Observation of chiral state transfer without encircling an exceptional point.

The adiabatic theorem, a corollary of the Schrödinger equation, manifests itself in a profoundly different way in non-Hermitian arrangements, resulting in counterintuitive state transfer schemes that have no counterpart in closed quantum systems. In particular, the dynamical encirclement of exceptional points (EPs) in parameter space has been shown to lead to a chiral phase accumulation, non-adiabatic jumps and topological mode conversion1-8. Recent theoretical studies, however, have shown that contrary to previously established demonstrations, this behaviour is not strictly a result of winding around a non-Hermitian degeneracy9. Instead, it seems to be mostly attributed to the non-trivial landscape of the Riemann surfaces, sometimes because of the presence of an EP in the vicinity9-11. Here, in an effort to bring this counterintuitive aspect of non-Hermitian systems to light and confirm this hypothesis, we provide a set of experiments to directly observe the field evolution and chiral state conversion in an EP-excluding cycle in a slowly varying non-Hermitian system. To do so, a versatile yet unique fibre-based photonic emulator is realized that utilizes the polarization degrees of freedom in a quasi-common-path single-ring arrangement. Our observations may open up new avenues for light manipulation and state conversion, as well as providing a foundation for understanding the intricacies of the adiabatic theorem in non-Hermitian systems.

Experimental demonstration of single-mode topological valley-Hall lasing at telecommunication wavelength controlled by the degree of asymmetry.

Topology plays a fundamental role in contemporary physics and enables new information processing schemes and wave device physics with built-in robustness. However, the creation of photonic topological phases usually requires complex geometries that limit the prospect for miniaturization and integration and dispossess designers of additional degrees of freedom needed to control topological modes on-chip. By controlling the degree of asymmetry (DoA) in a photonic crystal with broken inversion symmetry, we report single-mode lasing of valley-Hall ring cavities at telecommunication wavelength. The DoA governs four photon confinement regimes at the interface of topologically distinct valley-Hall domains and evidences an interplay between the width of the topological bandgap and the quality factor of ring-like modes for single-mode operation. Our results open the door to novel optoelectronic devices and systems based on compact topological integrated circuits.

Efficient design of random metasurfaces.

Random media introduce large degrees of freedom in device design and can thus address challenges in manipulating optical waves. Wave shaping with metasurfaces has mainly utilized periodic or quasi-periodic grids, and the potential of random arrangement of particles for devices has only come under investigation recently. The main difficulty in pursuing random metasurfaces is the identification of the degrees of freedom that optimize their efficiencies and functions. They can also encode information using the statistics of particle distribution. We propose a phase-map that accounts for the statistical nature of random media. The method takes into account effects of random near-field couplings that introduce phase errors by affecting the phase shift of elements. The proposed approach increases the efficiency of our random metasurface devices by up to ∼20%. This work paves the way toward the efficient design of random metasurfaces with potential applications in highly secure optical cryptography and information encoding.

Terahertz bistability and multistability in graphene/dielectric Fibonacci multilayer.

Here, we benefit from the strong nonlinear response of graphene and the rich variety of resonances provided by a graphene/dielectric Fibonacci multilayer to realize bistability and multistability in the terahertz (THz) frequency range. Toward this pursuit, we employ the nonlinear transfer matrix method. We examine the suitability of resonances in the Fibonacci multilayer for the bi/multistability purposes and determine the proper working point. We report various switching up/down manners via single or stepwise jumps between states of the same or different contrasts upon increasing followed by decreasing the intensity of the incident wave. We show that graphene samples of high quality are preferred for bi/multistable switching in terms of reducing the switch-up/-down thresholds and widening the multistable region. We also explore the possibility of tuning the bi/multistable behavior via the frequency and angle of the incident wave as well as the graphene Fermi level. We envision precious applications in THz switching, realizing logic gates, and so on for this system.

Nonlinear optics of surface plasmon polaritons in subwavelength graphene ribbon resonators.

We study the propagation characteristics of surface Plasmon polaritons (SPPs) on a patterned graphene sheet incorporating a subwavelength ribbon resonator and a Kerr nonlinear bounding medium (substrate or top cladding) which provides tunable bandpass filtering in the THz regime. We study theoretically and via modeling the tunability of maxima in the transmission spectrum, corresponding to the resonant frequencies of the ribbon resonator, by tuning the graphene Fermi level (via an applied gate voltage) and by altering the intensity of the incident THz wave. We determine the intensity-dependent increase in the refractive index of a Kerr nonlinear medium bounding graphene, via self-phase modulation and via the more efficient process of cross-phase modulation, revealing a noticeable red-shift in the resonant frequencies of the ribbon resonator. These concepts lead to ultrafast switching of SPP transmission through the ribbon (from a high to a low state). Using Kerr nonlinear media to bound graphene increases the tunability of graphene-based devices, enabling nonlinear plasmonic and ultrafast processing in the THz regime.

Quasi-phase matching for efficient long-range plasmonic third-harmonic generation via graphene.

We propose and numerically investigate an efficient method for long-range third-harmonic generation (THG) of propagating surface plasmon polaritons (SPP) waves on graphene sheets for nonlinear plasmonic purposes in the terahertz (THZ) gap region of the electromagnetic spectrum via a developed nonlinear finite-difference time-domain technique. We reveal that although extended and unmodulated graphene sheets with low Fermi levels can offer high-conversion efficiency (CE) for SPP THG at short distances, suitable for miniaturized plasmonic circuits, they suffer from inherent absorption loss induced by graphene that noticeably reduces the CE of the THG at long ranges. We suggest a structure benefiting from low Fermi-level graphene regions of strong nonlinear response as oscillators and high Fermi-level ones of low loss as a propagating medium in a periodic manner, which satisfies the quasi-phase matching condition and shows considerable efficiency improvement at long propagation distances. We predict that such a configuration can find valuable potential applications in the realm of nonlinear THz plasmonics for generating new frequencies and also in spectroscopy, signal processing, and so on.