Chirality, the property describing objects that cannot be superimposed onto their mirror images, is a ubiquitous concept spanning disciplines such as chemistry, physics, and biology. In optics, chirality emerges as a fundamental characteristic of light polarization states, distinguishable as right-handed or left-handed circular polarization, describing the helical rotation direction of the electric field as light propagates. Materials and structures that selectively interact with these chiral states form the basis for chiral photonics, which traditionally relies on static components like wave plates and polarizers. The innovation by the Harvard team transcends these limitations, offering a tunable platform that mechanically modulates the optical interaction with chiral light, an achievement with profound implications.

At the core of this technology lie photonic crystals—precisely nanofabricated periodic structures designed to manipulate electromagnetic waves on scales comparable to optical wavelengths. By stacking two layers of silicon nitride photonic crystals and twisting one layer relative to the other, the researchers exploit the concept of twistronics, originally prominent in the study of twisted bilayer graphene, to achieve emergent properties inaccessible to untwisted configurations. This twist imposes a left-right asymmetry, fundamentally enabling the bilayer composite to exhibit intrinsic optical chirality and thereby differentially respond to right- and left-circularly polarized light.

Conventional photonic crystal devices are inherently static, limited by their fixed geometry and material composition. The innovation here is the marriage of MEMS technology with photonic crystal structures, allowing for dynamic, nano-scale adjustment of both the twist angle and the spacing between the bilayer membranes. Through electrostatic actuators embedded within the MEMS platform, adjustments can be made with high precision and repeatability, tuning the degree of optical chirality in real time. This capability moves beyond proof-of-concept, offering a versatile mechanism to achieve nearly perfect selectivity in distinguishing the handedness of incident light.

The implication of controlling chirality dynamically is particularly significant in chiral sensing applications. Many biological molecules exhibit chirality—most famously pharmaceutical agents where mirror-image isomers have profoundly different biological effects, such as the tragic thalidomide case from the 1950s. Devices capable of discriminating molecular chirality optically can revolutionize diagnostics and pharmaceutical development by providing real-time, label-free sensing at the chip scale. The tunability of the Harvard device means it can be optimized for different wavelengths and molecular targets without mechanical overhaul or complex optical component substitution.

Beyond sensing, the dynamic chiral photonic crystal platform could advance optical communication technologies through on-chip light modulation that exploits polarization states. Conventional modulators typically do not distinguish chirality, but introducing this degree of freedom enables additional encoding channels and potentially enhances data bandwidth and security in quantum communication protocols. The integration of MEMS actuators within the chip ensures compatibility with existing photonic integrated circuit manufacturing pathways, promoting scalability and practical deployment.

From a theoretical perspective, the twisted bilayer photonic crystal represents a fascinating physical system where strong interlayer coupling between optical modes creates a complex landscape of photonic band structures influenced by twist angle and spacing. This tunability allows exploration of chiral optical phenomena near theoretical extremes, producing strong contrasts in transmission for circularly polarized light—a hallmark for effective chiral discrimination. The researchers’ methodology provides a systematic framework for designing such twisted photonic systems to harness intrinsic chirality as a controllable photonic property.

The experimental study published in the peer-reviewed journal Optica demonstrates not only the fabrication of high-fidelity bilayer silicon nitride photonic crystals but also the precise MEMS integration that enables dynamic actuation without compromising optical performance. Characterization under normal incidence illumination confirms the device’s ability to selectively transmit light of one circular polarization while suppressing the other, with modulation depths tunable via mechanical control. Such advances highlight the promise of merging microfabrication, photonics, and mechanics to unlock richer control over light-matter interactions than previously achievable.

Eric Mazur’s group at Harvard SEAS, led by graduate student Fan Du, represents a vanguard in exploring “twistronics-inspired” approaches for photonic materials. By borrowing concepts from 2D material physics, notably twisted bilayer graphene, this work extends the paradigm into silicon photonics with direct applications. The synergy between mechanical tuning and optics realized in this design introduces an unprecedented capability to engineer the photonic environment dynamically on a chip, setting the stage for a new class of reconfigurable photonic devices.

This pioneering research may also catalyze advancements in quantum photonics. The ability to tailor optical chirality dynamically at the microscale could enable new schemes for controlling spin-photon interactions, photon entanglement mechanisms, and chiral quantum state preparation with potential applications in quantum computing and secure communications. The integration of MEMS elements within photonic architectures represents an exciting pathway to add functionality and adaptability to otherwise static quantum platforms.

As the technology matures, further miniaturization and optimization of the device architecture could improve response times and reduce power consumption, fulfilling critical requirements for integration into real-world systems. The tunable chiral photonic crystal device stands at the nexus of physics, materials science, and engineering, embodying the evolving landscape of next-generation photonic technologies that leverage the interplay of structure, symmetry, and mechanical actuation for unparalleled control over light.

In summary, the dynamic control of optical chirality at the chip scale through MEMS-integrated twisted bilayer photonic crystals ushers in a new era of tunable, scalable, and highly selective photonic devices. By navigating the complex interdependence of geometry, optical mode coupling, and mechanical actuation, this work sets a compelling precedent for future research and technological innovation in chiral photonics and beyond.

Subject of Research: Not applicable

Article Title: Dynamic control of intrinsic optical chirality via MEMS-integrated photonic crystals

News Publication Date: 4-Mar-2026

Web References:
https://opg.optica.org/optica/fulltext.cfm?uri=optica-13-3-449

References:
Du, F., Tang, H., Liu, Y., Zhang, M., Lou, B., Gao, G., Li, X., Enriquez, A., Fan, S., & Mazur, E. (2026). Dynamic control of intrinsic optical chirality via MEMS-integrated photonic crystals. Optica, 13(3), 449. DOI: 10.1364/OPTICA.578880

Image Credits: Mazur group at Harvard SEAS

Keywords: Chirality, Twisted bilayer photonic crystals, MEMS actuators, Optical chirality, Circular polarization, Tunable photonics, Twistronics, Silicon nitride photonic crystals, Dynamic light modulation, Quantum photonics, Optical sensing, Micro-electromechanical systems

At the core of this innovation lies the ability to produce photons that are indistinguishable from one another on demand, a quality that has remained elusive until now. Unlike the ordinary distinctions valued in daily life, the realm of quantum technology demands absolute uniformity among photons — identical in every property and produced precisely when required. Such indistinguishability enables photons to interfere quantum mechanically, an effect analogous to noise-cancelling headphones where perfectly inverted sound waves cancel out unwanted noise. This interference is the linchpin for cutting-edge quantum phenomena integral to technologies ranging from quantum computing to secure quantum communication.

The team’s breakthrough, spearheaded by scientist Nico Hauser, addresses this precise need by developing a photon source that operates deliberately within the telecommunications C-band, around 1550 nm wavelength. This spectral region is favored for quantum technologies aiming to integrate with existing fibre-optic networks due to its minimal optical loss within silica fibres — the infrastructure backbone of modern communication systems. Historically, achieving deterministic operation with high-quality photons at this wavelength has been fraught with technical difficulties, as prior quantum dot-based sources typically excelled at shorter wavelengths (780 to 960 nm) but faltered in the telecom regime.

The technical challenge is compounded by the nature of alternative photon generation methods such as spontaneous parametric down-conversion (SPDC), which, despite delivering photons of excellent quality, do so probabilistically. In other words, SPDC sources cannot reliably emit photons at predetermined times, complicating synchronization necessary for many quantum protocols requiring simultaneous multi-photon interactions. In contrast, deterministic sources produce photons precisely when triggered but had hitherto struggled to achieve the same level of photon indistinguishability in the telecom C-band, with interference visibilities peaking below 75%, insufficient to fulfill the stringent demands of quantum information processing.

Hauser and colleagues have now engineered a highly refined photon source utilizing indium arsenide quantum dots nested within an indium aluminium gallium arsenide matrix, strategically integrated into a sophisticated circular Bragg grating resonator. This resonator significantly enhances the photon emission efficiency, a crucial factor for practical usage. Through an exhaustive comparison of excitation schemes, the team discovered that phonon-assisted excitation—the process of leveraging elementary lattice vibrations—yields superior photon indistinguishability compared to conventional higher-energy optical pumping. Operating in this mode, they achieved a remarkable raw two-photon interference visibility approaching 92%, setting a new record for deterministic single-photon sources at these telecom wavelengths.

This achievement not only narrows the performance gap between probabilistic and deterministic photon sources but also unlocks notable practical advantages. Generating identical photons on demand at telecom wavelengths directly facilitates scalable photonic quantum systems capable of synchronizing large numbers of photons. This capability is a critical enabler for advanced quantum computing architectures relying on measurement-based protocols, as well as quantum repeater networks designed to extend the reach of quantum communication over continental distances.

The synergy between the Stuttgart and Würzburg research groups underscores the collaborative nature of this achievement. Professor Sven Höfling’s team in Würzburg expertly fabricated the quantum dot samples, integrating their material science prowess with the photonic engineering expertise of Professor Barz’s group in Stuttgart. Both teams are integral parts of the PhotonQ consortium, funded by the German Federal Ministry of Research, Technology, and Space (BMFTR). This collaborative framework aims not just to pioneer individual photonic devices but to lay the groundwork for fully operational photonic quantum processors. Deploying these cutting-edge photon sources at the University of Stuttgart, researchers look forward to demonstrating practical quantum computing and facilitating distributed quantum networks through the Quantenrepeater.Net project, which ambitiously seeks to link multiple processors for networked quantum information tasks.

The advances reported by Hauser et al. thus herald a new era in photon source technology, bringing deterministic telecom photon generation into conformity with the stringent demands of scalable quantum information systems. The implications for quantum optics laboratories worldwide are profound: what once was a chronic limitation now stands resolved, ushering in practical pathways for widespread quantum computational and communicative applications. As quantum technologies race toward real-world deployment, such fundamental hardware innovations are instrumental in transitioning the field from theoretical promise to technological reality.

In light of these achievements, the paper detailing this breakthrough was published in Nature Communications on January 14, 2026. It documents not only the technical specifics of the device design and experimental results but also offers a compelling vision for the deployment of these sources in future quantum networks and computing platforms. The article provides a beacon for researchers aiming to overcome the tradition-bound constraints in photon source engineering, and it will no doubt inspire a wave of innovation in quantum photonics.

From a broader perspective, this research epitomizes the quest for harnessing the quantum realm to build fundamentally new technologies. By securing on-demand, indistinguishable single photons at telecom wavelengths, it aligns photonic quantum devices with existing global communication infrastructures, thereby bridging the gap between laboratory innovation and scalable industrial application. This alignment is crucial for the forthcoming commercial and scientific landscapes where quantum technologies are poised to revolutionize computing, cybersecurity, and information processing.

Ultimately, the collaboration and scientific ingenuity encapsulated in this work reflect a milestone in quantum technology development. The confluence of quantum dot engineering, photonic resonator design, and precise excitation control has culminated in a source that reliably produces single photons with the coveted properties demanded by the next generation of quantum systems. As the quantum revolution unfolds, this breakthrough paves the way for interoperable, networked quantum devices that can operate seamlessly within our existing communication frameworks, suggesting a future where the extraordinary potential of quantum information becomes ubiquitously accessible.

Subject of Research: Photonic quantum technologies, single-photon sources, quantum dots, telecommunications C-band.

Article Title: Deterministic and highly indistinguishable single photons in the telecom C-band.

News Publication Date: 14 January 2026.

Web References: DOI: http://dx.doi.org/10.1038/s41467-026-68336-0

Image Credits: Barz Group, University of Stuttgart / Ludmilla Parsyak

Keywords: quantum photonics, single-photon source, deterministic photon generation, indistinguishable photons, telecommunications C-band, quantum dots, quantum computing, quantum communication, photonic quantum processors, photon interference, quantum networks, quantum repeaters.

At the heart of this scientific advancement is the concept of Fano interference, a quantum mechanical phenomenon first described in the context of atomic physics, which emerges from the interaction between a discrete quantum state and a continuum of states. Applied to photon pairs, this interference pattern offers a rich terrain of quantum correlations and coherence effects that can be actively modulated in practical photonic systems. The innovative aspect of this research lies in harnessing these quantum interferences within a planar metamaterial, thereby transcending conventional limitations imposed by bulk optics and conventional nonlinear crystals.

The metasurface employed in this study is a sophisticated two-dimensional array of subwavelength nanoantennas, engineered with extreme precision to tailor light-matter interaction at the quantum scale. These nanoantennas function as resonant scatterers, supporting localized plasmonic modes that couple strongly with incident electromagnetic fields. When photon pairs, generated through spontaneous parametric down-conversion (SPDC) or other nonlinear optical processes, interact with this structured environment, their quantum states experience modulation leading to distinctive Fano interference patterns. This marks a paradigm shift in the ability to design quantum light sources with enhanced functional attributes.

Fundamentally, the phenomenon relies on balancing the discrete resonances of the metasurface’s nanoantennas with the broad continuum of photonic modes, resulting in asymmetric interference line shapes that are highly sensitive to environmental parameters and device geometry. The researchers meticulously characterized the spectral and quantum properties of the emitted photon pairs, employing advanced coincidence counting techniques and Hong-Ou-Mandel interferometry to verify the presence and tunability of the Fano resonances. These experimental validations underscore the robustness and reproducibility of the interference effects in realistic device architectures.

The implications of achieving controlled Fano interference in photon pairs are multifold. Quantum coherence and entanglement properties intrinsic to the photon pairs can be fine-tuned, allowing for enhanced control over quantum state preparation and measurement. This tunability is crucial for the development of scalable quantum networks, where the efficient routing and manipulation of quantum information carriers dictate the overall system performance. By integrating metasurfaces into chip-scale quantum photonic circuits, the study effectively paves the way for ultra-compact, versatile platforms capable of quantum state engineering on demand.

Moreover, the research highlights the adaptability of metasurfaces in tailoring energy transfer processes at the quantum level. Their planar nature enables seamless integration with existing semiconductor and photonic technologies, enhancing compatibility and fostering cross-disciplinary innovations. The ability to engineer Fano interference patterns in situ offers unprecedented control over the photonic density of states, which can be exploited to interact with a variety of quantum emitters beyond photon pairs, such as quantum dots and color centers, potentially revolutionizing quantum light-matter interfaces.

This work also addresses fundamental questions pertaining to decoherence mechanisms in quantum systems. By manipulating interference through precise design of the metasurface, the researchers demonstrated pathways to mitigate environmental noise and loss channels, thereby preserving the fragile quantum correlations essential for high-fidelity quantum operations. These insights contribute valuable knowledge to the broader endeavor of achieving fault-tolerant quantum information processing.

On the theoretical front, the integration of Fano interference principles with metasurface physics enriches the conceptual framework for describing open quantum optical systems. The interplay between discrete resonant states and continuum modes now finds a tangible experimental embodiment in engineered nanostructures, bridging gaps between abstract quantum theories and applied photonics. This synergy of theory and experiment forms the cornerstone for future explorations into nonlinear and quantum optical phenomena within artificially structured media.

The article further explores the spectral response and emission dynamics of the photon pairs, elucidating how geometrical parameters of the metasurface elements influence the resonance positions, linewidths, and interference contrasts. This deep understanding invites customizable designs, where metasurfaces can be tuned to specific operational wavelengths and quantum protocols, enhancing their functionality in practical applications ranging from secure quantum key distribution to quantum metrology.

Importantly, the work transcends pure scientific inquiry to hint at real-world technological impact. Controllable quantum interference effects realized through metasurfaces could lead to breakthroughs in creating on-chip quantum light sources with tailored emission profiles, critical for quantum computing architectures reliant on indistinguishable photons. Additionally, this technology may facilitate the development of quantum sensors with superior accuracy by exploiting interference-based sensitivity enhancements inherent to Fano resonances.

In terms of fabrication, the study leverages state-of-the-art nanolithography and material deposition techniques to achieve the requisite precision in metasurface construction. The reproducibility and scalability of these fabrication methods underscore the feasibility of industrial-scale applications and open a viable route toward commercialization of metasurface-enabled quantum photonic devices. This practical perspective ensures the research is not confined to laboratory curiosity but advances the frontier of next-generation quantum technologies.

The team’s interdisciplinary approach, combining expertise in quantum optics, plasmonics, and materials science, exemplifies the collaborative efforts needed to accelerate progress in quantum photonics. Their experimental strategies, alongside comprehensive theoretical modeling, form a comprehensive toolkit for exploring complex quantum phenomena in nanostructured environments. This holistic methodology further establishes metasurfaces as versatile platforms for exploring new physics and engineering challenges at the quantum scale.

Looking forward, this discovery is poised to inspire extensive research into hybrid metasurface-based quantum systems that incorporate active control mechanisms such as electrical tuning or optical modulation. Such advancements would propel adaptive quantum devices capable of dynamic response and reconfiguration, essential for complex quantum networks and quantum machine learning applications. The foundational work presented here is thus anticipated to catalyze a vibrant area of quantum photonics research over the coming decade.

In conclusion, the demonstration of Fano interference of photon pairs from metasurfaces epitomizes a landmark achievement in controlling quantum light-matter interactions with nanoscale precision. This convergence of quantum optics and nanophotonics not only enhances our capability to engineer quantum states but also unveils new avenues for practical quantum technologies. As metasurfaces continue to evolve, their integration with quantum systems will likely redefine the landscape of quantum information science, ultimately leading to transformative advances in secure communication, computation, and sensing.

Subject of Research: Quantum photonics; Fano interference of photon pairs; metasurfaces; quantum light manipulation

Article Title: Fano interference of photon pairs from a metasurface

Article References:
Noh, J., Santiago-Cruz, T., Doiron, C.F. et al. Fano interference of photon pairs from a metasurface. Light Sci Appl 14, 371 (2025). https://doi.org/10.1038/s41377-025-01998-5

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01998-5

The NV center, a point defect where a nitrogen impurity sits adjacent to a vacant site within diamond’s crystal lattice, serves as a natural quantum emitter whose spin states can be manipulated via optical and microwave excitation. It possesses remarkable capabilities for quantum sensing and communication, largely due to its long coherence times and stable spin-dependent fluorescence. Nevertheless, the NV center’s photoluminescence typically exhibits a broad and complex emission profile spanning multiple wavelengths, posing challenges for spectral indistinguishability and integration with photonic circuitry. The City College team, however, leveraged this characteristic in an ingenious way, revamping a supposed disadvantage into an advantage by coupling the NV emission to specially designed topological photonic waveguides.

Topological photonics has emerged as a vibrant field that exploits concepts from topological insulators in condensed matter physics to create photonic structures exhibiting robust, defect-immune modes. These modes propagate along edges or interfaces without backscattering, even in the presence of imperfections, thereby promising highly efficient and stable photon transport. By integrating NV-hosting nanodiamonds onto atomic force microscope (AFM) tips, the researchers developed a unique scanning probe methodology that permits spatially resolved interaction studies between the emitter and the photonic architecture. The AFM tip serves not only to position the quantum emitter with nanoscale precision but also to interrogate local photonic modes by analyzing emitted photons’ spectral and polarization properties.

A remarkable aspect of this investigation is the application of polarization-resolved spectroscopy to decode the complex interplay between the NV center’s emission and the underlying topological waveguide modes. Through suppression of right circularly polarized (RCP) emission at 710 nm—a key wavelength associated with the NV center’s zero-phonon line and phonon sideband—the team mapped regions supporting chiral photonic modes, manifesting as distinctive intensity fringes. This selective filtering unveiled spatial patterns within the waveguide’s electromagnetic environment that are intimately connected to the underlying topological properties, revealing domains where light fields exhibit one-way propagation determined by their handedness.

The implications of coupling the broad-band NV emission with topological photonic modes resonate deeply within quantum technology realms. Traditional efforts to generate deterministic single-photon sources with narrow linewidths often struggle with spectral diffusion and decoherence. By contrast, the demonstrated topological shaping of NV emission suggests avenues to mitigate these effects, potentially stabilizing photon frequencies and enabling robust spin–photon entanglement essential for quantum networks. Furthermore, the robustness of chiral photonic modes against scattering and fabrication imperfections indicates practical advantages in constructing scalable, integrated quantum optical circuits capable of maintaining coherence over extended distances.

Beyond quantum information processing, the team’s work pioneers a new, highly sensitive form of nanoscale sensing. The polarization sensitivity inherent in their approach permits reconstruction of photonic mode distributions with exceptional contrast, enabling detailed imaging of electromagnetic fields at the nanoscale. This methodology extends the capabilities of NV centers beyond traditional magnetometry or thermometry, positioning them as versatile probes for assessing the complex local photonic environment with both spatial and polarization resolution. Such advanced imaging capabilities could accelerate progress in nanophotonics, optical metamaterials, and biosensing.

At the interface of physics and biology, the polarization-resolved approach to detecting chiral photonic modes hints at transformative applications in molecular sensing. Chiral molecules, ubiquitous in living organisms and pharmaceuticals, often evade detection through conventional techniques due to subtle optical activity signatures. The researchers anticipate that adapting their method might enable direct sensing and characterization of molecular chirality with heightened sensitivity and spatial resolution, providing crucial insights into biomolecular structures and interactions. This integration of quantum photonics and molecular spectroscopy promises a fertile domain for future interdisciplinary research.

Follow-up investigations are set to delve deeper into the fundamental physics governing emitter–structure interactions and to push technological boundaries for device implementation. Optimization efforts will target enhancing the coupling efficiency between NV centers and topological waveguides, improving the spatial precision of emission control, and extending the approach to other quantum emitters and wavelength ranges. The synergy between experimental innovation and theoretical modeling will illuminate mechanisms underlying chiral emission, spontaneous emission modulation, and photon routing, fueling the design of next-generation photonic devices.

The versatility of the AFM-nanodiamond scanning probe technique also opens avenues for exploring a wide array of engineered photonic environments, including those with dynamically tunable topologies or nontrivial quantum properties. Researchers envision employing this platform to investigate photon-mediated spin interactions, coherent spin–spin coupling, and nonreciprocal light–matter phenomena crucial for quantum computation and secure communication. Such comprehensive studies promise to bridge gaps between fundamental quantum science and practical device engineering.

This work epitomizes a shift in quantum nanophotonics: transforming broad emission spectra from a limitation into a resource for advanced quantum control. The convergence of topological concepts with quantum emitter technology signifies a forward-looking paradigm wherein robust, defect-immune photonic states can harness complex quantum phenomena for real-world applications. As the field evolves, the City College team’s achievements mark a significant milestone, underscoring the power of interdisciplinary approaches to solve longstanding challenges in quantum engineering.

In sum, the research by Professor Carlos Meriles and collaborators provides profound insights into manipulating nitrogen-vacancy centers within topological photonic architectures. By meticulously coupling the NV center emission with chiral, topologically protected modes and employing nanoscale scanning probes, the team has charted a novel landscape where quantum emitters and advanced photonic materials coalesce. This synergy holds promise not only for enhancing quantum information technologies but also for innovating nanoscale sensing tools with far-reaching implications in medicine, materials science, and beyond.

The publication, appearing in the prestigious journal Nature Nanotechnology, highlights experimental advances accomplished through rigorous spectroscopy, nanofabrication, and probe scanning methodologies. As quantum photonics continues to steer toward integrated on-chip solutions, the principles and techniques established by this study will likely influence a broad spectrum of future research, catalyzing innovations that harness the nuanced interplay between quantum defects and engineered light.

Subject of Research: Not applicable

Article Title: Emission of Nitrogen–Vacancy Centres in Diamond Shaped by Topological Photonic Waveguide Mode

News Publication Date: 28-Aug-2025

Image Credits: Carlos Meriles

Keywords

Nitrogen-vacancy center, quantum emitter, topological photonics, chiral photonic modes, nanodiamond, scanning probe microscopy, quantum information, spin–photon entanglement, nanoscale sensing, polarization-resolved spectroscopy, quantum coherence, integrated photonic circuits

Photon avalanche, a rare and powerful photophysical phenomenon, diverges fundamentally from conventional fluorescence behaviors through its exceptionally nonlinear emission response to light excitation. Unlike linear fluorescence where emitted light intensity directly correlates with the excitation source, photon avalanche systems undergo a dramatic, abrupt surge in emission intensity once the excitation threshold is crossed. This effect is analogous to a runaway chain reaction or avalanche, propelled by intricate energy transfer dynamics within the material, amplifying light output exponentially rather than incrementally.

Central to this advancement is a meticulous redesign of lanthanide-doped nanocrystals, achieved through sublattice reconstruction by substituting lutetium ions within the crystal lattice framework. This subtle yet profound alteration induces significant local crystal field distortions, effectively enhancing interionic interactions that are pivotal for photon avalanche phenomena. Such structural manipulation enables a robust positive feedback loop among excited-state absorption (ESA) and cross-relaxation (CR) processes involving thulium ions (Tm³⁺), which mutually reinforce each other to escalate the excited-state population exponentially.

The research, spearheaded by Professor Liu Xiaogang of the NUS Department of Chemistry in collaboration with Professor Liang Liangliang from China’s Xiamen University, breaks new ground by demonstrating the capacity to engineer photon avalanche effects in nanocrystals with optical nonlinearities that leap beyond 500, a figure that eclipses prior benchmarks by an order of magnitude. This ultrahigh-order nonlinear optical response transcends limitations historically imposed on fluorescent probes and nanomaterials, ushering in new possibilities for ultrasensitive detection technologies.

At the core of the photon avalanche mechanism lies a sophisticated interplay of photophysical processes. Initially, ground-state absorption excites electrons to intermediate energy levels, which subsequently undergo excited-state absorption to higher energy states. This excited population then facilitates cross-relaxation energy transfer between neighboring lanthanide ions, effectively creating a regenerative cycle. The positive feedback loop generated between ESA and CR engenders a rapid and exponential increase in photon emission, akin to an optical cascade, which amplifies even minuscule fluctuations in excitation into significant emission shifts.

The nanoscopic scale engineering achieved by the team is particularly notable. By introducing lattice distortions at the nanoscale, the materials demonstrate enhanced energy transfer dynamics that magnify nonlinear responses. For instance, 27-nanometer nanocrystals exhibited optical nonlinearities exceeding 150, sufficient for super-resolution imaging modalities using only a single beam of excitation light. Notably, these materials achieved spatial resolution around 33 nanometers laterally and 80 nanometers axially—rivaling the performance of complex techniques such as Stimulated Emission Depletion (STED) microscopy without demanding elaborate instrumentation or high excitation intensities.

Scaling up to larger nanodisks of approximately 170 nanometers revealed even more pronounced nonlinearities exceeding 500. Fascinatingly, these larger structures exhibited spatially heterogeneous emission patterns within individual nanocrystals, enabling imaging capabilities that surpass the physical dimensions of the emitters themselves. Such spatial resolution and emission patterning introduce novel avenues for precise manipulation and detection at the nanoscale, leveraging the intrinsic nonlinear dynamics of photon avalanche materials.

The implications of this pioneering work extend far beyond imaging. The ultrahigh nonlinear response of these engineered nanomaterials opens pathways toward creating cost-effective and compact optical amplifiers capable of transducing subtle changes across a variety of stimuli—including light intensity, temperature fluctuations, and environmental variables—into amplified optical signals. These properties render photon avalanche nanomaterials ideal candidates for next-generation chemical and biosensors with extraordinary sensitivity.

Moreover, the research lays a strong foundation for advancements in quantum photonics. The precise control over nonlinear emission and feedback mechanisms underpinning photon avalanche phenomena may be harnessed to develop novel optical switches and photon sources critical for quantum information processing. The ability to spatially and intensity-specifically control emission at the nanoscale also hints at potential breakthroughs in high-density data storage and encryption technologies, where information can be encoded and retrieved through modulated optical signals with sub-wavelength precision.

Professor Liu reflects on the broader significance of the findings: “By integrating photon avalanche effects with atomically precise nanomaterials design, we are challenging the conventional thresholds of nonlinear optics. This convergence paves the way for an entirely new generation of light-driven technologies that promise unprecedented speed, compactness, and sensitivity.”

Published in the journal Nature on June 18, 2025, this research represents a milestone in photonics and materials science, potentially catalyzing a suite of technological innovations across biomedical imaging, sensing, and quantum device engineering. The combination of experimental insight and nanoscopic control achieved in this study exemplifies the growing interdisciplinarity driving modern scientific frontiers.

In conclusion, the NUS-led team’s success in pushing optical nonlinearity beyond previously attainable limits through careful lattice engineering not only enhances fundamental understanding of photophysical mechanisms but also unlocks myriad practical applications. As photon avalanche nanophotonics matures, it promises to transform how scientists and engineers exploit light-matter interactions for precision imaging, sensing, and information technologies in the years to come.

Subject of Research: Not applicable

Article Title: Optical nonlinearities in excess of 500 through sublattice reconstruction.

News Publication Date: 18 June 2025

Web References:
https://www.nature.com/articles/s41586-025-09164-y
http://dx.doi.org/10.1038/s41586-025-09164-y

Image Credits: National University of Singapore

Keywords

Physical sciences, Chemistry

At the heart of this breakthrough lies a unique fabrication process that employs ultrafast laser inscription to carve intricate waveguide architectures directly into a transparent substrate. This approach allows for unmatched three-dimensional (3D) configurability and miniaturization, overcoming the spatial limitations characteristic of traditional planar photonic integrated circuits. By integrating these reconfigurable waveguides with a high-density SPAD array, the team has effectively merged light manipulation and detection into a compact, scalable module capable of processing single photons with exceptional efficiency and precision.

The laser-writing technique facilitates dynamic, in situ tuning of photonic pathways. Unlike static lithographically defined circuits, whose designs cannot be altered post-fabrication, laser inscription allows multiple reconfiguration cycles through appropriate thermal or optical control mechanisms. This adaptability is a game-changer for applications requiring real-time optical routing, switching, or mode reshaping. Devices based on this method can dramatically enhance the flexibility and functionality of photonic networks, particularly in intricate quantum experiments where photon coherence and path integrity are paramount.

Critically, the direct coupling to a SPAD array addresses a long-standing bottleneck in integrated photonics: the efficient detection of single photons with high temporal and spatial resolution. Single-photon avalanche diodes are semiconductor devices capable of detecting individual photons by amplifying the resultant electron-hole pairs in a controlled avalanche process, producing a measurable electrical pulse. Incorporating the SPAD array directly adjacent to the waveguides reduces coupling losses and timing jitter, thereby boosting overall system performance for photon counting and quantum state measurements.

The research team meticulously optimized the waveguide-to-detector interface, balancing optical mode profiles with electrical constraints to maximize photon detection probability. By engineering the waveguide cross-sections and refractive index profiles through precision laser parameters, the device ensures efficient mode matching with the sensitive detection region of each SPAD pixel. This alignment is critical to achieving low dark count rates and maintaining high signal-to-noise ratios, essential for applications in quantum cryptography and photon-starved imaging environments.

One especially striking feature of this integrated platform is its potential to be fabricated on a compact chip scale using widely accessible laser inscription setups. Unlike conventional semiconductor fabrication processes that require expensive cleanroom facilities and complex lithography steps, ultrafast laser writing offers a more cost-effective, maskless alternative with rapid turnaround times. This democratization of photonic device engineering could accelerate the adoption of sophisticated quantum photonic systems in industry and academia, empowering a new wave of experimental possibilities.

The versatility of the laser-written PIC extends beyond quantum information science. Reconfigurable optical circuits are foundational components in next-generation optical computing architectures, neuromorphic photonic processors, and advanced sensors capable of adaptive spectral and spatial filtering. The seamless integration with SPAD technology further opens new horizons for time-resolved spectroscopy, fluorescence lifetime imaging microscopy (FLIM), and LIDAR systems, where single-photon sensitivity is indispensable.

Moreover, the realized system exhibited impressive scalability potential. The modular nature of the SPAD array, combined with the extensible architecture of the laser-written PIC, enables the construction of large-scale photonic networks that preserve quantum coherence and minimize interconnection losses. This integrated approach is expected to facilitate the development of complex quantum simulators and processors with enhanced fault tolerance and configurability, paving the way towards practical quantum advantage in computation and communication.

Beyond its immediate technological merits, this research embodies a profound shift in how photonic circuits are conceptualized and developed. The co-fabrication of reconfigurable components and detector arrays in a single, cohesive environment represents an important step towards monolithic integration of quantum photonics. This synergy mitigates signal degradation caused by interconnects and external coupling, a persistent challenge in hybrid devices that often impedes system stability and scalability.

The laser-written approach also permits the incorporation of novel material platforms that traditionally pose fabrication challenges for photonic integration. Exotic nonlinear optical materials, biocompatible glasses, or crystalline substrates can be tailored with waveguides and coupled to detectors, broadening the functionality and application scope of the devices. This flexibility encourages interdisciplinary research, bridging photonics with fields such as biophotonics, environmental sensing, and medical diagnostics.

Importantly, the research presents promising results validating the device’s performance under operational conditions relevant to quantum optics experiments. The integrated circuit demonstrated low propagation losses, high extinction ratios for waveguide switches, and temporal resolution compatible with sub-nanosecond photon events. Such performance metrics confirm the practicality of the platform for demanding applications requiring precise photon timing and routing control.

The design also anticipates integration with external electronic control circuitry, enabling active feedback loops and programmable operation modes. Future iterations could incorporate real-time reprogramming of photonic functions through microelectromechanical systems (MEMS), thermal phase tuning, or electro-optic modulation. This would significantly enhance the adaptability and user control of integrated quantum photonic devices, promoting multifunctionality within a single chip footprint.

In the grander scheme, the advent of this laser-written reconfigurable photonic integrated circuit directly coupled to a SPAD array is a vivid illustration of how photonics and quantum technologies are converging to unlock new regimes of information processing and measurement. As quantum communication protocols and quantum sensing schemes mature, the demand for compact, efficient, and tunable photonic platforms will only intensify. This innovation is well-positioned to meet these aspirations, acting as a cornerstone technology for the burgeoning quantum photonics industry.

Looking ahead, the team envisions expanding their work by integrating on-chip sources of single photons, efficient quantum memories, and error-correcting circuits, moving closer to fully integrated quantum photonic processors. Collaborations with material scientists and device engineers will further refine fabrication techniques and device architectures. Such multidisciplinary efforts are poised to catalyze the next generation of photonic technologies with unprecedented performance and versatility.

In summary, this groundbreaking research ushers in a new paradigm where laser-written reconfigurable photonic integrated circuits synergize seamlessly with single-photon detector arrays, culminating in a potent tool for advanced quantum photonics applications. The fusion of manufacturability, configurability, and single-photon sensitivity in a single platform heralds a future where quantum technologies become more accessible, robust, and scalable. The impact of this innovation is poised to resonate across scientific disciplines and industrial sectors, accelerating progress toward practical quantum-enabled devices.

Subject of Research: Reconfigurable photonic integrated circuits and single-photon detection integration

Article Title: Laser-written reconfigurable photonic integrated circuit directly coupled to a single-photon avalanche diode array

Article References:
Gualandi, G., Atzeni, S., Gardina, M. et al. Laser-written reconfigurable photonic integrated circuit directly coupled to a single-photon avalanche diode array. Light Sci Appl 14, 199 (2025). https://doi.org/10.1038/s41377-025-01854-6

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01854-6