In a groundbreaking advance poised to revolutionize photonic integration, researchers have reported the successful reduction of optical losses in waveguide resonators to levels approaching those found in optical fibers. This achievement, detailed in a new study published in Nature, demonstrates a novel method for depositing high-quality silica cladding that preserves ultra-high quality factors (Q) crucial for photonic devices, particularly in the violet to near-infrared spectrum.
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
