Parity-Time Symmetry Breaking in Coupled Nanobeam Cavities

Exceptional Precision of a Nonlinear Optical Sensor at a Square-Root Singularity

Exceptional points (EPs)-spectral singularities of non-Hermitian linear systems-have recently attracted interest for sensing. While initial proposals and experiments focused on enhanced sensitivities neglecting noise, subsequent studies revealed issues with EP sensors in noisy environments. Here we propose a single-mode Kerr-nonlinear resonator for exceptional sensing in noisy environments. Based on the resonator's dynamic hysteresis, we define a signal that displays a square-root singularity reminiscent of an EP. However, our sensor has crucial fundamental and practical advantages over EP sensors: the signal-to-noise ratio increases with the measurement speed, the square-root singularity is easily detected through intensity measurements, and both sensing precision and information content of the signal are enhanced around the singularity. Our sensor also overcomes the fundamental trade-off between precision and averaging time characterizing all linear sensors. All these unconventional features open up new opportunities for fast and precise sensing using hysteretic resonators.

Dual-frequency unidirectional reflectionless propagation in a non-Hermitian graphene plasmonic waveguide-cavity coupling system

We theoretically investigate a controllable dual-frequency unidirectional reflectionlessness at exceptional points by applying external voltage in a graphene plasmonic waveguide system. The system consists of a graphene waveguide and two end-coupled resonators. COMSOL simulation results show that the reflection of edge fundamental graphene surface plasmon polaritons mode for forward (backward) incidence is near to zero at frequency 24.418 THz (20.865 THz), while that for backward (forward) incidence is 24.71% (22.945%), respectively. In addition, the non-Hermitian scattering matrix is proposed to verify the existence of double exceptional points, and the tunable unidirectional reflectionless phenomenon is also achieved by changing the Fermi level (E_f) of graphene.

[Formula: see text] symmetry of the Su-Schrieffer-Heeger model with imaginary boundary potentials and next-nearest-neighboring coupling

By introducing the next-nearest-neighboring (NNN) intersite coupling, we investigate the eigenenergies of the [Formula: see text]-symmetric non-Hermitian Su-Schrieffer-Heeger (SSH) model with two conjugated imaginary potentials at the end sites. It is found that with the strengthening of NNN coupling, the particle-hole symmetry is destroyed. As a result, the bonding band is first narrowed and then undergoes the top-bottom reversal followed by the its width's increase, whereas the antibonding band is widened monotonously. In this process, the topological state extends into the topologically-trivial region, and its energy departs from the energy zero point, accompanied by the emergence of one new topological state in this region. All these results give rise to the complication of the topological properties and the manner of [Formula: see text]-symmetry breaking. It can be concluded that the NNN coupling takes important effects to the change of the topological properties of the non-Hermitian SSH system.

Highly-dispersive unidirectional reflectionless phenomenon based on high-order plasmon resonance in metamaterials

In this work, we design a structure of metamaterials that consists of double sliver-ring resonators, in which highly-dispersive unidirectional reflectionlessness and absorption are achieved based on high-order plasmon resonance. Reflections of +z and -z directions at 461.34 THz (456.68 THz) are ∼0 (0.82) and ∼0.85 (0) when the distance d=222.9 nm (259.8 nm), respectively. High absorption of ∼0.97 and the quality factor of ∼435 can be obtained in the loss metal structure at room temperature. What's more, unidirectional reflectionlessness is investigated at low temperature.

Enhanced nonlinear characteristics with the assistance of a [Formula: see text]-symmetric trimer system

We study the parity-time (PT) symmetry characteristics and the applications to nonlinear optics in an optical trimer system consisting of two indirectly coupled standing-mode micro-cavities and a two-level quantum emitter (QE) placed at the intersection of two cavities. We find this trimer system can exhibit analogical phenomena as those in typical [Formula: see text]-symmetric dimer systems composed of a passive cavity directly coupled to an active cavity. This system, whose [Formula: see text] symmetry is demonstrated by our analytic results, can be transformed between the [Formula: see text]-symmetric phase and the [Formula: see text]-broken phase by adjusting relevant system parameters. Then, with this system, we observe both the linear and nonlinear parts of the transmission field become remarkably enhanced and can further reach peak values around the [Formula: see text] breaking point. In addition, we show the negative correlation between the gain degree of the linear (nonlinear) transmission part and decay rate of the QE. This trimer proposal is feasible for experiments and may provide a promising platform for [Formula: see text]-symmetric optics of low-light levels. Moreover, novel phenomena arising from the QE-cavity-coupling induced nonlinearity gain could be explored to fabricate photonic devices and controllable nonlinear optical media for quantum information process and communication of photons.

Switching the unidirectional reflectionlessness by polarization in non-ideal PT metamaterial based on the phase coupling

An effective scheme on switching the exceptional point(EP) where unidirectional reflectionlessness occurs is firstly proposed in non-ideal PT metamaterial via the polarization of incident light. The unidirectional reflectionlessness could be effectively controlled only by adjusting the phase coupling of the two resonators which are consisted of two identical but vertically placed crosses and are excited by incident light as an effective gain. Besides, the unidirectional perfect absorber occurs in the vicinity of EP.

Dual-wavelength single-frequency laser emission in asymmetric coupled microdisks

The gain and loss in a microcavity laser play an important role for the modulation of laser spectrum. We show that dual-wavelength single mode lasing can be achieved in an asymmetric coupled system consisted of two size-mismatched microdisks. The amount of eigenmodes in this coupled-microdisk system is reduced relying on the Vernier effect. Then a single mode is selected to lase by controlling the gain branching in the supermodes. The supermodes are formed by the coupling between different transverse whispering-gallery modes (WGMs). When the gain/loss status between the two mirodisks is changed through selectively pumping process, the modulated gain branching for various supermodes leads to the switchable single-frequency laser emission. The results obtained in this work will provide the further understand for the spectral modulation mechanism in the coupled microcavity laser system.

Parity-time-symmetric circular Bragg lasers: a proposal and analysis

We propose a new type of semiconductor lasers by implementing the concept of parity-time symmetry in a two-dimensional circular Bragg grating structure, where both the real and imaginary parts of the refractive index are modulated along the radial direction. The laser modal properties are analyzed with a transfer-matrix method and are verified with numerical simulation of a practical design. Compared with conventional distributed-feedback lasers with modulation of only the real part of refractive index, the parity-time-symmetric circular Bragg lasers feature reduced threshold and enhanced modal discrimination, which in combination with the intrinsic circularly symmetric, large emission aperture are clear advantages in applications that require mode-hop-free, high-power, single-mode laser operation.

Absence of Exceptional Points in Square Waveguide Arrays with Apparently Balanced Gain and Loss

The concept of parity-time (PT) symmetry in the field of optics has been intensively explored. This study shows the absence of exceptional points in a three-dimensional system composed of a square waveguide array with diagonally-balanced gain/loss distribution. More specifically, we show that an array of four coupled waveguides supports eight fundamental propagation supermodes, four of which are singlet, and the other two pairs are double degenerated. It is found that the singlet states follow the routine PT phase transition; however, the double-degenerated modes never coalesce as the gain/loss-to-coupling strength level varies, showing no actual PT symmetry-derived behavior. This is evident in the phase rigidity which does not approach zero. The absence of exceptional points is ascribed to the coupling of non-symmetric supermodes formed in the diagonal waveguide pairs. Our results suggest comprehensive interplay between the mode pattern symmetry, the lattice symmetry, and the PT-symmetry, which should be carefully considered in PT-phenomena design in waveguide arrays.