Giant two-photon upconversion from 2D exciton in doubly-resonant plasmonic nanocavity

Photon upconversion through high harmonic generation, multiphoton absorption, Auger recombination and phonon scattering performs a vital role in energy conversion and renormalization. Excitons, the hydrogen-like bosonic quasiparticles formed by electron−hole pairs through Coulomb attraction, generally dominate the optical and electrical properties of low-dimensional semiconductor materials. As an important class of two-dimensional semiconductors, transition metal dichalcogenides monolayers provide a platform of enormous potential to explore photon upconversion because of the reduced dielectric screening and enhanced Coulomb interactions. Additionally, two-photon upconversion was recently demonstrated as an emerging technique to probe the excitonic dark states due to the extraordinary selection rule compared with conventional excitation. However, highly efficient two-photon upconversion still remains challenge due to the limited multiphoton absorption efficiency and long radiative lifetimes.

 

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Pengfei Qi and Weiwei Liu from Institute of Modern Optics, Nankai University, Key Laboratory of Micro-scale Optical Information Science and Technology, Tianjin 300350, China, and co-workers have observed an unprecedented 2440-fold amplification in the two-photon upconverted emission from 2D excitons, which can be attributed to the elevated excitation rate, magnified light collection, and quantum efficiency amplification arising from the Purcell effect. Figure 1a presents the device architecture of the Au nanocubes/WS₂/substrate plasmonic upconverter, designed using three-dimensional finite-difference time-domain (3D-FDTD) simulations. Figure 1b shows the spectral matching between plasmonic resonances and the characteristic excitation/emission bands of 2D excitons. The nanocavity's resonance wavelength can be adjusted by the spacer distance and the nanoparticle size. PL measurements reveal an intensity enhancement for cavity-coupled WS₂ relative to the monolayer WS₂ on Au/SiO₂/Si, confirming the predicted plasmonic amplification (Figure 1c). Considering the excitation area and the cavity hotspot area, an enhancement range of 2330~2440-fold was estimated (Figure 1d).

 

Fermi's golden rule establishes that the emission ratefor an emitter in a confined environment scales with the local density of states at the emission frequency. Subsequently, the excited-state decay dynamics were characterized by time-resolved photoluminescence (PL) spectroscopy. The nanocavity-coupled WS₂ demonstrated a significantly reduced luminescence lifetime (Figure 2a), due to the enhanced spontaneous emission rate for the emitter in nanocavity, known as the Purcell effect. The plasmonic nanocavity can also be regarded as a nanoscale patch antenna, effectively improving emission directionality to optimize light collection in optical systems with a fixed numerical aperture (NA). The objective lens (NA = 0.5) collects 59.1% of emitted light in nanocavity (Figure 2b), representing a 1.6-fold improvement over monolayer WS₂ on Au/SiO₂/Si (Figure 2c). Additionally, 3D-FDTD simulations reveal the charge and field distributions within nanocavity (Figures 2d-2i). Within the plasmonic cavity, Au nanoparticles interact with underlying Au film, generating image dipoles that couple with SPPs, to produce strong field enhancement in both in-plane and out-of-plane components.

 

To gain more insight into the attractive doubly resonant enhancement in such plasmon−exciton coupling system, the intriguing thermal tuning excitonic upconversion with optimized amplification factor exceeding 3000 at 350 K was realized. Meanwhile, it was demonstrated that the conventional PL is enhanced (~890-fold) only by the Purcell effect induced by the cavity mode at 2.0 eV, and second-harmonic generation (SHG) signals can be efficiently enhanced (~134-fold) by localized field of the fundamental wave which is resonant with the cavity mode at 1.55 eV. The measured enhancement factors of SHG and PL are significantly lower than two-photon upconversion in the same nanocavity, confirming the contribution of the double-resonance mechanism.

 

In summary, due to optimized light collection efficiency, enhanced excitation rates, and increased spontaneous emission efficiency, a 2440-fold two-photon upconversion enhancement was obtained. To clarify the physical mechanism, single-resonance-enhanced SHG and PL alongside thermally tuned dual-resonance upconversion are investigated. These results establish a foundation for developing cost-effective, high-performance nonlinear photonic devices and probing fine excitonic states via configuring plasmonic nanocavities.

---

---