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Hydrodynamic phenomena can be observed with light thanks to the analogy between quantum gases and nonlinear optics. In this Letter, we report an experimental study of the superfluid-like properties of light in a (1+1)-dimensional nonlinear optical mesh lattice, where the arrival time of optical pulses plays the role of a synthetic spatial dimension. A spatially narrow defect at rest is used to excite sound waves in the fluid of light and measure the sound speed. The critical velocity for superfluidity is probed by looking at the threshold in the deposited energy by a moving defect, above which the apparent superfluid behaviour breaks down. Our observations establish optical mesh lattices as a promising platform to study fluids of light in novel regimes of interdisciplinary interest, including non-Hermitian and/or topological physics.

Thin film samples of Ge9SnSb2Te9Se4, Ge4.5SnSb2Te4.5Se4, Ge2.5SnSb2Te2.5Se4, and GeSnSb2TeSe4 were prepared via co-sputtering of GeTe and SnSb2Se4 and compared to the well-investigated phase change material GeTe. All samples were obtained in an amorphous state. Temperature-dependent in situ X-ray diffraction experiments reveal a crystallization temperature that increases with an increasing SnSb2Se4 content, leading to a higher stability of the amorphous phase. The electrical contrast between the amorphous and metastable crystalline state investigated via the van der Pauw method is as large as 4 orders of magnitude up to a 4.5:1 GeTe:SnSb2Se4 ratio. Increasing the SnSb2Se4 content leads to a decrease in the electrical contrast. Investigations of the samples by applying Fourier transform infrared spectroscopy and variable incident angle spectroscopic ellipsometry show that the optical properties of the amorphous phase are not affected by changes in stoichiometry. In striking contrast, the impact of SnSb2Se4...

Geometrical properties of energy bands underlie fascinating phenomena in a wide-range of systems, including solid-state materials, ultracold gases and photonics. Most famously, local geometrical characteristics like the Berry curvature can be related to global topological invariants such as those classifying quantum Hall states or topological insulators. Regardless of the band topology, however, any non-zero Berry curvature can have important consequences, such as in the semi-classical evolution of a wave packet. Here, we experimentally demonstrate for the first time that wave packet dynamics can be used to directly map out the Berry curvature. To this end, we use optical pulses in two coupled fibre loops to study the discrete time-evolution of a wave packet in a 1D geometrical "charge" pump, where the Berry curvature leads to an anomalous displacement of the wave packet under pumping. This is both the first direct observation of Berry curvature effects in an optical system, and, more generally, the proof-of-principle demonstration that semi-classical dynamics can serve as a high-resolution tool for mapping out geometrical properties.

Over the last few years, parity-time ($\mathcal{P}\mathcal{T}$) symmetry has been the focus of considerable attention. Ever since, pseudo-Hermitian notions have permeated a number of fields ranging from optics to atomic and topological physics, as well as optomechanics, to mention a few. Unlike their Hermitian counterparts, nonconservative systems do not exhibit a priori real eigenvalues and hence unitary evolution. However, once $\mathcal{P}\mathcal{T}$ symmetry is introduced, such dissipative systems can surprisingly display a real eigenspectrum, thus ensuring energy conservation during evolution. In optics, $\mathcal{P}\mathcal{T}$ symmetry can be readily established by incorporating, in a balanced way, regions having an equal amount of optical gain and loss. However, thus far, all optical realizations of such $\mathcal{P}\mathcal{T}$ symmetry have been restricted to a single transverse dimension (1D), such as arrays of optical waveguides or active coupled cavity arrangements. In most cases, only the loss function was modulated---a restrictive aspect that is only appropriate for linear systems. Here, we present an experimental platform for investigating the interplay between $\mathcal{P}\mathcal{T}$ symmetry and nonlinearity in two-dimensional (2D) environments, where nonlinear localization and soliton formation can be observed. In contrast to typical dissipative solitons, we demonstrate a one-parameter family of soliton solutions that are capable of displaying attributes similar to those encountered in nonlinear conservative arrangements. For high optical powers, this new family of $\mathcal{P}\mathcal{T}$ solitons tends to collapse on a discrete network---thus giving rise to an amplified, self-accelerating structure.

We investigate linear and nonlinear light propagation in weakly perturbed AlGaAs slab waveguides whose fabrication induced local defects give rise to random multiple scattering. The statistical evaluation of the intensity distribution at the rear facet reveals an abundance of high-amplitude events in the linear regime, contradicting the predictions of the central limit theorem by far. In this article we propose weak successive backscattering to be a generator of high-amplitude events, while a focusing nonlinearity of the Kerr type counterintuitively diminishes them, thus effectively randomizing the intensity distribution. We support our experimental findings with numerical simulations based on the stray matrix approach introduced by Edrei et al. [Phys. Rev. Lett. 62, 2120 (1989)PRLTAO0031-900710.1103/PhysRevLett.62.2120]. This novel proposed mechanism for the formation of extreme events in the context of linear wave dynamics represents the first encounter of the phenomenon in the limit of weak scattering.

As time flow dictates all evolution, its effective reversal is a topic of active research in a broad range of disciplines, including acoustics, hydrodynamics and optics. This multifarious set of environments is reflected by a great diversity of approaches to observe various echoes of wave functions. Here, we experimentally demonstrate time reversal of a pulse sequence propagating through a photonic mesh lattice realized by two coupled loops of telecommunication fibres. Our system features a symmetric band structure, which allows for almost perfect reversal of its evolution by exchanging the population between two opposing bands. The protocol applied is based on a non-adiabatic and instantaneous exchange of eigenstates resulting in highly efficient time reversal of a pulse chain.

We experimentally demonstrate nonlinear wave collapse at mW-power levels in a genuine 1D system formed by coupled optical fibers. Based on time-multiplexing we realize an effective two-dimensional lattice and observe strong nonlinearly driven field compression.

Photonic mesh lattices provide a versatile experimental platform for studying light propagation in a discrete environment. By using standard telecommunication equipment it is possible to influence the evolution of light by actively modulating its amplitude and phase. Fiber components thus offer a ready to use solution for creating a \( \mathbb{PT} \)-symmetric optical network. Here, we analyze two classes of \( \mathbb{PT} \)-symmetric networks: those with local \( \mathbb{PT} \) symmetry fulfilling the conditions of \( \mathbb{PT} \) symmetry in each time step but not globally and those with global \( \mathbb{PT} \) symmetry providing completely real eigenvalues. We investigate light propagation in these dissipative media in the linear and nonlinear regime and discuss nonlinear localization as well as the formation of soliton. Furthermore, Bloch oscillations are induced by an external phase modulation and are found to restore pseudo-Hermitian propagation.

RSC Advances 7(28), 17164-17172 (2017). doi:10.1039/C7RA01140G, Published by RSC Publishing, London