Alois Regensburger

Parity-time synthetic photonic lattices

The development of new artificial structures and materials is today one of the major research challenges in optics. In most studies so far, the design of such structures has been based on the judicious manipulation of their refractive index properties. Recently, the prospect of simultaneously using gain and loss was suggested as a new way of achieving optical behaviour that is at present unattainable with standard arrangements. What facilitated these quests is the recently developed notion of ‘parity–time symmetry’ in optical systems, which allows a controlled interplay between gain and loss. Here we report the experimental observation of light transport in large-scale temporal lattices that are parity–time symmetric. In addition, we demonstrate that periodic structures respecting this symmetry can act as unidirectional invisible media when operated near their exceptional points. Our experimental results represent a step in the application of concepts from parity–time symmetry to a new generation of multifunctional optical devices and networks.

Observation of Defect States in PT-Symmetric Optical Lattices

We provide the first experimental demonstration of defect states in parity-time (PT) symmetric mesh-periodic potentials. Our results indicate that these localized modes can undergo an abrupt phase transition in spite of the fact that they remain localized in a PT-symmetric periodic environment. Even more intriguing is the possibility of observing a linearly growing radiation emission from such defects provided their eigenvalue is associated with an exceptional point that resides within the continuum part of the spectrum. Localized complex modes existing outside the band-gap regions are also reported along with their evolution dynamics.

Hybrid Colloidal Plasmonic‐Photonic Crystals

We review the recently emerged class of hybrid metal-dielectric colloidal photonic crystals. The hybrid approach is understood as the combination of a dielectric photonic crystal with a continuous metal film. It allows to achieve a strong modification of the optical properties of photonic crystals by involving the light scattering at electronic excitations in the metal component into moulding of the light flow in series to the diffraction resonances occurring in the body of the photonic crystal. We consider different realizations of hybrid plasmonic-photonic crystals based on two- and three-dimensional colloidal photonic crystals in association with flat and corrugated metal films. In agreement with model calculations, different resonance phenomena determine the optical response of hybrid crystals leading to a broadly tuneable functionality of these crystals.

Observation of optical solitons in PT-symmetric lattices

Controlling light transport in nonlinear active environments is a topic of considerable interest in the field of optics. In such complex arrangements, of particular importance is to devise strategies to subdue chaotic behaviour even in the presence of gain/loss and nonlinearity, which often assume adversarial roles. Quite recently, notions of parity-time (PT) symmetry have been suggested in photonic settings as a means to enforce stable energy flow in platforms that simultaneously employ both amplification and attenuation. Here we report the experimental observation of optical solitons in PT-symmetric lattices. Unlike other non-conservative nonlinear arrangements where self-trapped states appear as fixed points in the parameter space of the governing equations, discrete PT solitons form a continuous parametric family of solutions. The possibility of synthesizing PT-symmetric saturable absorbers, where a nonlinear wave finds a lossless path through an otherwise absorptive system is also demonstrated.

Optical mesh lattices with PT symmetry

We investigate a class of optical mesh periodic structures that are discretized in both the transverse and longitudinal directions. These networks are composed of waveguide arrays that are discretely coupled, while phase elements are also inserted to discretely control their effective potentials and can be realized both in the temporal and the spatial domain. Their band structure and impulse response are studied in both the passive and parity-time (PT)-symmetric regime. The possibility of band merging and the emergence of exceptional points, along with the associated optical dynamics, are considered in detail both above and below the PT-symmetry breaking point. Finally, unidirectional invisibility in PT-synthetic mesh lattices is also examined, along with possible superluminal light transport dynamics.

Photon Propagation in a Discrete Fiber Network: An Interplay of Coherence and Losses

We study light propagation in a photonic system that shows stepwise evolution in a discretized environment. It resembles a discrete-time version of photonic waveguide arrays or quantum walks. By introducing controlled photon losses to our experimental setup, we observe unexpected effects like subexponential energy decay and formation of complex fractal patterns. This demonstrates that the interplay of linear losses, discreteness and energy gradients leads to genuinely new coherent phenomena in classical and quantum optical experiments. Moreover, the influence of decoherence is investigated.

Nonlinear Dynamics in Photonic Mesh Lattices

Photonic mesh lattices are easily realized in the temporal domain using standard telecom equipment. This allows for the experimental demonstration of quantum walks and of various effects of linear and nonlinear dynamics in discrete systems.

Observation of PT-symmetric optical solitons in time-domain photonic lattices

We report the first experimental observation of optical solitons in PT-symmetric temporal lattices. By utilizing a judicious balance between gain, loss and nonlinear effects, such self-trapped states remain stable over long distances.

Bound states in a temporal fiber network with parity-time symmetry

Summary form only given. We report on the first experimental investigation of light propagation around defects in parity-time (PT) symmetric photonic lattices. An extended PT mesh lattice is realized using a setup consisting of two coupled fiber loops with different lengths and time dependent modulation of net gain/loss. Time-multiplexing is applied to map a 1D grid of transverse positions into the temporal domain thus transferring concepts of PT symmetric systems to pulse propagation. To guide bound light modes through this mesh lattice, we introduce a phase defect φd which occupies a single site in the periodic lattice.

Beam dynamics in optical mesh lattices

We study propagation dynamics in a new class of optical lattices which are bi-periodic and discrete in both coordinates. These mesh structures exhibit peculiar linear and nonlinear properties which are unattainable in traditional optical lattices.