated gain values, energy, and the oscillation frequency's sensitivity in the vicinity of the third-order SS-EPD, providing a comprehensive analysis of the system's various SS regimes. We determine that the stable and bistable regions in parameter space directly depend on the saturated gain values; that the dynamic range of high sensitivity around degenerate conditions is extended by increasing losses, consequently reducing the system's stored energy; and that, to exploit the cubic-root-like sensitivity associated to the third-order SS-EPD, the suggested working regime is best confined to operation within the weakly coupled regime and not exactly at the third order SS-EPD. Finally, we apply the model to two electronic circuits that exhibit cubic-root sensitivity, demonstrating the application and limitations of this analysis.

physics.app-ph↗

arXiv2025

Exceptional Points in the Scattering Resonances of a Sphere Dimer

We investigate exceptional points of degeneracy (EPDs) in electromagnetic scattering of a sphere dimer from the electroquasistatic limit to the fully retarded regime. In the quasistatic limit, we prove that $\parity\trev$-symmetric configurations, realized by spheres with complex-conjugate susceptibilities, host EPDs. Beyond this limit, retardation breaks $\mathscr{PT}$-symmetry; nevertheless, by jointly tuning the material dispersion of the two spheres, we derive analytic synthesis conditions for realizing EPDs at \textit{real frequencies}. Near an EPD, we show that single-parameter perturbations yield the characteristic square-root splitting of the eigenfrequencies, and we quantify its impact on scattering, extinction, and absorption, clarifying sensing implications.

physics.optics↗

arXiv2025

Lasing at a Stationary Inflection Point: erratum

This erratum provides an updated fitting function for the lasing threshold of finite-length cavities operating at a stationary inflection point (SIP) or regular band edge (RBE) resonance, clarifying their asymptotic scaling with the number of unit cells of the periodic cavity.

physics.optics↗

arXiv2025

Experimental Measurement of Enhanced Group Delay Silicon Photonic Waveguides Indicative of the Frozen Mode Regime Around the Stationary Inflection Point

The dispersion engineering of periodic silicon photonic waveguides presents opportunities for significant group delay enhancement compared to uniform waveguides of comparable length. We describe the spectral response characteristics for measured devices and compare their properties to modeled data. These waveguides support the frozen mode regime (FMR) around near infrared wavelengths and are expected to show enhanced group delays around the FMR resonances. Measurements of fabricated devices provide evidence for enhanced delays and spectral properties associated with the FMR. We study how perturbations to the waveguide model impact agreement with measurements and its meaning for these devices operating in the FMR.

physics.optics↗

arXiv2025

Third Harmonic Generation in Transparent Longitudinal Epsilon-Near-Zero Multilayers

Epsilon near zero (ENZ) materials can dramatically enhance local optical fields, enabling nonlinear interactions at relatively low intensities. Yet, near their plasma frequency, conventional isotropic ENZ media remain highly absorptive, limiting nonlinear operations that require good transparency. Longitudinal epsilon near zero metamaterials (LENZ), characterized by a vanishing permittivity along the optical axis provide an exceptional platform for field enhancement while mitigating absorption losses and impedance mismatch. We experimentally show that a Si and ITO multilayer engineered for a LENZ resonance in the near-infrared enables broadband, high pump transmission while still harnessing ENZ enhanced nonlinearity to generate a strong third harmonic signal. This demonstrates that efficient nonlinear processes can be driven without the high-loss conditions typical of isotropic ENZ media and regardless of intrinsic absorption at the harmonic frequency. The resulting third harmonic efficiency is comparable to isotropic ENZ films but without the absorption-induced heating constraints of ENZ operation. The high pump transmission enables transparent LENZ (TLENZ) stacks to be integrated into optical cavities, where resonant field buildup could amplify the nonlinear response without compromising thermal management. These results establish TLENZ multilayers as a robust, versatile platform for transparent, field enhanced nonlinear nanophotonics, combining strong light matter interaction with low-loss operation.

physics.optics↗

arXiv2025

Experimental Analysis of Biasing Voltage Generation in Wave-Controlled RIS

Reconfigurable intelligent surfaces (RISs), an emerging technology proposed for inclusion in next generation wireless communication systems, are programmable surfaces that can adaptively reflect incident electromagnetic radiation in different desired directions. To reduce the complexity and physical profile of conventional RIS designs, a novel concept known as Wave-Controlled RIS has been proposed, in which standing waves along a transmission line are used to generate the required dc bias for reflective control. This paper shows the design of such a Wave-Controlled RIS and its biasing transmission line. The effectiveness of this approach in generating the correct dc bias from a single standing wave frequency is analyzed through both theoretical modeling and experimental validation, which uncovered a dependence on impedance matching not accounted for by the theory. Additionally, the potential for reflective control using only a single standing wave frequency on the biasing transmission line is explored, demonstrating the ability of single-beam steering toward angles near broadside.

eess.SP↗

arXiv2025

Berry Curvature Dipole-Induced Chiral Terahertz Gain and Lasing Threshold in Bulk Tellurium

We investigate the use of Berry curvature dipole in $n$-doped Tellurium as a mechanism for achieving terahertz amplification and lasing by applying a DC electric field. When the electrical bias and wave vector are aligned along the trigonal $c$-axis, the right-handed circularly polarized mode experiences amplification at relatively low bias, while the left-handed mode is attenuated. Furthermore, when the electrical bias and wave vector are orthogonal to the $c$-axis, the structure supports elliptically polarized eigenmodes that also exhibit gain under suitable bias conditions, where the degree of ellipticity is tunable by the applied bias. We also investigate lasing conditions for a Fabry-Perot cavity incorporating biased Te as an active medium. Due to the resonance in the dielectric permittivity of Tellurium, there are discrete lasing intervals. Our results show that bulk chiral Tellurium could be used as an electrically tunable, polarization-selective gain medium for micrometer-scale terahertz lasers, with lasing achievable at bias fields below the material's breakdown threshold, paving the way towards new terahertz devices.

physics.optics↗

arXiv2025

Advancements in Degenerate Distributed Feedback Lasing

We advance the concept of degenerate distributed feedback (DDFB) lasing in a double grating photonic structure that operates near a degenerate band edge (DBE) to achieve a robust single-frequency lasing regime. The DBE is an exceptional point of degeneracy (EPD) of fourth order involving four coalescing Bloch eigenmodes. A DDFB photonic cavity operating close to the DBE frequency is shown to display a large quality factor that scales with the fifth power of the cavity length. Upon the inclusion of gain, the DDFB cavity displays a low lasing threshold with the exceptional scaling as the inverse of the fifth power of the double grating length, i.e., ${\alpha}_{D,th} \propto 1/N^5$, where $N$ is the number of waveguide unit cells making the mirrorless cavity. The work proposed here shows a path to single-frequency lasing mode using a mirrorless cavity with minimal device footprint. The DDFB laser is very attractive for various applications including communications, sensing, and spectroscopy.

physics.optics↗

arXiv2025

Double-Corrugated Degenerate Band-Edge Oscillator Driven by an Electron Beam

The properties of a degenerate band-edge (DBE) oscillator based on the synchronous interaction between a linear electron beam synchronized with four degenerate electromagnetic modes are presented. The DBE in the cold slow-wave structure is implemented in a rectangular waveguide with a staggered double corrugation in it. At the DBE, four modes coalesce into a single degenerate mode, forming a very flat dispersion diagram with vanishing or very small group velocity. The DBE oscillator frequency is robust to variations in electron beam current, electrons' velocity, and cavity length. The DBE oscillator concept provided here is expected to be extendable to several other geometries as long as the slow wave structure supports a DBE.

physics.plasm-ph↗

arXiv2025

Wideband Glide-Symmetric Slow-Wave Structure for Millimeter-Wave Sheet Beam TWTs

We introduce a slow-wave structure (SWS) for a millimeter-wave sheet-beam traveling-wave tube (TWT) with wide bandwidth. The wideband and stable operation is enabled through the topological properties associated with glide-symmetry that close the bandgap at the $3\pi$-point and also make the on-axis interaction impedance negligible for the backward wave. This space harmonic structure is designed to operate in the $V$-band over 55-68 GHz with synchronism to a 5.2 kV, 11 mA sheet electron beam that will be produced by a diamond field-emitter array.

physics.plasm-ph↗

arXiv2025

AI-Driven Optimization of Wave-Controlled Reconfigurable Intelligent Surfaces

A promising type of Reconfigurable Intelligent Surface (RIS) employs tunable control of its varactors using biasing transmission lines below the RIS reflecting elements. Biasing standing waves (BSWs) are excited by a time-periodic signal and sampled at each RIS element to create a desired biasing voltage and control the reflection coefficients of the elements. A simple rectifier can be used to sample the voltages and capture the peaks of the BSWs over time. Like other types of RIS, attempting to model and accurately configure a wave-controlled RIS is extremely challenging due to factors such as device non-linearities, frequency dependence, element coupling, etc., and thus significant differences will arise between the actual and assumed performance. An alternative approach to solving this problem is data-driven: Using training data obtained by sampling the reflected radiation pattern of the RIS for a set of BSWs, a neural network (NN) is designed to create an input-output map between the BSW amplitudes and the resulting sampled radiation pattern. This is the approach discussed in this paper. In the proposed approach, the NN is optimized using a genetic algorithm (GA) to minimize the error between the predicted and measured radiation patterns. The BSW amplitudes are then designed via Simulated Annealing (SA) to optimize a signal-to-leakage-plus-noise ratio measure by iteratively forward-propagating the BSW amplitudes through the NN and using its output as feedback to determine convergence. The resulting optimal solutions are stored in a lookup table to be used both as settings to instantly configure the RIS and as a basis for determining more complex radiation patterns.

cs.ET↗

arXiv2025

Super-exponential Amplification of Wavepacket Propagation in Traveling Wave Tubes

We analyze wavepacket propagation in traveling wave tubes (TWTs) analytically and numerically. TWT design in essence comprises a pencil-like electron beam in vacuum interacting with an electromagnetic wave guided by a slow-wave structure (SWS). In our study, the electron beam is represented by a one-dimensional electron flow and the SWS is represented by an equivalent transmission line model. The analytical considerations are based on the Lagrangian field theory for TWTs. Mathematical analysis of wavepacket propagation in one-dimensional space is based on the relevant Euler-Lagrange equations which are second-order differential equations in both time and space. Wavepacket propagation analysis is not simple and we develop a numerically efficient algorithm to perform the analysis efficiently. In particular, when the initial pulse has a Gaussian shape at the input port, it acquires non-Gaussian features as it propagates through the TWT. These features include: (i) super-exponential (faster than exponential) amplification, (ii) shift of the pulse frequency spectrum toward higher frequencies, and (iii) change in the shape of the pulse that becomes particularly pronounced when the pulse frequency band contains a transitional point from stability to instability.

physics.plasm-ph↗

arXiv2025

Experimental generation of optimally chiral azimuthally-radially polarized beams

We implement a paraxial azimuthally-radially polarized beam (ARPB), a novel class of structured light beams that can be optimal chiral (OC), leading to maximum chirality density at a given energy density. By using vectorial light shaping techniques, we successfully generated a paraxial ARPB with precise control over its features, validating theoretical predictions. Our findings demonstrate the ability to finely adjust the chirality density of the ARPB across its entire range by manipulating a single beam parameter. Although our experimental investigations are primarily focused on the transverse plane, we show that fields whose transverse components satisfy the optimal chirality condition are optimally chiral in all directions, and our results highlight the promising potential of OC structured light for applications in the sensing and manipulation of chiral particles. We show that optical chirality is more general than the concept of handedness. This work represents a significant advancement toward practical optical enantioseparation and enantiomer detection at the nanoscale.

physics.optics↗

arXiv2025

Array oscillator in coupled waveguides with nonlinear gain and radiation resistances saturating at exceptional point

A periodically loaded waveguide composed of periodic discrete nonlinear gain and radiating elements supports a stable oscillation regime related to the presence of an exceptional point of degeneracy (EPD). After reaching saturation, the EPD in the system establishes the oscillation frequency. We demonstrate a synchronization regime at a stable oscillation frequency, resulting in uniform saturated gain across the array and uniform radiating power. Unlike conventional one-dimensional cavity resonances, the oscillation frequency is independent of the array length. Our investigations further show that when small-signal gain is non-uniformly distributed across the array, the saturated gain results in having a uniform distribution at a gain value that generates an EPD. Experimental validation using the measured board confirmed that the system saturates at an EPD, with a measured spectrum exhibiting very low phase noise. This low noise allows for operation at a clean oscillation frequency. Additionally, the measured uniform power across the array corresponds to the simulation results. The proposed scheme can pave the way for a new generation of high-power radiating arrays with distributed active elements.

physics.app-ph↗