In the rapidly evolving realm of nanophotonics, a recent breakthrough heralds transformative potential for two-dimensional (2D) excitonic systems and their interaction with light. Scientists led by Professors Pengfei Qi and Weiwei Liu at Nankai University have unveiled a groundbreaking approach to dramatically enhance two-photon upconverted emission by leveraging doubly-resonant plasmonic nanocavities. Their pioneering work, published in Light: Science & Applications, reveals an extraordinary 2440-fold amplification in excitonic upconversion processes within monolayer transition metal dichalcogenides (TMDs), specifically WS₂, opening new frontiers for high-performance nonlinear photonic devices.
At the heart of this innovation lies the strategic design of a plasmonic nanocavity composed of meticulously engineered gold (Au) nanocubes positioned atop ultrathin WS₂ monolayers and Au films separated by dielectric spacers. Utilizing sophisticated three-dimensional finite-difference time-domain (3D-FDTD) simulations, the team tailored nanocavity resonances to match both excitation and emission wavelengths of 2D excitons. This dual spectral resonance, or “doubly resonant” configuration, is critical in enhancing the two-photon absorption and subsequent exciton recombination processes, overcoming inherent inefficiencies associated with multiphoton excitation and long radiative lifetimes in 2D semiconductors.
Two-dimensional TMD monolayers such as WS₂ have garnered intense research interest due to their tightly bound excitons resulting from reduced dielectric screening and pronounced Coulomb interactions. Excitons, quasiparticles formed by electron-hole pairs bound through Coulomb attraction, govern the optoelectronic properties in these materials and present unique avenues for nonlinear optical phenomena such as photon upconversion. This process, whereby low-energy photons are converted into higher-energy emissions via mechanisms like multiphoton absorption and Auger recombination, is pivotal for energy conversion and optical information technologies. However, realizing efficient two-photon upconversion has been a longstanding challenge, limited primarily by low absorption cross sections and inefficient radiative pathways.
The introduction of the plasmonic nanocavity fundamentally changes this paradigm. By engineering the nanocavity’s resonance wavelengths to coincide precisely with both the excitation laser and the emission bands of WS₂ excitons, the researchers achieved an unprecedented magnification of the local electromagnetic fields at the nanoscale. This electromagnetic confinement significantly boosts the excitation rate, while simultaneously enhancing the collection efficiency of emitted photons—a necessity for translating theoretical enhancements into practical device performance.
Photoluminescence (PL) measurements underscored the remarkable performance of the Au nanocube/WS₂/Au film heterostructure. Compared to WS₂ monolayers on conventional Au/SiO₂/Si substrates, the plasmonic nanocavity system exhibited a 2440-fold increase in upconverted PL intensity under identical excitation conditions. This dramatic enhancement stems from multiple synergistic effects: intensified excitation from local plasmonic fields, the Purcell effect-induced acceleration of spontaneous emission rates, and improved collection efficiency due to the antenna-like emission pattern shaped by the nanocavity geometry.
Fermi’s golden rule provides a theoretical foundation for understanding these enhancements. It posits that the spontaneous emission rate of an emitter is proportional to the local density of electromagnetic states surrounding it. Within the plasmonic nanocavity, the localized surface plasmon polaritons (SPPs) generated by the interaction of Au nanoparticles with the underlying Au films create concentrated electric fields and image dipole effects, markedly increasing the photonic density of states. Time-resolved PL spectroscopy further confirmed this behavior: the luminescence lifetime of excitons in the cavity-coupled WS₂ was notably shortened, indicating an accelerated decay rate that benefits the overall quantum efficiency.
Beyond enhancing the emission rate, the plasmonic nanocavity functions as a nanoscale patch antenna, directing emitted photons into predefined angular distributions matched to the numerical aperture of collection optics. This antenna effect registers as a 1.6-fold improvement in emitted light collection efficiency for the nanocavity device relative to bare WS₂ systems, circumventing common inefficiencies arising from isotropic photon emission.
Comprehensive 3D-FDTD simulations elucidated the intricate interplay of charge oscillations and electromagnetic field distributions within the nanocavity. The spatial separation controlled by spacer layers and the size of the gold nanocubes were shown to be crucial parameters dictating the resonance wavelengths and field enhancement factors. Both in-plane and out-of-plane field components were significantly strengthened, providing the multidimensional enhancement necessary to maximize upconversion efficiency.
Interestingly, the researchers also explored the temperature dependence of the system and reported a thermal tuning of the excitonic upconversion properties. At an elevated temperature of 350 K, the amplification factor intriguingly surpassed 3000-fold, highlighting the feasibility of exploiting temperature as a control parameter to optimize plasmon-exciton coupling dynamics.
The study draws a compelling contrast between the massively amplified two-photon upconverted emission and other nonlinear optical processes such as second-harmonic generation (SHG) and conventional photoluminescence. While SHG and PL signals exhibited enhancement factors of approximately 134-fold and 890-fold respectively, these values were significantly lower than those observed for two-photon upconversion. This disparity underscores the distinctive double-resonance mechanism underpinning upconversion amplification, which requires simultaneous spectral matching of both excitation and emission resonances—a condition not obligatory for single-resonance processes like SHG or PL.
Collectively, this work offers profound insights into the manipulation of exciton-photon interactions within plasmonic nanostructures. By optimizing excitation rate enhancement, spontaneous emission acceleration via the Purcell effect, and directional photon collection in a single integrated platform, the team demonstrated a new paradigm for generating giant two-photon upconversion signals. These findings pave the way toward cost-effective, highly efficient nonlinear photonic devices with versatile applications in quantum optics, nanoscale lasing, and advanced optical sensing.
Future research is poised to expand upon this foundation by exploring diverse material systems, alternative plasmonic geometries, and dynamic tuning schemes such as electrical gating and strain engineering to further tailor exciton-plasmon coupling. Moreover, the ability to probe excitonic dark states through enhanced upconversion heralds exciting prospects in the fundamental study of quasiparticle dynamics and emerging optoelectronic functionalities.
This investigation unequivocally demonstrates how intricate nanofabrication combined with rigorous theoretical modeling can unlock exceptional nonlinear optical phenomena in low-dimensional materials. Ultimately, the integration of doubly-resonant plasmonic nanocavities with 2D excitonic materials represents a monumental stride toward harnessing the full potential of light-matter interactions at the nanoscale for next-generation photonic technologies.
Subject of Research: Enhancement of two-photon upconverted emission in 2D excitons using doubly-resonant plasmonic nanocavities
Article Title: Giant two-photon upconversion from 2D exciton in doubly-resonant plasmonic nanocavity
Web References: https://doi.org/10.1038/s41377-025-02010-w
Image Credits: Fangxun Liu et al.
Keywords
2D excitons, plasmonic nanocavity, two-photon upconversion, transition metal dichalcogenides, WS₂ monolayer, Purcell effect, nonlinear photonics, finite-difference time-domain simulation, plasmon-exciton coupling, photoluminescence enhancement, nanoscale antenna, quantum efficiency amplification
