Yannis Kominis | National Technical University of Athens (original) (raw)

Papers by Yannis Kominis

Research paper thumbnail of Radically tunable ultrafast photonic oscillators via differential pumping

Journal of Applied Physics, 2020

We present the controllability capabilities for the limit cycles of an extremely tunable photonic... more 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 dyamical 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.

Research paper thumbnail of Antiresonances and Ultrafast Resonances in Coupled Semiconductor Lasers

Frontiers in Optics + Laser Science APS/DLS, 2019

Research paper thumbnail of Abundance of Exceptional Points in Two Dissimilar Coupled Diode Lasers

Frontiers in Optics + Laser Science APS/DLS, 2019

Research paper thumbnail of Spectral Line Shape and Modulation Response of Asymmetric Non-Hermitian Photonic Meta-Molecules

CLEO Pacific Rim Conference, 2018

A photonic dimer of two optically coupled lasers is analyzed as an optical meta-molecule for non-... more A photonic dimer of two optically coupled lasers is analyzed as an optical meta-molecule for non-Hermitian photonic applications. Spectral signatures of exceptional points and Hopf bifurcations are shown for generic asymmetric configurations. A significant enhancement in the small signal modulation response under differential pumping is observed.

Research paper thumbnail of Modelling Runaway Electrons Dynamics in Tokamak Plasmas: Progresses and Challenges

The sudden termination of a plasma discharge known as a major disruption is a well-identified dif... more The sudden termination of a plasma discharge known as a major disruption is a well-identified difficulty from the beginning of tokamak research, which remains still today particularly problematic for the design of a reliable fusion reactor. The key questions are principally related to the growth rate of the relativistic electron population, closely linked to the level of the critical electrical field for an electron to run-away and the upper energy limit that it can reach. Recently, very important achievements have been obtained in this domain. The introduction of the synchrotron radiation reaction force in kinetic calculations is shown to limit the upper energy of the runaway beam to a few tens of MeV, a level consistent with observations. Refined studies have also included the effect of bremsstrahlung radiation. The calculation of the runaway avalanche growth rate has been improved by accurately considering the large-angle Coulomb collisions, while the role played by magnetic fiel...

Research paper thumbnail of A European Effort for Kinetic Modelling of Runaway Electron Dynamics

  1. IRFM-CEA, Cadarache, France 2) National Technical University of Athens, Greece 3) BME – NTI, B... more 1) IRFM-CEA, Cadarache, France 2) National Technical University of Athens, Greece 3) BME – NTI, Budapest, Hungary 4) CRPP, Swiss Federal Institute of Technology, Switzerland 5) Chalmers University, Göteborg, Sweden 6) Institute for Plasma Physic, Prag, Czech Republic 7) Princeton Plasma Physics Laboratory, Princeton, USA 8) Aalto University, Finland 9) Max-Planck Institute for Plasma Physics, Garching, Germany 10) Culham Centre for Fusion Energy, UK

Research paper thumbnail of Basins of Attraction in Asymmetric Photonic Couplers

2019 International Conference on Electromagnetics in Advanced Applications (ICEAA), 2019

Two nonlinear RF photonic waveguides, one active and one lossy, are paired in a coupler setup. Th... more Two nonlinear RF photonic waveguides, one active and one lossy, are paired in a coupler setup. The stability of the configuration is investigated for various degrees of asymmetry between gain and loss, while the role of distance between the two components is examined. The enhancing influence of the gain saturation on system's stability and bistability is identified, while the basins of attraction for each fixed point are determined. The reported results unveil a wealth of dynamical features which are essential in various applications like non-reciprocal propagation, ultra-sensitive microwave sensing and tunable power teleportation.

Research paper thumbnail of Spectral Degeneracies in Optically Injected Quantum Well Lasers

Frontiers in Optics + Laser Science APS/DLS, 2019

Research paper thumbnail of Analytical calculation of the orbital spectrum of the guiding centre motion in axisymmetric magnetic fields

Journal of Plasma Physics, 2021

Charged particle motion in axisymmetric toroidal magnetic fields is analysed within the context o... more Charged particle motion in axisymmetric toroidal magnetic fields is analysed within the context of the canonical Hamiltonian guiding centre theory. A canonical transformation to variables measuring the drift orbit deviation from a magnetic field line is introduced and an analytical transformation to action-angle variables is obtained, under a zero drift width approximation. The latter is used to provide compact formulas for the orbital spectrum of the drift motion, namely the bounce/transit frequencies as well as the bounce/transit averaged toroidal precession and gyration frequencies. These formulas are shown to have a remarkable agreement with numerically calculated full drift width frequencies and significant differences from standard analytical formulas based on a pendulum-like Hamiltonian description. The analytical knowledge of the orbital spectrum is crucial for the formulation of particle resonance conditions with symmetry-breaking perturbations and the study of the resultin...

Research paper thumbnail of Spatial control of localized oscillations in arrays of coupled laser dimers

Physical Review E, 2020

Arrays of coupled semiconductor lasers are systems possessing radically complex dynamics that mak... more 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.

Research paper thumbnail of Enhanced stability, bistability, and exceptional points in saturable active photonic couplers

Physical Review A, 2019

A generic photonic coupler with active and lossy parts, gain saturation and asymmetric characteri... more 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.

Research paper thumbnail of Antiresonances and Ultrafast Resonances in a Twin Photonic Oscillator

IEEE Photonics Journal, 2019

We consider the properties of the small-signal modulation response of symmetrybreaking phase-lock... more We consider the properties of the small-signal modulation response of symmetrybreaking 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 antiresonances as well as efficient modulation at very high frequencies exceeding the free running relaxation frequencies by orders of magnitude.

Research paper thumbnail of Exceptional points in two dissimilar coupled diode lasers

Applied Physics Letters, 2018

We show the abundance of Exceptional Points in the generic asymmetric configuration of two couple... more 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.

Research paper thumbnail of Spectral signatures of exceptional points and bifurcations in the fundamental active photonic dimer

Physical Review A, 2017

The fundamental active photonic dimer consisting of two coupled quantum well lasers is investigat... more 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.

Research paper thumbnail of Controllable asymmetric phase-locked states of the fundamental active photonic dimer

Physical Review A, 2017

Coupled semiconductor lasers are systems possessing complex dynamics that are interesting for num... more 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.

Research paper thumbnail of The Asymmetric Active Coupler: Stable Nonlinear Supermodes and Directed Transport

Scientific Reports, 2016

We consider the asymmetric active coupler (AAC) consisting of two coupled dissimilar waveguides w... more We consider the asymmetric active coupler (AAC) consisting of two coupled dissimilar waveguides with gain and loss. We show that under generic conditions, not restricted by parity-time symmetry, there exist finite-power, constant-intensity nonlinear supermodes (NS), resulting from the balance between gain, loss, nonlinearity, coupling and dissimilarity. The system is shown to possess non-reciprocal dynamics enabling directed power transport functionality. Energy transport between coupled systems or different modes of the same system is one of the most fundamental problems in physics and the controlled and directed transport is of great importance in many technological applications such as electronic and optical devices. For the latter, the design and implementation of integrated photonic devices is a major challenge requiring the realization of a set of fundamental elements for photonic circuitry, such as couplers, switches, diodes and isolators for the directed transport of the optical power 1. The nonlinear coherent coupler 2,3 has been widely studied as a basic photonic component allowing for power-sensitive energy transport. The presence of nonlinearity, in principle, allows for the breaking of Lorentz-reciprocity which is a key mechanism for various applications related to unidirectional dynamics and optical isolation 4,5. It has been shown 6,7 that the presence of gain and/or loss in this system renders its dynamics more complex and enriches its functionality. Moreover, in the case where the gain in one channel is exactly equal to the loss in the other channel, the coupler can be considered as a PT-symmetric dimer, and has been shown to possess unidirectional dynamics 8,9 which is the key property for an optical diode. Similar properties have been studied for a large variety of such PT-symmetric photonic structures, extending the theoretical interest on these systems 10-14 , to realistic experimental studies on light propagation in coupled waveguide structures based either on AlGaAs heterostructures 15 or on Fe-doped LiNbO 3 16 at wavelengths of 1550 nm and 514.5 nm, respectively. The PT-symmetric systems have been considered for important applications such as the non-reciprocal light transmission 17-19 , the observation of asymmetric transport 20,21 , the study of active coupling mechanisms 22 , and the synthesizing of unidirectionally invisible media 23. Also, PT-symmetric cavities have been studied with respect to interesting properties of resonant mode control and selection, which is of crucial importance in laser physics 24-26. The presence of gain and loss along with the nonlinearity of a photonic structure has also been shown to support bright and dark solitons in dual-core systems 27-30 and to provide soliton control capabilites in photonic structures with homogeneous gain and loss 31,32 as well as in structures with symmetric 33 or nonsymmetric 34,35 spatially inhomogeneous gain and loss. Finally, we stress the relation of the underlying model of active photonic structures, consisting of coupled mode equations, with similar models used in the study of quantum systems including Bose-Einstein and exciton-polariton condensates 36-38. The PT-symmetric dimer is known to generate unstable dynamics above the parameter threshold which separates the PT-exact phase from the PT-broken phase 39. One way to regain stability is to use the analogy to dissipatively coupled exciton-polariton condensates in the weak lasing regime 36,37. In the optical coupler case this implies to place an active medium in the evanescent wave region of the coupler 22. This is a rather complicated and intricate experimental task, because the pumping in the evanescent wave region can easily lead to an overpumping, which will substantially modify the used underlying model equations.

Research paper thumbnail of Dynamic power balance for nonlinear waves in unbalanced gain and loss landscapes

Physical Review A, 2015

The presence of losses in nonlinear photonic structures is a crucial issue for modern application... more The presence of losses in nonlinear photonic structures is a crucial issue for modern applications. Active parts are introduced for wave power compensation resulting in unbalanced gain and loss landscapes where localized beam propagation is, in general, dynamically unstable. Here we provide generic sufficient conditions for the relation between the gain-loss and the refractive index profiles in order to ensure efficient wave trapping and stable propagation for a wide range of beam launching conditions such as initial power, angle of incidence and position. The stability is a consequence of an underlying dynamic power balance mechanism related to a conserved quantity of wave dynamics.

Research paper thumbnail of ANNEX IX Hamiltonian map description of electron dynamics in gyrotrons

Research paper thumbnail of <title>Solitary waves in photonic structures: analytical solutions of the nonlinear Kronig-Penney model</title>

ABSTRACT A novel method is presented for the analytical construction of solitary wave solutions o... more ABSTRACT A novel method is presented for the analytical construction of solitary wave solutions of the nonlinear Kronig-Penney model in a photonic structure. In order to overcome the restrictions of the coupled-mode theory and the tight-binding approximation and study the solitary wave formation in a unified model, we consider the original NLSE, with periodically varying coefficients, modeling a waveguide array structure. The analytically obtained solutions correspond to gap solitons and form a class of self-localized solutions existing under quite generic conditions. A remarkable robustness of the solutions under propagation is shown, thus providing potentiality for various applications.

Research paper thumbnail of Fokker–Planck description of the scattering of radio frequency waves at the plasma edge

Physics of Plasmas, 2010

In magnetic fusion devices, radio frequency ͑rf͒ waves in the electron cyclotron ͑EC͒ and lower h... more In magnetic fusion devices, radio frequency ͑rf͒ waves in the electron cyclotron ͑EC͒ and lower hybrid ͑LH͒ range of frequencies are being commonly used to modify the plasma current profile. In ITER, EC waves are expected to stabilize the neoclassical tearing mode ͑NTM͒ by providing current in the island region ͓R. Aymar et al., Nucl. Fusion 41, 1301 ͑2001͔͒. The appearance of NTMs severely limits the plasma pressure and leads to the degradation of plasma confinement. LH waves could be used in ITER to modify the current profile closer to the edge of the plasma. These rf waves propagate from the excitation structures to the core of the plasma through an edge region, which is characterized by turbulence-in particular, density fluctuations. These fluctuations, in the form of blobs, can modify the propagation properties of the waves by refraction. In this paper, the effect on rf due to randomly distributed blobs in the edge region is studied. The waves are represented as geometric optics rays and the refractive scattering from a distribution of blobs is formulated as a Fokker-Planck equation. The scattering can have two diffusive effects-one in real space and the other in wave vector space. The scattering can modify the trajectory of rays into the plasma and it can affect the wave vector spectrum. The refraction of EC waves, for example, could make them miss the intended target region where the NTMs occur. The broadening of the wave vector spectrum could broaden the wave generated current profile. The Fokker-Planck formalism for diffusion in real space and wave vector space is used to study the effect of density blobs on EC and LH waves in an ITER type of plasma environment. For EC waves the refractive effects become important since the distance of propagation from the edge to the core in ITER is of the order of a meter. The diffusion in wave vector space is small. For LH waves the refractive effects are insignificant but the diffusion in wave vector space is important. The theoretical model is general enough to study the effect of density blobs on all propagating cold plasma waves.

Research paper thumbnail of Radically tunable ultrafast photonic oscillators via differential pumping

Journal of Applied Physics, 2020

We present the controllability capabilities for the limit cycles of an extremely tunable photonic... more 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 dyamical 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.

Research paper thumbnail of Antiresonances and Ultrafast Resonances in Coupled Semiconductor Lasers

Frontiers in Optics + Laser Science APS/DLS, 2019

Research paper thumbnail of Abundance of Exceptional Points in Two Dissimilar Coupled Diode Lasers

Frontiers in Optics + Laser Science APS/DLS, 2019

Research paper thumbnail of Spectral Line Shape and Modulation Response of Asymmetric Non-Hermitian Photonic Meta-Molecules

CLEO Pacific Rim Conference, 2018

A photonic dimer of two optically coupled lasers is analyzed as an optical meta-molecule for non-... more A photonic dimer of two optically coupled lasers is analyzed as an optical meta-molecule for non-Hermitian photonic applications. Spectral signatures of exceptional points and Hopf bifurcations are shown for generic asymmetric configurations. A significant enhancement in the small signal modulation response under differential pumping is observed.

Research paper thumbnail of Modelling Runaway Electrons Dynamics in Tokamak Plasmas: Progresses and Challenges

The sudden termination of a plasma discharge known as a major disruption is a well-identified dif... more The sudden termination of a plasma discharge known as a major disruption is a well-identified difficulty from the beginning of tokamak research, which remains still today particularly problematic for the design of a reliable fusion reactor. The key questions are principally related to the growth rate of the relativistic electron population, closely linked to the level of the critical electrical field for an electron to run-away and the upper energy limit that it can reach. Recently, very important achievements have been obtained in this domain. The introduction of the synchrotron radiation reaction force in kinetic calculations is shown to limit the upper energy of the runaway beam to a few tens of MeV, a level consistent with observations. Refined studies have also included the effect of bremsstrahlung radiation. The calculation of the runaway avalanche growth rate has been improved by accurately considering the large-angle Coulomb collisions, while the role played by magnetic fiel...

Research paper thumbnail of A European Effort for Kinetic Modelling of Runaway Electron Dynamics

  1. IRFM-CEA, Cadarache, France 2) National Technical University of Athens, Greece 3) BME – NTI, B... more 1) IRFM-CEA, Cadarache, France 2) National Technical University of Athens, Greece 3) BME – NTI, Budapest, Hungary 4) CRPP, Swiss Federal Institute of Technology, Switzerland 5) Chalmers University, Göteborg, Sweden 6) Institute for Plasma Physic, Prag, Czech Republic 7) Princeton Plasma Physics Laboratory, Princeton, USA 8) Aalto University, Finland 9) Max-Planck Institute for Plasma Physics, Garching, Germany 10) Culham Centre for Fusion Energy, UK

Research paper thumbnail of Basins of Attraction in Asymmetric Photonic Couplers

2019 International Conference on Electromagnetics in Advanced Applications (ICEAA), 2019

Two nonlinear RF photonic waveguides, one active and one lossy, are paired in a coupler setup. Th... more Two nonlinear RF photonic waveguides, one active and one lossy, are paired in a coupler setup. The stability of the configuration is investigated for various degrees of asymmetry between gain and loss, while the role of distance between the two components is examined. The enhancing influence of the gain saturation on system's stability and bistability is identified, while the basins of attraction for each fixed point are determined. The reported results unveil a wealth of dynamical features which are essential in various applications like non-reciprocal propagation, ultra-sensitive microwave sensing and tunable power teleportation.

Research paper thumbnail of Spectral Degeneracies in Optically Injected Quantum Well Lasers

Frontiers in Optics + Laser Science APS/DLS, 2019

Research paper thumbnail of Analytical calculation of the orbital spectrum of the guiding centre motion in axisymmetric magnetic fields

Journal of Plasma Physics, 2021

Charged particle motion in axisymmetric toroidal magnetic fields is analysed within the context o... more Charged particle motion in axisymmetric toroidal magnetic fields is analysed within the context of the canonical Hamiltonian guiding centre theory. A canonical transformation to variables measuring the drift orbit deviation from a magnetic field line is introduced and an analytical transformation to action-angle variables is obtained, under a zero drift width approximation. The latter is used to provide compact formulas for the orbital spectrum of the drift motion, namely the bounce/transit frequencies as well as the bounce/transit averaged toroidal precession and gyration frequencies. These formulas are shown to have a remarkable agreement with numerically calculated full drift width frequencies and significant differences from standard analytical formulas based on a pendulum-like Hamiltonian description. The analytical knowledge of the orbital spectrum is crucial for the formulation of particle resonance conditions with symmetry-breaking perturbations and the study of the resultin...

Research paper thumbnail of Spatial control of localized oscillations in arrays of coupled laser dimers

Physical Review E, 2020

Arrays of coupled semiconductor lasers are systems possessing radically complex dynamics that mak... more 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.

Research paper thumbnail of Enhanced stability, bistability, and exceptional points in saturable active photonic couplers

Physical Review A, 2019

A generic photonic coupler with active and lossy parts, gain saturation and asymmetric characteri... more 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.

Research paper thumbnail of Antiresonances and Ultrafast Resonances in a Twin Photonic Oscillator

IEEE Photonics Journal, 2019

We consider the properties of the small-signal modulation response of symmetrybreaking phase-lock... more We consider the properties of the small-signal modulation response of symmetrybreaking 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 antiresonances as well as efficient modulation at very high frequencies exceeding the free running relaxation frequencies by orders of magnitude.

Research paper thumbnail of Exceptional points in two dissimilar coupled diode lasers

Applied Physics Letters, 2018

We show the abundance of Exceptional Points in the generic asymmetric configuration of two couple... more 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.

Research paper thumbnail of Spectral signatures of exceptional points and bifurcations in the fundamental active photonic dimer

Physical Review A, 2017

The fundamental active photonic dimer consisting of two coupled quantum well lasers is investigat... more 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.

Research paper thumbnail of Controllable asymmetric phase-locked states of the fundamental active photonic dimer

Physical Review A, 2017

Coupled semiconductor lasers are systems possessing complex dynamics that are interesting for num... more 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.

Research paper thumbnail of The Asymmetric Active Coupler: Stable Nonlinear Supermodes and Directed Transport

Scientific Reports, 2016

We consider the asymmetric active coupler (AAC) consisting of two coupled dissimilar waveguides w... more We consider the asymmetric active coupler (AAC) consisting of two coupled dissimilar waveguides with gain and loss. We show that under generic conditions, not restricted by parity-time symmetry, there exist finite-power, constant-intensity nonlinear supermodes (NS), resulting from the balance between gain, loss, nonlinearity, coupling and dissimilarity. The system is shown to possess non-reciprocal dynamics enabling directed power transport functionality. Energy transport between coupled systems or different modes of the same system is one of the most fundamental problems in physics and the controlled and directed transport is of great importance in many technological applications such as electronic and optical devices. For the latter, the design and implementation of integrated photonic devices is a major challenge requiring the realization of a set of fundamental elements for photonic circuitry, such as couplers, switches, diodes and isolators for the directed transport of the optical power 1. The nonlinear coherent coupler 2,3 has been widely studied as a basic photonic component allowing for power-sensitive energy transport. The presence of nonlinearity, in principle, allows for the breaking of Lorentz-reciprocity which is a key mechanism for various applications related to unidirectional dynamics and optical isolation 4,5. It has been shown 6,7 that the presence of gain and/or loss in this system renders its dynamics more complex and enriches its functionality. Moreover, in the case where the gain in one channel is exactly equal to the loss in the other channel, the coupler can be considered as a PT-symmetric dimer, and has been shown to possess unidirectional dynamics 8,9 which is the key property for an optical diode. Similar properties have been studied for a large variety of such PT-symmetric photonic structures, extending the theoretical interest on these systems 10-14 , to realistic experimental studies on light propagation in coupled waveguide structures based either on AlGaAs heterostructures 15 or on Fe-doped LiNbO 3 16 at wavelengths of 1550 nm and 514.5 nm, respectively. The PT-symmetric systems have been considered for important applications such as the non-reciprocal light transmission 17-19 , the observation of asymmetric transport 20,21 , the study of active coupling mechanisms 22 , and the synthesizing of unidirectionally invisible media 23. Also, PT-symmetric cavities have been studied with respect to interesting properties of resonant mode control and selection, which is of crucial importance in laser physics 24-26. The presence of gain and loss along with the nonlinearity of a photonic structure has also been shown to support bright and dark solitons in dual-core systems 27-30 and to provide soliton control capabilites in photonic structures with homogeneous gain and loss 31,32 as well as in structures with symmetric 33 or nonsymmetric 34,35 spatially inhomogeneous gain and loss. Finally, we stress the relation of the underlying model of active photonic structures, consisting of coupled mode equations, with similar models used in the study of quantum systems including Bose-Einstein and exciton-polariton condensates 36-38. The PT-symmetric dimer is known to generate unstable dynamics above the parameter threshold which separates the PT-exact phase from the PT-broken phase 39. One way to regain stability is to use the analogy to dissipatively coupled exciton-polariton condensates in the weak lasing regime 36,37. In the optical coupler case this implies to place an active medium in the evanescent wave region of the coupler 22. This is a rather complicated and intricate experimental task, because the pumping in the evanescent wave region can easily lead to an overpumping, which will substantially modify the used underlying model equations.

Research paper thumbnail of Dynamic power balance for nonlinear waves in unbalanced gain and loss landscapes

Physical Review A, 2015

The presence of losses in nonlinear photonic structures is a crucial issue for modern application... more The presence of losses in nonlinear photonic structures is a crucial issue for modern applications. Active parts are introduced for wave power compensation resulting in unbalanced gain and loss landscapes where localized beam propagation is, in general, dynamically unstable. Here we provide generic sufficient conditions for the relation between the gain-loss and the refractive index profiles in order to ensure efficient wave trapping and stable propagation for a wide range of beam launching conditions such as initial power, angle of incidence and position. The stability is a consequence of an underlying dynamic power balance mechanism related to a conserved quantity of wave dynamics.

Research paper thumbnail of ANNEX IX Hamiltonian map description of electron dynamics in gyrotrons

Research paper thumbnail of <title>Solitary waves in photonic structures: analytical solutions of the nonlinear Kronig-Penney model</title>

ABSTRACT A novel method is presented for the analytical construction of solitary wave solutions o... more ABSTRACT A novel method is presented for the analytical construction of solitary wave solutions of the nonlinear Kronig-Penney model in a photonic structure. In order to overcome the restrictions of the coupled-mode theory and the tight-binding approximation and study the solitary wave formation in a unified model, we consider the original NLSE, with periodically varying coefficients, modeling a waveguide array structure. The analytically obtained solutions correspond to gap solitons and form a class of self-localized solutions existing under quite generic conditions. A remarkable robustness of the solutions under propagation is shown, thus providing potentiality for various applications.

Research paper thumbnail of Fokker–Planck description of the scattering of radio frequency waves at the plasma edge

Physics of Plasmas, 2010

In magnetic fusion devices, radio frequency ͑rf͒ waves in the electron cyclotron ͑EC͒ and lower h... more In magnetic fusion devices, radio frequency ͑rf͒ waves in the electron cyclotron ͑EC͒ and lower hybrid ͑LH͒ range of frequencies are being commonly used to modify the plasma current profile. In ITER, EC waves are expected to stabilize the neoclassical tearing mode ͑NTM͒ by providing current in the island region ͓R. Aymar et al., Nucl. Fusion 41, 1301 ͑2001͔͒. The appearance of NTMs severely limits the plasma pressure and leads to the degradation of plasma confinement. LH waves could be used in ITER to modify the current profile closer to the edge of the plasma. These rf waves propagate from the excitation structures to the core of the plasma through an edge region, which is characterized by turbulence-in particular, density fluctuations. These fluctuations, in the form of blobs, can modify the propagation properties of the waves by refraction. In this paper, the effect on rf due to randomly distributed blobs in the edge region is studied. The waves are represented as geometric optics rays and the refractive scattering from a distribution of blobs is formulated as a Fokker-Planck equation. The scattering can have two diffusive effects-one in real space and the other in wave vector space. The scattering can modify the trajectory of rays into the plasma and it can affect the wave vector spectrum. The refraction of EC waves, for example, could make them miss the intended target region where the NTMs occur. The broadening of the wave vector spectrum could broaden the wave generated current profile. The Fokker-Planck formalism for diffusion in real space and wave vector space is used to study the effect of density blobs on EC and LH waves in an ITER type of plasma environment. For EC waves the refractive effects become important since the distance of propagation from the edge to the core in ITER is of the order of a meter. The diffusion in wave vector space is small. For LH waves the refractive effects are insignificant but the diffusion in wave vector space is important. The theoretical model is general enough to study the effect of density blobs on all propagating cold plasma waves.