arXiv subjects

Vassilios Kovanis

Publications and source records attributed to Vassilios Kovanis.

At least 19 recordsLinked to original sources

arXiv2025

Optimizing disorder with machine learning to harness synchronization

Disorder is often considered detrimental to coherence. However, under specific conditions, it can enhance synchronization. We develop a machine-learning framework to design optimal disorder configurations that maximize phase synchronization. In particular, utilizing the system of coupled nonlinear pendulums with disorder and noise, we train a feedforward neural network (FNN), with the disorder parameters as input, to predict the Shannon entropy index that quantifies the phase synchronization strength. The trained FNN model is then deployed to search for the optimal disorder configurations in the high-dimensional space of the disorder parameters, providing a computationally efficient replacement of the stochastic differential equation solvers. Our results demonstrate that the FNN is capable of accurately predicting synchronization and facilitates an efficient inverse design solution to optimizing and enhancing synchronization.

nlin.AO↗

arXiv2024

Deep-learning design of graphene metasurfaces for quantum control and Dirac electron holography

Metasurfaces are sub-wavelength patterned layers for controlling waves in physical systems. In optics, meta-surfaces are created by materials with different dielectric constants and are capable of unconventional functionalities. We develop a deep-learning framework for Dirac-material metasurface design for controlling electronic waves. The metasurface is a configuration of circular graphene quantum dots, each created by an electric potential. Employing deep convolutional neural networks, we show that the original scattering wave can be reconstructed with fidelity over 95$\%$, suggesting the feasibility of Dirac electron holography. Additional applications such as plane wave generation, designing broadband, and multi-functionality graphene metasurface systems are illustrated.

physics.comp-ph↗

arXiv2024

Coexisting periodic attractors under the influence of noise in injected photonic oscillators

The optical power spectrum is the prime observable to dissect, understand and design the long-time behavior of arrays of optically coupled semiconductor lasers. A long-standing issue has been identified within the literature on injection locking: how the thickness of linewidth and the lineshape waveform envelope correlates with the deterministic evolution of the slave laser oscillator and how the presence of noise and the dense proximity of coexisting limit cycles are shaping and influencing the overall spectral behavior. In addition we are critically interested in the regions where the basin of attraction has self-similar and fractal-like behavior, still, the long-time orbits are limit cycles, in this case, period-one (P1) and period-three (P3). Via numerical work, we find that the overall optical power spectrum is deeply imprinted with a strong influence from the underlying noise when the system lives in the region of the P1 and P3 limit cycles. Additionally, a qualitative comparison is made between the stochastic Langevin simulations and the recorded optical power spectra taken from older experiments that we revisit. Finally, we point to a possible set of next-generation applications in photonics and quantum technologies where such findings may have substantial implications.

physics.optics↗

arXiv2023

Nonlinear Multi-Resonant Cavity Quantum Photonics Gyroscopes Quantum Light Navigation

We propose an on-chip all-optical gyroscope based on nonlinear multi-resonant cavity quantum photonics in thin film $χ^{(2)}$ resonators -- Quantum-Optic Nonlinear Gyro or QONG in short. The key feature of our gyroscope is co-arisal and co-accumulation of quantum correlations, nonlinear wave mixing and non-inertial signals, all inside the same sensor-resonator. We theoretically analyze the Fisher Information of our QONGs under fundamental quantum noise conditions. Using Bayesian optimization, we maximize the Fisher Information and show that $\sim 900\times$ improvement is possible over the shot-noise limited linear gyroscope with the same footprint, intrinsic quality factors and power budget.

quant-ph↗

arXiv2023

Radically Uniform Spike Trains in Optically Injected Quantum Cascade Oscillators

It has been found that noise-induced excitability in quantum well and quantum dot semiconductor laser systems usually produces spike patterns of non-uniform amplitude. In this letter, we experimentally record that an inter-subband quantum cascade laser injected with a monochromatic laser exhibits a series of highly-uniform spike trains in the time domain. Theoretical analysis demonstrates that such high uniformity has its origin in the ultrashort carrier lifetime of the quantum cascade laser gain medium that is typically close to one picosecond.

physics.optics↗

arXiv2023

Time Crystals transforming Frequency Combs in Tunable Photonic Oscillators

The response of a tunable photonic oscillator, consisting of an Optically Injected Semiconductor Laser, under an injected Frequency Comb is considered with the utilization of the concept of the Time Crystal that has been widely used for the study of driven nonlinear oscillators in the context of mathematical biology. The dynamics of the original system reduce to a radically simple one-dimensional circle map with properties and bifurcations determined by the specific features of the Time Crystal fully describing the phase response of the limit cycle oscillation. The circle map is shown to accurately model the dynamics of the original nonlinear system of ordinary differential equations and capable for providing conditions for resonant synchronization resulting to output frequency combs with tunable shape characteristics. Such theoretical developments can have potential for significant photonic signal processing applications.

physics.optics↗

arXiv2022

Isochrons, Phase Response and Synchronization Dynamics of Tunable Photonic Oscillators

The global structure of the Isochrons and the corresponding Phase Response Curves are, for the first time, investigated and numerically computed for the fundamental photonic oscillator consisted of an Optically Injected Laser. Their crucial role in the synchronization dynamics under a periodic modulation of the injection beam is shown, along with their capability for providing conditions for stable phase-locking and periodic outputs with discrete equidistant spectra in the form of frequency combs.

physics.optics↗

arXiv2020

Spatial Control of Localized Oscillations in Arrays of Coupled Laser Dimers

Arrays of coupled semiconductor lasers are systems possessing radically complex dynamics that makes them useful for numerous applications in beam forming and beam shaping. In this work, we investigate the spatial controllability of oscillation amplitudes in an array of coupled photonic dimers, each consisting of two semiconductor lasers driven by differential pumping rates. We consider parameter values for which each dimer's stable phase-locked state has become unstable through a Hopf bifurcation and we show that, by assigning appropriate pumping rate values to each dimer, large-amplitude oscillations coexist with negligibly small amplitude oscillations. The spatial profile? of the amplitude of oscillations across the array can be dynamically controlled by appropriate pumping rate values in each dimer. This feature is shown to be quite robust, even for random detuning between the lasers, and suggests a mechanism for dynamically reconfigurable production of a large diversity of spatial profiles of laser amplitude oscillations.

physics.optics↗

arXiv2019

Radically Tunable Ultrafast Photonic Oscillators via Differential Pumping

We present the controllability capabilities for the limit cycles of an extremely tunable photonic oscillator, consisting of two coupled semiconductor lasers. We show that this system supports stable limit cycles with frequencies ranging from a few to more than a hundred GHz that are characterized by a widely varying degree of asymmetry between the oscillations of the two lasers. These dynamical features are directly controllable via differential pumping as well as optical frequency detuning of the two lasers, suggesting a multi-functional oscillator for chip-scale radio-frequency photonics applications.

physics.optics↗

arXiv2019

Enhanced stability, bistability, and exceptional points in saturable active photonic couplers

A generic photonic coupler with active and lossy parts, gain saturation and asymmetric characteristics is examined. Saturable activity is shown to be able to enhance the overall stability of the steady states, prevent evolution to undesirable unbounded modes and allow for bistable operation in specific regions of parametric space. Both stability and bistability are studied in the phase space of the system, where the basins of attraction of each state are identified, providing an accurate description of the dependence of the electric fields on the initial conditions. Continuous families of exceptional points are detected via suitable regulation of the coupling and asymmetry features of the configuration. In this way, a complete description of the nonlinear dynamics landscape is provided, which should be crucial for multiple application-driven designs incorporating such a ubiquitous optical component.

physics.optics↗

arXiv2019

Metasurface-Enabled Interference Suppression at Visible-Light Communications

Light can be used for wireless information transmission apart from illumination; that is the key idea behind visible-light communication (VLC), one of the disruptive technologies of our days. It combines remarkably high data rates due to ultrashort wavelengths with huge reliability and security due to small distances; nevertheless, it substantially suffers from interference of neighboring light sources in multiple-link configurations. In this manuscript, we investigate a pair of light emitting diodes (LEDs) interfering each other and propose a simple nanoslit metasurface that radically increases the directivity of the transmitting beams. As a result, enhancement of the signal-to-interference ratio by several orders of magnitude is reported. The considered generic setup retains its beneficial features in the presence of realistic design defects and, accordingly, may inspire standardization efforts towards the adoption of VLC in next-generation heterogeneous communication networks.

eess.SP↗

arXiv2018

Antiresonances and Ultrafast Resonances in a Twin Photonic Oscillator

We consider the properties of the small-signal modulation response of symmetry-breaking phase-locked states of twin coupled semiconductor lasers. The extended stability and the varying asymmetry of these modes allows for the introduction of a rich set of interesting modulation response features, such as sharp resonances and anti-resonance as well as efficient modulation at very high frequencies exceeding the free running relaxation frequencies by orders of magnitude.

physics.optics↗

arXiv2018

Exceptional Points in Two Dissimilar Coupled Diode Lasers

We show the abundance of Exceptional Points in the generic asymmetric configuration of two coupled diode lasers, under nonzero optical detuning and differential pumping. We pinpoint the location of these points with respect to the stability domains and the Hopf bifurcation points, in the solution space as well as in the space of experimentally controlled parameters.

physics.optics↗

arXiv2017

Spectral Signatures of Exceptional Points and Bifurcations in the Fundamental Active Photonic Dimer

The fundamental active photonic dimer consisting of two coupled quantum well lasers is investigated in the context of the rate equation model. Spectral transition properties and exceptional points are shown to occur under general conditions, not restricted by PT-symmetry as in coupled mode models, suggesting a paradigm shift in the field of non-Hermitian photonics. The optical spectral signatures of system bifurcations and exceptional points are manifested in terms of self-termination effects and observable drastic variations of the spectral line shape that can be controlled in terms of optical detuning and inhomogeneous pumping.

physics.optics↗

arXiv2017

Controllable Asymmetric Phase-Locked States of the Fundamental Active Photonic Dimer

Coupled semiconductor lasers are systems possessing complex dynamics that are interesting for numerous applications in photonics. In this work, we investigate the existence and the stability of asymmetric phase-locked states of the fundamental active photonic dimer consisting of two coupled lasers. We show that stable phase-locked states of arbitrary asymmetry exist for extended regions of the parameter space of the system and that their field amplitude ratio and phase difference can be dynamically controlled by appropriate current injection. The model includes the important role of carrier density dynamics and shows that the phase-locked state asymmetry is related to operation conditions providing, respectively, gain and loss in the two lasers.

physics.optics↗

arXiv2017

Judiciously distributing laser emitters to shape the desired far field patterns

The far-field pattern of a simple one-dimensional laser array of emitters radiating into free space is considered. In the path of investigating the inverse problem for their near fields leading to a target beam form, surprisingly we found that the result is successful when the matrix of the corresponding linear system is not well-scaled. The essence of our numerical observations is captured by an elegant inequality defining the functional range of the optical distance between two neighboring emitters. Our finding can restrict substantially the parametric space of integrated photonic systems and simplify significantly the subsequent optimizations.

physics.optics↗

arXiv2017

Turbulent chimeras in large semiconductor laser arrays

Semiconductor laser arrays have been investigated experimentally and theoretically from the viewpoint of temporal and spatial coherence for the past forty years. In this work, we are focusing on a rather novel complex collective behavior, namely chimera states, where synchronized clusters of emitters coexist with unsynchronized ones. For the first time, we find such states exist in large diode arrays based on quantum well gain media with nearest-neighbor interactions. The crucial parameters are the evanescent coupling strength and the relative optical frequency detuning between the emitters of the array. By employing a recently proposed figure of merit for classifying chimera states, we provide quantitative and qualitative evidence for the observed dynamics. The corresponding chimeras are identified as turbulent according to the irregular temporal behavior of the classification measure.

nlin.AO↗

arXiv2012

Bragg solitons in nonlinear PT-symmetric periodic potentials

It is shown that slow Bragg soliton solutions are possible in nonlinear complex parity-time (PT) symmetric periodic structures. Analysis indicates that the PT-symmetric component of the periodic optical refractive index can modify the grating band structure and hence the effective coupling between the forward and backward waves. Starting from a classical modified massive Thirring model, solitary wave solutions are obtained in closed form. The basic properties of these slow solitary waves and their dependence on their respective PT-symmetric gain/loss profile are then explored via numerical simulations.

physics.optics↗