Gain-Free Parity-Time Symmetry for Evanescent Fields.

Parity-time (PT) symmetry, satisfied when a system commutes under combined parity and time-reversal operations, enables extreme optical responses in non-Hermitian systems with balanced distributions of gain and loss. In this Letter, we propose a different path for PT symmetry utilizing the evanescent field excitation of anti-PT-symmetric structures, which anticommute with the PT operator and do not necessarily require gain. Beyond offering a robust platform to explore PT symmetry, our study showcases an important link between non-Hermitian physics and near-field interactions, with implications in nanophotonics, plasmonics, and acoustics for nanoimaging, sensing, and communications.

Virtual Parity-Time Symmetry.

Parity-time (PT) symmetry has recently been opening exciting directions in photonics, yet the required careful balance of loss and gain has been hindering its widespread applicability. Here, we propose a gain-free route to PT symmetry by extending it to complex-frequency excitations that can mimic gain in passive systems. Based on the concept of virtual absorption, extended here to implement also virtual gain, we implement PT symmetry in the complex-frequency plane and realize its landmark effects, such as broken phase transitions, anisotropic transmission resonances, and laser-absorber pairs, in a fully passive, hence inherently stable, system. These results open a path to establish PT symmetry and non-Hermitian physics in passive platforms.

Beyond Chu's Limit with Floquet Impedance Matching.

Chu's limit determines the minimum radiation quality factor Q of an electrically small resonator, and hence its maximum operational bandwidth, which is inversely proportional to its volume. This bound imposes severe restrictions in several areas of technology, from wireless communications to nanophotonics and metamaterials. We show that a suitably tailored temporal modulation of the matching network, combined with proper detuning of the feeding impedance, can overcome this limit and enable radiation over broader bandwidths, which scale as 1/sqrt[Q], ensuring at the same time stability. Our findings open opportunities for communication systems, nanophotonics, and sensor technology.

Overcoming Intensity Saturation in Nonlinear Multiple-Quantum-Well Metasurfaces for High-Efficiency Frequency Upconversion.

Engineered intersubband transitions in semiconductor heterostructures featuring multiple quantum wells (MQWs) are shown to support record-high second-order nonlinear susceptibilities. By integrating these materials in metasurfaces with tailored optical resonances, it is possible to further enhance photonic interactions, yielding giant nonlinear responses in ultrathin devices. These metasurfaces form a promising platform for efficient nonlinear processes, including frequency upconversion of low-intensity thermal infrared radiation and harmonic generation, free of phase-matching constraints intrinsic to bulk nonlinear crystals. However, nonlinear saturation at moderately large pump intensities due to the transfer of electron population into excited subbands facilitated by strongly enhanced light-matter interactions in metasurfaces fundamentally limits their overall efficiency for various nonlinear processes. Here, the saturation limits of nonlinear MQW-based metasurfaces for mid-infrared frequency upconversion are significantly extended by optimizing their designs for excitation with a strong pump coherently coupled with unpopulated upper electron subbands. This counterintuitive pumping scheme, combined with tailored material and photonic engineering of the metasurface, avoids saturation at practical levels of continuous-wave pump intensities, yielding significantly larger upconversion efficiencies than in conventional approaches. The present results open new opportunities for nonlinear metasurfaces, less limited by saturation mechanisms, with important implications for night-vision imaging and compact nonlinear wave mixing systems.