Ultrafast photoluminescence and multiscale light amplification in nanoplasmonic cavity glass

The quest for enhanced nanoscale light sources has driven significant research into the integration of plasmonic nanoparticles (NPs) with nanoscale emitters. Localized surface plasmon resonance (LSPR) in metallic NPs, excited at resonant wavelengths, generates amplified electromagnetic fields, thereby boosting the optical response of nanosystems. This enhancement impacts fluorescence/photoluminescence, optical nonlinearities, and Raman scattering. Plexcitonic systems, where plasmonic NPs interact with excitonic systems (quantum dots or molecules), are particularly promising due to their potential in optoelectronics, ultrafast optical switches, and quantum information science. Enhanced emission through exciton-plasmon coupling has been extensively studied in micro- and nanoscale structures, but analogous demonstrations in bulk materials have been largely unexplored. This study aims to address this gap by investigating a novel bulk nanocomposite glass material.

Existing literature showcases significant advancements in plasmonically enhanced luminescence, particularly in plexcitonic systems. Studies demonstrate the use of exciton-plasmon coupling to build spasers (surface plasmon amplification by stimulated emission of radiation devices), which, when synchronized, generate coherent radiation. Plexcitonics also facilitates controlled coupling and room-temperature strong coupling, leading to potential applications in low-threshold lasers, biomedical sensing, and quantum information processing. Common luminescence sources include semiconductors (quantum wells, nanowires, quantum dots) and fluorescent molecules, with quantum dots (QDs) being highly efficient emitters due to their chemical/thermal stability and high photobleaching threshold. However, spectral resolution (FWHM) in current QD systems is limited (20-30 nm). Most previous work focused on colloids or surface-assembled systems, neglecting the potential of bulk materials. This study leverages the advantages of glass as a matrix material, overcoming challenges associated with polymer matrices, such as agglomeration and separation of NPs.

The researchers fabricated bulk plexcitonic nanocomposites by employing the Nanoparticle Direct Doping (NPDD) method. This method allowed for the simultaneous doping of sodium borophosphate glass (NBP) with cadmium telluride quantum dots (CdTe QDs) and silver nanoparticles (nAg). NBP glass was chosen for its broad transparency range and low melting point, enabling the incorporation of NPs without damage during solidification. The 1.5 nm diameter CdTe QDs served as excitonic emitters, while the 20 nm diameter nAg NPs acted as plasmonic sources, with their absorption and LSPR overlapping. The NPDD method involved melting and solidifying a mixture of raw materials via a directional solidification process. As a control, nanocomposites with only QDs (NBP:CdTe) were also fabricated. The resulting materials were characterized using various techniques. Photoluminescence (PL) spectra were obtained using 405 nm or 488 nm continuous-wave laser excitation, recorded with a monochromator and photomultipliers. Temperature-dependent measurements were conducted using a cryostat. PL lifetime measurements utilized a streak camera with an optical parametric amplifier and a femtosecond laser. Finite-difference time-domain (FDTD) simulations were performed to model the interaction between CdTe QDs and nAg NPs.

The NBP:CdTe,nAg nanocomposite demonstrated significantly enhanced optical properties compared to the control samples. It exhibited ultranarrow (FWHM = 13 nm) and ultrafast (60-90 ps) emission at room temperature. The exciton emission at 503 nm was approximately 50 times stronger in the composite containing both nAg and QDs compared to the QD-only sample. The FWHM of the exciton emission was substantially reduced (13 nm vs 34 nm for QDs in water). The PL decay time was three orders of magnitude faster than for water-dispersed QDs (60-90 ps vs 30 ns). FDTD simulations confirmed that the enhanced PL stemmed from the electromagnetic field enhancement in the vicinity of QDs situated in nanocavities between two nAg particles. The simulations showed a massive PL emission enhancement factor (nearly 70,000) for QDs within the nanocavity formed by an nAg dimer. Temperature-dependent studies revealed a multicomponent PL emission, with exciton (X) and donor-acceptor (D-A) contributions. The analysis showed strong electron-phonon coupling, with a higher phonon energy (43 meV) than typical for CdTe (21.7 meV), indicating lattice distortion. Power-dependent measurements indicated coherent light amplification above 100 mW, with a distinct saturation point observed above 165 mW. An extinction dip, characteristic of systems exhibiting coupling between QDs and plasmonic microcavities, corroborated the coherent light amplification.

The findings demonstrate the successful creation of a bulk, volumetric plexcitonic material with significantly improved optical properties compared to previously reported systems. The ultranarrow and ultrafast emission at room temperature is a key advancement, addressing limitations of previous QD-based systems. The observed coherent light amplification opens possibilities for the development of novel optical devices with high repetition rates, such as telecommunication components. The ultranarrow PL also enables the creation of light sources with precisely defined emission wavelengths, important for applications like screen pixels. The short PL decay times suggest potential for single-photon emission with high repetition rates. The facile fabrication method based on NPDD offers a scalable and versatile approach for creating these materials. Future work could focus on further optimization of the system's parameters, for example, integrating the nanocomposite with whispering gallery mode resonators, to potentially achieve even narrower emission and explore single-photon emission with precise control over spin interactions in a magnetic field, potentially useful in spintronics.

This study successfully fabricated a bulk nanocomposite glass exhibiting ultrafast and ultranarrow photoluminescence at room temperature, arising from multiscale light amplification within plasmonic nanocavities. The facile fabrication method, combined with the superior optical properties, opens exciting avenues for high-speed optical devices and other applications requiring precisely defined emission wavelengths. Future research could explore the integration of this material with different resonator types to further enhance its performance and explore its potential in single-photon emission and spintronics.

While the study demonstrates significant advances, potential limitations exist. The random distribution of QDs and nAg NPs might lead to variations in the optical properties across the material. Further investigation into the long-term stability of the nanocomposite under continuous operation is needed. The simulations, while providing insightful understanding, are based on certain assumptions that might not perfectly reflect the complex interactions within the material. A more detailed analysis considering various factors, such as the specific distribution of nanoparticles and QDs, could further refine the accuracy of the theoretical model.