Imperial College London


Faculty of Natural SciencesDepartment of Mathematics

Research Associate







6M44Huxley BuildingSouth Kensington Campus





Publication Type

6 results found

Bosch M, Malzard S, Hentschel M, Schomerus Het al., 2019, Nonhermitian defect states from lifetime differences, Publisher: arxiv

Nonhermitian systems provide new avenues to create topological defect states.An unresolved general question is how much the formation of these statesdepends on asymmetric backscattering, be it nonreciprocal as in thenonhermitian skin effect or reciprocal as encountered between the internalstates of asymmetric microresonators. Here, we demonstrate in a concrete,practically accessible setting of a lossy coupled-resonator optical waveguidethat nonhermitian defect states can exist in open optical systems due tolifetime differences, without the need for asymmetric backscattering within orbetween the individual resonators. We apply our findings to a finite system ofcoupled circular resonators perturbed by nanoparticles, following the conceptof creating an interface by inverting the position of the nanoparticles in halfof the chain. We compare a coupled-mode tight-binding approximation tofull-wave numerical simulations, showing that spectrally isolated defect statescan indeed be implemented in this simple nonhermitian photonic device.

Working paper

Malzard S, Schomerus H, 2018, Bulk and edge-state arcs in non-Hermitian coupled-resonator arrays, Physical Review A, Vol: 98, ISSN: 2469-9926

We describe the formation of bulk and edge arcs in the dispersion relation of two-dimensional coupled-resonator arrays that are topologically trivial in the Hermitian limit. Each resonator provides two asymmetrically coupled internal modes, as realized in noncircular open geometries, which enables the system to exhibit non-Hermitian physics. Neighboring resonators are coupled chirally to induce non-Hermitian symmetries. The bulk dispersion displays Fermi arcs connecting spectral singularities known as exceptional points and can be tuned to display purely real and imaginary branches. At an interface between resonators of different shape, one-dimensional edge states form that spectrally align along complex arcs connecting different parts of the bulk bands. We also describe conditions under which the edge-state arcs are freestanding. These features can be controlled via anisotropy in the resonator couplings.

Journal article

Malzard S, Cancellieri E, Schomerus H, 2018, Topological dynamics and excitations in lasers and condensates with saturable gain or loss, Optics Express, Vol: 26, Pages: 22506-22518, ISSN: 1094-4087

We classify symmetry-protected and symmetry-breaking dynamical solutions for nonlinear saturable bosonic systems that display a non-hermitian charge-conjugation symmetry, as realized in a series of recent groundbreaking experiments with lasers and exciton polaritons. In particular, we show that these systems support stable symmetry-protected modes that mirror the concept of zero-modes in topological quantum systems, as well as symmetry-protected power-oscillations with no counterpart in the linear case. In analogy to topological phases in linear systems, the number and nature of symmetry-protected solutions can change. The spectral degeneracies signalling phase transitions in linear counterparts extend to bifurcations in the nonlinear context. As bifurcations relate to qualitative changes in the linear stability against changes of the initial conditions, the symmetry-protected solutions and phase transitions can also be characterized by topological excitations, which set them apart from symmetry-breaking solutions. The stipulated symmetry appears naturally when one introduces nonlinear gain or loss into spectrally symmetric bosonic systems, as we illustrate for one-dimensional topological laser arrays with saturable gain and two-dimensional flat-band polariton condensates with density-dependent loss.

Journal article

Malzard S, Schomerus H, 2018, Nonlinear mode competition and symmetry-protected power oscillations in topological lasers, NEW JOURNAL OF PHYSICS, Vol: 20, ISSN: 1367-2630

Topological photonics started out as a pursuit to engineer systems that mimic fermionic single-particle Hamiltonians with symmetry-protected modes, whose number can only change in spectral phase transitions such as band inversions. The paradigm of topological lasing, realized in three recent experiments, offers entirely new interpretations of these states, as they can be selectively amplified by distributed gain and loss. A key question is whether such topological mode selection persists when one accounts for the nonlinearities that stabilize these systems at their working point. Here we show that topological defect lasers can indeed stably operate in genuinely topological states. These comprise direct analogues of zero modes from the linear setting, as well as a novel class of states displaying symmetry-protected power oscillations, which appear in a spectral phase transition when the gain is increased. These effects show a remarkable practical resilience against imperfections, even if these break the underlying symmetries, and pave the way to harness the power of topological protection in nonlinear quantum devices.

Journal article

Zhao H, Miao P, Teimourpour MH, Malzard S, El-Ganainy R, Schomerus H, Feng Let al., 2018, Topological hybrid silicon microlasers, Nature Communications, Vol: 9, ISSN: 2041-1723

Topological physics provides a robust framework for strategically controlling wave confinement and propagation dynamics. However, current implementations have been restricted to the limited design parameter space defined by passive topological structures. Active systems provide a more general framework where different fundamental symmetry paradigms, such as those arising from non-Hermiticity and nonlinear interaction, can generate a new landscape for topological physics and its applications. Here, we bridge this gap and present an experimental investigation of an active topological photonic system, demonstrating a topological hybrid silicon microlaser array respecting the charge-conjugation symmetry. The created new symmetry features favour the lasing of a protected zero mode, where robust single-mode laser action in the desired state prevails even with intentionally introduced perturbations. The demonstrated microlaser is hybrid implemented on a silicon-on-insulator substrate, and is thereby readily suitable for integrated silicon photonics with applications in optical communication and computing.

Journal article

Malzard S, Poli C, Schomerus H, 2015, Topologically Protected Defect States in Open Photonic Systems with Non-Hermitian Charge-Conjugation and Parity-Time Symmetry, PHYSICAL REVIEW LETTERS, Vol: 115, ISSN: 0031-9007

Journal article

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