In a groundbreaking advancement poised to reshape the future of photonic technologies, researchers have unveiled a phenomenon of giant two-photon upconversion emanating from a two-dimensional (2D) exciton confined within a sophisticated doubly-resonant plasmonic nanocavity. This innovation marks a significant leap in harnessing the often elusive nonlinear optical interactions at the nanoscale, potentially revolutionizing applications ranging from ultrafast optical communication to quantum information processing.
At the heart of this discovery lies the delicate interplay between 2D excitons and plasmonic nanostructures. Excitons, quasiparticles representing bound electron-hole pairs, exhibit remarkable optical properties when confined in atomically thin semiconductor layers. These 2D materials, characterized by their reduced dimensionality, offer enhanced Coulomb interactions and markedly increased binding energies, enabling pronounced excitonic effects even at room temperature. By embedding such excitons within a nanocavity engineered to embrace dual resonances, the research team has effectively amplified nonlinear optical processes, resulting in an unprecedented efficiency of two-photon upconversion.
Two-photon upconversion refers to the nonlinear optical process where two photons of lower energy are simultaneously absorbed, combining their energies to emit a single photon of higher energy. This phenomenon, highly coveted in photonics for its potential in frequency conversion and bioimaging, is typically hampered by inefficiencies due to the need for strict phase matching and weak light-matter coupling in conventional materials. Overcoming such limitations demands strategic engineering at the nanoscale, a challenge adeptly addressed by leveraging the plasmonic nanocavity’s unique capabilities in this study.
The doubly-resonant plasmonic nanocavity constructed by the authors exhibits two discrete resonance modes precisely matched to both the excitation and emission wavelengths involved in the two-photon process. This carefully tuned resonator design ensures that the local electromagnetic fields at these frequencies are intensely confined and significantly enhanced, boosting the interaction strength between the incident photons and 2D excitons. Such dual resonance not only magnifies the absorption probability but also facilitates efficient emission, thereby optimizing the entire upconversion cycle.
Material-wise, the choice of 2D semiconductor material is pivotal. The research utilized monolayer transition metal dichalcogenides (TMDs), a class of 2D semiconductors known for their direct bandgaps and pronounced excitonic resonances in the visible spectrum. These properties allow the 2D excitons to couple strongly with the localized surface plasmons generated within the metallic nanocavity, resulting in a remarkable interplay that profoundly influences the nonlinear optical response. This strong coupling regime is instrumental in realizing the giant upconversion effect reported.
From an experimental perspective, the authors meticulously fabricated and characterized the doubly-resonant nanocavities, employing advanced nanolithography techniques to achieve nanoscale precision in cavity dimensions. Structural characterization confirmed the cavity’s geometric parameters, while spectral measurements validated the dual resonance modes’ positions. Subsequent nonlinear optical experiments revealed an extraordinary enhancement in two-photon upconversion efficiency—orders of magnitude greater than in isolated 2D materials or conventional plasmonic systems lacking such resonance engineering.
The mechanics behind this giant upconversion can be understood through the concept of Purcell enhancement, where the spontaneous emission rate of an emitter—here, the 2D exciton—is amplified by its photonic environment. In the doubly-resonant plasmonic nanocavity, the local density of optical states is tailor-made, leading to a synergistic enhancement of both two-photon absorption and exciton radiative recombination. This synergy culminates in a nonlinear optical process of unprecedented scale and efficiency, which until now had been largely theoretical.
The implications of these findings are vast and multifaceted. In the realm of optical computing and telecommunications, the ability to convert photons across different energies with high efficiency and at the nanoscale can lead to novel, compact photonic devices capable of ultrafast signal processing and wavelength multiplexing. Furthermore, applications in bioimaging stand to benefit greatly, as two-photon upconversion enables deeper tissue penetration with reduced photodamage, promising advancements in medical diagnostics and live imaging techniques.
Another notable facet of this work is the potential to integrate such 2D exciton-plasmonic nanocavity systems with emerging quantum technologies. Nonlinear optical processes are central to generating entangled photon pairs and single-photon sources, essential components for quantum cryptography and computing. Here, the giant two-photon upconversion response could serve as a platform for efficient quantum light sources at room temperature, significantly advancing practical quantum photonics.
Beyond the immediate technological landscape, the study provides crucial insights into the fundamental physics governing light-matter interactions in reduced dimensions under extreme confinement. Understanding how excitons behave and interact with plasmonic fields opens new avenues for exploring exciton-polariton phenomena, many-body interactions, and quantum coherence effects in 2D heterostructures, which remain at the frontier of condensed matter physics and nanophotonics.
The research also highlights the importance of precise nanofabrication and materials synthesis to tailor the optical environment rigorously. Achieving doubly-resonant conditions demands a harmonious balance between cavity design, material choice, and experimental conditions—a triad that, when optimized, unlocks phenomena previously unattainable in single-resonance or less controlled settings.
Looking ahead, the team envisions that their approach can be generalized to other 2D materials and hybrid nanostructures, paving the way for customizable nonlinear optical devices operating across a broad spectral range. This adaptability is crucial as photonic technologies evolve towards multifunctional, integrable platforms for sensing, energy harvesting, and information processing.
Moreover, this giant two-photon upconversion mechanism can inspire new strategies for enhancing other nonlinear processes such as harmonic generation and four-wave mixing in 2D systems, further expanding the toolkit for engineering light at the nanoscale. As such, the findings are not confined to a single phenomenon but rather illuminate a broader paradigm of nanoscale nonlinear optics capability.
In sum, the study presents a compelling demonstration of how meticulously engineered plasmonic nanocavities can unlock extraordinary nonlinear optical phenomena in atomically thin semiconductors. By marrying the unique excitonic properties of 2D materials with the electromagnetic prowess of plasmonics, this research sets a new benchmark for photonic device performance, promising a future where light manipulation at the quantum level is both practical and scalable.
This breakthrough not only enriches the fundamental understanding of exciton-plasmon coupling but also propels the field towards real-world applications, signalling an exciting era where two-photon upconversion and related nonlinear processes are harnessed with unprecedented efficiency, fidelity, and versatility.
As the scientific community digests the full impact of these findings, further explorations into tuning resonance conditions, improving material quality, and integrating such nanocavities in device architectures will undoubtedly accelerate the transition from proof-of-concept demonstrations to impactful technologies shaping the next generation of photonic systems.
Subject of Research: Giant two-photon upconversion from 2D excitons in a doubly-resonant plasmonic nanocavity
Article Title: Giant two-photon upconversion from 2D exciton in doubly-resonant plasmonic nanocavity
Article References:
Liu, F., Liu, H., Chi, C. et al. Giant two-photon upconversion from 2D exciton in doubly-resonant plasmonic nanocavity. Light Sci Appl 14, 312 (2025). https://doi.org/10.1038/s41377-025-02010-w
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