Abstract
The optical resonance of metal nanoparticles can be used to enhance the generation and detection of their main vibrational mode. In this work, we show that this method allows the accurate characterisation of the particle’s size because the vibrational frequency of plasmonic nanoparticles only depends on their mechanical properties. Moreover, by a careful selection of the particle size and/or probe laser wavelength, the detected signal can be increased by a large factor (∼9 for the particles used in this work) under the same illumination conditions. Finally, we show experimentally that particles of different sizes inside the point spread function can be observed due to the differences in their vibrational states, which could provide a feasible route to super-resolution.
Highlights
The physical and optical properties of nanoparticles have been heavily studied in the last few decades
The optical response of plasmonic nanoparticles was studied by the backscattered signal from the center of gold nanoparticles, by comparing the modulation of the backscattering cross-section when the probe wavelength is changed
The described behaviour shows a method to characterise sub-optical size nanoparticles (Section 5.2), and it might allow high-resolution detection of several nanoparticles inside the point spread function based on their vibrational mode frequencies and their localisation if we find the centroid for the amplitude maps at each particle frequency
Summary
The physical and optical properties of nanoparticles have been heavily studied in the last few decades. This interest stems from their specific behaviour that depends on the particle size and shape and is very different from that of the bulk material. The optical scattering properties of nanoparticles have been well-known since the development of Mie theory [1] and, more recently, the stability of numerical solutions has allowed reliable calculations on spherical structures [2,3]. Nanoparticles can work as sources of ultrasounds when a short optical pulse (pump laser) is used to thermally excite them producing GHz acoustic waves [14,15]. The vibrational response of a nanoparticle can be detected in the time domain by measuring the changes in the scattered light
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