Spatiotemporal microscopy (SPTM) techniques enable the probing of the evolution of photogenerated energy carriers in both space and time [1]. Through the application of ultrafast lasers and time-resolved detection methods, these techniques reach the femtosecond to picosecond time scales of the natural transport phenomena. Nonetheless, in the spatial domain, SPTM is still fundamentally diffraction-limited. Although sensitive to nanoscale signal variations, the diffusion information is averaged over many hundreds of nanometers, preventing the access to the real scale of exciton transfer, where diffusion lengths typically range from 10 to 100nm [2]. To bridge this size gap and access the real nanoscale structure of materials, we present a method to localize the excitation down to a few tens of nanometers 

Our method is based on a nanostructured platform consisting of rectangular nanoslits fabricated through electron-beam lithography on an opaque aluminum thin film on a glass cover slide. Upon illumination, only the light transmitted through a slit reaches the sample on the platform, resulting in a background-free excitation spot confined to the dimension of the slit – as small as 50nm.

Here we show the first successful proof-of-concept experiments using this near-field approach to study the exciton dynamics in the organic semiconductor Y6. By combining this platform with a photoluminescence SPTM configuration, we retrieve the diffusivity constant, the luminescence lifetime, and the diffusion length. Varying excitation fluence and sub-wavelength slit dimensions we easily reached the fluence regime where nonlinear effects, such as singlet-singlet annihilation (SSA), become negligible. As a result, consistent diffusion constants were obtained.

In conclusion, we present a versatile platform where the limits of excitation localization are no longer imposed by diffraction, but only by the fabrication capabilities at hand. Given the cover slide-based design, it is easy to implement with other experimental setups, such as pump-probe SPTM, and thus not limit the sample pool to photoluminescent materials only. Furthermore, the expansion of these sub-diffraction capabilities also to the detection should be possible by combining the slits with nanoantennas strategically fabricated in their vicinity. These will act as localized detection spots, leading to the next generation of ultra-high resolution and sensitivity spatiotemporal studies.