arXiv subjects

Nathaniel Furman

Publications and source records attributed to Nathaniel Furman.

8 recordsLinked to original sources

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↗

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

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

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↗

arXiv2023

Low-Threshold Lasing with Frozen Mode Regime and Stationary Inflection Point in Three Coupled Waveguide Structure

The frozen mode regime is a unique slow-light scenario in periodic structures, where the flat-bands (zero group velocity) are associated with the formation of high-order stationary points (aka exceptional points). The formation of exceptional points is accompanied by enhancement of various optical properties such as gain, Q-factor and absorption, which are key properties for the realization of wide variety of devices such as switches, modulators and lasers. Here we present and study a new integrated optical periodic structure consisting of three waveguides coupled via micro-cavities and directional coupler. We study this design theoretically, demonstrating that a proper choice of parameters yields a third order stationary inflection point (SIP). We also show that the structure can be designed to exhibit two almost-overlapping SIPs at the center of the Brillouin Zone. We study the transmission and reflection of light propagating through realistic devices comprising a finite number of unit-cells and investigate their spectral properties in the vicinity of the stationary points. Finally, we analyze the lasing frequencies and threshold level of finite structures (as a function of the number of unit-cells) and show that it outperforms conventional lasers utilizing regular band edge lasing (such as DFB lasers).

physics.optics↗

arXiv2022

Frozen Mode Regime in an Optical Waveguide With Distributed Bragg Reflector

We introduce a glide symmetric optical waveguide exhibiting a stationary inflection point (SIP) in the Bloch wavenumber dispersion relation. An SIP is a third order exceptional point of degeneracy (EPD) where three Bloch eigenmodes coalesce to form a so-called frozen mode with vanishing group velocity and diverging amplitude. We show that the incorporation of chirped distributed Bragg reflectors and distributed coupling between waveguides in the periodic structure facilitates the SIP formation and greatly enhances the characteristics of the frozen mode regime. We confirm the existence of an SIP in two ways: by observing the flatness of the dispersion diagram and also by using a coalescence parameter describing the separation of the three eigenvectors collapsing on each other. We find that in the absence of losses, both the quality factor and the group delay at the SIP grow with the cubic power of the cavity length. The frozen mode regime can be very attractive for light amplification and lasing, in optical delay lines, sensors, and modulators.

physics.optics↗

arXiv2022

Lasing at a Stationary Inflection Point

The concept of lasers based on the frozen mode regime in active periodic optical waveguides with a 3rd-order exceptional point of degeneracy (EPD) is advanced. The frozen mode regime in a lossless and gainless waveguide is associated with a stationary inflection point (SIP) in the Bloch dispersion relation, where three Bloch eigenmodes coalesce forming the frozen mode. As a practical example, we consider an asymmetric serpentine optical waveguide (ASOW). An ASOW operating near the SIP frequency displays a large group delay of a non-resonant nature that scales as the cube of the waveguide length, leading to a strong gain enhancement when active material is included. Therefore, a laser operating in the close vicinity of an SIP has a gain threshold that scales as a negative cube of the waveguide length. We determine that this scaling law is maintained in the presence of small distributed losses, such as radiation associated with waveguide bends and roughness. In addition, we show that although gain causes a distortion in the modes coalescing at the SIP, the properties of the frozen mode are relatively resistant to such small perturbations and we still observe a large degree of exceptional degeneracy for gain values that bring the system above threshold. Finally, our study also reveals that lasing near an SIP is favored over lasing near a photonic band edge located in close proximity to the SIP. In particular, we observe that an SIP-induced lasing in an ASOW displays lower gain threshold compared to lasing near the photonic regular band edge (RBE), even though the SIP resonance has a lower quality factor than the RBE resonance.

physics.optics↗