The team focused on germano-silicate resonators, employing an inductively coupled plasma chemical vapor deposition (ICP-PECVD) technique to deposit the upper silica cladding at a relatively low temperature of 250 °C. This process used a deuterated silane precursor combined with oxygen plasma, which allowed precise control over film thickness and composition. Notably, the researchers implemented a rapid thermal annealing step at 1,000 °C for 20 minutes following every 500 nm of cladding deposited. This annealing served a dual purpose: it mitigated stress-induced optical losses and repaired damage caused by direct plasma exposure during deposition.

The effectiveness of this approach was confirmed by depositing a 6-micron thick upper cladding, which fully encapsulated the resonator’s coupling gap, thereby significantly shielding the device from environmental contaminants. This protective encapsulation contributed to maintaining ultrahigh Q values—up to 160 million over several months. Although this represented some reduction from the pristine, unclad Q of approximately 250 million, the results affirmed that partial recovery and long-term stability were attainable through the annealing treatment. The observed trade-offs align well with previous findings in ultralow-loss (ULL) silicon nitride, suggesting that further refinement in deposition methods or precursor materials might eliminate this Q degradation entirely.

To analyze the Brillouin gain spectrum, the researchers utilized a highly sensitive dual-intensity-modulation pump-probe technique. The test devices had upper claddings made of 1.5 mol% P₂O₅-doped silica deposited via plasma-enhanced chemical vapor deposition (PECVD). This slightly phosphorus-doped glass demonstrated excellent ability to produce thick, stress-free films critical for confining both optical and acoustic modes effectively. Post-fabrication characterization revealed waveguide propagation losses below 0.5 dB/m and facet coupling losses around 1.4 dB, underscoring the exceptional optical quality achieved.

In their experimental setup, counterpropagating pump and probe lasers operating near 1560 nm were used. The pump laser was intensity-modulated at 10 MHz, while the probe was modulated slightly off-frequency at 10.075 MHz. A lock-in amplifier measured the probe transmission signal referencing a 75 kHz beatnote originating from the modulation difference. Scanning the probe over a 20 GHz detuning range from red to blue relative to the fixed pump frequency allowed precise acquisition of the stimulated Brillouin scattering (SBS) gain spectrum.

Complementing experimental work, numerical simulations based on finite element methods were employed to calculate the optical and acoustic fields. Material parameters derived from prior studies formed the basis of their models, including indices of refraction, material densities, Poisson ratios, Young’s moduli, Brillouin linewidths, and photoelastic coefficients for the core, upper cladding, and bottom cladding layers. This multi-parameter simulation framework facilitated a holistic understanding of the interaction between optical and acoustic waves within the device structure.

To investigate thermorefractive noise (TRN), the study also employed sophisticated COMSOL Multiphysics simulations using a fluctuation-dissipation theorem-based model. Simulations compared Ge-silica waveguides with both thin and thick silicon nitride (SiN) structures, each modeled as 3 mm diameter microresonators with specific rectangular waveguide cross sections. These simulations incorporated detailed material thermal properties such as thermo-optic coefficients, thermal conductivities, specific heat capacities, and densities, all calibrated to an ambient temperature of 300 K. The Ge-silica waveguides featured air cladding, while the SiN devices were silica-clad, reflecting realistic fabrication conditions.

The combination of meticulous material engineering, thermal processing, and rigorous experimental verification offers a promising route toward integrated photonic devices with loss figures rivalling those of bulk optical fibers. Such advancements are critical in enabling the next generation of on-chip lasers, modulators, and frequency combs, which rely heavily on ultralow-loss resonators to achieve unprecedented performance in communications, sensing, and quantum technologies.

However, challenges remain. The slight but persistent reduction in Q factor upon cladding deposition indicates that further innovation in deposition chemistry or approaches may be necessary. Alternatives such as low-pressure chemical vapor deposition or novel precursors like tetraethoxysilane PECVD may hold the key to minimizing plasma-induced damage and residual stress. The researchers highlight these potential pathways, underscoring that the current work lays a solid foundation for ongoing optimization.

This study heralds a significant milestone for photonic integration, demonstrating that fibre-like loss performance across a broad spectral range is achievable, connecting violet to near-infrared wavelengths seamlessly. Such capability opens new horizons for compact, high-performance photonic chips, integrating functionalities once thought to require bulky and fragile fiber setups.

In summary, by innovating on cladding deposition methods and leveraging advanced characterization and modeling, the researchers have resolved longstanding challenges in photonic device losses. Their results promise transformative impacts across telecommunications, precision metrology, and quantum information science, where low-loss photonics are essential cornerstones. Future efforts will doubtless extend and refine these techniques, pushing photonic integration toward new frontiers in performance and scalability.

Subject of Research: Photonic integrated circuits, ultralow-loss waveguide resonators, and deposition techniques for high-quality silica cladding.

Article Title: Towards fibre-like loss for photonic integration from violet to near-infrared.

Article References:
Chen, HJ., Colburn, K., Liu, P. et al. Towards fibre-like loss for photonic integration from violet to near-infrared. Nature 649, 338–344 (2026). https://doi.org/10.1038/s41586-025-09889-w

DOI: 08 January 2026

Keywords: ultralow-loss resonators, photonic integration, silica cladding, ICP-PECVD, thermal annealing, Brillouin scattering, thermorefractive noise, waveguide propagation loss, phosphorus-doped silica, COMSOL Multiphysics simulations

At the core of their research is the bound state in the continuum—a counterintuitive physical concept originally proposed in quantum mechanics, which has since permeated into photonics and wave physics. BICs represent localized states embedded within the continuum spectrum of radiation modes, defying the conventional expectation that such modes inevitably couple to radiative channels and thereby leak energy. By meticulously engineering photonic structures to support these delicate states, the team succeeds in creating optical modes that remain perfectly confined despite their coexistence with open radiation channels.

The practical impact of this discovery is profound. Conventionally, coupling between waveguides or resonant cavities is limited by the exponential decay of near fields, which intrinsically confines effective interaction to immediate neighbors. In their innovative approach, the researchers harness the unique properties of BICs to mediate long-range interactions that can span distances far exceeding typical evanescent decay lengths. This extension of coupling range translates to new design freedoms, enabling photonic circuits with elements physically decoupled yet functionally linked through the BIC-enabled channels.

To realize this concept experimentally, the team constructed tailored photonic architectures with precise control over geometry and refractive index profiles. Through rigorous computational modeling and fabrication, these structures were realized with nanometer-scale accuracy, ensuring the delicate conditions for BIC formation. The resulting systems exhibited sharp resonance features corresponding to bound states embedded in the radiation continuum, affirming the theoretical predictions with high fidelity.

Beyond static existence, the study delves into dynamic tunability, a hallmark that elevates the utility of BIC-enabled coupling in practical applications. By integrating external stimuli such as optical pumping, electric fields, or mechanical modulation, the researchers demonstrated real-time control over the coupling strength and spectral position of the bound states. This dynamic adjustability paves the way for advanced photonic devices capable of adaptive functionality, including tunable filters, reconfigurable interconnects, and active sensors.

The physical mechanisms underpinning the tunability stem from shifts in effective refractive indices and symmetry-breaking perturbations that influence the interference pathways sustaining the BICs. By finely balancing these factors, the coupling can be either enhanced or suppressed, granting a versatile handle on signal propagation and interaction within integrated photonic platforms.

Their findings not only illustrate a novel route for coupling control but also shed light on the fundamental physics governing wave localization and interference in complex media. The delicate interplay between symmetry, topology, and modal interference defining BIC formation offers fertile ground for further exploration, including potential applications in quantum information processing and light-matter interaction engineering.

Significantly, the realization of dynamically tunable long-range coupling opens unprecedented avenues in scalable photonic circuitry, bridging the gap between nanoscale integration and macroscopic functional interconnectivity. Traditionally, scaling up photonic networks has been hindered by spatial constraints and crosstalk issues; the BIC approach effectively mitigates these challenges by enabling interaction across non-adjacent components without direct physical proximity.

Moreover, the robust confinement and minimal radiative losses inherent to BICs contribute to enhanced device performance, including higher quality factors and lower insertion losses. These improvements are critical for applications spanning telecommunications, signal processing, and sensing technologies, where precise control over light propagation dynamics is paramount.

An intriguing aspect of this research is the potential for multiplexing and routing complex signals through engineered arrays of BIC-supporting elements. By selectively activating or deactivating coupling channels through external modulation, intricate networks with customizable interaction topologies become conceivable, advancing the field of programmable photonics.

Critically, the scalability of their fabrication techniques aligns with contemporary semiconductor manufacturing capabilities, suggesting that BIC-enabled devices can be feasibly integrated into existing photonic and optoelectronic platforms. This compatibility accelerates the pathway from laboratory demonstration to commercial deployment, amplifying the impact of the discovery.

The study also contributes to the expanding theoretical framework characterizing non-radiating states in open systems, interlinking with emerging disciplines such as topological photonics and non-Hermitian physics. Understanding and controlling BICs within these broader contexts could unlock new functionalities, including robust signal transport immune to imperfections and perturbations.

In essence, Tang and colleagues have delivered a landmark advancement that challenges conventional wisdom regarding coupling limitations in photonics. By leveraging the subtle physics of bound states in the continuum, they have crafted a versatile, dynamically controllable long-range coupling scheme that promises to revolutionize photonic circuitry design and implementation.

Looking ahead, the exploration of hybrid systems combining BIC effects with nonlinearities, active gain media, or other quantum phenomena stands poised to further augment the capabilities of photonic devices. These multidisciplinary pursuits will likely yield transformative breakthroughs in communications, sensing, and information processing.

The research not only enriches our fundamental understanding of wave phenomena but also drives practical innovation in optical technologies. As data demands soar and integration densities intensify, such tunable, long-range coupling strategies will be instrumental in shaping the next generation of photonic networks with unmatched performance and resilience.

This pioneering work, published in Light: Science & Applications, has set a new benchmark for what is physically achievable in photonic coupling, heralding an era where the constraints of proximity are no longer an insurmountable barrier but a surmountable design choice.

Subject of Research: Dynamically tunable long-range optical coupling enabled by bound states in the continuum.

Article Title: Dynamically tunable long-range coupling enabled by bound state in the continuum.

Article References:
Tang, H., Huang, C., Wang, Y. et al. Dynamically tunable long-range coupling enabled by bound state in the continuum. Light Sci Appl 14, 278 (2025). https://doi.org/10.1038/s41377-025-01975-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01975-y