Nonreciprocal Transmission in Nonlinear PT‐Symmetric Metamaterials Using Epsilon‐Near‐Zero Media Doped with Defects

Abstract

point exists, named exceptional point (EP), where the two eigenvalues of the system merge and the effective non-Hermitian Hamiltonian becomes defective. [24,25] The EP is the critical state found exactly before the system experiences a PT-breaking transition [24,[26][27][28][29][30][31] leading to larger than one transmission and eventually to spectral singularities or lasing. It has been demonstrated that the reflection becomes asymmetric at the EP by illuminating from opposite directions, while the transmittance is equal to unity due to reciprocity. [14] PT-symmetric systems have recently been designed by using different photonic and optical metamaterial schemes, such as zero index metamaterials (ZIMs), epsilon-near-zero (ENZ) materials, hyperbolic metamaterials, and multilayer Bragg gratings. [14,17,18,32,33] Optical nonlinearity is an inevitable phenomenon in the material response when the input power of the incident radiation is increased. [34][35][36][37] By accounting the effect of nonlinearity into PT-symmetric devices operating at the broken PT-symmetric phase, away from the EP, where a spectral singularity exists, or other passive asymmetric systems, the transmission can become nonreciprocal and the Lorentz reciprocity law is broken. [38][39][40][41][42][43][44][45] The asymmetric transmission is of great importance to many photonic applications, such as optical diodes, isolators, and cloaks. [11,19,24] However, passive nonlinear nonreciprocal systems suffer from fundamental physical bounds between their nonreciprocal transmission contrast and asymmetric field distribution. [41,43,46,47] In addition, lasing or other detrimental instabilities can occur for nonlinear PT-symmetric devices operating at the broken PT-symmetric phase that will limit their nonreciprocal response even in the presence of saturable nonlinearities. [40] In this work, the aforementioned limitations on nonlinear self-induced nonreciprocity are broken by using a scalable and compact PT-symmetric active metamaterial constructed by two ENZ media separated by an ultrathin gap filled with air and photonically doped with gain and loss defects. [48] We demonstrate that the extremely asymmetric field distribution, combined with the enhanced nonlinear effects inside both defects, will lead to almost perfect nonreciprocal transmission with a pronounced contrast. The permeability of the two ENZ media Nonreciprocal transmission forms the basic operation mechanism of optical diodes and isolators and requires the tantalizing task of breaking the Lorentz reciprocity law. In this work, strong nonreciprocal transmission is demonstrated by using a compact nonlinear parity-time (PT) symmetric system based on epsilon-near-zero (ENZ) materials photonically doped with gain and loss defects and separated by an ultrathin air gap. The nonlinear response of this scalable configuration is triggered at relatively low optical intensities due to the strong electric field confinement in the defects. The extreme asymmetric field distribution achieved upon excitati...

Non-Hermitian doping of epsilon-near-zero media

“…This enables, for instance, the engineering of extreme parameter responses, such as perfectly magnetic conductors and epsilon-and-mu-near-zero, without the need of magnetic material constituents. These results, validated experimentally at microwave frequencies (with a near-cutoff waveguide mimicking the ENZ host medium), have paved the way for new exciting developments in reconfigurable and flexible photonics (16), nonlinear (9, 17) and nonreciprocal (17,18) optics, and quantum (19) and transformation-invariant (20) metamaterials.…”

“…Non-Hermitian optics concepts are particularly intriguing in ENZ media, where the effects of relatively low levels of loss and/ or gain can be dramatically enhanced (30)(31)(32). For instance, recent studies have demonstrated a variety of interesting effects including tunneling (33), waveguiding (34), coherent perfect absorption (35), immunity to impurities (36), exceptional points and spectral singularities (37,38), and nonlinear and nonreciprocal effects (17).…”

Strong self-induced nonreciprocal transmission by using nonlinear PT-symmetric epsilon-near-zero metamaterials

“…The (3)  values used here can provide a conservative estimate of the input intensity required to obtain self-induced nonreciprocal transmission. Note that ENZ materials often have large nonlinear susceptibilities and the nonreciprocal transmission can occur at lower input intensities compared to the simulation results shown in the next paragraph [16].…”

Up‐And‐Coming Advances in Optical and Microwave Nonreciprocity: From Classical to Quantum Realm

“…Recently novel avenues to achieve nonreciprocity of electromagnetic fields without magnets but using new scattering effects and a new class of materials and metamaterials have been implemented. [ 16–21 ] The examples include dynamic spatiotemporal modulation of parameters, [ 22–25 ] synthetic magnetic field, [ 25–27 ] angular momentum biasing in photonic or acoustic systems, [ 21,28,29 ] nonlinearity, [ 30–33 ] interband photonic transitions, [ 34,35 ] optomechanics, [ 36–40 ] optoacoustics, [ 41,42 ] parity‐time (PT)‐symmetry breaking, [ 43–45 ] unidirectional gain and loss, [ 46–53 ] moving/rotating cavities [ 54–56 ] and emitters, [ 57 ] Doppler‐shift, [ 58 ] chiral light‐matter coupling and valley polarization, [ 59–63 ] and quantum nonlinearity. [ 64–67 ] Furthermore, quantum systems based on superconducting Josephson junctions attract much attention as they hold a great promise for quantum computing.…”