Exceptional points (EPs) are singularities in open (non-Hermitian) systems that occur when two or more complex resonant modes coalesce, forming a degenerate state and altering the system's topology. EPs have attracted significant interest due to their wide range of applications, including asymmetric mode switching, light stopping, and most notably, sensing. Operating near an EP provides enhanced sensitivity to external perturbations, resulting in a non-trivial square-root scaling behavior, where the sensitivity slope theoretically approaches infinity. Traditionally, achieving EPs in photonics has relied on gain-loss balanced systems, which involve complex fabrication processes and are challenging to implement at the nanoscale. Our group was the first to demonstrate EP formation in nanostructured plasmonic periodic arrays by breaking the symmetry of a double-layer metasurface structure (Park et al., Nature Physics, 2020). Although this plasmonic EP metasurface showed two orders of magnitude higher sensitivity for human anti-IgG protein detection, it faced several limitations, including complex fabrication processes, the need for bulky instrumentation, and the inability to detect biomarkers at the single-molecule level. To overcome these challenges, we recently introduced a novel approach to realize EP in a compact, four-port integrated Mach-Zehnder interferometer using a single coupled plasmonic nanoantenna (see Figure 1). Relevant research results were recently published in Photonics Research, Volume 13, Issue 3, 2025. [Kamyar Behrouzi, Zhanni Wu, Liwei Lin, Boubacar Kante, "Single plasmonic exceptional point nanoantenna coupled to a photonic integrated circuit sensor," Photonics Res. 13, 632 (2025)]

Figure. 1. Integrated EP Sensor: (a) Schematic of a nanoantenna EP with a four-port MZI for biosensing. (b) Energy diagram showing eigenmode tuning to achieve EP. (c) Microfluidic encapsulation for low-volume sample handling. (d) Sensing mechanism: Captured biomarkers alter eigenvalues, enabling detection.

To achieve exceptional points (EPs) at the single-particle level without the need for a 3D nanoantenna arrangement, we introduced a new design approach by tuning two key parameters: the nanoantenna length and its relative lateral displacement, eliminating the necessity for any periodic lattice. Kamyar Behrouzi, Ph.D., commented, "It was previously thought that achieving plasmonic EPs necessitated periodic lattice configurations, constraining the device miniaturization to scales of tens of microns. However, our approach reveals that the ultra-high sensitivity of EP can now be realized at the single coupled nanoantenna level." To validate our approach, we systematically swept the nanoantenna parameters and observed that the real and imaginary parts of the complex eigenmodes (resonant frequencies and loss rates) coalesced at the EP location in Riemann space, confirming the formation of an EP.

While demonstrating EP formation in a single coupled nanoantenna was achieved, measuring its response in free space remained a significant challenge. To overcome this, we designed a novel waveguide, referred to as the junction waveguide, which enables strong coupling between the guided optical mode and the plasmonic nanoantenna specifically at the junction location. This approach provided a practical solution for obtaining measurable responses, as simply placing the nanoantenna on top of a waveguide did not yield sufficient coupling strength. Additionally, we integrated a four-port Mach-Zehnder interferometer to capture the complete complex response of the system at the EP, allowing simultaneous measurements of both the amplitude and phase. To maximize system performance, the photonic integrated circuit (PIC) components were optimized using inverse design techniques. The collected data was further processed through S-matrix fitting to extract the complex eigenmodes, enabling ultra-sensitive perturbation sensing in the eigenmode space.

We then investigated the sensing capabilities of the proposed integrated plasmonic EP system by analyzing the eigenmode evolution under refractive index perturbations in two scenarios: bulk perturbations and local perturbations. The results confirmed the well-known square-root scaling behavior of EPs in response to perturbations, with significantly higher sensitivity compared to conventional linear systems. To demonstrate a practical sensing scenario, we further evaluated the system's response to protein-like and exosome-like particle attachments on the single coupled nanoantenna, considering the integration of the PIC with a microfluidic channel. Our statistical analysis of the complex mode splitting revealed the potential for single-molecule detection, further highlighting the capabilities of EP-based sensing.

Moving forward, we anticipate several promising research directions: (1) experimental realization of the proposed integrated plasmonic EP system, (2) improving the device's robustness against fabrication imperfections and environmental fluctuations, and (3) extending the design to achieve higher-order EPs in integrated platforms. Each of these directions presents unique challenges, and addressing them would significantly advance the understanding and application of EPs in ultra-sensitive optical sensing.