Exceptional singular resonance in gain mediated metamaterials

Nonlocal Metasurface with Chiral Exceptional Points in the Telecom-Band

The exceptional point (EP) is the critical phase transition point in parity-time (PT) symmetry systems, offering many unique physical phenomena, such as a chiral response. Achieving chiral EP in practical applications has been challenging due to the delicate balance required between gain and loss and complicated fabrication, limiting both working band and device miniaturization. Here, we proposed a nonlocal metasurface featuring orthogonal gold nanorods, where loss modulation is achieved through rod size and lattice pitch. By tuning the coupling strength, we experimentally observed the PT symmetry phase transition and chiral EP in the telecom-band. The experimental and simulated circular conversion dichroism at EP reach 0.79 and 0.99, respectively. We also demonstrated an abrupt phase flip of a specific component near EP theoretically. This work provides a feasible scheme for exploring EP in polarized space within the telecom-band, which may find applications in polarization control, wavelength division multiplexing, ultrasensitive sensing, imaging, etc.

Parity-time symmetry breaking optical nanocircuit

Gain and loss balanced parity-time (PT) inversion symmetry has been achieved across multiple platforms including acoustics, electronics, and photonics. Tunable subwavelength asymmetric transmission based on PT symmetry breaking has attracted great interest. However, due to the diffraction limit, the geometric size of an optical PT symmetric system is much larger than the resonant wavelength, which limits the device miniaturization. Here, we theoretically studied a subwavelength optical PT symmetry breaking nanocircuit based on the similarity between a plasmonic system and an RLC circuit. Firstly, the asymmetric coupling of an input signal is observed by varying the coupling strength and gain-loss ratio between the nanocircuits. Furthermore, a subwavelength modulator is proposed by modulating the gain of the amplified nanocircuit. Notably, the modulation effect near the exceptional point is remarkable. Finally, we introduce a four-level atomic model modified by the Pauli exclusion principle to simulate the nonlinear dynamics of a PT symmetry broken laser. The asymmetric emission of a coherent laser is realized by full-wave simulation with a contrast of about 50. This subwavelength optical nanocircuit with broken PT symmetry is of great significance for realizing directional guided light, modulator and asymmetric-emission laser at subwavelength scales.

Optical super-resonance in a customized P T-symmetric system of hybrid interaction

We investigate the optical resonances in coupled meta-atoms with hybrid interaction pathways. One interaction pathway is the directly near-field coupling between the two meta-atoms. The other interaction pathway is via the continuum in a waveguide functioned as a common bus connecting them. We show that by properly introducing gain or loss into the meta-atoms, the hybrid optical system becomes parity-time (P T) symmetric, in which the effective coupling rate can be customized by manipulating the length of the waveguide. At the exact phase of the customized P T symmetry, the coupled meta-atoms support discrete super-resonant modes that can be observed from the transmission spectra as extremely sharp peaks. At an exception point where the eigenmodes coalesce, albeit the transmission curve is flat, a high-Q factor of the localized field in the meta-atoms can be obtained. Similarities of the super-resonance with the bound states in the continuum (BICs) are discussed. This investigation promotes our understanding about the ways in realizing high-Q optical resonance especially by manipulating the distributions of loss and gain via the concepts of P T and BICs. Many attractive applications are expected.

Manipulating the critical gain level of spectral singularity in active hybridized metamaterials

In this paper, we investigate the spectral singularity in an active hybridized metamaterial, which manifests itself as ultra-high transmission and reflection at the same frequency in the far-field. A transmission line combined with lumped element model is utilized to describe the proposed active metamaterial. With this model, we reveal that the critical gain level for triggering the spectral singularity is related to the coupling strength between different components of the system. Through optimizing the coupling coefficients between different components, we demonstrate the spectral singularity of the proposed structure at very low gain level, which can bring active metamaterials systems a step closer to their practical implementation. Furthermore, we demonstrate rapid switching between two spectral singularities at different frequencies in the same structure by adding or reducing small amount of gain. The exotic properties of the proposed sub-wavelength structure promise applications in switching, sensing, spaser and nonlinear optics areas.