arXiv subjects

T. Kottos

Publications and source records attributed to T. Kottos.

At least 19 recordsLinked to original sources

arXiv2023

Optical Limiter Based on PT-Symmetry Breaking of Reflectionless Modes

The application of parity-time (PT) symmetry in optics, especially PT-symmetry breaking, has attracted considerable attention as a novel approach to controlling light propagation. Here, we report optical limiting by two coupled optical cavities with a PT-symmetric spectrum of reflectionless modes. The optical limiting is related to broken PT symmetry due to light-induced changes in one of the cavities. Our experimental implementation is a three-mirror resonator of alternating layers of ZnS and cryolite with a PT-symmetric spectral degeneracy of two reflectionless modes. The optical limiting is demonstrated by measurements of single 532-nm 6-ns laser pulses. At fluences below 10 mJ/cm2, the multilayer exhibits a flat-top passband at 532 nm. At higher fluences, laser heating combined with the thermo-optic effect in ZnS leads to cavity detuning and PT-symmetry breaking of the reflectionless modes. As a result, the entire multilayer structure quickly becomes highly reflective, protecting itself from laser-induced damage. The cavity detuning mechanism can differ at much higher limiting thresholds and include nonlinearity.

physics.optics↗

arXiv2020

Coherent virtual absorption of light in microring resonators

Light trapping and radiation process from linear reciprocal photonic resonators is one of the fundamental processes in optical science and engineering. Recently, the concept of coherent virtual absorption (CVA) of light was introduced and investigated for planar and cylindrical optical structures. The key feature of CVA is that by engineering the time-dependence of the excitation waveform, one can temporarily store all the input energy into the optical structure without any leakage. Here we further explore this novel concept in integrated photonic setups made of microring resonators. By using coupled-mode theory (CMT), we derive an analytical expression for CVA in this platform. This in turn allows us to make the connection with the notion of coherent perfect absorption (CPA) as well as extending our analysis to active resonators (having optical gain). We next provide a physical insight into this process by using a simple model made of cascaded beam splitters. Importantly, we confirm our results using a full-wave analysis of realistic material systems. Finally, we discuss the limitation on the CVA process due to waveform mismatch and nonlinear effects.

physics.optics↗

arXiv2019

Environmentally-induced exceptional points in elastodynamics

We study the nature of an environment-induced exceptional point in a non-Hermitian pair of coupled mechanical oscillators. The mechanical oscillators are a pair of pillars carved out of a single isotropic elastodynamic medium made of aluminum and consist of carefully controlled differential losses. The inter-oscillator coupling originates exclusively from background modes associated with the "environment", that portion of the structure which, if perfectly rigid, would support the oscillators without coupling. We describe the effective interaction in terms of a coupled mode framework where only one nearby environmental mode can qualitatively reproduce changes to the exceptional point characteristics. Our experimental and numerical demonstrations illustrates new directions utilizing environmental mode control for the implementation of exceptional point degeneracies. Potential applications include a new type of non-invasive, dfferential atomic force microscopy and hypersensitive sensors for the structural integrity of surfaces.

physics.app-ph↗

arXiv2019

Light-induced optical switching in an asymmetric metal-dielectric microcavity with phase-change material

We propose an infrared power switch based on an asymmetric high-Q microcavity incorporating a metallic nanolayer in close proximity to a layer made of a phase-change material (PCM). The microcavity is designed so that when the PCM layer is in the low-temperature phase, the metallic nanolayer coincides with a nodal plane of the resonant electric field component, to allow a high resonant transmittance. As the light intensity exceeds a certain threshold, light-induced heating of the PCM layer triggers the phase transition accompanied by an abrupt change in its refractive index in the vicinity of the transition temperature. The latter results in a shift of the nodal plane away from the metallic nanolayer, rendering the entire microcavity highly reflective over a broad frequency range. The nearly binary nature of the PCM refractive index allows for the low-intensity resonant transmission over a broad range of ambient temperatures below the transition point.

physics.optics↗

arXiv2018

Statistical Description of Transport in Multimode Fibers with Mode-Dependent Loss

We analyze coherent wave transport in a new physical setting associated with multimode wave systems where reflection is completely suppressed and mode-dependent losses together with mode-mixing are dictating the wave propagation. An additional physical constraint is the fact that in realistic circumstances the access to the scattering (or transmission) matrix is incomplete. We have addressed all these challenges by providing a statistical description of wave transport which fuses together a free probability theory approach with a Filtered Random Matrix ensemble. Our theoretical predictions have been tested successfully against experimental data of light transport in multimode fibers.

physics.optics↗

arXiv2018

Asymmetric Transmission in Photonic Structures with Phase-Change Components

We consider the scattering problem for an asymmetric composite photonic structure with a component experiencing a thermally driven phase transition. Using a numerical example, we show that if the heating is caused by the incident light, the transmittance can become highly asymmetric within a broad range of light intensities. This effect can be utilized for directional light transmission, asymmetric optical limiting, or power switching.

physics.app-ph↗

arXiv2016

Waveguide photonic limiters based on topologically protected resonant modes

We propose a concept of chiral photonic limiters utilising topologically protected localised midgap defect states in a photonic waveguide. The chiral symmetry alleviates the effects of structural imperfections and guaranties a high level of resonant transmission for low intensity radiation. At high intensity, the light-induced absorption can suppress the localised modes, along with the resonant transmission. In this case the entire photonic structure becomes highly reflective within a broad frequency range, thus increasing dramatically the damage threshold of the limiter. Here we demonstrate experimentally the principle of operation of such photonic structures using a waveguide consisting of coupled dielectric microwave resonators.

physics.optics↗

arXiv2015

Wavepacket Self-imaging and Giant Recombinations via Stable Bloch-Zener Oscillations in Photonic Lattices with Local ${\cal PT}$-Symmetry

We propose a family of {\it local} $\cal{PT}$-symmetric photonic lattices with transverse index gradient $ω$, where the emergence of {\it stable} Bloch-Zener oscillations are controlled by the degree of non-Hermiticity $γ$ of the lattice. In the exact $\cal{PT}$-symmetric phase we identify a condition between $ω$ and $γ$ for which a wavepacket self -imaging together with a cascade of splittings and giant recombinations occurs at various propagation distances. The giant wavepacket recombination is further enhanced by introducing local impurities.

physics.optics↗

arXiv2015

Light scattering in pseudo-passive media with uniformly balanced gain and loss

We introduce a class of metamaterials with uniformly balanced gain and loss associated with complex permittivity and permeability constants. The refractive index of such a balanced pseudo-passive metamaterial is real. An unbounded uniform pseudo-passive medium has transport characteristics similar to those of its truly passive and lossless counterpart with the same real refractive index. However, bounded pseudo-passive samples show some unexpected scattering features which can be further emphasized by including such elements in a photonic structure.

physics.optics↗

arXiv2014

PT-symmetry in macroscopic magnetic structures

We introduce the notion of PT-symmetry in magnetic nanostructures and show that they can support a new type of non-Hermitian dynamics. Using the simplest possible set-up consisting of two coupled ferromagnetic films, one with loss and another one with a balanced amount of gain, we demonstrate the existence of a spontaneous PT-symmetry breaking point where both the eigenfrequencies and eigenvectors are degenerate. Below this point the frequency spectrum is real indicating stable dynamics while above this point it is complex signaling unstable dynamics.

cond-mat.mes-hall↗

arXiv2014

Critical States Embedded in the Continuum

We introduce a class of critical states which are embedded in the continuum (CSC) of one-dimensional optical waveguide array with one non-Hermitian defect. These states are at the verge of being fractal and have real propagation constant. They emerge at a phase transition which is driven by the imaginary refractive index of the defect waveguide and it is accompanied by a mode segregation which reveals analogies with the Dicke super -radiance. Below this point the states are extended while above they evolve to exponentially localized modes. An addition of a background gain or loss can turn these localized states to bound states in the continuum.

cond-mat.dis-nn↗

arXiv2014

Light Localization induced by Random Imaginary Permittivities

We show the emergence of light localization in arrays of coupled optical waveguides with randomness only in the imaginary part of their permittivity and develop a one-parameter scaling theory for the normalized participation number of the Floquet-Bloch modes. This localization introduces a new length scale in the decay of the autocorrelation function of a paraxial beam propagation. Our results are relevant to a vast family of systems with randomness in the dissipative part of their impedance spatial profile.

cond-mat.dis-nn↗

arXiv2013

Optical Asymmetry Induced by PT-symmetric Nonlinear Fano Resonances

We introduce a new type of Fano resonances, realized in a photonic circuit which consists of two nonlinear PT-symmetric micro-resonators side-coupled to a waveguide, which have line-shape and resonance position that depends on the direction of the incident light. We utilize these features in order to induce asymmetric transport up to 47 dBs in the optical C-window. Our set-up requires low input power and does not compromise the power and frequency characteristics of the output signal.

physics.optics↗

arXiv2013

Unidirectional Lasing Emerging from Frozen Light in Non-Reciprocal Cavities

We introduce a class of unidirectional lasing modes associated with the frozen mode regime of non-reciprocal slow-wave structures. Such asymmetric modes can only exist in cavities with broken time-reversal and space inversion symmetries. Their lasing frequency coincides with a spectral stationary inflection point of the underlying passive structure and is virtually independent of its size. These unidirectional lasers can be indispensable components of photonic integrated circuitry.

physics.optics↗

arXiv2013

Experimental Observation of Stabilization and Lasing Death via Asymmetric Gain

Using a pair of coupled LRC cavities we experimentally demonstrate that instabilities and amplification action can be tamed by a spatially inhomogenous gain. Specifically we observe the counter-intuitive phenomenon of stabilization of the system even when the overall gain provided is increased. This behavior is directly related to lasing death via asymmetric pumping, recently proposed in [M. Liertzer {\it et al}., Phys. Rev. Lett. {\bf 108}, 173901 (2012)]. The stability analysis of other simple systems reveals the universal nature of the lasing death phenomenon.

physics.optics↗

arXiv2013

Perfect Impedance-Matched Isolators and Unidirectional Absorbers

A broad-band reflectionless channel which supports unidirectional wave propagation originating from the interplay between gyrotropic elements and symmetrically placed gain and loss constituents is proposed. Interchange of the active elements together with a gyrotropic inversion turns the same structure to a unidirectional absorber where incoming waves from a specific direction are annihilated. When disorder is introduced asymmetric Anderson localization is found. Realizations of such multi-functional architectures in the frame of electronic and photonic circuitry are discussed.

physics.optics↗

arXiv2013

Observation of Asymmetric Transport in Structures with Active Nonlinearities

A mechanism for asymmetric transport based on the interplay between the fundamental symmetries of parity (P) and time (T) with nonlinearity is presented. We experimentally demonstrate and theoretically analyze the phenomenon using a pair of coupled van der Pol oscillators, as a reference system, one with anharmonic gain and the other with complementary anharmonic loss; connected to two transmission lines. An increase of the gain/loss strength or the number of PT-symmetric nonlinear dimers in a chain, can increase both the asymmetry and transmittance intensities.

physics.optics↗

arXiv2008

Avalanches of Bose-Einstein Condensates in Leaking Optical Lattices

One of the most fascinating experimental achievements of the last decade was the realization of Bose-Einstein Condensation (BEC) of ultra-cold atoms in optical lattices (OL's). The extraordinary level of control over these structures allows us to investigate complex solid state phenomena and the emerging field of ``atomtronics'' promises a new generation of nanoscale devices. It is therefore of fundamental and technological importance to understand their dynamical properties. Here we study the outgoing atomic flux of BECs loaded in an one-dimensional OL with leaking edges, using a mean field description provided by the Discrete Non-Linear Schrodinger Equation (DNLSE). We demonstrate that the atom population inside the OL decays in avalanches of size $J$. For intermediate values of the interatomic interaction strength their distribution ${\cal P}(J)$ follows a power law i.e. ${\cal P}(J)\sim1/J^α$ characterizing systems at phase transition. This scale free behaviour of ${\cal P}(J)$ reflects the complexity and the hierarchical structure of the underlying classical mixed phase space. Our results are relevant in a variety of contexts (whenever DNLSE is adequate), most prominently the light emmitance from coupled non-linear optics waveguides.

cond-mat.stat-mech↗