Photonic honeycomb lattices have edge states that are protected against weak perturbations in the couplings as long as it respects certain symmetries. This topological property is the result of the existence of Dirac points in the band structure of honeycomb lattices. Here, we show by adding a certain degree of non-Hermiticity we can move an edge state and relocate it at the position of the non-Hermitian defect. To be specific the non-Hermitian defect is a local one-dimensional Parity-Time symmetric gain and loss mechanism added along the lattice in parallel to the edge. Our proposal enriches the engineering of topological states.
Using exceptional point dynamics in time and space modulated lattices we propose a new mechanism for generation of asymmetric robust higher harmonics. Specifically we design a metallic-silicon system with a complex optical potential spatiotemporally modulated along the length of the waveguide for this robust higher harmonic generation. In our system the dynamical equations associated with modal field amplitudes are described by a Jordan form Hamiltonian. This ultimately will allow for a unidirectional higher frequency generation which always has a maximum value for a specific length of the waveguide irrespective of the geometrical imperfections in the design of the waveguide. Furthermore, the phase of the generated higher harmonic mode becomes independent of the coupling between the fundamental frequency and higher harmonic one. Unlike other proposed spatiotemporal modulated systems when the system has a Jordan form Hamiltonian, the fundamental mode remains reciprocal while the harmonic generation is non-reciprocal. Consequently, while the proposed device cannot be used as an optical isolator it can be used for many other devices such as laser cavities, interferometry, and holographic processes.
Asymmetric transport is an uneven physical response of counter-propagating signals that has significantly contributed to fundamental science and revolutionized advanced technology via a variety of significant devices including diodes and isolators in electronics, optics, acoustics, and heat transfer. Photonic metasurfaces are two-dimensional ultrathin arrays of engineered subwavelength meta-atoms, acting as local phase shifters, which unprecedentedly mold wavefronts at will with a virtually flat optical element. While such an architecture can be potentially harnessed to achieve two-way asymmetric response of free-space light at an optically thin flatland, asymmetric light transport cannot be fundamentally achieved by any linear system including linear metasurfaces. Here, we report asymmetric transport of free-space light at nonlinear metasurfaces, with harmonic generation, upon transmission and reflection. We also derive the nonlinear generalized Snell’s laws of reflection and refraction which were experimentally verified by angle-resolved anomalous refraction and reflection of the nonlinear light. The asymmetric transport at optically thin nonlinear interfaces is revealed by comparing the original path of light through the metasurface with its corresponding reversed propagation path. Such a two-way asymmetric response at metasurfaces opens a new paradigm for free-space ultrathin lightweight optical devices with one-way operation including unrivaled optical valves and diodes.
We introduce a new family of spectral singularities with highly directional response in parity-time (PT) symmetric cavities. These spectral singularities support modes with infinite reflection from one side and zero reflection from the other side of the cavity, results in simultaneous unidirectional laser and unidirectional reflectionless parity-time symmetric cavity. Such unidirectional spectral singularities emerge from resonance trapping induced by the interplay between parity-time symmetry and Fano resonances.
We introduce a class of unidirectional lasing modes associated with the frozen mode regime of non-reciprocal slow-wave
structures.1 Such asymmetric modes can only exist in cavities with broken time-reversal and space inversion
symmetries. The lasing frequency coincides with a spectral stationary inflection point of the underlying passive
structure and it is virtually independent of the size of the cavity. These unidirectional lasers can be indispensable
components of photonic integrated circuitry.
We show that nonlinear optical structures involving a balanced gain-loss profile can act as optical diodes. This is made
possible by exploiting the interplay between the fundamental symmetries of parity (P) and time (T), with optical
nonlinear Kerr effects. This unidirectional propagation is demonstrated for the case of a PT -symmetric nonlinear coupler
and a PT-symmetric Bragg grating.
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