Controlling Light propagation in non-Hermitian Photonics

The goal of our research project CLIP, funded by the Austrian Science Funds (FWF) Lise- Meitner programme, will be the development of novel theoretical ideas for guiding light in micro- and nano-scopic structures involving active elements, which create or absorb photons. The properties of these active structures can be modulated in space to enable light to propagate in previously unexpected ways, even allowing for complete removal of scattering and interference inside highly irregular disordered structures. The existing strategies for manipulating light with so called metamaterials, which are artificially created structures with engineered optical properties, are mostly based on guiding light around objects, and are based on the ideas of transformation optics, which connect the propagation of light rays to the optical properties of materials. These strategies however require the use of materials that are hard to manufacture, and generally also deal with the properties of light away from the object, rather than inside of it. An advantage of our approach is that it requires only the use of dielectric materials, commonly found in nature, which are then modified by adding active elements. Another appealing aspect of our theoretical framework, which will be studied in this project, is the fact that the properties of light not only outside, but also inside the scattering medium can be controlled an aspect which is particularly interesting for applications in light focusing, generation and imaging. A special focus of our project will be to provide a bridge between the long-established methods of transformation optics with the newly acquired understanding of systems with spatially modulated active elements. In this way we aim to create a new understanding of light propagation in such novel materials based on modifying the geometry of space by adding gain and loss elements a feature, which we expect should generate a large interest from the broader optics community. In addition to these purely theoretical problems, the project also aims to explore the experimental realization of our proposals in two different platforms: time- multiplexed photonic lattices based on the pulse propagation in optical fibers, and laser- induced waveguides in hot atomic vapours. Both systems offer great flexibility and control such that they should be able to serve as reliable platforms for convincing demonstrations of our theoretical concepts in the near-term future.

1. Summary of https://doi.org/10.1103/PhysRevLett.131.163602: "In this work, using a fully quantum description, we theoretically and numerically demonstrate that spontaneous generation of significant multiparticle entanglement in atomic momentum states can occur in the transient, for Hamiltonians with continuous translational symmetry. Two theoretical models were studied, one based on light-mediated interaction of ultracold atoms placed in a ring resonator, and the other based on direct collisions between ultracold atoms in an oscillating magnetic field. Numerical calculations show that this method could potentially be very efficient in generating quantum entangled atoms even in so-called "bad resonators". The discovery opens the door to many exciting applications in quantum metrology, simulations and computing, not only using atomic condensates, but other types of dipole particle ensembles as well." 2. Summary of https://doi.org/10.1126/sciadv.abl7412: "Propagation through an inhomogeneous medium, such as fog, can severely degrade the shape of a beam of light due to complex scattering processes. In this Article, we have extended the framework of constant-intensity waves to a novel system - a synthetic photonic lattice built by interfering pulses of light propagating through a fiber, in order to remove the beam degradation caused by the scattering. The mechanism preserves the intensity distribution of the incoming beam, by mitigating destructive interference by gain and constructive interference by loss, with a taylored non-Hermitian lattice. This experiment marks a first demonstration of shape-preserved propagation and induced transparency for electromagnetic waves on a discrete non-Hermitian lattice, and is a step towards potential real-world realizations of transparent media based on gain and loss." 3. Summary of https://doi.org/10.1103/PhysRevLett.128.183901: "Transformation optics is a fascinating framework connecting the geometry of space with propagation of light through a medium. It is commonly known that materials with isotropic optical response can be generated by conformal transformations of free space, which locally preserve the angles between lines. In our recent article, we have demonstrated that, with certain caveats, also non-conformal transformations can be used to mould light propagation in isotropic materials. The condition is that now, in contrast to the previous case, the medium generated by the transformation is tied to the electric field solution in free space. Also, in contrast to the previous case, the non-conformality leads now to a modulation in the imaginary part of the refractive index distribution, i.e. gain and loss. We have used these insights to design a two-dimensional unidirectional cloak with an isotropic non-Hermitian dielectric material, which now works also for light pulses, in contrast to similar cloaks generated with conformal transformations."