non-Hermitian physics – Science

Scientists Develop Photoswitchable Exceptional Points Using Bound States in the Continuum

Katie Riggs — Mon, 03 Nov 2025 18:16:31 +0000

In a groundbreaking advancement at the intersection of non-Hermitian physics and photonic metamaterials, researchers from Nanjing University have demonstrated the first observation of the transition from a single bound state in the continuum (BIC) singularity to a two-dimensional exceptional ring. This milestone represents a significant leap in understanding topological singularities within non-Hermitian systems and paves the way for novel applications across terahertz (THz) technology and integrated optics.

Bound states in the continuum (BICs) originated as purely quantum mechanical phenomena characterized by modes that remain localized despite existing within a continuous spectrum of radiative modes. Exceptional points (EPs), on the other hand, are non-Hermitian spectral singularities where two or more eigenstates coalesce, resulting in dramatic physical consequences such as enhanced sensitivity and unusual wave dynamics. Although both concepts individually have been extensively explored, the intricate relationship and interplay between BICs and EPs has eluded comprehensive experimental verification until now.

Dielectric metasurfaces—engineered arrays of nanoscale dielectric structures—have become invaluable platforms for manipulating electromagnetic waves with unprecedented precision. Their inherent low loss and high structural tunability render them ideal candidates for studying complex wave phenomena such as BICs and EPs within photonics. The research team exploited these features by fabricating a metasurface design that enables the controlled evolution of a BIC singularity into a two-dimensional exceptional ring through precise angular manipulation of the incident electromagnetic wavevector.

At the heart of their experiment lies the Friedrich–Wintgen interference mechanism, wherein destructive interference between resonant modes facilitates the creation of BICs. By carefully tuning the incident angle of excitation, the team induced symmetry breaking in the system, triggering a transition that transforms the initially localized BIC point into an extended exceptional ring — a closed curve of degeneracies in momentum space. This transition from zero-dimensional singular points to one-dimensional topological features reveals a new dimension in the topological landscape of non-Hermitian photonics.

The research further delves into the complex eigenvalue spectrum of the system, capturing both real and imaginary components of eigenmodes as functions of momentum. This detailed spectral mapping elucidates the nontrivial topology of exceptional rings, reinforcing the connection between interference-induced BICs and non-Hermitian degeneracies. Such insights herald a new era in the dynamic manipulation of photonic states, transcending conventional Hermitian constraints.

Moreover, the team innovatively employed optical pumping techniques to modulate the carrier concentration within silicon components integrated into the metasurface. This approach enables active control over the system’s non-Hermitian properties by dynamically breaking the degeneracy responsible for EP formation. The ability to switch exceptional point configurations on demand constitutes a versatile platform for reconfigurable photonic devices, an advance that holds substantial promise for real-world applications.

Leveraging this mechanism, the researchers subsequently developed a practical terahertz transmission beam deflector capable of dynamic operation via optical pumping. Such a device exemplifies the translation of abstract topological concepts into tangible technological tools, underscoring the impact of fundamental physics on next-generation optoelectronic innovation. This integration of theory and device fabrication heralds a paradigm shift in how light manipulation can be achieved at terahertz frequencies.

The implications of these findings extend across multiple domains, notably in integrated optics where the compactness and tunability of EPs derived from BICs can revolutionize device functionalities. The sensitivity enhancement near exceptional points holds profound potential for ultraprecise sensors capable of detecting minute environmental changes. Additionally, dynamic wavefront shaping facilitated by EP modulation introduces a versatile methodology for on-chip light control, vital for advanced optical communication systems.

This work thus marks a seminal contribution to topological photonics, offering unprecedented control strategies for electromagnetic wave behavior in non-Hermitian regimes. By establishing a connection between bound states in the continuum and exceptional rings, the research opens pathways for engineering complex photonic landscapes with tailored spectral singularities and topological characteristics.

Future exploration is anticipated to expand the operational bandwidth and environmental robustness of such systems, facilitating their integration into scalable optoelectronic circuits and possibly quantum information platforms. The integration of photoswitchability into exceptional point dynamics represents a new horizon in adaptive photonics, where device properties can be programmatically modified in real time.

In summation, the experimental realization of the BIC-to-EP transition within dielectric metasurfaces not only confirms foundational theoretical predictions but also drives forward the practicality of topological photonics in applications ranging from sensing to dynamic light modulation. This nexus of topological physics and materials engineering promises to redefine the capabilities and complexities of photonic devices in the coming years.

Subject of Research: Topological physics of non-Hermitian photonic systems; transition between bound states in the continuum and exceptional points.

Article Title: Photoswitchable exceptional points derived from bound states in the continuum

Web References: https://doi.org/10.1038/s41377-025-02036-0

Image Credits: Caihong Zhang et al.

Keywords

Bound States in the Continuum, Exceptional Points, Non-Hermitian Physics, Dielectric Metasurfaces, Topological Photonics, Terahertz Technology, Optical Pumping, Silicon Photonics, Eigenmode Dynamics, Friedrich–Wintgen Interference, Dynamic Wavefront Control, Integrated Optics

On-Chip All-Dielectric Metasurface Enables Creation of Topological Exceptional Points

Katie Riggs — Thu, 21 Aug 2025 14:53:15 +0000

In a groundbreaking advancement in photonics, researchers from Wuhan University and Tsinghua University have pioneered a revolutionary platform that integrates topological exceptional points (EPs) within an all-dielectric, on-chip metasurface. This innovative strategy dispenses with traditional metallic components, thereby circumventing the notorious Ohmic losses associated with metal-based metasurfaces. By leveraging precise extraction methods for guided-wave optical fields, the study heralds a new era of compact, efficient, and highly integrable photonic devices, promising tremendous impacts on augmented reality (AR) visualization, optical information encoding, and next-generation optical display systems.

Metasurfaces, consisting of ultra-thin layers decorated with subwavelength meta-atoms, have long held promise for manipulating light at a fundamental level, enabling precise control over amplitude, phase, and polarization of optical waves. Their unique capacity to facilitate non-Hermitian physics, particularly the realization of topological EPs — points in parameter space where eigenvalues and eigenvectors coalesce — has been mostly demonstrated using metallic structures. However, metals inherently introduce undesirable energy dissipation via Ohmic losses, severely constraining device efficiency and hindering integration with purely dielectric photonic platforms.

Recognizing these constraints, the team spearheaded by Professor Zhongyang Li and Professor Qinghua Song has proposed and experimentally realized an all-dielectric on-chip metasurface capable of exciting topological EPs without resorting to metal components. Their approach ingeniously manipulates the geometry and spatial arrangement of dielectric meta-atoms atop dielectric waveguides to precisely extract and shape guided optical waves. This methodology not only eradicates absorption losses but also eliminates zero-order diffraction backgrounds, which often plague holographic projection systems and degrade image quality.

Central to their design is the selective generation and manipulation of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) components. The research highlights an emergent topological singularity exclusively within the LCP channel, where amplitude and phase singularities coincide spatially, marking the presence of a topological exceptional point. Notably, such singular behavior is absent in the RCP component, a feature deftly exploited by the team to enable independent encoding and precise polarization decoupling through the synthesis of Pancharatnam-Berry phase modulation and topological phase accumulation around the EPs.

The practical realization of these principles culminated in a sophisticated on-chip meta-holography system that projects distinct holographic images—depicting a “Key” and a “Lock”—corresponding to LCP and RCP components, respectively. This dual-channel holographic encoding highlights the metasurface’s unparalleled capability for multiplexed optical information processing, a critical requirement for advanced photonic applications including data storage, secure communications, and dynamic displays.

Beyond mere demonstrations of optical manipulation, the platform’s robustness extends into augmented reality applications. Through an elaborate experimental configuration, AR images float vividly and distinctly above the real-world background without any parasitic visual artifacts. The suppression of undesired diffraction orders ensures unprecedented image clarity and fidelity, a key hurdle in translating holographic AR technology from laboratory concept to commercial wearable devices.

This research exemplifies the successful convergence of topological photonics with integrated on-chip dielectric platforms. It thereby expands the degrees of freedom available for optical system design, offering not only enhanced control over light-matter interaction but also compatibility with existing silicon photonics infrastructure. The all-dielectric configuration enables miniaturization and scalability, essential attributes for the commercialization of photonic chips in consumer electronics, optical computing, and quantum information processing.

Of particular note is the team’s ability to harness non-Hermiticity—an often challenging regime characterized by energy exchange with the environment—within a purely dielectric system. This contrasts sharply with prevailing approaches that rely on inherent losses from metallic elements. Their breakthrough paves the way for low-loss, tunable, and topologically robust photonic devices capable of sustaining exceptional point dynamics vital for sensing, lasing, and optical switching functionalities.

The metasurface’s waveguide integration marks a significant stride toward functional photonic circuits, as it allows seamless interplay between guided modes and free-space optical fields. By engineering the meta-atom shape and layout, the researchers achieve phase and amplitude control with extreme precision, leading to unparalleled manipulation of spatial and polarization degrees of freedom. Such versatility is indispensable for next-generation holographic displays, data multiplexing, and complex wavefront shaping.

Looking forward, the implications of this work are vast. The demonstrated all-dielectric topological metasurface could spearhead the development of compact, energy-efficient wearable AR devices offering superior image quality and interaction fidelity. Moreover, the platform’s ability to encode multiple optical channels independently holds promise for multiplexed information storage systems, setting new standards for data density and retrieval speed in photonic memory architectures.

In conclusion, this pioneering research melds the intricate physics of topological exceptional points with cutting-edge metasurface engineering to unlock new potentials in integrated photonics. By surmounting the loss-related limitations of metal-based systems through all-dielectric design, the team sets a formidable precedent for the creation of high-performance, scalable, and multifunctional optical devices that can redefine future technologies in AR, quantum photonics, and beyond.

Subject of Research: Topological exceptional points in all-dielectric on-chip metasurfaces and their applications in meta-holography and augmented reality.

Article Title: Creating topological exceptional point by on-chip all-dielectric metasurface

Web References:
10.1038/s41377-025-01955-2

Image Credits: Cheng Yi et al.

Keywords

Topological exceptional points, all-dielectric metasurface, on-chip photonics, non-Hermitian optics, guided-wave extraction, polarization decoupling, meta-holography, augmented reality, Pancharatnam-Berry phase, integrated photonic circuits, optical encoding, low-loss photonics