Molecule-by-molecule nanoresolved picosecond Purcell-enhanced emission at room-temperature in hollow plasmonic nanocone metamaterials

The gist: A novel platform coupling molecular single emitters to 3D hollow plasmonic nanocones enables precise engineering of spontaneous emission changes in molecular single emitters through 3D nanofields, leading to nanometer-scale maps of the Purcell factor for individual molecules.

Platform and Geometry

The research introduces a novel material platform based on a 3D hollow plasmonic nanomaterial arranged in a square lattice which is uniformly scalable to the centimeter scale while maintaining unit cell geometry The nanostructures were created using a redeposition method that combines subtractive hybrid lithography (SHL) with Talbot lithography and I-line photolithography for precise nano- and macroscale patterning This platform is scalable to centimetersized samples while maintaining precise morphological control The hollow structures exhibit a rich variety of modes due to their inherit asymmetric geometry along the longitudinal axis

Light-Matter Interaction Enhancement

The system couples molecular single emitters to these 3D plasmonic material platform which supports "rich optical modes and nanoscale hotspots with extreme field enhancement (>600x according to simulations) Using far-field single-molecule fluorescence lifetime imaging microscopy (FLIM), the technique achieves nanometer-scale localization maps that reveal up to 100× enhancement in spontaneous emission decay rates at room temperature The enhancement depends strongly on the emitter’s position and dipole orientation, as well as the illumination angle"

Sensing Methodology

The study employs a custom-built single-molecule super-resolution microscope which correlates signals from an electron-multiplying CCD (EMCCD) camera with those from an array of eight single-photon avalanche diodes (SPADs) integrated with a time-correlated single photon counting (TCSPC) system This setup provides picosecond temporal resolution and allows for the measurement of the total spontaneous emission decay rate, denoted as Γt, by fitting the decay curves with a single-exponential model By correlating the arrival times of intensity bursts recorded by the SPADs with events detected by the EMCCD camera, researchers can reconstruct a super-resolved Purcell-enhanced map of single emitters with precise super-localization The resulting precision is approximately 14 nm within a 10 µm2 field of view

Spatial and Orientation Dependence

The analysis reveals that the decay rate enhancement is highly dependent on the emitter's spatial position within the nanostructure, as shown by examining the normalized decay rate, Γ⁄Γ, as a function of its distance from the pillar’s center Under normal incidence, the decay rate enhancement is mostly below 20 In contrast, under oblique illumination at approximately θ ≃ 30°, the decay rate significantly increases, with many values exceeding the maximum temporal resolution of our setup (GMax= 50 ns-1) Furthermore, filling the cavity with a polymer results in a pronounced decrease in the occurrence of decay enhancement values between 20 and 30 This reduction occurs because the polymer prevents molecules from binding inside the cavity, where moderate field enhancement occurs The highest decay rate enhancements are observed for dipoles placed on the top ring of the truncated cone (position 1) under oblique illumination, which is attributed to the excitation of a longitudinal mode of the hollow truncated cone

Emission Characteristics and Interpretation

The measurements show that emission remains largely unquenched despite the close proximity of the fluorophores to the metallic structures The spatial distribution of detected fluorescence intensity exhibits a smooth increase from the edges toward the center, with a distribution sharply peaked at the supercell’s center—demonstrating the remarkable super-localization precision of our method The results confirm that lifetime measurements alone are sufficient for molecular sensing of the 3D electromagnetic field within or outside the nanostructure using these 3D hollow plasmonic systems The study concludes that this platform can enhance emitter quantum yields and achieve picosecond-scale spontaneous emission lifetimes at room temperature This approach enables detection of subtle electromagnetic field changes—both at the surface and deep within the 3D near-field landscape

Simulation Validation

Numerical simulations using the Finite Element Method (FEM) were performed to analyze the near-field distribution and enhancement under both normal and oblique incidence of p-polarized plane waves in an infinite square array of Au hollow truncated cones Simulations revealed that at approximately 600 nm, the electromagnetic field is broadly distributed along the 3D structure, while at around 700 nm, the field is highly localized at the top of the truncated cone The calculated total decay rate enhancement values vary depending on emitter position: "the highest values are observed for dipoles placed on the top ring of the truncated cone (position 1), relatively high for those on the outer walls (position 2), moderate for dipoles inside the cavity (position 3), and the lowest for dipoles near the base of the cone (position 4) The results confirm that the position- and orientation-dependent behavior in this noncanonical conical geometry explains the significant variation observed in decay rate enhancements"

Fabrication Details

The fabrication of hollow Au nanopillars utilizes subtractive hybrid lithography (SHL), combining displacement Talbot lithography (DTL) which is utilized to generate a selfimage by exposing a periodic phase-shift mask with a monochromatic light source Gold is deposited onto the patterned resist through the sputter-redeposition process, forming dense arrays of hollow nanopillars arranged in a square lattice with a 250 nm pitch The fabrication process allows for centimeter scale production with wafer-scale precision and morphology control

Conclusion

In this work, the authors have introduced a novel platform and a methodology to precisely control and investigate spontaneous emission lifetimes at the nanoscale This enables detailed observation of light-matter interactions beyond the diffraction limit, facilitating molecular localization with nanometric precision The findings demonstrate that emission performance can be substantially improved through simultaneous enhancement of both radiative and non-radiative decay rates of individual molecular emitters This platform is produced as a wafer-scale device, with reproducible emission-rate enhancements patterned at the nanoscale across centimeter-sized areas The study confirms that the hollow plasmonic conical geometry offers a robust solution for mitigating plasmonic losses, while simultaneously supporting diverse 3D dispersive modes This work provides unprecedented, high non-averaged Purcell factors, demonstrating significant potential for quantum applications and sensing technologies

Methods Summary

The methodology involves several key steps:

1. Hollow Conical Pillars Fabrication: Utilizing SHL and DTL to create the structures on a substrate, followed by gold deposition via sputter-redeposition and angled ion beam etching (IBE) to control the redeposition rate The fabricated Au hollow nanopillars feature a solid base with a thickness of 90 ± 2 nm and a diameter of 140 ± 4 nm, while the nano-rims are 240 ± 10 nm high

2. Molecule-functionalized Sample Preparation and Single-Molecule Regime Optimization: Molecules were labeled by adding a mixture of PBS, sodium bicarbonate solution and photoactivatable Abberior CAGE 635 in Dimethylsulfoxide The photoactivation power was optimized so that the PSFs recorded on the EMCCD were sparse enough to be localized individually (i.e. separated by more than the diffraction limit, around 250 nm) and each SPAD collects the signal of at most one molecule at each time

3. Single-emitter Fluorescence Lifetime Experiments: smFLIM was carried out using a pulsed excitation laser with a repetition rate of 39 MHz, and decay rates were obtained by fitting the decay curves with a single-exponential model The system utilized an EMCCD camera and an SPADs array connected to a time-correlated single photon counting (TCSPC) module

1. Advanced Molecular Sensing Platforms: The system can enable molecular position sensing with resolution surpassing the diffraction limit by mapping nanometer-scale map of the Purcell factor for individual molecules within a 3D electromagnetic landscape.

2. Single-Molecule Quantum Control: The platform allows for fine-tuned control of light-matter interactions, specifically enabling the ondemand control of fast single-photon sources at room temperature.

3. In Situ Field Mapping: Researchers can perform ondemand control of fast single-photon sources and gain insights into the local density of states (LDOS) by generating a map that depends on both molecular position and dipole orientation, offering extreme position sensitivity within the 3D electromagnetic landscape.

4. Quantum Application Development: The technique provides a tool for molecular sensing and quantum applications at the single-emitter level, potentially enabling detection of minute electromagnetic field variations with unprecedented sensitivity for future quantum sensors.

Nanoscale variations in the electromagnetic environment strongly influence quantum emission processes, yet direct quantitative visualization of these effects at the single-emitter level remains challenging. By combining single-molecule super-resolution microscopy with time-correlated single-photon counting (TCSPC), we read the local electromagnetic environment of each molecule from its position and its excited-state lifetime. Using wafer-scale arrays of hollow plasmonic nanocones, we map position-dependent variations in the local density of optical states (LDOS) along the nanocone axis. We observe pronounced LDOS gradients that evidence a shortening of the fluorescence lifetime from nanoseconds to few picoseconds. The measured decay rates along the nanocone axis with maximum value at the top ring, yielding a total spontaneous-emission rate enhancement of more than two orders of magnitude at the single-molecule level, reaching an enhancement of the total spontaneous-emission rate of up to two orders of magnitude at the single-molecule level, in agreement with numerical simulations. These results provide direct experimental access to three-dimensional electromagnetic landscapes and establish a non-perturbative platform for quantitative molecular nanosensing in complex photonic architectures. The approach opens new opportunities for engineered room-temperature photon sources, vectorial field sensing, and scalable integrated nanophotonic technologies.