Writing in Light: Science & Applications, Peng and colleagues report a device they describe as a robust single-mode laser based on merging bound states in the continuum, or BICs. Their platform is a photonic-crystal slab laser built on InGaAsP multiple quantum wells. By tuning the diameter of the air holes etched into the slab, the researchers move accidental BICs in momentum space and merge them with a symmetry-protected BIC at the Gamma point, the center of the Brillouin zone. In an ideal, infinite photonic crystal, this merging broadens the high-Q region around Gamma: instead of a narrow, isolated high-Q point, radiative loss remains suppressed over a larger domain of in-plane wave vector. The concept builds on the topological picture of BICs as polarization singularities carrying conserved topological charges, and on the earlier experimental demonstration that merging multiple BICs can create ultrahigh-Q guided resonances that are robust against out-of-plane scattering.

At first glance, one might expect the exact merging point to be the optimum laser design, because it delivers the broadest radiative protection. Peng and colleagues show that this intuition is incomplete, and their correction is the most consequential finding of the study. In a finite photonic crystal, the continuous BIC band of the ideal infinite slab is quantized into discrete, cavity-like modes. The desired fundamental state and the nearby higher-order states are therefore not independent of the BIC landscape; they both sample it. If the high-Q region becomes too broad, it can protect the competing modes just as effectively as the target mode. The result is a lower absolute threshold, but a weaker single-mode hierarchy, because the very mechanism that shields the wanted mode also shields its rivals.

The key message of the paper is that the most robust single-mode laser is obtained not exactly at the merging condition, but slightly before it. In this pre-merging regime, the fundamental s-like BIC-derived mode is already strongly protected, while the nearest p-like competing mode still experiences substantially larger loss. The authors calculate that in the pre-merging device, the threshold gain of the p-like competitor is more than three times larger than that of the s-like lasing mode. At exact merging, this contrast collapses to only about 1.3. Experimentally, the difference is decisive: the pre-merging device remains strictly single-mode up to 80 times threshold, whereas at the merging condition a higher-order BIC-derived mode begins to lase at high pump power. The pre-merging point is, in effect, a sweet spot where protection is strong enough for the target mode but no longer generous enough for its competitors.

This reframes the role of BIC merging in active devices. Merging BICs were originally attractive because they suppress radiative leakage and make high-Q resonances more robust against fabrication disorder and finite angular spread of the pump. Super-BIC lasers have already exploited this principle to reduce threshold by merging symmetry-protected and accidental BICs in momentum space. What Peng and colleagues add is an explicitly active-laser criterion: one should not maximize Q blindly, but should instead maximize the useful threshold contrast between the desired mode and the nearest competing modes. The relevant figure of merit is therefore not simply the quality factor itself, but something closer to the ratio of the modal gains of the target and competing modes, or more generally the separation between the first and second lasing thresholds under realistic gain saturation.

The distinction matters most for miniaturized BIC lasers. Ideal BICs are Bloch modes of extended periodic structures, and their high-Q character is usually compromised when the device is made small. Reducing the lateral size increases edge leakage, broadens the momentum distribution of the confined field, and weakens the infinite-lattice protection that makes BICs attractive in the first place. Early BIC lasing experiments demonstrated that lasing action could persist in finite arrays, even down to a few periods of the crystal. The broader mini-BIC problem, however, is more subtle: how can one retain BIC-like vertical confinement while also providing in-plane confinement in a genuinely small footprint, without sacrificing the very quality factors that motivated the design?

Several research groups have addressed this question by combining BIC physics with additional lateral confinement mechanisms. One team demonstrated miniaturized BIC cavities with ultrahigh quality factors by combining lateral photonic-bandgap confinement with vertical BIC protection, reaching very high Q in small modal volumes. Others then used mini-BIC cavities with quantum-dot gain media to demonstrate low-threshold single-mode nanolasers, and subsequently achieved ultra-low-threshold continuous-wave operation with quantum-dot mini-BIC lasers. A further approach achieved confinement through topological band inversion, producing electrically pumped compact bulk lasers driven by band-inverted bound states in the continuum. These works established that mini-BICs can overcome the apparent incompatibility between BIC physics and small mode volume, but they often rely on a designed cavity region embedded within a larger photonic environment rather than on the patterned region alone.

Peng and colleagues attack the problem from a different angle. In their device, the entire patterned photonic-crystal region spans only five by five periods, an area of roughly 15 square micrometers. At this scale, one cannot simply assume that the structure behaves as a truncated version of an infinite BIC lattice; edge leakage becomes a dominant part of the cavity physics. The authors therefore engineer the boundary holes, reducing their diameter relative to the central holes, to suppress leakage and recover BIC-derived lasing in an extremely small patterned region. The significance is not merely that a small cavity tends to have fewer modes. Reducing the area generally increases mode spacing and can make single-mode operation easier in a trivial spectral sense. The real challenge is different: a very small BIC laser may lose the high-Q advantage that made the BIC useful in the first place. The new work shows that merging-BIC physics, finite-size quantization and edge engineering can be combined so that compactness does not simply destroy BIC protection.

The findings also connect to a parallel development in passive BIC photonics. Recently, the merging of accidental BICs with net-zero topological charge was shown to reshape the polarization texture of the emitted field, stretching the range of circularly polarized emission over a sizeable momentum-space domain. Taken side by side with the new laser result, this suggests that BIC engineering may eventually move beyond selecting a single frequency and spatial mode, toward selecting a single handedness or spin channel. That would open a possible conceptual route toward chiral or spin-selective BIC lasers, in which the same topological machinery that suppresses radiation loss also dictates the polarization state of the output beam.

The broader implication is that BIC lasers are entering a regime where topology, finite-size physics and gain competition must be designed together. Passive BIC photonics asks how radiation can be canceled. Active BIC photonics asks a harder question: after radiation is canceled for one state, which other states are also helped, and will any of them lase? Peng and colleagues answer by identifying a pre-merging sweet spot, demonstrating that the best laser is not located at the point of maximum radiative protection, but at the point where radiative protection is most selective. That principle, established here in a photonic-crystal slab small enough to fit comfortably within a few wavelengths of light, may become a standard design rule for compact, stable and eventually electrically pumped BIC lasers, and a reminder that in laser engineering, as in many fields, the optimum rarely coincides with the maximum.

Subject of Research: Modal selectivity in compact merging-bound-state-in-the-continuum photonic-crystal lasers

Article Title: Single-mode lasers before the merger

Article References: Zito, G. (2026). Single-mode lasers before the merger. Light: Science & Applications, 15(1), Article 372. https://doi.org/10.1038/s41377-026-02467-3

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02467-3

Keywords: bound states in the continuum, photonic crystals, semiconductor lasers, single-mode lasing, quality factor, modal selectivity, InGaAsP quantum wells, miniaturized lasers, topological photonics, threshold discrimination, gain competition, nanophotonics

Polaritons are hybrid light–matter quasiparticles formed when photons become strongly coupled to electronic excitations in a semiconductor. Because they combine the low effective mass of photons with interactions inherited from matter, polaritons can accumulate in a coherent quantum state known as a polariton condensate. This state behaves in several ways like a fluid, supporting collective phenomena such as superfluid flow, quantized vortices, soliton-like structures and other defects. Unlike ordinary fluids, however, polaritons also possess an internal degree of freedom associated with the polarization of light. This polarization acts as a pseudospin, giving the condensate an additional landscape in which complex textures can form.

Controlling that pseudospin has been one of the central challenges in polariton physics. Conventional strategies often introduce external gauge fields or carefully shaped optical potentials to manipulate the polarization state. Although these methods can be effective, the resulting textures may be weakly tied to the physical structure of the cavity. They can therefore be vulnerable to disorder, fabrication imperfections and fluctuations in the excitation conditions. The new work takes a different approach: instead of treating the cavity as a passive container, it uses the cavity’s topology and symmetry as an intrinsic mechanism for generating and stabilizing the condensate’s spin texture.

The central platform is a bound state in the continuum, or BIC, engineered in a metasurface made from a halide-perovskite film. A BIC is a photonic state that, despite existing at an energy where it could ordinarily couple to and radiate into the surrounding environment, remains confined because of interference and symmetry-related constraints. In an ideal structure, the mode can possess an extremely high quality factor, meaning that light remains trapped for a comparatively long time. The researchers used a metasurface with broken inversion symmetry, designing its geometry so that the confined optical mode carries a nontrivial polarization structure in momentum space. This spin–momentum locking links the direction of propagation to the polarization of the optical field.

When the perovskite metasurface is optically excited, the confined mode can reach the conditions needed for polariton condensation. The condensate does not simply form in a featureless spot. Instead, its spatial and polarization properties reflect the topology of the underlying photonic mode. According to the study, geometry-driven condensation produces pairs of half-vortices with opposite spin. Each half-vortex combines a singularity in the condensate phase with a rotation of its polarization, creating a defect that is fundamentally different from a conventional scalar vortex. In a full vortex, the phase changes by 2π around the core. In a half-vortex, the phase and polarization evolve together so that a half-quantum winding remains physically consistent.

A striking feature of the observed defects is that the half-vortices are connected to polarization strings extending from their cores. These strings can be understood as narrow regions across which the condensate’s polarization changes sharply, marking a topological connection between the defect and the surrounding spin texture. Rather than allowing the two defects to behave as independent points that move freely through the condensate, the strings constrain their motion. The resulting configuration resembles a pair of connected topological objects whose location and dynamics are determined by the polarization field imposed by the metasurface.

The researchers also found that the positions of the half-vortices can be tuned by changing the excitation density. Increasing or decreasing the pump conditions alters the condensate population, interactions and spatial distribution, allowing the defects to move along their associated polarization strings. This controlled displacement is important because topological defects in many systems can annihilate when defects with opposite charges meet. In the reported configuration, an intervening topological domain wall prevents the opposite-spin half-vortices from simply crossing the structure and annihilating. The domain wall therefore acts as a barrier embedded in the condensate’s spin landscape, preserving the defects while still allowing their positions to be adjusted.

The result is significant because it shifts the source of topological control from external fields to the architecture of the optical cavity itself. A geometry-defined spin texture can remain tied to the mode structure even when the material contains imperfections that would otherwise perturb the condensate. Halide perovskites are attractive for this purpose because they offer strong light–matter coupling and can be processed into photonic structures, but they can also exhibit structural disorder and spatial variations. By encoding the desired behavior into the metasurface geometry, the researchers aim to reduce the sensitivity of the topological state to such irregularities.

The study could open new directions for polariton-based devices in which information is carried not only by intensity or frequency, but also by the position, charge and spin of topological defects. Half-vortices and polarization strings may be useful for exploring nonequilibrium quantum fluids, spinor condensates and topological photonics, where light is manipulated through its polarization and phase. More broadly, the work demonstrates how carefully engineered photonic topology can impose order on a driven, dissipative quantum system. The ability to displace defects without destroying them could be particularly valuable for studying defect interactions and for developing robust methods to route excitations through complex optical landscapes.

The researchers describe their platform as a route toward deterministic control of polariton spin textures, but important questions remain. Future experiments will need to establish how rapidly the half-vortices can be moved, how stable they are under continuous operation, and how their behavior changes with temperature, disorder and pump geometry. It will also be important to determine whether more elaborate metasurface designs can create larger networks of strings, domain walls and vortices, or support programmable topological states. For now, the findings show that a condensate’s geometry can do more than confine light: it can dictate how quantum-fluid defects are born, where they travel and whether they survive.

Subject of Research: Spin polaritons, polariton condensates, half-vortices, polarization strings, bound states in the continuum and topology-guided excitations.

Article Title: Topology-guided vortices in a polariton condensate

Article References: Zacheo, A., Marangi, M., Mata-Cervera, N. et al. “Topology-guided vortices in a polariton condensate.” Nature Materials (2026). https://doi.org/10.1038/s41563-026-02693-5

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41563-026-02693-5

Keywords: polariton condensate, spin polaritons, half-vortices, topological excitations, bound state in the continuum, metasurface, halide perovskite, spin–momentum locking, polarization strings, topological domain walls.

At the heart of this discovery are plasmonic metasurfaces — ultrathin, artificially structured materials designed to manipulate electromagnetic waves in ways not possible with natural substances. The research team employed cutting-edge laser printing techniques to fabricate these metasurfaces with meticulous precision, enabling the harnessing of BICs to significantly enhance nonlinear optical processes and infrared detection efficiency.

Bound states in the continuum represent a peculiar class of resonances where localized modes remain perfectly confined despite residing within the energy spectrum of radiative waves. This counterintuitive phenomenon allows for exceedingly high quality (Q) factors, reflecting extended photon lifetimes and intensified electromagnetic fields. By integrating BICs into plasmonic metasurfaces, the researchers have engineered an optical platform where light can be trapped and manipulated with extraordinary finesse.

One of the most remarkable achievements of this study lies in the demonstration of efficient nonlinear light conversion on a compact, scalable platform. Nonlinear optical processes such as second-harmonic generation or sum-frequency mixing are pivotal for applications ranging from quantum information processing to advanced microscopy. Conventionally, achieving strong nonlinear responses necessitates bulky setups or complex material systems, but the laser-printed plasmonic metasurfaces provide a planar and integrable alternative with enhanced performance.

Infrared photodetection, crucial for telecommunications, environmental sensing, and security, also stands to benefit from these innovations. The metasurfaces’ near-field enhancement, enabled by BICs, amplifies the interaction between incoming infrared radiation and the detector material. This leads to increased responsivity without the need for cryogenic cooling or complicated signal amplification, paving the way for lightweight, energy-efficient infrared sensors.

The fabrication process itself is a testament to the transformative role of modern nanotechnology. Utilizing femtosecond laser printing, the researchers sculpted arrays of nanostructures with subwavelength precision directly onto metallic films. This method affords not only high throughput and reproducibility but also enormous flexibility in tailoring the metasurface geometry, crucial for tuning the BIC modes and optimizing their optical responses.

To elucidate the underlying physics, the team combined rigorous numerical simulations with experimental measurements. They observed how the metasurface’s geometry influences the emergence and spectral position of BICs, controlling the light localization and its coupling to free-space radiation. This fundamental understanding enables rational design strategies for metasurfaces tailored to specific nonlinear or photodetective functionalities.

Moreover, the exceptional field confinement at BIC resonances results in a dramatic enhancement of the local electromagnetic environment. This boost underpins the increased efficiency of both harmonic generation and photodetection, as nonlinear susceptibilities and photoresponse scales with the field intensity. Such synergy marks a notable leap in metasurface technology, pushing the boundaries of light manipulation beyond prior limitations.

The research also highlights the robustness of the laser-printed metasurfaces against fabrication imperfections. Bound states in the continuum exhibit inherent tolerance to minor structural deviations, which translates into consistent performance even when scaled to larger areas or integrated with other photonic components. This robustness is vital for real-world applications where manufacturing variability is inevitable.

Notably, the use of plasmonic materials, which inherently suffer from dissipative losses, is mitigated by the BIC-induced suppression of radiation leakage. By confining the optical energy more efficiently, the plasmonic losses become less detrimental, allowing for practical exploitation of metals in high-Q photonic devices. This represents a crucial advance over earlier BIC implementations that favored dielectric architectures with lower field confinement.

Application-wise, the implications extend across diverse technological domains. In optoelectronics, these metasurfaces could serve as compact frequency converters or coherently-driven light sources. In environmental monitoring, the enhanced infrared detection capabilities promise more sensitive and selective sensors for gas analysis or thermal imaging. Furthermore, in quantum computing and communications, the ability to engineer precise nonlinear interactions at the nanoscale opens avenues for novel photonic circuits.

The demonstrated combination of laser printing and BIC-enabled plasmonic metasurfaces also underscores the broader trend toward on-chip integration of complex optical functionalities. As integrated photonic systems grow increasingly sophisticated, the demand for miniaturized, efficient, and tunable components escalates. This work positions metasurfaces, fabricated by scalable laser techniques, as prime candidates for next-generation photonic chips.

Additionally, the research team explored the tunability of the metasurface response by varying structural parameters, such as lattice periodicity and nanoparticle shapes. This versatility enables dynamic adjustment of resonance wavelengths and nonlinear efficiencies, potentially allowing on-the-fly reconfiguration of device functions without physical alterations.

From a theoretical standpoint, the insights gained into the interplay between plasmonic resonances and BIC phenomena enrich the fundamental understanding of light confinement mechanisms. This could inspire novel designs that exploit topological photonics or hybrid material platforms, pushing nonlinear optics and photodetection into uncharted territories.

In conclusion, this pioneering work by Pavlov, Sergeeva, Seredin, and colleagues marks a significant milestone in nanophotonics, melding advanced laser fabrication techniques with the enigmatic physics of bound states in the continuum. The resultant plasmonic metasurfaces not only showcase impressive nonlinear light-conversion capabilities and broadband infrared detection but also establish a versatile platform for future integrated photonic devices and sensors. As these concepts mature towards commercialization, they may herald a new era of compact, efficient, and multifunctional optical technologies fundamentally reshaping our interaction with light.

Subject of Research: Nonlinear light conversion and infrared photodetection using laser-printed plasmonic metasurfaces supporting bound states in the continuum.

Article Title: Nonlinear light conversion and infrared photodetection with laser-printed plasmonic metasurfaces supporting bound states in the continuum.

Article References:
Pavlov, D.V., Sergeeva, K.A., Seredin, A.A. et al. Nonlinear light conversion and infrared photodetection with laser-printed plasmonic metasurfaces supporting bound states in the continuum. Light Sci Appl 15, 23 (2026). https://doi.org/10.1038/s41377-025-02040-4

Image Credits: AI Generated

DOI: 10.1038/s41377-025-02040-4

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

At the heart of this study is the novel coupling of two fundamental concepts in wave physics and optics. Bound states in the continuum are peculiar, non-radiating states that paradoxically exist within the spectrum of radiative modes yet remain localized and trapped due to interference effects. Exceptional points, on the other hand, are singularities in the parameter space of non-Hermitian systems where both eigenvalues and eigenvectors coalesce, leading to fascinating phenomena such as asymmetric mode switching and enhanced sensitivity. Wang et al. have ingeniously demonstrated how BICs can be engineered into EPs whose properties can be dynamically manipulated by light, thereby creating a photoswitchable platform with transformative potential.

The crux of the innovation lies in designing photonic structures where bound states coexist with radiative continua, linked by carefully tailored perturbations that allow the system to reach exceptional points under optical excitation. By introducing a photosensitive element into the setup, the researchers gain the ability to swiftly modulate the system’s refractive index and dissipation rates via external light stimuli. This modulation transforms the static BICs into dynamic, tunable EPs, essentially enabling the on-off switching of exceptional point behavior with optical control. Such dynamism opens avenues for innovative applications like ultrafast optical switches, sensors with amplified responsiveness, and lasers with controllable emission properties.

The experimental framework deployed by the team leverages state-of-the-art nanofabrication to realize metasurface arrays with embedded nonlinear materials. These metasurfaces exhibit tailored symmetry properties that determine the emergence and accessibility of bound states and exceptional points within the photonic band structure. By illuminating the metasurface with a secondary control laser, the refractive index changes locally, breaking certain symmetries and steering the system directly into the exceptional point regime. Monitoring this transition reveals telling alterations in transmission spectra and modal intensity profiles that confirm the successful realization of photoswitchable EPs.

Beyond experimental verification, the researchers have implemented rigorous theoretical modeling to underpin their observations. Their approach encapsulates non-Hermitian coupled-mode theory adapted to include nonlinear photo-induced refractive changes. Theoretical simulations map out the parametric conditions required for photoswitchability, pinpointing the threshold intensities and geometric configurations that optimize the fidelity and responsiveness of switching. This detailed understanding empowers the design of bespoke photonic devices with finely tuned functionalities operating at the nexus of quantum and classical regimes.

Importantly, the researchers address the broader implications of integrating BIC-derived exceptional points into functional photonic platforms. Unlike conventional EP-based devices, which often rely on static structural features or temperature tuning, the mechanically and electrically stable photoswitching mechanism mediated by light introduces unmatched versatility. This innovation enables real-time reconfiguration of device properties without altering the physical structure, thereby enhancing the robustness, miniaturization potential, and integration capability into optical communication and sensing systems.

The implications for sensing technology are profound. Exceptional points are famed for their ability to enhance sensor sensitivity by orders of magnitude compared to traditional resonant systems due to their non-Hermitian degeneracy. By making these EPs photoswitchable, it becomes possible to recalibrate sensors dynamically, optimizing detection thresholds for a variety of chemical, biological, and environmental signals. This adaptive sensing ability could fundamentally change the landscape of monitoring systems that require rapid, precise, and on-demand reconfiguration.

On the laser front, the merging of photoswitchability and exceptional points harbors the promise of controllable lasing thresholds and directionality. Lasers operating near EPs possess unique emission properties, including unidirectional output and mode selectivity. The additional capability to turn these exceptional point features on and off optically translates into unprecedented control over laser modes and powers in integrated photonic circuits. Such flexibility is especially advantageous in creating compact, low-energy photonic chips for optical computing and on-chip light manipulation.

Moreover, the study describes how the intrinsic topological properties associated with exceptional points and BICs can be leveraged for robust light transport immune to defects and disorder. Photoswitchable EPs enable switching the topology of the system on demand, facilitating novel schemes for topological photonics that dynamically control edge states and defect-immune pathways. This area holds considerable promise for future quantum information processing and robust photonic networks where coherent light manipulation and protection against perturbations are essential.

The integration of nonlinear optical materials into the structure is pivotal for achieving high-performance photoswitching. The nonlinear response enhances the contrast between ON and OFF EP states by amplifying refractive index changes under relatively low-intensity light. Such materials not only reduce the energy cost for switching but also shorten response times to sub-nanosecond scales, enabling ultrafast control of EP phenomena. This rapid adaptability places these devices at the forefront of modern photonics where speed and energy efficiency are critical.

In terms of fabrication, the researchers have demonstrated that their approach is compatible with existing semiconductor and dielectric metasurface technologies, suggesting a clear path toward scalability and industrial adoption. The low fabrication complexity and the use of well-known materials mean that these photoswitchable EP devices can integrate seamlessly into current photonic platforms, facilitating widespread deployment in telecommunications, sensing, and computing architectures.

Looking ahead, Wang and colleagues emphasize that their discovery opens unexplored design spaces for reconfigurable photonic devices that exploit non-Hermitian physics in multifunctional ways. By extending this concept to other wave systems such as acoustics and mechanics, the photoswitchable exceptional point mechanism may catalyze new classes of adaptive metamaterials and signal processors. The universality of the underlying physics ensures that the approach will inspire cross-disciplinary innovation.

This report from Wang et al. is not merely an incremental advance; it is a conceptual leap that redefines how we think about controlling wave phenomena in complex systems. By merging the enigmatic bound states in the continuum with the powerful non-Hermitian exceptional points and adding photoswitchability, the work unlocks a versatile toolkit for next-generation photonics with transformative societal and technological impacts. From ultrafast optical switches and resilient sensors to tunable lasers and topological devices, the horizon is rich with possibilities catalyzed by this novel photonic paradigm.

The scientific community has already begun to recognize the far-reaching implications of this work. The demonstration that light itself can be used to dynamically access, tune, and switch exceptional point regimes derived from BICs invites new interdisciplinary inquiries, linking nonlinear optics, topological physics, and materials science in unprecedented ways. The reported photoswitchable EPs could become foundational elements for the photonic technologies of tomorrow, offering unprecedented versatility, precision, and adaptability.

This seminal contribution sets a new standard for exploring non-Hermitian physics in realistic, operational devices. It beckons further experimental studies into material optimization, device miniaturization, and integration with electronic and quantum systems. As research intensifies around photoswitchable exceptional points and their unique capabilities, we can anticipate an accelerated wave of innovation, propelling photonic technologies into realms once thought inaccessible.

Ultimately, the discovery of photoswitchable exceptional points originating from bound states in the continuum illustrates the power of marrying fundamental physics with engineering ingenuity. It exemplifies how seemingly abstract mathematical concepts manifest as tangible, controllable phenomena that will shape the future of optics and photonics in extraordinary ways, positioning light at the forefront of technological evolution.

Subject of Research: Photoswitchable exceptional points and bound states in the continuum in photonic systems

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

Article References:

Wang, L., Liu, H., Liu, J. et al. Photoswitchable exceptional points derived from bound states in the continuum. Light Sci Appl 14, 377 (2025). https://doi.org/10.1038/s41377-025-02036-0

Image Credits: AI Generated

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

Phonon-polaritons, hybrid quasiparticles arising from the strong coupling between photons and optical phonons in polar crystals, have garnered immense scientific interest due to their ability to confine electromagnetic energy at subwavelength scales within the THz frequency domain. This spectral region is notoriously challenging to harness because it sits between the traditionally accessible electronic and photonic frequencies. The current research addresses this challenge head-on by engineering an interaction between phonon-polaritons and electromagnetic modes that enters the ultrastrong coupling regime—where the interaction strength rivals or surpasses the energies of the uncoupled systems—facilitating new physical phenomena otherwise unobservable in weak or moderate coupling scenarios.

Central to the reported study is the concept of bound states in the continuum, exotic wave modes that remain confined and non-radiative despite existing in the energy spectrum continuum where free propagation is permitted. By integrating BICs into a carefully designed photonic platform, the authors achieve a remarkable level of control over phonon-polariton properties. This innovative coupling scheme generates an unprecedented degree of tunability in the polaritonic dispersion and enhances the coherence and lifetime of the hybrid states.

The experimental framework combines advanced nanofabrication techniques with sophisticated spectroscopic measurements, enabling the precise observation of ultrastrong coupling phenomenology. The research team engineered metasurfaces patterned on polar dielectric substrates exhibiting Reststrahlen bands, where intrinsic phonon-polariton resonances are naturally supported. By tailoring metasurface geometries to support BIC modes overlapping spectrally and spatially with the phonon-polaritons, an efficient hybridization channel is established. This approach manipulates the near-field coupling landscape, offering a new degree of control over light-matter interactions in the THz regime.

One of the most striking outcomes of the study is the emergence of distinctly modified dispersion curves for the coupled modes, characterized by anticrossing behavior and large Rabi splittings, quintessential signatures of ultrastrong coupling. These observations confirm that the system departs fundamentally from linear response theory and enters a nonlinear domain where conventional perturbative methods fail. Such non-perturbative effects open avenues to explore novel quantum optical phenomena within solid-state platforms.

Another critical advantage arising from the BIC-enhanced coupling is the dramatic suppression of radiative losses. Bound states, by definition, decouple from the far-field continuum, rendering the polariton lifetimes significantly longer and the resonances sharper. This quality factor enhancement is essential for applications where coherence and low dissipation are paramount, such as quantum information processing, THz sensing, and nonlinear harmonic generation. The study thus not only pushes theoretical boundaries but also fosters practical innovation in device engineering.

Furthermore, the research elucidates the tunable nature of the hybrid modes. By varying parameters such as metasurface lattice constants, dielectric environment, and excitation angles, the team demonstrated control over the coupling strength and spectral positions of the phonon-polariton resonances. This flexible platform provides an experimental knob to dynamically program optical responses in the THz range, enabling bespoke photonic component designs that can be reconfigured on demand.

Beyond fundamental physics insights, the implications of this work resonate strongly with emerging quantum technologies. Ultrastrong coupling between light and matter is a cornerstone for realizing robust qubits and gates in quantum circuits, as it facilitates rapid coherent exchanges and entanglement protocols. Simultaneously, the enhanced field localization in phonon-polariton systems is conducive to sensing molecular vibrations and detecting minute environmental changes with exceptional sensitivity, paving the way for next-generation THz spectroscopy tools.

Remarkably, the authors documented the emergence of non-trivial topological features within the coupled mode spectrum, hinting at potential links to topological photonics. The interplay between BICs and phonon-polaritons forms a fertile ground for exploring protected edge states immune to backscattering, which can revolutionize waveguiding and robust signal transmission in integrated photonic circuits.

From a materials standpoint, the experiment leveraged well-established polar dielectric materials, such as silicon carbide and hexagonal boron nitride, known for their robust Reststrahlen bands and optical phonon modes. The compatibility of these substrates with existing semiconductor fabrication processes ensures that the new coupling paradigm can be seamlessly integrated into photonic chips, accelerating the translation from laboratory proof-of-concept to real-world applications.

Looking ahead, the findings open multiple research directions. One intriguing prospect is harnessing the ultrastrong coupling regime mediated by BICs for quantum simulators that can emulate complex many-body interactions and phase transitions in condensed matter physics. Moreover, nonlinearity inherent in the ultrastrong regime could be exploited for ultrafast optical switches, modulating THz signals with unprecedented speed and efficiency.

The theoretical framework developed in this study merges classical electrodynamics with quantum optics, deploying a hybrid modeling approach that accounts for the non-perturbative coupling Hamiltonian and electromagnetic boundary conditions governing BICs. Such rigorous modeling not only supports the experimental observations but also serves as a predictive tool for designing future metasurface architectures optimized for specific functionalities.

In conclusion, the manipulation of terahertz phonon-polaritons in the ultrastrong coupling regime via bound states in the continuum stands as a masterpiece of modern photonics research. It transcends traditional engineering limits, unveiling uncharted physical effects with promising practical applications. As the terahertz gap steadily narrows through innovations of this caliber, we anticipate a surge in transformative technologies spanning communication, sensing, and quantum information science.

As the scientific community digests these results, it is clear that the ultra-strong coupling of phonon-polaritons facilitated by BICs is not just a niche discovery but a cornerstone that will redefine how we harness light and vibrations in solid-state platforms. This work exemplifies how careful structuring at the nanoscale enables control over phenomena at the quantum level, charting a course toward unprecedented manipulation of electromagnetic waves in practically relevant regimes.

The implications for future devices are profound. With this approach, engineering platforms that operate beyond conventional limits of speed, size, and efficiency is within reach. From ultra-sensitive biochemical sensors to compact, integrated quantum optical systems, the terahertz domain is poised for a renaissance driven by the principles illuminated in this spectacular study. The fusion of advanced photonics, materials science, and quantum physics witnessed here marks an exciting milestone in the journey toward mastering light-matter interactions.

Subject of Research: Manipulation of terahertz phonon-polaritons in the ultrastrong coupling regime using bound states in the continuum

Article Title: Manipulating terahertz phonon-polariton in the ultrastrong coupling regime with bound states in the continuum

Article References:
Yang, J., Zhang, L., Wang, K. et al. Manipulating terahertz phonon-polariton in the ultrastrong coupling regime with bound states in the continuum. Light Sci Appl 14, 360 (2025). https://doi.org/10.1038/s41377-025-02044-0

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-02044-0

The quest to comprehend and manipulate mechanical waves is no trivial endeavor. Such waves are foundational to numerous technologies pervasive in our daily lives, whether it be smartphones, ultrasound equipment, or radios, where resonance plays a critical role. Though resonance—a phenomenal amplification of waves at designated frequencies—offers exciting applications, the typical resonators employed gradually lose energy, necessitating continuous power input to maintain effective functions. The potential for a system that can confine energy without leakage sparks the imagination and, now, scientific reality does not disappoint.

Delving into the past, historical figures like Nobel laureates John von Neumann and Eugene Wigner proposed a perplexing yet tantalizing idea over 90 years ago: the theoretical entrapment of waves, enabling their indefinite confinement without energy dissipation. Known as Bound States in the Continuum, these states serve as fascinating metaphors for whirlpools existing in flowing rivers. Despite extensive investigation, many believed the realization of BIC in compact, single-particle systems remained unattainable—until now.

Employing a strategically designed system consisting of cylindrical granular particles—essentially small quartz rods—the researchers constructed a finely tunable mechanical platform where the interaction of mechanical waves at contact points is under their precise control. By systematically adjusting the alignment of these cylinders, the team observed that under specific conditions, one could achieve complete confinement of a wave mode within a single cylinder without any energy escaping into the neighboring environment. This notable observation constitutes the first empirical evidence of a polarization-protected BIC, shattering long-held beliefs.

The implications of this discovery extend beyond merely trapping energy. Remarkably, the researchers succeeded in achieving exceptional quality factors (Q-factors) exceeding 1,000, a crucial metric that reflects how proficiently a resonator captures and stores energy while minimizing energy loss. This innovative achievement signifies not only a theoretical triumph but also a practical advancement in energy-efficient systems.

But what occurs when these special cylinders are interconnected to form a chain? The research team made a further remarkable finding—the trapped wave modes within individual cylinders could propagate throughout the chain without dispersing. This behavior is referred to as a flat band, an intriguing phenomenon suggesting that energy can travel without dissipating. Such movement bears striking similarities to throwing a pebble into a still pond, wherein the ripples remain stationary, oscillating only in place—a captivating visual of energy dynamics encapsulated in a Bound Band in the Continuum (BBIC).

Lead researcher Dr. Yeongtae Jang profoundly articulated the essence of their findings, stating that the experimentation reveals an ability to allow wave motion while simultaneously keeping energy precisely contained. This characteristic of perfect confinement paves the way for transformative applications ranging from energy harvesting devices to ultra-sensitive sensors, and even futuristic communications technologies.

The achievement of realizing BIC in a single cylindrical particle breaks long-standing theoretical barriers that have previously defined the limits of wave mechanics. Professor Junsuk Rho, leading the research group, emphasized the foundational importance of their findings, hinting at a future where low-loss energy systems could revolutionize technology. While emphasizing that the ongoing research remains rooted in fundamental exploration, the prospects for advanced sensing technologies and signal transmission systems appear tangible.

This trailblazing research receives support from the Mid-Career Research Program of the National Research Foundation of Korea (NRF), generously funded by the Ministry of Science and ICT, alongside the POSCO-POSTECH-RIST Convergence Research Center. The intersection of high-caliber academic investigation and cutting-edge technology fosters an environment ready to harness innovative capabilities unleashed by these scientific pursuits.

The accomplishment stands not only as a testament to the commitment of the research teams but also as a beacon of hope for future technologies, synthesizing the principles of theoretical physics and practical engineering in ways that could redefine our understanding of resonance and energy transfer. With the world increasingly reliant on efficient and sustainable technologies, the implications of this study are poised to resonate throughout various sectors, laying the groundwork for future innovations.

Ultimately, the intersection of rigorous academic endeavor and transformative practical application heralds a thrilling future for engineering and applied sciences. As research continues to unfold, society awaits the innovations that will arise from understanding and manipulating wave mechanics—with this study marking a fascinating chapter in that ongoing narrative.

Subject of Research: Bound States in the Continuum (BIC) in Mechanical Systems
Article Title: Bound States to Bands in the Continuum in Cylindrical Granular Crystals
News Publication Date: April 3, 2023
Web References: Link to DOI
References: Published in Physical Review Letters
Image Credits: POSTECH

Keywords

Mechanical energy, Mechanical systems, Resonance, Basic research, Fluid mechanics, Wave mechanics, Particle theory, Theoretical physics, Ultrasonic waves, Scientific publishing, Smartphones, Ultrasound, Signal amplification, Vortices, Particulate matter, Control theory.