In 1973 Klaus Hepp and Elliott Lieb showed that a tightly confined ensemble of atoms placed in a quantised single-mode optical cavity will have their emission of light enhanced by constructive interference of emission events if the photons emitted are indistinguishable. This effect, known as superradiance, has proven difficult to realise experimentally due to the strong light-matter coupling required to produce such behaviour. However, over the last decade there have been several successful realisations which engineer superradiance by driving Raman transitions between atomic ground states of multilevel atoms. In this case, superradiance emerges via a quantum phase transition mediated by the laser driving power. In this work, we consider a generalisation of this model referred to as the unbalanced Dicke model. It has recently been implemented experimentally and is characterised by two lasers driving two distinct Raman transitions. We show that, in the semiclassical limit of a large number of atoms, the unbalanced Dicke model exhibits a wealth of nonlinear dynamics which describe novel dynamical phases of matter. Examples of nonlinear behaviour observed are pitchfork, saddle-node, and Hopf bifurcations, as well as two types of chaos. We explore the quantum mechanical manifestations of these bifurcations and additional physical effects which appear in the presence of quantum fluctuations. In this way, the model creates a playground in which to explore the onset of classical nonlinear dynamics from an open quantum system.

We report the phase and amplitude ultrafast measurement of nonlinear random light by using a heterodyne time-lens technique in its digital time holography configuration (spatial encoding arrangement with hologram observation for recording in single-shot the electric field, SEAHORSE) [1]. We have developed an algorithm in order to compensate the aberrations generally induced by the high order dispersion in time lens devices. This enables an aberration-free single-shot recording of the phase and of the amplitude over a large temporal window (200 ps) with a resolution of the order of 300fs [2] We apply this new version of the SEAHORSE to investigate the spontaneous (or noise induced) modulation instability (MI) in the anomalous regime of dispersion of single mode fibers. We observe phase and amplitude signatures of Akhmediev breather-like in very good agreement with numerical predictions based on the focusing one-dimensional nonlinear Schrödinger equation. Note that the recording of the amplitude dynamics (without the phase information) had already been observed by using conventional time lens devices [3]. Finally, taking advantage of the phase and amplitude single-shot recording, we compute the inverse scattering transform (nonlinear Fourier transform) of the modulation instability. This allows us to measure for the first time the Weyl’s distribution of the amplitude of the solitons, which has been predicted theoretically in the framework of the inverse scattering transform [4] [1] A. Tikan, S. Bielawski, C. Szwaj, S. Randoux, and P. Suret, Nat. Photonics 12, 228 (2018). [2] A. Lebel, A. Tikan, S. Randoux, P. Suret and F. Copie Opt. Lett. 46, 2 (2021) [3] M. Närhi, B. Wetzel, C. Billet, S. Toenger, T. Sylvestre, J.-M. Merolla, R. Morandotti, F. Dias, G. Genty, and J. M. Dudley, Nat. Commun. 7, 13675 (2016). [4] A Gelash, D Agafontsev, V Zakharov, G El, S Randoux, P Suret Phys. Rev. Lett., 123, 23, 234102 (2019)

This conference presentation, “Temporal solitons in coherently driven active resonators” was recorded for the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

Optical resonators are becoming increasingly important due to their nonlinear properties and potentially small sizes. In recent years, it was discovered that these structures are able to support different kinds of localized solutions depending on the competition between the group velocity dispersion and higher-order nonlinearities. However, as the resonators become smaller, the number of modes decreases until the group velocity dispersion effect can be ignored. Here, we study the dynamics of two symmetric optical modes coupled in a single-mode ring resonator. We demonstrate the existence of localized pulse and kink solutions in this set-up, owing to the intrinsic Kerr nonlinearity. We show that pulse solutions lie on the boundary between different oscillating regimes in phase space, while kink solutions are found at the transition from symmetric to asymmetric pulse solutions. When perturbing the localized solutions, they undergo critical bifurcations to form persistent periodic and chaotic oscillations, some of which exhibit self-switching behaviours. We also show that these localized solutions are the organizing centre of various dynamical regimes in the cavity, including spontaneous symmetry breaking, optical bi-stability and multi-stability of homogeneous states, and Hopf bifurcation. We discuss how the ratio between the self- and cross-phase modulation parameters influences these dynamical regimes and, therefore, the existence of pulse and kink solutions. We use analytical techniques and numerical continuation methods to compute the loci of all these dynamical behaviours and present our results as comprehensive phase diagrams in the parameter plane. Overall, our work showcases the distinct and exotic behaviour that should be expected from this optical device.

We study theoretically the formation and dynamics of phase-locked temporal localized states (TLSs) and frequency combs that can be generated from a high finesse Fabry-Perot microcavity containing a Kerr medium that is coupled to a external cavity in the presence of optical injection. These TLSs possess a strongly asymmetrical oscillating tail which results from third order dispersion induced by the quasi-resonant cavity. Using a first principle model based upon delay algebraic equations and applying a combination of direct numerical simulations and path continuation methods, we disclose sets of multistable dark and bright TLSs coexisting on their respective bistable homogeneous backgrounds. In particular, we show that the detuning of the injection with respect to the microcavity resonance controls the amount of normal dispersion as well as the region of existence of TLSs. We identify a scenario of period-doubling route to chaos. Further we discuss a transformation of the original delay model to a normal form given by a partial differential equation using a rigorous multiple time scale analysis and a functional mapping approach. The analysis is performed in two experimentally sound situations. The first one is adapted to cavities with large losses and we consider the onset of bistability for normal dispersion. There, one obtain a real order parameter equation akin to the Swift-Hohenberg model that is modified by the presence of third order dispersion and self-steepening. The second case consists in the weakly nonlinear regime obtained in the good cavity limit for abnormal dispersion. This situation leads to the generalized Lugiato-Lefever model with third order dispersion and self-steepening. We finally discuss the lossless case and the more general question of how to link the conservative, time reversible, nonlinear Schrodinger equation with the dynamics of nonlinear time delayed systems.

This conference presentation, “Spatiotemporal complexity: multimode fiber light sources and their applications” was recorded for the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

We generalized the well-known Lugiato-Lefever Equation to unify the description of combs and localized structures formation in nonlinear optical systems such as Kerr micro-resonators (passive systems) and Quantum Cascade Lasers (QCL) (active systems). In particular this model was applied to the study of pattern formation in a unidirectional ring QCL driven by a coherent injected field. We showed the existence of Dissipative Solitons (DS) and Turing rolls associated to standard and harmonic Optical Frequency Combs (OFC) in the system. We also provided a proof of principle demonstration of the possibility to deterministically control the spectral properties of these OFC by switching-on one or more DS with suitable addressing pulses. These results considerably increase the theoretical insight in chip-scale combs sources in the mid-infrared region of the electromagnetic spectrum for timely applications in the field of e.g. high resolution and/or time resolved molecular spectroscopy, long range and high bit rate wireless communications.

We numerically and experimentally demonstrate the spontaneous emergence of conical waves when an ultrashort pulse propagates nonlinearly in a multimode fiber, i.e. a discretization of conical emission phenomenon (e.g., X-wave) in bulk media.

This conference presentation, “Nonlinear topological photonics” was recorded for the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

This conference presentation, “Parametric phase-sensitive amplification in silicon nitride waveguides” was recorded for the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

Laser radiation sources with wide wavelength tunability are of high demand in many applications including nonlinear microscopy, material science, spectroscopy, gas sensing, etc. Laser wavelength tunability is usually very limited, so optical parametric amplifiers (OPA) and optical parametric generators (OPGs) are used as sources of tunable laser radiation. Rapid progress in nonlinear optics enabled creation of a wide range of optical parametric devices with wavelength tunability from VIS to mid-IR spectrum range using pump from CW radiation to femtosecond pulses. However, there are very few optical parametric devices that operate in the subnanosecond duration range are (300 ps - 1 ns) and the few that have been demonstrated operate mainly in the IR spectral range. Since there is a demand of such sources in applications like laser-induced fluorescence, detection of chemical materials, etc., developing subnanosecond optical parametric devices remains an important task. In this study we report, to the best of our knowledge, the first subnanosecond OPAs systems generating widely-tunable radiation in the visible spectrum range. The first demonstrated OPA is based on LBO crystal and the other one - on BBO crystal. To overcome difficulties related to low pump intensity and laser induced damage threshold (LIDT) of nonlinear crystals, we use continuum generated in a photonic crystal fiber as a seed source for the OPAs. Experimental data and numerical simulation reveal that good spectral, temporal and particularly energy characteristics of the seed radiation are crucial in order to achieve efficient parametric generation of signal radiation. This work has received funding from European Regional Development Fund (project No. 01.2.2-LMT-K-718-03-0004) under grant agreement with the Research Council of Lithuania (LMTLT).

This conference presentation, “Pulse dynamics in microlasers” was presented at the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

We present a detailed numerical bifurcation analysis of the dual polarisation, sustained pulsing behavior in the spin-flip model of an excitable, vertical cavity surface emitting semiconductor micro-laser with saturable absorber (VCSEL-SA) and coherent optical feedback from an external reflecting mirror. These efforts are motivated by experimental observations showing sustained polarization mode competition between successive pulses generated, and re-generated, by the delayed feedback. The bifurcation analysis suggests that this competition is stress-induced, and a result of symmetry-breaking torus bifurcations mediated by amplitude and phase anisotropy. Our results inform the wider research on the pulse regeneration properties of such devices, which can be integrated into two-dimensional arrays and, as such, are potentially relevant for various applications, such as optical communication, signal processing and neuro-morphic computing. Of particular interest to us is the device's capacity for dual-channel pulse propagation~\textemdash~the independent transmission of pulses in orthogonal polarization modes~\textemdash~which can be used to realize inhibitory coupling between purely excitable micro-lasers.

Optical parametric light generation is a unique way to obtain continuously tunable laser radiation in a wide spectral region. Relatively inexpensive and compact subnanosecond (100 ps - 1 ns) optical parametric generators (OPG) with broad wavelength tunability are needed for a variety of applications that do not require the high temporal resolution supplied by expensive and sophisticated ultrashort (<10 ps) laser systems, but nanosecond (>1 ns) time resolution is not sufficient. Such applications are spectroscopy, laser-induced fluorescence, detection of chemical materials, nonlinear microscopy, biochemical research and differential absorption lidar to name a few. Development and the reduced cost of the passively Q-switched micro-laser manufacturing technologies permits them to be used as a high energy pump source for subnanosecond OPGs. Such micro-laser pumped subnanosecond OPGs would allow the implementation of low-cost, compact, continuously tunable laser radiation sources suitable for the aforementioned applications, especially in near-infrared (NIR) and visible (VIS) spectral ranges where the demand is high. To date, no subnanosecond OPGs based on short (up to 2 cm in length) periodically poled (PP) crystals have been demonstrated that could also be continuously tunable over a wide spectral range. Long PP crystals are more expensive, therefore implementation and investigation of such OPGs would be commercially attractive. Here we investigate parametric light generator based on short (up to 2 cm in length) periodically poled lithium niobate (PPLN) crystal pumped by 473 ps pulses from a 1 mJ energy micro-laser. Experimentally measured spatial, temporal and energy characteristics of the generated light from different length PPLN crystals are shown and supplemented by numerical simulations of parametric generation in a nonlinear dispersive medium. This work has received funding from European Regional Development Fund (project No. 01.2.2-LMT-K-718-03-0004) under grant agreement with the Research Council of Lithuania (LMTLT).

This conference presentation, “Multimode effects in nonlinear fibre optics: from telecommunications to high-harmonic generation” was presented at the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

This conference presentation, “Structural and optical properties of atomically engineered Ir/Al2O3 nanocomposites” was presented at the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.

We present a novel concept for optical switches, which is based on the optical Kerr-effect. In contrast to previous approaches, the switching is achieved by a combination of strongly Kerr-active materials with specially designed and produced optical interference filters. Intense laser irradiation causes refractive index changes in the sensitive Kerr-active layers of the components and the interference filter changes its spectral characteristics, i.e. becomes reflective, depending on design factors. The concept offers several advantages when compared to currently applied switching methods, such as the easier integration into photonic systems because of their compact nature and wide spectral application range.

Subwavelength size of nanophotonic devices in use with laser pulses at the few-cycle limit raises new questions about the spatial distribution of the carrier-envelope phase (CEP). It has been shown that the complexity of the CEP distribution for few-cycle laser pulses can go far beyond the axial phase flip, known as the Gouy phase. Moreover, the phase landscape is governed by various laser parameters, some of which can be deliberately changed in order to control the CEP distribution. To be able to fully grasp the control over CEP an accessible and reliable way to measure the distribution is needed. The measurement of CEP distribution of few-cycle laser pulses is challenging and the method so far relies on vacuum apparatus. Recently discovered light-driven CEP sensitive currents in dielectrics, which can be detected with microsized on-chip electrodes, offer a new perspective for the phase detection. In this work, we present a proof-of-principle method for measuring the CEP spatial distribution. With on-chip scanning the focal volume of tightly focused oscillator laser pulses we obtain the three-dimensional map of the phase with resolution down to 500 nm.

We have introduced the concept of rotation-time (RT) symmetry in bosonic systems, in which we have demonstrated coalescence of energy values [2], which is known as an exceptional point [1]. We have demonstrated that the RT symmetry is a generalisation of the parity-time (PT) symmetry, which was first presented by Bender and Boettcher [3]. We also established general principles for constructing non-Hermitian RT-symmetric Hamiltonians. This provides a foundation for studying the physics of spectral singularities in bosonic systems, including nonlinear interactions between modes. We investigated the effect of laser pumping on the spectral singularity. It has been shown that also systems without gain components, that include only losses, can exhibit EPs. When only losses are included, a system is known to have passive PT symmetry. Eigenenergies for such systems have a common imaginary part and its presence is not an obstacle in observing EPs. The first experimental realisations of passive PT systems were already performed. To reveal the desirable symmetry, the appropriate relation between the lossy components of both coupled modes has to be obtained. The result of that interplay is the increase of transmitted power in one of the modes despite the fact that only lossy mechanisms are included. We have shown that if the whole non-Hermitian Hamiltonian, being not PT-symmetric nor RT-symmetric, can be expressed as a composition of two parts: (i) RT-symmetric term and (ii) a term commuting with (i), then the hidden RT symmetry is present in this non-Hermitian Hamiltonian [4]. We refer to the frame, where the hidden PT or RT symmetry is clearly seen, as to equilibrium frame (EF). Here, we present non-linear, RT-symmetric passive system with spectral singularities. This system exhibit photon blockade and thus can be considered as single photon source as the nonlinearity is crucial for obtaining such phenomena. References [1] Hossein Hodaei, Absar U Hassan, Steffen Wittek, Hipolito Garcia-Gracia, Ramy El-Ganainy, Demetrios N Christodoulides, and Mercedeh Khajavikhan. Enhanced sensitivity at higher-order exceptional points. Nature, 548(7666):187–191, 2017. [2] Ewelina Lange, Grzegorz Chimczak, Anna Kowalewska-Kudlaszyk, and Karol Bartkiewicz. Rotation-time symmetry in bosonic systems and the existence of exceptional points in the absence of PT symmetry. Sci. Rep., 10(1):19906, Nov 2020. [3] Carl M. Bender and Stefan Boettcher. Real spectra in non-hermitian hamiltonians having PT symmetry. Phys. Rev. Lett., 80:5243–5246, Jun 1998. [4] G Chimczak, A Kowalewska-Kudlaszyk, E Lange, and K Bartkiewicz. Equilibrium frame reveals hidden PT symmetry of passive systems. preprint arXiv:2008.06532, 2020

This conference presentation, “Evaluation of aerodynamic levitation laser heating technique for synthesis of silicate glasses with gold nanoparticles” was recorded for the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.