Optical detection of single sub-15 nm objects using elastic scattering strong coupling

TL;DR

This paper demonstrates the first observation of strong elastic light scattering coupling from sub-15 nm nano-objects using plasmonic nanocavities, enabling optical detection of extremely small, non-fluorescent objects.

Contribution

It introduces a novel method to observe strong elastic scattering coupling from nano-objects smaller than 15 nm, expanding detection capabilities beyond existing techniques.

Findings

Achieved detection of 1.8 nm objects inside nanocavities.

Observed anti-crossing scattering modes indicating strong coupling.

Scattering cross-section scales with electric field to the fourth power.

Abstract

Metallic nano-objects play crucial roles in diverse fields, including biomedical imaging, nanomedicine, spectroscopy, and photocatalysis. Nano-objects with sizes that are less than 15 nm exhibit extremely low light scattering cross-sections, posing a significant challenge for optical detection. A possible approach to enhance the optical detection is to exploit nonlinearity of strong coupling regime, especially for elastic light scattering, which is universal to all objects. However, there is still no observation of the strong coupling of elastic light scattering from nanoobjects. Here, we demonstrate the strong coupling of elastic light scattering in self-assembled plasmonic nanocavities formed between a gold (Au) nanoprobe and an Au film. We employ this technique to detect individual objects with diameters down to 1.8 nm inside the nanocavity. The resonant mode of the nano-object on the Au film strongly couples with the nanocavity mode, revealing anti-crossing scattering modes under dark-field spectroscopy. The experimental result agrees well with numerical calculations, which we use to extend this technique to other metals, including silver, copper, and aluminum. Furthermore, our results show that the scattering cross-section ratio of the nano-object scales with the electric f ield to the fourth power, similar to surface-enhanced Raman spectroscopy. This work establishes a new possibility of elastic strong coupling and demonstrates its applicability for observing small, non-fluorescent, Raman inactive sub-15 nm objects, complementary to existing microscopes.